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Photocatalytic Oxidation for Indoor Air Purification: A Literature Review
Abstract
This paper presents a literature review of using photocatalytic oxidation (PCO) to destruct volatile organic compounds (VOCs) in indoor air. TiO2 is used extensively as a photocatalyst due to its superior characteristics. Through kinetic experiments, the dependence of reaction rate on some key influencing factors (moisture, light intensity, initial contaminant concentration) has been studied, and kinetic models have been developed to aid the optimal reactor design. In general, the final products of PCO include CO2 and H2O. However, the intermediates, which are produced in the process of PCO, shouldn't be ignored because they can occupy the active sites of catalyst and lead to the deactivation of the catalyst.
... However, compared to g-C 3 N 4 /TiO 2 , under the low-light and no-light conditions, the improvement of g-C 3 N 4 /TiO 2 /SrAl 2 O 4 : Eu 2+ , Dy 3+ in CO degradation rate was too small to show regular changes. The reason for this phenomenon may be that a reversible reaction occurs between CO and CO 2 when the catalyst is irradiated with light [43]. The two are both reactants and products, and after a certain period of time, their concentrations reach a relative equilibrium state. ...
... As shown in Figure 6, the results indicated an overall decrease in CO con However, compared to g-C3N4/TiO2, under the low-light and no-light conditio provement of g-C3N4/TiO2/SrAl2O4: Eu 2+ , Dy 3+ in CO degradation rate was to show regular changes. The reason for this phenomenon may be that a reversib occurs between CO and CO2 when the catalyst is irradiated with light [43]. T both reactants and products, and after a certain period of time, their concentra a relative equilibrium state. ...
This study examines the use of SrAl2O4: Eu2+, Dy3+ long-afterglow materials doped into g-C3N4/TiO2 coatings for photodegradation. The prepared sample was tested for the purification of automotive exhaust fumes, with the optimal mass ratio of g-C3N4/TiO2 and SrAl2O4: Eu2+, Dy3+ determined to be 1:1. Characterization tests, including XRD, FT-IR, XPS, and TG-DSC, were conducted to evaluate the microstructure and properties of the samples. Under poor lighting conditions, g-C3N4/TiO2 reduced CH and NOx by 59 ppm and 13 ppm within 4 h, respectively, while g-C3N4/TiO2/SrAl2O4: Eu2+, Dy3+ decreased CH and NOx by 98ppm and 34ppm, respectively, resulting in a significant improvement in degradation efficiency. The addition of long-afterglow materials significantly improves the efficiency of photocatalysts in purifying exhaust fumes in low-light environments, providing potential value for all-weather exhaust treatment in the future.
... Using the Tauc plot analysis, the band gaps of the Sp and commercial materials were estimated to be 3.2 eV for TiO 2 Sp10 and TiO 2 Sp5 compared to 3.1 eV for TiO 2 P25 (Fig. S6c †), which corresponded well to the reported values. 56 While Sp5 ...
... Further inspection using the 1st derivative of the Tauc plot revealed two levels of transition energy at 3.3 eV and 3.1 eV representing typical values for the anatase and rutile phases of TiO 2 . 56 The fitting analysis revealed the different anatase/rutile compositions of the TiO 2 material (Fig. 9b). While TiO 2 Sp10 contained about 15% of rutile, the Sp5 sample consisted of mainly anatase crystalline phases. ...
Environmentally friendly and waste-free production of photocatalytic filters involving spark ablation and calcination processes for enhanced and durable photocatalytic performance.
... The reactive species attack the organic dye molecules, leading to their mineralization and subsequent water molecules and carbon dioxide formation. The reactive species primarily govern the photodegradation mechanism of methylene blue dye • OH and • O2 − [38,39]. ...
... The reactive species attack the organic dye molecules, leading to their mineralization and subsequent water molecules and carbon dioxide formation. The reactive species primarily govern the photodegradation mechanism of methylene blue dye • OH and • O 2 − [38,39]. Photocatalytic degradation of methylene blue (MB) dye was studied using recombinant E. coli displayed with CP1 after cobalt adsorption. ...
A novel Escherichia coli strain, created by engineering its cell surface with a cobalt-binding peptide CP1, was investigated in this study. The recombinant strain, pBAD30-YiaT-CP1, was structurally modeled to determine its cobalt-binding affinity. Furthermore, the effectiveness and specificity of pBAD30-CP1 in adsorbing and extracting cobalt from artificial wastewater polluted with the metal were investigated. The modified cells were subjected to cobalt concentrations (0.25 mM to 1 mM) and pH levels (pH 3, 5, 7, and 9). When exposed to a pH of 7 and a cobalt concentration of 1 mM, the pBAD30-CP1 strain had the best cobalt recovery efficiency, measuring 1468 mol/g DCW (Dry Cell Weight). Furthermore, pBAD30-CP1 had a higher affinity for cobalt than nickel and manganese. Field Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), and Energy-Dispersive X-ray Spectroscopy (EDS) were used to examine the physiochemical parameters of the recombinant cells after cobalt adsorption. These approaches revealed the presence of cobalt in a bound state on the cell surface in the form of nanoparticles. In addition, the cobalt-binding recombinant strains were used in the photocatalytic reduction of methylene blue, which resulted in a 59.52% drop in the observed percentage. This study shows that modified E. coli strains have the potential for efficient cobalt recovery and application in environmental remediation operations.
... Photocatalysis is a process in which light activates a catalyst. The mechanism for photocatalysis is described by many researchers [10][11][12][13][14][15], and it is pictorially described in Figure 1 and in Equations (1)- (13). Figure 1. ...
... This suggests that ZnO in the composite materials was either amorphous or the ZnO crystal structures were below the detection limit of the XRD. As shown in Figure 2b The crystallite sizes of all photocatalysts were calculated based upon the predominant peaks in the XRD profiles (25.3° for anatase TiO2, 32.7° for ZnTiO3, and 36.2° for ZnO) and using the Scherrer equation, which is provided in Equation (14) The crystallite sizes of all photocatalysts were calculated based upon the predominant peaks in the XRD profiles (25.3 • for anatase TiO 2 , 32.7 • for ZnTiO 3 , and 36.2 • for ZnO) and using the Scherrer equation, which is provided in Equation (14) [77,78]. ...
The aim of this project is to investigate the photocatalytic activity of ZnO/TiO2 composite films for the gas-phase oxidation of ethanol. Pure TiO2, pure ZnO, and their composites were formulated using a sol-gel synthesis method, and the resulting powders were cast and dried as thin films in a flat-plate ultraviolet light-emitting diode (UV LED) reactor. P25 TiO2 and commercially available ZnO were also used for comparison. The structural, morphological, and optical characteristics of the materials were characterized. The photocatalytic oxidation of ethanol vapors in air after 24 h of reactor operation was used to assess the relative photocatalytic activity of the ZnO/TiO2 composite films. Our results show that as ZnO content increased in the coupled semiconductor materials, the apparent photocatalytic activity decreased. In this study, pure ZnO (both sol-gel and purchased) had the least photocatalytic activity for vapor-phase ethanol oxidation in our test apparatus. For gas-phase photocatalysis, surface area was shown to be a critical feature for photocatalytic activity. However, the inherent photocatalytic activity of the materials was not decoupled from the effects of surface area in this study.
... Heterogeneous photocatalysis with TiO 2 presents a wide range of applications (Zhao and Yang, 2003;Gaya and Abdullah, 2008;López-Ojeda et al., 2011;Morales-Zárate et al., 2018). The degradation of phenolic compounds in water from the phenolic resin industry or the cellulose industry has been reported (Parra et al., 2003), as well as the degradation of a wide variety of organochlorine compounds from PVC production plants or wood industries (Thu et al., 2005). ...
... As it is observed (Table 1), the efficiencies of 2,5-DCP degradation (degradation %) by solar photocatalysis, measured both by COD and UV-visible at 280 nm, for conditions of high intensity of solar radiation, are always superior to those obtained at low intensity, independently of the photocatalyst/support surface slope (TiO 2 /glass) and the flow (L/h) chosen. These results coincide with theories of maximum efficiency of degradation of organic matter on TiO 2 , by heterogeneous solar photocatalysis (Feitz et al., 2000;Zhao and Yang, 2003). Furthermore, average degradation efficiencies of up to 95% for conditions of high solar radiation in short reaction times (30 min) were reached, and are similar to those reported by Ba-Abbad et al. (2010), who degraded 2,4-DCP in aqueous solution (50 mg/L) by solar photocatalysis on ZnO, obtained also by the sol-gel method, determining a degradation efficiency greater than 98% during 60 min of reaction and measuring pollutant concentration by molecular absorbance (λ = 285 nm). ...
Modelado de la biodegradación en biorreactores de lodos de hidrocarburos totales del petróleo intemperizados en suelos y sedimentos (Biodegradation modeling of sludge bioreactors of total petroleum hydrocarbons weathering in soil and sediments)
... hours. Upon photon (hυ) activation of porous CdS, highly reactive electron-hole pairs are released, which can be written as equqtions 2-5 [22,23] The photodegradation rate estimate using the formula [24]: ...
How to cite this article Porous CdS nanocrystalline thin film was synthesized via chemical bath deposition. The XRD (X-ray diffraction) pattern of the porous shows the cubic structure. FESEM (Field emission scanning electron microscopy) analysis shows that the surface of Cadmium sulfide (CdS) is a porous shape. The EDX (energy dispersive X-ray spectroscopy) Technique assures the presence of Cd and S elements in the sample. To determine the band gap energy of porous CdS, UV-visible absorption spectra were analyzed, revealing an estimated value of approximately 2.39 eV. Subsequently, the photocatalytic degradation of methylene blue was using porous CdS as the catalyst. This degradation process took place under visible light irradiation from a 200-W xenon lamp. The utilization of porous CdS in this photocatalytic reaction demonstrates its potential as an effective catalyst for the degradation of methylene blue, a commonly used dye in various industries. To our knowledge, there is no previous study about porous CdS acting as catalysis to remove methylene blue dye. In seven hours, porous CdS removed 72% of methylene blue.
... Consequently, researchers are paying much attention to indoor air disinfection. Various methods have been employed for indoor air disinfection, such as chemical disinfection, gaseous disinfection and ultraviolet (UV) irradiation, to eradicate harmful airborne microorganisms [1][2][3][4]. Still, it is difficult to purify entire spaces using these methods because of various disadvantages associated with them, as reported elsewhere [5,6]. ...
Atmospheric pressure plasma (APP) has intrigued the interest of researchers for various applications such as disinfection, air purification, etc. In this context, a deeper understanding of the correlation between APP’s characteristics like discharge parameters, active species composition, and eradication of airborne microorganisms with varying relative humidity (RH) has been examined using a dielectric barrier discharge based atmospheric pressure plasma (DBD-APP) source. One of the electrodes of the developed DBD-APP source has been coated with TiO2 nanoparticles to enhance the generation of reactive species during the discharge process. The results show that, even with the same peak-to-peak applied voltage, the peak-to-peak current and discharge power decrease with increasing RH. Optical emission spectroscopy (OES) is used to determine the relative emission intensity of the reactive species, whereas spectrophotometry is used to quantify the reactive species produced by the plasma at various parameters. Instead of UV radiation, the plasma-produced highly energetic electrons activates the TiO2 nanoparticles for electron–hole pair generation. The geometry of the plasma device has played an important role in generating high energy electrons. From the developed DBD-APP source, the airborne microorganism’s disinfection efficiency of ∼95.8% and ∼98.7% has been achieved in the total bacterial counts (TBCs) and total fungal counts (TFCs) at an RH range of 70%–90%, in just 20 min of continuous operation. However, in the RH range of 20%–40%, the inactivation efficiency dropped to ∼78.8% and ∼87.5% for the TBCs and TFCs, respectively. The outcome indicates that higher humidity levels are better for indoor air purification using DBD-APP sources and that plasma with a circulation system can effectively disinfect indoor environments.
... Among catalysts, titanium dioxide (TiO 2 ) is known as the most suitable catalyst due to its low production cost, stability and non-toxicity, high photocatalytic activity, and corrosion resistance. 10,15,16 Despite these desirable properties, its practical application is still limited due to: (1) the considerable energy band gap of TiO 2 (3.2 eV), which limits its light absorption to the ultraviolet (UV) region; (2) the fast recombination of electron-hole pairs and the slow transfer of charge carriers; (3) difficulties in regeneration, [17][18][19] The difficulties in regeneration can arise due to several factors, such as: (I) deactivation is a common issue, where the material catalytic activity decreases over time, possibly due to the accumulation of reaction byproducts or the formation of surface passivation layers, 20,21 (II) fouling and poisoning can also occur, as impurities or unwanted products may adsorb onto the TiO 2 surface, blocking active sites and reducing effectiveness, 22 (III) structural changes, the TiO 2 material can undergo structural changes, such as phase transformations or defect formation, during use, 23 (IV) energy-intensive regeneration processes, like thermal treatment or chemical washing, may limit the practical feasibility of regeneration. 24 Attempts have been made to solve these limitations by increasing the light absorption range and decreasing the band gap of TiO 2 . ...
In this study, a novel environmentally friendly route was explored for the synthesis of a tin-doped titanium dioxide/calcium oxide (Sn-TiO2/CaO) composite using eggshell as a ternary photocatalyst. The composite was prepared via a simple hydrothermal method, resulting in a unique material with potential applications in photocatalysis. The prepared photocatalysts were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, UV-vis/diffuse reflectance spectroscopy, scanning electron microscopy, X-ray fluorescence, and the Brunauer–Emmett–Teller techniques. At the same time, the Sn-TiO2/CaO composite shows excellent degradation activity for toxic dyes. The degradation efficiencies for alizarin red, bromophenol blue, methylene blue, malachite green, and methyl red are 68.38%, 62.39%, 76.81%, 86.93%, and 17.52%, respectively, under ultraviolet light irradiation for 35 min at pH = 3. In addition, the best photocatalytic degradation efficiency for zero charge (pH 7) and basic pH is for AR 98.21% and 68.38%, MR 33.01% and 17.52%, BPB 73.17% and 17.52%, MB 72.32% and 76.81%, and MG 85.59% and 86.93%, respectively, under UV light irradiation for 35 min. The increase in photocatalytic activity of the ternary photocatalyst is accredited to the enhancement of electron–hole pair separation. Simultaneous photodegradation and photoreduction of organic dyes show that ternary photocatalysts could be used in real wastewater applications.
... Nonetheless, the photodeposition process has been found applicable in synthesizing noble metal/semiconductor compounds in different research areas, such as photocatalytic water splitting, [28,29] wastewater treatment, [30] and air purification. [31] While first successes with photodeposited iridium oxide on TiO 2 have been reported, [32] they are still scarce in the research for PEMWE. Albeit this demonstrates the general applicability of photodeposition to this research field, neither a continuous shell was achieved so far nor was the catalyst characterized on PEMWE single-cell level. ...
The widespread application of green hydrogen production technologies requires cost reduction of crucial elements. To achieve this, a viable pathway to reduce the iridium loading in proton exchange membrane water electrolysis (PEMWE) is explored. Herein, a scalable synthesis method based on a photodeposition process for a TiO2@IrOx core–shell catalyst with a reduced iridium content as low as 40 wt.% is presented. Using this synthesis method, titania support particles homogeneously coated with a thin iridium oxide shell of only 2.1 ± 0.4 nm are obtained. The catalyst exhibits not only high ex situ activity, but also decent stability compared to commercially available catalysts. Furthermore, the unique core–shell structure provides a threefold increased electrical powder conductivity compared to structures without the shell. In addition, the low iridium content facilitates the fabrication of sufficiently thick catalyst layers at decreased iridium loadings mitigating the impact of crack formation in the catalyst layer during PEMWE operation. It is demonstrated that the novel TiO2@IrOx core–shell catalyst clearly outperforms the commercial reference in single‐cell tests with an iridium loading below 0.3 mgIr cm⁻² exhibiting a superior iridium‐specific power density of 17.9 kW gIr⁻¹ compared to 10.4 kW gIr⁻¹ for the commercial reference.
... MoS based on tin, titanium, iron, zinc and cerium oxides are widely used for gas sensing. These sensors also show extraordinary performance in detecting volatile organic compounds (VOC) present in the air (Zhao and Yang, 2003;Fu and Liao, 2012;Gulati et al., 2022). The titanium dioxide (TiO₂) nanoparticles are usually used in photocatalytic activities but in nano sensor system it has emerged as a vital constituent. ...
IoT-based Sensors networks play a pivotal role in improving air quality monitoring in the Middle East. They provide real-time data, enabling precise tracking of pollution trends, informed decision-making, and increased public awareness. Air quality and dust pollution in the Middle East region may leads to various health issues, particularly among vulnerable populations. IoT-based Sensors networks help mitigate health risks by offering timely and accurate air quality data. Air pollution affects not only human health but also the region’s ecosystems and contributes to climate change. The economic implications of deteriorated air quality include healthcare costs and decreased productivity, underscore the need for effective monitoring and mitigation. IoT-based data can guide policymakers to align with Sustainable Development Goals (SDGs) related to health, clean water, and climate action. The conventional monitor based standard air quality instruments provide limited spatial coverage so there is strong need to continue research integrated with low-cost sensor technologies to make air quality monitoring more accessible, even in resource-constrained regions. IoT-based Sensors networks monitoring helps in understanding these environmental impacts. Among these IoT-based Sensors networks, sensors are of vital importance. With the evolution of sensors technologies, different types of sensors materials are available. Among this carbon based sensors are widely used for air quality monitoring. Carbon nanomaterial-based sensors (CNS) and carbon nanotubes (CNTs) as adsorbents exhibit unique capabilities in the measurement of air pollutants. These sensors are used to detect gaseous pollutants that includes oxides of nitrogen and Sulphur, and ozone, and volatile organic compounds (VOCs). This study provides comprehensive review of integration of carbon nanomaterials based sensors in IoT based network for better air quality monitoring and exploring the potential of machine learning and artificial intelligence for advanced data analysis, pollution source identification, integration of satellite and ground-based networks and future forecasting to design effective mitigation strategies. By prioritizing these recommendations, the Middle East and other regions, can further leverage IoT-based systems to improve air quality monitoring, safeguard public health, protect the environment, and contribute to sustainable development in the region.
... The construction industry has benefited from the use of TiO 2 , as it imparts advantageous properties to surfaces, including self-cleaning, de-polluting, and self-sterilizing capabilities. It is a semiconductor material that has been used in a variety of sectors, particularly in the anatase form, including water and air purification, cosmetics, paints, glassware, and photo-degrading a diverse range of organic pollutants like organic dyes and inorganic pollutants under UV radiations [9][10][11][12][13][14][15][16][17][18][19]. However, as the photodegradation ability of these particles is only activated under UV irradiation, studies are tending to expand the boundaries. ...
The use of nanoparticles has attracted widespread attention in surface science to improve the characteristics of
substrates such as thermal stability, corrosion resistance, self-cleaning, and antibacterial effects. Nonetheless,
porous materials like wood have received limited attention due to their inherent absorbency. This study aims to
evaluate the antibacterial and photo-activity of modified wood employing Ag-doped TiO2 coating applied on the
wood surface. The main characteristics of the modified wood were measured through XRD, SEM, AFM, TGA, and
DRS analysis to determine structure and crystallinity, shape of particles, physical modifications of the wood
surface, degradation rate, band gap alterations, and light absorbance, respectively. Methylene blue indicator
witnessed a photo-degradation rate of 90 percent under UV irradiation after 21 h. This rate was observed at
around 38 percent under irradiation of visible light. All in all, the modified wood underwent less color change
whereas the parent wood witnessed a major change regarding its color. The antibacterial properties of wood
were also measured. Two bacterial infections, S. Aureus and E. Coli, were employed for the following experiments.
The results illustrated that the coating was able to reduce the amount of colonization and adhesion of
E. Coli by 50 % and S. Aureus to zero.
... Typically, this process consists of two stages: the gas's mass transfer to the wall and its conversion on the surface of the active mortar. Furthermore, research has verified that the conversion process serves as a rate-limiting step [44,45]. ...
In this paper, an advanced photocatalyst, B-TiO 2 /MgAl-CLDH, was synthesized and applied in cement mortar. The photocatalytic NO x degradation ability of cement mortar containing B-TiO 2 /MgAl-CLDH under different initial NO x concentrations and flow rates was investigated. The results show that B-TiO 2 /MgAl-CLDH has stronger visible light absorption, lower recombination rate of electron-hole pairs and narrower optical band gap than commercial nano-TiO 2 (P25), resulting in enhanced NO x removal efficiency. Specifically, the maximum NO x degradation ratio of B-TiO 2 /MgAl-CLDH is 23.4% within 30 min, which is about 5 times that of P25. For pho-tocatalytic mortar, the initial NO x concentration (from 1.0 ppm to 2.0 ppm) and flow rate (from 1 L/min to 3 L/ min) were positively correlated with NO x removal amount, while negatively correlated with NO x removal ratio. Based on the internal molecular diffusion properties of NO x , Langmuir-Hinshelwood and the power law kinetic models were used to predict the NO x degradation ability of photocatalytic mortar. The modeling of NO x degradation process presents a reliable prediction of the NO x removal ability of photocatalytic mortar, serving as a valuable tool for assessing the mortar's NO x removal effectiveness for policymakers and engineers.
... Nonetheless, the photodeposition process has found application in the synthesis of noble metal/semiconductor compounds in different research areas, such as photocatalytic water splitting, 28,29 wastewater treatment, 30 and air purification. 31 While first successes about photodeposited iridium oxide on TiO2 are reported, 32 they are still scarce in the context of PEMWE. Albeit this demonstrates the general applicability of photodeposition to this research field, neither a continuous shell was achieved so far, nor was the catalyst characterized on PEMWE single-cell level. ...
The widespread application of green hydrogen production technologies requires cost reduction of crucial elements. To achieve this, a viable pathway to reduce the iridium loading in proton exchange membrane water electrolysis (PEMWE) is explored. Herein, we present a scalable synthesis method based on a photodeposition process for a TiO2@IrOx core-shell catalyst with a reduced iridium content as low as 40 wt%. Using this synthesis route, we obtain titania support particles homogeneously coated with a thin iridium oxide shell of only 2.1 ± 0.4 nm. The catalyst exhibits not only high ex situ activity, but also decent stability compared to commercially available catalysts. Furthermore, the unique core-shell structure provides a threefold increased electrical powder conductivity compared to structures without the shell. In addition, the low iridium content facilitates the fabrication of sufficiently thick catalyst layers at decreased iridium loadings mitigating the impact of crack formation in the catalyst layer during PEMWE operation. We demonstrate that the novel TiO2@IrOx core-shell catalyst clearly outperforms the commercial reference in single-cell tests with an iridium loading below 0.3 mgIr cm-2 exhibiting a superior iridium-specific power density of 17.9 kW gIr-1 compared to 10.4 kW gIr-1 for the commercial reference.
... It follows that the photocatalytic activity of Titanium Dioxide is dispatched in an excellent manner if it is exposed to the solar radiation, the first natural source of UV rays, or to artificial UV sources (e.g., special lamps). Moreover, studies demonstrated that the reaction rate differently depends on some key influencing factors, such as moisture, light intensity, initial contaminant concentration (Zhao et Yang, 2003). The air pollution is mainly caused by chlorofluorocarbons, VOC, and Nitrogen oxides, while the photocatalytic process of TiO2 is used for purification of air as it can react with these pollutants converting them into eco-friendly substances (Dong et al., 2014;Saif et al., 2014). ...
Nowadays, reducing air pollution is a critical and urgent issue. Among strategies to be pursued to improve the air quality, photocatalysis is quite interesting and viable. Titanium Dioxide TiO2 is recognized as having an excellent manner if it is exposed to solar radiation in terms of photocatalytic behavior. This article reports the results of a research regarding the evaluation of photocatalytic activity of degradation of organic components, activated by Titanium Dioxide (TiO2), with the aim to assess the behavior of different dyes. As explained in the methodology, two organic dyes contrast liquids were selected and in parallel deposited on a sample of untreated stone. The reaction and degradation speed and the organic decomposition were assessed on the basis of a direct observation and colorimetric analysis. The results showed that the coated samples generally show faster degradation of the dye under solar irradiation compared to the uncoated one sand that a greater quantity of TiO2 in these conditions does not lead to a significant improvement in the action enough to justify the use of a more concentrated solution.
... In past decades, a large number of investigations have focused on semiconductor photocatalysts due to their potential applications in solar energy conversion and environmental purification [1][2][3][4][5][6][7][8]. The wellknown semiconductors, such as TiO 2 and ZnO, have been close to be ideal photocatalysts due to their low-cost, large surface area, durability, abundance and non-toxicity. ...
... The hydroxyl radical, which is produced adsorbed water or adsorbed OHand oxygen are oxidized, is the major oxidant involved in degradation of organic compounds (MB), and can stop hole-electron pairs from re-combining. The ultimate byproducts of the complete reaction include CO 2 and H 2 O, among others (Zhao and Yang, 2003;Das et al., 2011): Fig. 2 (a, b, c) show the absorption spectra of photo degradation of MB (5 × 10 − 6 M concentration) solution containing CdS nanorods at different pH values under visible light. The degradation efficacy was used to track the photo degradation of MB as the normalized fluctuation in its concentration. ...
CdS nanorods were successfully synthesized using a hydrothermal method and their photocatalytic degradation
activity was tested on methylene blue (MB) at different pH levels. The synthesis of CdS nanorods was confirmed
using various techniques including UV–Vis and FTIR spectrometers, X-ray diffraction (XRD), dynamic light
scattering (DLS), and high-resolution transmission electron microscope (HRTEM). UV–Vis analysis showed an
absorption peak at 385 nm in the CdS nanorods suspension. FTIR spectroscopy revealed the presence of different
functional groups responsible for stabilizing the nanorods. TEM analysis showed that the particles had a rod-like
morphology with crystallite sizes ranging from 2 to 10 nm, consistent with the DLS studies. XRD analysis
confirmed the crystalline nature of CdS. A photocatalytic reactor was used to degrade methylene blue at different
pH values, and the residual concentration was monitored using UV–Vis spectroscopy. The photocatalytic activity
of CdS nanorods was found to be improved by adjusting the pH of the solution. The nanorods showed high
efficiency in degrading methylene blue, with degradation performance of approximately 32 % and 35 % after just
30 min under sunlight at pH 4 and pH 10, respectively. This research has potential applications in the photodegradation of harmful pollutants in both industrial wastewater and domestic water supplies.
... As an alternative approach, photocatalysis could be applied to cellulose conversion as a cost effective and simple method for harnessing valuable sugars from cellulose biomass, while simultaneously producing hydrogen. Since the initial report on water splitting by Fujishima and Honda [10], photocatalysis has been extensively utilised in environmental remediation applications such as water [11] and air [12] purification along with systems for H 2 production [13]. Despite the continued growth and expansion of the field, photocatalytic driven cellulose conversion has rarely been reported. ...
Cellulose is is made up of linear polymers of glucose monomers that could be a crucial source for valuable chemicals and sustainable liquid fuels. Cellulose is however, very stable and its conversion to a useful fuel or platform chemical products remains a significant challenge [3,4]. Photocatalysis is a versatile technology which has demonstrated potential for solar driven processes such as water splitting or solar fuels production and has also been applied to the degradation of pollutants in air and water and for the production of useful products from biomass. Here, we focus on the products that are produced from cellulose (a glucose (C6) based polymer) photocatalysis that compliment hydrogen production. Probing the initial steps via UV-TiO2 photocatalysis, we remarkably find that an array of oligosaccharides containing only five (C5) carbon units is initially produced. As the process continues C6 oligo oligosaccharides grow to dominate. The photocatalytic process is generally not viewed as a controllable synthetic process; however, these findings show, on the contrary that photocatalysis at semiconductor surfaces can achieve novel reaction pathways yielding new products.
... Studies have proven the potential of photocatalytic oxidation (PCO) to improve air quality by destruction of harmful air pollutants such as nitrogen oxides (NOx), sulphur oxides and volatile organic compounds (VOC) from a wide variety of chemical classes [4]. The combination of TiO 2 photocatalyst and UV radiation (sunlight) has been shown to destroy VOCs, and some GHGs [5,6]. ...
In order to decrease their time return on investment, all the solar chimney power projects (SCPP) describe alternative income sources like: certificates for carbon dioxide (CO2) compensation, farming under the greenhouse, tourism, R&D centres, telecommunications...
Other synergies could arise from photo-catalysed oxidation of atmospheric methane (CH4) and from CO2 capture. From all the long lived and globally mixed greenhouse gases (GHG), CO2 and CH4 account respectively for 63 and 18% of the total radiative forcing.
... It is well documented in the literature that the crystal size and shape as well as the surface area of the photocatalysts have important implications for its photoactivity. [35,38,50] In this regard, we mea- sured the surface areas of the FeOOH polymorphs by using the Brunauer-Emmett-Teller (BET) method. N 2 adsorption-desorption isotherms of all FeOOH samples were collected at 77 K (Figure 8). ...
... Heterogeneous photocatalytic oxidation (PCO) is a potential air puri cation method that may e ciently eradicate indoor air contaminants at room temperature. [9] HEPA lters are designed to capture very small particles, including PM2.5, which can have negative impacts on human health when breathed in. By using a HEPA lter in the home, it is possible to reduce exposure to PM2.5 and potentially improve cardiorespiratory outcomes. ...
The inhalation of airborne particles can endanger the health of any human being. Natural fiber and natural fiber reinforced with natural matrix material are employed in this work to create an indoor air purifier. Various natural fiber and natural composite combinations are used to purify the interior environment by eliminating particulate matter of various sizes and volatile organic chemicals. An air purifier is created using four distinct natural fibers, including hemp, jute, silk cocoon, and coir fibers, as well as neem and alovera gel as natural filler materials. An air quality-monitoring instrument is used to validate the performance of the designed natural fiber/natural plant-based filler material-equipped air purifier. Particulate matter of various sizes and volatile organic compounds in the indoor environment are detected at various time intervals. The efficacy of the air purifier is afterwards determined in human lungs of various ages utilizing health impact simulation studies. The current product is utilized to effectively purify indoor air by eliminating particulates and volatile organic chemicals.
... Previous studies have proved the concept utilizing PCO in reactors and found substantial conversion rates (>90% for several VOCs) [23,30,31]. However as for adsorption, PCO efficiency have been found to depend on several factors including the targeted VOC, humidity, pollutant concentrations, flow rate as well as light intensity and wavelength [23,32]. Szczotko et al. [17] identified three main barriers associated with utilization of PCO: 1) By-product formation, 2) Possible ozone emissions due to UV lamps, and 3) Limited lifespan of the photocatalyst. ...
Air cleaners are becoming increasingly popular as a response to the increased focus on the impact of indoor air quality on human health, comfort and performance. In this study, the removal of particulate matter (PM) and volatile organic compounds (VOCs) by mobile air cleaners is investigated in an unoccupied university classroom. From time-resolved measurements by proton-transfer-reaction time of flight mass spectrometer (PTR-TOF-MS) the study reports on VOC removal and by-product formation from different air cleaning technologies. Results revealed an inconsistency between expected and experimentally obtained clean air delivery rates (CADRs). For seven out of eight air cleaners examined, the CADRs for all VOCs tested (methanol, acetaldehyde, acetone, acetic acid, isoprene, butanone, toluene, benzaldehyde and limonene) were significantly lower than the corresponding CADRs for PM. In general, single pass removal efficiencies (REs) for VOCs were found to decrease as a function of time. The VOCs demonstrated very distinctive dynamic behavior. Results indicated adsorption as the main removal mechanism of VOCs for all air cleaners for which a significant removal (RE ≥ 5%) was observed. Formation of by-products (incl. CH2O and C4H6O) was associated with technologies containing an UV lamp. Reemission of specific VOCs was observed for adsorption-based air cleaners during operation in a newly vented room. The results highlight the inadequacy of how we currently report air cleaner performance.
... The high volume (where pollutants are dispersed) to surface (where the photocatalyst is applied) ratio is a drawback in the application of this surface-based technology. However, many laboratory scale studies have been reported in the literature both for outdoor (Liu et al., 2013) (Devahasdin et al., 2003) (Diamanti et al., 2008) (Folli et al., 2012) and indoor (Zhao and Yang, 2003) (Wang et al., 2007) (Zhihui Ai, Wingkei Ho, Shuncheng Lee, 2009 (Ao and Lee, 2005) applications with promising results. Standardized procedures are often employed in such studies in order to quantify the activity of the photocatalytic material. ...
... In recent years, photocatalytic oxidation using semiconductors that are activated in visible light has been proposed as a potential technique for removing pollutants arising from organic chemical agents. [10][11][12] Photocatalytic degradation, known as one of the advanced oxidation processes, consists of three components: light, semiconductor photocatalyst, and oxygen, which is used to produce free radicals. The photocatalytic process begins with the absorption of light by the photocatalyst. ...
Background: The International Agency for Research on Cancer (IARC) identified formaldehyde as a carcinogen in 2004, yet formaldehyde is widely used in health care settings and various industries. In recent years, photocatalytic oxidation has been developed as a potential technique for removing pollutants arising from organic chemical agents and consequently promoting the health indices. This study investigated the effect of operational factors in optimizing formaldehyde removal from the air using Ag3 PO4 /TiO2 photocatalyst.
Methods: An experimental study was designed to investigate the effect of operational factors on the efficiency of formaldehyde degradation. The variables investigated in this study include pollutant retention time, initial pollutant concentration and relative humidity. Sol-gel method was used to synthesize the nano-composite photocatalyst. An ideal experimental design was carried out based on Box-Behnken design (BBD) with response surface methodology (RSM). The sample size in this study includes all the glasses coated with Ag3 PO4 /TiO2 photocatalyst.
Results: The maximum formaldehyde degradation of 32% was obtained at the initial concentration of 2 ppm, 20% relative humidity, and 90 minutes of retention time. Based on the statistical results, the correlation coefficient of the present study for the impact of operational factors on formaldehyde degradation was 0.9635, which means that there is only 3.65% probability of error in the model.
Conclusion: The operational factors examined in this study (retention time, relative humidity, and initial formaldehyde concentration) were significantly influential in the degradation efficiency of formaldehyde by the photocatalyst. Due to the high exposure of employees and clients of health and treatment centers to formaldehyde as a carcinogenic substance, the results of this study can be used in ventilation systems to remove environmental pollutants in health care centers and other occupational settings.
... En la figura 3 inserta, se muestra el valor de la apertura de banda de energía del semiconductor, obtenido por el método modificado de Kubelka-Munk, F(R)hv 2 (eV 2) contra hv (eV) Exp i = experimentos realizados en distintas condiciones de pendiente y flujo del reactor, ħʋ = radiación solar, (º) = pendiente de la superficie del reactor, [2,5-DCF]i = concentración inicial de 2,5-diclorofenol, DQO = demanda química de oxígeno, λ = longitud de onda. Error estándar para ambas variables de respuesta ± 3.55 las teorías de máxima eficiencia de degradación de materia orgánica sobre TiO 2 por fotocatálisis solar heterogénea (Feitz et al. 2000, Zhao y Yang 2003. Las eficiencias promedio de degradación superiores a 98 % para condiciones de alta radiación solar son similares a las reportadas por Ba-Abbad et al. (2010), quienes degradaron 2,4-DCF en solución acuosa (50 mg/L) por fotocatálisis solar sobre ZnO obtenido por el método sol-gel durante 60 min de reacción. ...
Palabras clave: fotocatálisis heterogénea, método sol-gel, dinámica del reactor RESUMEN La degradación de compuestos orgánicos no biodegradables presentes en aguas resi-duales es un punto central de la fotocatálisis solar debido a su alta eficiencia y acep-tables costos de operación. Se estudió la degradación de 2,5-diclorofenol (2,5-DCF) en solución acuosa con concentración inicial variable (C0 ≤ 98 mg/L), por fotólisis y fotocatálisis solar con un reactor de placa plana (1 m 2) y vidrio impregnado con TiO2 sintetizado por el método sol-gel. Se establecieron dos variables de respuesta, la de-manda química de oxígeno y la concentración de 2,5-DCF determinada a 280 nm, por tres factores: ángulo de inclinación (20º y 26º), flujo (355 L/h y 407 L/h) y dos procesos (fotólisis y fotocatálisis). Lo anterior en condiciones de alta (ħʋprom = 847.4 W/m 2) y baja intensidad (ħʋprom = 453.6 W/m 2) de radiación solar, para lo cual se recirculó la solución acuosa sobre la placa de vidrio durante 60 min, bajo régimen laminar y con muestras cada 5 min. Se determinaron las constantes de velocidad K1 (min-1) y K2 (mol/L)-1 de orden primero y cero de reacción, respectivamente. Se consideraron ele-mentos de la dinámica del reactor en la evaluación de la cinética química de reacciones fotocatalíticas. Se alcanzaron porcentajes de degradación de 2,5-DCF superiores al 98 % por fotocatálisis solar. ABSTRACT The interest in solar photocatalysis degradation of non-biodegradable organic compounds in wastewater has raised due to its high efficiency and acceptable operation costs. The degradation of an aqueous 2,5-Dichlorophenol (2,5-DCP) solution with variable initial concentration (C0 ≤ 98 mg/L) was performed by photolysis and solar photocatalysis, using a flat glass plate (1 m 2) impregnated with TiO2 synthetized by
... The CoO nanoparticles comprise valence states such as Cu (I) and Cu (II). Cu (I) species disrupt the protein structures of the virus, thus causing significant antiviral properties even if a source of light is not available [63,64]. In the PCO reaction, semiconductors of a doped or pure metal oxide including Zno, CdS, TiO 2 , and Fe (III)-doped TiO 2 are frequently used as the photo-catalysts TiO 2 which can destroy the bacteria and viruses, at the same time. ...
The outbreak of COVID‐19 disease, the cause of severe acute respiratory syndrome, is considered a worldwide public health concern. Although studies indicated that the virus could spread through respiratory particles or droplets in close contact, current research have revealed that the virus stays viable in aerosols for several hours. Numerous investigations have highlighted the protective role of air purifiers in the management of COVID‐19 transmission, however, there are still some doubts regarding the efficiency and safety of these technologies. According to those observations, using a proper ventilation system can extensively decrease the spread of COVID‐19. However, most of those strategies are currently in the experimental stages. This review aimed at summarising the safety and effectiveness of the recent approaches in this field including using nanofibres that prevent the spread of airborne viruses like SARS‐CoV‐2. Here, the efficacy of controlling COVID‐19 by means of combining multiple strategies is comprehensively discussed.
... Photocatalytic materials like titanium dioxide can even capture harmful organic compounds from the air. "Various research projects have demonstrated the photocatalytic effect within the laboratory (e.g., Beeldens, 2006 (12) , Mueller, N., and B. Nowack, 2008 (13) , Zhao, J., and X. Yang, 2003 (14) ) . In these tests, the conversion of nitrogen dioxide as a result of a single contact between the air and the photocatalytic material was determined and reductions of between 30 and 95% were measured. ...
Developing paint products that help save the environment and address climate change started several years ago, but people who are exposed to toxic air pollutants at sufficient concentrations whether indoor or outdoor, for long periods of time, their chances of getting cancer have increased also they might experience other serious health effects, such as reproductive system problems, birth defects, and aggravated asthma. There are many types of coatings that purify the atmosphere, whether indoor or outdoor. These coatings can be used in advertising industry, especially external advertising and graphic ads. This technology has not been used in Egypt yet though its optimal use will provide fresh air and better life, reduce allergic chest diseases, and help purify the air for being able to breathe cleaner air. This paper aims at clarifying the impact of air purification by heterogeneous photo catalysis (ultrafine titanium dioxide or Nanocrystalline titanium dioxide (TiO2) particles with diameters less than 100 nm) that can be coated on advertising outdoor materials and painted on street walls in Egyptian industrial cities. Coating outdoor advertisements with TiO2 is an advanced surface coating technology designed to self-clean the surface, remove air and water contaminates, suppression of microbial growth, degrading hazardous organic pollutants into harmless substance. This paper also seeks to contribute in reducing air pollution rates by using advertising as an intermediary to realize the fastest results in the shortest time. Producing safe and healthy living environment is what this paper hopes to achieve in Egypt in a short period of time by help applying these coatings to industrial cities. The effect of using titanium dioxide nanoparticles, identification of its applications, usage on advertising materials and its impact on environment were studied. Some examples have been discussed and illustrated about using titanium dioxide in advertising and street paintings in air purification beside the statistical method used in analyzing the questionnaire presented to individuals. This paper follows the descriptive approach in collecting theoretical information related to Titanium dioxide and its role in converting outdoor advertisements to giant air purifying advertising systems, followed by presenting some international advertising models that used purifying titanium dioxide in ad painting. اإلنسانية والعلوم والفنون العمارة مجلة-الثامن المجلد-والثالثون الثامن العدد مارس 2023 642 المستخلص: ، سنوات عدة منذ المناخ تغير ومعالجة البيئة على الحفاظ في تساعد التي الطالء منتجات تطوير بدأ الذين األشخاص ولكن اإلصابة فرص من يزيدون قد ، طويلة لفترات ، وخارجها المباني داخل كافية بتركيزات السامة الهواء لملوثات يتعرضون المتفاقم. والربو الخلقية والعيوب اإلنجاب مشاكل مثل ، أخرى خطيرة صحية آلثار التعرض أو بالسرطان التي الطالءات من عديدة أنواع هناك ، اإلعالن صناعة في الطالءات هذه استخدام يمكن خارجيا. أو داخليا سواء الجو تنقي مستخدمة غير تقنية ا ً أيض وهي الحوائط. وعلى الشوارع فى المرسومة الجرافيكية واإلعالنات الخارجية اإلعالنات وخاصة من سيقلل كما أفضل، وحياة ًا نقي ً هواء األمثل استخدامها وسيوفر ، مصر في تنقية على ويساعد ، والحساسية الصدر أمراض أنظف. هواء واستنشاق الهواء أكسيد ثاني (جزيئات المتجانس غير الضوئي التحفيز طريق عن الهواء تنقية تأثير توضيح إلى البحثية الورقة هذه تهدف (الصغر متناهية التيتانيوم TiO2 من أقل بأقطار) 100 اإلعالنا على المطلية نانومتر) الشوارع جدران وعلى الخارجية ت باستخدام الخارجية اإلعالنات طالء إن المصرية. الصناعية المدن في TiO2 مصممة السطح لطالء متقدمة تقنية عن عبارة مادة إلى الخطرة العضوية الملوثات وتحويل ، الميكروبات نمو وقمع ، والماء الهواء ملوثات وإزالة ، للسطح الذاتي للتنظيف غير ضارة. في النتائج أسرع لتحقيق كوسيط اإلعالنات باستخدام الهواء تلوث معدالت تقليل في المساهمة إلى ا ً أيض الورقة هذه تسعى
... To eradicate VOCs from indoor air, various air purifier technologies, including adsorption, chemical oxidation, and biodegradation, have been proposed and implemented [14,15]. An alternative remediation technique, called photocatalytic oxidation (PCO), has considerable promise for the energy-efficient degradation of a wide spectrum of pollutants, especially VOCs at ppb concentration levels, into final harmless products such as CO 2 and H 2 O [16,17]. In the PCO process, a photocatalyst is used under ultraviolet (UV) light illumination and in presence of oxygen/water molecules in order to produce highly reactive hydroxyl radicals [16]. ...
Many studies have been conducted to produce composite materials that possess the ability to transform volatile organic compounds (VOCs) to other nontoxic forms economically and environmentally by means of photocatalytic method. However, the main drawbacks of these materials include restricted surface area, low affinity towards organic molecules, and wide band gap energy which dramatically inhibit their performance. In this research, a composite material that surpasses the above disadvantages has been successfully synthesized from TiO 2 - carbon nanodots (CDs) - zeolite ZSM-5. Particularly, CDs synthesized by bottom-up method were coated on TiO 2 before being uniformly distributed on zeolite ZSM-5. The powder samples with varying CDs/TiO 2 and zeolite ZSM-5 content were tested for their photocatalytic oxidization capability to determine the appropriate ratio. The results revealed that samples with higher zeolite content improved photocatalytic activity. Under other survey conditions such as low toluene flowrate, high relative humidity as well as high UV intensity, the photocatalytic performance was enhanced notably. The newly produced material has corrected most disadvantages of traditional photocatalysts, however, further researches need to be made to improve the removal stability.
With the use of photocatalytic materials, it was understood that self-cleaning and anti-bacterial properties are gained in cementitious systems and indoor air quality is improved. It was determined that Nano-TiO2 (nT) is used as the most common photocatalyst due to its advantages. Anatase phase, which is one of the phases of nT, was found to be the most preferred photocatalyst because it has a faster photocatalytic effect. In this study, a detailed literature search was conducted on the photocatalysis process, the photocatalyst selection criteria, the materials used in the production of photocatalytic concrete and their mix design. In addition, the effects of the use of nT on the setting time, flow performance, rheological behavior, degree of hydration, pore structure and strength performance of cementitious systems were investigated. It was determined that nT is used at rates ranging from 1-10% for the production of photocatalytic concrete. It was observed that the hydration rate of cementitious systems increases with the increase in the use of nT, and the flow performance and rheological behavior are adversely affected. Also, it was emphasized that the use of nT improves the mechanical properties by reducing the void volume of the system. However, it was reported that the use of nT above a certain rate causes the strength performance of cementitious systems to be adversely affected.
The material prepared by synthesizing titanium dioxide (TiO2) powder via sol-gel refluxing using titanium tetraisopropoxide (TTIP) as a precursor was doped with varying concentrations of tin (Sn) at 0.25%, 0.50%, and 0.75%. Characterization techniques including scanning electron microscopy (SEM) were employed to analyse particle sizes, revealing dimensions ranging between 12-20 nm. Subsequent X-ray diffraction (XRD) and Raman spectroscopy confirmed the formation of phase-pure nanocrystalline anatase, corroborated by Fourier transform infrared spectroscopy (FTIR) that identified Ti-O-Ti bonding in anatase titania. Surface area analysis using Brunauer-Emmett-Teller (BET) analysis showed a gradual increase in surface area with increasing Sn doping levels, from 100.5 m2/g for pure TiO2 to 116.9 m2/g for 0.75% Sn-doped TiO2. UV-Vis spectroscopy demonstrated distinct absorption peaks and corresponding band gaps, with 0.75% Sn-doped TiO2 exhibiting the broadest absorption in the range of 400 nm – 800 nm. Photocatalytic decomposition of methylene blue was then evaluated, revealing enhanced performance with Sn-doped titania, particularly with 0.75% Sn doping, suggesting its superiority in organic pollutant degradation. This observation underscores the potential of Sn-doped TiO2 as an effective photocatalyst.
The sustainable use of solar energy is critical to addressing the global energy crisis and environmental problems caused by the overconsumption of fossil fuels. However, satisfactory and efficient conversion of sunlight into energy is impossible without developing and using efficient photocatalysts. Environmental remediation by solar-based photocatalytic systems is a promising solution to many environmental problems, including efficiently removing pollutants by decomposition. Photocatalysts are crucial for the broad utilization of solar energy in various fields. Semiconductor photocatalysts are important materials that can be classified into metal oxide and non-oxide materials. Solar photocatalytic water and wastewater treatment, remediation of polluted soil, solar photocatalytic greenhouse gas removal, and solar photocatalytic air purification are some the environmental application of photocatalysts. Photocatalysts for solar energy conversion can be prepared using electrospinning, solid-phase methods, gas-phase methods, and liquid-phase methods. Most of the currently developed photocatalysts have poor quantum efficiencies and fail to utilize the visible spectrum of the light. In addition, most methods available for preparing photocatalysts are expensive or fail to produce photocatalysts with satisfactory performance for specific applications. This study presents recent advances in the development of photocatalysts for solar energy conversion and the synthesis methods for such nanomaterials. In addition, various applications of such photocatalysts in solar-based systems are discussed, emphasizing environmental applications. Finally, challenges in developing and using photocatalysts for solar energy conversion and research prospects in areas of photocatalytic solar energy conversion for environmental applications are presented.
In the last few years, increasing interest from researchers and companies has been shown in the development of photocatalytic coatings for air purification and self-cleaning applications. In order to maintain the photocatalyst’s concentration as low as possible, highly active materials and/or combinations of them are required. In this work, novel photocatalytic formulations containing g-C3N4/TiO2 composites were prepared and deposited in the form of coatings on a-block substrates. The obtained photocatalytic surfaces were tested for NOx and acetaldehyde removal from model air. It was found that the addition of only 0.5 wt% g-C3N4 towards TiO2 content results in over 50% increase in the photocatalytic activity under visible light irradiation in comparison to pure TiO2 coating, while the activity under UV light was not affected. The result was related to the creation of a g-C3N4/TiO2 heterojunction that improves the light absorption and the separation of photogenerated electron-hole pairs, as well as to the inhibition of TiO2 particles’ agglomeration due to the presence of g-C3N4 sheets.
More and more attention has been drawn to condensable particulate matter (CPM) since its emission has surpassed that of filterable particulate matter (FPM) with the large-scale application of ultra-low-emission reform. CPM is gaseous material in the flue stack but instantly turns into particles after leaving the stack. It is composed of inorganic and organic components. Organic components are an important part of CPM and they are irritant, teratogenic and carcinogenic, which will trigger photochemical smog, urban haze and acid deposition. CPM organic components can aggravate air pollution and climate change, so consideration should be given to them. Based on existing methods for removing atmospheric organic pollutants and combined with the characteristics of CPM organic components, we summarize a critical overview from the aspects of (i) Fundamental cognition of CPM, (ii) Common methods to control CPM organic components, and (iii) Catalytic oxidation of CPM organic components. As one of the most encouraging methods, catalytic oxidation is discussed in detail, especially in combination with selective catalytic reduction (SCR) technology to meet the growing demands for multipollutant control (MPC). We believe that the review is inspiring for a fuller understanding and deeper exploration of promising approaches to control CPM organic components.
The inhalation of airborne particles can endanger the health of any human being. Natural fiber and natural fiber reinforced with natural matrix material are employed in this work to create an indoor air purifier. Various natural fiber and natural composite combinations are used to purify the interior environment by eliminating particulate matter of various sizes and volatile organic chemicals. An air purifier is created using four distinct natural fibers, including hemp, jute, silk cocoon, and coir fibers, as well as neem and aloe vera gel as natural filler materials. An air quality-monitoring instrument is used to validate the performance of the designed natural fiber/natural plant-based filler material-equipped air purifier. Particulate matter of various sizes and volatile organic compounds in the indoor environment are detected at various time intervals. The efficacy of the air purifier is afterward determined in human lungs of various ages utilizing health impact simulation studies. The current product is utilized to effectively purify indoor air by eliminating particulates and volatile organic chemicals.
Addressing low-occurrence problems from low
concentrations, we demonstrate amorphous TiO2−0.5x(OH)x nanofilms
(amT) that feature abundant hydroxyl groups (of bulk and
surface) for formaldehyde oxidation under UVC irradiation. Such
hydroxyl groups can significantly enrich bound water (of adsorbed
and produced) and formaldehyde at a hydration layer on the
surface. With this structure, mechanistic insights and reaction
kinetics are correlated. Compared to the anatase one (anT), our
amT exhibits a remarkable decrease in the recombination of charge
carriers, complete formaldehyde oxidation even at dry conditions, a
nearly zero-order reaction that elucidates the approximately
independent kinetics, and an unprecedentedly 104-fold increase in the reaction rate. Based on photoluminescence and Fourier
transform infrared spectroscopy, light indispensability and moisture enhancement to formaldehyde oxidation are corroborated.
Consequently, due to abundant hydroxyl groups, the mass transfer (of formaldehyde and water) at its hydration layer is
strengthened, as evidenced by the mechanistic insights; hence, fascinating kinetics is achieved for environmental applications in
practice.
Catalytic conversion of greenhouse gas carbon dioxide to value-added chemicals and fuels powered by solar energy is envisioned to be a promising strategy to realize both energy security and environmental protection. This work demonstrates that earth abundant, low cost nanomaterials based on silicon and iron can be used to harvest both light and heat energy from the sun to reduce CO2 and generate solar fuels. Herein, we have demonstrated that ruthenium supported ultra-black silicon nanowires can drive the Sabatier reaction both photochemically and photothermally where both incident photons absorbed by and heat generated in the black silicon nanowires accelerate the photomethanation reaction. This allows the reaction to be activated at ambient temperatures removing the need for external heating that could cause sintering, mechanical degradation and eventual catalyst deactivation and therefore improves the overall energy efficiency of the process. Additionally, we have shown that the rate of photomethanation is greatly enhanced when highly dispersed nanocrystalline RuO2 is chemically deposited onto the black silicon nanowires support. Furthermore, we have iii demonstrated that other silicon structures such as three-dimensional silicon photonic crystals can be used as an efficient support for CO2 hydrogenation. Unlike other insulating supports, these silicon nanostructured supports are particularly attractive for solar-powered catalysis because, with a band-gap of 1.1 eV, they can potentially absorb 80% of the solar irradiance. Moreover, they exhibit excellent absorption strengths and low reflective losses across the entire solar spectral wavelength range of the ultraviolet, visible and near-infrared portion of the solar spectrum. Finally, we demonstrated a comprehensive comparative study of the physical, electronic, and photocatalytic properties of ironoxyhydroxide (FeOOH) polymorphs by studying the extent of methylene blue photodegradation. This work led to the transformation of these FeOOH polymorphs into magnetite (Fe3O4) which effectively reduced CO2 to CO via the reverse water gas shift (RWGS) reaction. iv
According to World Health Organization, air pollution kills millions of people worldwide every year. In addition, several epidemiological findings have uncovered the impacts of air pollution on respiratory and cardiovascular systems. This chapter presents current knowledge of human health concerns caused by volatile organic compounds (VOCs) and biological contaminants. These contaminants contribute to air pollutants that impair all environmental elements. Heterogeneous photocatalytic processes using semiconductor photocatalyst would serve as a promising technology and an efficient approach for removing VOCs and airborne pathogens. Considering the potentially toxic effect of these air pollutants, emerging mitigation approaches such as the photocatalysis process are explained elaborately in this chapter, including fundamental principles of photocatalysis, reaction mechanism, reaction kinetics, and photoreactor designs suitable for air purification. Furthermore, the photocatalytic process as a paradigm explores existing techniques utilized in research and commercial applications. Significant efforts have been made to include information from worldwide sources for this investigation.KeywordsAirborne pathogensAir pollutionHydroxyl radicalPhotocatalysisPhotoreactorPMROSTiO2VOCs
TiO2 (anatase) powder irradiated by near-UV light was used for carrying out the ethanol oxidation in gas phase. The reactivity experiments, performed at atmospheric pressure and low temperature, were carried out in a continuous annular photoreactor of Pyrex glass whose internal surfaces were covered by a thin layer of photocatalyst. The influence on the photoreactivity of the following operative parameters: gas flow rate, amount of catalyst, ethanol concentration, oxygen concentration, and presence of water in the reacting mixture was investigated. A Langmuir-Hinshelwood kinetics well expresses the dependence of the ethanol degradation rate on the ethanol and oxygen concentration. The presence of water does not affect the photoreactivity. The absence of catalyst deactivation during the photoprocess was also checked. The main product of ethanol photooxidation was acetaldehyde; the mass balance of reacted ethanol was closed by small amounts of formaldehyde and carbon dioxide.
The purpose of the investigation is to analyze the pros and cons of existing sorption models, as well as experimental methods. In addition, it summarizes existing sorption data in order to understand the phenomenon of VOC sorption on building materials better and to direct future research. We found that existing sorption models can be generally classified into either first order adsorption/desorption rate models or equilibrium-interface models. The adsorption/desorption rate models are simple and easy to use, but the model parameters have to be obtained from experimental data by curve fitting. The parameters of the equilibrium-interface models can be obtained through independent measurements, but lacking material property data and high computing costs hinder their applications. The sorption of VOCs on building materials is usually measured by the widely used small-scale chamber test method. The sink strength can be affected by the chemical properties of VOCs, physical properties of materials, and environmental conditions, such as temperature, humidity and air velocity.
The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. Problems with hazardous wastes at military installations are related in part to the disposal of chemical wastes in lagoons, underground storage tanks, and dump sites. Typical wastes of concern include heavy metals, aviation fuel, military-vehicle fuel, solvents and degreasing agents, and chemical byproducts from weapons manufacturing. In the civilian sector, the elimination of toxic and hazardous chemical substances such as the halogenated hydrocarbons from waste effluents and previously contaminated sites has become a major concern. General classes of compounds of concern include: solvents, volatile organics, chlorinated volatile organics, dioxins, dibenzofurans, pesticides, PCB's, chlorophenols, asbestos, heavy metals, and arsenic compounds. Advanced physicochemical processes such as semiconductor photocatalysis are intended to be both supplementary and complementary to some of the more conventional approaches to the destruction or transformation of hazardous chemical wastes such as high-temperature incineration, amended activated sludge digestion, anaerobic digestion, and conventional physicochemical treatment. 441 refs.
It is concluded that the success that was achieved in the partial (and total) heterogeneous oxidation of hydrocarbons under ultraviolet irradiation at room temperature may be explained by the fact that '%'thermal'%' catalytic oxidation (at temperatures in the 200 to 500 C range) and photon excitation were used instead. Such a procedure is liable to avoid the homogeneous free radical oxidation that seems to be responsible for the degradation of hydrocarbons.
Rates of photocatalytic removal of the components for 14 binary mixtures of vaporized organic components over Degussa P-25 TiO2 at 360 nm in the presence of air were measured. In some cases, intermediate products were identified and their rates of removal measured. Seven binary mixtures were reacted in a static reactor: CH3OH strongly inhibited removal of CH2Cl2 while CH2Cl2 somewhat inhibited removal of CH3OH. CH3OH and trichloroethylene (TCE) strongly inhibited each other's removal. Strong memory effects were observed in inhibition of removal of CH2Cl2 from its mixture with CH3OH and of CH3OH from its mixture with TCE. CCl4 significantly promoted removal of CH3OH while CH3OH had little effect on the rate of removal of CCl4. Either 2-propanol or t-butanol and CCl4 had little effect on each other's rates of removal. CH2Cl2 somewhat diminished only the initial rate of removal of C2Cl3F3 (Freon 113). The Freon somewhat inhibited the much faster removal of CH2Cl2. Methanol somewhat inhibited removal of acetone while acetone had a negligible effect on the removal of methanol. Seven binary mixtures were reacted in flow reactors: TCE and perchloroethylene (PCE) somewhat promoted each other's removal. TCE strongly promoted removal of iso-octane, CH2Cl2 and CHCl3. A strong memory effect occurred with iso-octane and probably with methylene chloride and chloroform. TCE substantially inhibited removal of acetone. The onset of inhibition of removal of acetonitrile by TCE was gradual and a memory effect occurred. Iso-octane somewhat reduced the rate of removal of benzene. Benzene had no effect on the rate of removal of iso-octane but greatly diminished the yield of acetone produced as an intermediate. Rationalizations of some of the data in terms of adsorption effects and chemical mechanism are presented.
The body of information presented in this paper is directed to engineers and scientists concerned with control of automobile emissions and exhaust gases from some industrial processes. The differential equations describing heat and mass transfer in a monolithic honeycomb catalyst are developed. Following transport mechanism is considered: convective heat and mass transfer in the holes of the structure, longitudinal thermal conductivity of the honeycomb support and gas-to-solid heat and mass transfer. The magnitudes of governing parameters for monolithic modules in use are discussed. Two methods for the numerical solution of a system of coupled, nonlinear ordinary differential equations with split boundary conditions are proposed. The first method-shooting procedure can be used only for problems with low values of Peclet number. For high values of Peclet number finite-difference approach along with the Newton-Raphson algorithm is suggested. It is shown that two stable steady states exist in certain regions of operation of a particular monolithic structure. The all-metal monolithic supports are more prone to multiplicity of steady states than the ceramic ones. For ceramic supports the two-phase piston-flow model is sufficiently accurate.
Heat and mass gas-to-solid coefficients associated with the vaporisation of water and some hydrocarbons from the surface of a porous, monolithic structure were experimentally established. The mass transfer results are correlated using Reynolds and Schmidt number and the length-to-diameter ratio. The data for heat transfer are correlated on the Nu-Re(d/L) plot. The experimental results can be used for design of afterburner reactors utilizing monolithic structures.
The photocatalytic degradation of gaseous formaldehyde--a major cause of sick building syndrome--was studied using a TiOâ thin film. TiOâ thin films have many unique photoinduced properties, for example, self-cleaning, anti-fouling, and anti-bacterial functions. UV illumination of the TiOâ thin film placed in a gaseous formaldehyde/air environment resulted in the total mineralization of formaldehyde to COâ and HâO. The authors invoked a Langmuir-Hinshelwood kinetic model to analyze the dependence of reaction rates on the concentration of formaldehyde. In addition, the overall decomposition rate constant for formaldehyde was comparable to that of acetaldehyde (a standard test reactant) for initial concentrations of up to 1000 ppmv. However, the apparent adsorption constant K{sub app} of formaldehyde onto TiOâ was ca. 2.5 times larger than that of acetaldehyde. Thus, in the low concentration regime, the reactivity of formaldehyde appeared to be greater than that of acetaldehyde. In like manner, a dark adsorption experiment also showed the high adsorption capacity of TiOâ for formaldehyde. Therefore, the authors conclude that TiOâ serves as both a good adsorbent and a photocatalyst for the elimination of gaseous formaldehyde.
Three types of films were prepared by a dip-coating process for the photocatalyzed decomposition of 1-butene in a gas-solid reaction. Under UV illumination (λ > 300 nm, 352 nm peak intensity), ultrasmall SnOâ with a diameter of 5 nm exhibited initial photoactivity as high as 3 times that of P-25 TiOâ (30 nm) in the absence of water vapor whereas SnOâ-l (22 nm) did not show photoactivity. Quantum size effects were mainly responsible for the high photoactivity achieved by SnOâ. Inactivity of SnOâ-l film was due to the absence of active hydroxyl groups on the catalyst surface and low surface areas. By investigation of the effects of humidity, water has two different functions: maintaining constant oxidation rates at low water levels by replenishing hydroxyl groups and decreasing the photoactivity at high water levels by competitive adsorption with butene on active sites. Compared with TiOâ, SnOâ cannot withstand high humidity since it is very sensitive to water concentration. Hydroxyl groups on catalyst surfaces are the active centers for the reaction. The occurrence of obvious deactivation on SnOâ films was due to the depletion of hydroxyl groups and the accumulation of carbonate species on particle surfaces, i.e., M-OCOOR species in place of M-OH. The kinetic data correlate with a Langmuir-Hinshelwood single-site model. XRD, UV-vis spectroscopy, and FTIR techniques were employed to characterize the particle size, band gaps, and surface properties of the catalysts.
Photooxidation of triethylamine (TEA) in the presence of O2, N2, and H2O over titanium oxide (TiO2) was investigated using a flat plate reactor. TEA was photocatalytically oxidized to CO2 and some by-products on TiO2 thin film catalysts. The intrinsic oxidation rate of the reaction was determined and was dependent on TEA concentration, humidity level, and light intensity. Photocatalytic deactivation was observed in these reactions. Fourier transform infrared (FTIR) and temperature-programmed desorption with a mass spectrometer as a detector (TPD-MS) were used to characterize the surface of the catalyst and study the deactivation mechanism. FTIR and TPD-MS results suggest that accumulation of carboxylic acid species, –N–N=O, and some other carbonaceous species occurred during the reaction. These by-product species or intermediates were chemisorbed on the catalyst surface. They were stable under reaction conditions and might be responsible for deactivation of TiO2 by either poisoning the active sites directly or blocking the adsorption of TEA on the catalyst surface.
The effects of preparation methods, humidity, and calcination temperatures on the behavior of nanoscale TiO2 photocatalysts were investigated in this study. Application of these photocatalysts in the gas-phase decomposition of 1-butene demonstrated that the nanoscale catalysts, prepared by the acid-assisted sol–gel technique, showed higher photocatalytic reactivity than commercially available Degussa P-25 TiO2, while lower photoactivity was obtained on TiO2 catalysts prepared without adding acid to titanium isopropoxide. Our experimental results revealed that the oxidation rates of 1-butene decreased exponentially with increasing water concentrations in the flowing stream. However, a trace amount of water vapor was indispensable in maintaining the stability of the catalysts. Proper calcination temperature (300°C) could promote the resistance of catalysts against the poisoning effects of humidity. Higher calcination temperature (400°C) unfavorably lowered the photoactivity due to phase transformations occurring at such a temperature. Amorphous and rutile-typed TiO2 showed less photocatalytic reactivity. XRD patterns and BET measurements indicated that moderate surface areas (from 100 to 160 m2/g), appropriate crystallite sizes (5∼6 nm), and crystallinity of anatase were beneficial to the photoactivity of TiO2 catalysts. In situ FTIR studies indicated that catalyst surfaces contained large amounts of chemisorbed water and hydroxyl groups, which are considered to be active sites in photocatalytic reactions. The accumulation of carbonate species on active sites resulted in the deactivation of TiO2 catalysts under dry conditions. Quantum size effects were thought to be responsible for the high photoactivity achieved on the nanoscale TiO2 catalysts prepared by sol–gel methods.
Photocatalyzed degradation of trace level trichloroethylene (TCE) and toluene in air were carried out over near-UV-illuminated titanium dioxide (anatase) powder in a flow reactor using a residence time of about 5–6 ms. Concentration ranges for TCE and toluene were 0–800 mg/m3. TCE photooxidation was very rapid under our experimental conditions, and ∼100% conversion was achieved for TCE concentration examined up to 753 mg/m3as a single air contaminant. Initial photodegradation rates for toluene in humidified air were fitted by a Langmuir–Hinshelwood rate form. Toluene photooxidation sole contaminant rates were of the same order of magnitude as reported previously form-xylene and acetone. No significant intermediates were detected by GC/FID during TCE or toluene photooxidation reactions. Humidification had significant influence: Toluene photooxidation rate increases with the water concentration up to about 1650 mg/m320% relative humidity) and decreases thereafter. The presence of sufficient TCE (225–753 mg/m3) promoted the toluene photooxidation reaction rate to achieve 90–100% toluene conversion in 5.6 ms. When feed toluene levels measured below ∼90 mg/m3. Higher toluene feed levels “quenched” this TCE promotion effect and also depressed TCE conversion very strongly, but the toluene conversion fell just to the toluene-only levels observed in single contaminant experiments. Photooxidation kinetics of TCE and toluene mixtures in air are thus shown to exhibit strong promotion and inhibition behavior vs that expected from the single-species kinetic degradation data. A previously suggested chlorine radical oxidation of TCE is modified to rationalize the TCE enhancement of the toluene photooxidation rate and the corresponding toluene “quench” of TCE destruction. Time-dependent catalyst activation (by TCE) and deactivation (by toluene or toluene oxidation products and, eventually, even by TCE products) were observed. Carboxylate formation and carboxylic acid accumulation postulated by previous investigators could be a major cause of such catalyst deactivation.
Transient reaction techniques were used to study the room-temperature, photocatalytic oxidation (PCO) of 2-propanol on a thin catalyst film of titania (Degussa P25) in an annular reactor. Adsorbed 2-propanol was photocatalytically oxidized in the absence of gas-phase 2-propanol, and the species remaining on the TiO2 surface were characterized by temperature-programmed desorption (TPD) and oxidation (TPO). Nonphotocatalytic decomposition (TPD) and oxidation (TPO) of 2-propanol and acetone were also studied. The initial PCO of 2-propanol at room temperature rapidly forms acetone and water; water can displace acetone into the gas phase. Adsorbed acetone is subsequently oxidized photocatalytically to CO2 and H2O at a slower rate than 2-propanol photocatalytically oxidizes to acetone and it may form a surface intermediate before complete oxidation. Thus, at steady-state, the TiO2 surface is expected to be covered with H2O and strongly bound acetone or an intermediate. The surface reaction steps require near-UV light, but desorption does not. Formation of gas-phase acetone and water are desorption limited, but CO2 and CO formation are reaction limited. The rate of PCO to form acetone is essentially independent of O2 pressure, and acetone forms at room temperature even in 30 ppm O2, apparently utilizing lattice oxygen. In contrast, complete oxidation to CO2 is first order in O2 at low concentrations and zero order at higher O2 concentrations. The coverage of photoadsorbed oxygen is low (less than 1 μmol/ g TiO2), and the oxygen is strongly bound to the surface (∼ 200 kJ/mol binding energy). There are no indications that photoadsorbed oxygen was important for PCO; gas phase O2 is needed to replace lattice oxygen, but this may or may not be photoadsorbed. In the absence of near-UV light, titania is an oxidation catalyst only at much higher temmperatures. Thermal oxidation of 2-propanol also proceeds through acetone formation, but acetone thermally oxidizes faster than 2-propanol. Methanol and ethanol also undergo PCO on TiO2 at room temperature to form aldehydes, CO2, and H2O.
The effects of initial NO concentration, gas-residence time, reaction temperature and ultraviolet (UV) light intensity on the photocatalytic decomposition of NO have been determined in an annular flow-type and a modified two-dimensional fluidized-bed photoreactors. The decomposition of NO by photocatalysis increases with decreasing initial NO concentration and increasing gas-residence time. The reaction rate increases with increasing UV light intensity. The light transmission increases exponentially with the bed voidage at superficial gas velocity above 1.3 times the minimum fluidizing velocity (Umf) in the two-dimensional fluidized-bed photoreactor. In the two-dimensional fluidized-bed photoreactor, NO decomposition reaches >70% at the gas velocity of 2.5Umf. A two-dimensional fluidized-bed photoreactor is an effective tool for high NO decomposition with efficient utilization of photon energy.
Q-sized CdS semiconductors have been synthesized in several nonaqueous solvents without the use of added stabilizers. The effects of solvent viscosity and dielectric strength, concentration of excess cadmium ions, temperature, and stirring upon the colloids' absorption spectra have been examined. In addition, efficiency of photoinitiation of polymerization of several vinylic monomers has been examined using both bulk and quantum-sized CdS, ZnO, and TiO2. The Q-sized semiconductors demonstrated significantly higher quantum yields for photopolymerization than their bulk-sized counterparts. A correlation between the reactivity of a monomer toward polymerization and its Alfrey and Price Q and e values was observed. Hole scavenging by the solvent was necessary for efficient polymerization to occur. A correlation between the semiconductor photoinitation efficiency, and the reduction potential of its conduction band electrons was also found. On the basis of these observations, a mechanism of an anionic initiation step followed by free radical chain propagation steps has been proposed.
The effect of temperature, water vapor concentration, and ethylene concentration on the photooxidation of ethylene on titania was investigated. Ultraviolet radiation from a black-light lamp together with a glass-plate reactor were used to develop intrinsic oxidation rates. Ethylene oxida tion rates decreased significantly as the water vapor concentration was increased from 1000 ppmv for the three temperatures (2, 27, and 48 °C) investigated. The influence of water vapor on the reaction rate derived from the low adsorption of ethylene due to its low adsorption affinity relative to water. Over the range of water vapor concentration of 1000−25000 ppmv, ethylene oxidation rates increased as the temperature increased. An Arrhenius plot of the measured ethylene oxidation rates indicated an apparent activation energy of 3.4 kcal/mol. A Langmuir−Hinshelwood expression displaying an explicit temperature dependence was used to correlate the entire set of rate data. Based on this correlation, an enthalpy of adsorption for ethylene of −2.6 kcal/mol was found.
The focus of this study was to investigate the role of water vapor in the photocatalytic degradation of trichloroethylene (TCE) on anatase titanium dioxide films immobilized on the surfaces of ring-roughened annular reac tors. Experimental variables included TCE concentration (0.7−7 parts per million by volume or ppmv), oxygen concentration (13−120 000 ppmv), residence time (2.77−9.81 s), relative humidity (0−100%), and reactor length. TCE conversion was not affected by relative humidities up to 20%, but it deteriorated as the gas mixture approached saturation with respect to water vapor. Major intermediates and products from TCE degradation were the same as those previously reported for dry conditions: carbon tetrachloride, chloroform, hexachloroethane, pentachloroethane, and tetrachloroethylene. The formation rates for these compounds increased with increasing water vapor concentrations at relatively low humidities as a result of a stronger deteriorating rate effect of water vapor on atomic oxygen oxidation reactions as compared to that on competing chlorine atom attack reactions. The presence of moderate concentrations of water vapor resulted in greater conversions of chloroform and pentachloroethane as compared to dry conditions due to rate enhancement of hydrogen extraction reactions. In contrast, tetrachloroethylene conversions decreased with increasing humidity primarily because of a deteriorating rate effect of water vapor on chlorine extraction reactions.
The effects of humidity and sub-parts-per-million by volume contaminant levels on the photooxidation of formaldehyde and toluene are presented. UV radiation from a black-light lamp and a titania-coated glass-plate reactor were used to develop kinetic oxidation rates. The data indicated that competitive adsorption between water and the two contaminants has a significant effect on the oxidation rate. A simple differential design model is developed and validated using data from two ceramic foam photocatalytic reactors. The utility of rate data developed from the glass-plate reactor as the basis for reactor design models is demonstrated.
The kinetics of photocatalytic oxidation of ethanol has been studied, and the formation and destruction rates of products have been measured. The important intermediates were identified as acetaldehyde, acetic acid, formaldehyde, and formic acid. Minor reaction channels resulted in the formation of methyl formate, ethyl formate, and methyl acetate. Kinetic and “dark” adsorption parameters were measured for ethanol and all of the important intermediates. We have modeled the complete oxidation process using a sequential chemical reaction mechanism. This mechanism is based primarily upon known homogeneous chemistry. However, the formation of acids from aldehydes in photocatalytic oxidation is different from known homogeneous chemistry and demonstrates the ability of the solid surface to stabilize energized transient species. We have measured the adsorp tion isotherms for ethanol and the oxidized intermediates and have concluded that there are two adsorption sites for some of the chemical species. We think it is possible that the adsorption properties of the compounds will change when the solid is illuminated.
Gas-solid heterogeneous photocatalytic oxidation of trichloroethylene (TCE) in humid airstreams is achieved at high conversion levels in a bench-scale flat-plate fluidized-bed photoreactor, which provides efficient continuous contact of near-ultraviolet photons, silica-supported titania photocatalyst, and gaseous reactants. Silica-supported titania catalysts prepared through sol-gel methods require approximately 1 h on stream time to develop their maximum photocatalytic oxidation activity. Steady-state reaction rates as high as 0.8-mu-mol of TCE (g of catalyst)-1 min-1 [2-mu-mol (g of TiO2)-1 min-1] and quantum efficiencies of 13% have been achieved over conditioned catalysts. High conversion of dilute levels (< 10 ppm) of TCE can be sustained for extended periods of operation, although the catalyst reversibly deactivates during conversion of streams containing higher TCE concentrations. Once activated, the fluidized catalyst bed responds rapidly to step changes in feed flow rate, composition, and photon flux.
A first-principles mathematical model describes performance of a titania-coated honeycomb monolith photocatalytic oxidation (PCO) reactor for air purification. The single-channel, 3-D convection–diffusion–reaction model assumes steady-state operation, negligible axial dispersion, and negligible homogeneous reaction. The reactor model accounts rigorously for entrance effects arising from the developing fluid-flow field and uses a previously developed first-principles radiation-field submodel for the UV flux profile down the monolith length. The model requires specification of an intrinsic photocatalytic reaction rate dependent on local UV light intensity and local reactant concentration, and uses reaction-rate expressions and kinetic parameters determined independently using a flat-plate reactor. Model predictions matched experimental pilot-scale formaldehyde conversion measurements for a range of inlet formaldehyde concentrations, air humidity levels, monolith lengths, and for various monolith/lamp-bank configurations. This agreement was realized without benefit of any adjustable photocatalytic reactor model parameters, radiation-field submodel parameters, or kinetic submodel parameters. The model tends to systematically overpredict toluene conversion data by about 33%, which falls within the accepted limits of experimental kinetic parameter accuracy. With further validation, the model could be used in PCO reactor design and to develop quantitative energy utilization metrics.
A research review of gas–solid heterogeneous photocatalysis is presented, ranging from details of pioneering works, which dealt with basic phenomena like oxygen and water vapor adsorption, to recent applications to pollutant removal in contaminated atmospheres. Special interest is taken in describing the different reactor configurations studied so far in this emerging and promising field. © 1997 SCI
Photocatalyzed oxidation of acetone (70-400 mg/m3) in air was carried out using near-UV illuminated TiO2 (anatase) coated on the surface of a ceramic honeycomb monolith. Considerable adsorption of acetone and water was noted on the catalyst coated monolith; these uptakes were described with a Langmuir adsorption isotherm for acetone and a modified BET adsorption isotherm for water. The acetone photocatalyzed disappearance kinetics on the TiO2 were determined with initial rate differential conversion, recycle reactor data and were analyzed using a Langmuir-Hinshelwood rate form coupled with a reactant mass balance including appreciable acetone monolith adsorption. The model, with parameters evaluated from initial rate data, is then shown to satisfactorily predict reactor behavior at all conversions. These kinetics and design results, together with earlier literature for photocatalytic oxidation of alkanes, 1-butanol, toluene, trichloroethylene, and odor compounds, indicate a potential for use of the photocatalytic monolith configuration for removal of all major classes of air contaminants.
To establish a promising method for the purification of air containing volatile organic compounds, photocatalytic decompositions of gaseous acetaldehyde over TiO2 deposited on porous silica (TiO2/SiO2 catalyst) and over a platinized TiO2/SiO2 catalyst (Pt-TiO2/SiO2 catalyst) have been investigated including the capture of intermediates on the catalyst surface and regeneration of the deactivated catalyst by heating. Results of kinetic analysis show that these photocatalytic decompositions obey Langmuir-Hinshelwood kinetics. A comparison between the amounts of acetaldehyde decomposed and CO2 produced reveals that about 10 % of acetaldehyde is missing. From the observation of the photocatalyst surface before and after the reaction by FT-IR spectroscopy, we conclude that this is due to the adsorption of intermediates such as formic acid and acetic acid on the porous catalyst as well as deposition of coke-like substances. When the Pt-TiO2/SiO2 catalyst is heated to a temperature above 473 K, these substances can be removed and discharged as CO2. A series of results obtained in the present work suggests that the use of a Pt-TiO2/SiO2 catalyst will enable us to construct a multifunctional reaction process for air purification, in which volatile organic compounds are photocatalytically decomposed. The harmful intermediates formed during the reaction are partly adsorbed on the porous catalyst, remain in the reactor system, and together with deposited coke-like substances are converted into CO2 by heat treatment of the catalyst. The catalyst is thus regenerated.
This investigation used a packed-bed reactor filled with coated titanium dioxide glass beads which was activated by a 365 nm UV light for heterogeneous photocatalytic decomposition of trichloroethylene under different conditions of flowrate, oxygen content, moisture and light intensity. For the flowrate of feed gas between 100–500 mL/min, the reaction rates increased with an increasing flowrate below 300 mL/min. When the flowrate was more than 300 mL/min, the reaction rate was not affected by the flowrate. For the oxygen content in the range of 0–20 %(v/v), it indicated that under 10 %(v/v) oxygen content the reaction rate increased as the oxygen content increased. The increase of moisture content lowered the reaction rates in the range of 230–30000 ppmv. The reaction rate of 6μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. The deactivation of the TiO2 described in the literature was not observed in this study.
The gas–solid heterogeneous photocatalytic oxidation of 1-butanol and 1-butylamine in air was investigated using supported TiO2 as a catalyst. The supported catalyst was prepared using a sol–gel method and irradiated employing two different light sources, a medium pressure mercury lamp or a xenon-chloride excimer lamp. The experimental set-up was especially designed for generating a gas stream containing stable and defined concentrations of the model pollutants. The gas stream at the reactor exit was analyzed on line by gas chromatography and the structures of the intermediates were established by gas chromatography coupled with mass spectrometry. Six major intermediates (butanal, butanoic acid, 1-propanol, propanal, ethanol and ethanal) were identified in the case of the photocatalytic degradation of 1-butanol. 1-Butylamine was less efficiently adsorbed on the catalyst and its degradation was slower. Three intermediates could be identified in this case (N-butylidene-1-butylamine, N-ethylidene-1-butylamine and N–butylformamide). Based on these results, a degradation mechanism is proposed for both compounds. Mineralization could be achieved under various conditions of concentrations and flow rates and was confirmed by infrared spectroscopy.
The development of a comprehensive 2-D finite-element model for a single channel of a honeycomb type monolith catalytic reactor is described. This includes a description of the weak variational form of the heat and mole balance equations derived using the Galerkin method. A description of the complex nonlinear boundary conditions is given and various schemes are compared for dealing with the convection term, including the method of characteristics, the standard Galerkin method and the Petrov—Galerkin method. The best results were obtained using a standard quadratic discretization for both temperature and concentration. For the transient case, several time discretization schemes were tested, including implicit Gear and Euler methods, the Crank—Nicholson method, as well as a fourth-order Runga—Kutta scheme. The Runga—Kutta scheme gave the most stable solution. The effects of radiation, homogeneous reaction, heterogeneous reaction and solid-phase conduction are addressed. Radiation and conduction were found to affect the outlet temperature, with axial conduction being more significant.
Nanoscale TiO2 catalysts prepared using a sol–gel method exhibit higher initial activity than commercially available P-25 TiO2 for the photocatalytic oxidation of toluene. Unlike P-25 TiO2, nonporous, nanoscale TiO2 catalysts are composed mainly of mesopores with pore sizes in the range of 35–44 nm. Calcination at 420°C leads to agglomeration of nanoscale TiO2 particles, formation of rutile, a decrease in pore capacity, and an enlargement of the mesopores. Catalysts treated at such a temperature display relatively low activity. Results of competitive adsorption of water and toluene on TiO2 samples confirm that TiO2 has a highly hydrophilic surface, which intrinsically suppresses the oxidation rate of toluene at high water content in the feed stream. Severe deactivation of TiO2 catalysts is due to the accumulation of partially oxidized intermediates, such as benzaldehyde and benzoic acid, on active sites. Complete recovery of catalytic activity requires a regeneration temperature above 420°C. Using platinum loaded on TiO2 results in lower oxidation rates of toluene, but facilitates the removal of poisonous intermediates from the deactivated TiO2 surface. Kinetic studies of the deactivation process indicate that the adsorption of poisonous intermediates in the initial stage of the photocatalytic reaction is almost irreversible. The initial oxidation rates on the catalysts are proportional to their surface areas. The surface concentration of illuminated active sites on TiO2 catalysts is estimated to be 0.85–0.96 μmol/m2.
The reaction engineering of suspended solid photocatalytic reactors is presented and the heterogeneous reactions in aqueous media, involving the presence of both fine particles of titanium dioxide and UV radiation are analyzed. Suspended solid photocatalytic reactors are just a family of the well-known slurry reactors wherein considerable progress are reported. To model the radiation field and also the initiation step, the radiative transfer equation (RTE) for heterogeneous media must be solved. The description of radiation field distribution in a typical flat plate, photocatalytic solar reactor simulator illustrates the method to be used for scaling-up purposes. With this objective the photon absorption is obtained as a function of position.
In recent years, photocatalytic degradation mediated by illuminated TiO2 has received considerable attention as an alternative for treating polluted water. In the present study, a new two-phase swirl-flow monolithic-type reactor was used to study the kinetics of heterogeneous photocatalytic processes. Photocatalytic degradation of phenol, 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) both in aqueous suspensions and over immobilized Degussa P25 TiO2 has been studied in laboratory scale. Experiments were conducted to investigate the effects of parameters such as catalyst dosage, pollutant concentration, temperature, partial pressure of oxygen, UV light intensity, catalyst-layer thickness, circulation flowrate and catalyst annealing temperature. Simple model for predicting the optimal catalyst dosage in aqueous suspensions for different photo-systems was proposed. Pseudo first-order kinetics with respect to all the parent compounds was observed. Experimental data obtained under different conditions were fitted with kinetic equation to describe the dependency of degradation rate as a function of the above mentioned parameters. Consequently, kinetic parameters were experimentally determined. Adsorptive properties of all the organics were also experimentally measured and fitted with Langmuir equation. The extreme low surface coverage of the organics on the catalyst may be one of the main factors that result in the low efficiency of the photocatalytic process. Besides, mass transfer of organics and oxygen in the photocatalytic process has also been discussed in detail.
The photo-assisted catalytic degradation of ethylene was studied in the tubular photoreactor packed with TiO2 pellets prepared by sol–gel method. The dependence of the reaction rate on the light intensity, feed composition (ethylene, oxygen and water vapor) and temperature were investigated. More than 95% ethylene was always completely mineralized irrespective of the reaction conditions. The reaction rate for ethylene conversion was greatly suppressed when the water vapor mole fraction was increased in the reactant gas stream. The rate determining step is the oxidation of C2H4OH radicals with oxygen on the catalyst surface. The rate law is expressed as follows to account for the results obtained:
The photochemical quantum efficiencies and oxidation performance of iron(III)-doped quantum-sized TiO_2 (Fe/Q-TiO_2), Degussa P25 (P25), and hybrid Fe/Q-TiO_2/P25 photocatalytic coatings are investigated using an optical fiber bundled array reactor. Fe/Q-TiO_2 coatings made from hydrosols of varying Fe/Q-TiO_2 content, 5–20 wt.%, a 13 wt.% P25 coating, and a hybrid, layered Fe/Q-TiO_2/P25 (5/13 wt.%) coating are tested. The light absorption efficiencies of the Fe/Q-TiO_2 coatings are inferior to the P25 coating, absorbing a maximum of only 80% of the input light compared with greater than 95% for P25 and 90% for the hybrid coating. The Fe/Q-TiO_2 coatings are found to increase the linear light transmission in a single optical fiber relative to P25 coated fibers by a factor of two owing to a reduced interfacial surface coverage of the photocatalyst particles on the quartz fiber. The hybrid coating does not significantly enhance linear light transmission.
Slurry-phase photoefficiencies for the photooxidation of 4-chlorophenol for the Fe/Q-TiO_2 photocatalyst are found to be significantly lower than those measured for P25, φFe/Q-TiO_2 = 0.002 vs.φ_(P25) = 0.012. In addition, the length of the coated fiber-bundle used in our reactor is insufficient to capitalize on the increased light transmission for the Fe/Q-TiO_2 coating. Thus, we are unable to investigate the effect of increased light transmission on the photoefficiency of the system. Initial reaction rates for the photooxidation of 4-chlorophenol range from 2.0 to 4.5 μM h^(−1) generally increasing with increasing hydrosol Fe/Q-TiO2 content and an average relative quantum efficiency of φ_(Fe/Q-TiO2) = 0.004 ± 0.001 is observed. These values are significantly low compared with initial rates and relative quantum efficiencies of 18.0 μM h^(−1), and φFe/_(Q-TiO2/P25) = 0.011 and 20.4 μM h^(−1) and φ_(P25) = 0.012 for the hybrid and P25 coatings, respectively.
Photocatalytic oxidations of toluene and benzene in air were carried out over water (TiO2/H2O) and HCl (TiO2/HCl) pretreated TiO2 as this latter pretreatment enhances the toluene removal rate. The main purpose was to identify intermediate products. No gas-phase by-products were detected by direct GC/FID analysis under our conditions despite the high aromatic concentration (50 mg m−3) and the short contact time (ms); this result illustrates an attractive capability of this air purification method. Adsorbed intermediate products recovered by extraction from the used photocatalysts were the same over TiO2/H2O and TiO2/HCl; i.e. no chlorinated products were found in this latter case. Benzoic acid, benzaldehyde, and benzyl alcohol were three major toluene intermediate products identified; trace derivatives of these products and of toluene that were monohydroxylated on the ring were also detected in catalyst extract samples. The benzene major by-product was phenol which was accompanied by hydroquinone and 1,4-benzoquinone. Acetic and formic acids were also formed from both benzene and toluene. For both aromatics, a water extraction of the used photocatalysts permitted separation of a yellow viscous material that settled between water and TiO2 after centrifugation. We have not identified the products contained in this material but believe them to be polymeric products which may be at the origin of the decreased photocatalytic activity of used TiO2 with respect to fresh TiO2. Finally, we discuss the photocatalytic oxidation pathways.
Persistent indoor air contaminants, those originating from emissions by interior furnishings, occupants, and materials of construction, typically exist in concentrations below 100 parts per billion (ppb) on an individual basis. The total of distinct contaminants may number in the hundreds with an equivalent accumulated concentration of one part per million. This study investigated the effects of humidity and trace (sub-ppmv) contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene. The evaluation also included variations in UV intensity and flow residence time. UV intensities from inexpensive mercury fluorescent lamps, those which are expected to be employed in a practical photocatalytic purifier, are in the mW/cm2 range. For this reason, the study included UV intensities in that range. The reactor element used in the study was a low pressure drop alumina reticulate, wash-coated with Degussa P25 titania. The data indicated that the reaction was first-order for the three reactants at the sub-ppmv level. An important finding was that competitive adsorption between water and trace (sub-ppmv) contaminants has a significant effect on the oxidation rate. The dependencies of humidity and contaminant concentrations on the oxidation rates are explained as being the results of competitive adsorption on available hydroxyl adsorption sites and of changes in hydroxyl radical population levels.
The photocatalytic oxidation of high levels of volatile chlorinated organic compounds in gas phase has been studied using a specially designed photoreactor. The influence of light intensity, initial water vapour concentration, temperature, inlet contaminant concentration and flow rate on destruction efficiency has been investigated. The performance of the titanium catalyst was strongly affected by the presence of water in the air stream. Experiments have been most successfully conducted at room temperature, low initial contaminant concentrations, low flow rates and high light intensities. Several by-products have been detected performing photocatalytic degradation of trichloroethylene (TCE) and tetrachloroethylene (PCE). No catalyst deactivation was observed.
The photocatalytic oxidation of methyl orange under weak illumination conditions was carried out in two different types of fluidized bed reactors. TiO2 powder was employed as the photocatalysts and a 15 W low pressure mercury lamp was used as the light source and the reactor volume was 2.5 l. The parametric study of photocatalytic oxidation of methyl orange in two different types of fluidized beds was investigated; effect of catalyst loadings, pH, air flow rate, initial concentration and oxygen concentration on the oxidation reaction rate. The experimental results were analyzed in conjunction with the characteristics of fluidized bed and the reactor geometry effect on the reaction rate was analyzed.
ALTHOUGH the possibility of water photolysis has been investigated by many workers, a useful method has only now been developed. Because water is transparent to visible light it cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm (ref. 1).
Destruction of trichloroethylene in air via semiconductor mediated gas-solid heterogeneous photocatalysis
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Destruction of volatile organic compound (VOC) emissions by photocatalytic oxidation (PCO): benchscale test results and cost analysis
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