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Photocatalytic Air Purification
Abstract
Titanium dioxide represents an effective photocatalyst for destruction of volatile organic compounds (VOC), NOx and SO2 in indoor and outdoor air. The mechanism of selected pollutants removal as well as the dependence of reaction rate on some key influencing factors are discussed. Recent papers and patents considering photocatalysts preparation and immobilization techniques including type of supports, surface pretreatment procedure are reviewed. Photocatalytic reactors are designed in order to provide strictly controlled conditions for photocatalytic air purification. Some reactors can be utilized in commercial applications as a part of HVAC systems (Heating, Ventilation and Air Conditioning) but most of them are used in laboratories to measure the activity of different types of photocatalysts applied for gas streams treatment. Examples of photoreactors used for gas phase treatment are presented.
... Airborne hazards such as chemicals, particulate matter, biological pollutants, and microorganisms may endanger not only human health but other living organisms. Major chemicals (also known as air pollutants) such as sulfur oxides (SO x ), nitrogen oxides (NO x ), and VOCs result from human activity [56]. The development of respiratory diseases or even premature deaths has been associated with exposure to these hazardous substances. ...
... Finally, it is capable of fully degrading VOCs into carbon dioxide (CO 2 ) and water (H 2 O) [15]. TiO 2 is a photosensitive semiconductor that has widely been employed in PCO technology for its effectiveness in absorbing ultraviolet (UV) light in the presence of oxygen and water vapor to form reactive hydroxyl radicals ( d OH) and superoxide radicals (O 2 d2 ) [56,58]. These free radicals may react with pollutants in a series of reactions, such as substitution, bond cleavage, and electron transfer and oxidize as well as decompose the pollutants into harmless end byproducts [58]. ...
... Thus, the immobilization of photocatalysts onto a support material would be convenient. However, the immobilization of the photocatalysts has not yet been satisfactorily achieved [1][2][3][4]. In the ideal case, the support material and matrix for immobilization should feature the following properties [5]: 1) Strong anchoring of the photocatalyst preventing detachment, 2) Promoting the activity of the photocatalyst (e.g., offering large surface area, preventing recombination, or transmitting irradiation), and 3) Being chemically stable during photocatalytic reactions. ...
... Photocatalysts can be synthesized by gas phase hydrolysis, chemical vapor deposition, sputtering, Hänel A., et al. electrochemical, direct oxidation of transition metals, micro-emulsion, sol-gel, or the hydrothermal method [3,6]. Two electrochemical methods can be distinguished: anodic oxidation of metal sheets and cathodic deposition. ...
The aim of our research was to develop an immobilization method for photocatalysts that is an alternative to the sol-gel or dip-coating methods and can be simply scaled up for technical applications. The investigated photocatalyst was TiO2, which was electrochemically deposited onto a cathode made of stainless steel. This deposited film was photocatalytically active. In order to enhance the photoactivity of the TiO2 film, commercially available P25 photocatalyst nanoparticles were occluded into the film. The effect of deposition current density as well as the amount of occluded nanoparticles on the photocatalytic activity and photoelectrochemical behavior was investigated. The photocatalytic activity was evaluated in a UV-LED reactor. The decomposition rate of toluene and cyclohexane in air was examined for all prepared stainless steel-photocatalyst composites. It was observed that deposits prepared with 5 g dm-3 of P25 in the deposition bath showed the best photocatalytic activity and highest photocurrent.
... From today's standpoint, the technology in question has a high economic potential due to its utilization of solar energy. Photocatalytic oxidation can remove the following pollutants: volatile organic compounds (VOC), unpleasant odors, ammonia (NH 3 ), hydrogen sulfide (H 2 S), methane (CH 4 ), nitrogen oxides (NO X ), sulfur oxides (SO X ), carbon monoxide (CO), and ozone (O 3 ) [1][2][3]. ...
An annular reactor is a type of reactor in which chemical reactants move through an annular gap between two concentric cylinders. This type of design is suitable for a variety of chemical processes because it facilitates efficient heat transfer and mixing of reactants. Previous research has shown that this type of reactor shows promising results in the photocatalytic decomposition of pollutants, which is why it is used in air purification tests. This paper presents a model of one such type of air purification reactor modeled in COMSOL Multiphysics simulation software and provides an overview of the steps that need to be taken in order to effectively model the photocatalytic oxidation. The purpose of modelling the reactor is to test its effectiveness in computer simulations of the decomposition of pollutants in the air using the process of photocatalytic oxidation, which is a combination of photooxidation based on the effect of UV radiation and catalytic oxidation. The resulting simulations allow the scaling of the system (its increase or decrease) so that it can be adapted to certain conditions and used in the real world as a method of air purification at the very sources of polluted air.
... UV-C LEDs lamps emit ultraviolet light with a wavelength in the range of 254 to 365 nanometres, coinciding with the peak UV absorption of virus RNA (Nunayon et al., 2019). When this latter interacts with the photocatalyst, it triggers a photocatalytic reaction, generating highly reactive oxygen radicals, which break down and oxidize a wide range of indoor air pollutants, including volatile organic compounds (VOCs), bacteria, viruses, and odorous compounds (Zaleska et al., 2010); ...
... Outdoors, the UVA fraction of the solar radiation supplies enough photons in the absorption spectrum of TiO 2 to drive photocatalytic reactions (Park et al., 2022). Indoors or specifically inside photocatalytic reactors, an artificial source of UVA photons is needed (Zaleska et al., 2010). For a long time, fluorescent tubes were used to supply the required UVA irradiation (Deutsches Institut für Normung, 2018). ...
Fluorescent tubes, a continuous source of UVA radiation, are increasingly being replaced by ultraviolet light-emitting diodes (UV LEDs or UVEDs), which emit an almost discrete spectrum (5 nm bandwidth). This creates both problems and opportunities from a photocatalytic point of view. In this paper, we report the influence of UVED radiation on the performance of an industrially produced TiO2 photocatalytic coating by measuring the degradation of nitrogen oxide (NO) and toluene (C6H5CH3) from a test atmosphere in a laboratory test setup. The influence of four commercially available UVED types (365 nm, 385 nm, 395 nm, and 415 nm) on the performance of a commonly used photocatalyst was compared. In a subsequent investigation, we switched from continuous to pulse-modulated LED operation and investigated its influence on the photocatalytic activity of the assembly. We could show that UVEDs are suitable replacements for fluorescent lamps when carefully chosen to the absorption spectrum of the used photocatalyst. In addition, the pulse width and pulse frequency modulation of the LED current show non-linear correlations with the resulting photocatalytic activity. The activity remains unexpectedly high with short pulse widths and low frequencies. By adjusting the control of the UVEDs accordingly, much energy can thus be saved during operation without reducing the catalytic activity.
... Depending upon target purity, various configurations can be adapted for air remediation process (Table 1). The overall photo-oxidation for volatile organic compounds (VOCs) is dependent on light intensity, humidity, temperature, and concentration and contaminant structure in the environment (Zaleska et al. 2010). The TiO 2 is used in powder form so that it can easily photocatalytically reduce nitrogen oxide (NO) at room temperature and also used as a coating material on highway walls for the absorption of nitrogen oxide emissions from vehicles. ...
Contaminated air is one of the greatest concerns that the world is facing in recent times, since it is bolstering with each passing year and consequently resulting in to grave and disastrous repercussion to the earth and its environment. The contamination of air pollution is caused by both natural and anthropogenic sources and apparently contribution of anthropogenic sources to air pollution surpasses the natural sources. According to the World Health Organization (WHO), presently the main air pollutants are particulate matter (PM), surface ozone (O3), nitrogen dioxide (NO2), sulphur dioxide (SO2) and carbon monoxide (CO). The green nanotechnology is an imminent technology that may administer a quick fix and withstand the air pollution. The present chapter aims to furnish comprehensive information about the role of emerging green nanomaterials for air pollutants control in three different steps viz. detection of air pollutants, source reduction or pollution prevention and remediation/degradation of air pollutants. The green nanomaterials are used as an excellent adsorbents, catalysts and sensing material due to their large specific surface areas and high reactivates. Owing to its large surface area and high surface energy, the nanomaterials have the competency to absorb large amount of air pollutants or catalyse reactions at a much faster rate. Hence, the energy consumption is reduced during degradation or it may aid to inhibit the release of contaminants. This chapter shed light on the application of green nanomaterials for the detection and remediation of air pollution also the future trends in this field.
... Immobilizing photocatalysts onto a support material would be convenient for recycling and reusing the photocatalyst. However, immobilization of the photocatalysts has not been satisfactorily achieved yet (Taranto et al., 2009;Hänel et al., 2010;Zaleska et al., 2010). The support material and matrix for immobilization should be characterized by the following properties (Pozzo et al., 1997): ...
Electrochemical deposition of photocatalyst, especially TiO 2 , onto several cathode materials was investigated and evaluated. The deposited films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDX) and electrochemical measurements. Photo-catalytic activity was determined by the decomposition of cyclo-hexane in air by irradiation with ultraviolet light emitting diodes. It was found that stainless steel is the best cathode material.
In this research, we highlight the honeycomb structure generated by electrochemical anodization and the effect of anodization voltage on the diameter and wall thickness of titania (TiO2) nanotubes. According to our observations, a tiny change in the anodization parameter causes the morphology of nanotubes to vary. As a result, nanotube production is an extremely delicate process. Following the experimental section, we obtained nanotubes with diameters of 38, 55, and 91 nm and anodization potentials of 20, 30, and 40 V, respectively. With constant time, temperature, and pH, nanotubes grew and became longer at 20 V. Such morphology significantly impacts titania nanotube applications like photocatalysis for wastewater treatment.
Pure and co-doped Titania thin films were prepared on aluminum substrates through the sol-gel method. The co-doped sample showed higher photocatalytic activity on benzene degradation compared to pure TiO2 under visible light illumination. XRD results showed the anatase phase for both TiO2 and co-doped TiO2 lattices with an average crystalline size of 12.9 and 10.4 nm, respectively. According to the UV-visible absorption spectra results, co-doped Titania showed higher visible light absorption compared to pure Titania. The synergistic effect of dopants caused a redshift to visible light absorption and also the lifetime of the photogenerated electron-hole were increased by induced electron levels in Titania lattice. The novelty of this study is the reactor’s specific design. We employed Al mesh as thin film substrate for 3 main reasons, first, the large surface area of the Al mesh causes to increase specific surface area of the photocatalysts, also it is a formable substrate which can be engineered geometrically to decrease the shadow spots so the thin films will receive the highest light irradiation. Also, the Al mesh flexibility facilitates the procedure of reactor design to reach a minimum pressure drop of airflow while it is installed in the air conditioners or HVAC systems.
The effects of CuO loading, reaction temperature, and surface gas velocity (Ug) on the photocatalytic reduction of NO have been determined in an annular flow type and a modified two-dimensional fluidized bed photoreactor. The optimum CuO loading was found to be 3.3 wt.% and NO degradation conversion in the modified two-dimensional fluidized bed photoreactor is more than 70% at 2.5 Umf.
Structureproperty relations for different borate glasses and their applications in the fields of optics and related techniques are presented. Optical glasses, with a high borate content mostly in the form of the boroxol ring-structure, with an sp2 configuration, possess anomalous partial dispersion which allows a minimisation of the secondary spectrum of objectives. Heavy metal borate glasses with high refractive index, nonlinear optical properties and doped with rare earth elements are discussed. Properties of two different types of borosilicate glasses, with high and low basicity caused by different M2O/B2O3 ratios, are described. They have large differences in their intrinsic ultraviolet absorption, excimer laser resistance and colouring behaviour with Ni2+ and Co2+ ions.
An incidence model is formulated from the basic principles of radiation heat transfer to predict the light intensity profile inside a photocatalytic monolith reactor, where a light-absorbing and light-reflecting catalytically active thin film is coated on the inner walls of monolith channels of either circular or square cross section. A diffuse external light source and diffusely reflecting wall coatings were assumed. The mathematical model representation of the light flux distribution to the monolith wall and through the cross section of the monolith channel take the form of integral equations. In dimensionless form, these equations reveal that, for a given channel type, light intensity profiles are controlled by channel aspect ratio and film reflectivity. The equations were solved numerically using Gauss–Legendre quadrature to give quantitative estimates of the radiation intensity profile down the length of the monolith channel for a specified incident light intensity distribution at the entrance of the channel and an assumed thin film reflectivity. Experimental cross-sectional light intensity profiles for square channeled, uncoated ceramic monoliths with two different cell densities confirmed the prediction of the model that dimensionless profiles are not dependent on absolute channel dimensions, but rather are uniquely determined by the channel aspect ratio. Experimental intensity data for titania-coated monoliths were well described by model predicted profiles assuming an average reflectivity of 40%. For identical aspect ratio channels, model predictions reveal that the light intensity profiles for square and circular channels are quite similar. Model predictions indicate that radiation field gradients are large, with relatively little light penetrating beyond a length equivalent to three channel widths. This prediction implies that, for monoliths with typical commercial aspect ratios, a large fraction of the coated channel wall is not effectively irradiated.
Photocatalytic purification of indoor air requires devices that are compact and ensure good contact between the air flow and the photocatalyst-coated material with a minimal pressure drop. These three conditions lead to materials-destined to be coated by a photocatalyst-having complex shapes. Accordingly, modeling becomes essential in optimizing the irradiance of the photocatalytic support, which obviously is a factor of paramount importance. We present here a methodical strategy to optimize irradiance on a regularly folded and a honeycomb-shaped material. The first step was to calculate the irradiance on a planar material. The surface of the UV lamp tubes was modeled as an ensemble of elemental lighting sources emitting according to a Lambert law. After the validity was verified by irradiance measurements, similar calculations were performed for a folded and a honeycomb-shaped material, using appropriate geometrical parameters. Using an experimental design (Doehlert matrix), the effects of the interval between the lamps, the distance between the lamps and the material, and the geometrical characteristics of the folds or the honeycomb cells upon the homogeneity and value of the irradiance were calculated. Finally, a multicriterion analysis was employed to determine the configuration producing optimal irradiance. The methodology can easily be applied to UV lamps and reactors having dimensions different from those used in our calculations.
The performance of the miniaturized photocatalytic air purifier including a continuous adsorption/desorption unit with a zeolite particles-loaded honeycomb rotor was investigated in the photocatalytic purification of 1m3 air containing toluene at concentrations of 3–11mgm−3 (about 1–3ppmv). While operating the continuous adsorption/desorption unit only, and for desorption temperatures controlled within the range of 90–120 and 130–160°C, the unit took approximately 10min to reduce the toluene concentration in a 1m3 room, to a value of almost zero. Almost the same time courses of toluene concentrations were obtained when the photocatalytic reactor was switched on. This result clearly shows that the rapid decrease in the toluene concentration in the 1m3 room is mainly due to the adsorption of toluene onto the zeolite rotor. When the reactor was switched off, the concentration of toluene desorbed into the 0.022m3 reactor box increased rapidly and then levelled off in 10min. On the other hand when the reactor was switched on, the toluene concentration in the reactor box increased rapidly, passing through a maximum in 10–150min, and then decreasing to a value near zero, leading to the toluene concentration in the 1m3 room becoming negligible. The apparent rates of decomposition determined from the decreasing toluene concentration in the reactor box were 10-fold smaller than the intrinsic rate constant in the Langmuir–Hinshelwood equation, which would suggest that the desorption process of toluene from the rotor is the rate-limiting. All the decomposition experiments were performed using the same zeolite rotor and photocatalytic reactor over a six month period; and the runs were repeated more than one hundred times. Nevertheless, there was no distinct decrease in decomposition activity of the photocatalyst or any significant loss in ability for the zeolite rotor to adsorb toluene could be seen, indicating that these materials can perform constantly over a long period of time. However, as the rotor will sooner or later become saturated with toluene, it may become necessary to introduce a regeneration procedure in order to periodically remove toluene from the rotor.
To develop a high-performance photocatalytic reactor for purification of indoor air, the photocatalytic decomposition of gaseous HCHO at a very low concentration is investigated both theoretically and experimentally. In this second paper, a photocatalytic reactor with a parallel array of nine light sources, designed on the basis of the experimental result in the first paper, is applied to the air containing HCHO at an indoor concentration level and the reactor performance is discussed. The experimental result indicates that this photocatalytic reactor can rapidly decompose HCHO toward zero concentration. The reason for this high reactor performance is explained by a mathematical model that takes into consideration a film-diffusional resistance in the neighborhood of the photocatalyst as follows: (1) the reaction field is irradiated with a high light intensity because the distance between the light source and photocatalyst surface is only 6 mm; (2) the rate of decomposition is increased by the UV light that permeates through a glass tube; (3) the film-diffusional resistance is remarkably reduced because of a high linear velocity (709 m min−1).
Mesoporous films of TiO2 prepared by a template-assisted procedure based on the evaporation-induced self-assembly mechanism were shown to be efficient photocatalysts for the oxidation of nitrogen oxide. Their high photocatalytic activity is explained by a local increase in the partial pressure of NO in the nanopores of the porous film close to the photocatalytic active sites. Owing to the strong adsorption of the intermediate products, a high selectivity towards nitric acid is achieved.
Sulfur-containing compounds are well-known catalyst poisons. To evaluate the feasibility of photocatalytic technology for indoor air purification, a typical atmospheric SO2 concentration of 200 parts per billion (ppb) was selected. In order to further evaluate the impact of SO2 on the photocatalytic activity of other typical indoor air pollutants, SO2 was co-injected with 200 ppb NO and 20 ppb benzene, toluene, ethylbenzene, and o-xylene (BTEX) using TiO2 (P-25) as photocatalyst coated on a glass fiber filter. A concurrent photodegradation of SO2 with NO, SO2 with BTEX, and SO2 with NO and BTEX was also conducted. Results showed that no photodegradation of SO2 was found. However, the blank glass fiber filter adsorbed more than 75% of the SO2. The conversion of NO decreased by 8% and the generation of NO2 increased by 10% with the presence of SO2. A similar inhibition effect was found on the photodegradation of BTEX with the presence of SO2. The presence of SO2 decreased the conversion of BTEX by more than 10%. Ion chromatography analysis on the TiO2 glass fiber filter showed that sulfate ion was formed from the adsorption of SO2. The formation of sulfate ion inhibited the formation of nitrate ion, which increased the generation of NO2. It is suggested that the inhibition effect of SO2 is due to the sulfate ion competing with the pollutant for adsorption sites on TiO2. The promotion effect of NO on BTEX was also reduced by the presence of SO2.
Titanium dioxide is one of the most important photocatalysts that allows the environmental purification of various toxic organic compounds in water and removal of harmful air pollutants. In this paper, two techniques of plasma spraying – plasma spraying from an agglomerated nanopowder and liquid suspension plasma spraying – were used to produce thin deposits. The microstructure of coatings was characterized using scanning electron microscopy and X-ray diffraction. The photocatalytic properties of the coatings were evaluated by the decomposition test of nitrogen oxides and compared with the efficiency of the initial agglomerated nanopowder and that of the Degussa P25 powder. Noticeable different behaviours in the photocatalytic tests were observed for coatings performed either from conventional or liquid plasma spraying. The TiO2 deposits realized by spraying of the liquid suspension exhibited higher photocatalytic efficiency. It is found that the plasma spraying of a slurry is an effective method to retain the original anatase phase and prevent the increase of the crystallite size. This new method of modified spraying is proved to be a promising technique to elaborate photocatalytic coatings for the removal of nitrogen oxide pollutants.
To develop a high-performance photocatalytic reactor for purification of indoor air, photocatalytic decomposition of HCHO at a very low concentration is investigated both experimentally and theoretically in two papers. This first paper reports the result of fundamental experiment that is necessary to design the photocatalytic reactor in the second paper. A combination of the absorption of trace HCHO into water with the quantitative analysis of HCHO by the 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (AHMT) method is found to be useful to accurately measure the HCHO concentration at a ppb level (less than 1.23 mg m−3). The experimental results obtained using three types of annular-flow photocatalytic reactors with different inside diameters (22–120 mm) and light sources (6 and 20 W blacklight blue fluorescent lamps) indicate that the photocatalytic reactor with a 6 W lamp and a glass tube of 28 mm in inside diameter (the distance between the light source and photocatalyst surface; 6.5 mm) gives a maximum rate of decomposition; HCHO at a ppb concentration level is rapidly decomposed to zero concentration when the photocatalyst surface kept at a low temperature (45 °C) is irradiated with UV light of a high light intensity.