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Abstract

Hydrotalcite like compounds containing Co, Al and Ce are synthesized by co-precipitation method. The mixed oxides issued from calcination step, have been characterized and tested for the toluene total oxidation with various concentrations of toluene and oxygen. The toluene conversion curves have been modelled in this paper with eight models. The Mars Van Krevelen mechanism shows the best accordance with the experimental data. Moreover, the determination of the Co3O4-CeO2 interface as the active center has been performed with the experimental observations. The Co3O4 allows obtaining high catalytic activity, and the CeO2 permits increasing the reoxidation of the Co3O4 to perform a new catalytic cycle.

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... The main novelty objective of this work is to evaluate the influence of the carbon monoxide on the toluene total oxidation. Indeed, studies of the VOC oxidation, especially toluene, have been widely reported in the literature [9,29,[30][31][32]. However, studies on the simultaneous catalytic oxidation of toluene and CO compounds in a mixture are very limited, even though this is an important step towards industrial environmental applications. ...
... The reducibility order according to the H 2 consumption at lower temperature (T < 400°C) is following the same order than the catalytic activity order observed for the toluene total oxidation. A relationship between the low-temperature reducibility and the toluene conversion suggests that the active phase of the catalyst corresponds to a Co 3 O 4 and a redox mechanism for this reaction can be suggested, which was confirmed in a previous paper [30]. This mechanism called also as Mars Van-Krevelen mechanism, is reported also in the bibliography for the total oxidation of VOC in presence of metal oxide catalysts [8,27,34]. ...
... This relationship shows that the high proportion of O II species results in higher catalyst activity that can be explained by the higher mobility of the adsorbed oxygen species compared to the bulk oxygen species. It have been already observed that for this type of catalytic materials, the Co 3 O 4 is the principal active phase for toluene total oxidation [27,28,30]. Moreover, the high quantity of oxygen adsorbed due to the presence ceria on the material, and the presence of Co 3 O 4 active phase create a synergistic effect to increase the reducibility of the materials that allow obtaining an active catalyst for the oxidation of toluene at low temperature. ...
... As various synergistic processes can occur concurrently on eggshell, Ag, and MnO 2 surface sites, it is not simple enough to pinpoint a specific rate-determining step. Nevertheless, the FA oxidation process on the Ag-MnO 2 /Eggshell catalyst can be assumed to occur through the Mars-Van Krevelen (MvK)-type mechanism (based on the FA oxidation pathway simulated through DFT), as is commonly observed for various gas-phase oxidation reactions (Genty et al., 2019;Misono, 2013;Lee et al., 2020). The interaction between the FA ads and O* species is often the rate-determining step in the MvK mechanism (Genty et al., 2019;Misono, 2013). ...
... Nevertheless, the FA oxidation process on the Ag-MnO 2 /Eggshell catalyst can be assumed to occur through the Mars-Van Krevelen (MvK)-type mechanism (based on the FA oxidation pathway simulated through DFT), as is commonly observed for various gas-phase oxidation reactions (Genty et al., 2019;Misono, 2013;Lee et al., 2020). The interaction between the FA ads and O* species is often the rate-determining step in the MvK mechanism (Genty et al., 2019;Misono, 2013). ...
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Under dark/low temperature (DLT) conditions, the oxidative removal of gaseous formaldehyde (FA) was studied using eggshell waste supported silver (Ag)-manganese dioxide (MnO2) bimetallic catalysts. To assess the synergistic effects between the two types of metals, 0.03%-Ag-(0.5-5%)-MnO2/Eggshell catalysts were prepared and employed for DLT-oxidation of FA. The steady-state FA oxidation reaction rate (mmol g⁻¹ h⁻¹), when measured using 100 ppm FA at 80°C (gas hourly space velocity (GHSV) of 5,308 h⁻¹), varied as follows: Ag-1.5%-MnO2/Eggshell-R (9.4) > Ag-3%-MnO2/Eggshell-R (8.1) > Ag-1.5%-MnO2/Eggshell (7.5) > Ag-5%-MnO2/Eggshell-R (7.2) > Ag-1.5%-MnO2/CaCO3-R (6.8) > MnO2-R (6) > Ag-0.5%-MnO2/Eggshell-R (3.2) > Ag/Eggshell-R (2.6). Here, ‘R’ denotes hydrogen-based thermochemical reduction pretreatment. The temperature required for 90% FA conversion (T90) at the same GHSV exhibited a contrary ordering: Ag/Eggshell-R (175°C) > Ag-0.5%-MnO2/Eggshell-R (123°C) > Ag-5%-MnO2/Eggshell-R (113°C) > MnO2-R (99°C) > Ag-1.5%-MnO2/Eggshell (96°C) > Ag-3%-MnO2/Eggshell-R (93°C) > Ag-1.5%-MnO2/Eggshell-R (77°C). The eggshell catalyst outperformed the commercial calcium carbonate variety due to the presence of defects in the former. The MnO2 co-catalyst enhances the capture and activation of atmospheric oxygen (O2) to boost the overall activity through rapid catalytic regeneration. Also, MnO2 favorably captures the hydrogen of the adsorbed FA molecules to make the oxidation pathway thermodynamically more favorable.
... As shown in Table 7, the toluene oxidation rate constant (to benzylic alcohol) is lower than the catalyst re-oxidation constant (to carbon dioxide). This is in accordance with the MVK model where the first step is the slower step [42]. Compared with the MVK-2 model, the MVK model (R 2 ≥0.9)has better consistency. ...
Article
To achieve efficient degradation of toluene, a series of Cu1-yMn2CeyOx/sepiolite catalysts (y = 0.1, 0.2, and 0.3) with different Cu1-yMn2CeyOx loadings (10%, 20%, and 30%) were prepared via the co-precipitation method. The structure-activity and surficial elemental species of Cu1-yMn2CeyOx/sepiolite were characterized by XRD, TEM, SEM, BET, ICP-MS and XPS. The catalytic activity of the catalysts was tested in the oxidation reaction of toluene, results showed that 20%Cu0.8Mn2Ce0.2Ox/sepiolite remains able to remove toluene completely with high efficiency at a temperature of 289 °C. Two kinetic models have been selected and tested to describe the oxidation of toluene, the Mars-van krevele (MVK) model provided a good fit (R2 ≥ 0.99). And the optimal relation of the surface oxidation activation energy (26.074 kJ mol-1) and surface reduction activation energy (23.591 kJ mol-1) were calculated.
... The formula used therefore takes into consideration the specific surface area and the mass of catalyst used. For the oxidation of toluene, previous work has shown a kinetic order of 1 for toluene 56 . The applied activity formula is therefore characteristic of a first order reaction and is calculated at low conversion (10%). ...
... revealed that MVK model compared to four others is the best mechanism to model total oxidation of toluene. E.Genty et al [177]. also confirmed that MVK is a best fit to describe the mechanism of the total oxidation of toluene on CoAlCe catalyst. ...
Article
Volatile organic compounds (VOCs) are a class of liquid compounds with low saturation vapor pressures, which enables them to change phases even at ambient conditions. This causes a huge problem when it comes to emissions. A large portion of VOCs cause catastrophic health effects such as cancer or liver damage even at very low concentrations (ppm), and the control of these compounds is vital in ensuring good air quality. Many VOCs are used in common household implements such as wood finishes or paints and can even be emitted off improperly ventilated cooking. This significantly alters indoor air quality, and as such, a large portion of research into these compounds have focused on indoor air. On the other hand, their widespread use as solvents in most industries along with their emission rates from vehicular exhausts results in contamination of outdoor air as well. The combination of these VOCs and NOx pollutants tends to cause ground level ozone formation, which has a large bio-toxicity and causes the formation of a smog. Densely populated cities have also seen a sharp rise in VOC emissions due to the increase in both vehicular traffic and industrialization. A myriad of options is available for the abatement of VOCs, and they are similar to other pollution control strategies. These methods can be broadly be classified into destruction, separation and hybrid methods. This review discusses the different perspectives to be considered for this scenario, along with descriptions of the different methods available to the industry, along with some novel technologies that have been introduced recently. The influential parameters for each method have been investigated, along with optimum catalyst/material selection. In addition to this, a mathematical modeling perspective has also been examined, with significance to simulation studies performed for the control of VOC emission technologies.
... Effectivement, Finocchio et al. [69] ont pu montrer d'après des études IR, que l'oxydation du propène sur le catalyseur Co3O4 a lieu selon le mécanisme MVK. De même, Genty et al. [70] ont montré d'après l'analyse des différentes données cinétiques que l'oxydation totale du toluène en présence d'un catalyseur à base de CoAlCe s'effectue selon le mécanisme Mars-Van Krevelen. ...
Thesis
Ce travail vise l'étude de l'oxydation catalytique des Composés Organiques Volatils (COVs). L'objectif principal est de trouver des matériaux catalytiques qui sont des alternatives aux métaux nobles très onéreux. La recherche des matériaux catalytiques actifs, sélectifs, stables dans le temps et à base de métaux de transition est abordée. Afin d'améliorer les performances catalytiques des métaux de transition, la dispersion de l'espèce métallique doit être optimisée en utilisant une méthode de synthèse adéquate. Les différents solides préparés ont été caractérisés par différentes techniques physico-chimiques : la Diffraction des Rayons X (DRX), les Analyses Thermiques (ATD/ATG), la Spectroscopie Infrarouge (IR), la Réduction en Température Programmée (H₂-RTP), la Microscopie Electronique à Balayage (MEB), la Spectroscopie de Photoélectrons induits par rayons X (SPX)...De plus, les oxydes obtenus ont été testés dans les réactions d'oxydation totale du COV propène. En premier lieu, nous avons focalisé notre travail sur la préparation d'une série d'oxydes mixtes Co-Mg/Al-Fe par voie hydrotalcite afin d'obtenir des propriétés intéressantes en catalyse hétérogène. Cette étude consiste surtout à évaluer la substitution des cations bivalents et trivalents. Il s'est avéré que le solide contenant à la fois du cobalt et du fer (CoFe) possède l'activité catalytique la plus élevée. L'intérêt de la voie hydrotalcite par rapport à d'autres méthodes de synthèse, telles que la méthode classique et le mélange mécanique des oxydes a été mis en évidence. En second lieu, l'utilisation des irradiations micro-ondes lors de la synthèse des matériaux hydrotalcites a été également étudiée comme une méthode de synthèse non-conventionnelle. Un effet bénéfique a été observé avec l'utilisation des micro-ondes dû principalement à des surfaces spécifiques plus élevées et à une meilleure réductibilité des espèces oxydes. En outre, cette étude vise également la mise au point de cette nouvelle méthode. De ce fait, l'optimisation de la température, durée et puissance des irradiations micro-ondes a été effectuée sur le solide CoFe, afin de montrer l'influence de chaque paramètre sur les propriétés physico-chimiques des matériaux. En troisième lieu, la synthèse d'une série de catalyseurs par voie hydrotalcite avec différentes teneurs en Co²⁺ et Fe³⁺ a été effectuée. L'influence du rapport molaire sur la structure hydrotalcite a été montrée. Une comparaison entre ces solides et ceux préparés par la méthode micro-ondes a été également abordée.
... Note that the intrinsic activity of the Co-free alumina supports (pure Al2O3, Mg-Al and Ce-Al) was negligible at 600 °C. The apparent activation energy of the four evaluated catalysts (Table 4) were estimated by the integral method, assuming that the reaction kinetics followed a pseudo-order one for methane and a zeroth order for oxygen, as it is usual for the Mars-van Krevelen mechanism in the presence of excess oxygen [46,47]. The corresponding linearized plots are depicted in Figure S2. ...
Article
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Spinel-type cobalt oxide is a highly active catalyst for oxidation reactions owing to its remarkable redox properties, although it generally exhibits poor mechanical, textural and structural properties. Supporting this material on a porous alumina can significantly improve these characteristics. However, the strong cobalt–alumina interaction leads to the formation of inactive cobalt aluminate, which limits the activity of the resulting catalysts. In this work, three different strategies for enhancing the performance of alumina-supported catalysts are examined: (i) surface protection of the alumina with magnesia prior to the deposition of the cobalt precursor, with the objective of minimizing the cobalt–alumina interaction; (ii) coprecipitation of cobalt along with nickel, with the aim of improving the redox properties of the deposited cobalt and (iii) surface protection of alumina with ceria, to provide both a barrier effect, minimizing the cobalt–alumina interaction, and a redox promoting effect on the deposited cobalt. Among the examined strategies, the addition of ceria (20 wt % Ce) prior to the deposition of cobalt resulted in being highly efficient. This sample was characterized by a notable abundance of both Co3+ and oxygen lattice species, derived from the partial inhibition of cobalt aluminate formation and the insertion of Ce4+ cations into the spinel lattice.
... Toutefois, dans le cadre de la politique de développement durable, la production de dioxygène natif constitue un enjeu majeur pour le développement de procédés qui mettent en oeuvre des [227], [228]. C'est la raison pour laquelle ce mécanisme est décrit par la suite. ...
Thesis
Ce travail de thèse s'inscrit dans le cadre du projet ADEME CALICE "Procédé de captage et libération du CO2 assisté par électrolyse utilisant des matériaux de type brucitique". Il vise à développer une technologie de rupture dans le domaine du traitement d'effluents gazeux contenant du CO2, en vue de sa valorisation. Le procédé repose sur la modification des valencedes espèces redox actives constitutives des feuillets du matériau pour moduler sa capacité d'échange anionique et assurer le captage et la libération du CO2, sous forme d'anions inorganiques. Décrits dans la bibliographie comme matériaux capables d'assurer jusqu'à 5000 cycles électrochimiques d'oxydation et de réduction, les HDLs Co/Fe ont été sélectionnés comme matériaux d'intérêt, susceptibles d'assurer le captage et la libération des anions HCO3- et/ou CO32- ; les atomes de cobalt, aux degrés d'oxydation II et III, constituent les entités électroactives. Le mécanisme de formation des HDLs Co/Fe, par coprécipitation à pH variable, a été étudié pour des valeurs de pH comprises entre 2 et 11. La ferrihydrite, sur laquelle Co2+ s'adsorbe, est l'intermédiaire réactionnel clé de la synthèse des HDLs Co/Fe. L'adsorption de Co2+ est responsable de phénomènes d'oxydoréduction qui induisent la formation d'un HDL métastable (FeII/CoIII) dont l'évolution conduit au matériau HDL final (CoII/FeIII). L'analyse du comportement thermique des matériaux a permis de préciser la formulation des HDLs synthétisés (Co3,14Fe2(OH)10,28(CO3), xH2O) et de définir leur domaine thermique d'application potentielle (inférieur à 160°C) en vue de leur utilisation dans un procédé de capture du CO2 contenu dans des gaz chauds issus d'installations industrielles dont la température peut atteindre 600°C. Dans le cadre de l'intégration énergétique du procédé, des échangeurs thermiques, pour la récupération d'énergie ou le préchauffage de certains flux, devront par conséquent être mis en œuvre. Le couplage des techniques électrochimiques, gravimétrique et de diffraction de rayons X, au synchrotron Soleil, a permis de déterminer le potentiel standard apparent du couple CoIII/CoII (1,23 V/ESH), la constante cinétique d'oxydo-réduction du couple CoIII/CoII constitutifs des feuillets du HDL (k° = 2,5 x10-4 m/s, a = 0,6, b = 0,4) et d'analyser les mécanismes physico-électrochimiques associés à l'oxydation de CoII et à la réduction de CoIII de telle sorte que : CoII6CoIIIFeIII2[(OH)16][CO3], zH2O + 0,5x CO32- ⇆ CoII6-xCoIIIxFeIII2[(OH)16][CO3](1+0,5x),(z-z')H2O + z'H2O + xe- Le procédé de captage et libération du CO2 a été mis en œuvre sur des solides de type HDLs utilisés sous forme de pulpes dans des réacteurs électrochimiques à deux compartiments, en milieu aqueux. Les analyses ont été conduites dans une solution tampon NaHCO3/Na2CO3 à pH 9 (FI 0,05M). Le couplage des électrolyses à potentiel imposé et du suivi de l'alcalinité a permis de calculer le rendement des réactions redox. Le pourcentage de Co redox actif est compris entre 5 et 12%. Le coefficient de transport de matière électroactive vers la surface de l'électrode, km, est égal à 4,72x10-5 m/s ; les constantes réactionnelles électrochimiques d'oxydation et de réduction du cobalt, qui quantifient l'efficacité de l'interaction matière/électrode, sont respectivement égales à 1,48x10-3 s-1 et 1,27x10-3 s-1. Dans les conditions optimales de cyclage (0,84 et 0,24 V/ESH respectivement pour l'oxydation et la réduction de Co, avec une concentration de Co de 4 10-3 M), le rendement du procédé est supérieur à 170 kg de CO2 par tonne de HDL et par cycle. Ce résultat laisse présager des perspectives intéressantes pour l'optimisation d'un procédé, assisté par électrolyse, de capture et de libération du CO2 par des HDLs Co/Fe.
... The above results demonstrated the Pt/Al 2 O 3 to be active and stable, and it is then necessary to obtain the oxidation kinetics for the purpose of determining the feasibility of the batch reactor-combustor recycling system. The catalytic toluene oxidation can be interpreted by the Mars van Krevelen mechanism [8,[32][33][34], which includes the reduction of the oxidized catalyst with the aromatic compound and the oxidation of the reduced catalyst by oxygen [32]: ...
Article
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A reactor‐combustor recycling system is proposed for safe operation of a hydrocarbon‐ and O2‐rich reactor by catalytically combusting excess O2and recycling generated CO2 for dilution. The Pt/Al2O3 catalysts are determined as active and stable, and the Mars van Krevelen mechanism was applied for the combustion kinetics. Modelling and simulation of the system involving reactor, heater, combustor, valve and cooler are performed, where multi‐tubular, monolith and micro‐structured reactors are compared for addressing the strong exo thermic combustion. The system is highly promising because 99.9% O2 can be consumed and explosion risk be eliminated via CO2 dilution. The micro‐structured combustor exhibits the highest potential by considering heat management, pressure drop and compactness.
... Toluene, phenol, and aniline, and are examples of these compounds, which are considered as water and air pollution precursors. These organic compounds are obtained from coal tar [31][32][33][34] and employed as the intermediate in the pesticide, munition, rubber, and pharmaceutical industries [35][36][37][38]. A matter of controversy about aromatic compounds is their high toxicity, which limits their advantages. ...
Article
Motivated by the enhancing interest of experimental research in synthesis of new borofullerenes, herein, density functional theory (DFT) calculations were employed to determine impact of substitutional doping with foreign atoms on the structural, electronic, and chemical properties of all-boron B38 nanocage. Accordingly, the transition metals of Mn, Fe, Co, and Ni are selected and the interplay between the chemical reactivity and bond strength of the formed nanocages is evaluated. Taking inspiration from the results on the role of dopants in the electronic properties of pristine nanocage, we also attempt to explore the adsorption capability of nanocages toward three aromatic pollutants viz. toluene, phenol, and aniline. After calculating the adsorption characteristics, it is found that each pollutant is strongly adsorbed over all examined doped nanocages, outperforming the pristine B38. Furthermore, the energy values for the adsorption of mentioned pollutants over doped nanocages have been indicated to be in the range of about −126 to −283 kJ mol⁻¹, depending on the type of dopant. Consequently, our results indicate that substitutional doping as an efficient strategy for modulation of the molecular arrangement possesses superior performance on the electronic structure of B38, which can provide opportunities to design suitable adsorbents for capturing harmful pollutants.
... The relatively weak Co-O bond, generates an easy interaction between the oxygen atoms of the lattice and the reagents of the oxidation reaction [2]. This redox property is often sought after, since the Mars-Van Krevelen mechanism takes place usually in these reactions [3]. Several factors influence the properties and the catalytic performance of cobalt species. ...
Article
Two series of mixed cobalt-iron oxides were synthesized by the conventional hydrotalcite route, without (CoxFe2-HT) or with microwave irradiations (CoxFe2-MW). Different molar ratios Co²⁺/Fe³⁺ were also investigated with 2 ≤ x ≤ 8. X-ray diffraction (XRD) analysis confirmed the formation of the desired hydrotalcite phase for all the materials before calcination whatever the molar ratio Co²⁺/Fe³⁺ is. After calcination of the latter materials at 500 °C, XRD evidenced the complete destruction of the hydrotalcite phase and, consequently, the formation of mixed oxides of cobalt and iron. After being characterized, the oxides materials were tested towards the total oxidation of the volatile organic compound (VOC) propene. Both the molar ratio and the preparation method have a significant effect on the catalytic properties. The light-off curves of CoxFe2-HT500 materials showed that the catalytic activity is improved for a molar ratio Co²⁺/Fe³⁺ > 2. Co6Fe2-HT500, with the ratio Co²⁺/Fe³⁺ equal to 3, showed the best catalytic performance. Furthermore, except for Co8Fe2-MW500, the materials that were subjected to microwave irradiations gave higher catalytic activities compared to those of the homologous ones prepared without such irradiations. Co6Fe2-MW500 was found to be the most catalytically active among all the materials of both series due to the higher specific surface area, the higher amount and easier reducibility of active species, higher proportion of mobile oxygen and of Co²⁺ on the surface. Furthermore, this catalyst did not show tendency to deactivation and coking even after 100 h under stream.
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A series of FeOx-MnO2-CeO2 catalysts were synthesized by the surfactant-templated coprecipitation method and applied for HCHO removal. The influence of Fe/Mn/Ce molar ratio on the catalytic performance was investigated, and the FeOx-MnO2-CeO2 catalyst exhibited excellent catalytic activity, with complete HCHO conversion at low temperatures (40 °C) when the molar ratio of Fe/Mn/Ce was 2/5/5. The catalysts were characterized by N2 adsorption and desorption, XRD, H2-TPR, O2-TPD and XPS techniques to illustrate their structure–activity relationships. The result revealed that the introduction of FeOx into MnO2-CeO2 formed a strong interaction between FeOx-MnO2-CeO2, which facilitated the improved dispersion of MnO2-CeO2, subsequently increasing the surface area and aiding pore development. This promotion effect of Fe enhanced the reducibility and produced abundant surface-active oxygen. In addition, a great number of Oα is beneficial to the intermediate decomposition, whereas the existence of Ce3+ favors the formation of oxygen vacancies on the surface of the catalyst, all of which contributed to HCHO oxidation at low temperatures.
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The completely selective oxidation of toluene to benzaldehyde with dioxygen, without the need to use H2O2, halogens, or any radical initiators, is a reaction long desired but never previously successful. Here, we demonstrate the enzyme-like mechanism of the reaction over hexadecylphosphate acid (HDPA)-bonded nano-oxides, which appear to interact with toluene through specific recognition. The active sites of the catalyst are related to the ability of HDPA to change its bonding to the oxides between monodentate and bidentate during the reaction cycle. This greatly enhances the mobility of the crystal oxygen or the reactivity of the catalyst, specifically in toluene transformations. The catalytic cycle of the catalyst is similar to that of methane monooxygenase. In the presence of catalyst and through O2 oxidation, the conversion of toluene to benzaldehyde is initiated at 70 °C. We envision that this novel mechanism reveals alternatives for an attractive route to design high-performance catalysts with bioinspired structures.
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Due to its excellent oxygen storage capacity, ceria is a well-known oxidation catalyst. However, its performance in the oxidation of volatile organic compounds can be improved by the introduction of gold. Depending on the type of VOC to be oxidized, the surface of gold nanoparticles and the gold/ceria interface may contribute to enhanced activity and/or selectivity. Choosing a proper preparation method is crucial to obtain optimal gold particle size. Deposition–precipitation was found to be more suitable than coprecipitation or impregnation. For industrial applications, monolithic catalysts are needed to minimize the pressure drop in the reactor and reduce mass and heat transfer limitations. In addition to the approach used with powder catalysts, the method employed to introduce gold in/on the washcoat has to be considered.
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This work deals with the extensive study of supported bimetallic Co-Ni catalysts for the complete oxidation of methane. The simultaneous incorporation of nickel and cobalt was found to enhance the redox properties by favouring the formation of nickel cobaltite-like species and partially inhibiting the interaction between Co3O4 and alumina. In turn, this resulted in an increase of the amount of Co³⁺ cations in the catalysts, which led to a more notable presence of active lattice oxygen species in the samples. When the nickel loading was increased, the generation of less active NiO was observed. The improvement of the redox properties resulted in a promotion of the specific reaction rate and a shift of around 50 °C in the T50 value over the bimetallic catalysts. The most active catalyst (25Co-5Ni) was found to be relatively stable during prolonged operation times, but suffered an appreciable and irreversible deactivation in the presence of water vapour.
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In this study, Fex−Mny/γ-Al2O3 catalysts were synthesized by reflux-coprecipitation method, and then these catalysts were characterized by BET, XRD, H2-TPR, XPS, SEM and TEM. Meanwhile, the toluene removal and secondary pollutants in a nonthermal plasma coupled with Fex−Mny/γ-Al2O3 reactor were studied, and then the reaction kinetic model and reaction pathway for the toluene removal were proposed. The results indicated that positive synergistic effect between the plasma and Fex−Mny/γ-Al2O3 catalysts greatly improved the removal performance, and reduced secondary pollutants and activation energy during toluene removal process in the plasma packed with Fex−Mny/γ-Al2O3 system. Meanwhile, the plasma packed with the Fe1−Mn1/Al2O3 exhibited the highest removal efficiency (98.59%) and CO2 selectivity (88.02%) with the lowest secondary pollutants and activation energy during toluene removal under the SED of 1500 J/L and the GHSV of 80,000 ml g ⁻ ¹ h ⁻ ¹. Finally, the toluene removal pathway in the plasma-catalysis system was also discussed based on the identified intermediates and synergistic effect between the plasma and Fe1−Mn1/γ-Al2O3.
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Marine biofouling is an expensive problem that needs evolved chemical or physical antifouling strategies. However, most of the current antifouling materials that would damage the environment through metal leaching and bacteria resistance are being halted. Nanozyme is one kind of environmental antifouling materials through generating reactive oxygen species (ROS). We prepared various contents of CeO2 that could uniform disperse compounding with Co3O4 and CoAl2O4 to form a stable Co-Al-Ce mixed metal oxide (MMO) by a layered double hydroxide derived method. We find that coupling with CeO2 can improve the peroxidase (POx) activity. When the molar ratio of Ce is 2.5% and the calcination temperature is 200°C, the POx activity of Co-Al-Ce MMO is the best caused by the good dispersion of catalytically active components and the high specific area (150.10±4.95 m2/g). This novel Co-Al-Ce MMO also exhibits an antibacterial mode of action Gram-negative bacteria in near-neutral pH solution through generating ROS (mainly ·O2−) in the presence of H2O2. Ce containing MMO can be utilized as potential green marine antifouling material.
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An investigation of the effect of oxygen and silver species on the oxidation of toluene has been launched to identify the reaction process and mechanism over MnOx/SBA-15 and Ag–MnOx/SBA-15 catalysts. Detailed knowledge of the effect and cycle of oxygen species under reaction is typically missing, and this study points at the behavior of oxygen species over MnOx/SBA-15 for toluene oxidation for the first time, using in situ mass spectrum techniques with the isotope ¹⁸O2. It is found that the Langmuir–Hinshelwood and Mars–van Krevelen mechanism coexist in toluene oxidation over MnOx/SBA-15 catalyst. In this study, surface and bulk lattice oxygen are shown to be vital active oxygen species in the whole reaction. The consumed oxygen species are replenished with gaseous oxygen, and the process follows the Mars–van Krevelen mechanism, as confirmed by detection of isotopic products. The adsorbed oxygen species with good mobility is also found to start up and participate in the reaction at lower temperatures, which follows the Langmuir–Hinshelwood reaction mechanism. The addition of Ag increases the amount of surface lattice oxygen species and improves the transformation rate and the migration of lattice oxygen, which effectively facilitates the low-temperature activity of toluene oxidation.
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To elucidate the role of oxygen vacancy defects, various Mn-based oxides with oxygen vacancy defects are employed to the toluene oxidation, which are synthesized by adjusting solvent and double-complexation routes. The MnOx-ET catalyst shows the highest catalytic activity (T90 = 225 °C) for toluene oxidation. Compared with other Mn-based oxides, the as-prepared MnOx-ET catalyst has more surficial oxygen vacancies and good oxygen storage capacity (OSC), which is the reason on its remarkable activity for toluene oxidation. In addition, in situ DRIFTS study reveals that both lattice oxygen and adsorbed oxygen species can participate in the activation-oxidation process of toluene, which results in two reaction routes for the toluene oxidation. The rich oxygen-vacancy concentration of catalysts accelerates the key steps for the activation and generation of oxidized products. Quasi-in situ XPS results further confirm that enrich adsorbed-oxygen species as active oxygen and increasing Mn⁴⁺ concentration enhance the superior activity for toluene oxidation.
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In order to study their synergistic catalytic effects in toluene degradation, CuMn2O4/HTS-1 (HTS-1 was a titanium silicon molecular sieve), Cu0.7Mn2Y0.3Ox/HTS-1 and Cu0.7Mn2Ce0.3Ox/HTS-1 catalysts were prepared by the impregnation method. The textural properties, redox properties and acidity of the catalysts were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), frustrated total internal reflection (FT-IR), ammonium temperature-programmed desorption (NH3-TPD) and pyridine adsorption internal reflection (Py-IR) measurements. The potential roles of Lewis acid sites (activating dioxygen) were discussed, and the experimental results indicated that the most efficient route for toluene degradation over Cu0.7Mn2Ce0.3Ox/HTS-1 (toluene conversion rate of 90% (T99)=295°C) was ascribed to regulation of the synergistic effects of redox properties (activating molecular toluene) and Lewis acid sites (activating dioxygen). The Mars-Van-Krevelen (MVK) model was adopted to describe the reaction process of toluene oxidation, which gave an in-depth view into the toluene degradation over CuMn2O4/HTS-1, Cu0.7Mn2Y0.3Ox/HTS-1 and Cu0.7Mn2Ce0.3Ox/HTS-1. In addition, the synergistic effects between redox properties and Lewis acid sites were studied in detail.
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Industrial volatile organic compounds (VOCs) are harmful to the environment and human health. Catalytic oxidation is the most effective method for end-of-pipe control. The challenges of VOC catalytic oxidation are the high space velocities and low oxidation temperatures. In this work, Mn cations were introduced into Ce-BTC supports by an impregnation method, after which the mixture was calcined. The as-synthesized CeO 2 -supported Mn catalyst MnO x -CeO 2 -s exhibited an outstanding activity for ethyl acetate (EtOAc) catalytic oxidation, which reached complete conversion at 210 °C (T 99 = 210 °C), even at a high space velocity (WHSV = 60,000 mL g ⁻¹ h ⁻¹ ). With in situ UV Raman and other characterization methods, the Frankel oxygen vacancy (F-OV) was determined to be the crucial active site for EtOAc catalytic oxidation. The excellent low temperature activity of MnO x -CeO 2 -s benefited from the enhanced F-OVs concentration. The high temperature stability was attributed to the stable F-OVs recovery potential of the Ce-BTC-derived CeO 2 support. The durability at a high space velocity was the result of the unique crossed channel structure inherited from Ce-BTC. These findings unraveled the crucial role of F-OVs in the EtOAc catalytic oxidation and the value of MOF-derived materials, which highlighted a feasible approach for the design of VOC-removal catalysts.
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Developing efficient catalysts with good resistance to complex flue gas is essential for VOCs removal in coal-fired flue gas. In this study, by exploring the effect of transition metal oxide additive, metal loading and bimetallic synergy on toluene oxidation in coal-fired flue gas, 10Cu-3V/γ-Al2O3 is identified as the optimal catalyst. It achieves 90% of CO2 generation at 350℃, which is decreased by ca. 46℃ compared with 13Cu/γ-Al2O3. And it also exhibits good resistance to H2O and good stability. ICP-OES, N2 adsorption-desorption isotherms, XRD, TEM, XPS, EPR and H2-TPR analyses were applied to characterize the catalyst composition and physicochemical properties. Doping V into 13Cu/γ-Al2O3 not only leads to better dispersity of CuO and homogeneous elements distribution that benefits to produce more active centers, but also constitutes the redox cycle of V⁵⁺ + Cu⁺ ↔ V⁴⁺ + Cu²⁺ which induces more surface chemical oxygen (Osur). Moreover, since SO2 is the main inhibiting factor in toluene oxidation, the SO2 poisoning mechanism was illustrated by XPS, TG and in situ DRIFT analyses in depth.
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In this work bimetallic Ni catalysts supported over Co-Al2O3 and monometallic Co-Al2O3 and Ni-Al2O3 catalysts were examined for the complete oxidation of methane. With a 30% total metallic loading, the samples were synthesized by a sequential precipitation route. All samples were characterized by nitrogen physisorption, X-ray fluorescence, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, scanning-transmission electron microscopy, X-Ray photoelectron spectroscopy, and temperature-programmed reduction with hydrogen and methane. Their catalytic performance was investigated in the temperature range of 200-600 °C with a space velocity of 60.000 h⁻¹. The bimetallic catalysts showed a better behavior in the oxidation reaction than the monometallic counterparts, mainly due to the good dispersion of Ni on the surface of the Co-Al2O3 samples. This has enabled the insertion of Ni²⁺ ions into the cobalt spinel lattice, which in turn provoked an increase in the amount of Co³⁺ species, and a subsequent enhanced mobility of oxygen species in the spinel. In this sense, the 5Ni/25Co catalyst showed the best performance, thus reducing the value of the T50 by 25 °C with respect to the monometallic catalysts.
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Catalytic total oxidation is an effective technique for the treatment of industrial VOCs principally resulting from industrial processes using solvents, and usually containing mono-aromatics (BTEX) and oxygenated compounds (acetone, ethanol, butanone). The catalytic total oxidation of VOCs on noble metal materials is effective. However, the cost of catalysts is a main obstacle for the industrial application of these VOC removal processes. Therefore, the aim of this work is to propose an alternative material to palladium-based catalysts (which are suitable for VOCs’ total oxidation): a mixed oxide synthesized in the hydrotalcite way, namely CoAlCeO. This material was compared to four catalytic materials containing palladium, selected according to the literature: Pd/α-Al2O3, Pd/HY, Pd/CeO2 and Pd/γ­Al2O3. These materials have been studied for the total oxidation of toluene, butanone, and VOCs mixtures. Catalysts’ performances were compared, taking into account the oxidation byproducts emitted from the process. This work highlight that the CoAlCeO catalyst presents better efficiency than Pd-based materials for the total oxidation of a VOCs mixture.
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The treatment of volatile organic compounds (VOC) emissions is a necessity of today. The catalytic treatment has already proven to be environmentally and economically sound technology for the total oxidation of the VOCs. However, in certain cases, it may also become economical to utilize these emissions in some profitable way. Currently, the most common way to utilize the VOC emissions is their use in energy production. However, interesting possibilities are arising from the usage of VOCs in hydrogen and syngas production. Production of chemicals from VOC emissions is still mainly at the research stage. However, few commercial examples exist. This review will summarize the commercially existing VOC utilization possibilities, present the utilization applications that are in the research stage and introduce some novel ideas related to the catalytic utilization possibilities of the VOC emissions. In general, there exist a vast number of possibilities for VOC utilization via different catalytic processes, which creates also a good research potential for the future.
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MMgAlO mixed oxide catalysts (M= Mn, Fe, Co, Ni, Cu, Zn, Ag and Pd) obtained from layered double hydroxide (LDH) precursors calcined at 1023K were used in the complete oxidation of methane and the oxidative dehydrogenation of propane. The catalysts were characterized by XRD, N2 adsorption, EDX and H2-TPR experiments. In the catalytic complete oxidation of methane, the Pd-based catalyst has been proved the most active, but the less stable catalyst. Other mixed oxides containing transition metals have been tested and their catalytic activity followed the order: MgAlO≈FeMgAlO < NiMgAlO < ZnMgAlO <MnMgAlO< CoMgAlO < AgMgAlO < CuMgAlO. Total conversion was achieved at 858K with CuMgAlO the most active and highly stable catalyst also able to perform the complete oxidation of ethane, propane and propene. It has been shown that highly reducible metal oxide species play an important role in the catalytic combustion of methane. In the oxidative dehydrogenation of propane in the temperature range from 723K to 873K the propene selectivity passed through a maximum for the Mn- and Fe-containing catalysts and decreased continuously for the other transition metal-containing catalysts to the benefit of COx for CuMgAlO and of cracking products for CoMgAlO, NiMgAlO and ZnMgAlO. Considering the propene yield, the catalysts can be ranked in the following order: CoMgAlO >MnMgAlO> NiMgAlO > ZnMgAlO > FeMgAlO > CuMgAlO. No straight correlation between the H2-TPR reducibility and the catalytic performances of the samples was found in the oxidative dehydrogenation of propane into propene. The effects of the contact time and of the propane-to-oxygen molar ratio on the catalytic performances of the most active CoMgAlO, MnMgAlO and NiMgAlO mixed oxides have been investigated.
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A uniform and highly dispersed CuFe-based catalyst was obtained via a calcination–reduction process of a CuFeMg-layered double hydroxide (LDH) precursor, which exhibits good activity and selectivity towards catalytic conversion of syngas to mixed alcohols. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal that the CuFeMg-LDH precursor possesses high crystallinity with a particle size of 40–60 nm. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) demonstrate a high dispersion of copper and iron species on the catalyst surface. The CuFe-based catalyst derived from CuFeMg-LDHs shows high CO conversion (56.89%) and the total alcohol yield (0.28 g mLcat.−1 h−1), as a result of the high dispersion of active species as well as the synergistic effect between the copper and the iron species revealed by X-ray photoelectron spectra (XPS) and H2 temperature-programmed reduction (H2-TPR) techniques. Therefore, this work provides a facile and effective method for the preparation of CuFe-based catalysts with high catalytic activity, which can be potentially used in syngas conversion to mixed alcohols.
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2% Au/Al2O3 catalysts were prepared by a novel method involving Direct Anionic Exchange (DAE). The method produces strong bonding of the gold complex (HAuCl4) to the alumina support with no loss of gold during the subsequent steps of preparation. The complete removal of chloride from the catalyst was achieved by washing with concentrated ammonia. This procedure ensures a better activity and prevents sintering during calcination as shown by TEM. The catalysts were tested for the oxidation of CO and of saturated and unsaturated hydrocarbons (C1 to C3). The catalysts showed high activities over a range of concentrations and temperatures relevant to applications in automotive exhaust cleaning. Furthermore, a remarkable resistance to thermal ageing at 600°C in the absence or presence of water was observed, due to the presence of the strongly anchored nanosized gold particles obtained during the preparation step.
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In this work, the catalytic oxidation of benzene, toluene andn-hexane in air, both alone and in binary mixtures, over a commercial Pt on γ-alumina catalyst was studied. Studies have been carried out at concentrations of up to 4200 ppmV, in a laboratory fixed-bed catalytic reactor. Results for single compounds show that temperature at which 50% conversion is attained (T50) increases as concentration increases for benzene and toluene, while the opposite behaviour is observed for n-hexane. Results for mixtures show that, while the presence ofn-hexane does not affect the conversion of benzene and toluene, the presence of benzene or toluene inhibits the combustion of hexane, and the aromatic compounds inhibits each other when are reacted together. Results obtained in absence of mass transfer limitations were fit to kinetic expressions: simple Mars–Van Krevelen kinetic expressions for single compounds, and a modified Mars–Van Krevelen mechanism, considering competitive adsorption of the hydrocarbons, for binary mixtures.
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A series of new Cu–Co–Fe compounds with the general formula Cux Co2−x Fe1HT (x = 0, 0.5, 1, 1.5 and 2) has been prepared by hydrotalcite coprecipitation method. The presence of hydrotalcite phase is revealed by XRD analysis for x values of 0, 0.5 and 1. When the copper quantity is higher than 1, the malachite phase is preferentially formed. These results are confirmed by TG-DTA, FT-IR and XPS analysis. After calcination at 500 °C in air of all samples, XRD analysis reveals the presence of spinel phases such as Co3O4, CoFe2O4, CuFe2O4, Cux Coy O4 in the solids, monoclinic CuO phase when the copper content is greater or equal to 1 and haematite phase for the sample where x is equal to 2. The presence of these phases is also confirmed by XPS results. For comparison, a Co2Fe1OH sample has been synthesized by classical coprecipitation method and although Co2Fe1HT sample and Co2Fe1OH form a similar phase after calcination at 500 °C, Co2Fe1HT500 presents a higher BET value than Co2Fe1OH500 sample.
Article
Co6Al2HT hydrotalcite-like compounds were synthesized by three different methods: co-precipitation, microwaves-assisted and ultrasound-assisted methods. The mixed oxides obtained after calcination were studied by several techniques: XRD, TEM, H2-TPR and XPS. They were also tested as catalysts in the reaction of total oxidation of toluene. The physico-chemical studies revealed a modification of the structural characteristics (surface area, morphology) as well as of the reducibility of the formed mixed oxides. The solid prepared by microwaves-assisted synthesis was the most active. Furthermore, a relationship between the ratio of Co2+ on the surface, the reducibility of the Co-Al mixed oxide and the T50 in toluene oxidation was demonstrated. This suggests a Mars Van Krevelen mechanism for toluene total oxidation on these catalysts.
Article
The competitive combustion of propane and carbon monoxide was investigated with regard to its selectivity for propane oxidation. Based on variations in the sol-gel synthesis and the composition, the TiCrOx catalysts were optimized. Specifically, the addition of small amounts of cerium led to a catalyst, which converted 89% propane at 375 degrees C but no carbon monoxide. The origin of this excellent and unusual selectivity has been investigated. The method of choice was diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS), which allowed recording adsorbed species on the catalysts surface. Hopcalite was taken as reference catalyst due to its selectivity (although opposite to TiCrOx) for the combustion of carbon monoxide in the presence of hydrocarbons. The spectroscopic investigations showed that carbon monoxide is linearly adsorbed on Cu+ at lower temperatures and formed carbonates at higher temperatures, whereas propane formed no detectable adsorbed species on hopcalite. The TiCrOx catalyst formed formate species with carbon monoxide above 250 degrees C and carboxylates with propane above 150 degrees C. The mixture of propane and carbon monoxide led to the formation of both species. By the addition of oxygen to the gas feed, only propane formed adsorbates on the catalysts surface, whereas CO seemed not to interact with the surface. Pre-adsorbed oxygen probably inhibits the adsorption of carbon monoxide but is not necessary for propane activation. The formation of carboxylates from propane also takes place in the absence of molecular oxygen, pointing to a Mars-van-Krevelen-like mechanism, in which lattice oxygen is involved in the oxidation process. A physical mixture of TiO2 and Cr2O3 showed no catalytic activity, and spectroscopic investigations showed no adsorbates on the sole oxides. Since only anatase and eskolaite could be identified in the powder diffraction patterns of the catalysts, a third phase has to be responsible for the propane oxidation, most likely surface decorations or solid solutions of the metal ions.
Article
Commercial VOC oxidation catalysts can be used as comparative materials during development of new or improved catalysts. The objective of this study was to investigate physicochemical properties of EnviCat® commercial catalysts and their performance in total oxidation of three model compounds (dichloromethane, toluene and ethanol) at laboratory scale. The reactivity of model VOC was decreasing in the order ethanol > toluene > dichloromethane. The Cu–Mn/Al catalyst was found to be the most active and selective catalyst in ethanol oxidation. In oxidation of dichloromethane, the Pt–Pd/Al–Ce catalyst with 0.10 wt% Pt + Pd was the most active, while the most selective one (giving the highest HCl yield) was the Pt–Pd/Al catalyst containing 0.24 wt% Pt + Pd. In toluene oxidation, the Pt–Pd/Al catalyst with 0.24 wt% Pt + Pd possessed the highest activity; the selectivity to CO2 was 100% for all investigated catalysts. Obtained results showed that the performance of commercial catalysts in laboratory scale tests can be different from that declared by catalyst supplier. A possible difference in catalytic performance at industrial and laboratory scale should be taken into account when industrial catalysts are used in laboratory scale tests.
Article
The catalytic oxidation of toluene, chosen as VOC probe molecule, was investigated over Co3O4, CeO2 and over Co3O4–CeO2 mixed oxides and compared with the catalytic behavior of a conventional Pt(1 wt%)/Al2O3 catalyst. Complete toluene oxidation to carbon dioxide and water was achieved over all the investigated systems at temperatures below 500 °C. The most efficient catalyst, Co3O4(30 wt%)–CeO2(70 wt%), showed full toluene conversion at 275 °C, comparing favorably with Pt/Al2O3 (100% toluene conversion at 225 °C).
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The catalytic properties of palladium and gold nanoparticles deposited on mesoporous TiO2 are investigated in toluene and propene oxidation. The catalysts, containing Pd and Au deposited on mesoporous TiO2 have been prepared by different order of metal deposition (Pd(shell)–Au(core)/TiO2, Pd–Au(alloy)/TiO2 and Au(shell)–Pd(core)/TiO2. For both toluene and propene oxidation reactions, the catalytic activity was found significantly higher when palladium is deposited on already-deposited gold (Pd(shell)–Au(core)/TiO2). This enhanced activity could be explained by the core–shell morphology (Pd–shell and Au-core) observed by UV–vis spectra, TPR profiles and XPS spectra. It was suggested that the oxidation reaction follows a Langmuir–Hinshelwood mechanism where the molecules of oxygen and VOC are in competition for adsorption on the surface of catalyst. Operando DRIFT spectroscopy was carried out to test the catalytic activity in a mixture of volatile organic compounds (toluene and propene). It was demonstrated that there is a competition between the molecules of VOC for adsorption but also the toluene has an inhibition effect for oxidation of propene.
Article
The oxidation of n-hexane and toluene mixtures in air over Pt/Al2O3 catalysts with mean Pt crystallite sizes of 1.0 and 15.5 nm has been studied. Measurements were performed in an external recycle reactor at temperatures in the range of 120–181 °C, in a concentration range of one hydrocarbon from 500 to 10,000 ppm. The mutual inhibition of the oxidation activity of n-hexane and toluene in the mixtures was observed for both Pt crystallite sizes. The structure sensitivity previously established for the oxidation of single components remains valid for their mixture. The mixture model oxidation based on a Mars van Krevelen mechanism fits very well experimental data at lower temperatures (for n-hexane: 148–160 °C; for toluene: 120–140 °C), whereas at higher temperatures the model predicts higher values for catalysts with both Pt crystallite sizes. The mixture model based on equilibrium adsorption of hydrocarbons and strong adsorption of oxygen fits exceptionally well experimental data in the whole investigated temperature range. For catalysts with a mean Pt crystallite size of 1.0 nm the evaluated ratio of adsorption coefficients of n-hexane and toluene is 9, and for the catalysts with a mean Pt crystallite size of 15.5 nm the ratio is 0.13. The evaluated ratios of adsorption coefficients point out that n-hexane is more strongly bound than toluene for catalysts with smaller Pt crystallites, while at larger Pt crystallites toluene is more strongly adsorbed than n-hexane.
Article
In situ infrared spectra and in situ laser Raman spectra of Co–Al and Ni–Al hydrotalcites (HTlcs) have revealed several novel aspects of the structure, and physico-chemical transformations upon thermal decomposition of these materials in air. The infrared spectra of the Co–Al-HTlc at room temperature show the presence of a fraction of the carbonate in a low symmetry, which was not observed for Ni–Al-HTlc. Furthermore, compared to Co–Al-HTlc, a relatively smaller ratio of carbonate over hydroxyls is present in the as-synthesized Ni–Al-HTlc, as revealed by the infrared and Raman spectra. This is in agreement with a mass spectrometric analysis of the as-synthesized materials upon thermal decomposition, and the relatively small c-parameter determined from the X-ray diffraction (XRD) pattern of the as-synthesized Ni–Al-HTlc. An extraordinary stability of interlayer water in Ni–Al-HTlc was found, which was only completely removed above 300°C, a temperature of 100°C higher than found for Co–Al-HTlc. Dehydroxylation of the octahedral layers and carbonate reorganization and decomposition also require higher temperatures for Ni–Al-HTlc (300–350 and >550°C) than for Co–Al-HTlc (150–200 and 450–500°C). Raman spectra indicate the presence of an intermediate cobalt oxide (CoOx) phase upon decomposition of Co–Al-HTlc in the temperature range of 175–250°C, before formation of a solid solution of cobalt spinels (Co(Co, Al)2O4) occurs. Highly dispersed NiO strongly interacting with the Al2O3 support was identified after decomposition of Ni–Al-HTlc at 550°C. The spectroscopically determined transition temperatures are in excellent agreement with XRD, thermogravimetric, and mass spectrometric analyses.
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Among the possible substitutes for noble metals, cobalt-based catalysts represent promising alternative systems. In recent years, many articles have been devoted to the synthesis, characterization and reactivity of cobalt oxides. This article provides a comprehensive review of the state-of-the-art activities that concentrate on the synthesis, structural properties and catalytic applications of Co3O4 nanocrystals and Co3O4/MOx binary oxides in CO, CH4 and VOCs oxidation at low temperatures. It begins with the major synthetic approaches and basic properties of Co3O4 nanocrystals and Co3O4/MOx binary oxides and subsequently highlights the relationship between the peculiar structure and redox properties of Co3O4 nanocrystals and Co3O4/MOx binary oxides and the catalytic activity as well. Finally, the feature article is concluded with the comparison of reaction sites or key factors determining the catalytic oxidation over Co3O4 and Co3O4/MOx. The authors’ outlook of the perspective with respect to future research on Co3O4 nanocrystals and Co3O4/MOx binary oxides is given.
Article
Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, SBET=100m2g−1) with different loading of ceria was proceeded using the impregnation method (assigned as CeX/R230, X=4, 12, 20, 35 and 50wt%). The CeX/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at −196°C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area (SBET=109m2g−1) of cobaltic oxide, further weaken the bond strength of CoO and lower the activation of CO oxidation among CeX/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T50 (temperature for 50% CO conversion) at 88°C.
Article
MnOx–CeO2 catalysts were prepared by a urea combustion method and their performance in the oxidation of ethanol, ethyl acetate and toluene was evaluated. XRD, XPS, H2-TPR and N2 physisorption were employed in catalyst characterization. Mn2+ and Mn3+ ions are present in the catalysts. In ceria-rich materials, crystalline manganese oxide phases are absent and Mn ions are homogeneously distributed between the bulk and the surface suggesting incorporation of Mn ions in ceria structure. In Mn-rich materials, segregation of a Mn3O4 phase takes place. The mixed oxides get reduced by H2 at lower temperatures than the corresponding single oxides and Mn ions promote reduction of ceria. The surface area of MnOx–CeO2 catalysts is larger than the one of single oxides prepared with the same method. The larger surface area of MnOx–CeO2 catalysts counterbalances their smaller specific activity allowing complete conversion of the examined VOCs at lower temperatures compared to the single oxides.
Article
Co3O4/CeO2 composite oxides with different cobalt loading (5, 15, 30, 50, 70wt.% as Co3O4) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co3O4 and CeO2 were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out.An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co3O4 in correspondence of Co3O4–CeO2 containing 30% by weight of Co3O4. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+–Co2+ redox couple. The presence in the sample Co3O4(30wt.%)–CeO2 of a high relative amount of Ce3+/(Ce4++Ce3+) as detected by XPS confirms the enhanced oxygen mobility.The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co3O4 phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750°C for 7h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co3O4 and Co3O4(30wt.%)–CeO2, performing the tests in the presence of 5vol.% H2O. A methane oxidation test upon hydrothermal ageing (flowing at 600°C for 16h a mixture 5vol.% H2O+5vol.%O2 in He) of the Co3O4(30wt.%)–CeO2 sample was also performed. All the results confirm the superiority of this composite oxide.
Article
The formation of nitrous oxide during the reaction of a mixture of CO, NO, C3H6, C3H8, H2 and O2 over supported rhodium, platinum and palladium catalysts has been investigated under near-stoichiometric conditions. Rhodium gives the highest amount of N2O with a peak selectivity near 70% at 250°C followed by a steady decline to low levels by 400°C. With Pt N2O is seen in a narrow region just above 300°C and is removed at higher temperatures during oxidation of propane. Ammonia, probably produced via isocyanic acid, is the chief nitrogen-containing product of the reaction over Pt/Al2O3 below 300°C, but this route is much reduced when CeO2/Al2O3 is the support. Palladium-containing catalysts give rise to N2O in two regimes. It arises below 200°C by the reaction of NO with H2 and above 300°C by another process which unlike Rh and Pt, persists through to 500°C. The behaviour of a trimetal catalyst containing Pd, Pt and Rh can be interpreted as a composite of that of the individual metals. Pt/Rh-containing catalytic converters recovered from vehicles show pronounced differences in N2O production which correlate with a reduced ability to remove propane. The most probable cause of such changes is sintering of the platinum component as a result of exposure to high temperature. This may be the origin of the increased N2O production seen in driving cycle tests with high mileage vehicles.
Article
Catalytic activity of the Co–Mn–Al mixed oxide catalyst (Co:Mn:Al molar ratio of 4:1:1) modified with various amounts of potassium (0–3wt%) was examined in total oxidation of toluene and ethanol. The prepared catalysts were characterized by chemical analysis (AAS), powder X-ray diffraction (XRD), surface area measurements, temperature programmed techniques (TPR, TPD), voltammetry of microparticles, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The non-modified catalyst was composed of spinel-like Co–Mn–Al mixed oxide as the only XRD crystalline phase. The surface concentrations of metal components obtained by XPS were different from the bulk ones determined by chemical analysis and the segregation of the metal components depended on the actual potassium content. The low K additions changed mainly the surface acid–base properties of the catalyst. According to XRD and voltammetry measurements, Mn oxides segregated from the original spinel-like phase at high concentration of potassium (2.7 and 3.0wt%); XPS showed an enrichment of the catalysts surface with Mn. The K addition caused significant changes in the catalyst efficiency. The highest conversion in toluene oxidation was achieved with the catalyst containing about 1wt% K; no reaction by-products were observed beside H2O and CO2. In ethanol oxidation, the catalysts activity gradually increased with increasing potassium content up to about 3wt% K, but the presence of excess potassium in the Co–Mn–Al catalyst negatively affected formation of reaction by-products: acetaldehyde production steeply increased with potassium concentration higher than 1wt%.
Article
This review intends to describe and critically analyze the growing literature dealing with the use of supported gold catalysts in the catalytic deep oxidation of volatile organic compounds (VOC). Among the wide family of VOC, attention has been given to the oxidation of saturated (methane, ethane, propane, isobutane, n-hexane) and unsaturated (acetylene, ethylene, propene) aliphatic compounds, aromatic hydrocarbons (benzene, toluene, xylenes, naphthalene), alcohols (methanol, ethanol, n- and iso-propanol), aldehydes (formaldehyde), ketones (acetone), esters (ethylacetate). Moreover, the oxida- tion of chlorinated VOC (dichloromethane, o-dichlorobenzene, o-chlorobenzene), as well as of nitrogen- (trimethylamine) and sulphur-containing (dimethyldisulfide) compounds has been addressed. The reaction mechanism and the influence of different factors, such as the nature and the properties of the support, the Au particle size and shape, the electronic state of gold, the preparation method and the pretreatment conditions of catalysts, the nature and the concentration of the organic molecule, are discussed in detail.
Article
Catalytic combustion of volatile organic compounds (VOCs: benzene and toluene) was studied over manganese oxide catalysts (Mn3O4, Mn2O3 and MnO2) and over the promoted manganese oxide catalysts with alkaline metal and alkaline earth metal. Their properties and performance were characterized by using the Brunauer Emmett Teller (BET), temperature programmed reduction (TPR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The sequence of catalytic activity was as follows: Mn3O4 > Mn2O3 > MnO2, which was correlated with the oxygen mobility on the catalyst. Each addition of potassium (K), calcium (Ca) and magnesium (Mg) to Mn3O4 catalyst enhanced the catalytic activity of Mn3O4 catalyst. Accordingly, K, Ca and Mg seemed to act as promoters, and the promoting effect might be ascribed to the defect-oxide or a hydroxyl-like group. A mutual inhibitory effect was observed between benzene and toluene in the binary mixture. In addition, the order of catalytic activity with respect to VOC molecules for single compound is benzene > toluene, and the binary mixture showed the opposite order of toluene > benzene.
Article
The present investigation was undertaken in an endeavor to study the effect of the cobalt content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation by varying the Co2+/Al3+ atomic ratio, thermal treatment of the samples and the reaction temperature. The samples (Co2+/Al3+ = 0.5, 1.5, 3.0) have been synthesized by the co-precipitation method. The unsupported Co3O4 has been prepared according to the same procedure as the reference compound in order to reveal the role of Al3+ ions presence.
Article
The kinetics of the complete combustion of propane and toluene at very low concentrations in air (0.45 and 0.3 mol % in air, respectively) over iron-doped ZrO2 (cubic) catalyst (Fe/Zr = 0.25) at different temperatures (598−723 K) in kinetic control regime have been investigated. The combustion rate data could be fitted very well to both the power-law and redox (Mars−Van Krevelen) models. However, the redox model provided a better fit to the kinetic data for the propane combustion. It also showed a better fit to the toluene combustion data at the lower temperatures (623 K). The reaction order (with respect to the hydrocarbon), apparent activation energy, and frequency factor (from the power-law model) for the propane combustion were 0.96 (average), 21.16 kcal/mol, and 4.67 × 105 mol g-1 h-1 kPa-n, respectively, and those for the toluene combustion were 0.77 (average), 26.08 kcal/mol, and 1.48 × 107 mol g-1 h-1 kPa-n, respectively.
Article
Our recent study on the incorporation of Sn in the lattice of MgAl-layered double hydroxides (LDHs) indicated that about 30 atom % of Al3+ could be isomorphously substituted by Sn4+ to form a new MgAlSn ternary LDH. In the present study, similar NiAlSn- and CoAlSn-LDHs were synthesized by a coprecipitation method. The influence of Sn on the thermal transformation and redox properties of NiAl- and CoAl-LDHs and their thermally derived products were investigated by X-ray powder diffraction (XRD), thermogravimetry/differential thermal analyses (TG/DTA), and temperature-programmed reduction (TPR) methods. The thermal transformation and reducibility of NiAlSn-LDH were different from that of the CoAlSn-LDH. Sn crystallized out as a SnO2 phase along with NiO and NiAl2O4 phases from NiAlSn-LDH calcined above 900 °C. On the other hand, a mixture of nonstoichiometric Co-spinel and Co2SnO4 inverse spinel phases was noticed from CoAlSn-LDH. The TPR profiles of NiAl-LDH and its calcined products exhibited peaks for the reduction of Ni2+ species existing in different chemical environments while an additional peak for the reduction of Sn4+ → Sno was observed in the Sn-containing counterparts. The Sn incorporation greatly enhanced the reducibility of Ni-containing phases. The CoAl- and CoAlSn-LDH and their calcined products exhibited complex TPR profiles. At least three different reduction regions were identified. They were assigned to the reduction of Co2+−Co3+ (Co3O4-like) species (region I, between 250 and 450 °C), Co3O4-like species containing Al3+ or CoAl2O4-like species containing Co3+ (region II, 500−550 °C) and Co2+−Al3+ (CoAl2O4-like) species (region III, above 550 °C). In contrast to that observed in the Ni-containing analogues the reducibility of Co species in these samples was found to decrease upon Sn incorporation.
Article
Catalytic oxidation of toluene in low concentration (800 ppm) in air was carried out over catalysts prepared from a new copper vanadate phase calcined at different temperatures. The catalysts were composed of different crystalline phases, namely ziesite, fingerite, blossite and other copper vanadium oxides. The best catalytic performance was obtained with the material calcined at 320 degrees C (composed of ziesite phase and an amorphous copper vanadium oxide) that showed a light off temperature of 265 degrees C. H-2-TPR measurements indicated that the most active catalysts (calcined at 320 and 400 degrees C) are also the most easily reducible. (c) 2008 Elsevier B.V. All rights reserved.
Article
X6Al2HT hydrotalcites, where X represents Fe, Cu, Zn, Ni, Co, Mn or Mg, were synthesized as precursors of catalysts for the toluene and CO total oxidation reactions. Specific area, Fourier transformed infrared spectroscopy analysis, XRD measurements and Thermal analysis of these dried solids were performed. After calcination at 500 °C, different mixed oxides were obtained. The structural analysis (XRD, FTIR) and specific areas of these solids were done. Concerning the total oxidation of toluene, the best activity is obtained with Mn6Al2HT catalyst with T50 at 249 °C. X6Al2 nano-oxides synthesized using hydrotalcite-type solids as precursors, are then very promising candidates for an utilisation as CO and VOC oxidation catalysts. The nature of bivalent cation in these compounds is essential for the efficacy of the catalyst.
Article
A study is presented of the kinetics and oxidation selectivity of methyl-ethyl-ketone (MEK) in air over bimetallic PdOx(0–1 wt% Pd)–MnOx(18 wt% Mn)/Al2O3 and monometallic PdOx(1 wt% Pd)/Al2O3 and MnOx(18 wt% Mn)/Al2O3 catalysts. Reaction rate data were obtained at temperatures in the 443–523 K range and for MEK partial pressures in the reactor feed of between 6.5 and 126.6 Pa. Products of both MEK combustion and partial oxidation reactions were found. Monometallic Pd/Al2O3 was the most selective catalyst for complete oxidation whereas the partial oxidation of MEK in the presence of manganese oxides was significant. The maximum yield for the partial oxidation products (acetaldehyde, methyl-vinyl-ketone, and diacetyl) was always below 10%. Kinetic studies showed that the rates of CO2 formation over PdOx/Al2O3 were well-fitted by the surface redox Mars–van Krevelen (MvK) kinetic expression and also by a Langmuir–Hinshelwood (LH) model derived after considering the surface reaction between adsorbed MEK and oxygen as the rate-determining step. In the case of the Mn-containing catalysts the MvK model provides the best fit. Irrespective of the model, the kinetic parameters for the bimetallic Pd–Mn catalysts were between the values obtained for the monometallic samples, suggesting an additive rather than a cooperative effect between palladium and manganese species for MEK combustion.
Article
The selective oxidation of butan-2-one to diacetyl has been studied in the temperature range 200–380°C over a vanadium phosphorus oxide catalyst. In addition to diacetyl the principal reaction products detected were acetic acid, acetaldehyde, methyl vinyl ketone, propionaldehyde and carbon dioxide. A kinetic analysis indicated that diacetyl formation proceeds through a Mars and van Krevelen type mechanism and that there were two sources of acetaldehyde formation; the first predominating at high oxygen partial pressures involved an enol intermediate and adsorbed molecular oxygen; the second, which predominated at low oxygen partial pressures involved lattice oxygen and a diol intermediate. It is also proposed that acetoin, CH3COCHOHCH3, is a common intermediate in the formation of diacetyl and methyl vinyl ketone. High selectivities to diacetyl can be achieved by operation in anaerobic conditions.
Article
Gold catalysts of CO oxidation, supported on a solid base of MgxAlO (x = Mg/Al molar ratio) hydrotalcite, were prepared by a modified deposition precipitation method. The effect of various parameters that are involved in the preparation of catalysts were studied, including the pH and the concentrations of HAuCl4 in the initial solution, the Mg/Al molar ratio and the calcination temperature of the MgxAlO support as well as the calcination temperature of the Au/MgxAlO catalyst. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) and X-ray photoelectron spectroscopy (XPS) analyses. The pH and the HAuCl4 concentrations in the initial solution, and the Mg/Al molar ratio of MgxAlO affected the pH in the final solution, and determined the real gold loading in the resulting catalyst. The calcination temperatures of the MgxAlO support and the Au/MgxAlO catalyst dominated the ratio of gold states (Au3+/Au0) on the catalyst. The optimal catalyst, 2%Au/Mg2AlO(100), was obtained using the following preparation parameters: (1) 1 × 10−3 M HAuCl4, (2) pH 2 (without adjusting pH) in the initial solution, (3) Mg/Al = 2 (Mg2AlO) calcined at 100 °C as a support and (4) 2%Au/Mg2AlO catalyst calcined at 100 °C. This investigation confirms that not only gold loading of the catalyst is important, the ratio of gold states (Au3+/Au0) is also critical in determining the activity of the catalyst for CO oxidation.Graphical abstractGold was dispersed and stabilized on a solid base of MgxAlO hydrotalcite by a modified deposition precipitation method to obtain a good catalyst for CO oxidation. The calcination temperatures of the MgxAlO support and the Au/MgxAlO catalyst dominated the ratio of gold states (Au3+/Au0) that influenced the activity of the catalyst.
Article
In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–pvi ≫ pvi ≫ edf, nta–pvi ≫ edf ≈ pvi and edf > nta–pvi > pvi.The catalysts prepared were characterized using various techniques (BET, UV–vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on Co3O4-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned Co3O4 crystallites: the ratio ‘amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step’ closely related with the ratio ‘amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step’ as well as the composition of the precipitated phase.The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)62+·2NO3−. Upon calcination it is transformed into loosely bounded Co3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts.The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800 °C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content.The application of the nta–pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(II)–nta]− and [Co(II)–2nta]4− complexes, in which the Co(H2O)62+ complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta–pvi catalysts a precipitated phase containing the [Co(II)–nta]−·NH4+(or H+) and [Co(II)–2nta]4−·4NH4+ (or 4H+) complex salts predominates. Upon calcination these are transformed into Co3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity.
Article
Co–Mg–Al hydrotalcite type solids were synthesized as precursors of catalysts for the total oxidation of toluene. After calcination at 500°C, different mesoporous mixed oxides were obtained with high specific surfaces. The comparison of the catalytic activities of the calcined hydrotalcites with those of calcined hydroxides evidenced the superiority of the first oxides explained meanly by higher specific surfaces and more easily reducible particles. DRIFT “operando” allowed to follow the oxidation reaction and the formation of light coke and carbonate species.
Article
The Ni–Mg–Al and Mg–Mn–Al hydrotalcite-like compounds with various Ni and Mn contents (molar ratios Ni:Mg:Al in hydroxide layers of 4:0:2, 3:1:2, and 2:2:2 and Mg:Mn:Al of 4:2:0 and 4:1:1, respectively) were prepared by coprecipitation. The products were characterised by chemical analysis and X-ray diffraction (XRD) measurements. After calcination at different temperatures, the properties of the resulting oxides (surface area, porous structure and surface concentration of metals by XPS) were examined. From the catalysts calcined at 800 °C the Ni4Al2 sample showed the highest surface area and the highest catalytic activity. The oxidation activity in methane combustion (related to the surface area of mesopores) depended on the amount of surface nickel or manganese component linearly. The lower activity of manganese over nickel in methane combustion was proved.
Article
Mesoporous activated carbons were obtained by chemical activation of kraft lignin with H3PO4 and used as supports for the preparation of carbon-based Pd catalysts with low palladium content (0.5%). The catalytic properties of the carbon-based Pd samples were evaluated in the catalytic oxidation of toluene. The effect that a thermal treatment at 900 °C in inert atmosphere carried out before and after the Pd-deposition produced on the structure and activity of the catalysts was analyzed. The catalysts obtained show high external surface areas and mesopore volumes. The chemical activation with H3PO4 yielded carbon supports with a significant amount of surface phosphorus, in form of COPO3, CPO3 and C3PO groups. These phosphorus groups act like physical barrier, increasing the oxidation resistance of the catalysts and avoiding the burn-off of the carbon substrate during the oxidation of the VOCs. TEM analysis confirmed the presence of well-dispersed Pd particles, with sizes between 5 and 12 nm. A kinetic study of the catalytic oxidation of toluene was performed. The reaction seems to proceed through a Langmuir–Hinshelwood mechanism, whose rate-limiting step is the surface reaction between adsorbed toluene and oxygen adsorbed dissociatively, with an activation energy value of 83 kJ mol−1. Toluene and xylene were oxidized to CO2 and H2O in the temperature range of 150–400 °C at a space velocity of 19,000 h−1.
Article
LaMn1−xMgxO3 perovskite catalysts (x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO3 and LaMn0.8Mg0.2O3). The major results obtained are: (i) Mg substitution in the basic LaMnO3 perovskite has a positive effect on the catalytic activity only at low x values (x≤0.2); (ii) as opposed to the results of previous studies on the LaCr1−xMgxO3 system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO3, the catalytic combustion over LaMn0.8Mg0.2O3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.
Article
The synthesis, characterisation and catalytic behaviour of palladium compound derived from Mg/Al hydrotalcite were investigated in the toluene total oxidation. The hydrotalcite sample was prepared by co-precipitation of the corresponding chlorides (MgCl2, AlCl3 and H2PdCl4) with boiled fresh water and under N2 atmosphere. Under these conditions, the layered double hydroxide [Mg0.75Al0.25(OH)2]0.25+ (CO3)x2−·nH2O was formed. Palladium species were inserted as [PdCl2(OH)2]2− complex coming from the partial substitution of Cl with OH. The space interlayer was imposed by CO32− present also as balancing anion. This new hydrotalcite is less stable than classical hydrotalcite Mg/Al without Pd since its destruction took place at a lower temperature. The palladium hydrotalcite calcined at 290 °C exhibited the best catalytic behaviour (activity and selectivity) in the total oxidation of toluene compared with a conventional palladium catalyst.
Article
The oxidations of benzene, toluene and 1-hexene, alone and in mixtures with isooctane and carbon monoxide, have been investigated over alumina-supported platinum, palladium and rhodium under three different conditions. In the absence of CO, oxidation of all hydrocarbons is complete at the lowest temperature with platinum. However, inclusion of CO greatly inhibits hydrocarbon oxidation on platinum. There is also moderate inhibition of palladium, but with rhodium oxidation of the aromatics is somewhat enhanced by the presence of carbon monoxide. The activity order then becomes Rh>Pd>Pt. Conversely, unsaturates inhibit CO oxidation to a lesser extent, this effect being greatest for hexene and toluene with palladium and least with benzene on rhodium. In mixtures of the four hydrocarbons, with or without CO present, the order of removal is always hexene followed by toluene, benzene and isooctane with rhodium exhibiting the largest separation between toluene and benzene. This order appears to be largely governed by competitive adsorption between hydrocarbons of different adsorption strength since the pattern of reactivity is very different when the unsaturates are oxidised alone. Under very lean conditions platinum is the most active for the oxidation of benzene alone, palladium for toluene and rhodium for hexene alone. However, relative activities are strongly dependent on conditions since the reactions exhibit very different kinetic orders in oxygen with each metal. The oxidations of hexene and toluene are positive order in oxygen over Pt and Pd but negative order over Rh. On the other hand the oxidation of benzene seems to be positive order in oxygen for Pt, negative order with Pd and near zero order with Rh. Some of these trends can also be rationalised in terms of relative strengths of adsorption.
Article
All technically interesting reactions carried out with vanadium oxide catalysts are marked by their highly exothermic character, which forms an impediment to the investigation of the kinetics of these processes. In the present study use was made of a fluid bed, in which the temperature is uniform. The oxidation of the following substances: benzene, toluene, naphthalene, and anthracene has been studied. The partial pressures of the reacting substances were varied to the greatest possible extent. Both reaction components appeared to influence the reaction rate. A formula depicting this influence is derived. This formula may be interpreted by assuming two successive reactions, namely the reaction between the aromatic and the oxygen on the surface, and the re-oxidation of the partly reduced surface by means of oxygen. The formula may be reduced to an equation by which also the data on the oxidation of sulphur dioxide by means of vanadium oxide catalysts found in the literature are well described. Using kinetic data it is possible to determine the optimum temperature distribution in a fixed bed reactor used for the oxidation of sulphur dioxide and to make calculations of the ratio between the amounts of catalyst to be used in the various stages of a multiple-stage reactor. The results of these calculations have been compared with practical experience.
Article
A systematic investigation of the oxidation of 1,2-dichlorobenzene (o-DCB) was conducted over a series of vanadia/titania catalysts with different V2O5 loadings (i.e. 0.8, 3.6 and 5.8 wt%). The TiO2 support was also found to be active for this reaction, but its activity was significantly lower than that of the surface vanadia species. Reaction rates per vanadium atom (i.e. turnover frequencies) were calculated for these catalysts and found to be independent of the vanadia coverage. Kinetic measurements showed that under excess oxygen conditions, the reaction is approximately first order in o-DCB and zero order in oxygen. Carbon monoxide and carbon dioxide were the only products detected. The selectivity towards CO was approximately 40–45% and was found to be independent of vanadia loading and temperature. No deactivation of the catalyst was observed even after 100 h of operation at 723 K. Addition of WO3, MoO3 and ZnO to the V2O5/TiO2 system was found to have no significant effect on its activity for o-DCB oxidation.
Article
Palladium is the active component in several catalytic formulations for environmental technologies, due to its superior performances in the conversion of some hydrocarbons (for example, methane) and halocarbons, and the thermal stability and low volatility of Pd species. The properties and reactivity of Pd-based catalysts in the conversion of methane catalytic combustion for gas turbine applications, reduction of greenhouse gas (methane, N2O) emissions, hydrodehalogenation and oxidative destruction of halocarbons and their applications in the elimination of other pollutants from gaseous emissions are reviewed, with emphasis on the structure-activity relationships, reaction mechanism and sensitivity to poisoning.
Article
Mn2O3–Fe2O3 powders have been prepared by a coprecipitation method. The pure compounds have been characterized as constituted of α-Fe2O3 (haematite) and α-Mn2O3 (bixbyite) while the mixed oxides are constituted of a mixture of haematite- and bixbyite-structure solid solutions. The observed reciprocal solubilities calculated from the XRD patterns approach the thermodynamic ones. α-Mn2O3 is more active than α-Fe2O3 as catalyst for both propane and propene oxidation. However, α-Mn2O3 is less active than Mn3O4 powder. Propene oxidation is in all cases very selective to CO2 while propane oxidation gives rise to significant amounts of propene on α-Fe2O3. Mn2O3–Fe2O3 powders are slightly more active than α-Mn2O3 as combustion catalysts. The selectivities to propene upon propane oxidation decrease with increasing Mn content.
Article
In this paper is presented a kinetic study of the catalytic combustion of trichloroethylene (TCE) over Y-zeolites exchanged with several cations. The catalysts, based on zeolite, were prepared by ion exchange and characterized by means of physico-chemical techniques and then tested under kinetic conditions. The kinetic results obtained were interpreted using kinetic models of power-law type and Eley-Rideal. The results obtained indicate that catalyst Y-Cr is more active than Y-Co catalyst. The greater activity of catalyst exchanged with Cr can be attributed to the higher acidity that presented these catalysts.
Article
Reaction behaviors and kinetics of catalytic oxidation of benzene, toluene, and ethyl acetate with feed concentrations in the range of 700–5,000 ppm over Pd/ZSM-5 catalyst were investigated. Results for single components show that ethyl acetate (T 50 = 190–200°C) is more easily oxidized than benzene (T 50 = 215–225°C) and toluene (T 50 = 225–235°C). The conversion of ethyl acetate was increased with the increase of its feeding concentration, while the opposite behaviors were observed for benzene and toluene as their conversion rates were decreased with the increase of the inlet concentration. Different behaviors were observed in catalytic oxidation of volatile organic compound (VOC) multi-components, the presence of benzene or toluene inhibits the conversion of ethyl acetate, and the aromatic hydrocarbons inhibit each other in all cases. Ethyl acetate possesses obvious inhibitory effect on benzene oxidation, while it is interesting to note that ethyl acetate has a promotion effect on toluene conversion. The kinetic data were fitted by the Power-law and Mars–van Krevelen kinetic models. The fitting result shows that the Power-law model is more suitable for predicting the conversion of benzene than the other VOCs, and the Mars–van Krevelen model can accurately express the reaction rate of all investigated VOCs.
Article
Two trimetallic ZnCuAl and MnCuAl hydrotalcites have been successfully synthesized by a co-precipitation method. The manganese based material was identified as a new hydrotalcite phase. Both lamellar precursors were calcined at 450 and 600 degrees C and the resulting catalysts were tested on reaction of total oxidation of toluene. The solids were characterized by X-ray diffraction, thermal analysis, atomic absorption spectroscopy, Fourier transformed infrared spectroscopy, N(2) adsorption and H(2) temperature-programmed reduction. It was found that ZnCuAl materials are composed of copper and zinc oxides supported on alumina; while MnCuAl ones comprise basically spinel phases, which were not completely identified. The catalytic behavior of the calcined samples showed that Mn hydrotalcite calcined at 450 degrees C exhibited the best catalytic performance that corresponds to 100% toluene conversion into CO(2) at about 300 degrees C.
Article
Basic catalysis! The basic properties of hydrotalcites (see picture) make them attractive for numerous catalytic applications. Probing the basicity of the catalysts is crucial to understand the base-catalysed processes and to optimise the catalyst preparation. Various parameters can be employed to tune the basic properties of hydrotalcite-based catalysts towards the basicity demanded by each target chemical reaction. Hydrotalcites offer unique basic properties that make them very attractive for catalytic applications. It is of primary interest to make use of accurate tools for probing the basicity of hydrotalcite-based catalysts for the purpose of 1) fundamental understanding of base-catalysed processes with hydrotalcites and 2) optimisation of the catalytic performance achieved in reactions of industrial interest. Techniques based on probe molecules, titration techniques and test reactions along with physicochemical characterisation are overviewed in the first part of this review. The aim is to provide the tools for understanding how series of parameters involved in the preparation of hydrotalcite-based catalytic materials can be employed to control and adapt the basic properties of the catalyst towards the basicity demanded by each target chemical reaction. An overview of recent and significant achievements in that perspective is presented in the second part of the paper.
Article
Despite the success of adsorption and thermal incineration of (C)VOC emissions, there is still a need for research on techniques which are both economically more favorable and actually destroy the pollutants rather than merely remove them for recycling elsewhere in the biosphere. The catalytic destruction of (C)VOC to CO2, H2O and HCl/Cl2 appears very promising in this context and is the subject of the present paper. The experiments mainly investigate the catalytic combustion of eight target compounds, all of which are commonly encountered in (C)VOC emissions and/or act as precursors for the formation of PCDD/F. Available literature on the different catalysts active in the oxidation of (C)VOC is reviewed and the transition metal oxide complex V2O5-WO3/TiO2 appears most suitable for the current application. Different reactor geometries (e.g. fixed pellet beds, honeycombs, etc.) are also described. In this research a novel catalyst type is introduced, consisting of a V2O5-WO3/TiO2 coated metal fiber fleece. The conversion of (C)VOC by thermo-catalytic reactions is governed by both reaction kinetics and reaction equilibrium. Full conversion of all investigated VOC to CO2, Cl2, HCl and H2O is thermodynamically feasible within the range of experimental conditions used in this work (260-340 degrees C, feed concentrations 30-60 ppm). A first-order rate equation is proposed for the (C)VOC oxidation reactions. The apparent rate constant is a combination of reaction kinetics and mass transfer effects. The oxidation efficiencies were measured with various (C)VOC in the temperature range of 260-340 degrees C. Literature data for oxidation reactions in fixed beds and honeycomb reactors are included in the assessment. Mass transfer resistances are calculated and are generally negligible for fleece reactors and fixed pellet beds, but can be of importance for honeycomb monoliths. The experimental investigations demonstrate: (i) that the conversion of the hydrocarbons is independent of the oxygen concentration, corresponding to a zero-order dependency of the reaction rate; (ii) that the conversion of the hydrocarbons is a first-order reaction in the (C)VOC; (iii) that the oxidation of the (C)VOC proceeds to a higher extent with increasing temperature, with multiple chlorine substitution enhancing the reactivity; (iv) that the reaction rate constant follows an Arrhenius dependency. The reaction rate constant kr (s(-1)) and the activation energy E (kJ/mol) are determined from the experimental results. The activation energy is related to the characteristics of the (C)VOC under scrutiny and correlated in terms of the molecular weight. The kr-values are system-dependent and hence limited in design application to the specific VOC-catalyst combination being studied. To achieve system-independency, kr-values are transformed into an alternative kinetic constant K (m3/(m2u)) expressed per unit of catalyst surface and thus independent of the amount of catalyst present in the reactor. Largely different experimental data can be fitted in terms of this approach. Results are thereafter used to define the Arrhenius pre-exponential factor A*, itself expressed in terms of the activation entropy. Destruction efficiencies for any given reactor set-up can be predicted from E- and A*-correlations. The excellent comparison of predicted and measured destruction efficiencies for a group of chlorinated aromatics stresses the validity of the design approach. Since laboratory-scale experiments using PCDD/F are impossible, pilot and full-scale tests of PCDD/F oxidation undertaken in Flemish MSWIs and obtained from literature are reported. From the data it is clear that: (i) destruction efficiencies are normally excellent; (ii) the efficiencies increase with increasing operating temperature; (iii) the higher degree of chlorination does not markedly affect the destruction efficiency. Finally, all experimental findings are used in design recommendations for the catalytic oxidation of (C)VOC and PCDD/F. Predicted values of the a)VOC and PCDD/F. Predicted values of the acceptable space velocity correspond with the cited industrial values, thus stressing the validity of the design strategy and equations developed in the present paper.
Article
The performance of bulk chromium oxide is compared with that of a Mn commercial catalyst for the deep oxidation of trichloroethene (1000-2500ppmv, 55h(-1) space velocity) in air, in dry and wet (20000ppm of H(2)O) conditions, in terms of activity, selectivity and stability. Chromium oxide was found to be more active (on a catalyst weight basis), however its activity decreases continuously with time on stream. The presence of water increases its stability, the Mn catalyst showing the opposite behaviour. The effect of water on the Cr catalyst can be explained according to the Deacon equilibrium, as the presence of water tends to decrease the Cl(2) concentration, assumed to be responsible of the catalyst deactivation. Regarding to the selectivity, the Mn catalyst yields C(2)Cl(4), CCl(4) and CHCl(3) as organochlorinated by-products, whereas chromium oxide produces only trace amounts of CCl(4). Simple power-law kinetics expressions (first-order for Mn and zero-order for Cr) provide fairly good fits for the evolution of the conversion with the temperature. Furthermore, the kinetic behaviour of chromium oxide can be represented with a Langmuir-Hinshelwood model taking into account the chlorine inhibitory effect.
  • E Genty
  • L Jacobs
  • T Visart De
  • C Bocarmé
  • Barroo
E. Genty, L. Jacobs, T. Visart De Bocarmé, C. Barroo, Catalysts 134 (2017) 1-44.