Search Results (27)

The proposed study is devoted to highlighting the importance of mixed oxides preparation through the layered double hydroxide route for undesirable gas pollutants abatement. Different series of Cu/Al/Ce mixed oxides with similar or different stoichiometrics were prepared and compared for toluene and/or CO oxidation. Catalyst synthesis methods influence material properties and activity for oxidation reactions. The high activity for the oxidation reactions of mixed oxides derived from LDH is explained by the Cu/Ce synergy. The presence of CO in the CO/toluene mixture does not affect the total toluene oxidation, and the toluene does not affect the total oxidation of CO conversion at low temperatures. The most effective catalytic material (Cu₆Al_1.2Ce_0.8) presents a long lifetime stability for total toluene oxidation and resistance to CO poisoning in mixtures. Full article

Aromatic volatile organic compounds (VOCs) are toxic to public health and contribute to global air pollution; thus, it is urgent to control VOC emissions. Catalytic oxidation technology has been widely investigated to eliminate aromatic VOCs; this technology exhibits high catalytic efficiency even at low temperatures. However, the reaction mechanism of aromatic VOCs’ total oxidation over metal-oxide-based catalysts, which is of great significance in the design of catalysts, is not yet clear. In this study, we systemically calculated the catalytic oxidation mechanism of toluene over the Ce_0.875Zr_0.125O₂ catalyst using density functional theory (DFT). The results show that toluene first loses hydrogen from the methyl group via oxy-dehydrogenation and is gradually oxidized by lattice or adsorbed oxygen to benzyl alcohol, benzaldehyde, and benzoic acid following the Mars-van Krevelen (MVK) mechanism. Afterwards, there is a decarboxylation step to produce phenyl, which is further oxidized to benzoquinone. The rate-determining step then proceeds via the ring-opening reaction, leading to the formation of small molecule intermediates, which are finally oxidized to CO₂ and H₂O. This work may provide atomic-scale insight into the role of lattice and adsorbed oxygen in catalytic oxidation reactions. Full article

Designing highly active catalysts for the oxidation of volatile organic compounds (VOCs) involves fine-tuning the catalytic surface and improving its interaction with VOCs. The present review assesses various studies conducted in the last decade on Co₃O₄ catalysts for the complete oxidation of toluene (C₇H₈) and provides information on the synthesis and physicochemical characterization of these catalysts. Taking this one step further, data from the literature were carefully chosen for an extensive meta-analysis aiming at elucidating the structure sensitivity of toluene oxidation over Co₃O₄ catalysts. Specifically, the specific reaction rate was calculated and correlated with the physicochemical properties of the catalysts, namely, the specific surface area, faceting, and Co³⁺/Co²⁺ ratio. Based on this analysis, the specific surface area does not exert a significant impact on the specific activity of Co₃O₄ catalysts for the total oxidation of toluene. Instead, the specific reaction rates are influenced by the morphology, surface concentration, exposed crystallographic planes, and oxidation state of cobalt cations. These factors determine the catalyst’s specific activity by creating defects, oxygen vacancies, or oxygen species with different reactivities. It was also found that a high surface Co³⁺/Co²⁺ ratio is necessary for achieving high catalytic performance in the oxidation of VOCs. Full article

Increased levels and detrimental effects of volatile organic compounds (VOCs) stimulate research efforts to develop catalysts with high efficiency in complete hydrocarbon oxidation. This work is focused on the complete oxidation of benzene as a probe reaction for VOCs elimination over alumina-supported CuO-CeO₂ mixed oxide promoted by gold. The benzene molecule is the most stable among the aromatic hydrocarbons with toxic and often carcinogenic effects known as BTEX (benzene, toluene, ethylbenzene, and xylenes) owing to the symmetry and stability of the benzene ring. Use of low-cost materials as support is an appropriate strategy aimed at improving catalyst economic profitability. The effect of the Cu-Ce ratio, namely 2:1 and 1:5, and the role of supported gold in the catalyst performance were evaluated. Analysis of the impact of support composition in benzene oxidation was based on sample characterization by textural measurements, PXRD, EPR spectroscopy, and the TPR technique. Special attention was paid to the disturbed symmetry of the ceria crystallographic structure by defects formation and its implication for the catalytic activity. Gold on alumina-supported binary oxide catalysts exhibited a significantly higher activity than promoted supported monometallic oxides. The best performance of the Au/Cu-Ce 1:5 sample was related to the highest concentration of paramagnetic Cu²⁺ ions and the best copper species dispersion evidenced by PXRD, EPR, and TPR results. The catalyst achieved stable total oxidation to CO₂ and water by 94% benzene conversion at 250 °C, thus implying the potential of this composition in developing efficient catalytic materials for atmospheric pollutant abatement. Full article

Structured catalytic membranes with high porosity and a low pressure drop are particularly suitable for industrial processes carried out at high space velocities. One of these processes is the catalytic total oxidation of volatile organic compounds, which is an economically feasible and environmentally friendly way of emission abatement. Noble metal catalysts are typically preferred due to high activity and stability. In this paper, the preparation of a thermally stable polybenzimidazole electrospun membrane, which can be used as a support for a platinum catalyst applicable in the total oxidation of volatile organic compounds, is reported for the first time. In contrast to commercial pelletized catalysts, high porosity of the membrane allowed for easy accessibility of the platinum active sites to the reactants and the catalytic bed exhibited a low pressure drop. We have shown that the preparation conditions can be tuned in order to obtain catalysts with a desired platinum particle size. In the gas-phase oxidation of ethanol, acetone, and toluene, the catalysts with Pt particle sizes 2.1 nm and 26 nm exhibited a lower catalytic activity than that with a Pt particle size of 12 nm. Catalysts with a Pt particle size of 2.1 nm and 12 nm were prepared by equilibrium adsorption, and the higher catalytic activity of the latter catalyst was ascribed to more reactive adsorbed oxygen species on larger Pt nanoparticles. On the other hand, the catalyst with a Pt particle size of 26 nm was prepared by a solvent evaporation method and contained less active polycrystalline platinum. Last but not least, the catalyst containing only 0.08 wt.% of platinum achieved high conversion (90%) of all the model volatile organic compounds at moderate temperatures (lower than 335 °C), which is important for reducing the costs of the abatement technology. Full article

Hollow cathode plasma sputtering is an advantageous method of preparing catalysts in the form of thin oxide films on supports. Such catalysts are particularly suitable for processes such as catalytic total oxidation of volatile organic compounds (VOCs), representing an economically feasible and environmentally friendly method of VOC abatement. Catalysts with Ni:Co molar ratios of 1:4, 1:1, and 4:1 were prepared on stainless-steel meshes and compared with single-component Ni and Co oxide catalysts. The properties of the catalysts were characterized by EDX, SEM, powder XRD, temperature-programmed reduction (H₂-TPR), Raman spectroscopy, and XPS. Powder XRD revealed the formation of various crystalline phases that were dependent on molar the Ni:Co ratio. NiO and Co₃O₄ were identified in the single-component Ni and Co oxide catalysts, whereas Ni-Co mixed oxides with a spinel structure, together with NiO, were found in the catalysts containing both Ni and Co. Raman spectra of the catalysts prepared at high working pressures showed a slightly lower intensity of bands, indicating the presence of smaller oxide particles. The TPR profiles confirmed the improved reducibility of the Ni-Co oxide catalysts compared to the single-component Ni and Co catalysts. Catalytic activity was investigated in the deep oxidation of ethanol and toluene, which were used as model volatile organic compounds. In ethanol oxidation, the activity of sputtered catalysts was up to 16 times higher than that of the commercial Cu-Mn oxide catalyst EnviCat^® VOC-1544. The main benefits of the sputtered catalysts are the much lower content of Ni and Co oxides and a negligible effect of internal diffusion. Moreover, the process of plasma jet sputtering can be easily implemented on a large scale. Full article

Co₃O₄, MgCo₂O₄ and MgO materials have been synthesized using a simple co-precipitation approach and systematically characterized. The total conversion of toluene to CO₂ and H₂O over spinel MgCo₂O₄ with wormlike morphology has been investigated. Compared with single metal oxides (Co₃O₄ and MgO), MgCo₂O₄ with the highest activity has exhibited almost 100% oxidation of toluene at 255 °C. The obtained results are analogous to typical precious metal supported catalysts. The activation energy of toluene over MgCo₂O₄ (38.5 kJ/mol) is found to be much lower than that of Co₃O₄ (68.9 kJ/mol) and MgO ((87.8 kJ/mol)). Compared with the single Co and Mg metal oxide, the as-prepared spinel MgCo₂O₄ exhibits a larger surface area, highest absorbed oxygen and more oxygen vacancies, thus highest mobility of oxygen species due to its good redox capability. Furthermore, the samples specific surface area, low-temperature reducibility and surface adsorbed oxygenated species ratio decreased as follows: MgCo₂O₄ > Co₃O₄ > MgO; which is completely in line with the catalytic performance trends and constitute the reasons for MgCo₂O₄ high excellent activity towards toluene total oxidation. The overall finding supported by ab initio molecular dynamics simulations of toluene oxidation on the Co₃O₄ and MgCo₂O₄ suggest that the catalytic process follows a Mars–van Krevelen mechanism. Full article

Solar photothermo-catalysis is a fascinating multi-catalytic approach for volatile organic compounds (VOCs) removal. In this work, we have explored the performance and the chemico-physical features of non-critical, noble, metal-free MnO_x-ZrO₂ mixed oxides. The structural, morphological, and optical characterizations of these materials pointed to as a low amount of ZrO₂ favoured a good interaction and the ionic exchange between the Mn and the Zr ions. This favoured the redox properties of MnO_x increasing the mobility of its oxygens that can participate in the VOCs oxidation through a Mars-van Krevelen mechanism. The further application of solar irradiation sped up the oxidation reactions promoting the VOCs total oxidation to CO₂. The MnO_x-5 wt.%ZrO₂ sample showed, in the photothermo-catalytic tests, a toluene T₉₀ (temperature of 90% of conversion) of 180 °C and an ethanol T₉₀ conversion to CO₂ of 156 °C, 36 °C, and 205 °C lower compared to the thermocatalytic tests, respectively. Finally, the same sample exhibited 84% toluene conversion and the best selectivity to CO₂ in the ethanol removal after 5 h of solar irradiation at room temperature, a photoactivity similar to the most employed TiO₂-based materials. The as-synthetized mixed oxide is promising for an improved sustainability in both catalyst design and environmental applications. Full article

Red mud waste from the aluminium industry was modified by leaching using hydrochloric acid or oxalic acid with additives, followed by precipitation or evaporation. The prepared catalysts were characterized in detail and tested for toluene total oxidation. The samples prepared by precipitation of the leachate by adding a base gave a much better performance in catalytic oxidation than the ones prepared by just evaporating the leachate. These improved performances can be correlated to the enhanced textural and redox properties of the catalysts due to the better dispersion and higher enrichment of Fe oxides at their surface. The best performing catalyst had a light-off temperature of around 310 °C and complete oxidation took place at around 380 °C. Full article

The paper discusses a formation of Mt–PAA composite containing a natural montmorillonite structure partially exfoliated by poly(acrylic acid) introduced through intercalation polymerization of acrylic acid. Mt–PAA was subsequently modified by controlled adsorption of Co²⁺ ions. The presence of aluminosilicate packets (clay) and carboxyl groups (hydrogel) led to the deposition of significant amounts of Co²⁺ ions, which after calcination formed the Co₃O₄ spinel particles. The conditions of the Co²⁺ ions’ deposition (pH, volume and concentration of Co(NO₃)₂ solution, as well as a type of pH-controlling agent) were widely varied. Physicochemical characterization of the prepared materials (including X-ray fluorescence (XRF), X-ray powder diffraction (XRD), low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H₂-TPR)) revealed that the modification conditions strongly influenced the content as well as the distribution of the Co₃O₄ active phase, tuning its reducibility. The latter parameter was, in turn, very important from the point of view of catalytic activity in the combustion of aromatic volatile organic compounds (VOCs) following the Mars–van Krevelen mechanism. Full article

The structure–property relationship of catalytic supports for the deposition of redox-active transition metals is of great importance for improving the catalytic efficiency and reusability of the catalysts. In this work, the role of alumina support precursors of Cu-Fe/Al₂O₃ catalysts used for the total oxidation of toluene as a model volatile organic air pollutant is elucidated. Surface characterization of the catalysts revealed that the surface area, pore volume and acid site concentration of the alumina supports are important but not the determining factors for the catalytic activity of the studied catalysts for this type of reaction. The determining factors are the structural order of the support precursor, the homogeneous distribution of the catalytic sites and reducibility, which were elucidated by XRD, NMR, TEM and temperature programed reduction (TPR). Cu–Fe/Al₂O₃ prepared from bayerite and pseudoboehmite as highly ordered precursors showed better catalytic performance compared to Cu-Fe/Al₂O₃ derived from the amorphous alumina precursor and dawsonite. Homogeneous distribution of Fe_xO_y and CuO_x with defined Cu/Fe molar ratio on the Al₂O₃ support is required for the efficient catalytic performance of the material. The study showed a beneficial effect of low iron concentration introduced into the alumina precursor during the alumina support synthesis procedure, which resulted in a homogeneous metal oxide distribution on the support. Full article

A series of Co₃O₄ catalysts were synthesized by an ammonia precipitation method at various precipitating pH values (8.0, 8.5, 9.0, 9.5, and 10.0) and with different numbers of washings. Their performance in the total oxidation of two selected hydrocarbons, toluene and propane, was evaluated at a reactant/oxygen molar ratio of 1/210 and a Weight Hourly Space Velocity (WHSV) of 40,000 mL g⁻¹ h⁻¹. The physicochemical properties of the catalysts were characterized by thermogravimetric and differential thermal analysis (TG/DTA), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and N₂ absorption–desorption. The results show that the catalysts are in the cubic spinel phase (Fd-3m (227), a = 8.0840 Å) with average crystalline sizes of 29−40 nm and specific surface areas of 12–20 m² g⁻¹. All catalysts allowed 100% conversion of both toluene and propane at temperatures below 350 °C. The precipitating pH and the number of washings were observed to significantly affect the catalytic performance. The optimal synthesis condition was established to be pH 8.5 with two washings. The best catalyst gave 100% conversion of toluene and propane at 306 °C and 268 °C, respectively. Full article

CuAlCe oxides were obtained from hydrotalcite-type precursors by coprecipitation using a M²⁺/M³⁺ ratio of 3. The collapse of the layered double hydroxide structure following the thermal treatment leads to the formation of mixed oxides (CuO and CeO₂). The catalytic performance of the copper-based catalysts was evaluated in the total oxidation of two Volatile Organic Compounds (VOCs): ethanol and toluene. XRD, SEM Energy-Dispersive X-ray Spectrometry (EDX), H₂-temperature programmed reduction (TPR) and XPS were used to characterize the physicochemical properties of the catalysts. A beneficial effect of combining cerium with CuAl-O oxides in terms of redox properties and the abatement of the mentioned VOCs was demonstrated. The sample with the highest content of Ce showed the best catalytic properties, which were mainly related to the improvement of the reducibility of the copper species and their good dispersion on the surface. The presence of a synergetic effect between the copper and cerium elements was also highlighted. Full article

Metal-doped (Mn, Cu, Ni, and Fe) cobalt oxides were prepared by a coprecipitation method and were used as catalysts for the total oxidation of toluene and propane. The metal-doped catalysts displayed the same cubic spinel Co₃O₄ structure as the pure cobalt oxide did; the variation of cell parameter demonstrated the incorporation of dopants into the cobalt oxide lattice. FTIR spectra revealed the segregation of manganese oxide and iron oxide. The addition of dopant greatly influenced the crystallite size, strain, specific surface area, reducibility, and subsequently the catalytic performance of cobalt oxides. The catalytic activity of new materials was closely related to the nature of the dopant and the type of hydrocarbons. The doping of Mn, Ni, and Cu favored the combustion of toluene, with the Mn-doped one being the most active (14.6 × 10⁻⁸ mol g_Co⁻¹ s⁻¹ at 210 °C; T₅₀ = 224 °C), while the presence of Fe in Co₃O₄ inhibited its toluene activity. Regarding the combustion of propane, the introduction of Cu, Ni, and Fe had a negative effect on propane oxidation, while the presence of Mn in Co₃O₄ maintained its propane activity (6.1 × 10⁻⁸ mol g_Co⁻¹ s⁻¹ at 160 °C; T₅₀ = 201 °C). The excellent performance of Mn-doped Co₃O₄ could be attributed to the small grain size, high degree of strain, high surface area, and strong interaction between Mn and Co. Moreover, the presence of 4.4 vol.% H₂O badly suppressed the activity of metal-doped catalysts for propane oxidation, among them, Fe-doped Co₃O₄ showed the best durability for wet propane combustion. Full article

The thermocatalytic, photocatalytic and photothermo-catalytic oxidation of some volatile organic compounds (VOCs), 2-propanol, ethanol and toluene, was investigated over brookite TiO₂-CeO₂ composites. The multi-catalytic approach based on the synergistic effect between solar photocatalysis and thermocatalysis led to the considerable decrease in the conversion temperatures of the organic compounds. In particular, in the photothermo-catalytic runs, for the most active samples (TiO₂-3 wt% CeO₂ and TiO₂-5 wt% CeO₂), the temperature at which 90% of VOC conversion occurred was about 60 °C, 40 °C and 20 °C lower than in the thermocatalytic tests for 2-propanol, ethanol and toluene, respectively. Furthermore, the addition of cerium oxide to brookite TiO₂ favored the total oxidation to CO₂ already in the photocatalytic tests at room temperature. The presence of small amounts of cerium oxide allowed to obtain efficient brookite-based composites facilitating the space charge separation and increasing the lifetime of the photogenerated holes and electrons as confirmed by the characterization measurements. The possibility to concurrently utilize the photocatalytic properties of brookite and the redox properties of CeO₂, both activated in the photothermal tests, is an attractive approach easily applicable to purify air from VOCs. Full article