Effect of strong interaction between Co and Ce oxides in CoxCe1-xO2-δ oxides on its catalytic oxidation of toluene

Effect of Morphology-Dependent Oxygen Vacancies of CeO2 on the Catalytic Oxidation of Toluene

Catalytic oxidation is regarded as an effective, economical, and practical approach to remove volatile organic compounds such as important air pollutants. CeO2 catalysts with different morphologies exhibit different oxygen vacancies content, which plays a vital role in oxidation reaction. Herein, three distinct morphologies of CeO2 i.e., shuttle (CeO2 (S)), nanorod (CeO2 (R)), and nanoparticle (CeO2 (P)), were successfully fabricated by the SEM and TEM results, and investigated for toluene catalytic oxidation. The various characterizations showed that the CeO2 (S) catalyst exhibited a larger surface area along with higher surface oxygen vacancies in contrast to CeO2 (R) and CeO2 (P), which is responsible for its excellent toluene catalytic oxidation. The 90% toluene conversion temperature at 225 °C over CeO2 (S) was less than that over CeO2 (R) (283 °C) and CeO2 (P) (360 °C). In addition, CeO2 (S) showed a greater reaction rate (14.37 × 10−2 μmol∙g−1∙s−1), TOFov (4.8 × 10−4∙s−1) at 190 °C and lower activation energy value (67.4 kJ/mol). Furthermore, the CeO2 (S) also displayed good recyclability, long-term activity stability, and good tolerance to water. As a result, CeO2 (S) is considered a good candidate to remove toluene.

Gold-Based Catalysts for Complete Formaldehyde Oxidation: Insights into the Role of Support Composition

Formaldehyde (HCHO) is recognized as one of the most emitted indoor air pollutants with high detrimental effect on human health. Significant research efforts are focused on HCHO removal to meet emission regulations in an effective and economically profitable way. For over three decades, the unique electronic properties and catalytic abilities of nano-gold catalysts continue to be an attractive research area for the catalytic community. Recently, we reported that mechanochemical mixing is a relevant approach to the preparation of Co-Ce mixed oxides with high activity in complete benzene oxidation. A trend of higher surface defectiveness, in particular, oxygen vacancies, caused by close interaction between cobalt oxide and cerium oxide phases, was observed for a mixed oxide composition of 70 wt.% Co3O4 and 30 wt.% CeO2. These results directed further improvement by promotion with gold and optimization of mixed oxide composition, aiming for the development of an efficient catalyst for room temperature HCHO abatement. Support modification with potassium was studied; however, the K addition caused less enhancement of HCHO oxidation activity than expected. This motivated the preparation of new carrier material. In addition to Co3O4-CeO2 mixed metal oxides with preset ratio, γ-Al2O3 intentionally containing 33% boehmite and shortly named Al2O3-b was used for synthesis. Analysis of the role of support composition in HCHO oxidation was based on the characterization of nano-gold catalysts by textural measurements, XRD, HRTEM, XPS, and TPR techniques. Gold supported on mechanochemically treated Co3O4-CeO2-Al2O3-b (50 wt.% Al2O3-b) exhibited superior activity owing to high Ce3+ and Co3+ surface amounts and the most abundant oxygen containing species with enhanced mobility. This catalyst achieved oxidation to CO2 and H2O by 95% HCHO conversion at room temperature and 100% at 40 °C, thus implying the potential of this composition in developing efficient catalytic materials for indoor air purification.

Mechanochemically Prepared Co3O4-CeO2 Catalysts for Complete Benzene Oxidation

Considerable efforts to reduce the harmful emissions of volatile organic compounds (VOCs) have been directed towards the development of highly active and economically viable catalytic materials for complete hydrocarbon oxidation. The present study is focused on the complete benzene oxidation as a probe reaction for VOCs abatement over Co3O4-CeO2 mixed oxides (20, 30, and 40 wt.% of ceria) synthesized by the more sustainable, in terms of less waste, less energy and less hazard, mechanochemical mixing of cerium hydroxide and cobalt hydroxycarbonate precursors. The catalysts were characterized by BET, powder XRD, H2-TPR, UV resonance Raman spectroscopy, and XPS techniques. The mixed oxides exhibited superior catalytic activity in comparison with Co3O4, thus, confirming the promotional role of ceria. The close interaction between Co3O4 and CeO2 phases, induced by mechanochemical treatment, led to strained Co3O4 and CeO2 surface structures. The most significant surface defectiveness was attained for 70 wt.% Co3O4-30 wt.% CeO2. A trend of the highest surface amount of Co3+, Ce3+ and adsorbed oxygen species was evidenced for the sample with this optimal composition. The catalyst exhibited the best performance and 100% benzene conversion was reached at 200 °C (relatively low temperature for noble metal-free oxide catalysts). The catalytic activity at 200 °C was stable without any products of incomplete benzene oxidation. The results showed promising catalytic properties for effective VOCs elimination over low-cost Co3O4-CeO2 mixed oxides synthesized by simple and eco-friendly mechanochemical mixing.