Abstract
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.
Highlights
The volatile organic compounds (VOCs) are air pollutants from both outdoor and indoor sources
Sample pretreatment procedure in air at moderate temperature of 200 ◦C was chosen having in mind the finding of Yu et al [41] that at these conditions the p-type semiconductors like Co3O4 form abundance of loosely bonded molecular oxygen species adsorbed at surface oxygen vacancies
The lowest catalytic activity was observed over single Co3O4 as compared to Co3O4-CeO2 mixed oxides prepared via mechanochemical mixing of cerium hydroxide and cobalt hydroxycarbonate precursors (Figure 1a)
Summary
The volatile organic compounds (VOCs) are air pollutants from both outdoor and indoor sources. Catalytic deep oxidation is economically favorable demanding much lower temperatures than thermal incineration but it is an environmentally friendly technology preventing contamination with harmful side products of incomplete oxidation This determines the continuing efforts for the development of efficient combustion catalysts with high oxidation activity and long-time stability and high selectivity to harmless CO2. Co3O4 is characterized by high reducibility, abundance of oxygen vacancies providing active surface oxygen species and unique textural properties. It has advantages of low-cost, chemical and thermodynamical stability.
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