Geometrical-Site-Dependent Catalytic Activity of Ordered Mesoporous Co-Based Spinel for Benzene Oxidation: In Situ DRIFTS Study Coupled with Raman and XAFS Spectroscopy

Co3O4 spinel has been widely investigated as a promising catalyst for the oxidation of volatile organic compounds (VOCs). However, the roles of tetrahedrally coordinated Co2+ sites (Co2+Td) and octahedrally coordinated Co3+ sites (Co3+Oh) still remain elusive, because their oxidation states are strongly influenced by the local geometric and electronic structures of the cobalt ion. In this work, we separately studied the geometrical-site-dependent catalytic activity of Co2+ and Co3+ in VOC oxidation on the basis of a metal ion substitution strategy, by substituting Co2+ and Co3+ with inactive or low-active Zn2+(d0), Al3+(d0), and Fe3+(d5), respectively. Raman spectroscopy, X-ray absorption fine structure (XAFS), and in situ DRIFTS spectra were thoroughly applied to elucidate the active sites of a Co-based spinel catalyst. The results demonstrate that octahedrally coordinated Co2+ sites (Co2+Oh) are more easily oxidized to Co3+ species in comparison to Co2+Td, and Co3+ are responsible for the oxidative breakage of the benzene rings to generate the carboxylate intermediate species. CoO with Co2+Oh and ZnCo2O4 with Co3+Oh species have demonstrated good catalytic activity and high TOFCo values at low temperature. Benzene conversions for CoO and ZnCo2O4 are greater than 50% at 196 and 212 °C, respectively. However, CoAl2O4 with Co2+Td sites shows poor catalytic activity and a low TOFCo value. In addition, ZnCo2O4 exhibits good durability at 500 °C and strong H2O resistance ability.