Co 3 O 4 spinel has been widely investigated as a promising catalyst for the oxidation of volatile organic compounds (VOCs). However, the roles of tetrahedrally coordinated Co 2+ sites (Co 2+ T d ) and octahedrally coordinated Co 3+ sites (Co 3+ O h ) 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 Co 2+ and Co 3+ in VOC oxidation on the basis of a metal ion substitution strategy, by substituting Co 2+ and Co 3+ with inactive or low-active Zn 2+ (d 0 ), Al 3+ (d 0 ), and Fe 3+ (d 5 ), 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 Co 2+ sites (Co 2+ O h ) are more easily oxidized to Co 3+ species in comparison to Co 2+ T d , and Co 3+ are responsible for the oxidative breakage of the benzene rings to generate the carboxylate intermediate species. CoO with Co 2+ O h and ZnCo 2 O 4 with Co 3+ O h species have demonstrated good catalytic activity and high TOF Co values at low temperature. Benzene conversions for CoO and ZnCo 2 O 4 are greater than 50% at 196 and 212 °C, respectively. However, CoAl 2 O 4 with Co 2+ T d sites shows poor catalytic activity and a low TOF Co value. In addition, ZnCo 2 O 4 exhibits good durability at 500 °C and strong H 2 O resistance ability.

The introduction of oxygen vacancies (Ov) has been regarded as an effective method to enhance the catalytic performance of photoanodes in oxygen evolution reaction (OER). However,t heir stability under highly oxidizing environment is questionable but was rarely studied. Herein, NiFe-metal-organic framework (NiFe-MOFs) was conformally coated on oxygen-vacancy-richB iVO 4 (Ov-BiVO 4 )a st he protective layer and cocatalyst, forming ac ore-shell structure with caffeic acid as bridging agent. The as-synthesized Ov-BiVO 4 @NiFe-MOFs exhibits enhanced stability and aremarkable photocurrent density of 5.3 AE 0.15 mA cm À2 at 1.23 V( vs. RHE). The reduced coordination number of Ni(Fe)-O and elevated valence state of Ni(Fe) in NiFe-MOFs layer greatly bolster OER, and the shifting of oxygen evolution sites from Ov-BiVO 4 to NiFe-MOFs promotes Ov stabilization. Ovs can be effectively preserved by the coating of at hin NiFe-MOFs layer,l eading to ap hotoanode of enhanced photocurrent and stability.

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Ru/CeO2 catalysts with different amounts of surface oxygen vacancies were prepared by changing the morphology of CeO2. The conversion of Ce4+ to Ce3+ and the formation of Ru–O–Ce bonds led to enhancement of the amount of oxygen vacancies. Ru species of low crystallinity enriched with Ru4+ ions exist on the surface of CeO2 nanorods, while metallic Ru particles exist on CeO2 nanocubes. The low crystallinity of Ru species and high concentration of oxygen vacancies enhanced the adsorption of hydrogen and nitrogen and also led to desorption of surface hydrogen in the form of H2. Therefore, Ru/CeO2 nanorods showed high ammonia synthesis activities. On the contrary, lower catalytic activity was observed over Ru/CeO2 nanocubes catalyst because H2 and N2 adsorption was less favorable plausibly due to the large particle size of Ru species and low concentration of oxygen vacancies, and most of the hydrogen species were consumed in H2O formation.