Identifying the Geometric Site Dependence of Spinel Oxides for the Electrooxidation of 5‐Hydroxymethylfurfural

Co-based spinel oxides are considered as promising catalysts for the electrochemical oxidation of 5-hydroxymethylfurfural (HMF) to high-value chemicals. Co 3 O 4  is of mixed valences with the presence of both Co 2+  and Co 3+  at different atom locations. Identifying the activity of each atomic site is critical to propose the design principle of advanced electrocatalysts, and tuning the geometrical sites at the atom level in Co 3 O 4  could lead to significant modifications on its electronic and thus modulate the catalytic activities for the electrooxidation of HMF. A typical spinel lattice contains a tetrahedral Co 2+  site and two octahedral Co 3+  sites. In this work, the impact of different sites in Co 3 O 4 was uncovered by selectively building block in the tetrahedral site and the octahedral site by Zn 2+  and Al 3+  respectively. It was found that the Co 3+  in octahedral site tends to be oxidized to the high valence of cobalt oxyhydroxide as the active centers and thus has higher activity than tetrahedral Co 2+  sites for HMF oxidation. The NH 3  TPD results indicate that the tetrahedral Co 2+  has abundant Lewis acidic adsorption sites in Co 3 O 4  and CoAl 2 O 4,  and the tetrahedral sites can provide chemical adsorption sites for HMF oxidation. Based on the finding of the geometric site dependence, the HMF oxidation activity was further improved by the replacement with Cu 2+  in the tetrahedral site of spinel oxides (CuCo 2 O 4 ). The CuCo 2 O 4  shows 4-fold enhancement for the HMF oxidation relative to Co 3 O 4  and shows a record performance among Co-based catalysts.