Facet and defect-mediated Co₃O₄ morphology effects in the selective oxidation of benzyl alcohol to benzaldehyde

Catalyst morphology selection can effectively tune the performance of transition metal oxide heterogeneous catalysts by optimizing surface structure and defect chemistry. In this study, five distinct Co₃O₄ heterogeneous catalyst morphologies - nanochains, nanocubes, nanosheets, nanoplates and porous nanoplates - were synthesized using two chemical routes: hydrothermal synthesis with controlled reaction temperature and co-precipitation using bases of different strengths. Structural characterization confirmed the catalyst materials were phase-pure cubic spinel Co₃O₄, but that they possessed morphology-dependent differences in crystallinity, defect concentration, crystallite size, and porosity. The catalytic performance of the materials was evaluated for the selective oxidation of benzyl alcohol, where a pronounced morphology-dependent activity was observed. The five examined nanomaterial morphologies exposed different crystal facets, with the anisotropic Co₃O₄ nanochains preferentially exposing {111} planes being the most reactive. Co₃O₄ nanochains exhibited the highest catalytic activity and excellent selectivity, producing benzaldehyde as the sole product under the studied conditions. The enhanced performance is attributed to the combined effects of exposed {111} facets and the high density of accessible oxygen vacancies in the Co₃O₄ nanochains, which facilitate the redox cycle of cobalt active sites. Based on these observations, a plausible redox mechanism for the catalytic benzyl alcohol oxidation over Co₃O₄ is proposed. This work demonstrates how controlled morphology and surface structure can significantly influence and improve rational design of efficient metal oxide catalysts for selective oxidation reactions.