Facet-engineered spinel Co₃O₄ for homogeneous-like efficiency in selective toluene oxidation to benzaldehyde

Heterogenization of Co-based catalyst can effectively solve the catalyst recovery problem encountered by homogenous catalyst Co(OAc)₂, but there may generate an obvious performance decrease for the heterogenized cobalt site ascribed to the increased mass transfer resistance. Hence, how to alleviate or eliminate the performance decline for Co-based heterogeneous catalyst is an intriguing scientific question. In this study, we reported the application of crystal engineering strategy on Co₃O₄ to bridge the performance gap between homogeneous catalyst and heterogeneous catalyst in the selective oxidation of toluene into benzaldehyde. Four distinct spinel Co₃O₄ catalysts, predominantly exposing the {220}, {222}, {440}, and {311} crystal facets respectively, were prepared and utilized as heterogeneous catalysts. The Co₃O₄-P sample predominantly exposing the {220} facets exhibited the optimal catalytic activity for toluene oxidation, achieving a toluene conversion of 56.1 % and a benzaldehyde selectivity of 67.6 %, which was significantly superior to that of homogenous catalyst Co(OAc)₂. The enhanced performance of this material was primarily attributed to the abundant reversible Co²⁺/Co³⁺ redox couples on its surface, which maintained sufficient Co²⁺ active sites throughout the catalytic process and facilitated toluene oxidation reactions. In addition, Co₃O₄-P also exhibited the maximum specific surface area, the highest Co²⁺/Co³⁺ ratio, and the greatest oxygen vacancy density, which together gave rise to its best catalytic performance. Density functional theory calculations further corroborated these findings. According to quenching experiments and in-situ Fourier Transform Infrared Spectroscopy characterizations, the possible selective toluene oxidation mechanism over Co₃O₄-P catalyst was proposed. Moreover, the recycled Co₃O₄-P sample could be reused for more than five consecutive reactions, demonstrating its eminent re-usability.