Abstract: As one of the important transition metal oxides, Co₃O₄ possesses an excellent property, which leads to the fact that Co₃O₄ is widely used in optical, electrical, magnetic, thermal and other fields. The crystals of Co₃O₄ can be used as heterogeneous catalytic materials and thereby have an irreplaceable position in many reactions. With the continuous study of the catalytic mechanism, it is found that the catalytic reaction process is not only affected by the particle size of the crystal catalyst, but also is sensitive to the crystal facet. Therefore, the study on crystal facet effect of transition metal oxides is of great significance for the in-depth understanding of heterogeneous catalytic reactions and the effective design of high-active catalysts. Co₃O₄ crystal has spinel structure with a mixed valence state of octahedral coordinated Co³⁺ and tetrahedral coordinated Co²⁺, and it shows the obvious crystal facet effect in different catalytic reactions. In this paper, the crystal facet effects of Co₃O₄ as heterogeneous crystal catalyst in thermal catalysis, photocatalysis, electrocatalysis and peroxidase-like catalysis in recent years are systematically reviewed. In combination with theoretical calculations, the reasons of these crystal facet effects are explained from the atomic structure of different crystal facets of Co₃O₄. Finally, the general law of crystal facet effect of Co₃O₄ in the above reactions is summarized, the shortcomings in current study on Co₃O₄ crystal facet effect are discussed, and the future development trend is reasonably prospected.

Key words: Co₃O₄, transition metal oxide, crystal catalyst, crystal facet effect, atomic arrangement, heterogeneous catalysis, theoretical calculation