Comparative study on Co3+, Co2+, and oxygen vacancies in the photocatalytic ozonation of VOCs on Co3O4 crystal facets

Dichloromethane (DCM) is one of the most emitted volatile organic compounds (VOCs) and has been classified as a potential carcinogen. Co3O4 is a notable material for the photocatalytic ozonation of VOCs. However, the variation in cobalt ion sites across different crystal facets and their collaborative effects with O3 reaction sites remain unclear. This work synthesized Co3O4 materials with predominantly exposed (110), (111), and (112) crystal facets. Results indicated that Co3+ ions were active for C-Cl bond cleavage, while oxygen vacancies facilitated O3 conversion. The (111) facet, which possessed the highest density of Co3+ sites, exhibited superior DCM activation performance. Conversely, the (112) facet, which was characterized by the most substantial surface oxygen vacancy content, effectively transformed O3 into reactive center dot O and center dot O2- species. Moreover, the electron density was higher on the (112) and (110) facets compared with the (111) facet, enhancing the migration of photogenerated holes toward the surface and promoting the activation of oxygen vacancies.