Facet-dependent catalytic mechanism of Co3O4 for VOCs oxidation: Role of tetra-coordinated co on (220) Planes

Achieving efficient catalytic oxidation of VOCs by transition metal oxides at low temperatures is a major challenge. Controlling exposed crystal facets through crystal facet engineering is an efficient strategy for enhancing the catalytic activity of catalysts. Herein, we proposed an anion-inducted method for precisely modulating the exposed crystal facets on Co 3 O 4 catalysts by using different types of cobalt salts. The Co 3 O 4 -a catalyst with highly exposed (220) facets exhibits exceptional catalytic performance, achieving 90% conversion of benzene and toluene at 239 and 240 °C, respectively, which are significantly lower than the temperatures required for Co 3 O 4 -s (286 and 299 °C). The specific reaction rates of Co 3 O 4 -a for benzene and toluene catalytic oxidation were 19.7 × 10 −9  mol/m 2 /s and 15.5 × 10 −9  mol/m 2 /s, which were 3.1 times and 19.4 times than that of Co 3 O 4 -s (6.3 × 10 −9  mol/m 2 /s and 0.8 × 10 −9  mol/m 2 /s), respectively. The excellent activity of Co 3 O 4 -a catalyst was due to stronger interactions between the tetra-coordinated cobalt atoms (Co 4c ) on the (220) crystal plane with reactants by in situ DRIFTS and DFT results. The Co 4c sites act as bifunctional active centers, simultaneously inducing C C bond elongation in aromatic hydrocarbons and O O bond activation in O 2 , which the pre-activation of adsorbed molecules can lower the energy barriers for C H cleavage and ring-opening reactions, and ultimately promotes the deep oxidation of VOCs. This research deepens the understanding of surface structure-oriented catalysis in VOCs oxidation, while aiding the rational design of advanced catalysts.