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 Co3O4 catalysts by using different types of cobalt salts. The Co3O4-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 Co3O4-s (286 and 299 °C). The specific reaction rates of Co3O4-a for benzene and toluene catalytic oxidation were 19.7 × 10-9 mol/m2/s and 15.5 × 10-9 mol/m2/s, which were 3.1 times and 19.4 times than that of Co3O4-s (6.3 × 10-9 mol/m2/s and 0.8 × 10-9 mol/m2/s), respectively. The excellent activity of Co3O4-a catalyst was due to stronger interactions between the tetra-coordinated cobalt atoms (Co4c) on the (220) crystal plane with reactants by in situ DRIFTS and DFT results. The Co4c sites act as bifunctional active centers, simultaneously inducing CC bond elongation in aromatic hydrocarbons and OO bond activation in O2, which the pre-activation of adsorbed molecules can lower the energy barriers for CH 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.