Cobalt oxide-catalyzed CO oxidation under steady-state conditions: Influence of the metal oxidation state

Co3O4 has emerged as a promising alternative to noble metal-based catalysts for CO oxidation. However, the Co oxidation state (Co2+Co23+O4) responsible for the high activity is still being debated. This study provides a feasible approach to identify the active Co oxidation state by comparing the activity of three sets of cobalt oxides with different oxidation states: (i) pure cobalt oxide catalysts (CoO, Co2O3, and Co3O4), (ii) cobalt oxide-rich catalysts obtained by controlled non-isothermal H2-reduction of Co3O4 to various temperatures (400–700 °C), and (iii) cobalt metal-rich catalysts obtained by controlled isothermal H2-reduction of Co3O4 at 700 °C of Co3O4 for various time durations (31–125 min). The catalysts' composition, electronic transitions and transfers, redoxabiliy, CO oxidation activity, and stability under the reaction atmosphere were determined by ex- and in-situ Fourier-transform infrared spectroscopy, Ultraviolet-Visible diffuse reflectance spectroscopy, and cyclic temperature-programmed reduction (TPR) and oxidation (TPO) measurements. The study revealed that isolated Co2+ (in CoO) and Co3+ (in Co2O3) exhibited only marginal CO oxidation activity. The enhanced activity was observed only when they were coupled by the oxygen sub-lattice of normal spinel-structured or clustered Co3O4, where electron-mobile Co2+-O2--Co3+ linkages were formed. The cobalt metal-rich catalysts were relatively two times more active than the pure oxides and oxide-rich catalysts. Moreover, Co3O4 and Co2O3, as well as the cobalt metal-rich catalysts, were composition-invariant under the reaction atmosphere. In contrast, the stoichiometric and non-stoichiomeric CoO (i.e., the oxide-rich) catalysts suffered surface restructuring into Co3O4-type surface under the reaction atmosphere at 150 °C, rendering them more active in the CO oxidation reaction.