MnO2 polymorphs (α-, β-, and ε-MnO2) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO2 shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T90 = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO2 (0.135 moleculeCO·nm-2·s-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO2 and α-MnO2, respectively. The specific reaction rate order (β-MnO2 > ε-MnO2 > α-MnO2) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO2 were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.
Keywords: kinetics; low-temperature CO oxidation; mechanism; oxygen vacancy; β-MnO2.