Effect of MnO 2 Polymorphs' Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation.

MnO 2 polymorphs (α-, β-, and ε-MnO 2 ) 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-MnO 2 shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity ( T 90 = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO 2 (0.135 molecule CO ·nm -2 ·s -1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO 2 and α-MnO 2 , respectively. The specific reaction rate order (β-MnO 2 > ε-MnO 2 > α-MnO 2 ) 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 MnO 2 were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.