Defective Mn<i>_x</i>Zr_1–<i>x</i>O₂ Solid Solution for the Catalytic Oxidation of Toluene: Insights into the Oxygen Vacancy Contribution

Abstract

Oxygen vacancy is conducive to molecular oxygen adsorption and activation, and it is necessary to estimate its contribution on catalysts, especially the doped system for volatile organic compound (VOC) oxidation. Herein, a series of doped Mn _xZr_{1- x}O₂ catalysts with oxygen vacancy were prepared by partially substituting Zr⁴⁺ in a zirconia with low-valent manganese (Mn²⁺). Compared with the corresponding mechanically mixed samples (MB-x) without oxygen vacancy, Mn _xZr_{1- x}O₂ catalysts exhibited better toluene conversion and specific reaction rate, where the differential values were calculated to estimate the contribution of oxygen vacancy on catalytic performance. The increase in oxygen vacancy concentrations in Mn _xZr_{1- x}O₂ catalysts can boost the differential values, implying the enhancement of oxygen vacancy contribution. Density functional theory (DFT) calculations further confirmed the contribution of oxygen vacancy, and molecular oxygen is strongly absorbed and activated on a defective Mn-doped c-ZrO₂ (111) surface with oxygen vacancy rather than a perfect m-ZrO₂ (-111) surface or a perfect Mn-doped c-ZrO₂ (111) surface, thus resulting in the significant improvement in catalytic activity for toluene oxidation. In situ DRIFTS spectra revealed that the oxygen vacancy can alter the toluene degradation pathway and accelerate the intermediates to convert into CO₂ and H₂O, thus leading to a low activation energy and high specific reaction rate.