Elucidating structure-performance correlations in gas-phase selective ethanol oxidation and CO oxidation over metal-doped γ-MnO2

Abstract

Despite of considerable efforts on the MnO2-based catalytic combustion, the different structural and component requirements of MnO2 for gas-phase selective oxidation and complete oxidation largely remain unknown. By comparing four types of MnO2 with different crystal structures (α, β, γ and δ), γ-MnO2 was found to be the most efficient catalyst for both aerobic selective oxidation of ethanol and CO oxidation. The structural effect of γ-MnO2 was further investigated by doping metal ions into the framework and by comparing the catalytic performance in the gas-phase aerobic oxidation of CO and ethanol. Among ten M-γ-MnO2 catalysts, Zn-γ-MnO2 showed the lowest temperature (160 °C) for achieving 90% CO conversion. The CO oxidation activity of the M-γ-MnO2 catalysts was found to be more relevant to the surface acidity-basicity than the reducibility. In contrast, surface reducibility has been demonstrated to be more crucial in the gas-phase ethanol oxidation. Cu-γ-MnO2 with higher reducibility and more oxygen vacancies of Mn2+/Mn3+ species exhibited higher catalytic activity in the selective ethanol oxidation. Cu-γ-MnO2 achieved the highest acetaldehyde yield (75%) and space-time-yield (5.4 g gcat–1 h–1) at 200 °C, which are even comparable to the results obtained by the state-of-the-art silver and gold-containing catalysts. Characterization results and kinetic studies further suggest that the CO oxidation follows the lattice oxygen-based Mars-van Krevelen mechanism, whereas both surface lattice oxygen and adsorbed oxygen species involve in the ethanol activation.