Individual Functionality and Synergistic Effects of Redox Site–Acid Site in Propane Oxidation
Cai-Hao Wen, Lin‐Ya Xu, You‐Zeng Hao, Qian Zhou, Yi-Wei Xian, Mao-Di Wang, Wei Tan, Lin Dong, Jian Chen, Meng‐Fei Luo, Qihua Yang
Abstract
Catalytic combustion represents one of the most efficient technologies for light-alkane volatile organic compound abatement. However, the reaction mechanisms over heterogeneous catalysts remain controversial, which significantly hinders the rational design of highly efficient and stable catalysts. In this work, it is found that the T 50 (the temperature at which propane conversion reached 50%) of Pt/MoO 3 (220 °C) is much lower than that of Pt/CeO 2 ( T 50 = 320 °C) in propane combustion (C 3 H 8 + O 2 → CO 2 + H 2 O), indicating the possibility of synergistic catalysis occurring between Pt and MoO 3 sites. Based on a hybrid catalysis system composed of physically mixed MoO 3 and Pt/CeO 2 (denoted as Pt/CeO 2 +MoO 3 ), the critical roles of the acid site from MoO 3 for propane activation and the Pt site for oxygen-activation have been originally investigated in propane oxidation, even though Pt and MoO 3 sites are spatially separated. The results of the kinetic study, in situ diffuse reflectance infrared Fourier transform spectra of propane combustion, and X-ray photoelectron spectra experiments sufficiently evidenced that propane preferentially absorbs on the MoO 3 surface, and the oxygen spillover from Pt sites to the MoO 3 surface further facilitates the oxidation of propane. Density functional theory calculations reveal that Pt sites exhibit stronger O 2 adsorption, while the synergy between five-coordinated and four-coordinated Mo sites enables MoO 3 with an enhanced propane activation capability. That is, MoO 3 offers important extra sites for propane activation, effectively avoiding the competitive adsorption between oxygen and propane on the Pt sites. A large distance between MoO 3 and Pt sites negatively impacts catalytic activity. Additionally, via construction of defective MoO 3 containing more surface acid sites, it is disclosed that stronger surface acidity of MoO 3 significantly promotes the catalytic performance of Pt sites and MoO 3 sites. This work sheds light on the reaction mechanism of light-alkane oxidation over the metal oxide-supported Pt catalyst.