Unraveling the Mechanism of the CO2-Assisted Oxidative Dehydrogenation of Propane over VOx/CeO2: An Operando Spectroscopic Study

Abstract

The CO₂-assisted oxidative dehydrogenation (ODH) of propane is of great interest for the usage of CO₂ in chemical industry. Vanadia-based catalysts are a promising material class, which can replace highly toxic CrO_x, the current state-of-the-art catalyst. Ceria is a commonly used support material in CO₂ activation but has not yet been used as a vanadia support for CO₂-assisted propane ODH. In this study, we address the interplay between vanadia and ceria as well as the nuclearity-dependent reaction behavior of VO_x/CeO₂ catalysts using XRD, multi-wavelength Raman, UV-Vis, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). While the vanadia loading increases the selectivity, the catalysts exhibit a significant amount of side reactions, including most prominently the total oxidation over bare ceria on surface oxygen sites due to their high reducibility and propane dry reforming (PDR) over catalysts with high vanadia loading. Mechanistic analysis reveals that dimers can transfer hydrogen from propane to the ceria lattice, forming Ce-H or to a monodentate carbonate, facilitating the reverse water-gas shift reaction (RWGSR), whereas a transfer to bridged Ce-OH surface species leads to total oxidation due to the high reactivity of the formed surface species. Oligomers facilitate PDR due to their high reducibility and the active oxygen site shifts from ceria to vanadia. The catalyst can be regenerated via carbonates, which are highly stable and can subsequently deactivate the catalyst surface. Our results highlight the benefit of applying multiple operando spectroscopies to enhance the mechanistic understanding of materials relevant for CO₂ activation and further the knowledge-based optimization of catalytic performance.