Elucidating the Reaction Mechanism and Deactivation of CO2-Assisted Propane Oxidative Dehydrogenation over VOx/TiO2 Catalysts: A Multiple Operando Spectroscopic Study

Abstract

The CO₂-assisted oxidative dehydrogenation (ODH) of propane is of great technical importance and enables the use (and thus removal from the atmosphere) of CO₂, a greenhouse gas, in a value-adding process. Supported vanadium oxide (VO_x) catalysts are a promising alternative to more active but toxic chromium oxide catalysts. Despite its common use, TiO₂ has not been investigated as a support material for VO_x in the CO₂–ODH of propane. In this study, we elucidate the interaction between titania (P25) and vanadia in the reaction mechanism by analyzing the reaction network and investigating the catalyst using X-ray diffraction (XRD), multiwavelength Raman, UV–vis and diffuse reflectance IR Fourier transform (DRIFT) spectroscopy. Besides direct and indirect ODH reaction pathways, propane dry reforming (PDR) is identified as a side reaction, which is more prominent on bare titania. The presence of VO_x enhances the stability and the selectivity toward propylene by participating in the redox cycle, activating CO₂ and leading to a higher rate of regeneration. Additionally, VO_x catalyzes the conversion of anatase to rutile, which facilitates CO₂ activation, thereby leading to an encapsulation of vanadium. At higher loadings, reducible VO_x oligomers are present on the surface, facilitating some PDR, but less than on bare P25. As the main deactivation mechanisms of the catalyst system, we propose the reduction of the titania lattice and the consumption of vanadium, while carbon formation appears to be less relevant. Our results highlight the importance of analyzing the CO₂–ODH reaction network and applying a multispectroscopic approach to obtain a detailed mechanistic understanding of CO₂-assisted propane ODH over supported VO_x catalysts.