No abstract

The oxidative dehydrogenation (ODH) of propane over supported vanadia catalysts is an attractive route toward propene (propylene) with the potential of industrial application and has been extensively studied over decades. Despite numerous mechanistic studies, the active vanadyl site of the reaction has not been elucidated. In this work, we unravel the ODH reaction mechanism, including the nuclearity-dependent vanadyl and surface dynamics, over ceriasupported vanadia (VO x /CeO 2 ) catalysts by applying (isotopic) modulation excitation IR spectroscopy supported by operando Raman and UV−vis spectroscopies. Based on our loading-dependent analysis, we were able to identify two different mechanisms leading to propylene, which are characterized by isopropyland acrylate-like intermediates. The modulation excitation IR approach also allows for the determination of the time evolution of the vanadia, hydroxyl, and adsorbate dynamics, underlining the intimate interplay between the surface vanadia species and the ceria support. Our results highlight the potential of transient IR spectroscopy to provide a detailed understanding of reaction mechanisms in oxidation catalysis and the dynamics of surface catalytic processes in general.

In this work we elucidate the main steps of the CO oxidation mechanism over Au/CeO2(111), clarifying the course of CO adsorption at a broad variety of surface sites as well as of transmutations of one CO species into another. By combining transient spectroscopy with DFT calculations we provide new evidence that the active centers for CO conversion are single gold atoms. To gain insight into the reaction mechanism, we employ Modulation Excitation (ME) DRIFT spectroscopy in combination with the mathematical tool of Phase Sensitive Detection to identify the active species and perform DFT calculations to facilitate the assignments of the observed bands. The transient nature of the ME-DRIFTS method allows us to sort the observed species temporally, providing further mechanistic insight. Our study highlights the potential of combined transient spectroscopy and theoretical calculations (DFT) to clarify the role of adsorbates observed and to elucidate the reaction mechanism of CO oxidation over supported gold and other noble-metal catalysts.

The elucidation of reaction mechanisms is an essential part of catalysis research, providing approaches to improve catalysts or, ultimately, to design catalysts based on a profound understanding of their mode of operation. In the context of C1 processes, redox and/or associative mechanisms have been proposed in the literature, but their critical assessment has been a major challenge. Here, we highlight the importance of applying a combination of techniques suited to address both the redox properties and intermediate/adsorbate dynamics in a targeted manner. We illustrate our approach by exploring the mechanism of LT-WGS over low-loaded Cu/CeO2 catalysts using different ceria morphologies (sheets, polyhedra, cubes, and rods) to study the influence of the surface termination. While the results from operando Raman and UV–vis spectroscopy are consistent with a redox mechanism, there is no direct correlation of activity with reducibility. Probing the subsurface/bulk oxygen dynamics using operando Raman F2g analysis coupled with H2 18O highlights the importance of transport properties and the availability of oxygen at the surface. Transient IR spectra reveal the presence of different surface carbonates, none of which are directly involved in the reaction but rather act as spectator species, blocking active sites, as supported by the facet-dependent analysis. From transient IR spectroscopy there is no indication of the involvement of copper, suggesting that the catalytic effect of copper is mainly based on electronic effects. The results from the operando and transient analysis unequivocally support a redox mechanism for LT-WGS over Cu/CeO2 catalysts and demonstrate the potential of our combined spectroscopic approach to distinguish between redox and associative mechanisms in oxide-supported metal catalysts.