Approaching C1 Reaction Mechanisms Using Combined Operando and Transient Analysis: A Case Study on Cu/CeO2 Catalysts during the LT-Water–Gas Shift Reaction

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 H218O 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.