Molecular-level understanding of reaction path optimization as a function of shape concerning the metal–support interaction effect of Co/CeO2 on water-gas shift catalysis

Abstract

In this work, the active sites generated in hydrogen reduction and the reaction pathways for the water gas shift (WGS) reaction over Co/CeO₂ catalysts were studied by in situ XAS and XPS coupled with DFT+U calculations. In reactions over fresh CeO₂ and Co/CeO₂ of spindle-, cube-, rod- and sphere-morphology, the CO conversion was less than 30% and did not show much variation across the catalysts. After in situ H₂ treatment at 400 °C for 1 h, the Co/CeO₂ catalyst of spherical morphology shows an excellent reaction rate and stability. In situ XAS, XPS and H₂-TPR results revealed that the H₂ treatment results in the strengthening of cobalt–ceria interaction, which is essential for the WGS reaction. It is apparent that the shape of CeO₂ has a strong influence on the strength of metal–support interaction which affects the degree of cobalt reduction. According to the results of DFT+U calculations, the WGS reaction follows the associative mechanism on reduced Co/CeO₂ catalysts, and the formation of the carboxyl group is found to be the rate-limiting step with an activation barrier of 0.77 eV. Interestingly, the carboxyl intermediate species over pure reduced CeO₂(111) are unstable, while these intermediate species can exist stably over the reduced Co/CeO₂(111) surface via the strong interaction between Co and Ce, which will promote the WGS performance. The findings of the present study offer new insights into Ce-based catalysts that are suitable for the WGS reaction.