Mechanistic study of the ceria supported, re-catalyzed deoxydehydration of vicinal OH groups

Abstract

Deoxydehydration (DODH) is an emerging biomass deoxygenation process whereby vicinal OH groups are removed. Based on DFT calculations and microkinetic modeling, we seek to understand the mechanism of the Re-catalyzed deoxydehydration supported on CeO₂(111). In addition, we aim at understanding the promotional effect of Pd in a heterogeneous ReO_x–Pd/CeO₂ DODH catalyst system. We disentangle the contribution of the oxide support, the oxide-supported single ReO_x species, and a co-adsorbed Pd promoter that has no direct interaction with the Re species. In the absence of a nearby Pd cluster, a Re site is able to reduce subsurface Ce-ions of a hydroxylated CeO₂(111) surface, leading to a catalytically active Re +6 species. The effect of Pd is twofold: (i) Pd catalyzes the hydrogen dissociation and spillover onto CeO₂, which is an indispensable process for the regeneration of the Re catalyst, and (ii) Pd adsorbed in close proximity to Re on CeO₂(111) facilitates the oxidation of Re to a +7 oxidation state, which leads to an even more active Re species than the Re +6 site present in the absence of Pd. The latter promotional effect of Pd (and change in oxidation state of Re) disappears with increasing Pd–Re distance and in the presence of oxygen defects on the ceria support. Under these conditions, the ReO_x–Pd/CeO₂ catalyst system exhibits appreciable activity consistent with recent experiments. The established mechanism and role of various species in the catalyst system help to better understand the deoxydehydration catalysis. Also, the importance of the Re oxidation state and the identified oxidation state modification mechanisms suggest a new pathway for tuning the properties of metal-oxide supported catalysts.