(327a) Pd As an Oxidation State Modifier and Cocatalyst for the Re-Catalyzed Heterogeneous Deoxydehydration

Deoxydehydration (DODH) is an emerging biomass deoxygenation process whereby vicinal OH groups are removed. We elucidate the promotional effect of Pd for the DODH of 1,4-anhyrdoerythritol over a heterogeneous ReO_x-Pd/CeO₂ catalyst from first principles. For this mechanistic study, we performed DFT+U calculations and developed microkinetic models based on DFT data and transition state theory. Excellent agreement with experimental observations is obtained, confirming that single Re sites are the main active site for catalysis. We disentangle the contribution of the oxide support, the oxide-supported single ReO_x species, and the co-adsorbed Pd promoter that has no direct interaction with the Re species. A promotional effect of Pd on the scission of diol O-H bonds at ReO_x sites is observed. Analysis of electronic properties suggests that Pd is an oxidation state modifier of nearby Re via the CeO₂ substrate, changing the oxidation state of Re from +6 in the non-promoted catalyst to +7. Additionally, Pd serves as a catalyst for hydrogen dissociation and spillover onto CeO₂, which is an indispensable process for the regeneration of the Re catalyst. The established oxidation state modification mechanism suggests (medium-range) communication between the Pd and Re sites through the reduced ceria support.