Bimetallic palladium gold (PdAu) catalysts have been shown to be superior to monometallic ones in many reactions, but the reasons for the enhancement are not thoroughly understood. In this work, palladium decorated gold nanoparticles (Pd-on-Au NPs) are used as structured model catalysts, allowing for the precise control of both size and metal distribution with Pd surface coverage (sc%). By testing reactions on a range of these catalysts, we hope to gain insight into the active site for a given reaction.
In hydrodechlorination of perchloroethene (PCE), Pd surface coverage was found to be the key factor in catalyst activity, with the optimum at 80 sc%. A complete mechanistic model that coupled mass transfer processes with the surface reactions was further developed, consistent with the observed product profiles.
Carbon supported Pd-on-Au NPs were tested for liquid phase glycerol oxidation for the first time. The best catalyst (80 sc%) had an initial TOF of ~6000 h-1, >10 times more active than Au/C and Pd/C. Catalytic activity, selectivity, activation energy and deactivation rate constant exhibited strong volcano-shaped dependences upon Pd sc%. Ex situ XANES results showed no to little change in surface Pd-O% for Au based catalysts, suggesting the possibility of Au suppressing Pd oxidation during reaction.
Ex situ EXAFS results further confirmed the core-shell structures of 60 and 150 sc% Pd-on-Au/C catalysts via Punnett square analysis, and also ascertained no to little change in their oxidation states and coordination numbers post glycerol oxidation. EXAFS observations correlate with kinetics results, and lead to the conclusion that catalysts with a larger amount of 3-D Pd ensembles are more prone to oxidize during glycerol oxidation, making them less resistant to deactivation.
Finally, Pd-on-Au/C catalysts were tested for room temperature formic acid decomposition. In situ XAS revealed that core-shell structures of 60, 150 and 300 sc% Pd-on-Au NPs maintained while oxidized Pd species was partially reduced during reaction. Catalyst with higher fraction of 3-D Pd ensembles showed much higher dehydrogenation activity than those with mostly 1-D or 2-D, correlating to the proposed mechanism that the dehydrogenation pathway is favored over metal terrace sites.