The hydrogen evolution reaction and hydrogen oxidation reaction on thin film PdAu alloy surfaces

A high throughput synthetic and screening methodology has been applied to the study of the Hydrogen Evolution Reaction (HER) and Hydrogen Oxidation Reaction (HOR) activities of PdAu thin film alloy surfaces. A compositional gradient method has been used to produce non-equilibrium surfaces of PdAu solid solution alloys. XPS shows surface compositions for these un-annealed alloys to be the same as the bulk, in contrast to the observed and expected behaviour of annealed alloys. The surface redox behaviour of the alloys provides direct evidence for the existence of Pd ensembles with distinctive surface chemistry dominating the oxygen interaction at intermediate alloy compositions. CO stripping voltammetry indicates that alloying of Pd with Au results in a strong poisoning of CO adsorption with the exception of CO adsorbed on Pd monomers. The compositional dependence of the HER and HOR activities is strongly correlated on the alloy surfaces, with a maximum in geometric activity observed at 60 at% Au, corresponding to a maximum in specific activity at 70 at% Au, for both reactions. This activity is associated with Pd poor ensembles. There is some evidence that while the Pd monomer catalyses the HER, HOR is not sustained at compositions where the monomer is present. Small Pd ensembles show optimal activity for hydrogen oxidation on PdAu alloys.