Temperature dependence study of Pd-Cu supported bimetallic films by photoelectron spectroscopy and cyclic voltammetry

Vapor deposition method is used to prepare layered Pd-Cu bimetallic films, with different thickness of Pd over a Cu film supported on glassy carbon. X-ray photoelectron spectroscopy (XPS) measurements of core level binding energy (BE) and valence band region are used to investigate the contribution of charge transfer resulting from the bonding of these two dissimilar metals in a layered structure. As this layered bimetallic film is annealed at increasing temperature from 298 to 650 K, atomic inter-diffusion occurs to form an alloy. We differentiate this interfacial charge transfer effect between these two dissimilar metals from that of surface alloying resulting from the annealing effect. Cyclic voltammetry (CV) tests combined with XPS confirm that (1) the surface is free of Cu atoms for Pd films with thicknesses between 2.0 and 7.0 ± 0.3 nm at room temperature and (2) Pd-Cu inter-diffusion propagates to the surface and the formation of surface alloys take place at temperature greater than 350 K. For these Pd thicknesses, the difference in BE shifts of Cu 2p and Pd 3d peaks between layered and alloyed structures are +0.16 eV and −0.17 eV, respectively. This difference in BE shifts allows for a clear distinction between these two bimetallic structures and the tailoring of the optimum configuration that enhances catalytic activity toward formic acid. Additionally, angle resolved XPS data as a function of temperature and XPS depth profiling at room temperature provide insight about the inter-diffusion length. Furthermore, CV measurements in H2SO4 and formic acid confirm that for a Pd-Cu bimetallic surface with a 3.8 ± 0.3 nm Pd film over Cu shows significantly improved activity toward formic acid oxidation compared to bulk Pd at 298 K and after being annealed at temperature of 350 K.