Molecular and Dissociative Hydrogen Adsorption on Bimetallic PdAg/Pd(111) Surface Alloys: A Combined Experimental and Theoretical Study

Aiming at a systematic, quantitative understanding of the surface chemistry of bimetallic PdAg surfaces, as present in bimetallic PdAg catalysts, we have investigated the adsorption of hydrogen on structurally well-defined PdAg/Pd(111) surface alloys by a combination of density functional theory (DFT)-based electronic structure calculations and temperature-programmed desorption (TPD) measurements. By comparison of experimental TPD data and calculated adsorption energies and making use of results of previous high-resolution scanning tunneling microscopy (STM) measurements ( Phys. Chem. Chem. Phys. 2012, 14, 10754−10761, DOI: 10.1039/C2CP41104K), we could quantitatively correlate changes of the adsorption energy with increasing Pd content, as indicated by the formation and disappearance of additional desorption features in the TPD spectra, with the occupation of specific, mono- and bimetallic adsorption sites and ensembles on the PdAg surface. The results of this combined approach lead to a detailed understanding of the different effects affecting the hydrogen adsorption behavior on these bimetallic surfaces. Comparing the present results with previous findings for CO adsorption on similar PdAg surfaces and for hydrogen adsorption on PdAu/Pd(111) surface alloys allows us to place the resulting trends into a more general perspective, which is highly relevant also for the atomic-scale mechanistic understanding of catalytic reactions on bimetallic catalysts, specifically on a PdAg catalyst.