The interaction of CO with PdAg/Pd(111) surface alloys—A case study of ensemble effects on a bimetallic surface

Abstract

The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.

MnOx-Promoted PdAg Alloy Nanoparticles for the Additive-Free Dehydrogenation of Formic Acid at Room Temperature

“…CO vibrational signatures on monometallic Ag NPs deposited on oxide surfaces typically appear at ≥2169 cm 66 ). This is in perfect agreement with a former study, 69 demonstrating the correlation between the red-shift in ν CO upon Ag incorporation into the Pd lattice and weakening of the CO adsorption strength. Similarity in the magnitude of the red shift for atop and bridging Pd adsorption sites suggests that Ag is rather uniformly distributed in the bimetallic NP, influencing atop and bridging Pd adsorption sites alike.…”

Ensemble Effect Evidenced by CO Adsorption on the 3-Fold PdGa Surfaces

“…10 Additionally, the CO binding energies shown in Table 1 are comparable to those computed for PdAg/Pd(111) surface alloys. 11 Using the same PBE functionals, Mancera et al…”

“…23,24 This transition was also reported for Pd ensembles on PdAu and PdAg surface alloys. 3,8,10 In these alloys, the Pd ensemble size depends on the random local Pd concentration at the surface, whereas in the PdGa (111) and (−1−1−1) surfaces, size and separation are given by the crystal structure and do not allow for CO bridge site adsorption. However, the Pd ensembles found in the partially covalently bound IMC retain their adsorption properties, as adsorption sites, energy, and vibrational modes compare for CO adsorbed on the IMC and on the surface alloys, for Pd ensembles of equal size.…”

Atomic Ensemble and Electronic Effects in Ag-Rich AgPd Nanoalloy Catalysts for Oxygen Reduction in Alkaline Media

“…However, this electronic ligand effect was shown to play a minor role next to the ensemble effects in PdAg films toward CO adsorption and oxidation. 10,11 Thus, ligand effects alone are not responsible for the observed activity synergy for the AgPd alloys.…”