Interactions of Hydrogen and Carbon Monoxide on Pd–Au Bimetallic Surfaces

Abstract

Pd−Au bimetallic catalysts have shown potential applications in numerous heterogeneous reactions in which hydrogen and CO act as reactants, intermediates, or products. A fundamental understanding of the interplay between coadsorbed H and CO on the Pd−Au surface is necessary for improving the understanding of catalytic performance. In this study, the interactions of hydrogen and CO with Pd/Au(111) model surfaces were investigated by temperature-programmed desorption (TPD) and molecular beam scattering (MBS) experiments, carried out under ultra-high-vacuum conditions. Our results reveal that CO adsorbs competitively on the hydrogen-precovered Pd−Au surface, causing surface H adatoms to diffuse away from stronger-binding sites (e.g., Pd(111)-like islands) to weaker-binding sites (e.g., Pd−Au alloy sites and subsurface), as evidenced by a shift of the H 2 desorption feature to lower temperatures in TPD measurements. Additionally, evolution of H 2 was observed when a CO molecular beam was impinged onto the H-precovered Pd−Au surface, providing direct evidence that CO induces recombinative desorption of H adatoms. The presence of H adatoms on the Pd−Au surface was found to decrease the initial sticking probability of CO during MBS experiments but had little influence on CO desorption during subsequent TPD measurements.

Effect of annealing in oxygen on alloy structures of Pd–Au bimetallic model catalysts

“…52,[56][57][58] The growth of the Pd overlayer on the Au(111) surface at 77 K has been suggested to obey a layer-by-layer mechanism. 52,[56][57][58] The growth of the Pd overlayer on the Au(111) surface at 77 K has been suggested to obey a layer-by-layer mechanism.…”

“…7,36,40 As the annealing temperature is increased, the surface Pd morphology is transformed from a mixture of isolated and contiguous sites to a surface containing only isolated Pd sites. 52,57 The deposition rate of Pd was calibrated with a quartz crystal microbalance (QCM) controller (Maxtek Inc.) assuming a thickness of 1 ML Pd equals 0.274 nm. 52,57 The deposition rate of Pd was calibrated with a quartz crystal microbalance (QCM) controller (Maxtek Inc.) assuming a thickness of 1 ML Pd equals 0.274 nm.…”

“…52,56,57 The Pd−Au surface was first heated to 500 K at 1 K/s to desorb any surface contaminants such as CO. After the sample had cooled to 77 K, an IR background scan was taken. 52,56,57 The Pd−Au surface was first heated to 500 K at 1 K/s to desorb any surface contaminants such as CO. After the sample had cooled to 77 K, an IR background scan was taken.…”

“…Pd-Au bimetallic model surfaces were prepared by depositing 1.5 monolayer (ML) of Pd atoms from a homemade thermal evaporator onto the Au(111) surface at 77 K followed by annealing in either UHV or in an ambient of O 2 at 1 Â 10 À6 Torr to a specified temperature for 10 min. 52,[56][57][58] The growth of the Pd overlayer on the Au(111) surface at 77 K has been suggested to obey a layer-by-layer mechanism. 57 Upon annealing in UHV, some of the surface Pd atoms can diffuse into the bulk of the Au(111) sample, forming a Pd-Au alloy at the surface.…”

Effects of ensembles, ligand, and strain on adsorbate binding to alloy surfaces

“…For example, it is known that Pd(111) and Au(111), respectively, bind to H strongly and weakly, 20,21 while the PdAu(111) surfaces have a H binding energy intermediate to Pd and Au. 8,17,22 Our previous studies have shown that TPD can differentiate the H 2 desorption peaks from Pd(111)-like and Pd-Au interface sites on PdAu(111) single-crystals, 16,[22][23][24][25][26] but TPD cannot differentiate the sizes and geometries of Pd or Pd-Au ensembles due to an overlap of the desorption peaks. However, it was found from density functional theory (DFT) calculations that different sizes and geometries of Pd(111)-like and Pd-Au interface sites have different selectivities and activities for catalytic reactions.…”