A comparative study of the behaviour of the PdAg(111) and Pd(111) surfaces towards the interaction with hydrogen and carbon monoxide

Abstract

AES, LEED and TDS studies are reported describing the adsorption behaviour of a Pd67%-Ag33%(111) surface (with surface composition of 10% Pd, 90% Ag) towards CO and hydrogen. The results are compared to those obtained for a Pd(111) surface. In addition, the effect of preadsorbed hydrogen on subsequent CO dosing and the effect of preadsorbed CO on postdosed hydrogen are described. The amount of CO that can be adsorbed on PdAg(111) per unit surface area at 250 K is only 8±5% of that on Pd(111). CO desorption from PdAg(111) occurs in a single peak with a maximum shifting with exposure from 400 to 375 K (heating rate 30 K/s). This should be compared with a desorption peak from Pd(111) with a maximum shifting from 500 to 475 K and with a shoulder around 340 K. The results are explained in terms of size and relative concentration of ensembles of Pd atoms on the surface. It can be concluded that the heat of adsorption of CO adsorbed on the triplet sites of Pd(111) is much larger (∽ 30 kJ/mol) than that of CO adsorbed on the singlet sites of Pd(111). The effect of lateral interaction in the CO adlayer on Pd(111) is about 15 kJ/mol and, hence, much smaller than the effect of the sites. Hydrogen is desorbed from PdAg at a significantly higher temperature than from Pd(111). Postdosed CO does not modify the amount of desorbing hydrogen at subsequent heating. The presence of adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. The CO desorption spectra are not influenced by pre- or post-exposure of PdAg(111) to hydrogen. The results suggest that hydrogen is not adsorbed on PdAg(111) at 230 K but that it is absorbed into the bulk. This process of hydrogen dissolution is much easier than for Pd(111). We conclude that isolated Pd atoms on the PdAg(111) surface act as trapping and dissociation centres for hydrogen. The heat of adsorption of hydrogen on Pd singlet sites is too low for detection at 220 K.