Adsorption/desorption of H2 and CO on Zn-modified Pd(111).

The adsorption and thermal desorption of H(2) and CO on clean and Zn covered Pd(111) surfaces were studied using temperature programmed desorption (TPD), low energy electron diffraction, and Auger electron spectroscopy. The obtained H(2) and CO-TPD results reveal that thick Zn layers (approximately 10 ML) prepared at low temperature (150 K) block the adsorption of H(2) and CO. However, the ZnPd surface alloy which is formed at temperatures above 300 K shows a different behavior. The amount of hydrogen adsorbed on surface sites is reduced by about 1/2 on the ZnPd surface alloy whereupon the diffusion of hydrogen into the subsurface region is not influenced. The initial sticking coefficient decreases from 0.5 on the clean surface to 0.14 on the ZnPd alloy. The TPD spectra for CO on the ZnPd surface alloy show that the heat of adsorption is shifted to much lower values than on clean Pd, yielding a desorption energy of 71+/-2 kJ mol(-1) at low CO coverages. The saturation coverage equals 0.5 ML which means that each Pd atom of the ZnPd surface alloy is occupied by one CO admolecule. Interestingly, however, the initial sticking coefficient for CO on the ZnPd surface alloy is still unity, as on the clean Pd surface.