UPS and TDS studies of the adsorption of CO and H 2 on [formula omitted

Abstract

The adsorption of CO and H 2 on a Cu/Ni (110) surface has been studied utilizing a combination of techniques including ultraviolet photoemission spectroscopy (UPS), thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). UPS is found to be a useful chemical probe to study ensemble and ligand effects in chemisorption. The techniques of chemical shift analysis utilized in X-ray photoemission spectroscopy (XPS) are extended to the valence band region and used to detect changes in the chemical nature of the binding site. As the adsorption site for CO changes from principally Ni sites to a mixture of Cu, Cu/Ni, and Ni sites, the CO emission features are found to shift by ~0.8 eV. This is detected by comparing UPS spectra taken for CO chemisorbed at room temperature and at 160 K, and at various CO coverages, and by relating these spectra to TDS data. This is a direct reflection of the ensemble effect in chemisorption. A ligand effect was observed for CO adsorbed at Cu sites by comparing UPS spectra for CO on pure Cu and CO adsorbed at Cu sites on the alloy. The CO emission features for CO adsorbed at Cu sites on the alloy are shifted by ~0.6 eV compared to CO on pure Cu. Strong adsorbate-adsorbate interactions were observed in both the UPS and TDS data when hydrogen and carbon monoxide were coadsorbed. Preadsorbed hydrogen apparently blocks a number of CO adsorption sites, in particular, certain Ni and Cu/Ni sites. Because of this, CO is adsorbed at Cu sites at much lower CO exposures when H 2 is first preadsorbed.