Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

Abstract

The adsorption and coadsorption of CO and H 2 have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β 2, β 1, and β 0 were detected after adsorption at 250 K. The population of β 2 and β 1 states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O + and CO + ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H + signal was recorded following H 2 adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H + ESD signal, a decrease of hydrogen surface population and an arisement of a broad H 2 TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β 2 state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H + ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.