Study of CO adsorption on La–Rh(1 0 0) surface alloys

Abstract

Adsorption of carbon monoxide on La–Rh(1 0 0) surface alloys has been studied by photoelectron spectroscopy, low-energy electron diffraction (LEED) and temperature programmed desorption (TPD). The surface alloys were formed by depositing overlayers of La in the monolayer regime on a Rh(1 0 0) substrate, and by subsequent annealing to about 1350 K. Carbon monoxide (CO) was dosed to saturation at various temperatures. The LEED pattern of the dissociated CO overlayer shows a c(4×4) structure with respect to the Rh(1 0 0) substrate. The LEED structure for the sub-monolayer alloy takes on a ring structure superposed on a (1×1) structure. At higher temperatures an apparent (10×10) superstructure is superposed on the ring structure. The TPD results shows that four different peaks are present in the desorption traces. From the core level spectroscopic data it is argued that the two first peaks are a combination of direct CO desorption, CO dissociation and associative CO desorption. The remaining two peaks are assumed to be related to associative desorption from atomic carbon and oxygen in a combination of different bonding states and different adsorbate interactions. Rh 3d core level spectra shows that the two peaks related to the surface atoms are shifted towards the bulk peak for CO on the alloy. The La 5p core level spectra shows the splitting of the core level into two peaks for CO and dissociated CO on the alloy. The 4σ and 5σ/1π molecular levels of CO on the alloy are observed in the valence band spectra, with similar binding energies as for CO on a disordered La layer. In particular, a stronger rehybridisation of the CO 4σ state with the electronic states of the surface alloy as compared to CO on clean Rh(1 0 0), is argued to be responsible for dissociation of CO on the alloy at low temperatures.