Reaction of coadsorbed NO and CO on fcc ultra-thin Fe films grown on a Rh(100) surface

We have studied the reaction of coadsorbed NO and CO on ultra-thin Fe films (1-3 ML, ML = monolayer) grown on Rh(100) surface in order to examine the reactivity of Fe thin-films having fee structure. TD spectra obtained from NO adsorbed Fe thin-films gave an evolution of N-2 below 300 K concomitant with a desorption of NO at 350 K and a recombinative desorption of atomic nitrogen above 800 K. The peak temperature of the recombinative desorption shifted from 800 K to 1000 K with increasing Fe coverage from 1 to 3 ML. Similarly, the desorption temperature of dissociatively adsorbed CO moved from 700 K to 900 K upon the increase in film thickness. These are correlated with a growth of the electronic density of states just below the Fermi level observed for these Fe thin-films. The coverage of dissociatively adsorbed CO is always less than 0.05 on 1 and 3 ML films, which is only one-fourth of that reported on bcc Fe(100) surface. It indicates that the dissociation of CO does not effectively proceed on fee Fe thin-films. TPR spectra of NO and CO coadsorbed Fe thin-films show the dependence of position and shape of product peaks on the film thickness. In particular, TPR spectra from Fe thin-films with nearly equal amounts of coadsorbed NO and CO display a sharp desorption of CO and CO2 above 500 K followed by a broad evolution of CO2 and N-2. The increase of Fe coverage induced the higher temperature shift and intensity growth of this sharp desorption. XPS and UPS spectra show that this is derived from molecularly adsorbed CO coexisting with dissociated N and O atoms. These results strongly suggest an attractive interaction between a pair of CO molecule and N atom in a mixed phase through surface Fe layer or a formation of surface complex like NCO species. (C) 1997 Elsevier Science B.V.