Interaction of CO and NO with Fe thin films grown on Rh(1 0 0) surface

Abstract

Chemisorption of CO and NO on Fe thin films on Rh(1 0 0) has been studied by LEED, AES, HREELS and TDS methods. The analysis of the LEED (0 0) beam I( V) curves shows that a compressed interlayer spacing in the pseudomorphic fct Fe films is expanded at 4 ML thickness. HREELS for CO adsorbed 1 ML thick Fe film displays two C–O stretch bands at 2060 and 1950 cm −1 due to CO bonded at atop and bridge sites, respectively. These two CO bands merge into a broad band centered at 2000 cm −1 with increasing film thickness. In addition, a weak C–O stretch band stays at 1630 cm −1 in the film thickness range of 1–5 ML, which can be assigned to CO bound to fourfold hollow sites. TPD of CO saturated Fe films shows three molecular desorption states and one recombinative desorption as similarly as bulk bcc Fe(1 0 0) surface. The two desorption states at low temperature sequentially occur from atop sites initially and then from bridge sites. In contrast, NO adsorption displays an intense N–O stretch mode at 1825 cm −1 with a shoulder band at 1620 cm −1 in the entire thickness range of 1–5 ML. These bands are assigned as NO bound to atop and bridge sites. On heating, NO bonded at atop sites produces the desorption peak at 350 K and the dissociation of NO is completed at 450 K. In the adsorption of CO and NO, there is no appreciable change in the stretching frequencies accompanied with the increase in interlayer spacing within the resolution of HREELS.