A LEED, TDS, and HREELS study of CO adsorbed on the Rh(311) stepped surface

Abstract

The bonding of CO on the stepped Rh(311) surface has been investigated by high resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At saturation coverage (estimated to be 1 monolayer) an ordered (2 × 2) surface structure is formed in the range of 120 to 270 K. Thermal desorption spectroscopy measurements (23 K s −1) show first order desorption with a shift in peak temperature from 528 K at low coverage to 503 K at high coverage, accompanied by an additional peak at 388 K. The heat of desorption at low coverage is found to be 31.1 kcal/mol which is nearly identical to the heat of desorption of CO from Rh(110), (100), and (111) showing that CO bonding at low coverages is structure insensitive. From the HREELS spectra both the Rh-C and CO stretching frequencies are observed to shift from 524 to 443 cm −1 and 1935 to 2056 cm −1 respectively as a function of CO coverage. HREELS spectra of the (2 × 2) structure find a CO stretch at a frequency of 2056 cm −1 and a Rh-C stretch at 443 cm −1, which indicates that the CO molecule is bonded to the top site of the step edges. Based on these results a structural model with CO top site adsorption is proposed.