Carbon monoxide adsorption and desorption on Rh{111} and Rh{331} surfaces

The combined techniques of X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) were utilized to investigate the adsorption and desorption characteristics of carbon monoxide from the initially clean Rh{111} and Rh{331} surfaces. An effective procedure was established, which reduced the bulk carbon contamination to an undetectable level. The XPS results for saturation exposures show a strong O 1s spectral region; the Rh{111} surface exhibited two peaks with binding energies at 532.1 eV and 530.7 eV, whereas for the Rh{331} surface, the peaks were at 532.1 eV and 531.2 eV. These binding energies are characteristic of molecular species bonded to two different surface binding sites. SIMS results on the CO saturated Rh{111} and Rh{331} surfaces showed strong RhCO⁺ and Rh₂CO⁺ cluster ion intensities, which further substantiates the conclusion that CO molecularly adsorbs on these Rh surfaces at 300 K. Heating the CO saturated Rh{111} and Rh{331} surfaces to greater than 470 K, thermally desorbed the CO overlayer. SIMS and XPS analysis after heating showed no evidence of molecular CO or its dissociated products. Further results of this study showed that surface carbon and oxide do not diffuse into the Rh crystal bulk below the molecular CO desorption temperature. This eliminated the possibility that any dissociated products of CO had diffused into the bulk and were therefore undetected.