Adsorption and reaction of H2O and CO on oxidized and reduced Rh/CeOx(111) surfaces

The interaction of H2O and CO with thin films of highly oriented CeO2(111) with vapor deposited Rh was studied using soft X-ray photoemission spectroscopy and thermal desorption spectroscopy. Adsorbed surface states of CO and H2O and desorption products were identified in relation with the ceria substrate oxidation state, adsorption/annealing temperature and the presence of Rh. Only molecularly adsorbed H2O and CO were detected on Rh/oxidized ceria surfaces. Rh/reduced ceria surfaces are more reactive, and promote dissociation of both H2O and CO. CO is preferentially adsorbed on the metal and H2O on the ceria substrate. Two adsorbed states were identified following the low temperature water adsorption on Rh/reduced ceria, chemisorbed water and hydroxyls. In the absence of Rh, chemisorbed water desorbs below 300 K and hydroxyls decompose above 500 K to give H2 as a desorption product. Rh promotes the dissociation of chemisorbed water and formation of hydrogen in the 200–350 K temperature range. It also lowers the temperature of hydrogen formation from hydroxyls. The interaction of CO and H2O strongly depends on the ceria oxidation state. Reaction of atomic carbon (formed by CO dissociation) with oxygen from ceria is facilitated by the presence of coadsorbed water. On the other hand, hydrogen desorption is inhibited by the presence of coadsorbed CO.