A comparison of the chemistry of RhI(acac)(CO)2 and RhI(CO)2Cl adsorbed on TiO2[110]: development of particulate Rh and oxidative disruption by CO

Room-temperature metallo-organic chemical vapour deposition (MOCVD) of Rh(acac)2(CO)2 [(acac)=(CH3CO)2CH] to the TiO2{110} surface leads to the formation of a molecular adlayer. Thermal desorption (TPD) and XPS indicate that the geminal dicarbonyl species derived from the Rh(acac)(CO)2 species is the most thermally labile component of the adsorbed layer and is comparable in stability to that derived from [Rh(CO)2Cl]2 adsorption. cac) ligand undergoes a complex decomposition in the temperature region 500–775 K, and the carbon left behind appears to perturb the subsequent clustering Rh. The Rh 3d5/2 binding energy (BE) is approximately constant at ∼307.4–307.2 eV in this temperature region, whereas in the [Rh(CO)2Cl]2-derived system, a BE of ∼307 eV (corresponding to Rh0) is attained by 600 K. A Rh 3d5/2 BE of 307 eV is only attained at T>775 K, and the remaining C residues are reacted away as CO by substrate oxygen. difference spectra show the formation of two distinct features during thermal treatment. One appears at high temperature with a BE shifted by ∼−2.0 eV from the bulk Ti4+ photoelectron line and is associated with substrate reduction to Ti3+ and the oxidation of surface bound carbon residues. The second appears at ∼−1.5 eV from the Ti4+ line. This feature appears in concert with the decomposition of the Rh organometallics and is shown to be precursor-dependent both in magnitude and the temperature range in which it persists. re of the thermally decomposed (600 K, Rh 3d5/2 BE ∼307.3 eV) Rh(acac)(CO)2/TiO2 {110} to CO provides evidence for two stages in Rh redispersion. At room temperature, a species (C 1s BE∼286.6 eV: Rh 3d5/2 BE ∼307.5) indicative of CO adsorption upon small Rh particles is observed. Further exposure to CO at a slightly elevated temperature leads to both linear CO species (BE 286 eV) and regeneration of around 25% of a geminal dicarbonyl species (BE 287.7 eV); this occurs in the absence of Cl and despite a surface C/Rh stoichiometry of ∼2–3. The latter observation indicates a mixture of CO-induced Rh redispersion and agglomeration.