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

Evans, J., Hayden, B. E. and Newton, M. A. (2000) A comparison of the chemistry of Rh-I(acac)(CO)(2) and Rh-I(CO)(2)Cl adsorbed on TiO2 110 : development of particulate Rh and oxidative disruption by CO. Surface Science, 462 (1-3), 169-180. (doi:10.1016/S0039-6028(00)00604-X).

Abstract

Boom-temperature metallo-organic chemical vapour deposition (MOCVD) of Rh(acac)(2)(CO)(2) [(acac) = (CH3CO)(2)CH] 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)(2)Cl](2) adsorption. The (acac) 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 3d(5/2) binding energy (BE) is approximately constant at similar to 307.4-307.2 eV in this temperature region, whereas in the [Rh(CO)(2)Cl](2)-derived system, a BE of similar to 307 eV (corresponding to Rh-0) is attained by 600 K. A Rh 3d(5/2) BE of 307 eV is only attained at T > 775 K, and the remaining C residues are reacted away as CO by substrate oxygen. Ti 2p difference spectra show the formation of two distinct features during thermal treatment. One appears at high temperature with a BE shifted by similar to -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 similar to -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. Exposure of the thermally decomposed (600 K, Rh 3d(5/2) BE similar to 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 Is BE similar to 286.6 eV: Rh 3d(5/2) BE similar to 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 similar to 2-3. The latter observation indicates a mixture of CO-induced Rh redispersion and agglomeration.

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