8548
J. Phys. Chem. B 2000, 104, 8548-8553
Substrate-Mediated Oxidation of Carbon Residues by TiO2{110}-Supported Model Catalysts: Metal-, Precursor-, and Treatment-Dependent Labilization of Framework Oxygen Mark A. Newton,* John Evans, and Brian E. Hayden Department of Chemistry, The UniVersity of Southampton, Highfield, Southampton, SO17 1BJ U.K. ReceiVed: April 19, 2000; In Final Form: July 2, 2000
Thermal and chemical treatment of metallo-organic chemical vapor deposition (MOCVD)-prepared Rh and Pd model catalysts, supported on TiO2{110}, lead to the formation of carbon residues that are oxidized to CO at elevated temperatures. These processes are metal, metal precursor, and surface- treatment dependent. In the absence of surface Cl, Rh precursors exhibit surface carbon oxidation, at T > 775 K. In the case of Pd this is not seen until T > 825 K and some carbon is always found to remain. Room-temperature reduction of the model catalysts using hydrogen progressively lowers the temperature of carbon oxidation. However, reduction of a Cl-containing system dramatically promotes CO oxidation/Ar expulsion in a manner that is kinetically distinct from that observed in the absence of Cl. These observations are discussed in terms of the mobility of surface oxygen species on TiO2{110}, metal-support interactions, and the effect of Cl incorporation into the TiO2 substrate.
Introduction Supported Rh and Pd catalyze numerous important commercial processes. The type of support and the nature of the metal precursor used in each case can have a significant influence on the reactivity and selectivity of the final catalyst for a given process. For instance it has been shown that a wide variation of these parameters can be achieved in the hydrogenation of CO using Rh/SiO2 catalysts simply through variation of the catalyst precursor.1 Indeed, the two Rh organometallic species investigated here were among those which resulted in notable changes in selectivity in CO hydrogenation: catalysts produced from [Rh(CO)2Cl]2 yield an unusually high selectivity for C2-C3 hydrocarbons; Rh(acac)(CO)2-based systems show a relatively high selectivity for acetaldehyde. It is also the case that when using TiO2, and reducible supports in general, thermal history is crucial to determining the properties of the catalyst. This is principally due to the possibility of support reduction and the catalyst entering strong metal support interaction (SMSI) states. Such states may be due to electronic interactions between metal and support, and/or due to a physical migration and encapsulation of species such as oxygen or suboxides emanating from the substrate.2-12 Purely electronic interactions due to charge transfer from TiO2 to Rh have been invoked to explain the enhanced activity of Rh/TiO2 in CO hydrogenation compared to other less-reducible supports.10 However, it is also know that reaction above 623 K for these RhCl3-derived systems results in a lowering in the level of surface carbon maintained upon the catalyst. The level of carbonaceous material retained by the working catalyst has been linked to the selectivity the Rh catalyst displays in CO hydrogenation reactions.1 Most usually the onset of SMSI though encapsulation in Rh/ TiO2 catalysts occurs at ∼770 K, though the magnitude of this effect is also dependent upon the method of preparation used.7 The presence of Cl in Rh/TiO2 13 and Rh/CeO2 14 systems has * Corresponding author. Tel: + 44 (0)2380 59 67 44. Fax +44 (0)2380 59 37 81. E-mail:
[email protected].
also been found to perturb the onset of support reduction. Further Ti-hydrido species within defected TiO215a have also been implicated in SMSI and in the formation of oxygenated products in Fischer-Tropsch catalysis.15b Metal particle encapsulation has significant consequences for the adsorption of gas-phase species, most notably a severe reduction in the capacity for adsorption of CO and H2. The curtailment of adsorptive capacity does not necessarily mean, however, that the catalyst in question is less effective in facilitating a given process: examples exist where reaction may be promoted as well as poisoned by this effect.4,11,12a As such this phenomenon, first described in ref 2, has received much attention in the literature: the Rh/TiO2 system has attracted much interest,1-9 the Pd/TiO2 case, less so, though SMSI effects have been reported.2,12,19 In the case of the Rh/TiO2 system, SMSI states have been identified on single-crystal model catalysts prepared by metal vapor deposition (MVD). Evidence for the induction of such a state in UHV, and in the absence of H2, has been presented.9 However, further work16 has found suboxide encapsulation of the Rh particles was only induced in the presence of H2 at elevated temperatures; this encapsulation was inferred to be by species close to TiO2 in stoichiometry. Last, a recent study17 of the formation and migration of carbon adsorbed on model (MVD-synthesized) Rh particles supported upon the TiO2{110}(1 × 2) surface has provided evidence for the formation of threedimensional carbon species growing around and over Rh particles. Further, a spillover of some of these particulate carbon species was identified at T > 500 K, and gasification of this carbon at T > 800 K. In terms of model studies of Pd/TiO2{110} those of most direct relevance to this communication are the direct observations of a thermally induced sintering of the Pd particles,18a and a reverse spillover of oxygen from Pd particles onto the reduced rutile{110} support at 673 K.18b More recent measurements19a have indicated a lack of Pd particle encapsulation by suboxides at 973 K but clear evidence for such process occurring on a 10
10.1021/jp001499m CCC: $19.00 © 2000 American Chemical Society Published on Web 08/16/2000
Substrate-Mediated Oxidation of Carbon Residues Å film of Pd at 1170 K.19b In both of these cases substrate reduction to give a p(1 × 2) structure in LEED was observed. Another method for producing model catalyst systems is through MOCVD of volatile organometallics.20-27 These have an advantage in that they generally lead to the formation of a monodisperse adlayer. This method is also (generally) limited to adsorption of a single monolayer of organometallic precursor and intrinsically low (
