SPECTROSCOPIC STUDY OF THE EFFECTS OF CARRIER DOPING ON THE SURFACE-STRUCTURE OF RH/TIO2 CATALYSTS AND ON THEIR INTERACTION WITH CO AND H-2

FTIR and laser-Raman spectroscopies are employed to investigate the structure of Rh catalysts supported on W6+-doped TiO2 carriers and their interaction with CO and H-2. The results of Raman study show that the surface of the TiO2 carrier (sintered at 1173 K) contains both the rutile and anatase structures, in contrast to the bulk, which is 100% rutile. Doping of TiO2 with W6+ cations increases the proportion of the anatase structure on the surface. The concentration of CO adsorbed on the Rh surface (i.e., the sum of the gem-dicarbonyl Rh+(CO)(2), linear (RhCO)-C-0, and bridged (Rh2CO)-C-0), following exposure to 76 Torr of CO at 300 K, is found to exhibit a maximum with respect to dopant content at a level of 0.11-0.22 at. %. Blue shifts by 9 cm(-1) of the linear CO band on the Rh crystallites and by 11 cm(-1) of the gem-dicarbonyl band on the isolated Rh+ sites are observed when Ph is supported on the doped carriers. The alterations in the CO adsorption capacity and in the frequency of the C-O bond, upon carrier doping, are attributed to electronic interactions between the Rh crystallites and the doped TiO2 carrier, which result in alterations in the electronic configuration of the rhodium sites. Preadsorbed hydrogen on the doped catalyst is found to greatly retard the rate of subsequent CO adsorption, as compared to the corresponding behavior of the undoped catalyst, but to significantly enhance the CO adsorption capacity at equilibrium. This observation is attributed to the presence of two types of hydrogen species on the doped catalysts: one on the Rh surface which retards the rate of CO adsorption and one on the doped carrier surface, formed via hydrogen spillover from the Rh crystallites, which promotes the formation of the gem-dicarbonyl species.