Spectroscopic Study of the Effects of Carrier Doping on the Surface Structure of Rh/TiO2 Catalysts and On Their Interaction with CO and H2

Abstract

FTIR and laser-Raman spectroscopies are employed to investigate the structure of Rh catalysts supported on W[sup 6+]-doped TiO[sub 2] carriers and their interaction with CO and H[sub 2]. The results of Raman study show that the surface of the TiO[sub 2] carrier (sintered at 1173 K) contains both the rutile and anatase structures, in contrast to the bulk, which is 100% rutile. Doping of TiO[sub 2] with W[sup 6+] 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 gemdicarbonyl Rh[sup +](CO)[sub 2], linear Rh[sup 0]CO, and bridged Rh[sup 0][sub 2]CO), 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[sup [minus]1] of the linear CO band on the Rh crystallites and by 11 cm[sup [minus]1] of the gem-dicarbonyl band on the isolated Rh[sup +] sites are observed when Rh 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 Rhmore » crystallites and the doped TiO[sub 2] 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. 51 refs., 8 figs., 2 tabs.« less