Structural changes at the titania surface and their relationship to metal-support interactions in rhodium-titania catalysts

Abstract

NMR, XPS, and ESR spectra have been obtained for two different Rh/TiO/sub 2/ catalyst preparations (with dispersions of ca. 15% and 70%) which had been subjected to strong reduction with flowing H/sub 2/, and the results are compared with those obtained on catalysts reduced in closed-cell environments. NMR spectra of such strongly reduced samples reveal the well-known effect of strong hydrogen chemisorption suppression. The ESR spectra of Ti(III) ions indicate the onset of a structural change at the support surface during deep reduction in flowing hydrogen at T > 573 K, which is accompanied by a 0.5-eV shift of the Rh(3D) XPS peak to lower binding energies; both effects remain after reoxidation in O/sub 2/ at 673 K followed by mild re-reduction at 473 K. A model is proposed to explain these effects, in which anion vacancies generated in large amounts on the oxide by the reduction process are removed through their association with one another, leading to a restructured surface phase of the oxide with which the supported rhodium metal binds strongly. The possible influence of these changes on the previously proposed generation of hydride centers on the oxide is discussed.