ABSTRACT Reduction in H 2 of catalyst precursor formed by impregnation of TiO 2, with a rhodium salt produces formation of metal particles on the surface oxide. The use of 1H NMR spectroscopy has permitted the analysis of hydrogen adsorption on metal particles and hydrogen spill-over from the metal to the sύpport. When the sample is reduced in H 2 at temperatures below 200° C the support does not affect significantly both processes. However, at higher reduction temperatures hydrogen adsorption on the metal and spill-over are hindered. In this temperature range (200-500° C), the formation of oxygen vacancies at the metal-support interface favours hydrogen incorporation into them and formation of Rh-Ti bondings. Both processes induce the electronic perturbation and the loss of hydrogen chemisorption of metal particles (SMSI effect). When samples are reduced above 400° C, blockage of metal adsorption sites by TiO X suboxide species is also produced. Elimination of electronic perturbation of the metal requires an oxidation of cata lysts at 200° C while TiOX overlayers are removed by oxidation at 400° C. From the NMR analysis, different phenomena participating in inhibition of metal H 2 adsorption have been identified.
Buy this Article
|