NMR study of hydrogen adsorption on Rh/TiO2

The adsorption of hydrogen on reduced Cl3Rh/TiO2 catalysts originates a new line in the NMR spectrum, shifted toward high fields, that has been ascribed to hydrogen adsorbed on metal particles. The analysis of the intensity and position of this line vs. the H2 pressure indicates the presence of two types of adsorbed hydrogen on the metal and the modification of the metal-hydrogen interaction with the extent of the adsorption. The reduction of the sample at temperatures above 723 K produces a drop of the shifted line intensity, indicating the appearance of the strong metal-support interaction, but a subsequent reoxidation treatment permits to recover the intensity of this line and eliminates the SMSI effect. The adsorption of the most labile hydrogen on the metal is accompanied by a hydrogen transfer to the support, which is reflected in the NMR spectra of the oxidized sample as an intensity increase of the unshifted line with the H2 pressure. The incorporation of hydrogen to the support is favored when the temperature of the sample is increased above 400 K. 14 references, 8 figures, 1 table