Mobility of surface species on oxides .2. Isotopic exchange of D-2 with H of SiO2, Al2O3, ZrO2, MgO, and CeO2: Activation by rhodium and effect of chlorine

Exchange of D-2 With the OH groups of several oxides was carried out on the samples on which we investigated the O-18(2)/(OH)-O-16 exchange (part I, J. Phys. Chem. 1996, 100, 9429). Temperature-programmed isotopic exchange was first carried out on the bare oxides. In every case, the reaction follows a simple heteroexchange mechanism, with HD as a primary product. The maximal rates of exchange are obtained at the following temperature: CeO2 (prereduced), 100 degrees C > MgO, 120 degrees C > ZrO2, 145 degrees C > CeO2 (preoxidized), 160 degrees C > gamma-Al2O3, 190 degrees C > Cl-Al2O3, 200 degrees C much greater than SiO2, 540 degrees C. The first two samples exchange their hydrogen at a significant rate at room temperature. Secondary peaks and shoulders show the multiplicity of the OH groups on most oxides. Compared to oxygen, hydrogen exchange occurs at much lower temperatures, the shift of the temperature ranges of O and H exchange being between 250 and 430 degrees C. Isothermal isotopic exchange was carried out between 25 and 100 degrees C over rhodium catalysts supported on these oxides. The presence of rhodium accelerates the hydrogen exchange, at least by 2 orders of magnitude. As with oxygen exchange, three main steps must be considered: (i) adsorption-desorption on the metal, (ii) transfer of D atoms onto the support (spillover), and (iii) hydrogen diffusion. Contrary to what was observed with O-2 exchange, step i is never a rate-determining step (rds) for H-2 exchange, the rate of H-2 + D-2 equilibration being much higher than the rate of exchange. The results are in accordance with a model where at T < 75 degrees C the rds of exchange would be the hydrogen transfer from the metal to the support (spillover step), while at T greater than or equal to 75 degrees C, the rds would be the surface diffusion. This model is valid for all the supports except silica, for which most of the hydroxyl groups exchange at high temperatures. The rate of exchange depends linearly on the density of the hydroxyl groups and tends toward zero for a fully dehydroxylated support. Coefficients of surface diffusion give the following order for the hydrogen mobility at 75 degrees C (base 100 for gamma-Al2O3): CeO2, 770 > MgO, 230 > gamma-Al2O3, 100 > ZrO2, 23 much greater than SiO2, nd. There is no correlation between the hydrogen mobility and the surface acidity of the oxides, the highest hydrogen mobility being found on basic oxides with a very high oxygen mobility. A model of isotropic heterogeneous diffusion is proposed to explain certain discrepancies observed between the present isotopic exchange method and direct IR spectroscopic methods previously published.