Mobility of Surface Species on Oxides. 1. Isotopic Exchange of18O2with16O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by Noble Metals. Correlation with Oxide Basicity

Temperature-programmed isotopic exchange of 18 O 2 with 16 O of several oxides was carried out in the 200−900 °C temperature range. The oxides can be ranked according to their maximal rates of exchange obtained at the following temperatures: CeO 2, 410 °C ≫ CeO 2 -Al 2 O 3, 480 °C ≈ MgO, 490 °C > ZrO 2, 530 °C ≫ γ-Al 2 O 3, 620 °C ≫ SiO 2, 850 °C. Except on CeO 2 and on CeO 2 -Al 2 O 3 a simple exchange yielding initially 18 O 16 O can be observed. With ceria containing oxides, the reaction occurs in part via a multiple exchange mechanism yielding initially 16 O 2 which is indicative of the presence of binuclear species (O 2, O 2 -, or O 2 2- ) at the ceria surface. Chlorine-free rhodium catalysts supported on these oxides were prepared with metal dispersions between 32 and 89%. The presence of rhodium accelerates considerably the oxygen exchange with the support: the maximal rates of the exchange can be observed at much lower temperatures, by about 200−300 °C with respect to the bare oxides. This is attributed to a spillover of oxygen from the rhodium particles to the support. Isotopic exchange experiments carried out at temperatures (300−400 °C) at which the direct exchange is negligible allow for calculation of the coefficient of surface diffusion of oxygen on the oxides. At 400 °C, the relative mobility of oxygen (base 100 for γ-Al 2 O 3 ) is CeO 2, 28 100 ≫ MgO, 500 > ZrO 2, 280 > CeO 2 -Al 2 O 3, 180 > γ-Al 2 O 3, 100 ≫ SiO 2, 1.7. Oxygen mobility can be paralleled with the surface concentration of basic sites measured by CO 2 chemisorption (sites per nm -2 ): CeO 2, 3.23 > MgO, 1.77 > ZrO 2, 1.45 > CeO 2 -Al 2 O 3, 0.44 > γ-Al 2 O 3, 0.17 > SiO 2, ≈ 0. Actually, the basicity of CeO 2 cannot alone explain the exceptional mobility of oxygen on this oxide, due to a large part to the presence of oxygen vacancies. Above 400 °C, bulk oxygen diffusion can be observed on CeO 2, ZrO 2, γ-Al 2 O 3 and CeO 2 -Al 2 O 3 . Ceria possesses a very high internal mobility. The coefficient of bulk diffusion of oxygen in ZrO 2 is about two orders of magnitude higher than in γ-Al 2 O 3, which contrasts with the relatively close values of their surface mobility. Except for CeO 2, there is a good correlation between this surface mobility and the metal−oxygen bond strength in the oxide crystal.