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

Abstract

Temperature-programmed isotopic exchange of 18O2 with 16O 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: CeO2, 410 °C ≫ CeO2-Al2O3, 480 °C ≈ MgO, 490 °C > ZrO2, 530 °C ≫ γ-Al2O3, 620 °C ≫ SiO2, 850 °C. Except on CeO2 and on CeO2-Al2O3 a simple exchange yielding initially 18O16O can be observed. With ceria containing oxides, the reaction occurs in part via a multiple exchange mechanism yielding initially 16O2 which is indicative of the presence of binuclear species (O2, O2-, or O22-) 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-Al2O3) is CeO2, 28 100 ≫ MgO, 500 > ZrO2, 280 > CeO2-Al2O3, 180 > γ-Al2O3, 100 ≫ SiO2, 1.7. Oxygen mobility can be paralleled with the surface concentration of basic sites measured by CO2 chemisorption (sites per nm-2): CeO2, 3.23 > MgO, 1.77 > ZrO2, 1.45 > CeO2-Al2O3, 0.44 > γ-Al2O3, 0.17 > SiO2, ≈ 0. Actually, the basicity of CeO2 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 CeO2, ZrO2, γ-Al2O3 and CeO2-Al2O3. Ceria possesses a very high internal mobility. The coefficient of bulk diffusion of oxygen in ZrO2 is about two orders of magnitude higher than in γ-Al2O3, which contrasts with the relatively close values of their surface mobility. Except for CeO2, there is a good correlation between this surface mobility and the metal−oxygen bond strength in the oxide crystal.