Study of surface reaction mechanisms by 16O/18O and H/D isotopic exchange

Surface diffusion of active species is a general phenomenon in catalysis: rates of migration often being much higher than turnover frequencies of reaction, surface species can migrate and visit a great number of active sites in between two successive catalytic cycles. 16O/18O and H/D isotopic exchange is a very useful technique to elucidate reaction mechanisms involving mobility steps of O- and H-containing reactive species. The method is based on the measurement of the rates of exchange between gaseous 18O2 (or D2) and 16O species (or OH) species of the support via metal particles, small metallic clusters acting as porthole of O or H on the support. Simple kinetic models allow to calculate surface and bulk diffusivities on the oxide used as support. As a rule, a significant O mobility can only be observed above 200 °C for most oxides. Furthermore, very great differences of diffusivity (five orders of magnitude) are recorded between silica (on which OH groups are virtually immobile) and oxides like ceria exhibiting a very high oxygen mobility. Differences of diffusivity (two orders of magnitude) are much less for hydrogen than for oxygen. This method can give useful information about reactions catalyzed by metal/oxide systems in which surface diffusion can play a decisive role in the mechanism: steam reforming, water gas shift, hydrocarbon oxidation, aromatic hydrogenation, methanol synthesis, coke formation). For oxides such as CexZr1−xO2 mixed oxides, O mobility is so high that surface and bulk mobility can no longer be distinguished. More complex kinetic models requiring computer simulation of all the steps of adsorption, desorption, diffusion and exchange simultaneously have been developed.