Wetting/non-wetting phenomena during catalysis: evidence from in situ on-line EXAFS studies of Cu-based catalysts

In the present study we have used in situ EXAFS to provide experimental evidence for a reversible change in the morphology of the metallic particles in a high surface area, porous catalyst system, Cu/ZnO, containing small metallic copper particles. By changing the oxidation potential in the synthesis gas mixture, it is found that the apparent Cu-Cu coordination number changes in an essentially reversible manner suggesting that the small metallic Cu particles dynamically change morphology. This indicates that a wetting/non-wetting phenomenon takes place in the Cu/ZnO system with changing partial pressures of oxygen. Under similar conditions, these effects are not observed when copper is supported on, for example, SiO2. A model based on surface and interface free energies provides a simple explanation for the observed results. Since the wetting/non-wetting processes are accompanied by a change in the active surface area, the observed behavior has important general implications and such effects must be incorporated into microkinetic models in order to provide a proper description of the catalyst performance.