The structural properties of Cu/ZnO/Si model catalysts were investigated by X-ray photoelectron spectroscopy (XPS), Ar-ion XPS depth profiling and atomic force microscopy (AFM) after exposure to various methanol reforming conditions. The composition of the reactant mixture is found to influence drastically the chemical state and the morphology of the Cu overlayer. Oxidation of Cu to Cu(I) is observed upon increasing the oxygen content in the gas phase, with thinner Cu islands being less reactive, probably due to the stabilising influence of the support. In the absence of water in the reactant mixture the formation of Cu(I) is associated with Cu islands with greater lateral size covering a larger fraction of the ZnO surface. On the other hand, the formation of Cu(I) in the presence of water is accompanied by a drastic decrease of the dispersion of the Cu overlayer. The presence of water in the reactant mixture also favours the oxidation of Cu during methanol reforming and promotes the formation of Cu(II) on the surface of thin Cu islands (0.4 nm nominal thickness of the deposited Cu layer). On the other hand, no formation of Cu(II) was observed in the case of high Cu coverage Cu/ZnO/Si model catalysts (1.3 nm nominal thickness of the deposited Cu layer) for all tested compositions of the reactant mixture. The formation of Cu(II) is associated with an increase of the dispersion of the Cu overlayer and of the fraction of ZnO surface covered by Cu.

The formation of a ZnO layer partially covering the surface of metallic Cu islands is observed in the presence of oxygen in the reactant mixture. This modification, which is inhibited by the presence of water in the gas phase, is more pronounced in the case of thicker Cu islands. The relevance of the observed structural changes for the behaviour of industrial Cu/ZnO-based catalysts and their influence on the catalytic activity for the production of hydrogen from methanol are discussed.