Copper (sub)oxide formation: a surface sensitive characterization of model catalysts

Model studies on the catalytic methanol oxidation over single and polycrystalline copper have been performed. The catalytic activity was investigated by means of temperature-programmed techniques (thermal desorption and temperature-programmed reaction spectroscopy, TDS and TPRS, respectively). The TPRS experiments call for the existence of chemically inequivalent species of atomic oxygen accessibly for catalytic processes on the copper surface. The surface morphological changes after the combined action of oxygen and methanol were observed by using atomic force (AFM) and scanning electron miscroscopy (SEM) and indicate the participation of not only the surface but to a great extend also the bulk. Furthermore, ex situ X-ray absorption spectroscopy (XAS) at the O K-edge shows that a copper suboxide phase of Cu(x greater than or equal to 2.5)O is formed at the surface/near-surface region up to a depth of about 100 Angstrom. Core-level (XPS) and valence band (UPS) photoemission suggests that the suboxide phase can be viewed as an oxygen-deficient copper(I) oxide phase exhibiting an increased density-of-states at the Fermi level pointing to an electrically conducting phase. The depth-selective recording of X-ray absorption spectra gives clear evidence of the formation of a protective copper(I) oxide film underneath the suboxide layer covering the bulk metal phase.