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

Model studies on the catalytic methanol oxidation over single and polycryst alline copper have been performed. The catalytic activity was investigated by means of temperature-programmed techniques (thermal desorption and tempe rature-programmed reaction spectroscopy, TDS and TPRS, respectively). The T PRS experiments call for the existence of chemically inequivalent species o f atomic oxygen accessibly for catalytic processes on the copper surface. T he surface morphological changes after the combined action of oxygen and me thanol were observed by using atomic force (AFM) and scanning electron misc roscopy (SEM) and indicate the participation of not only the surface but to a great extend also the bulk. Furthermore, ex situ X-ray absorption spectr oscopy (XAS) at the O K-edge shows that a copper suboxide phase of Cu(x gre ater 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-d eficient copper(I) oxide phase exhibiting an increased density-of-states at the Fermi level pointing to an electrically conducting phase. The depth-se lective recording of X-ray absorption spectra gives clear evidence of the f ormation of a protective copper(I) oxide film underneath the suboxide layer covering the bulk metal phase.