Copper (sub)oxide formation: a surface sensitive characterization of model catalysts
Abstract
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(x2.5)O is formed at the
surface/near-surface region up to a depth of about 100 Å. 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.