Reactive Phase Separation during Methanol Oxidation on a V-Oxide-Promoted Rh(110) Surface

The distribution of ultrathin layers of vanadium oxide on Rh(110) (theta(V) <= 1 MLE, one monolayer equivalent corresponds to the number of Rh atoms in the topmost Rh(110) surface layer) after exposure to catalytic methanol oxidation in the 10(-4) mbar range has been investigated with x-ray photoelectron spectroscopy and spectroscopic low-energy electron microscopy (SPELEEM). The reaction is shown to cause a macroscopic phase separation of the VOx film into VOx-rich and into V-poor phases. For theta(V) = 0.8 MLE compact VOx islands develop whose substructure exhibits several ordered phases. At theta(V) = 0.4 MLE the VOx-rich phase consists of many small VOx islands (0.1-1 mu m). Laterally resolved x-ray photoelectron spectroscopy of V 2P(3/2) shows an oxidic component at 515.5 eV binding energy (BE) and a component at 513.0 eV BE attributed to metallic or strongly reduced V. On the V-poor phase only the reduced/metallic component is present. The results are compared with the distribution of ultrathin layers of vanadium oxide on Rh(111) after catalytic methanol oxidation. The presence of the metallic V on Rh(110) is at variance with the behavior of Rh(111), where V is found to be present only in high oxidation states during methanol oxidation.