Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V‑Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10^–4 mbar range during catalytic methanol oxidation on ultrathin VO_<i>x</i> films (θ_V ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [11̅0] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VO_<i>x</i> leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VO_<i>x</i>, almost no catalytic activity in formaldehyde production is found on Rh(110)/VO_<i>x</i>

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V‑Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10^–4 mbar range during catalytic methanol oxidation on ultrathin VO_<i>x</i> films (θ_V ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [11̅0] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VO_<i>x</i> leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VO_<i>x</i>, almost no catalytic activity in formaldehyde production is found on Rh(110)/VO_<i>x</i>

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V‑Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10^–4 mbar range during catalytic methanol oxidation on ultrathin VO_<i>x</i> films (θ_V ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [11̅0] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VO_<i>x</i> leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VO_<i>x</i>, almost no catalytic activity in formaldehyde production is found on Rh(110)/VO_<i>x</i>

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) (θ_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 VO_<i>x</i> film into VO_<i>x</i>-rich and into V-poor phases. For θ_V = 0.8 MLE compact VO_<i>x</i> islands develop whose substructure exhibits several ordered phases. At θ_V = 0.4 MLE the VO_<i>x</i>-rich phase consists of many small VO_<i>x</i> islands (0.1–1 μm). 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

Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)

Submonolayer coverages of V-oxide on Rh(111) condense during catalytic methanol oxidation into a pattern of macroscopic stripes or islands. Under reaction conditions, a phase separation occurs within the VOx islands that has been studied in a pressure range of 10–6–10–4 mbar with photoemission electron microscopy (PEEM), low-energy electron microscopy (LEEM), microspot-low-energy electron diffraction (μLEED), and microspot-X-ray photoelectron spectroscopy (μXPS). An oxidized outer ring with a (√7 × √7)R19.1° structure coexists with an inner (12 × 12) Moiré-type boundary layer and a reduced core exhibiting a (√3 × √3)R30° Moiré type pattern. The dependence of the substructure on the reaction conditions, on V coverage, and on island size was investigated. With μXPS, the V coverages of the different phases in the VOx islands were determined