5 research outputs found
Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a VâOxide Promoted Rh(110) Surface
Chemical wave patterns
and the formation of macroscopic vanadium
oxide islands have been investigated in the 10<sup>â4</sup> mbar range during catalytic methanol oxidation on ultrathin VO<sub><i>x</i></sub> films (θ<sub>V</sub> ⤠1 monolayer
equivalent) supported on Rh(110). At temperatures around 800 K, wave
fragments traveling along the [11Ě
0] direction and oxidation/reduction
fronts exhibiting different front geometries are observed with photoemission
electron microscopy. At â1000 K, a redistribution of VO<sub><i>x</i></sub> 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<sub><i>x</i></sub>, almost no catalytic activity
in formaldehyde production is found on Rh(110)/VO<sub><i>x</i></sub>
Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a VâOxide Promoted Rh(110) Surface
Chemical wave patterns
and the formation of macroscopic vanadium
oxide islands have been investigated in the 10<sup>â4</sup> mbar range during catalytic methanol oxidation on ultrathin VO<sub><i>x</i></sub> films (θ<sub>V</sub> ⤠1 monolayer
equivalent) supported on Rh(110). At temperatures around 800 K, wave
fragments traveling along the [11Ě
0] direction and oxidation/reduction
fronts exhibiting different front geometries are observed with photoemission
electron microscopy. At â1000 K, a redistribution of VO<sub><i>x</i></sub> 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<sub><i>x</i></sub>, almost no catalytic activity
in formaldehyde production is found on Rh(110)/VO<sub><i>x</i></sub>
Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a VâOxide Promoted Rh(110) Surface
Chemical wave patterns
and the formation of macroscopic vanadium
oxide islands have been investigated in the 10<sup>â4</sup> mbar range during catalytic methanol oxidation on ultrathin VO<sub><i>x</i></sub> films (θ<sub>V</sub> ⤠1 monolayer
equivalent) supported on Rh(110). At temperatures around 800 K, wave
fragments traveling along the [11Ě
0] direction and oxidation/reduction
fronts exhibiting different front geometries are observed with photoemission
electron microscopy. At â1000 K, a redistribution of VO<sub><i>x</i></sub> 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<sub><i>x</i></sub>, almost no catalytic activity
in formaldehyde production is found on Rh(110)/VO<sub><i>x</i></sub>
Reactive Phase Separation during Methanol Oxidation on a VâOxide-Promoted Rh(110) Surface
The
distribution of ultrathin layers of vanadium oxide on Rh(110)
(θ<sub>V</sub> ⤠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<sup>â4</sup> 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<sub><i>x</i></sub> film into VO<sub><i>x</i></sub>-rich and
into V-poor phases. For θ<sub>V</sub> = 0.8 MLE compact VO<sub><i>x</i></sub> islands develop whose substructure exhibits
several ordered phases. At θ<sub>V</sub> = 0.4 MLE the VO<sub><i>x</i></sub>-rich phase consists of many small VO<sub><i>x</i></sub> islands (0.1â1 Îźm). Laterally
resolved x-ray photoelectron spectroscopy of V 2p<sub>3/2</sub> 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 10â6â10â4 mbar
with photoemission electron microscopy (PEEM), low-energy electron
microscopy (LEEM), microspot-low-energy electron diffraction (ÎźLEED),
and microspot-X-ray photoelectron spectroscopy (ÎźXPS). An oxidized
outer ring with a (â7 Ă â7)R19.1° structure
coexists with an inner (12 Ă 12) MoireĚ-type boundary layer
and a reduced core exhibiting a (â3 Ă â3)R30°
MoireĚ type pattern. The dependence of the substructure on the
reaction conditions, on V coverage, and on island size was investigated.
With ÎźXPS, the V coverages of the different phases in the VOx islands were determined