In situ Low-Energy Electron Microscopy of Chemical Waves on a Composite V-oxide/Rh(110) Surface.

Chemical wave patterns and V-oxide redistribution in catalytic methanol oxidation on a VO _x /Rh(110) surface have been investigated in the 10 ^-4  mbar range with low-energy electron microscopy (LEEM) and micro spot low-energy electron diffraction (micro-LEED) as in situ methods. V coverages of θ _V =0.2 and 0.4 MLE (monolayer equivalents) were studied. Pulses display a c(2×2) pattern in the reduced part and (1×2) and c(2×8) structures in the oxidized part of the surface. At θ _V =0.4 MLE (1×2)/(1×4) patterns with streaks along the [001]-direction at the 1/8 positions are present on the oxidized part of the surface. This phase can be assigned to V-oxide. On a tentative basis, an excitation mechanism for pulses is presented, Annealing the surface to 990 K under reaction conditions results in a macroscopic hole pattern in which holes of low VO _x coverage are surrounded by a V-oxide layer. Chemical waves propagate inside the holes as well as on the VO _x covered parts of the surface. The results demonstrate for the first time that also in supported oxidic overlayers selforganization processes can take place leading to chemical waves and a large scale redistribution of the oxide.
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