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

Abstract

Chemical wave patterns and V-oxide redistribution in catalytic methanol oxidation on a VOx/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 θV=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 θV=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 VOx coverage are surrounded by a V-oxide layer. Chemical waves propagate inside the holes as well as on the VOx 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.