Dynamics of Ultrathin Vanadium Oxide Layers on Rh(111) and Rh(110) Surfaces During Catalytic Reactions

Over the past 35 years rate oscillations and chemical wave patterns have been extensively studied on metal surfaces, while little is known about the dynamics of catalytic oxide surfaces under reaction conditions. Here we report on the behavior of ultrathin V oxide layers epitaxially grown on Rh(111) and Rh(110) single crystal surfaces during catalytic methanol oxidation. We use photoemission electron microscopy and low-energy electron microscopy to study the surface dynamics in the 10^-6 to 10^-2 mbar range. On VO /Rh(111) we find a ripening mechanism in which VO islands of macroscopic size move toward each other and coalesce under reaction conditions. A polymerization/depolymerization mechanism of VO that is sensitive to gradients in the oxygen coverage explains this behavior. The existence of a substructure in VO islands gives rise to an instability, in which a VO island shrinks and expands around a critical radius in an oscillatory manner. At 10^-2 mbar the VO islands are no longer stable but they disintegrate, leading to turbulent redistribution dynamics of VO . On the more open and thermodynamically less stable Rh(110) surface the behavior of VO is much more complex than on Rh(111), as V can also populate subsurface sites. At low V coverage, one finds traveling interface pulses in the bistable range. A state-dependent anisotropy of the surface is presumably responsible for intriguing chemical wave patterns: wave fragments traveling along certain crystallographic directions, and coexisting different front geometries in the range of dynamic bistability. Annealing to 1000 K causes the formation of macroscopic VO islands. Under more reducing conditions dendritic growth of a VO overlayer is observed.