Reaction dynamics of metal/oxide catalysts: Methanol oxidation at vanadium oxide films on Rh(1 1 1) from UHV to 10⁻² mbar

Recent advances in in situ microscopy allow to follow the reaction dynamics during a catalytic surface reaction from ultra-high vacuum to 0.1 mbar, thus bridging a large part of the pressure gap. Submonolayer vanadium oxide films on Rh(1 1 1) have been studied during catalytic methanol oxidation in situ with spatially resolving imaging techniques. At 10⁻⁶–10⁻⁴ mbar VO_x condenses into macroscopic circular islands that exhibit a substructure, consisting of a reduced island core and an oxidized outer ring. This substructure arises due to an oxygen gradient inside the VO_x islands, which results in different coexisting 2D-phases of VO_x on Rh(1 1 1). This substructure is also responsible for a “breathing-like” oscillatory expansion and contraction that the islands undergo under stationary conditions. Using density functional theory, the 2D-phase diagram of VO_x on Rh(1 1 1) has been computed. The oscillatory behavior can be understood as a periodic phase transition between two 2D phases of VO_x. With a newly developed near ambient pressure – low-energy electron microscope, it was shown that VO_x islands disintegrate at 10⁻² mbar, resulting in turbulent dynamics.