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

Abstract

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 x /Rh(111) we find a ripening mechanism in which VO x islands of macroscopic size move toward each other and coalesce under reaction conditions. A polymerization/depolymerization mechanism of VO x that is sensitive to gradients in the oxygen coverage explains this behavior. The existence of a substructure in VO x islands gives rise to an instability, in which a VO x island shrinks and expands around a critical radius in an oscillatory manner. At 10 −2 mbar the VO x islands are no longer stable but they disintegrate, leading to turbulent redistribution dynamics of VO x. On the more open and thermodynamically less stable Rh(110) surface the behavior of VO x 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 x islands. Under more reducing conditions dendritic growth of a VO x overlayer is observed.

Structural Phase Transitions of NbO2: Bulk versus Surface

“…Note that quantitative assessment via peak fitting is unreliable as the Moiré reconstruction may introduce a complicated geometrical factor into the analysis. For instance, TiO x and VO x thin films with various metal oxidation states as observed with X-ray spectroscopy techniques have displayed numerous Moiré patterns. − The Moiré patterns observed were a result of a superposition of TiO x or VO x overlayers, which tended toward higher oxidation states, and the underlying substrates. − As Sedona et al suggest, however, the overlayers are comprised of numerous metal (V and Ti) environments and exact assignments of charge state are meaningless . Nevertheless, the increase in relative spectral weight in the higher-binding energy region (nominally Nb 5+ in Figure b) supports the idea that oxygen adsorption results in the presence of the Moiré pattern on NbO 2 surfaces as seen previously on TiO x and VO x surfaces.…”

Structural Transitions Driving Interface Pulses in Methanol Oxidation on Rh(110) and VOx/Rh(110): A LEEM Study

“…A strong hysteresis of a width of roughly 5 × 10 –6 mbar in p (O 2 ) is observed for the transitions between the oxidized and reduced states. In rate measurements (not shown) a low catalytic activity of the oxidized state was found, that can be attributed to an inhibition of methanol adsorption by a high oxygen coverage surface . This inhibition effect is presumably also responsible for the hysteresis in the transitions between the two stable states.…”

“…The system CH 3 OH + O 2 /Rh­(110) exhibits bistability in the 10 –4 mbar range at temperatures above ∼680 K. ,, Depending on the experimental parameters, the surface is either in an oxidized state, characterized by a high oxygen coverage and a low catalytic activity, or in a reduced surface state, exhibiting a low oxygen coverage and a high catalytic activity. The oxidized state displays a low intensity in PEEM and a high intensity in LEEM/MEM.…”

“…Since a high oxygen coverage is known to inhibit methanol oxidation, as demonstrated for Pd(110) and Rh(110), the oxygen covered Rh(110) surface should exhibit a low catalytic activity. , No data exist for methanol oxidation on a carbon covered Rh(110) surface, but presumably a carbon coverage should have a negative effect on both, on O 2 as well as on methanol adsorption. The highest catalytic activity should have a surface with a low adsorbate coverage which is the (1×1) phase after removal of the oxygen adlayer.…”

Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)

“…This problem is at the heart of the well-known pressure and materials gap in heterogeneous catalysis. Here we study the dynamic behavior of submonolayer coverages of VO x on Rh(111) during catalytic reactions. − …”