Reactivity and Stability of Ultrathin VOx Films on Pt(111) in Catalytic Methanol Oxidation

The growth of ultrathin layers of VOx (< 12 monolayers) on Pt(111) and the activity of these layers in catalytic methanol oxidation at 10−4 mbar have been studied with low-energy electron diffraction, Auger electron spectroscopy, rate measurements, and with photoemission electron microscopy. Reactive deposition of V in O2 at 670 K obeys a Stranski–Krastanov growth mode with a (√3 × √3)R30° structure representing the limiting case for epitaxial growth of 3D-VOx. The activity of VOx/Pt(111) in catalytic methanol oxidation is very low and no redistribution dynamics is observed lifting the initial spatial homogeneity of the VOx layer. Under reaction conditions, part of the surface vanadium diffuses into the Pt subsurface region. Exposure to O2 causes part of the V to diffuse back to the surface, but only up to one monolayer of VOx can be stabilized in this way at 10−4 mbar.

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 _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.

Simulation of traveling interface pulses in bistable surface reactions

A couple of bistable oxidation reactions on Rh(110), the CH_{3}OH+O_{2} and the NH_{3}+O_{2} reactions, exhibit localized excitations at the interface between oxygen-poor and oxygen-rich phase that propagate in a pulselike manner along the interface. A three-variable reaction-diffusion model is set up based on a mechanism that explains the localized excitations as being caused by temporary structural defects generated in the vicinity of the interface. The structural defects are a consequence of different densities of surface atoms in the oxygen-induced reconstruction phases and in the nonreconstructed (1×1) phase. One- and two-dimensional simulations show that traveling interface pulses (TIPs) exist in a region of so-called double metastability adjacent to the equistability point of the bistable system. As in the experiment, we observe triangular-shaped TIPs that move fast along the interface. Diffusional anisotropy is not required for the occurrence of TIPs. All essential features of the experiment are reproduced by the simulations.

Hole patterns in ultrathin vanadium oxide layers on a Rh(111) surface during catalytic oxidation reactions with NO

Various oxidation reactions with NO as oxidant have been investigated on a partially VO_x covered Rh(111) surface (θ_V = 0.3 MLE) in the 10^-4 mbar range, using photoelectron emission microscopy (PEEM) as spatially resolving method. The PEEM studies are complemented by rate measurements and by low-energy electron diffraction. In catalytic methanol oxidation with NO and in the NH₃ + NO reaction, we observe that starting from a homogeneous surface with increasing temperature first a stripe pattern develops, followed by a pattern in which macroscopic holes of nearly bare metal surface are surrounded by a VO_x film. These hole patterns represent just the inverse of the VO_x distribution patterns seen if O₂ instead of NO is used as oxidant.