Traveling interface modulations in the NH3 + O2 reaction on a Rh(110) surface

Tunable deep ultraviolet laser based near ambient pressure photoemission electron microscope for surface imaging in the millibar regime

A newly developed instrument comprising a near ambient pressure (NAP) photoemission electron microscope (PEEM) and a tunable deep ultraviolet (DUV) laser source is described. This NAP-PEEM instrument enables dynamic imaging of solid surfaces in gases at pressures up to 1 mbar. A diode laser (976 nm) can illuminate a sample from the backside for in situ heating in gases up to 1200 K in minutes. The DUV laser with a tunable wavelength between 175 nm and 210 nm is perpendicularly incident onto the sample surface for PEEM imaging of a wide spectrum of solids with different surface work functions. Using this setup, we have first demonstrated spatiotemporal oscillation patterns of CO oxidation reaction on Pt(110) from high vacuum to NAPs and gas-induced restructuring of metal nanostructures in millibar gases. The new facility promises important applications in heterogeneous catalysis, electrochemical devices, and other surface processes under nearly working conditions.

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.

Large amplitude excitations traveling along the interface in bistable catalytic methanol oxidation on Rh(110)

A PEEM image and an x–t-plot showing traveling interface modulations in a bistable surface reaction.

Traveling interface modulations and anisotropic front propagation in ammonia oxidation over Rh(110)

The bistable NH3 + O2 reaction over a Rh(110) surface was explored in the pressure range 10(-6)-10(-3) mbar and in the temperature range 300-900 K using photoemission electron microscopy and low energy electron microscopy as spatially resolving methods. We observed a history dependent anisotropy in front propagation, traveling interface modulations, transitions with secondary reaction fronts, and stationary island structures.

Heterogeneous catalytic activity of platinum nanoparticles hosted in mesoporous silica thin films modified with polyelectrolyte brushes

Platinum nanoparticles of 3 nm diameter were included in mesoporous silica thin films by controlling the mesopore surface charge with a short polymer brush. This metal-polymer-mesopore nanocomposite presents high catalytic activity toward ammonia oxidation at low temperatures with 4.5% weight platinum loading. An anomalous partial selectivity toward nitrous oxide is observed for the first time, which can be traced back to the synergy of the particles and modified surface. This effect opens a path toward the design of nanocomposite catalysts with highly controlled environments, in which the size- and function-controlled cavities can be tuned in order to lower the reaction barriers.