In situ Low-Energy Electron Microscopy of Chemical Waves on a Composite V-oxide/Rh(110) Surface.

Chemical wave patterns and V-oxide redistribution in catalytic methanol oxidation on a VO/Rh(110) surface have been investigated in the 10 mbar range with low-energy electron microscopy (LEEM) and micro spot low-energy electron diffraction (micro-LEED) as in situ methods. V coverages of θ=0.2 and 0.

Identification of the surface species in electrochemical promotion: ethylene oxidation over a Pt/YSZ catalyst.

The electrochemical promotion of the CH + O total oxidation reaction over a Pt catalyst, interfaced to yttrium stabilized zirconia (YSZ), has been studied at 0.25 mbar and = 650 K using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) as an method. The electrochemical promoter effect is linked to the presence of a several layers thick graphitic overlayer that forms on the Pt surface in the presence of CH.

Stationary stripe patterns and chemical waves on the bimetallic Rh(110)/Ni surface during the H + O reaction.

Chemical wave patterns that develop in the O + H reaction on a bimetallic Rh(110)/Ni surface have been studied with photoelectron emission microscopy (PEEM) in the 10 to 10 mbar range. The bifurcation diagram for Ni coverages up to 3 monolayers (ML) was mapped out for = 770 K. Stationary concentration patterns of macroscopic stripes as well as target patterns and irregular chemical waves were observed.

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 to 10 mbar range.

On the Promotion of Catalytic Reactions by Surface Acoustic Waves.

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis, sensor and nanotechnology. SAWs were shown to cause a strong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However, the promotion mechanism has not been unambiguously identified.

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.

Catalytic CO oxidation on Pt under near ambient pressure: A NAP-LEEM study.

A near ambient pressure low-energy electron microscope (NAP-LEEM) has recently been constructed, that allows in situ imaging of surfaces up to a pressure of 10 mbar. Here we report on pattern formation in catalytic CO oxidation on a Pt(110) single crystal surface and on a polycrystalline Pt foil in the 10 mbar range, operating the microscope in the mirror electron microscopy (MEM) and in the LEEM mode. Excitations localized at structural defects and spiral wave fragments have been observed.

Chemical waves in the O + H reaction on a Rh(111) surface alloyed with nickel. II. Photoelectron spectroscopy and microscopy.

Chemical waves in the H + O reaction on a Rh(111) surface alloyed with Ni [Θ < 1.5 monolayers (ML)] have been investigated in the 10 and 10 mbar range at T = 773 K using scanning photoelectron microscopy and x-ray photoelectron spectroscopy as in situ methods. The local intensity variations of the O 1s and the Ni 2p signal display an anticorrelated behavior.

Chemical waves in the O + H reaction on a Rh(111) surface alloyed with nickel. I. Photoelectron emission microscopy.

Chemical waves that arise in the H + O reaction on a bimetallic Rh(111)/Ni surface have been studied in the 10 and 10 mbar range at T = 773 K with photoelectron emission microscopy (PEEM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Nickel coverages of 0.3, 0.

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 covered Rh(111) surface (θ = 0.3 MLE) in the 10 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.

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.

Island Ripening via a Polymerization-Depolymerization Mechanism.

In catalytic methanol oxidation on ultrathin vanadium oxide layers on Rh(111) (Θ_{V}≈0.2 monolayer equivalent) we observe a 2D ripening of the VO_{x} islands that is controlled by the catalytic reaction. Neighboring VO_{x} islands move under reaction conditions towards each other and coalesce.

Excitability in the H₂+O₂ reaction on a Rh(110) surface induced by high coverages of coadsorbed potassium.

By means of photoemission electron microscopy as spatially resolving method, the effect of high coverages of coadsorbed potassium (0.16 ≤ θ(K) ≤ 0.21) on the dynamical behavior of the H2 + O2 reaction over a Rh(110) surface was investigated.

Origin of non-Faradayicity in electrochemical promotion of catalytic ethylene oxidation.

The electrochemical promotion of catalytic C(2)H(4) oxidation has been investigated under low pressure conditions (p approximately 10(-6)-10(-4) mbar) with a Pt film on yttrium stabilized zirconia (YSZ) as catalyst. All measurements were conducted with a UHV system with a differentially pumped quadruple mass spectrometer (QMS) for rate measurements and a photoelectron emission microscope (PEEM) for spatially resolved measurements. A pronounced rate hysteresis upon cyclic variation of p(C(2)H(4)) was observed under open circuit conditions which is attributed to carbonaceous CH(x) adlayer inhibiting O(2) adsorption and hence poisoning the reaction.

The electrochemical promotion of ethylene oxidation at a Pt/YSZ catalyst.

The electrochemical promotion of the C(2)H(4)+O(2) reaction to form CO(2) and H(2)O is studied in the 10(-5) and 10(-4) mbar range with a Pt catalyst interfaced as working electrode to yttrium-stabilized zirconia (YSZ). Photoemission electron microscopy (PEEM) was used as spatially resolving method. Under open-circuit conditions, that is, without an external voltage applied, the CO(2) production exhibits a pronounced hysteresis upon cyclic variation of p(C(2)H(4)).

Nucleation of spiral wave patterns at surface defects.

The nucleation of spiral waves at a surface defect during catalytic CO oxidation on Pt(110) has been studied with a low energy electron microscope system. It is found that reaction fronts originate from a boundary layer between the defect and the surrounding Pt(110) area. The findings are corroborated by numerical simulations within a realistic reaction-diffusion model of the surface reaction.