Spatially resolved measurements of electrochemically induced spillover on porous and microstructured Pt/YSZ catalysts

Pt/YSZ electrodes prepared from platinum (Pt) and yttria-stabilized zirconia (YSZ) can be applied in electrocatalytic experiments for the promotion of different heterogeneously catalyzed reactions. The strong increase in catalytic activity of Pt/YSZ assemblies on electrochemical polarisation has recently been attributed to the creation of catalytically active spill-over species under non-equilibrium conditions. Our experiments combine locally resolved surface sensitive techniques (SPEM, PEEM) with electrochemical methods in order to resolve the processes at the Pt/YSZ interface and to clarify the nature of the spillover species

Electrocatalysis on Pt/YSZ electrodes

Electrochemical polarization of porous metal electrodes on solid electrolytes often leads to an increase of their catalytic activity in heterogeneous reactions. Thin microstructured Pt films on single crystalline YSZ (yttria stabilized zirconia) are investigated as structurally and geometrically well-defined model systems in order to understand the origin of this effect. Photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM) have been applied as spatially resolving methods in situ to study the processes in the vicinity of the three-phase boundary (tpb). Measurements with SPEM show that atomic oxygen is created under anodic polarization and covers the Pt film homogeneously. A specific spillover species is not found, rather the formation of atomic oxygen is detected, which has the same O1s binding energy as chemisorbed oxygen from the gas phase

Fluctuations in catalytic surface reactions

The internal reaction-induced fluctuations which occur in catalytic CO oxidation on a Pt field emitter tip have been studied using field electron microscopy (FEM) as a spatially resolving method. The structurally heterogeneous Pt tip consists of facets of different orientations with nanoscale dimensions. The FEM resolution of roughly 2 nm corresponds to a few hundred reacting adsorbed particles whose variations in the density are imaged as brightness fluctuations. In the bistable range of the reaction one finds fluctuation-induced transitions between the two stable branches of the reaction kinetics. The fluctuations exhibit a behaviour similar to that of an equilibrium phase transition, i.e. the amplitude diverges upon approaching the bifurcation point terminating the bistable range of the reaction. Simulations with a hybrid Monte Carlo/mean-field model reproduce the experimental observations. Fluctuations on different facets are typically uncorrelated but within a single facet a high degree of spatial coherence is found

Tuning excitability by alloying: the Rh(111)/Ni/H2 + O2 system

The dynamic behavior of the O2 + H2 reaction on a Rh(111) surface alloyed with Ni has been studied in the 10(-5) mbar range using photoemission electron microscopy (PEEM) as a spatial resolving method. For T = 773 K and p(O2) = 5 x 10(-5) mbar the bifurcation diagram has been mapped out as a function of the Ni coverage in a range of 0 ML /= 1.3 ML. A critical Ni coverage of Theta(Ni,crit) = 0.13 monolayers (ML) is required for excitability. In the excitable parameter range pulse trains and irregular chemical wave patterns are found. Whereas the propagation speed of the pulses exhibits no clear-cut dependence on the Ni coverage, the frequency of the local PEEM intensity oscillations increases linearly with Ni coverage in the range from Theta(Ni) = 0.13 ML to Theta(Ni) = 1.3 ML.DF

Dynamics of ultrathin V-oxide layers on Rh(111) in catalytic oxidation of ammonia and CO

Catalytic oxidation of ammonia and CO has been studied in the 10(-4) mbar range using a catalyst prepared by depositing ultra-thin vanadium oxide layers on Rh(111) (thetaV approximately 0.2 MLE). Using photoemission electron microscopy (PEEM) as a spatially resolving method, we observe that upon heating in an atmosphere of NH3 and O2 the spatial homogeneity of the VOx layer is removed at 800 K and a pattern consisting of macroscopic stripes develops; at elevated temperatures this pattern transforms into a pattern of circular VOx islands. Under reaction conditions the neighboring VOx islands become attracted by each other and coalesce. Similar processes of pattern formation and island coalescence are observed in catalytic CO oxidation. Reoxidation of the reduced VOx catalyst proceeds via surface diffusion of oxygen adsorbed onto Rh(111). A pattern consisting of macroscopic circular VOx islands can also be obtained by heating a Rh(111)/VOx catalyst in pure O2

Catalytic CO Oxidation on Nanoscale Pt Facets: Effect of Inter-Facet CO Diffusion on Bifurcation and Fluctuation Behavior

We present lattice-gas modeling of the steady-state behavior in CO oxidation on the facets of nanoscale metal clusters, with coupling via inter-facet CO diffusion. The model incorporates the key aspects of reaction process, such as rapid CO mobility within each facet, and strong nearest-neighbor repulsion between adsorbed O. The former justifies our use a "hybrid" simulation approach treating the CO coverage as a mean-field parameter. For an isolated facet, there is one bistable region where the system can exist in either a reactive state (with high oxygen coverage) or a (nearly CO-poisoned) inactive state. Diffusion between two facets is shown to induce complex multistability in the steady states of the system. The bifurcation diagram exhibits two regions with bistabilities due to the difference between adsorption properties of the facets. We explore the role of enhanced fluctuations in the proximity of a cusp bifurcation point associated with one facet in producing transitions between stable states on that facet, as well as their influence on fluctuations on the other facet. The results are expected to shed more light on the reaction kinetics for supported catalysts.Comment: 22 pages, RevTeX, to appear in Phys. Rev. E, 6 figures (eps format) are available at http://www.physik.tu-muenchen.de/~natali

Surface Structure and Catalytic COCO Oxidation Oscillations

A cellular automaton model is used to describe the dynamics of the catalytic oxidation of $CO$ on a $Pt(100)$ surface. The cellular automaton rules account for the structural phase transformations of the $Pt$ substrate, the reaction kinetics of the adsorbed phase and diffusion of adsorbed species. The model is used to explore the spatial structure that underlies the global oscillations observed in some parameter regimes. The spatiotemporal dynamics varies significantly within the oscillatory regime and depends on the harmonic or relaxational character of the global oscillations. Diffusion of adsorbed $CO$ plays an important role in the synchronization of the patterns on the substrate and this effect is also studied.Comment: Latex file with six postscript figures. To appear in Physica

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.Fil: Rafti, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina. Leibniz-Universität Hannover; AlemaniaFil: Borkenhagen, Benjamin. Technische Universität Clausthal; AlemaniaFil: Lilenkamp, Gerhard. Technische Universität Clausthal; AlemaniaFil: Lovis, Florian. Leibniz-Universität Hannover; AlemaniaFil: Smolinsky, Tim. Leibniz-Universität Hannover; AlemaniaFil: Imbihl, Roland. Leibniz-Universität Hannover; Alemani

Homogeneous and Heterogeneous Front Nucleation in a Bistable Surface Reaction: The NO + CO Reaction on a Cylindrical Pt Crystal

Under continuous flow conditions the oscillatory NO + CO reaction on Pt(100) exhibits bistability in the temperature range below 440 K. In this range, an inactive stationary branch coexists with a reactive oscillatory branch. The transition from the inactive branch, which is only metastable, to the truly stable active branch proceeds via propagating reaction fronts. The associated nucleation process has been investigated in the 10-6 mbar range using photoemission electron microscopy (PEEM) as a spatially resolving method. A Pt sample of cylindrical shape whose surface comprises the highly active orientational range between (100) and (3 10) was used as a catalyst. Depending on the degree of supersaturation, both heterogeneous and homogeneous nucleation have been found. Homogeneous nucleation manifests itself in the occurrence of a so-called “surface explosion” at Tc = 440 K. Below this temperature, heterogeneous nucleation occurs. Depending on the preparation of the inactive state, we observe two different nucleation scenarios which we can tentatively assign to two different kinds of defects

The structure of atomic nitrogen adsorbed on Fe(100)

Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 × 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED I–V spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Å above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Å). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe4N, thus supporting the view of a “surface nitride” and providing a consistent picture of the structural and bonding properties of this surface phase