184 research outputs found
Quasi-Stationary Distributions for Models of Heterogeneous Catalysis
We construct the quasi-stationary (QS) distribution for two models of
heterogeneous catalysis having two absorbing states: the ZGB model for the
oxidation of CO, and a version with noninstantaneous reactions. Using a
mean-field-like approximation, we study the quasi-stationary surface coverages,
moment ratios and the lifetime of the QS state. We also derive an improved,
consistent one-site mean-field theory for the ZGB model.Comment: 18 pages, 13 figure
New type of metal ion source: Surface diffusion Li<sup>+</sup> ion source
A surface diffusion metal ion source, a new type of metal ion source, is explored. In this device a field desorption process is used to achieve an almost monoenergetic continuous flux of Li+ ions from a [111]âoriented W field emitter. Earlier difficulties with the continuous supply of adatoms, required to produce measurable desorption rates, were overcome by making use of solid state surface diffusion from the Li multilayer reservoir at the shank of the field emitter. The high density of the ion beam (an ion current of 10â12 A was achieved from a W trimer), the extremely narrow energy distribution (full width at halfâmaximum of 0.25 eV) and the stable geometric form of the emitter itself during the operation are advantages of the new ion source which may be important in different areas of nanotechnology
Interaction of CO and O<sub>2</sub> with Pt Studied by Field Ion Appearance Energy Spectroscopy
Using field ion appearance energy spectroscopy we have examined the interaction of CO and 02 with stepped platinum surfaces in the presence of electrostatic fields ranging between 10 and 20 V/nm. Mass-to-charge resolved retarding potential analyses have been carried out for single sites of [001] and [111]-oriented Pt field emitter exposed to a continuous flow of CO and 02. Applying a thermionic cycle, binding energies of molecularly adsorbed CO and O2, were derived from the appearance energies of field desorbed CO+ and O+2 . The data reveal an effect of the high field on the molecule-surface interaction, which is most pronounced for COPt(111) steps. Implications for FIM studies of catalytic CO and H2, oxidation reactions are discussed
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
Field desorption of lithium
Absolute appearance energies of field-desorbed Li+ ions were obtained from mass-to-charge resolved retarding potential analyses of Li+ emitted from the first and second Li layer on W(111). Activation energies for Li+ field desorption were derived from desorption rate measurements. The field-independent binding energy of Li adatoms has been found from field-dependent Li+ appearance and activation energy values, indicating a negligible field-induced charge transfer in the applied field range. We use the cluster embedded in jellium model, based on density-functional theory, to interpret the data by calculating local field enhancements, surface potentials, and activation energies for Li field desorption as a function of field strength and surface coverage as well as geometry
Local Catalytic Ignition during CO Oxidation on Low-Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study
Shedding light on light-off: Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (âlight-offâ) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis
Silicon Oxide Surface Segregation in CO Oxidation on Pd: An in situ PEEM, MS and XPS Study
The effect of silicon oxide surface segregation on the locally-resolved kinetics of the CO oxidation reaction on individual grains of a polycrystalline Pd foil was studied in situ by PEEM, MS and XPS. The silicon oxide formation induced by Si-impurity segregation at oxidizing conditions, was monitored by XPS and its impact on the global and local (spatially resolved) kinetics of the CO oxidation was determined by MS and PEEM. The results reveal a drastic inhibiting effect of silicon oxide on the Pd reactivity towards CO oxidation, manifested both in the collapse of the global CO2 formation rate and in the modified local reactive properties of individual Pd micrograins. The presence of adsorbed oxygen on the Pd surface effectively enhances the silicon segregation to the Pd surface
Fluctuation-Induced Transitions in a Bistable Surface Reaction: Catalytic CO Oxidation on a Pt Field Emitter Tip
Fluctuations which arise in catalytic CO oxidation on a Pt field emitter tip have been studied with field electron microscopy as the imaging method. Fluctuation-driven transitions between the active and the inactive branch of the reaction are found to occur sufficiently close to the bifurcation point, terminating the bistable range. The experimental results are modeled with Monte Carlo simulations of a lattice-gas reaction model incorporating rapid CO diffusion
Single-Particle Catalysis: Revealing Intraparticle Pacemakers in Catalytic H2Oxidation on Rh
Self-sustained oscillations in H2 oxidation on a Rh nanotip mimicking a single catalytic nanoparticle were studied by in situ field emission microscopy (FEM). The observed spatio-Temporal oscillations result from the coupling of subsurface oxide formation/depletion with reaction front propagation. An original sophisticated method for tracking kinetic transition points allowed the identification of local pacemakers, initiating kinetic transitions and the nucleation of reaction fronts, with much higher temporal resolution than conventional processing of FEM video files provides. The pacemakers turned out to be specific surface atomic configurations at the border between strongly corrugated Rh{973} regions and adjacent relatively flat terraces. These structural ensembles are crucial for reactivity: while the corrugated region allows sufficient oxygen incorporation under the Rh surface, the flat terrace provides sufficient hydrogen supply required for the kinetic transition, highlighting the importance of interfacet communication. The experimental observations are complemented by mean-field microkinetic modeling. The insights into the initiation and propagation of kinetic transitions on a single catalytic nanoparticle demonstrate how in situ monitoring of an ongoing reaction on individual nanofacets can single out active configurations, especially when combined with atomically resolving the nanoparticle surface by field ion microscopy (FIM)
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