Catalysis on microstructured surfaces: Pattern formation during CO oxidation in complex Pt domains

Irregular excitation patterns in reaction-diffusion systems due to perturbation by secondary pacemakers

Spatiotemporal excitation patterns in the FitzHugh-Nagumo model are studied, which result from the disturbance of a primary pacemaker by a secondary pacemaker. The primary and secondary pacemakers generate regular waves with frequencies f(pace) and f(pert), respectively. The pacemakers are spatially separated, but waves emanating from them encounter each other via a small bridge. This leads to three different types I-III of irregular excitation patterns in disjunct domains of the f(pace)-f(pert) plane. Types I and II are caused by detachments of waves coming from the two pacemakers at corners of the bridge. Type III irregularities are confined to a boundary region of the system and originate from a partial penetration of the primary waves into a space, where circular wave fronts from the secondary pacemaker prevail. For this type, local frequencies can significantly exceed f(pace) and f(pert). The degree of irregularity found for the three different types is quantified by the entropy of the local frequency distribution and an order parameter for phase coherence.

Optical imaging of pattern formation: reflection anisotropy microscopy applied to globally coupled oscillatory CO-oxidation

A reflection anisotropy microscope (RAM) creates the contrast in its images from a change in the polarization orientation of reflected light from a surface. This may stem from local variations of the reconstruction of a surface, being initiated by changes in an adsorption layer. The advantages and disadvantages of a recently improved RAM versus other optical imaging techniques are discussed. We demonstrate the unique features of RAM and present the first experimental findings of so called 2pi phase kinks in the globally coupled CO-oxidation on Pt(110).

Periodic heterogeneity-driven resonance amplification in density fingering

Periodic heterogeneity is introduced in experiments with thin solution layers where downward propagating planar autocatalytic fronts are hydrodynamically unstable and cellular patterns develop. The evolution of fingers is greatly affected by the spatial heterogeneity when the wave number associated with it falls in the vicinity of the most unstable mode of the reference system with uniform thickness. The imposed heterogeneity will drive the instability by amplifying the modes with the matching wave numbers as indicated by the experimentally constructed dispersion curves.

Guiding chemical pulses through geometry: Y junctions

We study computationally and experimentally the propagation of chemical pulses in complex geometries. The reaction of interest, CO oxidation, takes place on single crystal Pt(110) surfaces that are microlithographically patterned; they are also addressable through a focused laser beam, manipulated through galvanometer mirrors, capable of locally altering the crystal temperature and thus affecting pulse propagation. We focus on sudden changes in the domain shape (corners in a Y-junction geometry) that can affect the pulse dynamics; we also show how brief, localized temperature perturbations can be used to control reactive pulse propagation. The computational results are corroborated through experimental studies in which the pulses are visualized using reflection anisotropy microscopy.

Spatiotemporal oscillations and clustering in the Ziff-Gulari-Barshad model with surface reconstruction

We study the dynamics of the Ziff-Gulari-Barshad (ZGB) model on square (sq) and hexagonal-honeycomb (hex) lattices and when surface restructuring is introduced. We show that the ZGB model exhibits nonequilibrium phase transitions on the hex lattice similar to the ones already observed on the sq lattice, but the critical values of the kinetic parameters depend crucially on the substrate type. If surface reconstruction (sq<-->hex) is assumed for high lattice coverage of one of the reactive species then persistent spatiotemporal oscillations and clustering of homologous species are observed for kinetic parameter values 0.348<k1<0.393.

Voting and catalytic processes with inhomogeneities

We consider the dynamics of the voter model and of the monomer-monomer catalytic process in the presence of many "competing" inhomogeneities and show, through exact calculations and numerical simulations, that their presence results in a non-trivial fluctuating steady state whose properties are studied and turn out to specifically depend on the dimensionality of the system, the strength of the inhomogeneities, and their separating distances. In fact, in arbitrary dimensions, we obtain an exact (yet formal) expression of the order parameters (magnetization and concentration of adsorbed particles) in the presence of an arbitrary number n of inhomogeneities ("zealots" in the voter language) and formal similarities with suitable electrostatic systems are pointed out. In the non-trivial cases n = 1,2, we explicitly compute the static and long-time properties of the order parameters and therefore capture the generic features of the systems. When n > 2 , the problems are studied through numerical simulations. In one spatial dimension, we also compute the expressions of the stationary order parameters in the completely disordered case, where n is arbitrary large. Particular attention is paid to the spatial dependence of the stationary order parameters and formal connections with electrostatics.

Damped oscillation in the NO+CO/Pt(100) reaction system

Considering the influence of the surface defects formed in the surface restructuring phase transition in the CO+NO/Pt(100) reaction system, we propose a lattice gas model to investigate the damped oscillation in the high-temperature oscillatory regime by means of a Monte Carlo simulation. The simulation results show that the persistent oscillation can change into a damped one when the fraction of the defects increases. The production rate of CO2 is near to the maximum value when the oscillation is damped to the end. Furthermore, it is found, in the early stage of the oscillation, that the NO decomposition mainly occurs in the 1 x 1 phase and the hex phase is inactive for the reaction. However, as the reaction proceeds, defects are gradually formed in the 1 x 1 <==> hex phase transition, the hex phase becomes active and dominative for the NO decomposition, and then the oscillation becomes damping. The simulation results give an explanation for some previous experimental phenomena.

Identification of nonlinear spatiotemporal systems via partitioned filtering

The problem of identifying continuous spatiotemporal nonlinear systems from noisy and indirect observations is determined by its computational complexity. We propose a solution by means of nonlinear state space filtering along with a state partition technique. The method is demonstrated to be computationally feasible for spatiotemporal data with properties that occur typically in experimental recordings. It is applied to one component of the simulated chaotic data of a two-component reaction diffusion system, yielding estimates of both the unobserved state component and the diffusion constant.

Hysteresis phenomena in CO catalytic oxidation system in the presence of inhomogeneities of the catalyst surface

The hysteresis phenomena in the CO catalytic oxidation system are studied by Monte Carlo simulation in the presence of the inhomogeneities of the catalyst surface. We show that the O-passivated state is destroyed due to the inhomogeneities of the surface, in contrast to the classical Ziff-Gulari-Barshad model. The defects on the surface have a significant effect on the hysteresis transition points. Most importantly, the supercritical nucleation and growth of the O adatom island during the transition from a low reactivity to a high reactivity states are closely related to the inhomogeneities of the catalyst surface. It is shown that the width of the hysteresis loop shrinks as the scan rate beta(CO) of y(CO) (the fraction of CO in gas phase) decreases, but there exists a finite width of the hysteresis loop even if beta(CO) becomes infinitely small. On the other hand, the width of the hysteresis loop decreases with decreasing the diffusion rate, and even the hysteresis loop may disappear for a slow diffusion. These simulation results are in good consistency with the previous relevant experimental results.

Reconstruction and roughening of a catalytic Pt(110) surface coupled to kinetic oscillations

Three-dimensional reconstruction and roughening of a Pt(110) surface is studied with the help of a qualitative Monte Carlo model. A distinct CO adsorption uptake on different surface phases is taken into account. The computations show that a "missing row" structure with defects relaxes to a more stable (111)-faceted structure. The CO+O2 reaction kinetics is modeled by a phenomenological equation with a cubic nonlinearity reproducing a correct qualitative picture of oscillations. The surface roughening developing under the reaction conditions causes slow changes in catalytic activity of the surface. A nanoscale front between the 1x1 and 1x2 phases disintegrates due to repeated phase transitions caused by CO coverage oscillations. Defect formation and roughening dominate the dynamics of surface phase transitions. A one-dimensional extension of the model reproduces microscopic traveling waves on the CO diffusion scale.

Pattern formation on anisotropic and heterogeneous catalytic surfaces

We review experimental and theoretical work addressing pattern formation on anisotropic and heterogeneous catalytic surfaces. These systems are typically modeled by reaction-diffusion equations reflecting the kinetics and transport of the involved chemical species. Here, we demonstrate the influence of anisotropy and heterogeneity in a simplified model, the FitzHugh-Nagumo equations. Anisotropy causes stratification of labyrinthine patterns and spiral defect chaos in bistable media. For heterogeneous media, we study the situation where the heterogeneity appears on a length scale shorter than the typical pattern length scale. Homogenization, i.e., computation of effective medium properties, is applied to an example and illustrated with simulations in one (fronts) and two dimensions (spirals). We conclude with a discussion of open questions and promising directions that comprise the coupling of the microscopic structure of the surface to the macroscopic concentration patterns and the fabrication of nanostructures with heterogeneous surfaces as templates. (c) 2002 American Institute of Physics.

Catalysis on microstructured bimetallic surfaces

Microstructured bimetallic Pt/Rh and Pt/Ti surfaces have been employed to study the dynamics of catalytic NO reduction and the O(2)+H(2) reaction at low pressure (p<10(-3) mbar). Photoelectron emission microscopy and scanning photoelectron microscopy were used as spatially resolved in situ methods to image the local work function changes and to identify chemical changes in the substrate and in the adsorbate layer. It is shown that diffusional coupling leads to dynamic effects which are dependent on the macroscopic size (&mgr;m range). With alkali metals on the surface, stationary patterns form whose mechanism of formation has been studied in detail. (c) 2002 American Institute of Physics.

Front initiation on microdesigned composite catalysts

We first briefly review the subject of spatiotemporal pattern formation on microdesigned composite catalysts. One of the most significant interaction mechanisms between different reacting domains (consisting of different metal catalysts such as Pt and Rh, coupled through surface diffusion) is the initiation of reaction fronts at the interface between them. We then explore in some detail the effect of two-dimensional composite geometry on this basic building block of composite catalyst dynamics. (c) 2002 American Institute of Physics.

Self-organized nanostructures in surface chemical reactions: Mechanisms and mesoscopic modeling

Nanoscale patterns can form in reactive adsorbates on catalytic surfaces as a result of attractive lateral interactions. These structures can be described within a mesoscopic theory that is derived by coarse graining the microscopic master equation thus providing a link between microscopic lattice models and reaction-diffusion equations. Such mesoscopic models allow to systematically investigate mechanisms responsible for the formation of nanoscale nonequilibrium patterns in reactive condensed matter. We have found that stationary and traveling nanostructures may result from the interplay of the attractive lateral interactions and nonequilibrium reactions. Besides reviewing these results, a detailed investigation of a single reactive adsorbate in the presence of attractive lateral interactions and global coupling through the gas phase is presented. Finally, it is outlined how a mesoscopic theory should be constructed for a particular scanning tunneling microscopy experiment [the oxidation of hydrogen on a Pt(111) surface] in order to overcome the failure of a corresponding reaction-diffusion model to quantitatively reproduce the experiments. (c) 2002 American Institute of Physics.

Spatiotemporal addressing of surface activity

We have modified surface catalytic activity in real time and space by focusing an addressable laser beam to differentially heat a platinum (110) single-crystal surface. Ellipsomicroscopy imaging of local conditions (such as reactant and product local coverages) enabled us to close the loop between sensing and actuation (both spatiotemporally resolved). Pulses and fronts, the basic building blocks of patterns, could be formed, accelerated, modified, guided, and destroyed at will. Real-time image processing and feedback allow the design and implementation of new classes of nonlocal evolution rules.