Diffusive Propagation of Chemical Waves through a Microgap

We study the effect of microdesigned composite geometries on pattern formation during the catalytic oxidation of CO on Pt-Ti, Pt-Rh, and Pt-Pd composite catalysts. In particular, we find experimentally (and rationalize through modeling) that adsorbate surface transport through the second (active) component drastically affects the shapes and interactions of concentration patterns (traveling pulses) observed on pure Pt. DOI: 10.1103/PhysRevLett.86.6038 PACS numbers: 82.40.Bj, 05.45.-a, 82.40.Np, 82.45.Jn The use, over the last ten years, of spatially resolving techniques such as photoemission electron microscopy (PEEM) [1], and others [2,3], has allowed the real-time, in situ observation of spatiotemporal pattern formation on catalytic surfaces. These observations have important implications for the understanding of the transport and reaction mechanisms underlying catalytic reactions. At the same time, the generic features of the instabilities and bifurcations involved have made catalytic reactions a fruitful model medium for the study of such phenomena, leading to insights for pattern forming systems at large. A recent promising research avenue is the use of microfabrication techniques to construct microreacting domains with controlled shapes and sizes. The boundaries of these domains can be inert: in our case 2000 Å "tall" Ti walls, presumably oxidizing to TiO 2 under reaction conditions, which can be thought of as no-flux, inert, insulating boundaries. Shape, size, and dimensionality effects have been systematically studied on such microdesigned, inert boundary surfaces [4,5]. More recently, active boundaries are being constructed: the domains that surround the active Pt catalyst are no more inert, tall walls; they are, instead, covered by relatively thin layers (50-300 Å height) of another catalyst (e.g., Pd, Rh) [6][7][8]. The motivation behind the fabrication of these composite catalysts is the desire to combine different catalytic activities through surface transport (modulated through geometrical design) to improve the overall catalyst activity/selectivity [9,10].In this paper we study a crucial intermediate step of this program: an active, pattern forming catalyst [Pt(110) in the excitable regime], surrounded by an active, nonpattern forming catalyst (in our case Rh or Pd). Related studies with a passive "just diffusion, no reaction" companion medium have recently been reported [11,12].The evaporated, active, nonpattern forming catalyst effectively serves as an alternative supply for some of the reactants to the active, pattern forming one (the Pt). This additional source of reactants by surface diffusion through the interface boundary can drastically affect the shape, interactions, and speed of the patterns (pulses) on the Pt. The effect can be tuned through design of the composite geometry and the choice of its metal components.We study CO oxidation in the 400-480 K temperature range for partial pressure of oxygen of 4 3 10 24 mbar and for CO pressures in the 10 25 mbar range. The surfaces we emplo...

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“…Related studies with a passive "just diffusion, no reaction" companion medium have recently been reported [11,12].…”

mentioning

confidence: 88%

Formation of Two-Dimensional Concentration Pulses on Microdesigned Composite Catalyst Surfaces

et al. 2001

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“…Fig. 12) can be applied in a controlled pattern through ink-jet printing [197] or photolithography [198]. Imposing rigid spatial structures on the pseudo-planar BZ medium opened the path to a new range of chemical processors.…”

Section: Excitable Chemical Mediamentioning

confidence: 99%

Molecular Information Technology

Zauner

2005

Critical Reviews in Solid State and Materials Sciences

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ABSTRACT:Molecular materials are endowed with unique properties of unrivaled potential for high density integration of computing systems. Present applications of molecules range from organic semiconductor materials for low-cost circuits to genetically modified proteins for commercial imaging equipment. To fully realize the potential of molecules in computation, information processing concepts that relinquish narrow prescriptive control over elementary structures and functions are needed, and self-organizing architectures have to be developed. Investigations into qualitatively new concepts of information processing are underway in the areas of reaction-diffusion computing, self-assembly computing, and conformation-based computing. Molecular computing is best considered not as competitor for conventional computing, but as an opportunity for new applications. Microrobotics and bioimmersive computing are among the domains likely to benefit from advances in molecular computing. Progress will depend on both novel computing concepts and innovations in materials. This article reviews current directions in the use of bulk and single molecules for information processing. KEY WORDS:polymer electronics, molecular switches, self-assembly computing, conformation-based computation, self-organizing materials, bioimmersive computing

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“…31 Moreover, experiments with waves exiting from thin channels into large systems have proven to be a valuable tool for the analysis of reaction fronts. [32][33][34] These experiments yield the critical (i.e., minimal) radius of a superthreshold perturbation required for successful wave initiation. The existence of such a critical radius is well established for reaction-diffusion systems.…”

Section: Introductionmentioning

confidence: 96%

Electrochemical Waves on Patterned Surfaces: Propagation through Narrow Gaps and Channels

2001

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The dissolution and corrosion of iron and certain types of steel can involve the propagation of electrochemical waves. Low-carbon steel plates covered by nitric acid solutions self-organize pseudo-two-dimensional wave patterns which share many features with reaction-diffusion waves in excitable systems. Here, we investigate the dynamics of these electrodissolution waves in the presence of geometrical constraints. A simple procedure for the preparation of insulating, millimeter-scale obstacles is presented, which allows the controlled, local protection of the metal surface. This method is used in the measurement of the material consumption rate per wave [(850 ( 50) nm/wave or 6-7 g/(m 2 wave)] as well as for the study of front propagation through narrow gaps and long channels. In channels, the wave velocity decreases with decreasing width from 10 to 4 mm/s. Propagation failures occur within the channel if its width is smaller than 0.2-0.4 mm. These results indicate that the obstacles have sink-like boundaries. For the estimation of the critical nucleation size in this system, we prepared short and narrow gaps that induce wave blockage for openings smaller than 0.65 ( 0.1 mm.

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Diffusive Propagation of Chemical Waves through a Microgap

Cited by 18 publications

References 23 publications

Formation of Two-Dimensional Concentration Pulses on Microdesigned Composite Catalyst Surfaces

Formation of Two-Dimensional Concentration Pulses on Microdesigned Composite Catalyst Surfaces

Molecular Information Technology

Electrochemical Waves on Patterned Surfaces: Propagation through Narrow Gaps and Channels

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