Pattern Formation during the Oxidation of CO onPt{100}: A Mesoscopic Model

Abstract

Constantly changing irregular patterns of carbon monoxide (CO) and oxygen are seen during CO oxidation on platinum crystals in the [100] orientation. Ours is the first reaction-diffusion model to reproduce this pattern formation on physically feasible length and time scales, faithfully incorporating the available experimental data. Numerical simulations show patterns made up of CO and oxygen fronts moving at similar speeds to those seen in experiments [4][5][6].PACS numbers: 68.43. Bc,68.47.De,82.40.Ck Spatiotemporal pattern formation occurs in a number of catalytic reactions, such as the Belousov-Zhabotinsky reaction where oscillating spirals and targets are seen [1]. Similar structures form during the catalytic oxidation of carbon monoxide on the surface of single platinum crystals, where patterns comprise areas of different surface phase or covered by different adsorbates. During the reaction oxygen and CO are adsorbed onto the platinum surface. At high enough temperatures and for certain orientations of the surface relative to the bulk crystal, the presence of adsorbates leads to a phase change, i.e. a rearrangement of the surface platinum atoms. When the oxygen and CO react, carbon dioxide gas is released and the surface reverts to its original configuration. This cycle can lead to kinetic oscillations in the surface phase. If communication between different areas on the crystal is very rapid compared with the rate of oscillation, the whole surface oscillates in phase; otherwise there can be phase lags across the surface and spatial patterns arise [2]. There are two main spatial coupling mechanisms: diffusion of CO across the surface, important at low pressures, and global coupling through the gas phase, which dominates at high pressures. The surface orientation has a decisive effect on pattern formation: in the [111] orientation there is no phase change and hence no patterns, on Pt{110} classical spirals and targets are seen (e.g. [3]), while on Pt{100} the patterns are typically more irregular [4-6], though circular wavefronts are also seen [5].In this Letter, we model pattern formation in CO oxidation on a single platinum crystal in the [100] orientation. The kinetic mechanism for this reaction is well established and has been modelled extensively [7][8][9]. Spatial modelling has concentrated on Pt{110} using a simplified representation of the kinetic mechanism, extended to include diffusion terms (and sometimes gas global coupling) [10]. These models produce results that qualitatively resemble experimentally observed patterns, but they cannot be used for detailed quantitative comparison since both the kinetic and diffusion mechanisms are simplified. Monte Carlo methods are also used, but at present these must assume unrealistically low diffusion coefficients in order to allow pattern formation on computationally accessible length and time scales [11]. We present a mesoscopic spatial model based on the detailed kinetic mechanism [9] together with diffusion terms derived from a careful consideration of...

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