Spatiotemporal concentration patterns in a surface reaction: Propagating and standing waves, rotating spirals, and turbulence

Abstract

Laterally varying surface concentrations associated with the oscillatory oxidation of carbon monoxide on a Pt(l 10) surface were imaged by photoemission electron microscopy. Depending on the applied conditions, a large variety of spatiotemporal patterns were observed that are characteristic for the nonlinear dynamics of reaction-diffusion systems.PACS numbers: 68.35.-p, 82.65.-i Chemical reactions far from equilibrium may exhibit, even if operated under stationary continuous-flow conditions, various phenomena of temporal and spatial selforganization. Effects belonging in these categories have already been widely investigated, both experimentally and theoretically, with homogeneous reactions in solution, in particular, with the famous Belousov-Zhabotinskii (BZ) reaction. 1 Corresponding studies with heterogeneous reactions occurring at well-defined single-crystal surfaces were initiated only more recently. 2 Among the latter, the catalytic oxidation of CO on a Pt(llO) surface was found to exhibit a very rich variety of oscillatory kinetics. The underlying microscopic mechanism has been explored in great detail, enabling satisfactory theoretical modeling of the temporal behavior. 2,3 Generally, the occurrence of temporal variations of the concentrations of the species involved in the reaction implies that these also vary spatially, unless local differences are eliminated by "stirring," i.e., rapid convection. With the BZ reaction (and other homogeneous reactions in solution), the resulting spatiotemporal concentration patterns can easily be made visible by the associated color differences. More refined techniques are, however, required to image local variations of surface properties. For example, recent attempts to image patterns associated with the CO oxidation reaction include the application of scanning low-energy electron diffraction, 4 as well as scanning photoemission microscopy 5 (SPM) and photoemission electron microscopy (PEEM). 6 The latter technique is particularly versatile due to its high temporal (-10 ms) and lateral (-0.1 j/m) resolution, and a newly designed instrument of this type 7 was applied in the present work to image an unprecedented rich variety of spatiotemporal patterns associated with the oscillatory reaction of catalytic CO oxidation on Pt( 110).The principle of the PEEM technique consists in illuminating the sample surface with UV light from a deuterium lamp (with its cutoff near 6.9-eV photon energy) and to image the lateral intensity distribution of the photoemitted electrons through a system of electrostatic lenses after amplification by a channel plate onto a "back-view" fluorescence screen. The intensity of photoemitted electrons depends on the (local) work function O which in turn is affected by the dipole moments of the adsorbate complexes.In the current investigation, Oof a clean Pt(l 10) surface increases 0.3 eV when saturated with CO, and 0.5 eV through oxygen chemisorption. Therefore, areas covered by O a d appear dark in the images, while those covered by CO a d are brigh...

Macroscopic Synthesis of Self-Assembled Dissipative Structures

“…We have previously suggested 14 that the origin of these repulsions can be explained using ideas from low Reynolds number hydrodynamics. According to our analysis (described in full elsewhere 10 ), the hydrodynamic repulsion F ij j exerted by a disk of radius a j on a disk of radius a i depends on the radii of the disks, the distance d ij between their centers, the rotational speed ω, and the density of the fluid F. The hydrodynamic force is proportional to Fω 2 a j 5 a i 2 /d ij 3 and acts along the direction of d ij and away from disk j.…”

“…Self-assembly [1][2][3][4][5][6][7] in dynamic systems [8][9][10] sthat is, in systems that develop order only when dissipating energyscan lead to multiple final structures; the structure(s) reached are determined by energy fluxes through the system and by the history of its evolution. Dependence of structure on flux and history implies that the products of dynamic self-assembly are, in principle, sensitive to their internal configuration and to external conditions and that dynamic systems are plausible precursors to adaptiVe matter 11,12 sthat is, matter whose structure and properties change autonomously in response to external stimuli.…”

LEEM and PEEM as Probing Tools to Address Questions in Catalysis

“…One clear example is the pioneering work of Jakubith [23] and Rotermund [24] et al on the evolution of spatio-temporal patterns formed on Pt surfaces during the oxidation of carbon monoxide. On Pt surfaces the reaction proceeds via the recombination of CO with O previously formed from the dissociative adsorption of molecular oxygen.…”

Meandering Spiral Waves in the 1,4-Cyclohexanedione Belousov−Zhabotinsky System Catalyzed by Fe[batho(SO3)2]34-/3-

“…[1][2][3] The first spiral waves in a chemical system were observed in the BelousovZhabotinsky (BZ) reaction 4,5 shortly after the discovery of propagating excitation waves in thin layers of the BZ system. 6,7 These intriguing structures are also found in the CO oxidation on platinum surfaces, 8 during the aggregation of the slime mold Dictyostelium discoideum, 9 and as spreading depression waves on chicken retinas. 10 One of the crucial features of spiral waves is the motion of the spiral tip.…”