Surface Structure and Catalytic COCO Oxidation Oscillations

A cellular automaton model is used to describe the dynamics of the catalytic oxidation of $CO$ on a $Pt(100)$ surface. The cellular automaton rules account for the structural phase transformations of the $Pt$ substrate, the reaction kinetics of the adsorbed phase and diffusion of adsorbed species. The model is used to explore the spatial structure that underlies the global oscillations observed in some parameter regimes. The spatiotemporal dynamics varies significantly within the oscillatory regime and depends on the harmonic or relaxational character of the global oscillations. Diffusion of adsorbed $CO$ plays an important role in the synchronization of the patterns on the substrate and this effect is also studied.Comment: Latex file with six postscript figures. To appear in Physica

Spiral waves in a surface reaction : model calculations

A systematic study of spiral waves in a realistic reaction-diffusion model describing the isothermal CO oxidation on Pt(110) is carried out. Spirals exist under oscillatory, excitable, and bistable (doubly metastable) conditions. In the excitable region, two separate meandering transitions occur, both when the time scales become strongly different and when they become comparable. By the assumption of surface defects of the order of 10 µm, to which the spirals can be pinned, the continuous distribution of wavelengths observed experimentally can be explained. An external periodic perturbation generally causes a meandering motion of a free spiral, while a straight drift results, if the period of the perturbation divided by the rotation period is a natural number

Chemical waves in media with state-dependent anisotropy

In the reduction of NO with H2 on a Rh(110) surface rectangularly shaped target patterns and spirals with sharp corners have been observed. These patterns can be reproduced with a simple model assuming that the (anisotropic) diffusion is state dependent. Such a dependence is realized in the system Rh(110)/NO + H2 by the presence of different adsorbate-induced reconstructions with varying substrate geometries

Complex electrooxidation of formic acid on palladium

Herein, oscillatory formic acid (FA) electrooxidation on polycrystalline palladium is investigated and compared with the one on polycrystalline platinum; major differences between both are attributed to differences on the kinetics of sub-set chemical network as well as to preferential routes admitted on palladium surface. To presume the kinetic rate of poison accumulation on palladium, FA oxidation was accomplished in presence of occluded hydrogen and hydrogen-free electrodes. The preferential routes were presumed from the temporal pattern. Markedly, oscillations during FA electrooxidation have minimum potential at 0.2 V, which is linked to the fast rate of direct dehydrogenation; moreover, it has one of the largest induction period (ca. 60 min) and oscillatory period (20 min) observed in electrochemistry, since subsurface hydrogen slows down the rate of CO accumulation on the surface

On the origin of oscillations in the electrocatalytic oxidation of HCOOH on a Pt electrode modified by Bi deposition

We report experimental observations on the temporal dynamics in the electrocatalytic oxidation of formic acid (HCOOH) on a polycrystalline Pt electrode modified by deposition of bismuth. Bismuth modification significantly enhanced the current density of HCOOH oxidation, since it suppressed the poisoning branch and increased the apparent direct oxidation rate. Impedance spectroscopy and the galvanostatic scan in HCOOH oxidation on Bi/Pt exhibited a hidden negative differential resistance and a Hopf bifurcation. The electrocatalytic oxidation of HCOOH on Bi/Pt spontaneously underwent transitions from homogeneous catalytic activity to spatiotemporally inhomogeneous distributions of the interfacial electrode potential, in the form of traveling pulses of the interfacial potential

Autocatalysis in the open circuit interaction of alcohol molecules with oxidized Pt surfaces

We studied the open circuit interaction of methanol and ethanol with oxidized platinum electrodes using in situ infrared spectroscopy. For methanol, it was found that formic acid is the main species formed in the initial region of the transient and that the steep decrease of the open circuit potential coincides with an explosive increase in the CO(2) production, which is followed by an increase in the coverage of adsorbed CO. For ethanol, acetaldehyde was the main product detected and only traces of dissolved CO(2) and adsorbed CO were found after the steep potential decay. In both cases, the transients were interpreted in terms of (a) the emergence of sub-surface oxygen in the beginning of the transient, where the oxide content is high, and (b) the autocatalytic production of free platinum sites for lower oxide content during the steep decay of the open circuit potential.Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[04/04528-0]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[07/01575-6]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[06/01088-5]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)[450160/2008-4]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)[302698/2007-8

Spatiotemporal mixed-mode oscillations on a ring electrode

We present spatiotemporal mixed-mode oscillations in the electrocatalytic oxidation of formic acid on a platinum ring electrode modified by bismuth deposition. On the anodic scan, we observed spontaneous reversal of direction of a rotating activation pulse occurring repeatedly at the same location of the ring. In the constant potential method, spatiotemporal mixed-mode oscillations of pulse reversal were shown to constitute a fraction of an ordered Farey sequence

Mechanism and model of the oscillatory electro-oxidation of methanol

A mechanism for the kinetic instabilities observed in the galvanostatic electro-oxidation of methanol is suggested and a model developed. The model is investigated using stoichiometric network analysis as well as concepts from algebraic geometry (polynomial rings and ideal theory) revealing the occurrence of a Hopf and a saddle-node bifurcation. These analytical solutions are confirmed by numerical integration of the system of differential equations

Existence regions of spatiotemporal patterns in the electro-oxidation of formic acid

Experimental observations of different spatiotemporal patterns of the interfacial potential on a bismuth modified platinum ring working electrode (WE) with different positions of the reference electrode (RE) in the electrocatalytic oxidation of formic acid are presented. For small distances between WE and RE, inhomogeneous patterns (e.g., standing waves and rotating pulses) are obtained, while synchronized oscillations (i.e., in-phase active or passive oscillations of interfacial potential) take over at larger distances between WE and RE. The results can be rationalized in terms of the geometry dependence of the non-local migration coupling