Reaction hysteresis of the CO + O --> CO2 reaction on palladium(111).

Rate measurements of the reaction CO + O --> CO(2) on palladium(111) single crystal surfaces have been performed by means of mass spectroscopy under ultrahigh vacuum conditions. The total flux Phi of the impinging reactants CO and O(2) was held constant at 1 ML s(-1), whereas its CO fraction Y was varied between 0 (pure O(2)) and 1 (pure CO). The measurements have been performed for surface temperatures between 370 and 510 K and with a wide range of sampling times, evaluating the system parameter range for bistable behavior. Long-time measurements lasting several days proved the bistable behavior to result from two stable states rather than from slow processes not visible on usual experimental time scales. Pulselike modulations of the feed gas composition revealed the mechanisms confining the experimentally observed bistable range: the high CO fraction border of the bistability is given by the equistability condition of both states, whereas the other border is found to be associated with a saddle-node bifurcation in the corresponding system of reaction diffusion equations.

Noise-induced spatiotemporal patterns in a bistable reaction-diffusion system: photoelectron emission microscopy experiments and modeling of the oxidation reaction on Ir(111).

We use photoelectron emission microscopy (PEEM) measurements to study the spatiotemporal patterns obtained for the CO oxidation reaction on Ir(111) as a function of the noise strength we superpose on the CO and the oxygen fractions of the constant total reactant gas flux. The investigations are focused on the bistable regime this reaction displays including its monostable vicinity. Simultaneously we analyze numerically the underlying reaction-diffusion (RD) equations in two spatial dimensions. For intrinsic and/or small strength of the external noise we find transitions from the locally stable to the globally stable branch via slow nucleation and growth of islands of the globally stable state: oxygen or CO, respectively. With increasing noise strength the number of islands as well as their growth rate increases. These phenomena are very well reproduced by numerical calculations of the RD model. For sufficiently large noise strength we observe bursts from CO rich to oxygen rich and back as well as switching between the two states. While such phenomena are also obtained from the model calculations, their experimentally observed spatial scales were not satisfactorily reproduced using the same approach as for the lower noise strengths.

Spatiotemporal patterns of external noise-induced transitions in a bistable reaction-diffusion system: photoelectron emission microscopy experiments and modeling.

The rate of CO oxidation on Ir(111) surfaces exhibits bistability at T=500 K in a range of the CO fraction Y in the CO+O reactant gas flux. Measured CO2 rates as a function of the noise strength imposed on Y are well reproduced by parameter-free modeling. We present photoelectron emission microscopy measurements and 2D calculations of the spatiotemporal patterns of CO- and O-rich domains. The role of combined multiplicative and additive noise on Y for CO and O domain wall motion and island nucleation-growth-coalescence processes is analyzed.

External noise imposed on the reaction-diffusion system CO+O2-->CO2 on Ir(111) surfaces: experiment and theory.

We study experimentally and theoretically the influence of noise on the fractions of CO and oxygen in the constant gas flow directed at an Ir(111) surface during CO oxidation. Depending on the noise strength and the fraction Y of CO we observe in the deterministically bistable region a large variety of different types of behavior. These include bistable behavior for small noise intensities, transitions from the upper to the lower branch of the bistable loop and vice versa, island nucleation and growth and noise-induced switching. Near the boundary of the bistable region and in the presence of noise the transition between the two branches takes place via very slow domain wall motion with time scales of the order of 10(4)-10(5) s. The experiments were carried out in an UHV system for which the mass flow could be controlled very precisely. The modeling was using the reaction-diffusion system underlying the reaction studied for which all the kinetic coefficients are known rather precisely. Our numerical analysis was performed for one and two spatial dimensions showing qualitatively similar behavior. The comparison between the experimental results and the modeling shows semiquantitative to quantitative agreement.