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

Reaction-diffusion fronts and the butterfly set

A single-species reaction-diffusion model is used for studying the coexistence of multiple stable steady states. In these systems, one can define a potential-like functional that contains the stability properties of the states, and the essentials of the motion of wave fronts in one- and two-dimensional space. Using a quintic polynomial for the reaction term and taking advantage of the well-known butterfly bifurcation, we analyze the different scenarios involving the competition of two and three stable steady states, based on equipotential curves and points in parameter space. The predicted behaviors, including a front splitting instability, are contrasted to numerical integrations of reaction fronts in two dimensions.

Competing ternary surface reaction CO + O2 + H2 on Ir(111)

The CO oxidation on platinum-group metals under ultra-high-vacuum conditions is one of the most studied surface reactions. However, the presence of disturbing species and competing reactions are often neglected. One of the most interesting additional gases to be treated is hydrogen, due to its importance in technical applications and its inevitability under vacuum conditions. Adding hydrogen to the reaction of CO and O₂ leads to more adsorbed species and competing reaction steps towards water formation. In this study, a model for approaching the competing surface reactions CO+O ₂ + H₂ is presented and discussed. Using the framework of bifurcation theory, we show how the steady states of the extended system correspond to a swallowtail catastrophe set with a tristable regime within the swallowtail. We explore numerically the possibility of reaching all stable states and illustrate the experimental challenges such a system could pose. Lastly, an approximative first-principle approach to diffusion illustrates how up to three stable states balance each other while forming heterogeneous patterns.

Detailed analysis of transitions in the CO oxidation on palladium(111) under noisy conditions

It has been shown that CO oxidation on Pd(111) under ultrahigh vacuum conditions can suffer rare transitions between two stable states triggered by weak intrinsic perturbations. Here we study the effects of adding controlled noise by varying the concentrations of O₂ and CO that feed the vacuum chamber, while the total flux stays constant. In addition to the regime of rare transitions between states of different CO₂ reaction rates induced by intrinsic fluctuations, we found three distinct effects of external noise depending on its strength: small noise suppresses transitions and stabilizes the upper rate state; medium noise induces bursting; and large noise gives rise to reversible transitions in both directions. To explain some of the features present in the dynamics, we propose an extended stochastic model that includes a global coupling through the gas phase to account for the removal of CO gas caused by the adsorption of the Pd surface. The numerical simulations based in the model show a qualitative agreement with the noise-induced transitions found in experiments, but suggest that more complex spatial phenomena are present in the observed fluctuations.

Travelling fronts of the CO oxidation on Pd(111) with coverage-dependent diffusion

In this work, we study a surface reaction on Pd(111) crystals under ultra-high-vacuum conditions that can be modeled by two coupled reaction-diffusion equations. In the bistable regime, the reaction exhibits travelling fronts that can be observed experimentally using photo electron emission microscopy. The spatial profile of the fronts reveals a coverage-dependent diffusivity for one of the species. We propose a method to solve the nonlinear eigenvalue problem and compute the direction and the speed of the fronts based on a geometrical construction in phase-space. This method successfully captures the dependence of the speed on control parameters and diffusivities.

The Role of Defects in the Local Reaction Kinetics of CO Oxidation on Low-Index Pd Surfaces

The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10-5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.