CO oxidation on Ir(111) surfaces under large non-Gaussian noise

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.

1/f 2 noise in bistable electrocatalytic reactions on mesoscale electrodes

The formation of a self-organized spatial domain during current-controlled CO oxidation, a kinetically bistable reaction, is investigated experimentally and by deterministic simulations as a function of the electrode size and of the supporting electrolyte concentration. Decreasing the microelectrode size leads to the suppression of the spatial instability at the electrode and thus stabilizes the S-NDR branch of the reaction. The critical microelectrode size capable of supporting sustained domain formation is shown to be strongly affected by the sulfuric acid concentration, the characteristic time of the positive feedback loop increasing with the sulfate concentration. Furthermore, we demonstrate that for microelectrode diameters close to the instability threshold, small amplitude electrochemical potential fluctuations appear in the system. These potential fluctuations cannot be captured by deterministic mathematical models and are attributed to a strong enhancement of molecular fluctuations or intrinsic noise in the vicinity of the spatial instability. Analysis of the electrochemical noise revealed a 1/f ² frequency dependence and several common features with neuronal shot noise.

Enhanced conformational sampling technique provides an energy landscape view of large-scale protein conformational transitions

A novel in silico approach (NMA–ITS) is introduced to rapidly and effectively sample the configuration space and give quantitative data for exploring the conformational changes of proteins.