Kinetic phase transitions in simple reactions on solid surfaces

Abstract

In analogy to real thermodynamic phase transitions, the term “kinetic phase transition” physically means that the kinetic behaviour of the system under consideration changes qualitatively when a control parameter (e.g., temperature or pressure) passes through a critical point. Mathematically, this means that a bifurcation occurs at this point. If the change in the reaction rate is stepwise at the critical point, the kinetic phase transition belongs to the first-order class. If the change is softer, the transition is continuous. The present review is primarily focused on the first-order kinetic phase transitions connected with bistability and resulting in chemical waves. Transitions of this type, predicted and often well described by common, mean-field kinetic equations, are experimentally observed in rapid surface reactions such as CO or hydrogen oxidation on transition metals under UHV conditions, and at atmospheric pressure as well. Continuous kinetic phase transitions in heterogeneous reactions have been predicted by Monte Carlo simulations for systems with a high reaction rate, provided that the adsorbed species are immobile. In real systems, however, surface diffusion is usually rapid compared to reaction steps. Perhaps this is the main reason why continuous kinetic phase transitions have not been observed experimentally so far. Nevertheless, such transitions are of general theoretical interest, and they are also discussed in this review. The material presented may be useful for interpreting how the steady-state kinetics of surface reactions vary with the control parameters and also for understanding more complex time-dependent critical kinetic phenomena, such as oscilations and chaos, which may take place in strongly nonequilibrium heterogeneous systems.