Chemical reactions carried out under strongly non-equilibrium conditions can result in a variety of interesting effects such as oscillations, chemical waves, kinetic phase transitions, bistability, instabilities and chaos - all of these effects being mathematically due to the strong non-linearities in the kinetic rate equations describing these chemical processes. The present article reviews the various types of non-linear behaviour observed in several NO reduction reactions over Pt-group metals. A large part of this review deals with oscillations and spatiotemporal pattern formation in the reaction over Rh surfaces: the reaction dynamics of this system has been realized on the microscopic, mesoscopic and macroscopic scales using field emission microscopy, photoemission electron microscopy and macroscopic rate measurements respectively. The various mechanisms of this and the other reactions reviewed in this article are also considered.

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Interesting kinetic phenomena, such as multiple steady states and kinetic oscillations recently found in the NOϩH 2 reaction over Rh͑533͒ and Rh͑111͒ single crystal surfaces in the 10 Ϫ6 mbar pressure range have been studied by means of experiments and computer modeling. A mathematical model, consisting of five ordinary differential equations and taking into account the lateral interactions in the adlayer, has been developed for simulating the NOϩH 2 /Rh͑533͒ and NOϩH 2 /Rh͑111͒ reactions. The simulation results make it possible to explain in detail the underlying reasons for the experimentally observed complex dynamic behavior. In particular, the kinetic oscillations and their properties have been reproduced. It was found that accumulation of NH ads species, which serves as an intermediate in the pathway of NH 3 production, is an important step in the oscillatory mechanism. In addition, the same mathematical model is able to successfully reproduce the experimental data concerning temperature programmed desorption ͑TPD͒ spectra, hysteresis phenomena, and the dependence of selectivity upon temperature and reactant partial pressures. Lateral interactions in the adlayer are shown to play a crucial role in the adequate simulation of the experimental observations.