Simplest non-linear mechanisms of catalytic reactions producing critical phenomena

Critical phenomena in heterogeneous catalytic reactions (experiment, models) 

Critical effects in CO oxidation over Pt catalysts were obtained [33, 34, 63-85] in various catalytic systems over wires, foils and gauzes, on single pellets and fixed beds, in isothermal and adiabatic reactors (differential and integral). The literature also reported the oscillating behaviour of the homogeneous oxidation of CO [86, 87]. 

We must emphasize one essential peculiarity until recently practically the whole of the critical effects in CO oxidation (and in the other catalytic oxidation reactions) were obtained at normal ( 760 torr) or almost normal pressures. In ultrahigh-vacuum experiments, these effects have been observed, and have induced scepticism. Over the last several years the situa- 

Taylor et al. [90], when studying the oxidation of CO over Ir (110), also found hysteresis on decreasing and increasing the temperature. This hysteresis is ascribed primarily to the non-linear kinetic dependences of the conversions of surface substances [90]. However, in our opinion, this hysteresis is also most likely to be false since temperature variations of catalyst were sufficiently high ( 1.25Ks ) and the steady state could hardly be achieved. 

It is interesting that, in the experiments by Taylor et al. [90] after the repetition of the same standard experimental run (increase and then decrease of temperature), the hysteresis peculiarities of kinetic curves were preserved but not reproduced quantitatively. Apparently, this is also associated with the fact that the time to achieve a steady state was insufficient. It could also be due to the slow mass transfer processes between surface and bulk. 

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Simplest Non-linear Mechanisms of Catalytic Reactions Producing Critical Phenomena... 

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