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Temperature hysteresis is observed only in exothermic heterogeneous catalytic reactions (viz., oxidation and methanation of CO or propene hydrogenation) and is absent in the case of endothermic reactions (dehydrogenation of isobutane) or reactions with heat close to zero (viz.. 2-butene isomerization). Temperature hysteresis in hydrogenation reactions was discovered for the first time. The concept of local overheating of catalyst active sites caused by poor removal of the reaction heat is proposed to provide a noncontradictory interpretation of the appearance of hysreresis loops.Key words: heterogeneous catalysis, temperature hysteresis, oxidation, hydrogenation, methanation.Temperature hysteresis in heterogeneous catalysis belongs to the class of phenomena referred to as critical in theoretical kinetics, t The essence of this phenomenon is that some reaction parameter (for example, the rate or the degree of conversion of a reactant) measured during the change of temperature in one direction does not coincide with the same parameter measured during the change of temperature in the opposite direction. This produces the so-called hysteresis loop when the ascending and descending branches of the temperature dependence differ from each other.An explanation of hysteresis is based on the view that several steady states exist in a catalytic system. In this case, it is assumed that during gradual temperature change, one steady state is replaced, at a particular instant, by another state submitting to different kinetic features. Therefore, the reaction proceeds at a different rate, although the temperature measured in the reactor remains the same.This conclusion has been drawn, for example, after mathematical analysis of CO oxidation over platinum, z The possibility of existence of different steady states in this reaction proceeding over the 0.05%Pd/SiO 2 and (0.05Pd%+I%Cr203)/AI20 3 catalysts has been verified experimentally. 3 The appearance of hysteresis has been attributed 4 to the fact that CO oxidation over nickel catalyst occurs in the region of the Ni--NiO phase transition. It is believed 4 that the formation of the two-dimensional NiO phase is accompanied by substantial acceleration of the reaction, whereas the subsequent transition to the three-dimensional NiO phase results in its sharp retardation. In another study, the occurrence of hysteresis in CO oxidation over platinum was explained by the existence of different steady states. 5 Among the relevant publications, one can also mention studies in which this reaction was carried out at low pressures (10 -2 Ton') over the CuO/SiO 2 and Pd catalysts, respectively. 6,7 The hypothesis of multiple steady states as a reason for hysteresis has been considered fiairly comprehensively in a review. 8 The results of a systematic study of the mathematical models of chemical reactions, which can involve critical phenomena, are presented in a monograph, t in which the same CO oxidation is considered as an example.Other interpretations of the reasons for temperature hysteresi...

Abs~a~The temperature hysteresis observed in the oxidation of CO on both supported and unsupported catalysts can be explained by the local overheating of active centers rather than by transitions from one steady state to another,