The Brønsted−Evans−Polanyi Correlations in Oxidation Catalysis
Catalysis Reviews: Science and Engineering
, E-ISSN: 1520-5703
||Brønsted coefficient, Brønsted-Evans-Polanyi relationship, Compensation effect, Deep oxidation, H2, Heat of oxygen adsorption, Hydrogen abstraction, Hydrogen atom transfer, Isotopic exchange of O2, N2, Oxidation catalysis
Panov Gennady I.
Parfenov Michail V.
Parmon Valentin N.
Boreskov Institute of Catalysis, Novosibirsk, Russia
Novosibirsk State University, Novosibirsk, Russia
The experimental Brønsted–Evans–Polanyi (BEP) correlations in the field of oxidation catalysis, describing both the liquid-phase reactions on metal complexes and especially the gas-phase oxidations on metal oxides including the O2 isotopic exchange, were analyzed. It was shown that the rate of deep oxidations on metal oxides is determined by two thermodynamic parameters of a catalytic system: the heat of oxygen adsorption, QO2, and the heat of reaction, Qr, which constitute a unified BEP descriptive parameter Quni = (Qr – QO2). The correlations based on the energy of chemical bonds that are cleaved or formed in the course of reaction are more limited and less predictive.
Special attention was paid to the numerical value of the Brønsted coefficient β. It was found that for all oxidations, in both the liquid and gas phases, β ≈ 0.5. Using the unified BEP descriptor Quni with β = 0.5, a universal correlation was plotted describing the rates of all reactions over all catalysts under consideration.
An idea of considering the kinetic compensation effect as a part of an extended BEP correlation is suggested. One may think that this long-debated phenomenon may relate primarily to mechanistic features of the reaction rather than to the nature of catalyst.
In conclusion, difficult questions arising from analysis of the BEP correlations are summarized.