Spillover catalysis

Bifunctional and Spillover Catalysis. - It has been suggested above that the active involvement of the support in a catalytic reaction could appear as a metal-support interaction, and several recent papers demonstrate in some detail how this can come about. Catalysis of the hydrogenation of CO2 by Rh on various supports shows a strong dependence of turnover number on the kind of support used (see Table 2). Dispersions, D, are similar, as are activation energies, so the considerable differences in turnover number reside principally in the pre-exponential factor. A further study of this system by i.r. spectroscopy has identified CO on the metal and the formate ion on the support (although not in the case of Si02), and the favoured mechanism for CH4 formation involves a reaction of spillover H with CO2 chemisorbed on the support by insertion into surface OH groups. 

Bifunctional and spillover catalysis and reaction at interfacial sites may occur to an extent depending upon the components of the catalyst. 

The definition of promotion suggested above excludes cases where the support itself provides an essential service (e.g., in bifunctional and spillover catalysis) or where the promotion occurs unintentionally. Carbon deposition can however sometimes result in a selectivity improvement and of course a species may act at the same time as both an electronic and a selectivity promoter. For these reasons it is not always easy to classify what a particular promoter does, or how it works. 

According to the Catalytic Cycle (Figure 5.1), an active centre is a place where the reactants are adsorbed adjacently, ready to react. We may therefore attempt a working definition of an active centre as a location on the surface of a catalyst where reactants may be adsorbed in the right way and with the best strengths to give the desired products efficiently. It is not necessary for the reactants to adsorb directly onto the active centre one or both may adsorb elsewhere and then diffuse to the active point this is what happens in spillover catalysis. There are without doubt cases where two reactants do not adsorb in competition on the same type of site each has its own requirement, and it may be more closely restricted for one reactant than for the other. The active centre will then contain two sites of different character, on which each reactant is adsorbed non-competitively. Kinetic analysis should reveal when this situation arises. 

W.C. Conner Jr. in New Aspects of Spillover Catalysis (T.Inui, K. Fujimoto, T. Uchijima and M. Masai, eds.) Studies in Surface Science and Catalysis, Elsevier Amsterdam 77 (1993) 61. 

It is interesting to compare the mechanisms proposed for the hydrogenations of ethyne and benzene. With the former, there is no suggestion of a role for spillover catalysis with the latter there is no role for carbonaceous deposits in creating active centres or in acting as vehicles for hydrogen atom transfer. 

Wang, L. and Yang, R. T. Hydrogen storage on carbon-based adsorbents and storage at ambient temperature by hydrogen spillover. Catalysis Rev. Sci. Eng. 2010, 52(4), 411 61. 

Volume 77 New Aspects of Spillover Effect in Catalysis. For Development of Highly Active... 

The reader must have already identified some of the basic concepts which play a key role in understanding the electrochemical activation of heterogeneous catalysis catalysis, electrocatalysis, promotion, electrochemical promotion, spillover, backspillover. It is therefore quite important to define these terms unambiguously so that their meaning is clearly determined throughout this book. 

The effect of spillover plays an important role in heterogeneous catalysis and was extensively studied during recent years. It was first noticed in the 1950s by Kuriacose.62 Work in this area has been reviewed by Teichner63 and by Conner et al.64... 

In cases of spillover in heterogeneous catalysis the usual kinetic models can no longer be applied in a direct way. The creation of new surface sites or... 

C.G. Vayenas, and S. Brosda, Spillover-modified catalysis Experiment and mathematical modeling, Stud. Surf. Sci. Catal. in press, (2001). 

I. Metcalfe, Electrochemical Promotion of Catalysis II The role of stable spillover species and prediction of reaction rate modification,/. Catal. 199, 259-272 (2001). 

Since then Electrochemical Promotion of Catalysis has been proven to be a general phenomenon at the interface of Catalysis and Electrochemistry. More than seventeen groups around the world have made important contributions in this area and this number is reasonably expected to grow further as the phenomenon of electrochemical promotion has very recently been found, as analyzed in this book, to be intimately related not only to chemical (classical) promotion and spillover, but also to the heart of industrial catalysis, i.e. metal-support interactions of classical supported catalysts. 

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