In spillover

A variety of surfaces have been shown to react with spiltover hydrogen. The above discussion focused on oxide surfaces where polar association is possible. Boudart et al. found that spillover occurs from Pt to carbon at modest temperatures (114). Indeed, it has been suggested that a hydrocarbon bridge assists in spillover from Pt onto the supports (46,85). The spiltover... 

Each of the intermediate steps in the mechanistic sequence involved in spillover can differ. As an example, surface transport may occur as a two-dimensional gas, as species associated with an activated site, or as two-dimensional exchange (via hydroxyls). The literature has suggested a variety of mechanisms for spillover and mechanisms induced by spillover. The studies have not assumed that all of the mechanisms are possible and have not focused on discriminating between the alternative possibilities. This more open (albeit more disconcerting) approach is suggested to understand the phenomena of spillover. Further, novel experimental techniques are needed to isolate the catalytic effects occurring on the source of spillover (usually the metal) from the effects induced by spillover on the acceptor surface, and to access the relative contribution of each. Both the numbers of species involved and the relative rates need to be quantified. 

K. Fujimoto, I. Nakamura, In Spillover and Migration of Surface Species on Catalysts, Elsevier, Amsterdam, 1997, p. 29. 

The above framework points to the very subtle effects which may play a part in spillover, and to the necessity for the use of extreme care in characterizing surfaces and systems, as already emphasized. ... 

Masai M, Nakahara K, Yabashi M, Murata K, Nishiyama S, Tsuruya S, in Spillover of Adsorbed Species, Pajonk GM et al. (eds.). Stud Surf Sci Catal 17 89, 1983. 

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. 

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