Mechanisms Donor and Acceptor Phases

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 

Equations (3.16) and (3.17) describe the dissociative adsorption and, recombination of oxygen on a donor D. The transfer between the donor D and acceptor A is described by eq. (3.18). The spillover oxygen (O) is a mobile species which is present on the acceptor surface without being associated with a particular surface site. The mobile spillover species can interact with a particular surface site B forming an active site C (eq. 3.19). Eq. (3.20) represents the deactivation of the active site C by interaction with a reactant E. 

The rate of creation of active sites fo) on the acceptor is proportional to the fraction of the acceptor covered with spillover oxygen (0soa) and to the fraction of inactive Mo03 surface sites (1-a) 

The mathematical model was found to describe well the observed activity increase for numerous mechanical mixtures of a-Sb204/Mo03 at different oxygen partial pressures and temperatures.73 

It is now well established that spillover-backspillover phenomena play an important role in numerous catalytic systems. It is worth reminding that the effect of strong-metal-support interactions (SMSI), which was discovered by Tauster74 and attracted the intense interest of the catalytic community for the least a decade75 was eventually shown to be due to backspillover of ionic species from the Ti02 support onto the supported metal surfaces. 

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