Sorbed species, spillover

The spillover effect can be described as the mobility of sorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). In this way a seemingly inert material can acquire catalytic activity. In some cases, the acceptor can remain active even after separation from the donor. Also, quite often, as shown by Delmon and coworkers,65 67 simple mechanical mixing of the donor and acceptor phases is sufficient for spillover to occur and influence catalytic kinetics leading to a Remote Control mechanism, a term first introduced by Delmon.65 Spillover may lead, not only to an improvement of catalytic activity and selectivity but also to an increase in lifetime and regenerability of catalysts. 

Limited mobility of sorbed species has long been understood. The exchange of species from one position to another, either on the surface or through the bulk, has been well established (7). More unique is the mobilization of a sorbed species from one phase onto another phase where it does not directly adsorb. This has been defined as spillover. ... 

Studies of the effect of other sorbed species give considerable insight into the nature of spillover. Boudart et al. studied the influence of sorbed water on the rate of spillover (114). The ratio of W03 conversion at room temperature to the hydrogen bronzes and the sorption of hydrogen from the gas phase were dramatically increased by the presence of water. Levy and Boudart used a spectrum of alcohols and acids as coadsorbents, and found that the increase in the rate of reduction of W03 was related to the proton affinity of the coadsorbent (39,115). The proton, produced on the Pt, is proposed to be... 

This brings the discussion of the changes in the solid full circle. Spiltover hydrogen can exchange with the surface. It may react with and replace methoxyls with hydroxyls. It may be incorporated into the bulk with a change in the bulk crystal structure. Bulk reduction may occur. The species spilling over may react only with the surface, with coke, or with other sorbed species. In addition, spillover may promote or inhibit reaction on the surface. 

We find this explanation not totally satisfactory. Unless subsequent spillover displaces previous spillover, diffusion will involve a monotonic gradient from the source. Subsequent spillover should not displace but intermix with previously sorbed species. Since it is generally accepted that the spiltover species is atomic, it is difficult to accept that little HD is formed and the desorption peaks occur with a 250°C difference in temperature.These studies seem to give credence to the hypothesis that multiple states of spiltover hydrogen (or deuterium) exist on the surface. Further studies are needed to clarify the nature of the sorbed states, their energetics, and their number on the support surface. 

The variation in rate-determining step with coadsorbed species was noted by Bond in his recent review (9). Bond cited the extensive evidence that spillover (passage from the metal to the support) can be rate determining. For systems where H20 (or other sorbed species) promotes spillover, it may do so by increasing the interfacial transport, a potentially slow step in the process. 

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