Catalyst spillover effect

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. 

S.J. Teichner. New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts in Third International Conference on Spillover, 27 (1993) Amsterdam Elsevier. 

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical pumping of oxygen on catalytic behaviour of metal electrodes in methane oxidation, in New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts, Stud. Surf. Sci. Catal. 77 (T. Inui, K. Fujimoto, T. Uchijima,... 

NMR adsorption isotherms for Ru/SiOi catalysts have been obtained using explicit calibration (89). Although the pressure over the sample could be adjusted in situ, no volumetric data were taken simultaneously, probably because of the important spillover effects in this catalytic system (see Section III.A). The NMR study was performed at pressures between 10 and 760 Torr and at temperatures between 323 and 473 K (only the 323-K results are reviewed here). The dispersion of the catalyst was determined from the irreversible H NMR signal as 0.29. The metal loading was 8 wt% so that a monolayer coverage on 1 g of catalyst corresponds to 2.8 cm of H2 under standard conditions. It is typical for an NMR sample to contain 0.5 g of material in a 1-cm sample volume, and the pores in the powder make up about half the volume. If such a sample of this catalyst is under 760 Torr of hydrogen, the gas phase corresponds to one-third of a mono-layer, and it can make a detectable contribution to the NMR signal. 

Moreover, the described phenomena will bear relevance for the metal-promoter interaction in promoted supported transition or noble metal catalysts. Unless spillover effects play a decisive role, promotion can occur only if the active metal and promoter oxide are in contact. Obviously, in such complex systems the surface- and interface-free energies and the mobilities of individual components under preparation conditions critically will determine their morphology and distribution. For a deeper understanding of the detailed mechanisms of wetting and spreading in such complex systems as supported catalysts, additional fundamental studies are required, in which our basic knowledge in surface chemistry, surface spectroscopy, colloid and solid-state chemistry, and powder technology must be combined. 

In the reduction of NO by CO at low temperature, on Pd/MgO(100) model catalysts, the reaction rate is independent of CO pressure but increases with NO pressure [64,65], then the TON of the reaction is modified by the reverse-spillover effect and, in particular, depends on particle size. In that case, it is no longer possible to compare the TON value measured on different particle sizes, it is more appropriated to calculate the reaction probability [64], which takes into account for the real flux of molecules that reach the clusters which can be measured by molecular beam experiments. Reverse-spillover effects have also been recently observed for the CO oxidation on size-selected Pd clusters soft-landed on MgO epitaxial films [66]. 

Fig. 4.33. Measured CO oxidation rate on EBL-fabricated model catalysts with different support materials and particle sizes (a) The inlet gas mixture represented by the parameter (3 = Pco/ Pco + P02) has been scanned up/down at a constant temperature of 450 K, and the rate of CO2 production has been monitored during the gas scan in j3. This has been made for three different samples (a) 750-nm Pt/Si02 (blue), (b) 750-nm Pt/CeOj, (black), and (c) Pt/CeOj, sample (200mn, red, disintegrated particles). Results both from experiments (filled circles) and simulations solid lines) are shown. The arrows indicate which reaction rate branch that has been observed while scanning up/down in (3. A bistable region (hysteresis) is observed for all samples, (b) The bistability diagrams determined from a series of measurements as those shown in (a) at different temperatures (Pt/Si02, blue and open squares) Pt/CeOj, black hatch marks and crosses and the 200-nm Pt/CeOj, red hatched area and open squares). The observed differences can be traced back to a pronounced O-spillover effect on ceria. Note the logarithmic scale for the /3-value (from [29])...

In the past years, many independent investigators have shown strong evidences of the existence of oxygen spillover and of the positive role played by this species in improving the performance of catalysts. In the latter case, these authors often alluded to a "spillover effect" rather than a "remote control". These two different terms nevertheless reflect the same concept [11-15]. 

Hydrogen spillover effect over the oxide surfaces in supported nickel catalysts... 

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