Schwab effect, metal particles

Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the Schwab effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-121) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). 

The small number of charge carriers in the semiconductor is partially compensated for by the small size of metal particles. By using Schwab s magnetic measurements as a clue to the number of electrons transferred to the nickel across the nickel-alumina interface it can be estimated that a change of 0.05 electron per atom would produce detectable catalytic effects. By extrapolation to the small dispersed type of metal particle being considered here, it is seen that for a particle containing 2000 atoms the equivalent transfer would be produced by 100 electrons. Despite this, if one considers contact between a 2000-atom platinum particle and a 100 alumina particle (volume 4 x 10 cc) the flow of electrons to the metal would be drastically limited by the small number of charge... 

Already in 1929 it was proposed by Schwab and Pietsch that the catalytic reaction on supported metal catalysts often takes place at the metal-oxide interface. This effect is known as adlineation, however, up to the present there is only little direct experimental evidence. In one example, the oxidation of CO on nanoscale gold, it is presently discussed whether the catalytic action takes place at the particle upport interface. Adlineation is strongly related to the effect of reverse spillover, where the effective pressure of the reactants in a catalytic process is enhanced by adsorption on the oxide material within the so-called collection zone and diffusion to the active metal particle (see Fig. 1.55 and also The Reactivity of Deposited Pd Clusters). The area of the collection zone and thus the reverse spillover are dependent on temperature, on the adsorption and diffusion properties of the reactants on the oxide material, as well as on the cluster density. 

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