Support Effect Reverse-spillover

The spillover of reactant is a well-known phenomenon in heterogeneous catalysis. It is due to the diffusion of atomic or molecular intermediates, formed, e.g. by dissociation, from the catalyst to the support material [25]. The reverse-spillover is the opposite process and it corresponds to the diffusion of molecular species adsorbed on the support towards the catalyst s particles (see a complete discussion of this phenomenon in [58]). This phenomenon was first 

Within this model, the capture of adsorbed molecules physisorbed on the support is a second channel for chemisorption of reactants on the catalyst s particles, with the direct impact from the gas phase being the first channel (see Fig. 3.11). 

In a simple approach, one defines a capture zone around each cluster of width p, where all physisorbed molecules will become chemisorbed on the metal cluster by diffusion. In the first degree of approximation, p is equal to the mean diffusion of a molecule physisorbed on the substrate  

We can define a parameter Og that represents the fraction of the flux of molecules impinging on Icm of sample that becomes chemisorbed on the metal clusters  

We see the global adsorption to increase when the temperature decreases. At high temperature it is equal to Ac, the fraction of the substrate covered by the metal clusters. In that case, the width of the capture zone, p, is zero and only molecules directly impinging on the clusters are chemisorbed and Og is equal to nR n. When the temperature decreases, the mean diffusion length, Xg, grows and p increases. Finally, the global adsorption probability reaches saturation when the capture zones overlap. The maximum value of the adsorption probability is given by  

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The effect of the reverse spillover in the oxidation of CO on supported model catalysts has been observed by several other authors on various systems Pd/mica [133], Pd/alumina [103,131, 132, 144, 163] Pd/MgO [45, 161], Pd/silica [104] it can increase the reaction rate by a factor as large as 10. 

Platinum remains more active than rhenimn even in the presence of CPE and CPD, which confirms that the metals play a dual role in the formation of coke dehydrogenation giving coke precursors (non operating here since CPE and CPD are already present in the reactant) and consolidation of the coke deposited on the support by continuous elimination of hydrogen via a reverse spillover phenomenon. It is clear that Pt remains more effective than Re in coke consolidation. TPO profiles on Re (Fig.4) show a small... 

The support plays an important effect in the adsorption kinetics of CO on supported clusters. Indeed CO physisorbed on the support is captured by surface diffusion on the periphery of the metal clusters where it becomes chemisorbed. The role of a precursor state played by CO adsorbed on the support is a rather general phenomenon. It has been observed first on Pd/mica [173] then on Pd/alumina [174,175], on Pd/MgO [176], on Pd/silica [177], and on Rh/alumina [178]. This effect has been theoretically modeled assuming the clusters are distributed on a regular lattice [179] and more recently on a random distribution of clusters [180]. The basic features of this phenomenon are the following. One can define around each cluster a capture zone of width Xg, where is the mean diffusion length of a CO molecule on the support. Each molecule physisorbed in the capture zone will be chemisorbed (via surface diffusion) on the metal cluster. When the temperature decreases, Xg increases, then the capture zone increases to the point where the capture zones overlap. Thus the adsorption rate increases when temperature decreases before the overlap of the capture zones that occurs earlier when the density of clusters increases. Another interesting feature is that the adsorption flux increases when cluster size decreases. It is worth mentioning that this effect (often called reverse spillover) can increase the adsorption rate by a factor of 10. We later see the consequences for catalytic reactions. 

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. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

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