Spill-over mechanism

The spill-over is a well-known process in the field of heterogenous catalysis. It has been particularly studied in the case of platinum and palladium. This process is 

On contact with the metallic catalyst, the oxygen molecule dissociates. The atoms migrate towards the surface of the oxide support. Then, by reacting with an electron of the oxide, ionization occurs. This consumption amonnts to a modification of the oxide spaee-charge layer. 

Sm indicates an adsorption site on the metal, and s, an adsorption site on the 

Regarding kinetics, the presence of the catalyst makes the regeneration of the O s compound easier at the surface. 

As for as the response to H2 action, the proposed model implies a dissociation of H2 on the metal and a reaction with the absorbed oxygen. 

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In the presence of a catalyst and with a finite selectivity for CO production, soot oxidation is described with the following global reactions, one for the thermal and one for the catalytic path (i.e. oxidation of soot by oxygen transferred from the catalyst by a redox and/or spill-over mechanism). Konstandopoulos and Kostoglou (1999b, 2000) ... 

Figure 8.4. Spill-over mechanism according to S. Morrison...

The obverse of the spill-over mechanism, the reverse spill-over mechanism, mentioned by K. Grass, eonsists of an adsorption of the oxygen on tin dioxide, followed by a migration of the adsorbed species towards the metal, as is shown in Fignre 8.5. 

Figure 8.5. Reverse spill-over mechanism concerning oxygen, proposed by K. Grass and H. Lintz...

The phenomenon thns described is displayed in Figure 10.15. This scheme shows a phenomenon opposite to the classic spill-over phenomenon. According to the spill-over mechanism, the gas is dissociated at the surface of the metallic electrodes and the reactive species diffuse from the metal towards the material. ... 

Fig. 2.1. Plausible mechanisms of influence of metal particles deposited on adsorbent on adsorption-caused response of its electrophysical characteristics a - chemical sensitization the dope particles cause the activation and spill-over of adsorbate b - electronic sensitization the dope particles become donors or acceptors of electrons in dependence on the conditions in gas phase.

Also, a mechanism involving the surface diffusion of NO ad-species towards reduced Pt sites cannot be excluded (Figure 6.14b) [11], In this case, NO spills over the surface and is decomposed at reduced Pt sites. The role of the reductant in this mechanism is... 

It is well established, that the poor selectivity of tin-oxide sensors can partly be overcome by adding catalysts to the sensitive layer. Most common additives are noble metals like gold (Au), platinum (Pt) or palladium (Pd). They can be mixed with the tin oxide during paste formation before deposition. The influence of dopants on the gas sensor response is still subject to debates. The two most established mechanisms are the spill-over and the Fermi-level mechanism [82]. 

A new study with n-hexane, however, suggests a monomolecular mechanism with the participation of Lewis acid-Lewis base site pairs generating the carboca-tion. The role of Pt is to activate hydrogen which spills over to form hydride species for the desorption of the isocarbocation.311... 

Indisputable evidence for the occurrence of hydrogen spill-over exists although the mechanism whereby it occurs is still not clear. Benson et al. 

The mechanism of hydride formation of ZrMn and TiMn, is related to segregation and preferential oxidation of the Mn on the surface and precipitation of metallic Zr and Ti at the subsurface below the oxidized Mn. The Zr and Ti metallic precipitates catalyze the Hj 2 H reaction. In addition, the Zr and Ti precipitates themselves may form hydrides and spill over atomic H to the intermetallic compound lying below. 

The Podbielniak and Alfa Laval centrifugal extractors are essentially continuous differential contactors. The Westfalia and Robatel centrifugal extractors contain discrete mechanical stages, and flow from one to another is effected by spill over discs and skimmers according to usual centrifugal clarifier practice. As the number of discrete stages is increased, the allowable flow rates are proportionately decreased. 

The effect of temperature, contact time and reactant concentration on the kinetics of NO reduction by CsHs and by CsHg over Pt/Al203 under lean-bum conditions have been investigated and kinetic models which satisfactorily fit the data have been developed. The results suggest that with CsHs the Pt surface is dominated by carbonaceous species, while with CsHs adsorbed atomic oxygen is the main species on the Pt surface. This difference in the state of the Pt surface results in different mechanisms for NOx reduction. Thus, with CsHe, NOx reduction seems to occur via the dissociation of adsorbed NO on the Pt surface, while with CsHg, NOx reduction appears to occur via spill-over of NO2 from the Pt metal onto the AI2O3 support where it reacts with CjHs-derived species to form N2 and N2O. 

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