Reverse spillover

Examples of reverse spillover (or backspillover) are the dehydrogenation of isopentane and cyclohexane on active carbon. Deposition of a transition metal on the active carbon accelerates the recombination of H to H2 due to a reverse spillover or backspillover effect.72... 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

Hence the 30 % that are lost could be on the support and slowly react with 1-hexanol to form HA. However when aniline is reacted there is no significant loss of material, which suggests that aniline cannot interact directly with the surface hydroxyls. This suggests that the interaction between aniline and the support hydroxyls is not as simple as shown above, rather it is more likely that the reaction operates via a spillover mechanism involving an intermediate in the nitrobenzene hydrogenation sequence rather than aniline. The alkylation reaction between aniline and 1-hexanol takes place on the metal function, therefore the reaction with the missing aniline associated with the support will be slow as it requires a reverse spillover and a diffusion across the support surface. 

Restricted Hartree-Fock method, 34 136 Retinal pigment models, 32 451-452 Reverse spillover, 34 2, 4, 11, 16, 31, 44, 46,... 

Ba(NO3)2, decomposition of BaOa to BaO and oxygen and the reversible spillover of NO2 between Pt sites and BaO sites. Essentially the model assumes that the adsorption of NO proceeds through the nitrate route and does not consider the nitrite route. Olsson et al. [76] estimated part of the rate parameters in their model from theoretical considerations, part were taken from the literature or calculated from thermodynamic constraints and part were estimated by fitting a set of experimental data. 

An important aspect of the reverse spillover is its dependence with particle size, particle density and surface temperature. This complex dependence is depicted in Fig. 9a. 

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. 

In the experiments the small particles (2.8 and 6.8 nm) appear more active than the largest ones (13 nm) especially in the medium range of temperature. First we have tried to take into account for the reverse spillover of CO by using Eq. (13). The reaction probability has been fitted from the universal curve given from experiments on various Pd extended surfaces [124] ... 

In conclusion the existence of the second peak is not a genuine effect of one type of site but simply the occurrence of the second peak is not the same for the two types of sites. It is also important to note that the reverse spillover can also have an influence on the second peak because it increases the actual flux of CO reaching the particles then it is equivalent to an increase of CO pressure. However the reverse spillover by itself cannot explain all the experimental observations the intrinsic heterogeneity of the metal particles has to be taken into account. 

Scheme 9.1 A mechanism for metal-catalysed hydrogen spillover, shown by the exchange of support hydroxyls with deuterium. The process can extend to the whole surface (A), but HD is formed by reverse spillover (B), followed by desorption.

Reverse spillover or back-spillover is observed to proceed by surface migration of the spiltover species from the accepting sites to the metal, where it desorbs as H2 molecules or reacts with another hydrogen acceptor such as 02, pentene, ethylene, etc. Reverse or back-spillover (primary as well as secondary) is hindered by H20 (11), whereas secondary spillover is promoted by H20 (case B in Fig. 1). Hydrogen spillover depends on the acceptor surface it is thought to be easier on silica than on alumina (45) for hydrogen-molybdenum-bronze preparation. 

A spillover of protons was inferred from the IR data. The protons migrated onto the basic sites of the support. The origin of the Ni+ species could be either a disproportionation of Ni° and Ni2 + or reverse spillover according to Eq. (10) during evacuation. 

This mechanism was later shown to occur by reverse spillover (//,41) and a similar mechanism may be inferred for the dehydrogenation of n-heptane as discussed in Section V,D,4 and 5. 

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