Platinum ensembles

It was proposed that, in the platinum-rich range, large platinum ensembles are present so that a reaction pathway involving di- and tri-adsorbed intermediates is dominant. As ensemble size becomes small, and the number of platinum atoms much restricted, a second mechanism leading to hydrogenolysis predominates. This interpretation was supported by detailed kinetic evidence and by computer simulation of the ensembles proposed. 

In conclusion, bimetallic Pt-Sn/alumina catalysts prepared by successive impregnations with an intermediary reduction step and introduction of the tin salt (SnCU) under hydrogen are less sensitive to coke deactivation than catalysts prepared by coimpregnation. This behavior probably results from a more effective interaction between the two metals, leading to smaller platinum ensembles, as evidenced by the low hydrogenolysis activity. However, the amount of coke deposited on the whole catalyst depends on the nature of the feed and therefore on the nature of the dehydrogenated species which are more or less active precursors for coke deposition on the support. 

It has been pointed out that the coke formation on the metallic surface occurs mainly on the flat planes (large particles) [10]. If we assume that Au at high coverage is deposited on the dense planes of the platinum particles, we can propose that Au acts as a diluent of the platinum ensembles, and hence that coke formation is inhibited. The level of coke and its nature are under study, and the results will be reported in a subsequent paper. 

The addition of tin has an important effect on the selectivity toward dehydrogenation reaction, at the expense of hydrogenolysis reactions by decreasing the size of platinum ensembles needed [5]. The addition of tin has also been suggested to improve the stability of the catalytic systems by decreasing the adsorption energy of coke precursors. On the other hand, improving selectivity also requires to inhibit the acid catalyzed reactions on the... 

The improved selectivity of sulflded PtRe/Al203 for non-parasitic reactions requires a short discussion. Although the PtRe unit has been implicated in greater rates of hydrogenolysis (perhaps because the rhenium atom can more easily accommodate a multiple C—M bond), the higher selectivity for isomerisation and cyclisation must be a feature of a small platinum ensemble. This conforms to the effect of dispersion, where small particles also show high isomerisation selectivity. 

Divergent views have been expressed on the way in which the tin acts. Its role in limiting the size of platinum ensembles is not in question what is at issue is whether there is any electronic modification of the active centre. It was claimed that, in the reaction of MCP with hydrogen, tin produced positive effects on dehydrogenation and aromatisation that were not shown by either carbon or sulfur they were attributed to an electronic action, for which Mossbauer spectroscopy provided some evidence. In view of the proposal interpretation of the effect of sulfur on butadiene hydrogenation (Section 8.3) it would not be surprising if tin also influenced the platinum ensembles to some degree. 

The sulfurization of the Pt, or allojdng it with inactive compounds (Au, Sn, Pb) decreases the effective size of the platinum ensembles, and therefore decreases the activity for hydrogenolysis (geometric effect). In the case of bimetallic catalysts such as Pt-Re, and Pt-lr, due to the very high hydrogenol3rtic activity of the second metal, these catalysts have to be presulfided before the operation, to avoid the initial high production of methane with a dangerous increase in temperature. 

Supported bimetallic Re—Pt catalysts are important in selective reforming of petroleum. It is believed that sulhding the catalyst before use gives ReS units which act as inert diluents to reduce the size of a local ensemble of platinum atoms. Selectivity for desirable dehydrocyclization and isomerization reactions... 

In conclusion, hydrogenolysis processes and coke formation occur on large ensembles of surface platinum atoms [160], while dehydrogenation reactions would proceed on single (isolated) Pt atoms [169]. The presence of tin atoms regularly distributed on the metal surface diminishes the size of the ensemble [130,170-173], the same is observed for copper atoms on nickel surfaces [174] or tin atoms on rhodium and nickel surfaces [137,175-177], leading to site isolation and therefore to selectivity. 

Qiu, J.-D., et al., Controllable deposition of a platinum nanoparticle ensemble on a polyaniline/graphene hybrid as a novel electrode material for electrochemical sensing. Chemistry - A European Journal, 2012.18(25) p. 7950-7959. 

At the moment we have no good explanation for the observed acceleration except that it has a connection to the basic character of the quinuclidine part and the adsorption behavior of the cinchona molecule. In addition, we think that the rate and product determining steps occur on the platinum surface and that well defined interactions between the platinum surface (ensembles), one cinchona molecule and the a-ketoester are crucial. There are, of course, other possible explanations for the observed enantioselection. Wells and Thomas [80] have proposed that an array of... 

High toxicities were obtained with very low lead coverages (0pb < 0.05), equal to about 20-50 atoms of platinum deactivated by one lead adatom. Such high toxicities cannot be explained neither by ensemble effects nor by ligand effects. A fast diffusion of lead adatoms on the platinum surface could account for this result. A plateau in activity is found, for medium lead coverages (0.05 < 0pb < 0.30-0.50) which could be ascribed to the formation oflead islands on the platinum surface. 

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