Platinum ensemble effect

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. 

In conclusion by using rhenium as adsorbent instead of platinum, it is possible to achieve the ensemble control by sulfur passivation, at sulfur levels comparable to those applied for nickel and much lower than that which would have been required on a non-alloyed Pt-catalyst. A similar ensemble effect is achieved by alloying alone on Pt-Sn catalysts ... 

The high dispersity inside the nano-honeycomb matrix and the high surface area of the nanopartides leads to very good electrocatalytic activity. The electrocatalytic activities of nanosized platinum particles for methanol, formic add and formaldehyde electrooxidation have been recently reported [215]. The sensitivity of the catalyst particles has been interpreted in terms of a catalyst ensemble effect but the detailed microscopic behaviour is incomplete. Martin and co-workers [216] have demonstrated the incorporation of catalytic metal nanopartides such as Pt, Ru and Pt/Ru into carbon nanotubes and further used them in the electrocatalysis of oxygen reduction, methanol electrooxidation and gas phase catalysis of hydrocarbons. A related work on the incorporation of platinum nanopartides in carbon nanotubes has recently been reported to show promising electrocatalytic activity for oxygen reduction [217]. 

Finally, evidence of ensemble effects in Pt-Au/Si02-catalysed isomerization and hydrogenolysis of neopentane at 593 K have been described recently by Anderson and Foger. The selectivity for isomerization, S, passed through a maximum with mole fraction of surface platinum, X(s)pt as indicated below ... 

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. 

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. 

In this chapter, the investigations of abnormal infrared effects (AIREs) of nanometer-scale thin him material of platinum group metals and alloys were reviewed. The AIREs describe the ensemble of abnormal IR features that have been discovered recently for CO and other molecules (CN SCN PoPD, etc.) adsorbed on nano structured hlms. It was observed on nanometer-scale thin him of... 

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