PtSn alloy formation

Tin Mossbauer. 119Sn Mossbauer data also provide a bulk diagnostic and this method has been utilized in a number of studies (for example, 7,26-39). Direct evidence for PtSn alloy formation was obtained from Mossbauer studies (for example, 7,34,35,38,39) however, many of these studies were at high metal loadings and even then a complex spectrum was obtained so that there was some uncertainty in assigning Sn(0) to the exclusion of tin oxide phases. 

Alkane dehydroeyelization with Pt-Sn-alumina catalysts—Continued pressure effect, 120 PtSn alloy formation, 117-118 role of Sn, 117 Sn vs. carbon deposition, 120 Sn vs. coking, 118-119 Sn vs. n-octane conversion, 120-122 Sn vs. selectivity, 118 temperature effect, 119 Alkene hydroformylation, asymmetric catalysis, 24... 

As noted in the Introduction, the Pt-Sn system has been studied extensively, both from the standpoint of the structure of the catalyst as well as the reason for the superior catalytic properties. Davis utilized Pt-Sn complexes, including Pt3Sn8Cl22, in an organic solvent to prepare supported catalysts (42). It was proposed that the reason for the superior activity of catalysts prepared in this manner was the formation of a PtSn alloy (48,49). 

By means of SOMC/metals, weU-defined Pt-Sn alloys have been prepared. With sihca-deposited platintim-tin particles of nanometric size, the well-defined formation of PtsSn and PtSn alloys has been observed, as shown in the following high resolution electron micrograph (Fig. 18.9). 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

Thus, the reaction is poisoned by the formation of Pt-(CO)ads after complete methanol dehydrogenation. There have been intensive searches for other active materials, which can provide oxygen in its active form from water at low overpotentials, to increase the oxidation rate of chemisorbed CO. Pt-Ru alloys are reported as having excellent promotional effects, " and PtSn,[ l PtMo,t ° PtRuCo, and PtRuIrOs have also been studied for the MOR reaction of DMFC. 

Structure of Alloy Type Sn-Pt/Si02 Catalysts used in Low Temperature CO Oxidation. Both Mossbauer and FTIR spectroscopy provided sufficient proof of surface reconstruction during the low temperature CO oxidation. However, the above reconstruction appeared to be reversible as the reversible interconversion of PtSn Sn -I- Pt was demonstrated by both spectroscopic techniques. This reversibility can only be achieved if the segregation described above is within the supported nanoparticle, i.e., when surface reactions involved in CO oxidation do not result in formation of separate Pt and tin-oxide phases on the silica support. 

According to the results of in situ Mossbauer spectroscopy the formation and stabilization of the PtSn (1 1) alloy phase has been shown under condition of room temperature CO oxidation (see results presented in Table 12). One of the most active surfaces of the PtSn (1 1) alloy phase, the (110) phase, was chosen to model the interaction of the CO molecule with the metal surface. The computer modeling and the related calculations were made on density functional level. In this model a small cluster of the (110) surface of the PtSn phase as shown in Figure 25a, was selected in order to calculate and investigate the interaction of the CO molecule with the metal surface. 

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