Pt-Sn bimetallic catalysts

Catalytic reaction of CO2 with C2H4 on supported Pt-Sn bimetallic catalysts... 

Acetylene is a reactive molecnle with a low C H stoichiometry that can be used to evaluate the resistance of metal-based catalysts to the formation of carbonaceous residue (coking). Pt is very reactive, and the chemisorption of on Pt(lll) is irreversible under UHV conditions, with complete conversion of into surface carbon during heating in TPD. Alloying with Sn strongly reduces the amount of carbon formed during heating [49]. This is consistent with observations of increased lifetimes for commercial, supported Pt-Sn bimetallic catalysts compared to Pt catalysts used for hydrocarbon conversion reactions. 

Results from these studies are important for the ongoing debates on the existence and utility of Sn/Pt alloy phases in bimetallic Pt-Sn supported catalysts. For example, our observation of dramatically decreased carbon buildup on the alloy surfaces from acetylene (a coke-precursor), and the enhanced yield of aromatics and alkenes from alkane dehydrogenation mimics important aspects of the chemistry of commercial Pt-Sn supported catalysts used for reforming. On the contrary, it seems unlikely that Sn/Pt alloy phases are solely responsible for the high selectivity observed in crotonaldehyde hydrogenation using Pt-Sn bimetallic catalysts. 

Kaneko S, Arakawa T, Ohshima M, et al Dehydrogenation of propane combined with selective hydrogen combustion over Pt-Sn bimetallic catalysts, Appl Catal A Gen 356 (l) 80-87, 2009. 

The early XPS studies, including those from our laboratory, revealed that the tin is present only in an oxidized state (10,16). These results were consistent with those for the Pt-Re bimetallic catalysts where only oxidized Re was observed (9,22). Li et al. (23,24) reported that a portion of the tin in Pt-Sn-alumina catalysts was present in the zero valence state furthermore, it appears that the composition of the Pt-Sn alloy, based upon the amount of Pt in the catalyst and the Sn(0) detected by XPS, increases with increasing ratios of Sn/Pt. 

These results also demonstrate that zero-valent tin does affect catalytic performance in beneficial ways. So, although zero-valent tin is rarely detected in conventional Pt-Sn/Al203 catalysts, small amounts possibly formed on the Pt particles (by H2 reduction of Sn2+) may be at least partially responsible for beneficial changes in this important class of bimetallic catalysts. (44)... 

The essential application of LEISS analysis concerns supported bimetallic catalysts. For these solids, XPS analysis can no longer be considered as a surface analysis. The sizes of the particles present on the surface of these catalysts are around a nanometre, similar to the mean free path of the photoelectrons that determines the thickness probed by XPS. Certain pairs of elements do not lend themselves to LEISS analysis. This is the case, for example, with Pt-Rc for which the mass difference of the two metals is small compared to the resolution. One example of a pair that lends itself to LEISS analysis is provided by a study of Pt-Sn reforming catalysts. LEISS is ten times more sensitive for platinum than for tin. Despite this, an intensity of the tin peak at least comparable to that of platinum can be observed on a catalyst with a Pl/Sn ratio of three (Fig. 6.5), This study can be used to show a surface segregation of tin that is dilTicult to detect by XPS because of the small size of the platinum particles (2 nm). 

Table 4 presents the CO2/O2 ratio for the oxidation of the niobia catalysts. This ratio decreases in the bimetallic Pt-Sn/Nb20s catalyst forming less CO2, which suggests a more hydrogenated coke. Moreover, the compounds of the soluble coke, already shown in Table 3, indicate that coke was more hydrogenated on Pt/Nb20s than on Pt/Al20j catalyst. 

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. 

Fig. 1 shows the n-butane conversion on frwsh Ft/AliOj and Pt-Sn/AlzQj catalyst as a function of reaction Iiwe After reaction for 1 hr, the n-butane conversion on the PI/Al Oa catalyst was 22%, while that on PI.-ST1/AI2G3 was 45%. [ I. can be seen that the stability of the bimetallic catalyst was better than that of the monometallic catalyst. 

In the course of the catalytic reduction, deposition of the second metal (or the third) can occur on both the parent metal and the support, depending on the nature of the support and the operating conditions. Effectively, in the case of bimetallic Rh-Ge [41] and Pt-Sn [38] catalysts, it was observed that the amount of Ge or Sn deposited is higher on alumina than on silica supported catalysts (Fig. 

A similar set of experiments has been performed on the bimetallic Pt-Sn/Al203 catalyst and the results are summarized in Figure 12.22. It was noted that O2 oxidizes the Sn present in the Pt-Sn alloy during the coke bum-off, while subsequent reduction with H2 restores the Pt- Sn alloy. Metal nanoparticle sintering... 

Passes FB, Aranda DAG, Schmal M. Characterization and catalytic activity of bimetallic Pt-In/AI203 and Pt-Sn/AI203 catalysts. J Catal. 1998 178 478. 

Supported bimetallic catalysts find many industrial applications. Examples include Pt and Rh in automobile exhaust conversion catalysts and Pt and Re (or Pt and Sn or Pt and Ir) in naphtha reforming catalysts. 

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