SMAD catalysts

All of the bimetallic SMAD catalysts have shown unusual catalytic properties. Several laboratories are now reporting similar findings.(22,23) Both geometric/ensemble effects and electronic effects have been considered in trying to explain these unusual properties. 

Through the combined use of catalytic probe reactions, Mossbauer, EXAFS, XPS, XRD, it has been demonstrated that the anticipated particle structures for the half-SMAD and full SMAD procedures are close to reality.(40-42) Thus, 119Sn Mossbauer, a bulk solid analysis technique, revealed the relative amounts of Sn, Pt-Sn alloy, SnO, and Sn02 present in the catalysts. It was possible to differentiate Sn° from Pt-Sn alloy through supporting evidence of XPS and selective oxidation, since it was found that ultra-fine Sn° particles were much more susceptible to oxidation than Pt-Sn alloy particles. Also, since the full SMAD Pt°-Sn°/Al203 catalysts behaved much differently than Pt°/Al203, it is clear that the SMAD catalysts are not made up of separate Pt° and Sn particles. 

Indeed, with mono-metallic studies, we have found that solvent polarity and warm-up procedure can drastically affect resultant crystallite sizes,(29) magnetic properties,(29,31) and ability to form stable colloidal solutions.(32,34) However, a bimetallic such as Au-Sn has not been examined in this way before (although other bimetallics have yielded interesting SMAD catalysts).(40,60,61,62,63)... 

Solvated Metal Atom Dispersed (SMAD) Bimetallic Catalysts... 

Figure 1 illustrates these four catalyst preparation methods and the anticipated Pt-Sn cluster structures. Note that the conventional preparation methods are expected to yield Pt° and Sn4+/Sn2+ oxides on A1203, while the SMAD methods are expected to yield Pt° and Sn° combinations of differing morphologies. 

Metal-supported catalysts are among the important classes of synthetic materials developed in the past 50 years. The importance of these materials can be gauged from the fact that about 20% of the U.S. national gross product is dependent on such catalysts. Thus, the preparation of zero valent metal particles supported catalysts is very vital to the economy of any nation. Before discussing the advantages of the SMAD method, it is worthwhile to briefly review the traditional methods of preparation of metal-supported catalysts. 

In a subsequent report we shall present NEXAFS and EXAFS structural data for bimetallic supported catalysts prepared via the SMAD procedure and other techniques. 

A comprehensive comparison of the catalytic properties of nanopartides obtained via chemical and physical routes is difficult, because of the scarce information on nanoparticles prepared via physical methods. Catalytic applications were described for Pd nanopartides obtained via electrochemical route, and metal clusters obtained in a SMAD reactor.P3 l The differences in the catalytic behaviour relative to analogous conventional catalysts obtained via chemical route h concerning activity and selectivity in, e.g., the hydrogenation of 1-heptene or 1,4-butadiene lie not only in the size and shape of the partides, but also in specific interactions with the solvent. Thus, it is daimed that Pd nanopartides, e.g., react with polar solvent molecules and acquire a negative charge. The absence of other chemical species, which can interact with the metal and the small size of the nanopartides make them in some degree comparable with gas phase metal clusters and their unique reactivity. 4]... 

In order to discriminate between the activity of the different gold species on ceria, our group prepared ceria-supported gold catalysts by classical deposition-precipitation (DP) followed by drying at 393 K, and by the solvated metal atom dispersion (SMAD) technique... 

---