Supported Sn-Pt Catalysts

Anchoring of tin ions onto the surface of the support is based on the reactivity of surface OH groups of Si02 or AI2O3. This has been accomplished by using 

In solvated metal atom dispersion (SMAD) method solvated atoms prepared at very low-temperature are used as transient, highly reactive organometallic reagent for the deposition of Sn-Pt bimetallic particles onto different supports. In another approach chemical vapor deposition (CVD) using tin organometallic compounds was applied. For example, the selective reaction of Sn(CH3)4 vapour with Pt nanoparticles supported on Si02 appears to be very promising preparation method. 

Methods of Controlled Surface Reactions (CSRs) and Surface Organometallic Chemistry (SOMC) were developed with the aim to obtain surface species with Sn-Pt interaction. In CSRs two approaches have been used (i) electrochemical, and (ii) organometallic. Characteristic feature of the organometallic approach is that both CSR and SOMC results in almost exclusively supported alloy type bimetallic nanoclusters. Studies on the reactivity of tin organic compounds towards hydrogen adsorbed on different transition and noble metals have revealed new aspects for the preparation of supported bimetallic catalysts. 

The formation of surface alloys, phase segregation at the surface, site sensitive chemisorption, changes in electronic properties, surface reconstruction, etc. have been investigated by a range of surface science methods. 

2 Preparation of Alloy Type Sn-Pt/Si02 Catalysts. Supported bimetallic Sn-Pt catalysts can be prepared using different methods and approaches. However, exclusive formation of alloy type nanoclusters can be achieved by using methods of surface organometallic chemistry, namely by applying Controlled Surface Reactions (CSRs) between hydrogen adsorbed on platinum and tin tetraalkyls. 

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XPS analysis of silica-supported Sn-Pt catalysts shows that the binding energy of platinum shifted towards lower values of approximately 1 eV with respect to... 

Figure 15 Liquid phase hydrogenation of methylvinil ketone on Si02 supported Sn-Pt catalyst, Sna cfJPts= 1.43. Effect of the pretreatment with crotonaldehyde on the selectivity. Selectivity - conversion dependencies without and with pretreatment in crotonaldehyde, and , respectively. Temperature of the pretreatment in hydrogen = 300 °C, temperature of the pretreatment with crotonaldehyde = 80 °C, concentration of crotonaldehyde = 30 mmol/dm reaction temperature = 40 °C, initial concentration of the substrate = 120 mmollgca,-(Reproduced from ref. 107 with permission)...

Figure 23 Schematic view of silica supported Sn-Pt nanocluster after reduction in hydrogen at 300 °C. Catalyst type (H-3)...

It has been shown that new types of supported Au, Cu, Sn-Pt, Sn-Ru, Re-Pt, catalysts have been prepared and used for selective hydrogenation of different organic carbonyl compounds (unsaturated aldehydes, esters, carboxylic acids, carboxamides, etc.) and nitriles. Supported Sn-Pt and Au catalysts were also... 

Most studies coincide in that Sn-Pt/C catalysts show the highest Faradaic current for the electrooxidation of ethanol. Therefore, numerous studies have been devoted to the synthesis of carbon supported Sn-Pt/C electrocatalysts and to the understanding of the promotional... 

Pt-Sn Pt-Pb Initial catalyst lifetime, A1203 support Hexane reforming. 

Pt-Sn-Alumina Structure. No single model will adequately describe the above catalyst characterization data and the published data that has not been included because of space limitations. The relative distribution of both the Pt and Sn species depend upon a number of factors such as surface area of the support, calcination and/or reduction temperature, Sn/Pt ratio, etc. Furthermore, it appears that the "co-impregnated" and "co-precipitated" catalysts are so different that their structure should be considered separately. 

For a "co-impregnated" catalyst all, or the dominant fraction of, both Pt and Sn are located on the surface alumina support. For the following discussion we consider the role of only the support surface area, the metal concentration and the Sn/Pt ratio. First consider the case of a series of catalysts with a constant Pt loading but with variable Sn/Pt ratios. 

The situation appears to be very different for the catalyst where the support was prepared by coprecipitation of a mixed tin and aluminum hydroxide. In this case it appears that a high fraction of the tin is present in the interior of the solid, with the fraction located on the surface being small. Thus, when Pt is added to the surface by impregnation, even when the bulk Sn/Pt ratio is on the order of 1 to 4, the actual surface ratio of Sn/Pt will be very much smaller. 

Beltramini and Trimm (67) utilized Pt-, Sn- and Pt-Sn- supported on y-alumina for the conversion of n-heptane at 500°C and 5 bar. They observed that during six hours less coke per mole of heptane converted was deposited on the Pt-Sn-alumina catalyst than on Pt-alumina however, the total amount of coke formed during six hours was much greater on Pt-Sn-alumina than on Pt-alumina. The addition of tin increased the selectivity of dehydrocyclization. Since hydrocracking and isomerization activity of a Sn-alumina catalyst remained high in spite of coke formation, the authors concluded that there was little support for the suggestion that tin poisons most of the acid sites on the catalyst. These authors (68) also measured activity, selectivity and coking over a number of alumina supported catalysts Pt, Pt-Re, Pt-Ir, Pt-Sn and Pt-... 

Li and Klabunde (72) utilized a pulse reactor (normal pressure) to carry out n-heptane conversions. Pt and Sn were evaporated into a solvent at low temperature following evaporation the solvent was allowed to warm to room temperature where agglomeration of atoms took place to produce a dispersion of colloidal particles that were then added to an alumina support. These catalysts were compared to conventional Pt-Sn-alumina catalysts for n-heptane conversion. The authors proposed that the presence of small amounts of Sn° on the surface of Pt can cause both an increase in catalytic activity and a decrease in hydrogenolysis. 

A series of catalysts, each with 1 wt.% Pt but containing 1 to 8 atomic ratio of Sn/Pt on an acidic support, were utilized for the conversion of n-octane at 100 psig. As the data in Figure 5 clearly show, the addition of tin caused a... 

For PtSn supported on a nonacidic alumina the addition of Sn causes an increase in activity up to Sn/Pt = 4, and then a decline in activity for low pressure operation (42). The increase in activity is much less at 400 psig operation than the two-fold increase observed at atmospheric pressure. However, there is a change in the selectivity of aromatic isomers produced from n-octane at both 15 and 400 psig as Sn is incorporated into the catalyst. Thus, both Pt and Pt/Sn catalysts produce only (> 90-95%) ethylbenzene and o-xylene as the dehydrocyclization products from n-octane. However, Pt produces ethylbenzene o-xylene = 1 1 whereas a catalyst with Sn/Pt = 4 produces ethylbenzene o-xylene = 1 2. This change in aromatic isomerization leads to two postulates ... 

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. 

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