Pt-Sn-alumina catalysts

FIGURE 1. Schematic of a proposed structure for a Pt-Sn-alumina catalyst. 

In the following, we summarize some data from methods that make a more direct measure of the chemical or physical state of the Pt and/or Sn present in Pt-Sn-alumina catalysts. 

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. 

Transmission Electron Microscopy. An electron microdiffraction technique was employed to identify crystal structures developed in two Pt-Sn-alumina catalysts (47). One catalyst was prepared by co-precipitating Sn and A1 oxides and then impregnating the calcined material with chloroplatinic acid to give... 

Our view of the catalyst surface is schematically depicted in Figure 4. The indirect and direct characterization data for Pt indicates that it is present in a zero valence state. The Pt will therefore be distributed among Pt atoms, Pt clusters that are larger than one atom and Pt present as a Pt/Sn alloy. Thus, a description of the state of Pt in the Pt-Sn-alumina catalyst involves determining the fraction present in each of the three states. Furthermore, both of the direct methods for determining the Pt/Sn alloy composition, XRD and TEM, indicates that only the PtSn =1 1 alloy is formed. Thus,... 

Figure 4. Schematic of proposed surface structure of a low (1 1) Pt/Sn-alumina catalyst.

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. 

Lin et al. (77) reported that there are two types of sites on Pt-Sn-alumina catalysts sites adsorb hydrogen that can react with ethene while hydrogen... 

Alcohol and aldehyde decarbonylation on Rh(l 11), activation of C-H, C-C, and C-0 bonds, 345-353 Alkane dehydroeyelization with Pt-Sn-alumina catalysts aromatic formation, 120 preparation condition effect, 119... 

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... 

X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) Spectra of Pt—Sn—Alumina Catalysts... 

The state of tin in Pt/Sn/alumina catalysts was investigated bv Li and Shia (25) via Mossbauer spectroscopy (i/9Sn enriched isotopes) and XPS. The former technique indicated the presence of Sn+, Sn+2 and Sn, in proportions that depended on the method of preparation, but in all cases the Sn+4 component dominated. These conclusions were confirmed by the XPS experiments. Additional TPR tests on the reduced catalyst and on samples exposed to air showed that reoxidation of Pt/Sn/alumina reduced preparations was rather slow, confirming our EXAFS observations. The presence of zero valent tin in similar preparations, using the acetone complexation procedure, was recently confirmed by Li, Stencel and Davis (12) in an extended XPS investigation. For reduced samples, with a Pt Sn ratio 1 5, these authors estimated that approximately 68% of the tin was in the metallic state. However, they observed that exposure of the sample to air for 10 minutes entirely eliminated the XPS detectable Sn°. Their data also indicated that upon reduction, chlorine migrated from the surface to the alumina. Thus, XPS which measures surface composition indicates a higher sensitivity to oxidation than was demonstrated by our EXAFS experiments, which is a bulk diagnostic. 

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. 

Srinivasan R, De Angeles RJ, Davis BH. AUoy formation in Pt-Sn-alumina catalysts In situ X-ray diffraction study. J Catal 1987 106 449-57. 

Catalyst Pt-Sn-/-alumina catalyst for the catalytic reforming of naphtha. 

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