Pt-Sn / C catalysts

On the other hand, the yield of CO2 with a Pt/C catalyst is double that with a Pt-Sn/C catalyst (see Table 1.2). This can be explained by the need to have several adjacent platinum sites to adsorb dissociatively the ethanol molecule and to break the C-C bond. As soon as some tin atoms are introduced between platinum atoms, this latter reaction is disadvantaged. 

Antolini and Gonzalez recently proposed a simple empirical model to evaluate the contribution of alloyed and non-aUoyed platinum and tin to the ethanol oxidation reaction on Pt-Sn/C catalysts for DEFC [194]. On the basis of the model, the ethanol oxidation on partially alloyed catalysts occurs through a dual pathway mechanism, separately involving the PtsSn and Pt-SnOx phases. The model, validated by experimental data, can predict the performance of a DEFC by varying the Sn content and/or the degree of alloying of Pt-Sn/C catalysts used as the anode material (Fig. 8.20). 

Pt-Sn electrocatalysts only show modest improvement in catalyzing MOR compared to pure Pt catalysts, despite the superior performance of Sn as a cocatalyst for enhancing CO oxidation [86]. Generally, comparisons between ft-Ru and Pt- n catalysts indicated that the former are more active for the MOR, and DMFCs with Pt-Ru/C anode catalysts demonstrated substantially better performance compared to one with Pt-Sn/C catalysts under similar operating conditions [62, 87-89]. 

Pt-Sn/C catalysts with 10-20 at. % of Sn exhibited best activity at low potentials. Results from Jiang et al. [68] showed that the ft-SnOx/C catalyst with 30 at.% of Sn was the most active among four different Pt/Sn ratios. An integrated surface science and electrochemistry study of the SnOx/Pt(lll) model catalysts indicate a volcano dependence of the EOR activity on the surface composition, with the maximum at the SnOx coverage of 37 % [69]. Despite the improved overall EOR activity of optimized Pt- n system, on-line differential electrochemical mass spectroscopy (OEMS) studies have shown that acetic acid and acetaldehyde represent the dominant products with CO2 formation contributing only 1-3 % [68]. 

The experiments were carried out using Pt/C, Pt-Sn/C and Pt-Sn-Ru/C catalysts and in each case no other reaction products than AAL, AA and CO2 were detected. The addition of tin to platinum not only increases the activity of the catalyst towards the oxidation of ethanol and therefore the electrical performance of the DEFC, but also changes greatly the product distribution the formation of CO2 and AAL is lowered, whereas that of AA is greatly increased (Table 1.2). 

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-... 

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... 

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. 

Table 5 presents the heat of combustion of dispersed coke on the deactivated catalysts. The Pt-Sn/Nb205 catalyst exhibits the highest exothermicity during the coke bum, which is related to higher H/C ratios in coke. Similarly, the Pt/Nb20s catalyst evidences the presence of lighter hydrocarbons in coke than the Pt/Al203 catalyst. 

Comparison of the CO2 and CH4 conversions for the Pt and Pt-Sn catalysts under the two different reaction temperatures (linear ramp and constant at 650°C) shows that the conversion of both species was much higher when the reaction temperature was held constant at 650°C. Comparison of the TPO s for both the Pt and Pt-Sn/Si02 catalysts under the two reaction conditions show that the runs in which the temperature was held constant at 650°C resulted in... 

Although the Pt/Sn/PPha catalyst system has been described as highly active , this activity is relative to that of Pt catalysts without SnCl2, which are extremely slow. Hydroformylation rates for Pt/Sn/PPha range from about 25 to 50 turnovers/h for 1-alkenes at 70 °C and 1500psi. This rate can be compared to Rh/PPhs, which produces 500 turnovers/h with 1-hexene giving a linear to branched aldehyde product ratio of 17 1 at QOpsig and 90 °C. 

Tayal J, Rawat B, Basu S (2011) Bi-metallic and tri-metallic Pt-Sn/C, Pt-Ir/C, Pt-Ir-Sn/C catalysts for electro-oxidation of ethanol in direct ethanol fuel cell. Int J Hydrogen Energy 36 14884-14897... 

Pt-Sn electrocatalysts have been considered as the most active binary catalyst for the EOR, and their superior performance has been confirmed in fuel cell measurements [89-91]. Sn promotes the EOR activity of Pt and works even better than Ru. Polyol method [92, 93] and Bonneman method [94,95] were employed to synthesize alloy Pt-Sn/C and Pt-SnO c/C catalysts, and Jiang et al. [68] claimed that the greater activity was from Pt-SnO c/C due to the presence of both sufficiently... 

Jiang L, Cohnenaresa L, Jusysa Z, Sunb GQ, Behma RJ (2007) Ethanol electrooxidation on novel carbon supported Pt/SnOx/C catalysts with varied Pt Sn ratio. Electrochim Acta 53 377-389... 

Pd-Pb/C catalysts with different amounts of Pb were prepared using NaBH4 chemical reduction method in the presence of sodium citrate. Pd-Pb (4 1)/C showed better activity towards ethanol electrooxidation in alkaline electrolyte than Pd/C catalyst. The Arrhenius equation was used to calculate the activation energy, which showed a smaller value, thus implying a faster charge transfer process. The enhanced activity of Pd-Pb/C was explained by a bifunctional mechanism and the d-band theory [56]. Pd4-Au/C and Pd2.5-Sn/C catalysts prepared by He et al. [72] showed lower activity for ethanol electrooxidation in alkaline electrolyte than commercial Pt/C but were more tolerant to poisoning. 

In the electrochemical oxidation of methanol, carbon dioxide gas is the chief reaction product. The yields of other potential products of the oxidation reaction, such as formaldehyde, formic acid, and the like, are a few percent at most. Arico et al. (1998) concluded from a chromatographic analysis of the reaction products that the chief product of electrochemical oxidation of ethanol (with a yield of about 98%) is CO2, just as for methanol. This conclusion is inconsistent with the results obtained by other workers. Wang et al. (1995) studied the reaction products of ethanol and propanol oxidation by differential electrochemical mass spectrometry. They found that during the reaction, only 20 to 40% of the theoretical yield of CO2 is produced, whereas acetaldehyde is formed to 60 to 80% (even traces of acetic acid are formed). Rousseau et al. (2006) used a high-performance liquid chromatograph for analysis of the products of ethanol oxidation. According to their data, about 50% aldehyde, 30% acetic acid, and only about 20% CO2 are formed at a temperature of 90°C at a platinum catalyst. With Pt-Sn or Pt-Sn-Ru catalysts, somewhat different numbers were obtained 15% aldehyde, 75% acid, and 10% CO2. It follows from these data that the composition of the reaction products depends heavily on the catalyst used for the... 

S. Garcia-Rodriguez, F. Somodi, 1. Borbath, J.L. Margjtfalvi, M.A. Pena, J.L.G. Fierro, S. Rojas, Controlled synthesis of Pt-Sn/C fuel cell catalysts with exclusive Sn-Pt interaction apphcation in CO and ethanol electrooxidation reactions, Appl. Catal. B Environ. 91 (2009) 83-91. 

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