Pt-Sn alloyed surfaces

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

Baltruschat and coworkers [120] reported a large negative shift of CO oxidation to 0.25 V caused by Sn adsorbed at step sites on Pt (332) and Pt(755) stepped surfaces (Fig. 10). Sn adsorbs preferentially at the step sites, which can be deduced from the inhibition of H adsorption on the step sites from voltammetry curves [121]. The effect has been ascribed to a destabilization of the CO molecules due to repulsion between CO and Sn. This repulsion shifts the CO molecules into a so-called high-coverage state, which can be oxidized at low potentials as shown for Pt-Sn alloy surfaces [122]. (This state is not formed in oxidation of methanol on this alloy, which was offered as an explanation of the inhibition effect observed in that reaction.) The CO oxidation rates at low potentials are, however, very sluggish. The effect of Sn on the reaction on Pt(lll) was found to be negligible. The activity of stepped Pt surfaces is certainly related to the observed effect of Sn on polycrystalline Pt [97], which usually has a high density of steps. 

Sn (25 or 33%) in ordered structures. Dehydrogenation of hexene and hexadi-ene was less pronounced on Pt-Sn surfaces than on pure Pt. Dehydrogenation of hexadiene produced carbonaceous deposits rather than benzene on Pt-Sn alloy surfaces, where threefold Pt sites were blocked by Sn, pointing to their importance in aromatization. 

Most of the studies have involved the alloying of a second metal to platinum. The second metal was generally chosen because of its ability to increase the concentration of oxygenated species on the electrode surface, but also for its corrosion resistance. Even if some discrepancies exist in the literature, R-Ru is now widely accepted as the most interesting one, and hence our analysis will focus on this alloy in the next subsection. Other alloys such as R-lr, R-Os, or R-Re have also been reported to be good candidates, and R-Mo under specific conditions of preparation was claimed to have the desired properties. The Pt-Sn alloy is still a subject... 

In the presence of tin, the number of active platinum sites seems to be superior compared to catalyst B and thus an increase in the space velocity by a factor of ten does not seem to saturate all the sites. These results show the importance of the role played by tin since platinum loading was the same in both cases. It is reasonable to think that in the case of catalyst B, due to the method of deposition, some aggregates of platinum are formed on the surface of the catalyst. In the presence of tin, a part of the aggregates could disappear and some Pt/Sn alloy particles, better dispersed at the surface, could be formed. 

Figure 2.16 Characterization by high-resolution electron microscopy of a bimetallic nanoparticle of a Pt-Sn alloy obtained via the surface organometallic route.

Coking - regeneration cycle was examined. Surface area diminished while Pt-Sn alloy increased. Sn2+ and A1203 interaction important in catalyst stability. Ce02 addition enahnced stability.68 ... 

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,... 

Wang, K. et al.. On the reaction pathway for methanol and carbon monoxide electrooxidation on Pt-Sn alloy versus Pt-Ru alloy surfaces, Electrochim. Acta, 41, 2587, 1996. 

Abstract Thermally stable, ordered surface alloys of Sn and Pt that isolate threefold Pt, twofold Pt, and single-Pt atom sites can be produced by controlled deposition of Sn onto Pt single crystals and annealing. The strnctnre was established by characterization with several techniques, including ALISS, XPD, LEED, and STM. Chemisorption and catalysis studies of these well-defined, bimetallic surfaces also define the overall chemical reactivity of Pt-Sn alloys, clarify the role of a second-metal component in altering chemistry and catalysis on Pt alloys, and develop general principles that describe the reactivity and selectivity of bimetallic alloy catalysts. 

Studies on unsupported, bulk Pt-Sn alloys have been helpful in assessing the properties of Pt-Sn alloy clusters formed in catalysts [15, 16], A large number of surface science studies of Pt-Sn alloys have also been conducted the most extensive of these concerned the oxidation of PtjSn, in which much work was done by Hoflund and coworkers (as reviewed by Unger and Marton [17]). Bulk alloy samples can be problematic in fundamental chemisorption and catalytic reaction studies because of significant differences between the composition and structure of the surface and bulk. For example, exposure to or annealing in vacuum causes Sn... 

In this chapter, we will illustrate with a few selected examples how well-defined, ordered Pt-Sn surface alloys have been used to elucidate the overall chemical reactivity of Pt-Sn alloys, clarify the role of Sn in altering this chemistry and catalysis, and develop general principles for understanding the reactivity and selectivity of bimetallic alloy catalysts. Most studies have involved chemisorption under UUV conditions, but the use of these alloys as model catalysts for investigating catalysis at pressures up to one atmosphere will also be discussed. 

Atrei A, Bardi U, Rovida G, Torrini M, Zanazzi E, Ross PN (1992) Structure of the (001)- and (lll)-oriented surfaces of the ordered fee Pt Sn alloy by low-energy-electron-diffraction intensity analysis. Phys Rev B 46 1649... 

Pretreatment in a stream of 5vol% CO in He at 125 °C for Ih. This is assumed to form Pt/Sn alloy particles having surface hydroxyls. 

A more delicate item may be the effect of alloy formation between noble metals and the metal components of the supports. In the case of Pt/Sn02, low-temperature (at 120 °C) reduction is required, which leads to both the formation of Pt-Sn alloys and the formation of surface hydroxyls at the perimeter [68]. On the other hand, in the case of Au/Ti02, vacuum evacuation or reduction dramatically suppresses the initial catalytic activity, which can be recovered gradually during CO oxidation in excess O2. The removal of oxygen species at the perimeter interface is deleterious to supported Au catalysts. 

Wang K, Gasteiger HA, Markovic NM, Ross PN (1996) On the reaction pathway fm methanol and carbmi mmioxide electrooxidation mi Pt-Sn alloy versus alloy surfaces. Electrochim Acta 41 2587-2593... 

Shubina, T.E. and Koper, M.T. (2002) Quantum-chemical calculations of CO and OH interacting with bimetallic surfaces. Electrochim. Acta, 47, 22—23. Gasteiger, H.A., Markovic, N.M., and Ross, P.N. (1995) Electrooxidation of CO and H2/CO mixtures on a well-characterized PtjSn electrode surface./. Phys. Chem., 99, 16757-16767. Wang, K., Gasteiger, H.A., Markovic, N.M., and Ross, P.M. (1996) On the reaction pathway for methanol and carbon monoxide electrooxidation on Pt-Sn alloy versus Pt-Ru alloy surface. Electrochim. Acta, 41, 2587-2593. 

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