Pt-Sn surface alloys

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. 

In order to better understand ensemble-size requirements for reactions on Pt alloys and the role of Sn as a site-blocking agent, it is important to study other Pt-M surfaces that are isostructural with Pt-Sn surface alloys. However, this has turned out to be a difficult task. Different metals (M=Ge, Zn, Cu, Ag, Fe, and Ti) form different surface alloy structures. For example, on Pt(lll), Zn and Cu alloy but do not form long-range ordered structures, and Ge only forms a dilute, two-domain... 

Microreactor Studies of Catalysis over Pt-Sn Surface Alloys... 

Tsai YL, Koel BE (1997) Temperature programmed desorption investigation of the adsorption and reaction of butene isomers on Pt(lll) and ordered Pt-Sn surface alloys. J Phys Chem B 101 2895... 

Panja C, Saliba N, Koel BE (1998) Adsorption of methanol, ethanol and water on well characterized Pt-Sn surface alloys. Surf Sci 395 248... 

Olivas A, Jerdev D, Koel BE (2004) Hydrogenation of cyclohexanone on Pt-Sn surface alloys. J Catal 222 285... 

Pt-Sn surface. Even if hydrogen needs more than one transition metal atom in order to dissociate, this would only explain the large decrease in hydrogen adsorption if the possibility of spillover is largely reduced on the alloys. The ligand effect, to be discussed in Section IV, would have to be present for this assumption. 

A rapid survey of the methods utilized for the study of binary alloys, and specifically for the Pt-Sn system will be reported here. In the present review, we consider only studies performed in conditions of ultra high vacuum (UHV), where bimetallic Pt-Sn surfaces are stable. It is known that in air and in general in the presence of oxygen at pressures larger than ca. 10 Torr, tin alloyed with platinum tends to oxidize and de-alloy to form oxide phases, a phenomenon that will not be treated here. 

Measured rumpling amplitudes of Sn surface alloys on the (111) surfaces of Ni, Cu, Rh and Pt in a similar format to table 2. Note that Sn atomic radii corresponding to half the Sn-Sn distances are given for both semiconducting diamond-structure a-Sn and tetragonal p-Sn which is the stable phase at room temperature. 

In the case of the Ni/Sn system, data also exists for all three low index faces, forming c(2x2) 0.5 ML surface alloy phases on both Ni(lOO) and Ni(l 10), while on Pt(l(X)) there are also data for a c(2x2)-Sn surface alloy, so these systems allow a rather different comparison of mmpling amplitudes, shown in Table 5. Notice that on all these surfaces the Sn atoms must replace Ni or Pt atoms which, at least in one direction within the surface, are separated by their nearest-neighbour distance of twice their metallic radius. A simple... 

Measured rumpling amplitudes of Sn surface alloys on different orientation surfaces of Ni and Pt in a similar format to table 4. 

Xu C, Peck JW, Koel BE (1993) A new catalyst for benzene production from acetylene under UHV conditions - Sn/Pt(lll) surface alloys. J Am Chem Soc 115 751... 

The chemisorption of acetylene on the Sn/Pt(100) surface alloys revealed similar chemistry and provided additional information on the structure sensitivity of these reactions [53, 54]. While 15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3V2xV2)R45°-Sn/Pt(100) alloy, no such benzene desorption occurred from related surfaces, as shown in Fig. 2.6. 

Early higher pressure reaction smdies over Pt-Sn model catalysts by Paffett [62,63] and Somorjai [64, 65] and their coworkers revealed new insights into hydrocarbon catalysis in such systems. Szanyi et al. [62] showed that n-butane hydrogenolysis under moderate pressures (1-200 Torr H3/butane=20) and temperatures (up to 650 K) could be carried out without disruption of the ordered Sn/Pt(lll) surface alloys. This established that such catalytic reactions could be studied while maintaining the composition and geometric structure of these alloys under reducing reaction conditions (but not catalytic oxidation due to the aggressive interaction of O3 with Sn). These ordered Sn/Pt surfaces are qualitatively different from those in many studies of promoters and poisons, or disordered alloys, e.g., Au-Pt, in which the quantitative information on ensemble sizes available for reactions is difficult to determine. 

Xu C, Koel BE (1994) Probing the modifier precursor state adsorption of CO on Sn/Pt(lll) surface alloys. Surf Sci 304 L505... 

Samson P, Nesbitt A, Koel BE, Hodgson A (1998) Deuterium dissociation on ordered Sn/ Pt(lll) surface alloys. J Chem Phys 109 3255... 

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