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The influence of alloyed Sn on the chemistry of C4 butene isomers, including 1-butene, cis-2-butene, and isobutene, chemisorbed on Pt(111) was investigated by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Pt−Sn alloy chemistry was probed by investigation of two ordered surface alloys formed when Sn atoms were incorporated within the topmost layer on a Pt(111) substrate to form a (2 × 2) Sn/Pt(111) alloy with ϑSn = 0.25 and a (√3 × √3)R30° Sn/Pt(111) alloy with ϑSn = 0.33. Low-coverage states of chemisorbed 1-butene, cis-2-butene, and isobutene on Pt(111) have desorption activation energies of 17.5, 17, and 17 kcal/mol, respectively. These energies are reduced to 16, 15.5, and 15 kcal/mol on the (2 × 2) alloy and 13.5, 12, and 11 kcal/mol on the (√3 × √3)R30° alloy. Changing the surface Sn concentration from ϑSn = 0.25 to ϑSn = 0.33 causes a relatively larger decrease in the chemisorption bond strength of these alkenes, and we associate this with the importance of a pure Pt 3-fold site for strong alkene bonding. All three butenes undergo decomposition on Pt(111) during TPD which accounts for 50−60% of the chemisorbed monolayer. Alloying Sn into the surface causes a large reduction in the reactivity of the surface, and the fraction of the chemisorbed layer which decomposes is decreased to 3−7% on the (2 × 2) alloy, and no decomposition occurs on the (√3 × √3)R30° alloy. The strong reduction of decomposition on these two surface alloys may be due to the elimination of adjacent pure Pt 3-fold hollow sites. No large changes occur in the coverage of the chemisorbed monolayer of butenes in the presence of up to 33% of a monolayer of alloyed Sn, showing that the adsorption ensemble requirement for chemisorption of these alkenes on Pt(111) and the two Sn/Pt(111) alloys is at most a few Pt atoms. To the extent that alloying or direct Pt−Sn interactions occur in supported, bimetallic Pt−Sn catalysts, the chemistry reported here would lead to increased isobutene yields and decreased coking of the catalyst.

Two Pt−Sn surface alloys were oxidized at 300 K by ozone (O3) exposure in UHV. Both alloys were less reactive than Pt(111), and the p(2 × 2) alloy (ϑSn = 0.25) was more reactive than the (√3x√3)R30° alloy (ϑSn = 0.33). The relative O3 dissociative sticking coefficients on these surfaces at 300 K were 1.0:0.79:0.33, respectively. Ozone dissociation was inhibited more easily on the alloys than on Pt(111), and large O3 doses on the p(2 × 2) and (√3x√3)R30° surface alloys produced oxygen coverages of 1.2 and 0.87 monolayers, respectively, compared to 2.4 monolayers on Pt(111). Both chemisorbed and “oxidic” oxygen states were characterized by using Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). At 300 K, chemisorbed oxygen adatoms are formed at low exposures, but oxidation of Sn occurs at large oxygen coverages, as evidenced by a 1.6 eV downshift of the Sn(MNN) AES peak. Heating during TPD causes SnO x formation even at low coverages, and this decomposes to liberate O2 in desorption peaks at 1015 and 1078 K on the p(2 × 2) and (√3x√3)R30° surfaces, respectively. After oxidation of Sn, TPD indicates desorption of oxygen from chemisorbed adatoms bound at Pt sites and eventually formation of platinum oxide particles. SnO x particles formed in intimate contact with Pt by oxidation of these Pt−Sn alloys and high-temperature heating are easier (100 K) to reduce by heating in a vacuum than a corresponding thick SnO x film. We also find additional stability (130 K) imparted to PtO x particles by the presence of oxidized Sn following oxidation of these alloys. Heating these oxidized alloys to 1000 K produces a (4 × 1) LEED pattern that we have assigned to the formation of large domains of an SnO2 overlayer on both of the surface alloys.

The interactions of low-energy electrons with organic solids is of interest for a variety of processes. One particular application that we are interested in has to do with using low-energy electron induced dissociation (EID) of hydrocarbon multilayers to prepare monolayer coverages of adsorbed hydrocarbon intermediates on reactive metal surfaces under UHV conditions. To probe the mechanism of the formation of adsorbed alkyl species via EID of multilayer alkane films, we have investigated EID in a two-component, structured multilayer system on Pt(111) at 90 K. Temperature-programmed desorption (TPD) was mainly used to study the reaction products of EID of films comprised of cyclohexane-d 12 and n-butane. On the basis of these results, we conclude that diffusion of the EID-produced alkyl fragments to the surface is the main channel for the formation of surface alkyl species in these types of systems. We exclude a dominant role for transfer of the alkyl fragment to the surface indirectly through abstraction of hydrogen atoms from molecules in the chemisorbed layer to produce new reactive fragments adjacent to the surface. IntroductionElectron-induced dissociation (EID) and desorption (ESD) of adsorbed molecules has been extensively studied and both are well-known phenomena. 1-19 This is especially true in the case of weakly adsorbed molecules, because the cross sections for reactions caused by low-energy electrons are much larger for physisorbed molecules than for chemisorbed molecules. White and co-workers have effectively used the selectivity for C-H bond cleavage in EID of physisorbed molecules 18 using electron energies below 50 eV to prepare hydrocarbon intermediates on primarily Ag surfaces. This selectivity in EID using low-energy electrons to dissociate monolayers is unfortunately limited to weakly adsorbed molecules. This is simply because the metal substrate quenches excitations induced by low-energy electrons in cases of moderate and strong chemisorption. In common practice, however, the most interesting systems include moderately or strongly bonded adsorbates on reactive metal surfaces. We have recently described a simple but crucial modification of EID experiments which enables selective preparation of hydrocarbon intermediates on reactive metal surfaces containing moderately and strongly bound chemisorbed adsorbates. Instead of using monolayer coverages, we produce a multilayer. The additional layers are isolated from the substrate by the monolayer, and excitations induced by lowenergy electrons have a prolonged lifetime which opens a dissociation pathway. Our first experiments produced adsorbed cyclohexyl on Pt(111) and Sn/Pt(111) surface alloys 16,20 via EID of cyclohexane multilayers. In that work, the cyclohexane multilayer had a large cross-section for EID (on the order of 10 -15 cm 2 ), but the cross section for the chemisorbed monolayer was more than 1000 times smaller. Once the cyclohexyl intermediate was bonded to the surface after being formed from EID of the multilayer, the chemisorption bon...

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