Temperature-Programmed Desorption Investigation of the Adsorption and Reaction of Butene Isomers on Pt(111) and Ordered Pt−Sn Surface Alloys

Abstract

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.