TEMPERATURE-PROGRAMMED DESORPTION INVESTIGATION OF THE ADSORPTION ANDREACTION OF BUTENE ISOMERS ON PT(111) AND ORDERED PT-SN SURFACE ALLOYS

The influence of alloyed Sn on the chemistry of C-4 butene isomers, in cluding l-butene, cis-2-butene, and isobutene, chemisorbed on Pt(lll) was investigated by temperature-programmed desorption (TPD), Auger ele ctron spectroscopy (AES), and low-energy electron diffraction (LEED). Pt-Sn alloy chemistry was probed by investigation of two ordered surfa ce alloys formed when Sn atoms were incorporated within the topmost la yer on a Pt(lll) substrate to form a (2 x 2) Sn/Pt(lll) alloy with The ta(Sn) = 0.25 and a (root 3 x root 3)R30 degrees Sn/Pt(lll) alloy with Theta(Sn), = 0.33. Low-coverage states of chemisorbed l-butene, cis-2 -butene, and isobutene on Pt(lll) 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 x 2) alloy and 13.5, 12, and 1 1 kcal/mol on the (root 3 x root 3)R30 degrees alloy. Changing the sur face Sn concentration from Theta(Sn) = 0.25 to Theta(Sn) = 0.33 causes a relatively larger decrease in the chemisorption bond strength of th ese alkenes, and we associate this with the importance of a pure Pt 3- fold site for strong alkene bonding. All three butenes undergo decompo sition on Pt(lll) during TPD which accounts for 50-60% of the chemisor bed monolayer. Alloying Sn into the surface causes a large reduction i n the reactivity of the surface, and the fraction of the chemisorbed l ayer which decomposes is decreased to 3-7% on the (2 x 2) alloy, and n o decomposition occurs on the (root 3 x root 3)R30 degrees alloy. The strong reduction of decomposition on these two surface alloys may be d ue to the elimination of adjacent pure Pt 3-fold hollow sites. No larg e changes occur in the coverage of the chemisorbed monolayer of butene s in the presence of up to 33% of a monolayer of alloyed Sn, showing t hat the adsorption ensemble requirement for chemisorption of these alk enes on Pt(lll) and the two Sn/Pt(lll) alloys is at most a few Pt atom s. To the extent that alloying or direct Pt-Sn interactions occur in s upported, bimetallic Pt-Sn catalysts, the chemistry reported here woul d lead to increased isobutene yields and decreased coking of the catal yst.