Acetylene chemisorption on Sn/Pt(100) alloys

Adsorption and reaction of acetylene on a hexagonally reconstructed (5 x 20 )-Pt(100) surface and two ordered Sn/Pt(100) alloy surfaces were investigat ed using temperature programmed desorption spectrometry (TPD), Auger electr on spectroscopy (AES), low energy electron diffraction (LEED) and X-ray pho toelectron spectroscopy (XPS). Vapor deposition of Sn onto a Pt(100) single -crystal substrate was used to form two Pt-Sn alloys? the c(2 x 2) and (3 r oot 2x root2)R45 degrees Sn/Pt(100) structures with theta (Sn) = 0.5 and 0. 67 ML, respectively, depending on the initial Sn concentration and annealin g temperature. Acetylene nearly completely decomposed during TPD on Pt(100) in the absence of Sn, forming hydrogen, which then desorbs as H-2, and sur face carbon. This decomposition, associated with irreversible dissociative adsorption, was strongly suppressed on the two Pt-Sn alloy surfaces, and a large acetylene desorption peak in TPD was observed. Additionally, 15% of t he adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3 root 2x root2)R45 degrees Sn/Pt(100) alloy. No such benzene deso rption occurred from the c(2 x 2) alloy. Alloyed Sn in the c(2 x 2) alloy d ecreased the initial sticking coefficient of acetylene on Pt(100) at 100 K by similar to 40%, but additional Sn in the other alloy had no additional e ffect. The saturation coverage of C2D2 in the chemisorbed monolayer at 100 K decreased from that on Pt(100) by 35% on the c(2 x 2) alloy and 50% on th e (3 root 2x root2)-R45 degrees Sn/Pt(100) alloy. However, the c(2 x 2)-Sn adlayer eliminates acetylene chemisorption, illustrating that the effective ness of Sn to "block" sites depends crucially on its location as an adatom or alloyed atom on Pt surfaces. The acetylene chemisorption bond energy, es timated by the acetylene desorption activation energy measured in TPD, also decreased (45-65%) as the alloyed Sn concentration increased. Multiple TPD peaks for C2D2 desorption from both the c(2 x 2) and the (3 root 2x root2) R45 degrees Sn/Pt(100) alloy surfaces indicate either several energetically distinguishable adsorption sites for acetylene or the rate-limiting influe nce of more complex surface reactions on these surfaces.