Paper SS-WeA10
Modifying Selectivity of Hydrocarbon Conversion Reactions by Alloying Sn and Pt: Benzene Formation from Acetylene

Wednesday, October 31, 2012, 5:00 pm, Room 21

Bimetallic catalysts involving Sn and Pt have important applications in hydrocarbon conversion catalysis. We have performed experiments probing chemisorption and reaction kinetics on well-defined, ordered Pt-Sn surfaces in order to aid developments needed for improving catalyst selectivity and overall performance. One specific example from investigations of benzene formation from acetylene on Pt-Sn alloys with HREELS, TPD, and DFT calculations will be discussed. On Pt(111), μ₃-η²-acetylene chemisorbed in a three-fold site is the most stable configuration, as indicated by DFT calculations, and has a C-C stretching frequency (ν_CC) of 1310 cm^-1. This configuration becomes less thermodynamically favorable in the presence of Sn compared to the bridge-bonded μ₃-η² configuration (ν_CC = 1495 cm^-1). The ν_CC peaks at ~1600 cm^-1 are assigned to π-bonded acetylene, which dominate the spectra collected at 90 K. Absence of three-fold Pt sites on the Pt₂Sn alloy inhibits the transformation of acetylene to CCH₂, and the ν_CC peak at 1412 cm^-1 assigned to CCH₂ appears only in the spectra of the Pt₃Sn alloy. DFT calculations show that the destabilizing effect of Sn alloying is more significant for CCH₂ and CCH + H than for acetylene. This change in relative stability increases the barrier for acetylene decomposition and makes associative reactions more likely. Results from DFT calculations indicate that benzene formation on the Pt-Sn alloys proceeds through the formation of an upright cyclic C₄H₄ intermediate, which is predicted to produce benzene by reacting with an additional surface acetylene. This closely integrated experimental-computational study has enabled us for the first time to characterize the adsorption modes of acetylene on Pt-Sn alloys. In addition, we developed a molecular level reaction mechanism for benzene formation by consolidating HREELS and TPD experimental results with DFT calculations. The presence of Sn changes the preferential adsorption sites for hydrocarbons, decreases the stability of adsorbed species to varying degrees, and favors associative reactions, thus, enabling benzene production by cyclotrimerization of acetylene.

B.E.K. acknowledges support by NSF Grant No. CHE-1129417.