Microcalorimetric, infrared spectroscopic, and DFT studies of ethylene adsorption on Pt/SiO2 and Pt-Sn/SiO2 catalysts

Microcalorimetric and infrared spectroscopic measurements for adsorption of ethylene on Pt/SiO(2) and PtSn/SiO(2) (5-8 wt % Pt) have been carried out at temperatures from 173 to 300 K. Ethylene adsorption on Pt and Pt-Sn at temperatures lower than 233 K leads to the formation of pi-bonded and di-sigma-bonded ethylene species. Ethylidyne species begin to form at temperatures higher than 263 K. Formation of ethylidyne species is suppressed by the presence of Sn. The main surface species observed during ethylene adsorption at room temperature are ethylidyne species on Pt/SiO(2), di-sigma-bonded ethylene on 7Pt/Sn/SiO(2), and pi-bonded ethylene on 3Pt/Sn/SiO(2). The heat of formation of ethylidyne species and coadsorbed atomic hydrogen on Pt was found to be 157 kJ/mol, while the average heat for the formation of pi-bonded and di-sigma-bonded ethylene species on Pt was 125 kJ/mol. The heats for formation of pi-bonded and di-sigma-bonded ethylene on Pt-Sn surfaces were found to be between 98 and 106 kJ/mol. Results from quantum chemical calculations employing density functional theory (DFT) for Pt(19) and Pt(16)Sn(3) clusters indicate that Sn donates electrons to the 6sp and 5d orbitals of platinum. These calculations show that the bonding between ethylidyne species and 3-fold hollow sites composed of adjacent Pt atoms involves electron donation from ethylidyne into 6sp orbitals of Pt and back-donation of electrons from 5d orbitals of Pt to the ethylidyne species. The higher electron density on the Pt atoms caused by Sn leads to more repulsive interactions for the formation of ethylidyne species than for the formation for pi-bonded and di-sigma-bonded ethylene species, and the higher occupation of the Pt 5d orbitals in the presence of Sn requires more extensive back-donation of electrons from Pt to ethylidyne species. These electronic effects caused by Sn weaken the bonding of ethylidyne species at neighboring 3-fold hollow sites.