Microcalorimetric, infrared spectroscopic, and DFT studies of ethylene adsorption on Pd and Pd/Sn catalysts

Microcalorimetric and infrared spectroscopic (FTIR) measurements for the ad sorption of ethylene on Pd/SiO2 and Pd/Sn/SiO2 catalysts (4 wt % Pd, Pd/Sn = 3) have been performed at temperatures of 300, 263, and 233 K. In additio n, microcalorimetric measurements were made for H-2 and CO adsorption and F TIR studies were conducted of CO adsorption at 300 K on these catalysts. Qu antum chemical calculations employing density functional theory (DFT) were performed using Pd-10 and Pd6Sn4 clusters. Ethylene adsorption on the catal ysts results in the formation of ethylidyne species, di-sigma-bonded ethyle ne, and pi-bonded ethylene species at 300 K, with initial heats of adsorpti on of 160 and 110 kJ/mol for the Pd and Pd/Sn catalysts, respectively. Only di-sigma-bonded ethylene and Jr-bonded ethylene species form at 263 and 23 3 K, with the pi-bonded ethylene species dominating. The initial heats of e thylene adsorption are equal to 110 and 102 kJ/mol on Pd/SiO2 at 263 and 23 3 K, respectively; and these values are equal to 90 and 85 kJ/mol on Pd/Sn/ SiO2 at these lower temperatures. In addition to the lower heats of ethylen e adsorption caused by the addition of Sn, a new band at 1542 cm(-1) is obs erved in the IR spectra of ethylene on Pd/Sn/SiO2, and this band is represe ntative of a weakly adsorbed, pi-bonded ethylene species. Quantum chemical calculations indicate that the electronic effect of Sn addition to Pd is mo st significant for adsorption at 3-fold sites (e.g., formation of ethylidyn e species), the effect of Sn is smaller for adsorption at bridge-bonded sit es (e.g., formation of di-sigma-adsorbed ethylene), and the effect of Sn is smallest for adsorption at atop sites (e.g., formation of pi-adsorbed ethy lene).