Microcalorimetric, Infrared Spectroscopic, and DFT Studies of Ethylene Adsorption on Pd and Pd/Sn Catalysts

Microcalorimetric and infrared spectroscopic (FTIR) measurements for the adsorption of ethylene on Pd/SiO 2 and Pd/Sn/SiO 2 catalysts (4 wt % Pd, Pd/Sn = 3) have been performed at temperatures of 300, 263, and 233 K. In addition, microcalorimetric measurements were made for H 2 and CO adsorption and FTIR studies were conducted of CO adsorption at 300 K on these catalysts. Quantum chemical calculations employing density functional theory (DFT) were performed using Pd 10 and Pd 6 Sn 4 clusters. Ethylene adsorption on the catalysts results in the formation of ethylidyne species, di-σ-bonded ethylene, and π-bonded ethylene species at 300 K, with initial heats of adsorption of 160 and 110 kJ/mol for the Pd and Pd/Sn catalysts, respectively. Only di-σ-bonded ethylene and π-bonded ethylene species form at 263 and 233 K, with the π-bonded ethylene species dominating. The initial heats of ethylene adsorption are equal to 110 and 102 kJ/mol on Pd/SiO 2 at 263 and 233 K, respectively; and these values are equal to 90 and 85 kJ/mol on Pd/Sn/SiO 2 at these lower temperatures. In addition to the lower heats of ethylene adsorption caused by the addition of Sn, a new band at 1542 cm - 1 is observed in the IR spectra of ethylene on Pd/Sn/SiO 2, and this band is representative of a weakly adsorbed, π-bonded ethylene species. Quantum chemical calculations indicate that the electronic effect of Sn addition to Pd is most significant for adsorption at 3-fold sites (e.g., formation of ethylidyne species), the effect of Sn is smaller for adsorption at bridge-bonded sites (e.g., formation of di-σ-adsorbed ethylene), and the effect of Sn is smallest for adsorption at atop sites (e.g., formation of π-adsorbed ethylene).