Microcalorimetric, Infrared Spectroscopic, and DFT Studies of Ethylene Adsorption on Pt/SiO₂ and Pt−Sn/SiO₂ Catalysts

Abstract

Microcalorimetric and infrared spectroscopic measurements for adsorption of ethylene on Pt/SiO2 and Pt−Sn/SiO2 (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 π-bonded and di-σ-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/SiO2, di-σ-bonded ethylene on 7Pt/Sn/SiO2, and π-bonded ethylene on 3Pt/Sn/SiO2. 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 π-bonded and di-σ-bonded ethylene species on Pt was 125 kJ/mol. The heats for formation of π-bonded and di-σ-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 Pt19 and Pt16Sn3 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 π-bonded and di-σ-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.

Experimental and DFT Studies of the Conversion of Ethanol and Acetic Acid on PtSn-Based Catalysts

“…Ethylene adsorption on Pt catalysts at 300 K confirmed the presence of large Pt ensembles through formation of ethylidyne surface species. 35 Ethylidyne surface species were not detected on Pt-Sn alloy catalysts, confirming the breakup of large Pt ensembles through alloying.…”

Structure and reactivity of Pt–In intermetallic alloy nanoparticles: Highly selective catalysts for ethane dehydrogenation

“…Dehydrogenation is a structure insensitive reaction requiring a single active atom [46,47] while hydrogenolysis is a structure sensitive reaction requiring an ensemble of active atoms [47]. It has been shown that Pt 3-fold hollow sites present in large Pt ensembles are responsible for the formation of strongly adsorbed alkylidyne species which are precursors of hydrogenolysis and coke forming reactions [48][49][50][51]. The formation of Pt 3 In reduces the number of Pt 3-fold hollow sites and it has been shown that the formation of ethylidyne is suppressed on Pt 3 Sn alloys with equivalent structures to the Pt 3 In phase in Pt-In(0.7) [48,50,51].…”

“…It has been shown that Pt 3-fold hollow sites present in large Pt ensembles are responsible for the formation of strongly adsorbed alkylidyne species which are precursors of hydrogenolysis and coke forming reactions [48][49][50][51]. The formation of Pt 3 In reduces the number of Pt 3-fold hollow sites and it has been shown that the formation of ethylidyne is suppressed on Pt 3 Sn alloys with equivalent structures to the Pt 3 In phase in Pt-In(0.7) [48,50,51]. While the number of Pt 3-fold hollow sites is reduced, they are not completely eliminated, and trimers of Pt atoms are still present in the alloy structure.…”

Density functional theory studies of the adsorption of ethylene and oxygen on Pt(111) and Pt3Sn(111)

“…This dependence has important implications in the observed hydrocarbon chemistry on PtSn alloy catalysts. For example, we have recently combined results from experimental and theoretical investigations to interpret the observed reaction kinetics data for ethane hydrogenolysis over platinum, 55,60,61 which has been a probe reaction to investigate the reactivities of hydrocarbons over metal catalysts. 62 In our studies, we conclude that the preferred reaction pathways for cleavage of C-C bond involve activated C 2 H 5 and CHCH 3 species, which occupy two-fold and three-fold sites, respectively.…”