High-pressure adsorption of ethylene on cubic Pt nanoparticles and Pt(100) single crystals probed by in situ sum frequency generation vibrational spectroscopy

A model catalytic system of a monolayer consisting of 9-nm average size, cubic, single-crystal Pt nanoparticles and poly(vinylpyrrolidone) (PVP) polymer capping agent deposited on a sapphire prism was investigated by sum-frequency generation (SFG) vibrational spectroscopy in total internal reflection (TIR) geometry. Exposure of a clean nanoparticle monolayer after removal of PVP by cyclic oxidation-reduction treatment to high-pressure ethylene at room temperature led to the formation of ethylidyne and di-σ bonded ethylene. Low-pressure ethylene adsorption on a pseudohexagonal reconstructed Pt(100) single crystal resulted only in the formation of di-σ bonded ethylene. High-pressure adsorption of ethylene on Pt nanoparticle monolayers and Pt(100) led to the formation of both ethylidyne and di-σ bonded ethylene and stabilized the pseudohexagonal reconstruction of Pt(100) on both the single crystal and the surface of clean cubic nanoparticles. Restructuring of the PVP layer caused by CO adsorption indicated a small fraction of the Pt surface was available for adsorption. The stretching frequency of linear-bound CO red-shifted relative to CO adsorption on a clean Pt nanoparticle monolayer. PVP reversibly restructured upon the removal of CO by oxidation at room temperature. After the near complete removal of PVP by a cyclic low-temperature oxidation-reduction process, the peak position of the linear-bound CO blue-shifted to a frequency consistent with the adsorption of CO on a clean Pt surface. The successful application of TIR-SFG to catalytically relevant surfaces under high-pressure conditions demonstrated in this study is a significant advance in the detection of surface intermediates.