Combined DRIFTS and DFT Study of CO Adsorption and Segregation Modes in Pt-Sn Nanoalloys

This contribution aims at rationalizing the observations made by in situ IR spectroscopy during CO adsorption over ca. 1.8 nm Pt-Sn nanoparticles by density functional theory (DFT) calculations and describing the environment and electronic properties of surface Pt atoms modified by Sn. Pt surface enrichment upon CO exposure was observed by diffuse reflectance Fourier transform infrared (FT-IR) spectroscopy (DRIFTS) and rationalized by the theoretical calculations, which also indicated that isolated Pt was favored over Pt pairing in a Sn-rich alloy. DFT frequency calculations allowing the fine assignment of vibrational nu CO spectra show a gradual decay of the linear carbonyl wavenumber from plain Pt (2075 cm(-1)), through a Sn-poor Pt-Sn nanoparticle (2054 cm(-1)), and down to a Sn-rich Pt-Sn nanoparticle (2039 cm(-1)). This decay is accompanied by a weakening of Pt-CO bonds, which confirms the ability of Sn to prevent CO poisoning of Pt when present in a Pt-Sn nanoalloy. Moreover, electronic structure analyses evidence a predominant charge transfer from Sn to Pt atoms, which explains the weakening of both the Pt-CO and C-O bonds through back-donation to antibonding molecular orbitals. A weaker C-O bond would favor CO dissociation and the formation of Pt and SnOx ensembles, which is consistent with DRIFTS data recorded at low temperatures.