Investigation of the vibrational properties of CN– on a Pt electrode by in situ VIS–IR sum frequency generation and functional density calculations

Abstract

The vibrational properties of CN^– pseudohalide ions adsorbed on a Pt(110) single-crystal electrode in aqueous neutral solution are studied using in situ VIS–IR sum frequency generation (SFG) and compared to theoretical results obtained from density functional theory (DFT) calculations on PtNC^– and PtCN^– molecular ions. As observed previously on a polycrystalline electrode, the adsorption behaviour of both ions depends drastically on the electrode potential and on the immersion potential in a CN^–-containing solution. When the electrode is immersed at negative potential, two absorption bands are detected in the spectral range of the CN^– stretching vibration modes on a surface site. The first resonance at 2070 cm^–1 is already observed at –1.4 V (Ag/AgCl), in the hydrogen evolution potential region, with an initial potential tuning rate of 20 cm^–1. A second narrow resonance, peaking at 2150 cm^–1, starts growing at –0.6 V (Ag/AgCl). The occurrence of two resonances, corresponding to N-bound and C-bound CN^– species, is clearly evidenced for a single-crystal electrode, and appears as a genuine property of the system, not a side effect induced by surface defects. The polycrystalline structure and the defects influence the shape, the position and the width of the resonances, not the basic spectroscopic properties of the system. This result is further supported by new density functional calculations of the vibrational properties of the PtCN^– and PtNC^– molecular ions.