Vibrational sum-frequency generation study of the CO2 electrochemical reduction at Pt/EMIM-BF4 solid/liquid interfaces

Electrochemical reduction of CO2 on polycrystalline Pt catalysts in a 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM]BF4 room-temperature ionic liquid (RTIL) is not impaired by well-known effects such as high ohmic losses or vigorous hydrogen evolution reactions. Vibrational broadband sum-frequency generation (SFG) combined with cyclic voltammetry has been applied to identify the formation of a [CO2-EMINI] complex at the interface which gives rise to a SFG active vibrational mode at 2355 cm(-1). The [CO2-EMIM] complex serves as a stable precursor to CO2 reduction and is linked to a remarkable small potential barrier for CO2 reduction and subsequent CO production. Vibrational SFG spectra reveal CO linearly bonded to Pt atop sites as the predominant surface adsorbed reduction product. Extended electrochemical reduction of CO2 results in an intense CO band and an electrochemical Stark tuning rate of 33 cm(-1)/V at potentials >-0.2 V vs. SHE which is indicative of a close packed CO adlayer. Anomalous Stark tuning for potentials <-0.2 V is observed and attributed to hydrogen co-adsorption from residual H2O into the CO adlayer. CO modification of Pt leads to considerable reduction of the observed reduction current density that is caused by a poisoning effect of the CO adlayer that persists at the interface for +0.5 to -0.8 V. (C) 2016 Elsevier B.V. All rights reserved.