Interactions between adsorbed hydrogen and water molecules on a polycrystalline Pt electrode studied by in-situ ATR-FTIR spectroscopy

Abstract

In order to gain further insight into the interactions between hydrogen adsorbed on platinum and water in an acid electrolyte, in situ infrared spectra were examined in detail. A polycrystalline Pt electrode/sulfuric acid solution interphase was observed by in-situ attenuated total reflectance (ATR) FTIR spectroscopy in the potential region between the double layer and hydrogen evolution by use of the potential difference method. The background potential in the measurements was chosen first at 0.7 V vs. RHE to examine the effect of underpotentially deposited hydrogen (HUPD) adsorption on the interfacial water spectra. Analysis of the spectra revealed two well-defined HOH bending bands, a sharp one at 1620 cm−1 and a broad one at 1700 cm−1. The intensity of the 1620 cm−1 band was found to increase linearly with increasing coverage of HUPD determined by the cyclic voltammogram (CV) corrected for the HER current. This led to an assignment to water molecules co-adsorbed with HUPD with only weak interactions with the latter and between each other. The broad band at 1700 cm−1 was assigned to the HOH bending mode of water associated with hydronium ions. Its intensity was found to increase at lower potentials due to its electrostatic attraction to the increasingly negative surface charge. An intense, broad, negative-going band around 2970 cm−1 was found to develop below 0.4 V, which is assigned to water dimers adsorbed at on-top Pt sites in the double layer potential region and desorbed upon adsorption of HUPD. A new choice of the reference potential at 0.2 V was successful in detecting a distinct OH stretching band around 3580 cm−1, which developed in close correlation with the band assigned to overpotentially deposited hydrogen (HOPD) at ca. 2080 cm−1. The band was assigned to the asymmetric OH stretching mode of water molecules hydrogen-bonded with HOPD. Its band intensity was found to increase linearly with that of HOPD with a slope close to 1.0, which suggests a highly polar nature of the Pt-HOPD bond.