Infrared spectroscopic investigations of electrocatalysis

In situ infrared spectroscopy is a particularly powerful method for obtaining molecular specific information pertaining to structure and bonding at the electrode-electrolyte interphase. This study demonstrates the versatility of the technique by elucidating several problems of topical interest. Carbon monoxide adsorbed on a polycrystalline platinum electrode shows a shift in band position with potential. Evidence is provided that the intense electric field across the metal-solution interface interacts with the adsorbed CO, perturbing the bonding and resulting in the dependence of the vibrational frequency on electrode potential. The necessity for a thin layer of electrolyte, between the infrared transmitting window and the electrode, has previously constrained in situ infrared studies to processes in which there is no sustained faradaic current. A thin layer flow cell has been constructed which enables rapid replenishment of the electrolyte layer, while simultaneously obtaining spectra. Using such a flow cell reactive intermediates have been detected during the electrooxidation of methanol at platinum electrodes in aqueous acid electrolytes. These adsorbed intermediates have been identified as enspaceCOH and enspaceCHOH (or enspaceCH2OH). Investigations using in situ IR spectroscopy of adsorbed hydrogen on polycrystalline Pt, Rh, Ir and single crystal Pt(111), Pt(110) and Pt(100) electrodes in acid solution allow the identification of a new type of this adsorbate, which is shown to be the intermediate in H2 evolution. The absorption band at 2090 cm-1 enables it to be identified as a hydrogen atom singly coordinated on top of a surface metal atom. SNIFTIRS spectra of the adsorbed bisulphate anion have been obtained for Pt(100), (110) and (111) electrodes. For Pt(111) evidence is provided for the adsorbed anion being coordinated through the 3 non-equivalent oxygen atoms. For Pt(110) and (100) the spectra are assigned to the adsorbed anion bonding through either one or two oxygen atoms. Some preliminary data for Cl-(aq.) adsorption on Ag and Pt and calomel formation on Hg is presented, using SNIFTIRS in the far-IR region. These and other results indicate that it is possible to record SNIFTIRS spectra to as low as 40 cm-1. (DX86225)