POTENTIAL-DIFFERENCE SURFACE INFRARED-SPECTROSCOPY UNDER FORCED HYDRODYNAMIC FLOW CONDITIONS - CONTROL AND ELIMINATION OF ADSORBATE SOLUTION-PHASE INTERFERENCES

The utilization of a thin-layer spectroelectrochemical cell featuring forced hydrodynamic solution flow for the separation of potential-difference infrared (PDIR) spectral components arising from compositional changes at the electrode interface and in the bulk solution is illustrated for the adsorption of azide and cyanate ion at polycrystalline silver. In a conventional (stagnant) thin-layer configuration, bipolar PDIR bands are obtained, reflecting the coupled changes in the interfacial and solution-phase adsorbate composition that arise from potential-dependent adsorption-desorption equllibria. In the presence of sufficient hydrodynamic flow, however, only the unipolar band component for the adsorbed species remains since the solution composition remains invariant. Separate extraction of the solution-phase infrared component of the bipolar bands can be obtained by appropriate subtraction of corresponding spectra obtained in the presence and absence of solution flow. Such tactics enable a quantitative assessment of the coverage-dependent infrared band characteristics, since the solution-phase band absorbance, A1(sol), in the PDIR spectra provides a reliable monitor of the potential-induced change in adsorbate surface concentration. The relationships between the coverage-dependent integrated absorbances in the adsorbed and solution state are discussed for the asymmetric N-N-N and C-N stretching bands of azide and cyanate, respectively. The general utility of such flow-cell tactics for distinguishing between surface and solution-phase components of PDIR spectra is pointed out.