Disentangling Chemical\nInteraction and Electric Fields\nat Electrochemical Interfaces

The specific binding of solution components to a catalytic\nsurface\nand the interfacial electric field are critical to electrocatalysis,\nbut the two factors are entangled. Distinguishing whether adsorbate\nbinding to the surface is governed by through-bond charge transfer\nthat is modulated by a field or through-space electrostatic effects\nis essential for rational catalyst design. Here, we probed the binding\nof thiocyanate (SCN<sup>–</sup>) to Au and Pd as a function\nof applied potential (ϕ<sub>app</sub>) using surface-enhanced\ninfrared absorption spectroscopy (SEIRAS). The IR data showed distinct\nshifts of the CN stretching wavenumbers (ν̃<sub>CN</sub>) with ϕ<sub>app</sub> for S- versus N-bound SCN<sup>–</sup> and on Au versus Pd. These ν̃<sub>CN</sub> shifts with\nϕ<sub>app</sub> are the result of different extents of through-space\nelectric field (Stark) effects and through-bond chemical binding effects.\nThe presence of two binding modes of SCN<sup>–</sup> on the\nmetal surfaces provided two separate interfacial probes, which enabled\nestimating the contributions of the two effects to the overall ν̃<sub>CN</sub> versus ϕ<sub>app</sub> slopes quantitatively. Competitive\nadsorption experiments using SCN<sup>–</sup> and different\nhalides (Cl<sup>–</sup>, Br<sup>–</sup>, and I<sup>–</sup>) support our conclusions. While the through-bond effect for S-bound\nSCN<sup>–</sup> is more pronounced on Pd than on Au, that of\nN-bound SCN<sup>–</sup> is similar. Our data also suggest a\nsimilar electric field change with ϕ<sub>app</sub> on Pd and\nAu. This work offers a method to differentiate key contributors to\ninterfacial chemistry in an external electric field and provides important\ngroundwork for the development of future electrocatalysts.