Surface structure dependence of reactive chemisorption of acetonitrile on single-crystal Pt surfaces

Abstract

Acetonitrile (AN) is reductively chemisorbed at Pt at the beginning of the “double-layer” potential region in the cyclic voltammogram for this electrode material, taken between 0.06 V and ca. 0.80 V vs. RHE. In this adsorbed state, it then undergoes almost reversible stages of reoxidation or re-reduction, and slower further reduction over the underpotential deposition (UPD) H region. In this respect, Pt, bearing a sub-monolayer of chemisorbed acetonitrile, was one of the first “chemically modified electrodes” to be discovered. These processes, previously studied at polycrystalline Pt, were investigated by means of cyclic voltammetry at the (111), (110) and (100) planes, together with the (311) stepped plane, of well characterized single-crystal Pt surfaces which exhibit, in the absence of adsorbed acetonitrile, the characteristically distinguished cyclic voltammograms demonstrated by Clavilier for clean, well prepared surfaces. Complementary information on the reductive chemisorption of AN and competitive occupation of H sites is provided by recording of adsorption current transients when AN is added, at controlled potential, to an initially AN-free solution. In the UPD H region, the transient currents contain a component due to the “anodic H desorption effect”. The chemisorption and surface reactivity behaviour of acetonitrile at these crystal planes is found to be very specific to the geometry of the surfaces and provides one of the clearest examples of such surface specificity in electrochemical surface science. Similarities to and contrasts with the surface-specific behaviour of the four crystal planes with respect to UPD H behaviour and HSO 4 − or ClO 4 − anion adsorption are observed and their significance is discussed.