Nitric oxide reduction and oxidation on stepped pt[n(111)x(111)] electrodes

The structure sensitivity of the reduction and oxidation of saturated and subsaturated NO adlayers has been studied on a series of stepped Pt[n(111)x(111)] electrodes by cyclic and stripping voltammetry experiments in sulfuric and perchloric acid solution. In agreement with earlier experimental work, the NO adlayer on Pt(111) may be oxidized and subsequently reduced between 0.6 and 1.1 V (vs RHE) in a surface-bonded redox couple. From the assumption that in a reductive stripping experiment, the NO adlayer is completely reduced to ammonia, the NO saturation coverage on Pt(111) is ca. 0.4-0.5. Using this coverage, the NO oxidation-reduction couple at 0.6-1.1 V appears to be a one-electron reaction involving adsorbed NO, HNO2, and coadsorbed OH. From our analysis of the reductive stripping of the NO adlayer, the NO saturation coverage is observed to increase with the openness of the surface, from ca. 0.4-0.5 on Pt(111) to 0.9-1.0 on Pt(110). Even though the voltammetry due to the reductive process differs on the different surfaces, the reduction of chemisorbed NO does not appear to be a very structure sensitive process. The reduction takes place in two or three features on all surfaces, the corresponding charges of which may vary but the potentials of which do not seem to be very structure sensitive. Any observed structure sensitivity is believed to be due to different NO surface configurations and coverages combined with structure-sensitive hydrogen and anion adsorption, rather than a structure sensitivity of the NO reduction per se. Our conclusion regarding the structure insensitivity of the NO reductive stripping is also supported by the 40 mV/dec Tafel slopes obtained on the different surfaces, which is interpreted in terms of an EE mechanism, involving two concerted proton-electron-transfer reactions with the second being rate determining, leading to the formation of a H2NO species on the surface. Since this process takes place at the same potentials on all surfaces, with negligible hydrogen adsorbed on the surface, it is suggested that protons are transferred directly from a solution (hydronium) species, without the involvement of adsorbed hydrogen. Significantly, the lack of structure sensitivity for the surfaces studied indicates that neither the adsorption strength of NO to the surface nor the dissociation of the N-O bond plays a significant role in the overall rate of the electrochemical NO adsorbate reduction.