NITRIC-OXIDE AS A PROBE ADSORBATE FOR LINKING PT(111) ELECTROCHEMICAL AND MODEL ULTRAHIGH-VACUUM INTERFACES USING INFRARED-SPECTROSCOPY

The effects of water coadsorption on nitric oxide adlayers on Pt(111) in ultrahigh vacuum (uhv) are examined with infrared reflection-absorption spectroscopy (IRAS) along with work-function measurements with the objective of relating the uhv-based system to NO chemisorption at the Pt(111)-aqueous electrochemical interface as studied recently by in-situ IRAS. In contrast to the corresponding (and extensively studied) Pt(111)/CO system, solvent coadsorption apparently yields little or no change in the NO surface binding geometry at low as well as saturated chemisorbate coverages, the solvent-induced downshifts (ca. 35-70 cm(-1)) in the N-O stretching (upsilon(NO)) frequencies being consistent with the occurrence of only an electrostatic Stark effect. This behavior, along with the stability of the electrochemical NO adlayer at relatively high electrode potentials (E), facilitates intercomparison of the surface potentials for the aquated uhv acid in-situ interfaces by matching the upsilon(NO) spectrum for the former with the upsilon(NO) frequency-E data for the latter interface. This procedure yields an estimate of the ''absolute'' electrode potential, E(k), of the normal hydrogen electrode equal to 4.9 +/- 0.1 V. The approximate consistency of this value with some previous estimates of E(k) supports the essential validity of the low-temperature uhv-based approach for exploring chemisorbate solvation effects.