Spectroscopic study of the nitric oxide adlayers formed from nitrous acid solutions on palladium-covered platinum single-crystal electrodes

Palladium multilayers deposited on Pt(lll), Pt(100), and Pt(110) single-cry stal electrodes have been characterized by using adsorbed nitric oxide as a probe molecule. Nitric oxide adlayers can be easily formed on the palladiu m-covered electrodes from nitrous acid or NO-saturated solutions and are st able under electrochemical conditions in a wide range of electrode potentia ls. The in-situ FTIR spectra obtained for the saturated adlayers have been compared with those previously reported in the N-O stretching region for NO adsorbed on bulk palladium single-crystal surfaces both under UHV and elec trochemical conditions. A good agreement has been found in all cases regard ing the frequency of the bands observed. Changes in the spectra upon partia l reduction of the NO adlayer also fits with coverage-dependent changes pre viously reported. On the basis of all these structure-sensitive characteris tics of the vibrational spectra of adsorbed NO, we can conclude that pallad ium layers are grown epitaxially on the platinum substrate. Thus, the resul ting palladium-covered electrodes seem to be a good alternative for the stu dy of any structure-dependent process on palladium. The combination of in-s itu FTIR and on-line DEMS experiments reported in this paper has provided a dditional data on the electrochemical behavior of the palladium-covered ele ctrodes in the nitrous acid solution. Nitric oxide, nitrous oxide, and ammo nium have been shown to be the main reduction products formed during the re duction of nitrous acid at the palladium electrode surface. Nitrate and hip onitrate anions seem to be also formed from nitrous acid at 0.90 V. It has also been shown that dissolved ammonium is the only product formed during t he reductive stripping of adsorbed NO.