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-crystal electrodes have been characterized by using adsorbed nitric oxide as a probe molecule. Nitric oxide adlayers can be easily formed on the palladium-covered electrodes from nitrous acid or NO-saturated solutions and are stable under electrochemical conditions in a wide range of electrode potentials. 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 electrochemical conditions. A good agreement has been found in all cases regarding the frequency of the bands observed. Changes in the spectra upon partial reduction of the NO adlayer also fits with coverage-dependent changes previously reported. On the basis of all these structure-sensitive characteristics of the vibrational spectra of adsorbed NO, we can conclude that palladium layers are grown epitaxially on the platinum substrate. Thus, the resulting palladium-covered electrodes seem to be a good alternative for the study of any structure-dependent process on palladium. The combination of in-situ FTIR and on-line DEMS experiments reported in this paper has provided additional data on the electrochemical behavior of the palladium-covered electrodes in the nitrous acid solution. Nitric oxide, nitrous oxide, and ammonium have been shown to be the main reduction products formed during the reduction of nitrous acid at the palladium electrode surface. Nitrate and hiponitrate 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 the reductive stripping of adsorbed NO.