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The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.

The dissociation and electrooxidation of ethanol and acetaldehyde was studied in perchloric acid on platinum electrodes by employing Surface Enhanced Raman Spectroscopy (SERS). It is found that the scission of the carbon−carbon bond occurs readily at low potentials, leading to adsorbed C1 species. Besides adsorbed CO, chemisorbed CH has been observed for the first time as an adsorbed intermediate in the electrochemical oxidation of ethanol and acetaldehyde. In addition, it is found that the breaking of the C−C bond is easier in acetaldehyde than in ethanol. On the basis of the results obtained here, a reaction mechanism for the dissociation and oxidation of ethanol and acetaldehyde is suggested.

We present the construction and some first applications of an On-line electrochemical mass spectrometry system for detecting volatile products formed during electrochemical reactions at a single-crystal electrode in hanging meniscus configuration. The system is based on a small inlet tip made of porous Teflon and a Peek holder, which is brought in close proximity (ca. 10-20 lm) to the electrode surface. The tip is connected to the mass spectrometer by glass and metal tubing. Because of the small amount of gas entering the mass spectrometer, no differential pumping is needed during the measurement. The tip construction and preparation introduced here leads to reproducible voltammetry with very good cleanliness characteristics. The presence of the tip has no significant influence on the blank voltammetry of a Pt(111) in sulfuric acid, and on voltammetric responses for CO adlayer oxidation, methanol oxidation, and hydroxylamine electrochemistry on Pt(111). The formation of gaseous products in these reactions can be followed accurately and is in good agreement with earlier results obtained by other mass spectrometric or spectroscopic techniques. The time response and tailing of the setup is on the order of seconds and mainly determined by the distance between the tip and the electrode.

We present a combined in situ Fourier transform infrared reflection-absorption spectroscopy and voltammetric study of the reduction of saturated and subsaturated NO adlayers on Pt(111) and Pt(110) single-crystal surfaces in acidic media. The stripping voltammetry experiments and the associated evolution of infrared spectra indicate that different features (peaks) observed in the voltammetric profile for the electrochemical reduction of NO adlayers on the surfaces considered are related to the reduction of NO(ads) at different adsorption sites and not to different (consecutive) processes. More specifically, reduction of high- and intermediate-coverage (ca. 0.5-1 monolayers (ML)) NO adlayers on Pt(110) is accompanied by site switching from atop to bridge position, in agreement with the ultra-high-vacuum data. On Pt(111) linearly bonded (atop) NO and face-centered cubic 3-fold-hollow NO species coexist at high coverages (0.25-0.5 ML) and can be reduced consecutively and independently. On Pt(111) and Pt(110) electrodes, linearly bonded NO species are more reactive than multifold-bonded NO species. Both spectroscopic and voltammetric data indicate that ammonia is the main product of NO(ads) reduction on the two surfaces examined.

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The mechanism of electrocatalytic reduction of nitric oxide on Pt(100)-(1 x 1) in acidic media has been studied using voltammetry, in-situ infrared spectroscopy, and on-line mass spectroscopy, considering the effect of surface defects, NO coverage, and the nature of the supporting electrolyte (sulfate vs perchlorate). Related mechanistic aspects of hydroxylamine (HAM) transformations on the same surface have been also examined. The adsorption of nitric oxide on Pt(100) results in the formation of an adlayer with a structure similar to that formed under ultrahigh vacuum (UHV) conditions. Ammonia was shown to be the main product of NOads reduction on Pt(100). The saturation coverage of NO adsorbate on Pt(100) was found to be around 0.5 ML, in agreement with previous UHV and electrochemical studies. Two features observed in the voltammetric profile for the electrocatalytic reduction of saturated and subsaturated NO adlayers were tentatively ascribed to reactions of NO species having different reactivity. The Tafel slope analysis of these voltammetric features gives values of ca. 60 mV decade(-1). This value was interpreted in terms of an EC mechanism, in which the first electron/proton transfer is at equilibrium, resulted in formation of HNOads intermediate, while the second reaction step is a chemical rate-determining step. This chemical step is assumed to involve the N-O bond breaking in HNOads intermediate, which most probably requires a free neighboring site. From a comparison with NOads reduction on Pt(111) and Pt(110), it follows that (i) the reaction mechanism is structure sensitive and (ii) Pt(100) is the most active surface for breaking the N-O bond, which is in agreement with the trend observed under UHV conditions. As suggested in our previous studies, the electrocatalytic reduction of HAM is likely to proceed through its partial dehydrogenation. In this study, we further develop this idea, and, based on the mechanism for NOads reduction proposed here, we suggest HNOads to be the intermediate appearing both in HAM reduction/oxidation and in NOads reduction.

In this work we investigate the electro-oxidation of CO on Pt(111) in alkaline solution by using Fourier transform infrared spectroscopy (FTIRS), to determine the adsorption sites of the CO, the intermediate species and the final oxidation product as a function of the applied potential. Multiple CO vibration bands (on-top, bridge and 3-fold hollow site) are observed on the Pt(111) electrode, their distribution and potential dependence being strongly dependent on the surface treatment. Spectroscopic results show that the final reaction product is carbonate and suggest that adsorbed carbonate blocks the access of CO from the (111) terrace to the active sites (i.e., step and kink sites).