No abstract

No abstract

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.

The oxidation of formic acid and carbon monoxide was studied at a gold electrode by a combination of electrochemistry, in situ surface-enhanced Raman spectroscopy (SERS), differential electrochemical mass spectrometry, and first-principles DFT calculations. Comparison of the SERS results and the (field-dependent) DFT calculations strongly suggests that the relevant surface-bonded intermediate during oxidation of formic acid on gold is formate HCOO- ad*. Formate reacts to form carbon dioxide via two pathways: at low potentials, with a nearby water to produce carbon dioxide and a hydronium ion; at higher potentials, with surface-bonded hydroxyl (or oxide) to give carbon dioxide and water. In the former pathway, the rate-determining step is probably related to the reaction of surface-bonded formate with water, as measurements of the reaction order imply a surface almost completely saturated with adsorbate. The potential dependence of the rate of the low-potential pathway is presumably governed by the potential dependence of formate coverage. There is no evidence for CO formation on gold during oxidation of formic acid. The oxidation of carbon monoxide must involve the carboxyhydroxyl intermediate, but SERS measurements do not reveal this intermediate during CO oxidation, most likely because of its low surface coverage, as it is formed after the rate-determining step. Based on inconclusive spectroscopic evidence for the formation of surface-bonded OH at potentials substantially below the surface oxidation region, the question whether surface-bonded carbon monoxide reacts with surface hydroxyl or with water to form carboxyhydroxyl and carbon dioxide remains open. The SERS measurements show the existence of both atop and bridge-bonded CO on gold from two distinguishable low-frequency modes that agree very well with DFT calculations.

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)×(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.

No abstract

The electrocatalytic properties of small platinum nanoparticles were investigated for the oxidation of CO, methanol, and formic acid using voltammetry, chronoamperometry, and surface-enhanced Raman spectroscopy. The particles were generated by galvanostatic deposition of platinum on a polished gold surface from an H 2 PtCl 6 containing electrolyte and ranged between 10 and 20 nm in diameter for low platinum surface concentrations, 10 and 120 nm for medium concentrations, and full Pt monolayers for high concentrations. CO stripping and bulk CO oxidation experiments on the particles up to 120 nm in diameter displayed pronounced structural effects. The CO oxidation current-time transients show a current decay for low platinum coverages and a current maximum for medium and high coverages. These results were also observed in the literature for particles of 2-to 5-nm size and agglomerates of these particles. The similarities between the literature and our results, despite large differences in particle size and morphology, suggest that particle structure and morphology are also very important catalytic parameters. Surface-enhanced Raman spectroscopy data obtained for the oxidation of CO on the Pt-modified Au electrodes corroborate this conclusion. A difference in the ratio between CO adsorbed in linear-and bridge-bonded positions on the Pt nanoparticles of different sizes demonstrates the influence of the surface morphology. The oxidation activity of methanol was found to decrease with the particle size, while the formic acid oxidation rate increases. Again, a structural effect is observed for particles of up to ca. 120 nm in diameter, which is much larger than the particles for which a particle size effect was reported in the literature. The particle shape effect for the methanol oxidation reaction can be explained by a reduction in available "ensemble sites" and a reduction in the mobility of CO formed by decomposition of methanol. As formic acid does not require Pt ensemble sites, decreasing the particle size, and thus, the relative number of defects, increases the reaction rate.

No abstract