Francisco C. Nart

Vibrational spectroscopy of adsorbed sulfate on Pt(111)

The absorption of sulfate species was studied on well ordered Pt(111) surfaces at pH 2.8 and 0.23 using in situ external reflectance ir spectroscopy. Independently of the pH in the solution the spectra present two overlapped vibrational bands. A strong one is located at 1220–1280 cm−1 and is potential dependent. The second one is observed as a shoulder at 1190 cm−1. The analysis using group theory and the surface selection rule shows that only sulfate ions are adsorbed at both pH, suggesting a strong Lewis acid character of the Pt(111) surface. The randomly distributed sulfate species are relatively tightly adsorbed and present weak lateral interaction. At increasing coverage the sulfate adlayer becomes more organized, as attested by the observed band narrowing. The maximum adsorption is observed at 0.8 V vs. Pd/H2 ie at potentials higher than that of the so called anomalous states in the voltammogram. A quadratic dependence of the applied electric field on the SO vibrational band of adsorbed sulfate is observed, suggesting a second order Stark effect.

Performance of a co-electrodeposited Pt-Ru electrode for the electro-oxidation of ethanol studied by in situ FTIR spectroscopy

The ethanol electro-oxidation on a co-electrodeposited Pt-Ru catalyst was studied by in situ FTIR spectroscopy. In contrast with the formation of the known acetaldehyde, acetic acid and carbon dioxide reaction products on a Pt catalyst, on the prepared Pt-Ru electrode only carbon dioxide was detected under the same experimental conditions.

On the adsorption and reduction of NO3− ions at Au and Pt electrodes studied by in situ FTIR spectroscopy

The adsorption of nitrate ions on gold and platinum electrodes in acid solution has been studied in acidic solution using in situ FURS. It is found that nitrate is adsorbed probably with a two-fold coordination on gold electrodes as suggested by the adsorbate potential-dependent spectral feature centered between 1440 and 1460 cm−1. In addition, partial reduction of the nitrate ions at more negative potentials (ca. 0.05 V) generates nitrite ions in solution. These ions are co-adsorbed with nitrate ions in the double layer region of potentials. The nitrite ions seem to be adsorbed O-down with a one-fold or two-fold coordination. Platinum electrodes are found to be much more active catalytically to nitrate reduction than gold electrodes. The reduction of nitrate ions at potentials below 0.8 V generates an adsorbed product presenting a potential-dependent band at 1540 to 1580 cm−1 which has been identified as adsorbed NO.

Performance evaluation of porous electrocatalysts via normalization of the active surface

When comparing the rate of electrochemical processes at different porous electrocatalysts a surface normalization should be used. It is shown for the case of methanol oxidation at PtRu layers electrodeposited on gold substrates that substantially different data are obtained for current, mass spectrometry signals and integrated IR band intensities of the products, with and without normalization of the catalyst surface used. Using stripping of saturated CO coverage as a normalization tool, cyclic voltammograms, on line MS and in situ FTIR spectroscopy give reasonable agreement of catalytic activity towards methanol oxidation.

Sulfate adsorption on well-defined Pt(100) electrodes

The metal—solution interface of a single-crystal Pt(100) electrode in the presence of specifically adsorbed sulfate ions is investigated by means of in situ FTIR spectroscopy. Sulfate adsorption starts at about 0.2 V vs. Pd/H2 and goes through a maximum at ca. 0.5 V. Sulfate ions are adsorbed through two oxygen atoms presenting a C2v symmetry. The two strong bands observed at around 1100 and 1200 cm−1 are assigned to the symmetric SO stretches of the resulting SO2 groups (coordinated and non-coordinated (respectively).

On the adsorption of ionic phosphate species on Au(111)—an in situ FTIR study

The adsorption of phosphate species on the well ordered Au(111) single crystal surfaces was studied with in situ FTIR spectroscopy in order to identify the spectral features of the adsorbed H2PO−4, HPO2−4 and PO3−4 selectively by changing the OH− content in solution. H2PO−4 presents two potential dependent adsorbate features at 1100 and 980 cm− 1 due to the two symmetric stretching vibrations of the adsorbed ion under C2v symmetry. HPO2−4 shows only one potential dependent band at about 1120 cm−1 which can be attributed to the symmetric A1 stretching of the adsorbate, probably under C3v symmetry. For the adsorbed PO3−4 one potential dependent feature located between ca. 1060 and 1083 cm−1 was found. This band is assigned to the A1 mode due to the splitting of the vibration of the F mode under C3v or C2v symmetry.

On-line mass spectrometry investigation of the reduction of carbon dioxide in acidic media on polycrystalline Pt

Abstract Differential electrochemical mass spectrometry (DEMS) is used to investigate the reaction of electroreduction of CO2 on platinum porous electrode in acidic media. This technique, which gives molecular specificity, permits the reaction products to be followed concurrently with potential and time. These results showed for the first time the on-line production of methanol in acidic media using a Pt electrodeposited electrode. Reduction of CO2 in perchloric acid on Pt occurs in the H2 evolution region leading to the formation of formic acid methanol and traces of methane. Experiments using CO show that this substance is the intermediate of the pathway leading to methanol.

Uso de espectrometria de massas em medidas eletroquímicas - a técnica de DEMS

The combination of cyclic voltammetry and on line mass spectrometry, called differential electrochemical mass spectrometry (DEMS), is already a well established method for on line identification and quantitative detection of the products and intermediates of electrochemical reactions. This article aims to show the principles of the method and how this technique have been used to study electrochemical problems such the electrocatalytic oxidation of organic molecules, as methanol and ethanol, at noble metal electrodes.

“On line” mass spectrometric detection of ammonia oxidation products generated by polypyrrole based amperometric sensors

Polypyrrole (PPy) is the conducting polymer most widely employed in electrochemical sensors for ammonia detection in the last decade. Although sensors have been described in depth in the literature, the mechanism of ammonia detection by polypyrrole is still a matter of controversy. The differential electrochemical mass spectrometry (DEMS) technique, together with UV-Vis spectroscopy, gives direct and conclusive evidence with respect to the ammonia oxidation products formed in NaOH solution (pH = 10) at polyrrole or N-methyl polypyrrole film sensors polarized at 0.35 V (Ag/AgCl). Monitoring of the different possible ammonia oxidation products by on-line DEMS measurements indicates that only NO is formed and that other species such as N2 or N2O are absent. UV-Vis absorption spectroscopy also showed that ionic products such as nitrate or nitrite are not formed.

Sulfate ions adsorbed on Au(hkl) electrodes: in situ vibrational spectroscopy

Abstract The adsorption of sulfate and bisulfate ions has been studied on Au(111), Au(100) and Au(110) single crystal electrodes using in situ reflection absorption IR spectroscopy. It has been found that on Au(111) and Au(100) the adsorbed sulfate presents only two observable bands, coherent with C 3 v or C 2 v symmetries. Comparing the OO distances of the sulfate ion with the Au–Au distance at the surface, a single coordination for adsorbed sulfate on Au(111) and Au(100) has been proposed, since both electrodes present the same results for adsorbed sulfate. Only a very small amount of bisulfate ions has been detected for Au(111) electrodes at very low pH (0.23). For Au(100) a larger amount of bisulfate than on Au(111) co-adsorbed with sulfate ions has been observed. The onset of adsorption for both electrodes coincides with the lifting of reconstruction. At Au(111) the adsorption begins to be observable with in situ IR spectroscopy at ∼0.7 V, while for Au(100) at 0.6 V. The adsorbed sulfate on Au(110) electrode presents three IR bands, indicating that at least part of the adsorbed sulfate is tilted at the surface revealing the surface selection rule forbidden bands. The symmetric band for adsorbed sulfate on Au(110) surface has been observed at 1177–1198 cm −1 and 960 cm −1 and the unsymmetric stretching at 1130 and 1100 cm− 1 .