Structure sensitivity of methanol electrooxidation pathways on platinum:an on-line electrochemical mass spectrometry study

By monitoring the mass fractions of CO2 (m/z 44) and methylformate (m/z 60, formed from CH3OH + HCOOH) with on-line electrochemical mass spectrometry (OLEMS), the selectivity and structure sensitivity of the methanol oxidation pathways were investigated on the basal planesPt(111), Pt(110), and Pt(100)and the stepped Pt electrodesPt(554) and Pt(553)in sulfuric and perchloric acid electrolytes. The maximum reactivity of the MeOH oxidation reaction on Pt(111), Pt(110), and Pt(100) increases in the order Pt(111) < Pt(110) < Pt(100). Mass spectrometry results indicate that the direct oxidation pathway through soluble intermediates plays a pronounced role on Pt(110) and Pt(111), while, on Pt(100), the indirect pathway through adsorbed carbon monoxide is predominant. In 0.5 M H2SO4, introducing steps in the (111) plane increases the total reaction rate, while the relative importance of the direct pathway decreases considerably. In 0.5 M HClO4, however, introducing steps increases both the total reaction rate and the selectivity toward the direct oxidation pathway. Anion (sulfate) adsorption on (111) leads to a more prominent role of the direct pathway, but, on all the other surfaces, (bi)sulfate seems to block the formation of soluble intermediates. For both electrolytes, increasing the step density results in more methylformate being formed relative to the amount of CO2 detected, indicating that the [110] steps themselves catalyze the direct oxidation pathway. A detailed reaction scheme for the methanol oxidation mechanism is suggested based on the literature and the results obtained here

The reduction of substituted benzylamines by means of electrochemically generated solvated electrons in LiCl + methylamine

Methoxy-substituted and N-methylated benzylamines were reduced to their 1,4-dihydro derivs. using the electrochem. Benkeser redn. N,N-dimethylveratrylamine decompd. during the redn. The differences in current efficiencies can be explained by differences in the stabilization of the radical anions and by differences in protonation rates. Rotating ring-disk electrode (RRDE) expts. showed that in the redn. of benzylamines, the 1st protonation can be achieved either intramolecularly or intermolecularly. [on SciFinder (R)

On-line mass spectrometry system for measurements at single-crystal electrodes in hanging meniscus configuration

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 µm) 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

Electrochemical quartz crystal microbalance measurements of CO adsorption and oxidation on Pt in various electrolytes

The oxidation of ammonia on platinized platinum has been studied with cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). These techniques show the surface to be highly covered with adsorbates during the selective oxidation of ammonia to N-2 at potentials where platinum is free of oxides. These adsorbates are inactive in the formation of N-2 and consist of NHx, probably N-ads, whereas no NO adsorbates are present among these adspecies. These adsorbates remain present on the surface after exchange of the ammonia solution for base electrolyte and in a negatively directed potential scan N-2 and NH3 are formed. When this potential scan is interrupted by holding the potential at 0.55 V the current reverses from negative to positive, being accompanied by N-2 formation. These data support a mechanism in which NHx species, proposedly NHads, are the active intermediates and N-ads acts as a poison. (C) 1998 Elsevier Science Ltd. All rights reserve

Electrocatalytic reduction of carbon dioxide to carbon monoxide and methane at an immobilized cobalt protoporphyrin

The electrochemical conversion of carbon dioxide and water into useful products is a major challenge in facilitating a closed carbon cycle. Here we report a cobalt protoporphyrin immobilized on a pyrolytic graphite electrode that reduces carbon dioxide in an aqueous acidic solution at relatively low overpotential (0.5 V), with an efficiency and selectivity comparable to the best porphyrin-based electrocatalyst in the literature. While carbon monoxide is the main reduction product, we also observe methane as by-product. The results of our detailed pH-dependent studies are explained consistently by a mechanism in which carbon dioxide is activated by the cobalt protoporphyrin through the stabilization of a radical intermediate, which acts as Bronsted base. The basic character of this intermediate explains how the carbon dioxide reduction circumvents a concerted proton-electron transfer mechanism, in contrast to hydrogen evolution. Our results and their mechanistic interpretations suggest strategies for designing improved catalysts

Electrochemical carbon dioxide and bicarbonate reduction on copper in weakly alkaline media

The electrochemical reduction of CO2 on copper is an intensively studied reaction. However, there has not been much attention for CO2 reduction on copper in alkaline electrolytes, because this creates a carbonate buffer in which CO2 is converted in HCO3 (-) and the pH of the electrolyte decreases. Here, we show that electrolytes with phosphate buffers, which start off in the alkaline region and, after saturation with CO2, end up in the neutral region, behave differently compared to CO2 reduction in phosphate buffers which starts off in the neutral region. In initially alkaline buffers, a reduction peak is observed, which is not seen in neutral buffer solutions. In contrast with earlier literature reports, we show that this peak is not due to the formation of a CO adlayer on the electrode surface but due to the production of formate via direct bicarbonate reduction. The intensity of the reduction peak is influenced by electrode morphology and the identity of the cations and anions in solution. It is found that a copper nanoparticle-covered electrode gives a rise in intensity in comparison with mechanically polished and electropolished electrodes. The peak is observed in the SO4 (2-)-, ClO4 (-)-, and Cl-- containing electrolytes, but the formate-forming peak is not seen with Br- and I-