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