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l~lb~ el bu~rgrmic Chemi,~lr~' aml Catra!vsix, EindhoPen tJni,~6(1~) MB Eimlhocen. Netherlands Received 16 December 1Ot)0; revised 31 January 1997 AhsiraelThe: c~,clroca|alyliqr |vdBcIion of uilrale has been investigated on Pt, Pd and Pl k Pd elecu~od¢,~ covered wi|h a ,~ubmonolayeJ of gern~aniunk PI + Pd eleo,~des were prepared by electrol¢~s deposition of stubmnnolayer~ of Pd on Pl by exchange of PdCI~ for preadsod~ed eopl~,r, Underpote~ ~i,dly deposited germanium enhances the reduction rale of nitrate strongly. The reduction of nitrite i~ euhanced to a lesser extent, whezeas germanium is inactive for NO and hydroxylamine reduction. Furlher. cyclic voltammetry shows that the well known inhibition of the nitrate reduction at low potentials is absent fi~r germaniumomodified electrodes. Amperometry shows lhat the cun~nt densities for nitrate reduction at 0.1 V depend strongly on the composition of the electrode surface. The activities increase in the order Pd, PI and Pt + Pd and all electrodes display a plopurtional relation between the activity and the germanium coverage. This shows that gemmnium is involved in the rate determining step, which is the reduction of nitrate to nitrite and its role is to bind the oxygen atom of nitrate, The higher activities for Pt + Pd electrodes can be understood in terms of changes in the electronic structure of the metals as a result of alloying. Selectivity measurements with a rotating ring-disk electrode have shown for all electrodes that the hydroxylamine selectivity increases for increasing germanium coverage. Pd displays higher hydroxylamine ,;electivides than Pt and Pt + Pd electrode~. No gaseous products were observed for Pi. whereas for Pt + Pd and Pd N~O selectivitie~ up to 8% were found. © 1997 Elsevier Science S.A.

The irreversible adsorption of ethanol and 1,2-ethanediol on platinized platinum has been studied with Fourier transform IR spectroscopy (FTIRS) and differential electrochemical mass spectrometry (DEMS) in perchloric acid electrolyte. The adsorption was found to be dissociative for both ethanol and 1,2-ethanediol. During adsorption 1,2-ethanediol is completely dehydrogenated to adsorbed CO. For ethanol it was concluded that carbon species are formed in addition to adsorbed CO. Part of this residue is hydrogenated at low potential to methane. IntroductionThe electrochemical oxidations of 1,2-ethanediol and particularly ethanol on platinum are of considerable interest because of their role as model compounds for the study of the adsorption and electrooxidation behavior of organic species on platinum. Electrochemical studies can also provide useful information with respect to the platinum-catalyzed oxidation of alcohols with molecular oxygen in the liquid phase.The electrooxidation of ethanol over platinum yields carbon dioxide, acetic acid, and acetaldehyde, 1-6 as was found with spectroscopic methods. Several studies have been conducted to elucidate the structure of the irreversibly adsorbed species that are present on platinum after removal of the ethanol solution. The results are rather contradictory and can be divided into three categories. (i) Dissociation of the carbon-carbon bond occurs during adsorption, leading to the formation of adsorbed CO. This was suggested on the basis of differential electrochemical mass spectrometry (DEMS) 7 and voltammetry. [8][9][10][11] (ii) Dissociative as well as nondissociative adsorption was proposed on the basis of electrochemical thermal desorption mass spectrometry (ECTDMS) 2 and Fourier transform infrared spectroscopy (FTIRS). 12,13 (iii) Only nondissociative adsorption occurs, leaving the carbon-carbon bond intact. This conclusion was based on DEMS measurements. 1 The formation of adsorbed CO was also observed during bulk electrooxidation of ethanol with FTIR spectro-

The irreversible adsorption of several C3 alcohols, 1-butanol, and ethene on platinized platinum has been studied with Fourier transform IR spectroscopy (FTIRS) and differential electrochemical mass spectrometry (DEMS) in perchloric acid electrolyte. Apart from 2-propanol, all studied alcohols display C-C(O) dissociative adsorption as the only pathway active in the formation of irreversible adsorbates. 1,2,3-Propanetriol is the only compound that is fully converted to adsorbed CO, while all other primary alcohols generate hydrocarbon adspecies in addition to CO. No further C-C bond breaking is observed in these hydrocarbon adspecies, which were found to be highly dehydrogenated. The hydrocarbon species can be fractionally hydrogenated off the surface to form corresponding gaseous compounds. 2-Propanol and ethene do not undergo C-C dissociation to form CO but are dehydrogenated considerably. For 2-propanol and ethene it was found that a small amount of oxygen incorporation occurs on the C 1 position, followed by decarbonylation to form CO.

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