The role of Ru and Sn on the methanol oxidation over Pt was investigated for three different systems viz. Pt covered with adatom layers of Ru and Sn, electro-codeposited Pt-Ru and carbon supported Pt-Ru. By following the oxide growth on the Pt-promoter metal electrodes with ellipsometry it was found that in the presence of methanol the surface oxides of the promotor metal are no longer present on the surface. This supports the bifunctional model of the promotor action. DEMS measurements at Pt with submonolayer coverage of Ru or Sn revealed that the current efficiency of the methanol oxidation to CO 2 is increased in the presence of Ru or Sn and that the onset potential of the oxidation keeps lowering with increasing amounts of the promoting metal. On electrodeposited Pt-Ru systems the adsorption of methanol already takes place at potentials in the hydrogen range. These results seem to point to an electronic (ligand) effect. This is further corroborated by activity measurements at carbon supported Pt-Ru with very small particles, which show a tenfold higher activity compared with the Ru-free system.It is concluded that the promoting action of Ru and Sn may involve both a bifunctional and an electronic (ligand) effect.

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To obtain the surface content of Ru in rough electrocodeposited PtRu electrodes, the mass change of a Pt electrode during Ru deposition was measured with the electrochemical quartz crystal microbalance (EQCMB). It is shown that there is a correlation between the potential of the surface oxide reduction peak of the PtRu electrode and the surface Ru content. With the EQCMB also the methanol oxidation and oxide formation on both Pt and PtRu were studied. Implications for the oxidation of CO and CH 3OH are discussed, in the context of which some IR experiments on CO and PtRu are presented as well.

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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.