Whm Wil Visscher

Particle size effect of carbon-supported platinum catalysts for the electrooxidation of methanol

Abstract The effect of the particle size of carbon-supported Pt catalysts on the electrooxidation of methanol was studied. Different methods were used to prepare Pt C catalysts with particle sizes ranging between 1.2 and 10 nm. The possible interaction of Pt with carbon surface groups was also investigated by preparing catalysts on oxidized and non-oxidized carbon supports. The specific activity was found to decrease with decreasing particle size in the range 4.5–1.2 nm. Carbon-supported Pt particles appear to be more active than non-supported particles and the presence of an acidic group on the support slightly enhances this effect. The dependence of the activity on the particle size can be explained in terms of either its effect on the formation of an adsorbed hydroxy species or its effect on the number of methanol adsorption sites.

On the role of Ru and Sn as promotors of methanol electro-oxidation over Pt

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 PtRu and carbon supported PtRu. 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 CO2 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 PtRu 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 PtRu 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.

Electrochemical reduction of oxygen: an alternative method to prepare active CoN4 catalysts

An alternative method for preparing active catalysts for oxygen reduction with similar a performance as heat treated cobalt porphyrin on carbon is presented. The catalysts were prepared from cobalt acetate, carbon black and a nitrogen donor. Several nitrogen donors were investigated. The best results were obtained with 2,5-dimethylpyrrole. In this case the activity of the heat treated cobalt porphyrin could be matched. EXAFS indicated that similar active sites were found on both types of catalyst, although in the alternative catalyst some metallic cobalt is also present.

The electrocatalytic reduction of NO3− on Pt, Pd and Pt + Pd electrodes activated with Ge

he electrocatalytic reduction of nitrate has been investigated on Pt, Pd and Pt +Pd electrodes covered with a submonolayer of germanium. Pt + Pd electrodes were prepared by electroless deposition of submonolayers of Pd on Pt by exchange of PdCl2 for preadsorbed copper. Underpotentially deposited germanium enhances the reduction rate of nitrate strongly. The reduction of nitrite is enhanced to a lesser extent, whereas germanium is inactive for NO and hydroxylamine reduction. Further, cyclic voltammetry shows that the well known inhibition of the nitrate reduction at low potentials is absent for germanium-modified electrodes. Amperometry shows that the current densities for nitrate reduction at 0.1 V depend strongly on the composition of the electrode surface. The activities increase in the order Pd, Pt and Pt + Pd and all electrodes display a proportional relation between the activity and the germanium coverage. This shows that germanium 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 selectivities than Pt and Pt + Pd electrodes. No gaseous products were observed for Pt, whereas for Pt + Pd and Pd N2O selectivities up to 8% were found.

The selectivity of oxygen reduction by pyrolysed iron porphyrin supported on carbon

Abstract The selectivity of oxygen reduction at pyrolysed FeTPP-Cl on carbon black was studied in acidic electrolyte using the RRDE. First the applicability of RRDE theory to RRDE porous disk electrodes was tested. It was observed that the collection efficiency for these systems decreases with increasing rotation frequency. By compensating for these variations in collection efficiency a Wroblowa analysis could be performed. It was concluded that the desired 4-electron reduction plays a dominant role only at high potentials, where the current densities are low, at lower potentials the production of hydrogen peroxide predominates. The direct formation of water was related to the possibility of the Fe(IV)O intermediate adduct being formed. Hydrogen peroxide reduction and decomposition were also studied with the RRDE, using a CoTSPc/polypyrrole ring electrode. It was found that hydrogen peroxide decomposition plays a major role in the consecutive reactions of hydrogen peroxide. The decomposition reaction was found to be independent of the potential, making a radical decomposition catalysed by the FeN4-sites most likely.

A DEMS and cyclic voltammetry study of NH3 oxidation on platinized platinum

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 N2 at potentials where platinum is free of oxides. These adsorbates are inactive in the formation of N2 and consist of NHx, probably Nads, 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 N2 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 N2 formation. These data support a mechanism in which NHx species, proposedly NHads, are the active intermediates and Nads acts as a poison.

Ellipsometry and dems study of the electrooxidation of methanol at Pt and Ru- and Sn- promoted Pt

The oxidation of submonolayers of Ru and Sn on Pt in sulfuric acid was monitored with ellipsometry. In the presence of methanol the oxides on Ru and Sn disappear but the Pt-oxide is not affected by methanol. This signifies that Ru and Sn are present in their zero valent state during the methanol oxidation. DEMS measurements showed that both metals enhance methanol oxidation. The promoting role of these metals is explained with the bifunctional mechanism.

Oxygen reduction on pyrolysed carbon-supported transition metal chelates

Abstract A review is given of the pyrolysis behaviour of carbon-supported transition metal chelates and the developed theories explaining the increased activity and stability towards oxygen reduction. Additional measurements which a new carbon-modified rotating ring-disc electrode show that the central metal ionN 4 unit remains the active site after pyrolysis. The selectivity is increased after pyrolysis since relatively less H 2 O 2 is detected at the ring. The number of available active sites is an important parameter for the resulting activity. During the heat treatment, different processes take place: migration of the chelates over the carbon support, increasing the number of availableactive sites; reaction of the outer fringes of the organic skeleton retaining the metal ionN 4 unit; and degration of the catalyst with loss of activity. The condition of the pyrolysed catalyst and its corresponding activity are determined by the relative rate of these processes at the given pyrolysis temperature.

Redox potential and electrocatalysis of O2 reduction on transition metal chelates

Abstract The redox potentials of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and cobalt tetraazaannulene (CoTAA), irreversibly adsorbed on pyrolytic graphite, were investigated as a function of the pH by cyclic voltammetry. The reduction of oxygen was studied under the same conditions with the rotating disc electrode technique. At high pH, where the reduction to H2O2 on the cobalt-containing chelates is more reversible, the value of the redox potential is less important. At low pH, where this reduction is irreversible, there is a clear correlation between the redox potential of the central metal ion and the O2 reduction behaviour. In acid media, a 470 mV more positive redox potential of CoTAA compared to that of CoPc, resulted in a half-wave potential, E 1 2 , for the O2 reduction being 400 mV more positive than the E 1 2 of CoPc.

The effect of Sn on Pt/C catalysts for the methanol electro-oxidation

The effect of Sn for the methanol oxidation in sulfuric acid is investigated using electro-deposited Pt and carbon supported Pt. The preperation has a considerable influence, as the Sn effects range from a small increase to a decrease in methanol oxidation activity. Sn is believed to act through the activation of H2O. The optimum Sn surface coverage is found to be low; of the order of 10%.