van Jar Rob Veen

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.

The role of adsorbates in the electrochemical oxidation of ammonia on noble and transition metal electrodes

Abstract The activity for ammonia oxidation and the intermediates formed during the reaction have been studied on platinum, palladium, rhodium, ruthenium, iridium, copper, silver and gold electrodes. The activity in the selective oxidation to N2 is related directly to the nature of the species at the surface: the electrode is active if NHads (or NH2,ads) is present and deactivates when Nads is present. The potential at which NHads or Nads is formed is metal dependent. The observed trend in the strength of adsorption of Nads is Ru>Rh>Pd>Ir>Pt ≫ Au, Ag, Cu . This trend corresponds well with the trend observed in the calculated heat of adsorption of atomic nitrogen, with iridium being an exception. Platinum is the best catalyst for this reaction because Nads is formed at high potential, compared to rhodium and palladium, but seems to stabilize NHads rather well. Gold, silver and copper do not form NHads or Nads, and show only an activity when the surface becomes oxidized. The metal electrodissolution is enhanced by ammonia under these conditions. Most metals produce oxygen-containing products, like NO and N2O, at potentials where the surface becomes oxidized.

Electrocatalytic reduction of NO3− on palladium/copper electrodes

The reduction of NO3− on palladium/copper electrodes has been studied using differential electrochemical mass spectroscopy (DEMS), rotating ring-disk electrodes (RRDE) and quartz microbalance electrodes (ECQM). In acidic electrolytes, the activity increases linearly with Cu coverage, in alkaline electrolytes, a different dependence on coverage is observed. One monolayer of Cu gives a different selectivity from bulk copper. The adsorption of NO3− is competitive with SO42−, whereas Cl− adsorption blocks the reduction. Competitive adsorption lowers both the activity and the selectivity to N2. Copper activates the first electron transfer, the role of palladium is to steer the selectivity towards N2. The trends in activity and selectivity are explained in terms of coverage of N-species.

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.

A Refinement on the Notion of Type I and II (Co)MoS Phases in Hydrotreating Catalysts

The influence of Mo content on thiophene hydrodesulfurization for alumina- and carbon-supported (Co)Mo catalysts is investigated by TPR, XPS and H2-D2 equilibration. Activity trends for alumina-supported catalysts are explained by a gradual change of the type I MoS2 active phase from a large to a small number of Mo-O-Al anchors. A pure type II phase without such anchors and a high HDS activity is only obtained employing chelating ligands or a carbon support. At low Mo loading a separate oxysulfidic Mo species in strong interaction with alumina exhibits an unexpectedly high HDS activity, which, however, is difficult to promote by Co.

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.

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%.