Electrochemical promotion of CO combustion over non-percolated Pt particles supported on YSZ using a novel bipolar configuration

The electrochemical promotion of CO combustion over nano-dispersed Platinum particles supported on yttria stabilized zirconia (YSZ) is investigated at 300 degrees C under high vacuum conditions. By polarizing the nanoparticles, we were able to enhance the reaction rate by a factor of 500. Charging the Pt clusters promotes the catalytic rate of CO combustion over the surface of the catalyst. The experimental facts prove that the catalytic activity of platinum nanoparticles can be electrochemically activated. Having a tuning capability on every nanoparticle rather than on a thin film opens new possibilities for the application of nano-dispersed catalysts in electrochemical promotion. (C) 2010 Elsevier B.V. All rights reserved.

Iron oxide nanoparticles supported on activated carbon fibers catalyze chemoselective reduction of nitroarenes under mild conditions

Duncan Alexander, Oliver Anthony Beswick, Lioubov Kiwi

Highly dispersed iron oxide (Fe2O3) nanoparticles (NPs) have been immobilized on structured supports of activated carbon fibers (ACF). Systematic catalyst characterizations suggest that ACF-based catalysts remain ultra-microporous after Fe2O3 NPs (similar to 2 nm) deposition. The structured nano-catalysts show high activity and chemoselectivity in reduction of different functionalized nitroarenes to the corresponding anilines giving yields of the target products close to 100%. The reaction takes place under mild conditions at normal pressure and low temperatures (

Elsevier Science Bv2015

Etude de la promotion électrochimique de l'oxydation catalytique de l'éthylène sur des oxydes métalliques

The electrochemical promotion of heterogeneous catalytic reactions is a new development which draws the attention of the scientific community as much to fundamental research as to the possible direct applications of this phenomenon in automotive and in industrial catalysis. We have chosen to study the electrochemical promotion of the complete ethylene oxidation on metallic oxides catalysts. The principal oxides which we used has been the iridium oxide (IrO2) and iridium and titanium oxide mixtures (IrO2+TiO2), deposited on yttria-stabilized zirconia (YSZ). We have proved that the electrochemical promotion of ethylene combustion can be applied to these catalysts. Contrary to what was established before, we have demonstrated a certain persistence of activation of the reaction. mainly on the iridium oxide, after the end of polarization. The persistent reaction rate enhancement ratio is a function of the passed electric charge and levels off at a value of approximately 3. The other experimental significant fact for heterogeneous catalysis which has been discovered is the relation between chemical promotion and electrochemical promotion. Applying electrochemical promotion to a IrO2 + TiO2 composite catalyst, the two kinds of promotions have been confronted, since a IrO2 + TiO2 composite catalyst presents a chemical promotion, more precisely a synergy between the two components of the catalyst. The maximal amplitude of the electrochemical promotion of such catalysts has been demonstrated to be strongly reduced in comparison with the one of pure catalysts. As the maximal enhancement factor of the catalytic reaction rate was 12 for pure iridium oxide catalyst, the same factor dropped to less than 2 for a IrO2 + TiO2 composite catalyst. This result has enabled us to affirm that the mechanisms of chemical promotion and electrochemical promotion should involve a common type of surface activating species : a mobile surface spillover oxygen species. The catalyst work function on-line measurement with a Kelvin probe has enabled us to link the reaction rate with the work function change and to link the applied potential with the work function change, for al1 the phases of a transient polarisation experiment. The persistent activation of the catalyst after the end of a polarization has been linked to a persistent change in the work function. A major trend of this research consists in cyclic voltammetry. It is certainly one of the most powerful medium to extract experimental facts which have been useful to understand the electrochemical promotion principles. Due to the small amplitude of the potential which has been used in cyclic voltammetric experiments, the electrolyte-catalyst interface wasn't practically affected by these in situ measurements. By analogy to voltammetric charge formation mechanism in aqueous electrochemistry, we have taken the following chemical capacitance as responsible for the voltammetric charge in our system: IrO2 + δO- ⇄ IrO2+δ + δe- According to this mechanism, the reaction between O- and IrO2 can only occur at the interface between solid electrolyte and catalyst, where contact between the two species exists. In Our system, voltammetric charge has been demonstrated to be proportional to the thickness of the catalyst, proving by this way that the whole surface of the catalyst is in contact with the O- activating species coming from the solid electrolyte. So the implication of a spillover oxygen species during electrochemical promotion has been demonstrated by the way of cyclic voltammetry. The existence of this spillover oxygen species has been also directly proved by thermal desorption experiments under electrochemical promotion conditions. By considering the strong change in the decomposition temperature of the oxide induced by its polarisation, the thermal desorption technique has been used to display the influence of the polarization on the structure of the oxide catalyst itself.

EPFL1999

Investigation of platinum-based nanoparticles on boron-doped diamond substrate for applications in electrocatalysis

Guillaume Siné

The search of new energy sources has motivated during the last few years numerous research projects due to environmental, economical and political reasons. One of the main subjects of these works is the development of fuel cells. Within this framework the Direct Alcohol Fuel Cell (DAFC), mainly fed with methanol or ethanol, has received great attention. However one of the major limitations of the DAFC is the electrocatalysis of fuel oxidation; the electrocatalyst (dispersed Pt-based particles) is indeed readily poisoned by adsorbed intermediates, hence decreasing the fuel cell efficiency. Consequently the electrocatalysis of methanol and ethanol oxidation on different types of Pt-based nanoparticles, deposited on synthetic boron-doped diamond (BDD) thin film, has been studied during this thesis. This substrate has been chosen due to its outstanding properties of chemical inertness, illustrated by a very low capacitive current. In the first part of this thesis, electrodeposited Pt particles on BDD have been studied. Electrodeposition of Pt particles has been carried out both on bare BDD and on diamond pre-modified with gold nanoparticles deposited via a thermal decomposition technique. In both cases it was established that Pt particles were electrodeposited following a progressive mechanism during which formation of new nuclei and growth of primary nuclei simultaneously occur. The electrocatalytic activity of electrodeposited particles was also studied, and it was shown that the presence of Au particles together with Pt particles does not influence markedly the electrocatalytic behaviour of the later ones. Preliminary calcination of the co-deposit results in a dramatic decrease of its electrocatalytic activity, presumably due to the formation of a core (Pt)–shell (Au) structure less capable of alcohols dehydrogenation. Finally electrodeposition of Pt on BDD leads to particles of diameter in the 150-700 nm domain with very large size distribution. Obviously such particles are not consistent with the definition of nanoparticles in catalysis and electrocatalysis. In a second part of this work, Pt-based nanoparticles synthesized via the microemulsion method have been investigated once deposited on a diamond substrate. Attention has been mainly focused on Pt, Pt/Ru and Pt/Sn nanoparticles. All the synthesized nanoparticles are in the 2-5 nm size range with narrow size distribution. Moreover the compositions of bimetallic particles were very close to those expected. The electrocatalytic behaviour of bimetallic Pt-based nanoparticles toward both methanol and ethanol electrooxidation has been investigated. Bimetallic nanoparticles are more efficient than pure Pt, and it was shown that Pt/Ru nanoparticles were more indicated for electrocatalysis of methanol oxidation whereas Pt/Sn were more efficient in the case of electrocatalysis of ethanol oxidation. Both cooperative and electronic effects are involved in the enhancement of the electrocatalytic activity of bimetallic nanoparticles, and Pt-rich nanoparticles are the most efficient ones due to their superior alcohols adsorption properties. However the electronic effect is not of same nature in Pt/Ru than in Pt/Sn (Pt/Ru nanoparticles are alloyed ones, on the contrary of Pt/Sn), and this may explain the probable activation of the ethanol C-C bond scission by Pt/Sn electrocatalysts. Due to the different action modes of Pt/Ru and Pt/Sn electrocatalysts, a ternary Pt/Ru/Sn sample has also been synthesized by the microemulsion technique. This ternary electrocatalyst has exhibited very good activity toward methanol electrooxidation whereas that toward ethanol electrooxidation was significantly lower. It is believed that the electronic effect mainly occurs between Sn and Ru components of the sample, creating a new state of adsorbed oxygenated species that should be of higher mobility and reactivity than on bimetallic surfaces. In addition, a model for methanol electrooxidation at Pt/M (M = Ru, Sn...) surfaces has been developed in order to generalise the obtained results. The concepts and theories of heterogeneous catalysis have been extended to the specific case of this electrochemical reaction involving adsorbed intermediates. The donor-acceptor theory of heterogeneous catalysis has been more particularly considered and adapted, and within this framework Pt has been considered as the acceptor whereas Sn was regarded as the donor. The development of this model has made the establishment of a relation between the measured current to the applied potential possible in a number of real limiting cases. Validity and limitations to the proposed model has in due course been discussed in view of the obtained results.

EPFL2006

Etude de la promotion électrochimique de l'oxydation catalytique de l'éthylène sur des oxydes métalliques

EPFL1999

Investigation of platinum-based nanoparticles on boron-doped diamond substrate for applications in electrocatalysis

Guillaume Siné

EPFL2006