1,721,148 research outputs found
Anodic mineralization of organic substrates in chloride-containing aqueous media
No full textThe possibility of eliminating organic pollutants from industrial wastes, by anodic mineralization or “incineration” has been discussed in recent years [1-4]. The electrochemical wastewater treatment may be of particular interest when the effluent contains biorefractory organics and/or high amounts of organic carbon and requires some pre-treatment to allow further biological purification. The mineralization process takes place as an extreme case of anodic oxidation, together with the oxygen evolution reaction. The formation of adsorbed hydroxyl radicals is a necessary condition for the oxidative attack of the organic substrate to take place and also for the oxygen evolution [4]. At high oxygen overvoltage anodes, like PbO2, Sb- or F-doped SnO2 , typically the anodic mineralization of organic substrates takes place with better faradaic yields. In the case of lead dioxide, however, problems of service life and of release of lead ions in the treated effluent, may represent serious drawbacks in a practical application. For tin dioxide-based electrodes, the short service life is again an inconvenience.
While the improvement of these anodes is under investigation in several research groups, it may be of interest to investigate the possibility to use stable anodes, like galvanic platinum, iridium dioxide-based DSA’s®, improving the faradaic yield of the electrochemical mineralization. This aim may be achieved using inorganic mediators of the oxidation of the organic substrate. In this respect active chlorine may be of particular interest, and has been discussed for the case of electrochemical mineralization of phenol [5,6]. In the presence of chlorides the electrochemical treatment can be carried out at much lower potentials, compared with those required for the non-mediated (direct) anodic oxidation. Galvanic Pt and other DSA’s materials based on iridium and ruthenium oxides, can be used, the optimal choice depending on pH and other compositional features of the effluent. An important drawback of electrolytic treatments in chloride solutions may be the formation of chloroderivatives of the organic substrates and of their oxidation products. In such cases an electrochemical treatment would result in an increase in toxicity of the wastewater and, possibly, also in stability of the residual chlorinated substrates. Accordingly, it is important to study the mechanism of oxidative degradation of different organic molecules, in different chloride-containing media, and at different anodes, to find optimal conditions for the electrochemical treatment, ensuring good faradaic yields for mineralization process, avoiding the formation of chlorocompounds.
In the present work two model substrates have been studied: glucose and phenol. The first may be of interest considering that saccharides are important components in effluents from food industry (e.g.: olive mill wastewaters). The second has been studied in different researches on electrochemical abatement of organic pollutants and many results are available for comparison, including some aspects of its reactivity in chloride media.
Experiments on electro-oxidation of glucose have been carried out at Ti/Pt and Ti/PbO2 electrodes, in presence of NaCl concentrations between 0.5 and 5 g dm-3. The substrate, in COD units, was 10.000 mg O2 dm-3 have been used, in consideration of the high COD values nor-mally met in effluents like olive-mill ones. The initial solution pH was 12. Electrolyses were carried out at different temperatures between 15 and 50°C. At Ti/Pt electrodes complete mineralization of glucose could be achieved at current densities > of 500 A m-2, under different electrolysis conditions.The mineralization rate was larger the lower the electrolysis temperature. The mineralization rate of glucose was found to increase with the NaCl concentration for values between 0.5 and 3 g dm-3. Only a small influence could be observed above this value. At Ti-PbO2 electrodes a substantially similar situation was found. At Ti/Pt electrodes, in the absence of NaCl, the abatement of COD was very small even after long electrolysis times. At PbO2, on the contrary, complete mineralization was achieved, even without the active chlorine mediator. Interestingly, a treatment of glucose solutions with sodium hypochlorite allowed a decreas of COD to only one half of its initial value. An important part of the overall reaction is therefore a surface stage. No organic chlorinated compounds were detected during the electrolysis. At the end of the electrolysis it was found that part of the original chloride was converted to chlorate and hypochlorite. In the first stage of the electrolysis the main intermediate seems to be gluconic acid.
Under analogous conditions also the mineralization of phenol (1000 ppm) in alkaline media (pH 13) could be achieved. Two unidentified complex quinonic species were present during the first stages of the electrolysis. As expected from the literature, maleic, fumaric and oxalic acid are then formed. No chlorophenols or organochloro compounds were detected.
References
1 S. Stucki, R. Koetz, B. Carcer, W. Suter, J. Appl. Electrochem. 21, 99 (1991)
2 Ch.Comninellis and E. Plattner, Chimia 42, 250 (1988)
3 Ch.Comninellis and C. Pulgarin, J. Appl. Electrochem. 23, 108 (1993)
4 Ch. Comninellis, Electrochim. Acta 39, 1863 (1994)
5 A. Boscolo, F. Gottardi, M. Tavan, R. Amadelli, A. De Battisti, A. Barbieri, G. Battaglin, J. Appl. Electrochem.. 24, 1052 (1994)
6 Ch. Comninellis and A. Nerini, J. Appl. Electrochem. 25, 23 (1995
Sustainable base production towards phospate remobilisation: a modelling approach
No full textSCGCGR-53 1058; EPFL SB SCGC-GE. Consultable sur demande à la Bibliothèque de l'EPFL / Offered in consultation at the EPFL library
Investigation on an innovative pilot-scale steam decontamination process for heat-sensitive food powders
No full textSCGCGR-53 1055; EPFL SB SCGC-GE. Consultable sur demande à la Bibliothèque de l'EPFL / Offered in consultation at the EPFL library
Investigation of platinum-based nanoparticles on boron-doped diamond substrate for applications in electrocatalysis
No full textThe 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.GGE
Development of a selective electrolytic process for the recovery of heavy metals (Cu, Pb, Cd, and Zn) from secondary fly ash
No full textGGE
Etude de la promotion électrochimique de l'oxydation catalytique de l'éthylène sur des oxydes métalliques
Full text linkThe 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.GGE
Promotion électrochimique des catalyseurs à base de rhodium et d'iridium
No full textIn this thesis, the electrochemical promotion of lr and Rh based catalysts was studied for the complete combustion of ethylene and propylene. The study was performed by using two types of electrocatalytical cells: a classical, pellet type cell and a tubular cell. This work has two major goals. The first one is the optimization of the employed electrocatalytical systems from electrochemical point of view. The second one concerns the study of the mechanism of the electrochemical promotion. The major part of this work concerns the electrochemical characterization of two cell types: a pellet type cell and a tubular cell. On the basis of the obtained results the modélisation of the potential and current distribution was performed, providing key parameters for the development of more efficient cell configurations. On one hand, these configurations ensure an uniform current distribution on the working electrode and enable correct catalyst potential measurement. On the other hand, it minimizes the current bypass in the bipolar tubular cell. The aim of the investigation in a two-compartment reactor was to evaluate and to compare the Au and Ir electrodes used in this work. Under 400°C, gold was found to be much worse material than lrÛ2 both as reference and counter electrode. Its potential was affected by oxygen and propylene adsorption and no equilibrium of electrochemical reaction was obtained at such low temperature. On the base of this knowledge, several materials better adapted to electrochemical promotion conditions have been proposed. Using the two-compartment reactor potential measurements under steady state conditions allowed to verify the mechanism of the propylene combustion on Rh and that of ethylene combustion on lr based catalysts proposed in the literature. Ameliorations of the existent kinetic models were proposed. The kinetic measurements were then analyzed with the additional help of ex situ XPS analysis. This study allowed to link the oxidation state of the catalyst surface with the extent of electrochemical promotion of the catalysts. It also allowed the ex situ observation of the appearance of new oxygen (oxide) species on the gas exposed surface of Ir catalyst. This new type of oxygen was different from the oxygen of Ir02 oxide, and it can represent the promoting species, according to backspiïïover theory. The same oxygen species was observed on well oxidized Ir02 catalyst surface, even before the polarization of this catalyst. Its presence was associated to the catalyst-support effect. It explains the high activity of Ir02 catalyst by comparison with the catalytic activity of lr metal. XPS analysis of Rh based catalysts indicated the possibly formation of an inferior Rh oxide (close to RhO) due to polarization. This may be a possible explanation of the observed permanent electrochemical promotion. Voltammetric charge measurements in very narrow potential window were performed while changing the catalyst thickness. The charge was demonstrated to be proportional to the catalyst thickness, suggesting the presence of the charged species on the gas exposed catalyst surface. The analogy between this situation and that of the emerged electrodes in liquid state electrochemistry was emphasized. The voltammograms obtained in this study were interpreted in the quite new, original way, which allowed to calculate the capacity of the different catalyst interfaces, the gas/catalyst interface, and the triple phase boundary, in good agreement with reference values from impedance spectroscopy measurements. Finally, the reaction rate transients during galvanostatic polarization of lr based catalyst have been moralized. The model fits well the experimental results.GGE
Electrochemical reduction of carbon dioxide using pyridinium as catalyst
No full textSCGCGR-53 850 ; EPFL - SB - SCGC - GE. Consultable sur demande à la Bibliothèque de l'EPFL / Offered in consultation at the EPFL library
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