Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS


NENA Streszczenie raportu

Project ID: 505906
Źródło dofinansowania: FP6-NMP
Kraj: Finland

Final Report Summary - NENA (Nanostructures for Energy and Chemicals Production)

The aim of the NENA project was to develop advanced nanostructured materials and components for the cogeneration of chemicals and energy in a fuel cell. The main specific objectives of the project were:
- development of a theoretical and computational chemistry to investigate the pathway of the reactions of interest to provide guidelines for the design of nanoscale electrocatalytic materials;
- the synthesis and study of the reactivity of electrocatalytic materials for producing hydrogen peroxide and epoxides in a fuel cell using nanoparticles of different materials as well as the use of redox-catalytic cycles;
- to understand the relationship between crystal orientation of nanocrystalline materials and their electrochemical reactivity;
- synthesis of novel anionic exchange materials for use in fuel cells;
- development, modelling and testing a small fuel cell for co-generation of chemicals and energy.

The strategy followed for the development of the concept of cogeneration of energy and chemicals required the confluence of three strands of activity: experimental selection of electrocatalytic materials, theoretical investigations of reaction mechanisms and construction and testing of fuel cells and appropriate product collection hardware.

To better understand the important hydrogen oxidation reaction a theory for the electrocatalysis of bond-breaking electron transfer has been developed and a mechanism by which a d-band centred near the Ferrmi level of the electrode could lower the energy of activation for the reaction was proposed. This theory has been tested on a particularly important reaction: the oxidation of hydrogen. Potential energy surfaces for the hydrogen oxidation had been calculated, and from these the energy of activation was obtained. There was a good correlation between the calculated values and experiment. According to the model the platinum group metals are such good catalysts, because they have both a high density of states near the Fermi level, and high coupling constants. In contrast, the transition metals nickel and cobalt are bad catalysts in spite of their favourable density of states, because their coupling constants are smaller by a factor of three to four. The dependence of reactivity with pH is of fundamental importance in practical fuel cell equipment for cogeneration of energy and chemicals due to the need to define the type of membranes to be employed.

Above mentioned theory was based on the kinetics of O2 reduction which has been studied on anthraquinone and phenanthrenequinone modified GC electrodes at different pHs (pH=0.3-14). On the basis of these results it may be conclude that the highest electrocatalytic activity of these electrodes is observed in the solutions of high pH (pH>10) and therefore, these catalysts are good candidates for the production of peroxide only in alkaline media.

In addition, the mechanism of oxygen reduction on hydroxide clusters was investigated by quantum chemical calculations. A redox catalytic cycle was proposed for the formation of hydrogen peroxide using metal hydroxide clusters as a model for the active site.

Also a general model for the reduction of oxygen on oxide or hydroxide covered metals was developed taking into account the conductivity of the solid phase as well as the site specific reduction of oxygen. To elucidate the mechanism of oxygen reduction, a long range of experiments have been made investigating the effect of composition and morphology of the pulse plated layer as well as the effect of pH, electrolyte cations and temperatures.

A model catalyst approach was followed in order to establish basic relationships between structure and activity of the electrodes towards oxygen reduction reaction (ORR). The results indicated that the mechanism of ORR is basically the same for thin-film and bulk Pt electrodes.

Due to the excellent properties and processability of commercially available cation exchange membranes, acid media electrocatalysts were investigated. The use of transition metal compounds dispersed on carbon has shown high yields of peroxide in strongly acid media, both in measurements at a rotating disk electrode and when this material is incorporated within a fuel cell.

A series of membrane electrode assemblies (MEAs) were fabricated and continuously modified to improve the catalyst utilisation and cell performance. One particular catalyst / electrode configuration achieved a current density in excess of 500 mA/cm2 on oxygen at 30 psig. In common with MEA development for power applications, the water management properties of the MEA were found to be very important in controlling peroxide yields.

A 50cm2 single cell test station has been constructed and used to map out the space-time yield characteristics of peroxide generation in fuel-cell mode. The yield of the peroxide production process has been lifted up to new levels such that the single-cell can produce 8 % concentration. This figure could be improved further by a combination of development work on catalyst, electrode and cell design. The upper limit of peroxide concentration would be set in part by the humidification regime imposed upon the cell. It was concluded that with this technology, production of peroxide could reach levels of > 20 %.

The aim of this work was to develop electrocatalytic materials appropriate for oxygen insertion reactions to double bonds. Oxides and metals were investigated as electrocatalysts.

The surface and interfacial properties of metal oxide particles were studied in the context of corrected Debye-Hückel theory of surface complexation (CDH-SC). In this context, the theoretical developments continued with:
1) development of a new direct method in the context of CDH theory of electrolytes and
2) complementing CDH-SC calculations on oxide colloids with Monte Carlo simulations.

The partial ethene oxidation was investigated systematically and quantum chemical calculations to predict the mechanism on noble metal clusters were carried out. The modelling of the possible reaction pathways found the ethene oxidation to epoxide to be possible on both major noble metals - Au and Pt. The reactivity of the electrode material depends on its spin state and on the surface morphology. A flat surface seems to be preferential for ethene oxidation of over the edges.

The actual electrocatalytic activity of the nano-crystalline and nano-structured noble metals towards ethene oxidation in acid media was followed by electrochemical and differential electrochemical mass spectrometry (DEMS) techniques. The results confirmed the theoretical prediction of higher activity of Au towards ethene oxidation.

Alkene oxidation was also studied on single crystal electrodes and according to these experiments only active platinum surface for partial oxidation is Pt. No formation of epoxide was detected in the whole potential range. Carbon dioxide and adsorbed carbon monoxide were the only oxygen containing species observed.

Despite the promising results, it was clearly shown that the epoxidation reaction could not be used as an anode reaction in a fuel cell device. However, all the experimental and theoretical efforts made within the project to investigate the reactivity and mechanism of the epoxidation reaction on different materials has shown that the indirect way of epoxide formation may be feasible.

A positive side effect was discovered for the electrochemical oxidation of allyl alcohol on both Ni and NiZn in alkaline solution. The oxidation results in the formation of acrolein as the only product, which shows a new effective route for aldehyde formation. The inability of oxidising the double bond has further been proved by investigating the oxidation of propene. No additional electrochemical activity was observed on Ni and NiZn in the presence of propene.

Because of the problems encountered with the direct epoxidation, indirect hydrogen peroxide and halide ions assisted epoxidation reactions was investigated in theory and in practice, respectively. A new mechanism for hydrogen peroxide assisted epoxidation was formulated, where the stabilisation of the transitions state is emphasised. By micro-solvating with water the activation energy is lowered showing that the hydrogen bonded network can act as a catalytic site for the reaction. The mechanism of heterolytic ring opening for ethene oxide under micro-solvation conditions in acidic, neutral and alkaline environments was also studied. The crucial role of the nucleophile in both acidic and alkaline media puts constraints on any successful strategy to produce epoxides in aqueous media. However, the stability of the epoxide under neutral conditions was inspiring and could be utilised in the development of epoxidation catalysts.

The ethene oxidation was directed to ethene oxide only in the case of Ru based oxide electrodes in presence of halide ions. The reaction mechanism proceeded via halohydrine pathway.

The nanocrystalline Ru based oxides was developed to control the activity and selectivity of the surface towards halogen and oxygen transfer in the halohydrine forming reaction. The selectivity and reactivity of the surface was controlled by the particle shape and particle surface composition.

The most important achievements of this project were the following:
1. Catalyst selection for H2O2 production resulted in two materials that can be used in an acid and a basic cathode environment.
2. Fuel cells operating with electrocatalysts developed during the project were giving high yields of peroxide.
3. Quantum chemical calculations provided important guidelines for catalysts selection.
4. A new theoretical approach for analysing the class of reactions investigated, electron and proton transfer, was developed.

The work on hydrogen peroxide generation has advanced to the point where the technique could be industrially implemented in the next few years. Furthermore the work carried out received great attention in industry through publications. The work has also had an impact in the scientific community by establishing a new focus on electrocatalysis and by fostering the relationship between quantum chemical methods applied to analyse important practical problems, and experimental techniques.

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