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Content archived on 2024-04-16

Use of solid oxide fuel cells as a chemical reactor


Fuel cells which oxidize hydrogen to produce electricity and water can also be used to oxidize other chemical compounds such as methane, ethylene, propylene to produce valuable chemical products. It is the objective of the project to explore the possibilities for a SOFC to be used as a new highly selective and energy efficient chemical reactor for oxidation processes and CO hydrogeneration reactions. In this research it will be investigated how the selectivity and yield can be improved. In this research the NEMCA effect (discovered in research carried out under a previous EC contract) which can strongly improve the catalytic properties of metals in a SOFC, will play a central role.
I. New processes
The following systems were investigated under NEMCA effect conditions:
I1. Ethylene epoxidation on Ag films deposited on ytrria-stabilized-zirconia (YSZ), an O2- conductor, and on beta"-Al2O3, a Na+ conductor.
I2. Hydrogenation of CO2 on Pd deposited on YSZ and on beta"-Al2O3.
I3. Hydrogenation of CO on Pd interfaced to YSZ.
I4. Methanol synthesis via hydrogenation of CO2 over a commercial Cu/ZnO/Al2O3 catalyst interfaced to YSZ.
I5. Ethylene oxidation on Pd interfaced with YSZ or beta"-Al2O3.
I6. Oxidative coupling of methane over a manganese oxide catalyst interfaced with YSZ.
I7. Propene partial oxidation to acrolein on Cu2O or Au/Cu2O electrodes and CO oxidation on copper oxide electrodes interfaced to YSZ.
I8. The selective catalytic reduction (SCR) of NO by ammonia on Pt interfaced with YSZ. Significant nonfaradaic rate changes were observed (by up to 25000%) as well as changes in product selectivity. The most promising system is the ethylene epoxidation reaction over Ag/beta"-Al2O3 where a maximum ethylene oxide selectivity value of 88% was obtained for a sodium coverage equal to 0.03 and 1ppm dicloroethane in the gas phase. This is one of the highest values reported in the open or patent literature, thus the results may be of immediate technological importance.

II. General rules and origin of the NEMCA effect
II1. The following rules, for some of which experimental evidence already existed, were firmly established:
(a) The order of magnitude of the enhancement factor or Faradaic efficiency lambda defined from:
where deltar is the change in the catalytic reaction rate (expressed in mol O), I is the applied current (defined positive when anions are supplied to the catalyst) and F is the Faraday constant, can be estimated for any catalytic reactor from:
|lambda| is approximately equal to 2Fro/I0
where ro is the open-circuit (unpromoted) catalytic rate and I0 is the exchange current of the catalyst-solid electrolyte interface.
(b) The magnitude of the rate change relaxation time constant tau can be predicted by:
tau is approximately equal to 2FN/I
where N (mol) is the reactive oxygen uptake of the catalyst which expresses, approximately, the catalyst surface area.
(c) Depending on the polarity of the bond broken or formed in the rate limiting step electrophobic or electrophilic behaviour is observed. A catalytic reaction is termed electrophobic when its rate increases with increasing catalyst potential or work function, while a catalytic reaction is termed electrophilic when its rate increases with decreasing catalyst potential or work function
(d) the catalytic rates are exponentially dependent on catalyst potential and work function:
ln(r/ro)= alpha(delta(e{phi})/kbt) = alpha((F delta VWR)/RT)
where ro is the open-circuit catalytic rate

II2. Regarding the physicochemical origin of NEMCA the following studies were performed:
(a). X-ray photoelectron spectroscopy (XPS) was used to investigate the effect of electrochemical oxygen pumping on Pt catalyst films interfaced with YSZ. It was found that electrochemical oxygen pumping to the catalyst causes spillover of significant amounts of anionic oxygen from the solid electrolyte onto the platinum film surface. The spillover oxygen species has an XPS binding energy 528.8 eV compared to 530.4 eV for chemisorbed oxygen, which is also observed on the surface, and is less reactive than chemisorbed oxygen with the reducing ultrahigh vaccum background. The detection of the anionic oxygen species upon electrochemical pumping confirms the previously proposed explanation of the NEMCA effect.
(b). The effect of induced work function changes on the kinetics and energetics of the interaction of oxygen with polycrystalline Pt interfaced with YSZ, were studied, by means of the temperature programmed desorption technique (TPD). It was found that by increasing catalyst potential and work function the oxygen desorption peak shifts towards lower temperatures, showing that the binding strength of chemisorbed oxygen species weakness by increasing catalyst work function. The activation energy of desorption of adsorbed oxygen species was found to decrease linearly with slope -1 with increasing catalyst work function. This straightforward experimental correlation between catalyst work function and the binding energy of chemisorbed oxygen species is in absolute agreement with the observed linear dependence of the apparent activation energy on catalyst potential and work function, of all reactions studied.

As a result of the present studies it was firmly established that the NEMCA effect is due to the controlled migration (back-spillover) of promoting ions from the solid electrolyte to the gas-exposed catalyst-electrode surface under the influence of the applied current or potential. Some of the promoting ions, eg O{delta-} cannot be introduced from the gas phase. The ability of solid electrolytes to precisely tune the in situ introduction of dopants on catalyst surfaces creates several interesting technological possibilities and allows for a detailed study of the role of promoters in heterogeneous catalysis.
The proposed programme will concentrate on the study of NEMCA in SOFC reactors for a selected number of technologically important partial oxidation and CO hydrogenation reactions. The main goals are :

1. To examine the effect of changing catalyst potential and work function (via NEMCA) together with temperature, space velocity and reactant composition on product selectivity and to compare the maximum obtainable selectivity via NEMCA with the maximum obtainable selectivity utilizing commercial promoted catalysts.

2. To establish general trends or rules governing the effect of NEMCA on product selectivity. Such trends or rules will be useful guidelines for extending future NEMCA studies to systems for which no satisfactory catalytic processes are currently available. Both metal and metal oxide catalyst-electrodes will be studies in the course of the investigation. Most of the work involving metals will be carried out at ICE-HT in Patras and most of the work involving oxide will be done at the University of Karlsruhe (UKARL), where extensive experience already exists on the deposition of metal oxide electrodes in SOFC reactors.

The following systems will be investigated :

Ethylene epoxidation on AG (ICE - HT) . SP 0 Partial oxidation of ethylene to acetaldehyde (UKARL)
Partial oxidation of propylene on Au (UK and Cu2O (UKARL)
Oxydative coupling of methane (ICE - HT)
Fischer-Tropsch synthesis (ICE - HT)
Methanol synthesis (ICE - HT)
Catalyst characterization and elucidation of the NEMCA mechanism (ICE-HT and UKARL)


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Foundation for Research and Technology-Hellas
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Stadiou 18, Platani
26500 Patras

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