ELECTROCHEMICAL OXIDATION OF METHANE OVER AN
IRON ELECTRODE IN A SOLID ELECTROLYTE CELL
A. Kungolosa, C. Athanassioub, K. Kalimeric, N. Kyratzisb, G. Marnellosc
a
Department of Planning and Regional Development, University of Thessaly, Volos 38334, Greece
Polution Control Technologies Department, Technological Education Institute of W. Macedonia,
50100 Kila, Kozani, Greece
c
Department of Engineering and Management of Energy Resources, University of West Macedonia,
Kastorias & Fleming, 50100 Kozani, Greece
Tel: +302310-991305
Fax: +302310-991304
e-mail: [email protected]
b
ABSTRACT
The electrochemical oxidation of methane was examined at 700 oC and atmospheric total pressure.
The reaction was studied in an Yttria-Stabilized Zirconia solid electrolyte cell using Fe as working
and Ag as reference and counter electrodes. Under closed circuit, the effect of electrochemical
oxygen “pumping” to the catalyst, on reaction rate was examined. Non-Faradaic phenomena were
observed but the obtained rate enhancement factor Λ values were low.
ΗΛΕΚΤΡΟΧΗΜΙΚΗ ΟΞΕΙΔΩΣΗ ΤΟΥ ΜΕΘΑΝΙΟΥ ΣΕ
ΗΛΕΚΤΡΟΔΙΟ ΣΙΔΗΡΟΥ ΣΕ ΚΕΛΛΙ ΣΤΕΡΕΟΥ ΗΛΕΚΤΡΟΛΥΤΗ
α
Α. Κούγκολοςα, Κ. Αθανασίουβ, Κ. Καλημέρηγ, N. Κυρατζήςβ, Γ. Μαρνέλλοςγ
Τμήμα Μηχανικών Χωροταξίας Πολεοδομίας και Περιφερειακής Ανάπτυξης, Πανεπιστήμιο
Θεσσαλίας, Βόλος 38334, Ελλάδα
β
Τμήμα Τεχνολογιών Αντρρύπανσης, Τεχνολογικό Εκπαιδευτικό Ίδρυμα Δ. Μακεδονίας,
50 100 Κοίλα, Κοζάνη, Ελλάδα
γ
Τμήμα Μηχανικών Διαχείρισης Ενεργειακών Πόρων, Πανεπιστήμιο Δυτικής Μακεδονίας,
Καστοριάς & Φλέμινγκ, 50100 Κοζάνη, Ελλάδα
Τηλ: +302310-991305
Fax: +302310-991304
e-mail: [email protected]
ΠΕΡΙΛΗΨΗ
H ηλεκτροχημική οξείδωση του CH4 μελετήθηκε στους 700C και ατμοσφαιρική πίεση. Ο στερεός
ηλεκτρολύτης που χρησιμοποιήθηκε ήταν ζιρκονία (ZrO2) σταθεροποιημένη με ύττρια (Υ2Ο3). Το
ηλεκτρόδιο εργασίας ήταν από Fe, ενώ τα ηλεκτρόδια αναφοράς και μέτρησης αποτελούνταν από
Ag. Σε συνθήκες κλειστού κυκλώματος εξετάστηκε η επίδραση της ηλεκτροχημικής άντλησης
οξυγόνου στον καταλύτη, στον ρυθμό της αντίδρασης. Παρατηρήθηκαν ασθενή μη-Φαρανταϊκά
φαινόμενα, με το συντελεστή ενίσχυσης Λ να λαμβάνει χαμηλές τιμές.
1. INTRODUCTION
Methane is the major component of natural gas and as a consequence is an abundant
inexpensive fuel. Therefore, many investigators have studied the partial or complete oxidation of
methane in solid electrolyte cells over the last 40 years. The original incentive for using CH4-fuelled
solid electrolyte cells was to operate in the fuel cell mode. Due to high efficiencies (over-doubled
compared to the conventional internal combustion engines), the combined separation of O2 from N2
(elimination of NOX emissions) in the same unit and the ability of hydrogen utilization, solid oxide
fuel cells (SOFC) are considered as the next generation technology for green energy production [1].
In such a cell, CH4 can be directly or indirectly (via reforming) oxidized to CO2 and H2O according
to the following reactions [1]:
Cathode:
Anode:
Overall:
2x [O2 + 4 e- ---> 2 O2-]
CH4 + O2- ---> CO + 2 H2 + 2 eCO + O2- ---> CO2 + 2 e2 H2 + 2 O2- ---> 2 H2O + 4 eCH4 + 2 O2 ---> CO2 + 2H2O
(1)
(2)
(3)
(4)
(5)
The majority of studies were targeted to the investigation of the CH4 combustion
mechanism. YSZ was solely used as a solid electrolyte, while in most cases various noble metals
(Pt, Pd, Ag, Au) and some perovskite type oxides were employed as electrodes-catalysts [2-6].
In the present communication, the effect of electrochemical oxygen pumping on the methane
oxidation rate on a Fe electrode-catalyst, is examined. The reaction was studied under open and
closed circuit operation in an YSZ cell at 700C and atmospheric total pressure.
2. EXPERIMENTAL
2.1. Experimental Apparatus
The apparatus used for the catalytic and electrocatalytic measurements has been described in
detail in previous communications [6, 7]. Reactant gases, CH4, O2 and diluent N2 were of 99.99%
purity. The total volumetric flowrate used was equal to 100 ml STP/min. The analysis of reactants
and products was done using an on-line, SHIMADJU GC-14B, gas chromatograph with a thermal
conductivity detector (T.C.D.) and a molecular sieve 5A and a Porapak-N columns. A paramagnetic
oxygen analyzer (Oxynos 100 by Rosemount) was also used for the continuous monitoring of
oxygen in both inlet and outlet streams. An EG&G model 363 potentiostat-galvanostat was used for
applying constant currents between the working and counter electrodes. Bar Graph HC-737 digital
multimeters were also used to measure the applied voltage and current. A schematic diagram of the
solid electrolyte cell-reactor used, is shown in figure 1. It consisted of a YSZ tube (19 mm OD, 16
mm ID, 15 cm long), closed flat at one end.
2.2. Catalyst Preparation
The Fe catalyst-electrode was prepared from Fe(CO)5 powder [8]. In order to prepare the
working electrode, the Fe(CO)5 powder was mixed with ethylene glycol (10 g of powder in 20 ml of
glycol). The Fe film was deposited on the inside bottom wall of the YSZ tube, by applying a thin
coating paste and then the catalyst was calcined at 800 °C (heating rate: 3 °C/min) for 2 h. The total
mass of iron catalyst used was 25 mg. The thickness of the Fe film was about 5–20 μm in both cases
and its superficial surface area was approximately 2 cm2. Silver instead of iron, was used for the
preparation of the counter- and reference-electrodes, since silver adheres to the YSZ surface much
stronger than iron. The preparation and characterization of the silver electrode has been described in
detail in previous communications [8].
Figure 1: Schematic diagram of the solid electrolyte cell reactor
3. THEORY
3.1. Solid Electrolyte Potentiometry (SEP) Technique
The basic principle and applicability of SEP have also been explained previously [1, 9]. SEP
utilizes an YSZ solid electrolyte cell with one of the electrodes exposed to the reacting mixture,
serving as a catalyst for the reaction. The other electrode is exposed to air, serving as a reference
electrode. The thermodynamic activity of atomic oxygen adsorbed on the catalyst surface is given
by the Nernst equation:
a o  (0.21)1 / 2 exp(
2 FE
)
RT
(6)
where F is the Faraday’s constant, R the ideal gas constant, T the absolute temperature, E the
electromotive force of the cell and α0 is the activity of atomically adsorbed oxygen on the catalyst
surface [9].
The validity of Eq. (6) is based on several assumptions [1, 9], among which the most
questionable for the present system, is that atomically adsorbed oxygen is the only species to
equilibrate rapidly with oxygen ions at the gas–electrode–electrolyte boundary. Although valid for
the reference-electrode, this assumption may not hold for the catalyst-electrode. If, in addition to the
O2- equilibrium with adsorbed oxygen [Eq. (1)] various charge transfer reactions with adsorbed
methane or CO also take place at a comparable rate [Eq. (2-5)] then a mixed potential is established
and the measured emf provides a qualitative and not a quantitative measurement of the surface
activities.
In any case, if we assume that Eq. (6) is valid, the thermodynamic activity of adsorbed
atomic oxygen can be continuously monitored during reaction [1, 9]. At the same time the gas-phase
oxygen partial pressure above the catalyst surface can be measured independently and the two
values can be compared. If thermodynamic equilibrium is established between adsorbed and
gaseous oxygen, then PO21/2=α0 [9]. If, on the other hand, the steady-state rates of oxygen adsorption
and reaction on the catalyst surface become comparable, then the surface reaction can pull down the
surface oxygen activity, α0, several orders of magnitude lower than PO21/2.
3.2. Electrochemical "pumping": Faradaic and non-Faradaic effects (NEMCA)
When the solid electrolyte cell-reactor (Figure 1) operates under closed circuit and the
primary goal is to effectively carry out chemical reactions, the cell operates as electrochemical
oxygen "pump" [1-3]. A current I corresponds to I/4F mol of oxygen per second transported through
the solid electrolyte, due to the Faraday's law. When the total amount of oxygen required for the
reaction is supplied electrochemically as O2-, the maximum attainable rate of oxygen consumption
at the anode is equal to the rate of O2- transport through the solid electrolyte. This is the case of
Faradaic operation. If, however, gaseous oxygen is cofed with reactants in the gas phase, then it
would be possible for the oxygen consumption rate to exceed the rate of electrochemical transport
of oxygen [1-3]. Vayenas and co-workers [2, 3] defined the rate enhancement factor, Λ, as:

r  ro
r

I / 2F I / 2F
(7)
where Δr is the increase in the catalytic rate of oxygen consumption, r is the electrocatalytic rate
(under closed circuit), r0 is the catalytic rate (under open circuit) and I/2F is the imposed flux of O2through the electrolyte. All the above rates are expressed in gram-atoms of oxygen per second. A
reaction exhibits NEMCA effect when |Λ|>1. When Λ>1, the reaction is termed electrophobic and
when Λ<1 is termed electrophilic [2, 3]. In the case of a Faradaic effect, all oxygen
electrochemically transported through the electrolyte reacts at the anode (Λ=1). In addition to the Λ
factor, the dimensionless parameter ρ, called rate enhancement ratio, is also used to describe
NEMCA effect and is defined [2, 3] as:

r
ro
(8)
4. RESULTS
The reaction was studied in the reactor-cell of figure 1 at 700C and atmospheric total
pressure, using Fe as anodic electrode-catalyst. In the outlet stream the major products were the
complete oxidation products except in fuel rich conditions were small amounts of higher
hydrocarbons (e.g. ethane, ethylene) were detected.
During the measurements, particular emphasis was given to the definition of the catalyst
oxidation state (e.g., Fe, FeO, Fe3O4 and Fe2O3), as this might have a pronounced and reproducible
effect on the catalyst activity. Iron is converted to various iron oxides according to the following
reactions:
⇄ FeO
3 FeO + ½ O2 ⇄ Fe3O4
2 Fe3O4 + ½ O2 ⇄ 3 Fe2O3
Fe + ½ O2
(9)
(10)
(11)
The above reactions are in thermodynamic equilibrium and, depending on the composition and
temperature, one of them becomes predominant.
Figures 2a and 2b show the effect of time on stream on the rate of carbon dioxide formation
and oxygen activity, respectively. The outlet partial pressures of the reactants were kept constant
and equal to 14 kPa in the case of methane and 1 kPa for oxygen. The temperature, Τ, was kept
constant at 700 °C. Under these reaction conditions, the main products detected were those of
complete oxidation and traces of C2’s hydrocarbons. During the reaction, it seems that rate
decreases drastically with reaction time and levels off to a low value.
4,00E-07
2,00E+03
(a)
1,50E+03
VWR, mV
3,00E-07
rCO2, mol/s
(a)
2,00E-07
1,00E+03
5,00E+02
1,00E-07
0,00E+00
0,00E+00
-5,00E+02
0
50
100
150
200
250
0
50
100
t, min
150
200
250
t, min
4,80E-08
1,00E-04
(b)
(b)
1,00E-06
rCO2, mol/s
Fe2O3
αο
Fe3O4
1,00E-08
I = 1.8 mA
Λ= 1.18
ρ = 1.14
4,40E-08
I = 1 mA
Λ= 1.38
ρ = 1.08
1,00E-10
r0= 4.084 10 -8 mol/s
4,00E-08
1,00E-12
0
0
50
100
150
200
t, min
Figure 2: Effect of time on stream, t, on the
(a) rate of CO2 formation and (b) oxygen
activity (PO2=1kPa, PCH4=14kPa, T=700C).
250
50
100
150
200
250
t, min
Figure 3: (a) Catalyst potential and (b) rate
response to step changes in applied current
(PO2=1kPa, PCH4=14kPa, T=700C).
The continuous horizontal line of Figure 2b corresponds to the thermodynamic stability limit
of the Fe2O3–Fe3O4 system at this temperature. Hence, in the area below the continuous line, only
Fe3O4 was stable. Similarly, in the area above this line, the formation of Fe2O3 was
thermodynamically favorable. It is obvious that, as the reaction time increases the oxygen activity
values followed the expected behaviour, i.e. an increase with the time on stream with a tendency to
reach the value predicted by Eq. (6) for thermodynamic equilibrium between gas-phase and
adsorbed oxygen. Concerning the oxidation state of iron, it seems that, initially the dominating state
of iron is Fe3O4 and after approximately 40 minutes, Fe2O3 dominates the catalyst surface at those
reaction conditions. Taking into account the above information it seems that Fe3O4, is more active
than Fe2O3, for methane combustion.
Figure 3 shows a typical Electrochemical Oxygen Pumping (EOP) experiment carried out in
the setup presented in figure 1. Figure 3 shows a typical galvanostatic transient, i.e. it depicts the
transient effect of a constant applied current on the rate of carbon dioxide formation at 700 °C. The
Fe catalyst film was deposited on YSZ and was exposed to a gas mixture with oxygen and methane
partial pressures equal to 1 and 14 kPa, respectively (whereas Fe2O3 is the dominant oxidative state
of iron after 40 minutes of operation).
Initially (t<0), the circuit was open (I=0). Then, at t=0, a constant current of +1 mA was
applied between the working (catalyst) and counter electrodes (Figure 1). As a result, oxygen ions
O2- were supplied to the catalyst–gas–solid electrolyte three-phase boundary (tpb) (current is defined
positive when O2- are supplied to the catalyst). The catalytic rate started increasing (Figure 3b) and
within approximately 15 min reached gradually the maximum value, which was 1.08 times higher
than the open-circuit rate. The increase in the catalytic rate Δr was 1.38 the value of I/2F.
After 60 min, the circuit was opened again and both the open-circuit voltage and carbon
dioxide’s formation rate returned to their initial values. After steady state was established, a
constant current of +1.8 mA was applied to the cell. The catalytic rate started increasing gradually
and within almost 60 min, it was 1.18 times higher than the open-circuit rate. The increase in the
catalytic rate, Δr, was almost Faradaic compared to the value of I/2F. This means that each O2supplied to the Fe catalyst causes, at steady state, 1.18 chemisorbed oxygen atoms to react with
methane and form carbon dioxide.
It is worth noticing here that although the open-circuit voltage responded almost
immediately after the interruption of the current, the time response of the rate was more than
20 min. This might be a temporary effect of the imposed current to the catalyst and therefore to the
reaction rate, as the total residence time of the system was less than 0.5 min.
Figure 4 depicts the effect of oxygen partial pressure on the rate enhancement factor, Λ, and
the rate enhancement ratio ρ. Methane partial pressure was kept constant at 14 kPa, the operating
temperature was equal to 700 oC, and the imposed current was 1 mA. It is obvious that both Λ and ρ
are increased by increasing oxygen partial pressure. The highest Λ value, equal to 6, was obtained at
2 kPa oxygen partial pressure, with a corresponding ρ value equal to 1.3.
8
100
1,5
1,4
6
1,2
I, mA
4
ρ=r/r0
Λ= ΔrCO2/(I/4F)
10
1,3
I0 = 9 mA
1
2
1,1
0,1
0
1
0
0,5
1
1,5
2
2,5
PO2in, kPa
Figure 4: Effect of oxygen inlet partial
pressure, PO2, on the rate enhancement
factor, Λ, and on the rate enhancement ratio,
ρ. (Ι=1mA, PCH4=14kPa, T=700C).
-2
-1
0
1
2
η, V
Figure 5: Effect of applied current, I, on cell
overpotential, η.
(PO2=1kPa, PCH4=14kPa, T=700C).
5. DISCUSSION
Figure 3 depicts a typical galvanostatic transient, where at t=0 a constant current was applied
and after steady-state was established, the system returned to open-circuit operation. It can be seen
that anodic polarizations of the iron electrode (O2- is pumped to the catalyst) enhanced the reaction
rates with respect to their open-circuit values. The observed changes in the rate formation versus the
applied current were detected under all experimental conditions and were quite reversible. When the
circuit was opened, the rates returned to their initial values. Specifically, under fuel-rich reaction
conditions (PCH4 = 14 kPa, PO2 = 1 kPa and T = 700C), where Fe2O3 dominates the catalytic surface
as shown in figure 2, anodic polarization of working electrode led to a slight increase in the reaction
rate. Analysis of the data obtained showed that this rate increase was nearly Faradaic (Λ=1.18–
1.38). The above results are in agreement with previous studies in respect of low Λ values observed
when Pt, Pd and Au were employed as catalysts-electrodes [3].
As far as the ρ values concern, the results obtained here are lower (less than 1.5) than the
corresponding ρ values calculated in previous studies, which are ranged between 3 and 70 [3]. The
NEMCA effect for those catalytic systems was explained by taking into account the increase in the
catalyst work function during oxygen pumping and the consequent weakening of the chemisorptive
bond of oxygen. One might expect that oxygen pumping to Fe (either in the form of Fe3O4 or Fe2O3)
as well as to Pt, Pd, and Au, would be accompanied by dramatic changes in the rate of methane
oxidation. However, according to the data obtained in the present study, the catalytic properties of
Fe in contrast to Pt, Pd, and Au electrodes [3] did not alter essentially.
Rate enhancement factors, Λ, are generally low, typically less than five, due to the high
operating temperatures and concomitantly high Ι0 values [3]. I0 is a measure of the overall
electrocatalytic activity of the metal/solid electrolyte interface or, equivalently, of the three-phase
boundary (electrode, solid electrolyte, gas phase). In order to induce NEMCA effect the
electrode/solid electrolyte interfaces must be highly polarizable.
An important characteristic of NEMCA is that, for any reaction, the magnitude of |Λ| can be
estimated by the equation:
 
2  F  ro
Io
(12)
where I0 is the exchange current density of the catalyst/solid electrolyte interface [2, 3]. In the
present study, at a temperature of 700C and when CH4 and O2 partial pressures were equal to 14
kPa and 1 kPa respectively, the open circuit reaction rate, ro, was found to be equal to 16,336  10-8
g-atom O/s. Therefore, in order to estimate Λ, from Eq. (12), we have to calculate the exchange
current density, I0, for these specific conditions.
Figure 5 shows a typical relation between the overpotential, η, and the current density, I, for
the Fe2O3/YSZ interface, at 700oC. The overpotential, η, is defined as:
0
n  VWR  VWR
 IRdrop
(13)
0
where VWR
is the open circuit catalyst potential relative to the reference electrode and IRdrop is the
ohmic loss. It is clear that the iron electrode exhibits quite symmetrical behaviour. In order to
estimate the value of anodic I0, we can use the corresponding Butler-Volmer equation:
ln I  ln I 0 
aa F

RT
(14)
Anodic exchange current density, I0, is calculated from the intercept of the anodic curve for
overpotentials higher than 120 mV, and was found equal to 9 mA. Thus, the theoretical |Λ| value
calculated from equation (12) is approximately equal to 3.5. It is obvious that both theoretical and
experimental Λ are of the same order of magnitude.
6. CONCLUSIONS
The present study shows the usage of solid electrolyte cells in the study of catalytic
reactions. Additionally, solid electrolytes can be used as active catalyst supports to reversibly
promote catalyst surfaces. The promoting effect of solid electrolytes is due to an electrochemically
driven and controlled spillover of ions on the catalyst surface. Specifically, in the present work,
under closed circuit operation, NEMCA behavior was observed but the rate enhancement factor (Λ)
values measured were not very high.
ACKNOWLEDGEMENTS
We gratefully acknowledge financial support of this research by the Center for Research and
Technology Hellas (C.E.R.T.H.).
7. REFERENCES
1. M. Stoukides (2000) ‘Solid electrolyte membrane reactors: Current experience and future
outlook’ Catalysis Reviews – Science and Engineering, Vol. 42(1&2), pp. 1.
2. Vayenas C.G., M.M. Jaksic, S.I. Bebelis and S.N. Neophytides (1996) ‘The electrochemical
activation of catalytic reactions’ in J.O’.M. Bockris, B.E. Conway Modern Aspects in
Electrochemistry, W.R.E. White (Ed.), Plenum Press, Vol. 29, pp. 57.
3. Vayenas C.G., S. Bebelis, C. Pliangos, S. Brosda and D. Tsiplakides (2001) ‘Electrochemical
Activation of Catalysis’ Kluwer Academic/Plenum Publishers, New York.
4. Seimanides S. and M. Stoukides (1984) ‘Solid electrolyte aided study of methane oxidation on
silver’ Journal of Catalysis, Vol. 88, pp. 490.
5. Mar’ina, O.A., V.A. Sobyanin, V.D. Belyaev and V.N. Parmon (1992) ‘The effect of
electrochemical oxygen pumping on catalytic properties of Ag and Au electrodes at gas-phase
oxidation of CH4’ Catalysis Today, Vol. 13(4), pp. 567.
6. Tsiakaras P., C. Athanasiou, G. Marnellos, M. Stoukides, J.E. ten Elshof and H.J.M.
Bouwmeester (1998) ‘Methane activation on a LSCF perovskite -Catalytic and electrocatalytic
results’ Applied Catalysis A, Vol. 169, pp. 249.
7. Kungolos A., P. Tsiakaras and M. Stoukides (1995) ‘Production of synthesis gas in a solid
electrolyte cell’ Ionics, Vol. 1, pp. 214.
8. Marnellos G. E., S. T. Zisekas and A. G. Kungolos (2002) ‘Catalytic and Electrocatalytic
Oxidation of CO on a Fe Electrode in a Solid Electrolyte Cell’, Applied Catalysis B, Vol. 42(3),
pp. 225.
9. M. Stoukides (1988) ‘The use of solid electrolytes in heterogeneous catalysts’ Industrial and
Engineering Chemistry Research, Vol. 27, pp. 1745.