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Catalytic and electrocatalytic oxidation of carbon monoxide on a Fe electrode in a solid electrolyte cell
Applied Catalysis B: Environmental 42 (2003) 225–236
Catalytic and electrocatalytic oxidation of carbon monoxide on a Fe electrode in a solid electrolyte cell G.E. Marnellos a,∗ , S.T. Zisekas a , A.G. Kungolos b a
Chemical Engineering Department, Chemical Process Engineering Research Institute, P.O. Box 361, 57001 Thermi-Thessaloniki, Greece b Department of Planning and Regional Development, University of Thessaly, Volos 38334, Greece Received 31 March 2002; received in revised form 7 July 2002; accepted 12 September 2002
Abstract The catalytic oxidation of carbon monoxide on Fe catalyst was studied at 300–500 ◦ C and atmospheric total pressure. The reaction was studied under both open- and closed-circuit operation in an yttria-stabilized zirconia solid electrolyte cell. The technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the Fe electrode under open circuit. Kinetic and potentiometric measurements were combined in order to elucidate the reaction mechanism. The results are in agreement with a Langmuir–Hinselwood type of adsorption-reaction with two different adsorption sites for carbon monoxide and oxygen. Under closed circuit, the effect of electrochemical oxygen “pumping” to the catalyst was examined. The operation of the cell was almost Faradaic as the rate enhancement factor (Λ) values measured were close to unity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide oxidation; Fe catalyst; Solid electrolyte cell; YSZ
1. Introduction The reaction of carbon monoxide oxidation has been studied extensively in the last three decades, primarily because carbon monoxide is one of the main pollutants of the auto exhaust emissions [1]. The reaction was studied on several metals, but mainly on Pt, since all catalytic converters are based on platinum. In addition to its practical importance, the catalytic oxidation of carbon monoxide on Pt attracted the interest of many investigators, because of the sustained oscillatory phenomena observed on the reaction rate under certain conditions [1]. Several researchers studied the reaction ∗ Corresponding author. Tel.: +30-310-498120; fax: +30-310-498190. E-mail address: [email protected] (G.E. Marnellos).
in a solid electrolyte cell-reactor, taking advantage of Solid Electrolyte Potentiometry (SEP) to provide an “in situ” and continuous measurement of the surface oxygen activity. In that way they combined kinetic and potentiometric data in order to explain the oscillatory phenomena and to propose a possible mechanism for the reaction of carbon monoxide oxidation [2–10]. Ehrhardt et al. studied the reaction of carbon monoxide oxidation on Pt at 500 ◦ C and pointed out, that a successful and reliable conjunction of kinetic and SEP measurements, depends strongly on the electrode preparation [5]. Okamoto et al. [11–14] observed a large difference between electromotive force (emf) measured experimentally and emf calculated from Nernst’s equation, assuming that gaseous and adsorbed oxygen are in equilibrium. They suggested
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that this was the result of mixed potentials. Vayenas [4] offered a different interpretation based, among others, on exchange current density measurements [15,16]. Yentekakis et al. [9] studied the reaction at 200–450 ◦ C using a wide range of reactants partial pressures, especially under those at which the reaction rate exhibited periodic behavior. In addition to the reaction rate oscillations, they observed that the cell emf was oscillating as well. The bifurcation between oscillatory states was very close to the dissociation pressure of PtO2 , an observation which strongly indicates that a periodic formation and decomposition of PtO2 is responsible for the oscillatory behavior [9]. Hildenbrand and Lintz [17] studied the reaction of carbon monoxide oxidation on a Cu–Cu2 O–CuO catalyst-electrode using for the first time SEP on oxide electrodes. They proved that the phase composition of the catalyst is not determined conclusively by thermodynamic restrictions but may also be dictated by kinetics [17]. In a similar manner, Petrolekas and Metcalfe [18] studied the reaction with the aid of SEP on a La–Sr–Mn mixed oxide electrode. Different catalyst states corresponded to different catalytic reactivity toward carbon monoxide oxidation [18,19]. The effect of Electrochemical Oxygen Pumping (EOP) and the appearance of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) on the reaction of carbon monoxide oxidation have been studied extensively on several metals including Pt, Pd, Au, and Ag, and on Ag–Pd alloys [1]. Yentekakis and Vayenas [20] studied the reaction on Pt at 250–600 ◦ C. In addition to a strong non-Faradaic behavior (Λ = 2000, ρ = 3) observed under certain conditions, it was found that reaction rate oscillations could be induced or stopped, adjusting properly the rate of O2− transport through the electrolyte. In later studies, Vayenas and co-workers [21–23] studied the reaction on Pt and, instead of O2− , they used F− (CaF2 ) and Na+ ( -Al2 O3 ) conductors. They proved that NEMCA effect is caused by the spillover of any ions, not only by those that participate in the catalytic reaction [22,23]. Although not as dramatic as on Pt, NEMCA effects were also observed on Au, Pd, and Ag electrodes [24–27]. In the present communication, the kinetics of the catalytic oxidation of carbon monoxide using simultaneous SEP measurements is studied on a Fe catalyst. The effect of Electrochemical Oxygen Pumping
on the oxidation rate of Fe catalyzed carbon monoxide is also examined.
2. Experimental 2.1. Experimental apparatus The apparatus used for the catalytic and electrocatalytic measurements has been described in detail in previous communications [28,29]. It consisted of the feeding unit, the cell-reactor and the analysis system. Reactant gases, carbon monoxide, oxygen and diluent nitrogen were of 99.99% purity. The total volumetric flowrate used was the same in all the experiments and equal to 100 ml/min STP. The analysis of reactants and products was done using an on-line, Hewlett-Packard 5890 Series II, gas chromatograph with a thermal conductivity detector (TCD). A molecular sieve 5-Å column was used to separate carbon monoxide, oxygen, and nitrogen, while a Porapak-N column was used for carbon dioxide. A paramagnetic oxygen analyzer (Oxynos 100 by Rosemount) was also used for the continuous monitoring of oxygen in both inlet and outlet streams. By performing the elemental carbon balance, the possibility of carbon to deposit on the catalyst surface was checked. Constant currents or voltages were applied between the working and counter electrodes using a EG&G model 363 potentiostat–galvanostat. 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 Fig. 1. It consisted of a YSZ tube (19 mm o.d., 16 mm i.d., 15 cm long), closed flat at one end. 2.2. Catalyst preparation The Fe catalyst-electrode was prepared from Fe(CO)5 powder. 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 in the SEP experiments was about 80 mg and during the application of the Electrochemical Oxygen
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227
Fig. 1. Schematic diagram of the solid electrolyte cell-reactor.
Pumping technique it was 25 mg. The thickness of the Fe film was about 5–20 m in both cases and its superficial surface area was approximately 7 and 2 cm2 , respectively. Scanning electron micrographs of the Fe electrode showed that the average diameter of the crystallites was approximately 3 m. Using the above crystallite size and the catalyst loading used, a total catalyst surface area of 310 cm2 was estimated. In this way, the total exposed catalyst film surface area was about 10−6 g-atom Fe. 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 [30].
3. Theory 3.1. Solid Electrolyte Potentiometry technique The basic principle and applicability of SEP have also been explained previously [16,30]. SEP utilizes a 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: 2FE 1/2 αO = (0.21) exp (1) RT where F is the Faraday constant, R the ideal gas constant, T the absolute temperature, E the electromotive force of the cell and αO is the activity of atomically adsorbed oxygen on the catalyst surface [16]. The validity of Eq. (1) is based on several assumptions [16,30], 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: O2− O + 2e−
(2)
a charge transfer reaction with adsorbed carbon monoxide also takes place at a comparable rate (especially under fuel-rich conditions where most of the catalyst surface is covered with carbon monoxide): COad + O2− CO2 + 2e−
(3)
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then a mixed potential is established and the measured emf provides a qualitative and not a quantitative measurement of the surface activities [9]. In any case, if we assume that Eq. (1) is valid, the thermodynamic activity of adsorbed atomic oxygen can be continuously monitored during reaction [16,30]. 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 1/2 adsorbed and gaseous oxygen, then PO2 = αO [16]. 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, αO , several orders 1/2 of magnitude lower than PO2 . 3.2. Electrochemical “pumping”: Faradaic and non-Faradaic effects (NEMCA) When the solid electrolyte cell-reactor (Fig. 1) operates under closed circuit and the primary goal is to effectively carry out chemical reactions, the cell operates as electrochemical oxygen “pump” [1]. 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]. Vayenas and co-workers [21,24,32–34] defined the rate enhancement factor, Λ, as:
r r − ro Λ= = (4) I/2F I/2F where r is the increase in the catalytic rate of oxygen consumption, r is the electrocatalytic rate (under closed circuit), ro is the catalytic rate (under open circuit) and I/2F is the imposed flux of O2− through 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 [21]. 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 [20–24] as: r (5) ρ= ro 4. Results 4.1. Open-circuit measurements In order to obtain a thorough understanding of the effect of O2− pumping on the kinetics of carbon monoxide oxidation on Fe, it is important to describe first the open-circuit kinetic behavior. The reaction rate of carbon monoxide oxidation and the surface oxygen activity behavior was studied in the reactor-cell of Fig. 1, between 300 and 500 ◦ C and atmospheric total pressure, using Fe as anodic electrode-catalyst. The carbon monoxide and oxygen partial pressures varied from 0 to 5 kPa and 0 to 20 kPa, respectively, and nitrogen was used as diluent. As shown in previous communications [30,31], the reactor behavior was very close to that of a well-mixed reactor (CSTR) for the range of volumetric flowrates employed in the present study. No products other than carbon dioxide were detected in the outlet stream. The YSZ tube itself, before the deposition of the Fe electrode, was totally inactive for carbon monoxide oxidation even at 500 ◦ C. During the kinetic measurements, particular emphasis was given to the definition of the catalyst oxidation state (e.g. Fe, FeO, Fe3 O4 and Fe2 O3 ), as this might have a pronounced and reproducible effect on the kinetic behavior. Iron is converted to various iron oxides according to the following reactions: Fe + 21 O2 FeO
(6)
3FeO + 21 O2 Fe3 O4
(7)
2Fe3 O4 + 21 O2 3Fe2 O3
(8)
The above reactions are in thermodynamic equilibrium and, depending on the oxygen partial pressure and temperature, one of them becomes predominant.
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Fig. 2. Dependence of reaction rate, rCO2 , on reciprocal absolute temperature, 1/T (PO2 = PCO = 2 kPa).
229
the reaction rate increases drastically for low oxygen partial pressures and after going through a maximum decreases to a level off value indicating that the rate is zero order in oxygen. Fig. 4a and b shows the effect of oxygen partial pressure on the rate of carbon dioxide formation and oxygen activity, respectively. The outlet partial pressure of carbon monoxide, PCO , was equal to 2 kPa and the temperature, T, was kept constant at 400 ◦ C. The continuous horizontal line of Fig. 4b corresponds to the thermodynamic stability limit of the Fe2 O3 –Fe3 O4 system at this temperature. Hence, in the area below the continuous line, only Fe3 O4 was stable. Similarly, in the area above this line, the formation of Fe2 O3 was thermodynamically favorable.
Fig. 2 shows the dependence of the total carbon dioxide production rate, rCO2 , on the inverse of absolute temperature, 1/T, at equal carbon monoxide and oxygen partial pressures (PCO = PO2 = 2 kPa). From the slope of this curve, the apparent activation energy of the reaction could be calculated and was found equal to 11.6 kcal/mol. Fig. 3 shows the dependence of the reaction rate, rCO2 , on the oxygen partial pressure at temperatures between 300 and 500 ◦ C. The outlet partial pressure of carbon monoxide, PCO , was kept constant at 2 kPa. Under these conditions, the only product detected was carbon dioxide. At all temperatures examined
Fig. 3. Effect of oxygen partial pressure, PO2 , on the rate of carbon dioxide formation, rCO2 (PCO = 2 kPa, T = 300, 400, 500 ◦ C).
Fig. 4. (a) Effect of oxygen partial pressure, PO2 , on the rate of carbon dioxide formation, rCO2 . (b) Effect of oxygen partial pressure, PO2 , on the surface oxygen activity, αO (PCO = 2 kPa, T = 400 ◦ C).
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and KCO
Fig. 5. Effect of carbon monoxide partial pressure, PCO , on the rate of carbon dioxide formation, rCO2 (PO2 = 1 kPa, T = 300, 400, 500 ◦ C).
2642 = 0.227 exp T
bar −1
(11)
Finally, Fig. 6a and b shows the effect of carbon monoxide partial pressure on the rate of carbon dioxide formation and oxygen activity when the outlet partial pressure of oxygen, PO2 , and the temperature, T, were kept constant at 1 kPa and 400 ◦ C, respectively. Reaction rate increased less than linearly with carbon monoxide partial pressure according to Eq. (9). The corresponding oxygen activity values seem to follow an opposite path. They initially decreased smoothly with carbon monoxide partial pressure until they essentially leveled off. Regarding the oxidation state of
It is obvious that, by increasing the oxygen partial pressure the reaction rate increases dramatically at first, reaches a maximum value and then drops off. The corresponding oxygen activity values followed the expected behavior, i.e. a sharp increase with the oxygen partial pressure with a tendency to reach the value predicted by Eq. (1) for thermodynamic equilibrium between gas-phase and adsorbed oxygen. Concerning the oxidation state of iron, it seems that, when the oxygen partial pressure is under 0.5 kPa and carbon monoxide partial pressure is equal to 2 kPa, the dominating state of iron is Fe3 O4 ; otherwise Fe2 O3 dominates the catalyst surface. Fig. 5 shows the dependence of the reaction rate, rCO2 , on the carbon monoxide partial pressure at temperatures between 300 and 500 ◦ C, at constant oxygen partial pressure, PO2 , equal to 1 kPa. It is obvious that, the formation rate of carbon dioxide, which was the only product, increases almost linearly with carbon monoxide partial pressure at the highest temperature (500 ◦ C) but the behavior is less than linear at lower temperatures. It was found that in the range where the rate becomes independent of PO2 all the kinetic data could be expressed quite accurately by Eq. (9): r = KR with
KCO PCO 1 + (KCO PCO )
KR = 0.00232 exp −
4322 T
(9) mol/s
(10)
Fig. 6. (a) Effect of carbon monoxide partial pressure, PCO , on the rate of carbon dioxide formation, rCO2 . (b) Effect of carbon monoxide partial pressure, PCO , on the surface oxygen activity, αO (PO2 = 1 kPa, T = 400 ◦ C).
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231
Fig. 7. Surface oxygen activity dependence on gas-phase composition.
iron, it seems that under these operating conditions, Fe2 O3 dominates the catalyst surface. An effort was made in order to use SEP data to describe the dependence of oxygen activity on gas-phase composition. It was found that αO values used in the Figs. 4b and 6b could be correlated quite accurately by the expression: αO
K = + K1 1/2 PCO PO2 − αO
(12)
1/2
Fig. 7 presents the dependence of (αO /(PO2 − αO )) on PCO , with K equal to: K = 2.343 × 10−6
(13)
4.2. Closed-circuit measurements Fig. 8 shows a typical EOP experiment carried out in the setup presented in Fig. 1. The figure shows a typical galvanostatic transient, i.e. it depicts the transient
Fig. 8. Rate and catalyst potential response to step changes in applied current during carbon monoxide oxidation on Fe (PO2 = 0.1 kPa, PCO = 2 kPa, T = 500 ◦ C).
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Fig. 9. Rate and catalyst potential response to step changes in applied current during carbon monoxide oxidation on Fe (PO2 = 1 kPa, PCO = 3 kPa, T = 500 ◦ C).
effect of a constant applied current on the rate of carbon dioxide formation at 500 ◦ C. The Fe catalyst film was deposited on YSZ and was exposed to a gas mixture with oxygen and carbon monoxide partial pressures equal to 0.1 and 2 kPa, respectively (whereas Fe3 O4 is the dominant oxidative state of iron). Initially (t < 0), the circuit was open (I = 0). Then, at t = 0, a constant current of +1.5 mA was applied between the working (catalyst) and counter electrodes (Fig. 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 (Fig. 8) and within approximately 40 min reached gradually the maximum value, which was 1.09 times higher than the open-circuit rate. The increase in the catalytic rate r was 0.6 the value of I/2F. In the following 60 min, the circuit was opened again and both the open-circuit voltage and formation rate of carbon dioxide returned to their initial values. After steady state was established, a constant current of +3 mA was applied to the cell. The catalytic rate started increasing gradually and within almost 60 min, it was 1.27 times higher than the open-circuit rate. The increase in the catalytic rate, r, was almost Faradaic compared to the value of I/2F. 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 [21]. Contrary to the previous experiment, Fig. 9 shows a typical galvanostatic transient, where Fe2 O3 dominates the catalyst surface (PO2 = 1 kPa, PCO = 3 kPa), as illustrated in Fig. 4b. Following the same procedure as described above, two different currents (+1 and +2 mA) were applied through the cell. In both cases, the increase in the catalytic rate was almost the same, about 2.3 times higher than the value of I/2F. This means that each O2− supplied to the Fe catalyst causes, at steady state, 2.3 chemisorbed oxygen atoms to react with carbon monoxide and form carbon dioxide. In the latter case, weak NEMCA effect was observed, but the low Λ value show that the operation of the cell was very close to Faradaic.
5. Discussion 5.1. Open-circuit measurements The steady-state open-circuit kinetics of carbon monoxide oxidation has been studied on a number of catalysts in the past. In those studies, it was found that the oxidation of carbon monoxide is characterized by limit cycle phenomena. Before 1980, four major
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233
candidates were considered as the cause for oscillations [9]: (i) strong dependence of the activation energy or heat of adsorption on surface coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to slow adsorption of an inert species, and (iv) periodic oxidation and reduction of the surface. After 1980, the work of Sales, Turner, Maple and co-workers provided experimental and theoretical evidence that the origin of oscillations is most likely related to (iv) and to some extend to (iii) [9]. The correct performance of the solid electrolyte cell was verified by introducing into the reactor various air–nitrogen mixtures of known oxygen partial pressure and by measuring open-circuit emfs in agreement with the Nernst equation: RT PO2 E= ln (14) 4F 0.21
The observed SEP and kinetic data allow us to make some conclusions about the mechanism of carbon monoxide oxidation on Fe. We assume a Langmuir type absorption for both atomic oxygen and carbon monoxide. If thermodynamic equilibrium is established between gas-phase and adsorbed carbon monoxide, the coverage of carbon monoxide, θ CO , is given by Eq. (15): KCO PCO θCO = (15) 1 + (KCO PCO )
within ±2 mV. Below 260 ◦ C, the time necessary for the establishment of steady-state emf measurements was quite long and no data could be obtained in this region. Catalyst exposure to air–carbon monoxide mixtures resulted in open-circuit emfs of −50 to −950 mV, indicating that the surface oxygen activity αO given by Eq. (1) was quite low and α2O < PO2 . Therefore, the above under reaction conditions adsorbed oxygen on the Fe surface was not in thermodynamic equilibrium with gaseous oxygen. It was observed that αO increased with increasing PO2 (Fig. 4b), and decreased with increasing PCO (Fig. 6b). Figs. 2–6 show the effect of the reactants concentration and temperature on the carbon dioxide formation rate. It can be seen (Fig. 2), that the reaction rate increased with increasing temperature and the apparent activation energy calculated under the specific reaction conditions was 11.6 kcal/mol. As shown in Figs. 3–6, the production of carbon dioxide was, in general, zero order with respect to oxygen (Figs. 3 and 4a). With respect to carbon monoxide, the reaction order is close to unity at the highest temperature, while this simple first order dependence disappears at lower temperatures (Figs. 5 and 6a) according to Eq. (9). The observed odd behavior of the formation rate of carbon dioxide in Figs. 3 and 4a could be attributed to the change of the catalyst surface. It can be seen that in all cases, the catalyst (iron) was in the form of Fe2 O3 , except under fuel-rich conditions (PO2 < 0.5 kPa) where Fe3 O4 dominated the catalyst surface (Fig. 4b).
where KO is the oxygen adsorption coefficient. The above equation correlates two surface properties and is valid irrespective of whether thermodynamic equilibrium exists between gaseous and surface oxygen. Considering that the reaction takes place between two adsorbed reactants (carbon monoxide, oxygen), the reaction rate could be expressed by the equation:
where KCO is the carbon monoxide adsorption coefficient. For atomic adsorbed oxygen the same equation can be written: K O αO θO = (16) 1 + (KO αO )
r = KR θCO θO2
(17)
According to Eqs. (15) and (16), Eq. (17) can be written as: KCO PCO KO αO r = KR (18) 1 + (KCO PCO ) 1 + (KO αO ) Eqs. (9) and (18) are equal if KO αO 1. From Eq. (11), giving the adsorption coefficient of carbon monoxide, one can calculate an enthalpy and entropy of adsorption using Eq. (19): KCO = e( S/R) e−( H/RT)
(19)
and
HCO ∼ = −22 kJ/mol,
SCO ∼ = −12.3 kJ/mol (20)
Writing down the steady-state mass balance for adsorbed atomic oxygen, the surface oxygen activity dependence on gas-phase composition might be explained: 1/2
KA PO2 (1 − θO ) = (KD θO ) + (KR θCO θO )
(21)
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where KA and KD are the oxygen adsorption and desorption coefficients, respectively. Dividing both terms by KA (1 − θO ) one obtains: 1/2
PO2 =
K D θO KR θCO θO + KA (1 − θO ) KA (1 − θO )
where KD 1 = KA KO
(22)
(23)
and θO = KO α O 1 − θO
(24)
Thus, Eq. (22) now can be written as: 1/2
PO2 − αO =
KR KCO PCO KA 1 + (KCO PCO )
(25)
and dividing by αO and taking the reciprocal of both terms the following equation can be obtained: αO 1/2 PO2
− αO
=
KD 1 KD + KR KCO PCO KR
(26)
Eq. (26) is similar to the Eq. (12) describing the experimental data, where K=
KD KR KCO
and
K1 =
KD KR
(27)
The value of KD can be calculated from the known values of K, KR and KCO and at the temperature of the experiment (T = 400 ◦ C) was found to be: KD = 1.3 × 10−11 mol/s. The observed kinetic data allow us to make some conclusions about the mechanism of carbon monoxide oxidation on Fe. It seems reasonable to assume that the reaction follows a simple Langmuir–Hinselwood mechanism, with two types of adsorption sites one for oxygen and the other for carbon monoxide. The above kinetic model explains both kinetic and SEP results in a satisfactory way. 5.2. Closed-circuit measurements Figs. 8 and 9 depict some typical galvanostatic transients, where at t = 0 a constant current was applied and after steady-state was established, the system returned to open-circuit operation. The selection of reaction conditions was not random. Particularly, in Fig. 8
catalyst-iron is in the form of Fe3 O4 , while in Fig. 9 Fe2 O3 dominates the catalyst surface. It can be seen that in both cases anodic polarizations of the iron electrode (O2− is pumped to the catalyst) enhance the reaction rates with respect to their open-circuit values. The observed changes in the rate formation vs. 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. In the first case, under fuel-rich reaction conditions (PCO = 2 kPa, PO2 = 0.1 kPa and T = 500 ◦ C), where Fe3 O4 dominates the catalytic surface, 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 purely Faradaic (Λ ≈ 0.6–1.5). The same conclusions were obtained in the second case (Fig. 9) where, from the SEP measurements, it was found that, under those reaction conditions (PCO = 3 kPa, PO2 = 1 kPa and T = 500 ◦ C), Fe2 O3 dominated the catalytic surface. In the latter case, the obtained Λ values were higher than in the previous experiment, but still quite low (Λ < 2.5) which implies that the increase in the catalytic rate was nearly Faradaic. The above results are in agreement with those, obtained when Au was used as a working electrode [25]. The different Λ values observed during the above two experiments might be explained by the different electronic conductivity of the two different iron oxidation states. It is well-known that Fe3 O4 exhibits higher electronic conductivity than Fe2 O3 . Therefore, Fe3 O4 , is expected to exhibit higher exchange current density, Io , as electrode than Fe2 O3 . Io 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. That is why, in our case, Fe2 O3 exhibits higher Λ values than Fe3 O4 . On the contrary, in recent studies [21] it has been found that oxygen pumping to Pt, Pd, and Ag electrodes, dramatically enhances the carbon monoxide oxidation rate. The published results showed that the observed behavior was non-Faradaic (Λ ≈ 102 –103 and r/ro ≈ 1.5–3). 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
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chemisorptive bond of oxygen. One might expect that oxygen pumping to Fe (either in the form of Fe3 O4 or Fe2 O3 ) as well as to Pt, Pd, and Ag, would be accompanied by dramatic changes in the rate of carbon monoxide oxidation. However, according to the data obtained in the present study, the catalytic properties of Fe in contrast to Pt, Pd, and Ag electrodes [21] did not alter essentially. An important characteristic of NEMCA is that, for any reaction, the magnitude of |Λ| can be estimated by the equation: |Λ| =
2Fro Io
(28)
where Io is the exchange current density of the catalyst/solid electrolyte interface [21]. In the present study, at the temperature of 500 ◦ C and when carbon monoxide and oxygen partial pressures were equal to 2 and 0.1 kPa, respectively, the open-circuit reaction rate, ro , was found to be equal to 2.75 × 10−8 mol of oxygen per second. From a previous work [35], the magnitude of the exchange current, Io , under similar reaction conditions has been calculated to be about 2–3 mA. Thus, the theoretical |Λ| value is between 1.5 and 2.5, which is very close to the experimental values.
6. Conclusions The reaction of carbon monoxide oxidation on Fe was studied at atmospheric pressure in a CSTR reactor with the simultaneous measurement of the oxygen activity on the catalyst surface. It was found that the dependence of the oxygen activity αO on gas-phase composition could be described quite accurately by the 1/2 expression: (αO /(PO2 − αO )) = (K/PCO ) + K1 . At all temperatures examined the reaction rate increases drastically for low oxygen partial pressures and after going through a maximum decreases to a level off value, indicating that the rate is zero order in oxygen. In the range where the rate becomes independent of PO2 , all the kinetic data could be expressed in a satisfactory way by the equation: r = KR (KCO PCO /(1 + KCO PCO )). Kinetic and SEP measurements were combined in order to elucidate the mechanism of carbon monoxide oxidation on Fe. The results are in agreement with
235
a Langmuir–Hinselwood type of adsorption-reaction with two different adsorption sites for carbon monoxide and oxygen. Additionally, the effect of electrochemical oxygen “pumping” was studied in order to enhance the carbon monoxide oxidation rate. Specifically, it was found that between the two different iron oxidation states Fe3 O4 and Fe2 O3 , Fe2 O3 exhibits higher rate enhancement factor values. Nevertheless, the operation of the cell in both cases was almost Faradaic as the (Λ) values measured were close to unity. Contrary, to the present results, in recent studies strong NEMCA effect was observed during oxygen pumping to Pt, Pd, and Ag electrodes [21].
Acknowledgements We gratefully acknowledge financial support of this research by the Center for Research and Technology Hellas (CERTH). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20]
M. Stoukides, Catal. Rev. Sci. Eng. 42 (1–2) (2000) 1. H. Okamoto, G. Kawamura, T. Kudo, J. Catal. 86 (1984) 437. H. Okamoto, G. Kawamura, T. Kudo, J. Catal. 87 (1984) 1. C.G. Vayenas, J. Catal. 90 (1984) 371. J.J. Ehrhardt, E. Hafele, H.-G. Lintz, A.F. Martins, Ber. Bunsenges. Phys. Chem. 89 (1985) 894. I.S. Metcalfe, S. Sundaresan, Chem. Eng. Sci. 41 (1986) 1109. E. Hafele, H.-G. Lintz, Solid State Ionics 23 (1987) 235. E. Hafele, H.-G. Lintz, Ber. Bunsenges. Phys. Chem. 92 (1988) 188. I.V. Yentekakis, S. Neophytides, C.G. Vayenas, J. Catal. 111 (1988) 152. I.S. Metcalfe, S. Sundaresan, AIChE J. 34 (1988) 195. H. Okamoto, G. Kawamura, T. Kudo, Solid State Ionics 1 (1980) 319. H. Okamoto, G. Kawamura, T. Kudo, Solid State Ionics 3–4 (1981) 453. H. Okamoto, G. Kawamura, T. Kudo, J. Catal. 86 (1984) 437. H. Okamoto, G. Kawamura, T. Kudo, J. Catal. 87 (1984) 1. P.J. Gellings, H.J.A. Koopmans, A.J. Burggraaf, Appl. Catal. 39 (1998) 1. M. Stoukides, Ind. Eng. Chem. Res. 27 (1988) 1745. H.-H. Hildenbrand, H.-G. Lintz, Appl. Catal. 49 (1989) L1. P. Petrolekas, I.S. Metcalfe, in: J.M. Winterbottom (Ed.), Proceedings of the International Chemical Engineering Research Event, Birmingham, 1993. I.S. Metcalfe, Catal. Today 20 (1994) 283. I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170.
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[21] C.G. Vayenas, M.M. Jaksic, S.I. Bebelis, S.N. Neophytides, in: J.O’.M. Bockris, B.E. Conway, W.R.E. White (Eds.), Modern Aspects in Electrochemistry, vol. 29, Plenum Press, New York, 1996, p. 57. [22] I.V. Yentekakis, C.G. Vayenas, J. Catal. 149 (1994) 238. [23] I.V. Yentekakis, G. Moggridge, C.G. Vayenas, R.M. Lambert, J. Catal. 146 (1994) 293. [24] C.G. Vayenas, S.I. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303. [25] O.A. Mar’ina, V.A. Sobyanin, Catal. Lett. 13 (1992) 61. [26] C. Karavasilis, S. Bebelis, C.G. Vayenas, Mater. Sci. Forum 76 (1991) 171. [27] T.I. Politova, V.V. Gal’vita, V.D. Belyaev, V.A. Sobyanin, Catal. Lett. 44 (1997) 75.
[28] P. Tsiakaras, C. Athanasiou, G. Marnellos, M. Stoukides, J.E. ten Elshof, H.J.M. Bouwmeester, Appl. Catal. A 169 (1998) 249. [29] A. Kungolos, P. Tsiakaras, M. Stoukides, Ionics 1 (1995) 214. [30] S. Seimanides, M. Stoukides, J. Catal. 88 (1984) 490. [31] A. Balian, G. Hatzigiannis, D. Eng, M. Stoukides, J. Catal. 145 (1986) 526. [32] C.G. Vayenas, S.I. Bebelis, S.N. Neophytides, J. Phys. Chem. 92 (1988) 5083. [33] C.G. Vayenas, S.I. Bebelis, S. Ladas, Nature 343 (1990) 625. [34] C.G. Vayenas, S.I. Bebelis, S.N. Neophytides, I.V. Yentekakis, J. Appl. Phys. 49 (1989) 95. [35] D. Koulioumpas, V. Tsirides, Diploma Thesis in Chem. Eng. Dept., 1996.
Catalytic and electrocatalytic oxidation of carbon monoxide on a Fe electrode in a solid electrolyte cell G.E. Marnellos a,∗ , S.T. Zisekas a , A.G. Kungolos b a
Chemical Engineering Department, Chemical Process Engineering Research Institute, P.O. Box 361, 57001 Thermi-Thessaloniki, Greece b Department of Planning and Regional Development, University of Thessaly, Volos 38334, Greece Received 31 March 2002; received in revised form 7 July 2002; accepted 12 September 2002
Abstract The catalytic oxidation of carbon monoxide on Fe catalyst was studied at 300–500 ◦ C and atmospheric total pressure. The reaction was studied under both open- and closed-circuit operation in an yttria-stabilized zirconia solid electrolyte cell. The technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the Fe electrode under open circuit. Kinetic and potentiometric measurements were combined in order to elucidate the reaction mechanism. The results are in agreement with a Langmuir–Hinselwood type of adsorption-reaction with two different adsorption sites for carbon monoxide and oxygen. Under closed circuit, the effect of electrochemical oxygen “pumping” to the catalyst was examined. The operation of the cell was almost Faradaic as the rate enhancement factor (Λ) values measured were close to unity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide oxidation; Fe catalyst; Solid electrolyte cell; YSZ
1. Introduction The reaction of carbon monoxide oxidation has been studied extensively in the last three decades, primarily because carbon monoxide is one of the main pollutants of the auto exhaust emissions [1]. The reaction was studied on several metals, but mainly on Pt, since all catalytic converters are based on platinum. In addition to its practical importance, the catalytic oxidation of carbon monoxide on Pt attracted the interest of many investigators, because of the sustained oscillatory phenomena observed on the reaction rate under certain conditions [1]. Several researchers studied the reaction ∗ Corresponding author. Tel.: +30-310-498120; fax: +30-310-498190. E-mail address: [email protected] (G.E. Marnellos).
in a solid electrolyte cell-reactor, taking advantage of Solid Electrolyte Potentiometry (SEP) to provide an “in situ” and continuous measurement of the surface oxygen activity. In that way they combined kinetic and potentiometric data in order to explain the oscillatory phenomena and to propose a possible mechanism for the reaction of carbon monoxide oxidation [2–10]. Ehrhardt et al. studied the reaction of carbon monoxide oxidation on Pt at 500 ◦ C and pointed out, that a successful and reliable conjunction of kinetic and SEP measurements, depends strongly on the electrode preparation [5]. Okamoto et al. [11–14] observed a large difference between electromotive force (emf) measured experimentally and emf calculated from Nernst’s equation, assuming that gaseous and adsorbed oxygen are in equilibrium. They suggested
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 2 3 4 - 5
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that this was the result of mixed potentials. Vayenas [4] offered a different interpretation based, among others, on exchange current density measurements [15,16]. Yentekakis et al. [9] studied the reaction at 200–450 ◦ C using a wide range of reactants partial pressures, especially under those at which the reaction rate exhibited periodic behavior. In addition to the reaction rate oscillations, they observed that the cell emf was oscillating as well. The bifurcation between oscillatory states was very close to the dissociation pressure of PtO2 , an observation which strongly indicates that a periodic formation and decomposition of PtO2 is responsible for the oscillatory behavior [9]. Hildenbrand and Lintz [17] studied the reaction of carbon monoxide oxidation on a Cu–Cu2 O–CuO catalyst-electrode using for the first time SEP on oxide electrodes. They proved that the phase composition of the catalyst is not determined conclusively by thermodynamic restrictions but may also be dictated by kinetics [17]. In a similar manner, Petrolekas and Metcalfe [18] studied the reaction with the aid of SEP on a La–Sr–Mn mixed oxide electrode. Different catalyst states corresponded to different catalytic reactivity toward carbon monoxide oxidation [18,19]. The effect of Electrochemical Oxygen Pumping (EOP) and the appearance of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) on the reaction of carbon monoxide oxidation have been studied extensively on several metals including Pt, Pd, Au, and Ag, and on Ag–Pd alloys [1]. Yentekakis and Vayenas [20] studied the reaction on Pt at 250–600 ◦ C. In addition to a strong non-Faradaic behavior (Λ = 2000, ρ = 3) observed under certain conditions, it was found that reaction rate oscillations could be induced or stopped, adjusting properly the rate of O2− transport through the electrolyte. In later studies, Vayenas and co-workers [21–23] studied the reaction on Pt and, instead of O2− , they used F− (CaF2 ) and Na+ ( -Al2 O3 ) conductors. They proved that NEMCA effect is caused by the spillover of any ions, not only by those that participate in the catalytic reaction [22,23]. Although not as dramatic as on Pt, NEMCA effects were also observed on Au, Pd, and Ag electrodes [24–27]. In the present communication, the kinetics of the catalytic oxidation of carbon monoxide using simultaneous SEP measurements is studied on a Fe catalyst. The effect of Electrochemical Oxygen Pumping
on the oxidation rate of Fe catalyzed carbon monoxide is also examined.
2. Experimental 2.1. Experimental apparatus The apparatus used for the catalytic and electrocatalytic measurements has been described in detail in previous communications [28,29]. It consisted of the feeding unit, the cell-reactor and the analysis system. Reactant gases, carbon monoxide, oxygen and diluent nitrogen were of 99.99% purity. The total volumetric flowrate used was the same in all the experiments and equal to 100 ml/min STP. The analysis of reactants and products was done using an on-line, Hewlett-Packard 5890 Series II, gas chromatograph with a thermal conductivity detector (TCD). A molecular sieve 5-Å column was used to separate carbon monoxide, oxygen, and nitrogen, while a Porapak-N column was used for carbon dioxide. A paramagnetic oxygen analyzer (Oxynos 100 by Rosemount) was also used for the continuous monitoring of oxygen in both inlet and outlet streams. By performing the elemental carbon balance, the possibility of carbon to deposit on the catalyst surface was checked. Constant currents or voltages were applied between the working and counter electrodes using a EG&G model 363 potentiostat–galvanostat. 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 Fig. 1. It consisted of a YSZ tube (19 mm o.d., 16 mm i.d., 15 cm long), closed flat at one end. 2.2. Catalyst preparation The Fe catalyst-electrode was prepared from Fe(CO)5 powder. 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 in the SEP experiments was about 80 mg and during the application of the Electrochemical Oxygen
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227
Fig. 1. Schematic diagram of the solid electrolyte cell-reactor.
Pumping technique it was 25 mg. The thickness of the Fe film was about 5–20 m in both cases and its superficial surface area was approximately 7 and 2 cm2 , respectively. Scanning electron micrographs of the Fe electrode showed that the average diameter of the crystallites was approximately 3 m. Using the above crystallite size and the catalyst loading used, a total catalyst surface area of 310 cm2 was estimated. In this way, the total exposed catalyst film surface area was about 10−6 g-atom Fe. 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 [30].
3. Theory 3.1. Solid Electrolyte Potentiometry technique The basic principle and applicability of SEP have also been explained previously [16,30]. SEP utilizes a 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: 2FE 1/2 αO = (0.21) exp (1) RT where F is the Faraday constant, R the ideal gas constant, T the absolute temperature, E the electromotive force of the cell and αO is the activity of atomically adsorbed oxygen on the catalyst surface [16]. The validity of Eq. (1) is based on several assumptions [16,30], 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: O2− O + 2e−
(2)
a charge transfer reaction with adsorbed carbon monoxide also takes place at a comparable rate (especially under fuel-rich conditions where most of the catalyst surface is covered with carbon monoxide): COad + O2− CO2 + 2e−
(3)
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then a mixed potential is established and the measured emf provides a qualitative and not a quantitative measurement of the surface activities [9]. In any case, if we assume that Eq. (1) is valid, the thermodynamic activity of adsorbed atomic oxygen can be continuously monitored during reaction [16,30]. 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 1/2 adsorbed and gaseous oxygen, then PO2 = αO [16]. 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, αO , several orders 1/2 of magnitude lower than PO2 . 3.2. Electrochemical “pumping”: Faradaic and non-Faradaic effects (NEMCA) When the solid electrolyte cell-reactor (Fig. 1) operates under closed circuit and the primary goal is to effectively carry out chemical reactions, the cell operates as electrochemical oxygen “pump” [1]. 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]. Vayenas and co-workers [21,24,32–34] defined the rate enhancement factor, Λ, as:
r r − ro Λ= = (4) I/2F I/2F where r is the increase in the catalytic rate of oxygen consumption, r is the electrocatalytic rate (under closed circuit), ro is the catalytic rate (under open circuit) and I/2F is the imposed flux of O2− through 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 [21]. 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 [20–24] as: r (5) ρ= ro 4. Results 4.1. Open-circuit measurements In order to obtain a thorough understanding of the effect of O2− pumping on the kinetics of carbon monoxide oxidation on Fe, it is important to describe first the open-circuit kinetic behavior. The reaction rate of carbon monoxide oxidation and the surface oxygen activity behavior was studied in the reactor-cell of Fig. 1, between 300 and 500 ◦ C and atmospheric total pressure, using Fe as anodic electrode-catalyst. The carbon monoxide and oxygen partial pressures varied from 0 to 5 kPa and 0 to 20 kPa, respectively, and nitrogen was used as diluent. As shown in previous communications [30,31], the reactor behavior was very close to that of a well-mixed reactor (CSTR) for the range of volumetric flowrates employed in the present study. No products other than carbon dioxide were detected in the outlet stream. The YSZ tube itself, before the deposition of the Fe electrode, was totally inactive for carbon monoxide oxidation even at 500 ◦ C. During the kinetic measurements, particular emphasis was given to the definition of the catalyst oxidation state (e.g. Fe, FeO, Fe3 O4 and Fe2 O3 ), as this might have a pronounced and reproducible effect on the kinetic behavior. Iron is converted to various iron oxides according to the following reactions: Fe + 21 O2 FeO
(6)
3FeO + 21 O2 Fe3 O4
(7)
2Fe3 O4 + 21 O2 3Fe2 O3
(8)
The above reactions are in thermodynamic equilibrium and, depending on the oxygen partial pressure and temperature, one of them becomes predominant.
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Fig. 2. Dependence of reaction rate, rCO2 , on reciprocal absolute temperature, 1/T (PO2 = PCO = 2 kPa).
229
the reaction rate increases drastically for low oxygen partial pressures and after going through a maximum decreases to a level off value indicating that the rate is zero order in oxygen. Fig. 4a and b shows the effect of oxygen partial pressure on the rate of carbon dioxide formation and oxygen activity, respectively. The outlet partial pressure of carbon monoxide, PCO , was equal to 2 kPa and the temperature, T, was kept constant at 400 ◦ C. The continuous horizontal line of Fig. 4b corresponds to the thermodynamic stability limit of the Fe2 O3 –Fe3 O4 system at this temperature. Hence, in the area below the continuous line, only Fe3 O4 was stable. Similarly, in the area above this line, the formation of Fe2 O3 was thermodynamically favorable.
Fig. 2 shows the dependence of the total carbon dioxide production rate, rCO2 , on the inverse of absolute temperature, 1/T, at equal carbon monoxide and oxygen partial pressures (PCO = PO2 = 2 kPa). From the slope of this curve, the apparent activation energy of the reaction could be calculated and was found equal to 11.6 kcal/mol. Fig. 3 shows the dependence of the reaction rate, rCO2 , on the oxygen partial pressure at temperatures between 300 and 500 ◦ C. The outlet partial pressure of carbon monoxide, PCO , was kept constant at 2 kPa. Under these conditions, the only product detected was carbon dioxide. At all temperatures examined
Fig. 3. Effect of oxygen partial pressure, PO2 , on the rate of carbon dioxide formation, rCO2 (PCO = 2 kPa, T = 300, 400, 500 ◦ C).
Fig. 4. (a) Effect of oxygen partial pressure, PO2 , on the rate of carbon dioxide formation, rCO2 . (b) Effect of oxygen partial pressure, PO2 , on the surface oxygen activity, αO (PCO = 2 kPa, T = 400 ◦ C).
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and KCO
Fig. 5. Effect of carbon monoxide partial pressure, PCO , on the rate of carbon dioxide formation, rCO2 (PO2 = 1 kPa, T = 300, 400, 500 ◦ C).
2642 = 0.227 exp T
bar −1
(11)
Finally, Fig. 6a and b shows the effect of carbon monoxide partial pressure on the rate of carbon dioxide formation and oxygen activity when the outlet partial pressure of oxygen, PO2 , and the temperature, T, were kept constant at 1 kPa and 400 ◦ C, respectively. Reaction rate increased less than linearly with carbon monoxide partial pressure according to Eq. (9). The corresponding oxygen activity values seem to follow an opposite path. They initially decreased smoothly with carbon monoxide partial pressure until they essentially leveled off. Regarding the oxidation state of
It is obvious that, by increasing the oxygen partial pressure the reaction rate increases dramatically at first, reaches a maximum value and then drops off. The corresponding oxygen activity values followed the expected behavior, i.e. a sharp increase with the oxygen partial pressure with a tendency to reach the value predicted by Eq. (1) for thermodynamic equilibrium between gas-phase and adsorbed oxygen. Concerning the oxidation state of iron, it seems that, when the oxygen partial pressure is under 0.5 kPa and carbon monoxide partial pressure is equal to 2 kPa, the dominating state of iron is Fe3 O4 ; otherwise Fe2 O3 dominates the catalyst surface. Fig. 5 shows the dependence of the reaction rate, rCO2 , on the carbon monoxide partial pressure at temperatures between 300 and 500 ◦ C, at constant oxygen partial pressure, PO2 , equal to 1 kPa. It is obvious that, the formation rate of carbon dioxide, which was the only product, increases almost linearly with carbon monoxide partial pressure at the highest temperature (500 ◦ C) but the behavior is less than linear at lower temperatures. It was found that in the range where the rate becomes independent of PO2 all the kinetic data could be expressed quite accurately by Eq. (9): r = KR with
KCO PCO 1 + (KCO PCO )
KR = 0.00232 exp −
4322 T
(9) mol/s
(10)
Fig. 6. (a) Effect of carbon monoxide partial pressure, PCO , on the rate of carbon dioxide formation, rCO2 . (b) Effect of carbon monoxide partial pressure, PCO , on the surface oxygen activity, αO (PO2 = 1 kPa, T = 400 ◦ C).
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231
Fig. 7. Surface oxygen activity dependence on gas-phase composition.
iron, it seems that under these operating conditions, Fe2 O3 dominates the catalyst surface. An effort was made in order to use SEP data to describe the dependence of oxygen activity on gas-phase composition. It was found that αO values used in the Figs. 4b and 6b could be correlated quite accurately by the expression: αO
K = + K1 1/2 PCO PO2 − αO
(12)
1/2
Fig. 7 presents the dependence of (αO /(PO2 − αO )) on PCO , with K equal to: K = 2.343 × 10−6
(13)
4.2. Closed-circuit measurements Fig. 8 shows a typical EOP experiment carried out in the setup presented in Fig. 1. The figure shows a typical galvanostatic transient, i.e. it depicts the transient
Fig. 8. Rate and catalyst potential response to step changes in applied current during carbon monoxide oxidation on Fe (PO2 = 0.1 kPa, PCO = 2 kPa, T = 500 ◦ C).
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Fig. 9. Rate and catalyst potential response to step changes in applied current during carbon monoxide oxidation on Fe (PO2 = 1 kPa, PCO = 3 kPa, T = 500 ◦ C).
effect of a constant applied current on the rate of carbon dioxide formation at 500 ◦ C. The Fe catalyst film was deposited on YSZ and was exposed to a gas mixture with oxygen and carbon monoxide partial pressures equal to 0.1 and 2 kPa, respectively (whereas Fe3 O4 is the dominant oxidative state of iron). Initially (t < 0), the circuit was open (I = 0). Then, at t = 0, a constant current of +1.5 mA was applied between the working (catalyst) and counter electrodes (Fig. 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 (Fig. 8) and within approximately 40 min reached gradually the maximum value, which was 1.09 times higher than the open-circuit rate. The increase in the catalytic rate r was 0.6 the value of I/2F. In the following 60 min, the circuit was opened again and both the open-circuit voltage and formation rate of carbon dioxide returned to their initial values. After steady state was established, a constant current of +3 mA was applied to the cell. The catalytic rate started increasing gradually and within almost 60 min, it was 1.27 times higher than the open-circuit rate. The increase in the catalytic rate, r, was almost Faradaic compared to the value of I/2F. 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 [21]. Contrary to the previous experiment, Fig. 9 shows a typical galvanostatic transient, where Fe2 O3 dominates the catalyst surface (PO2 = 1 kPa, PCO = 3 kPa), as illustrated in Fig. 4b. Following the same procedure as described above, two different currents (+1 and +2 mA) were applied through the cell. In both cases, the increase in the catalytic rate was almost the same, about 2.3 times higher than the value of I/2F. This means that each O2− supplied to the Fe catalyst causes, at steady state, 2.3 chemisorbed oxygen atoms to react with carbon monoxide and form carbon dioxide. In the latter case, weak NEMCA effect was observed, but the low Λ value show that the operation of the cell was very close to Faradaic.
5. Discussion 5.1. Open-circuit measurements The steady-state open-circuit kinetics of carbon monoxide oxidation has been studied on a number of catalysts in the past. In those studies, it was found that the oxidation of carbon monoxide is characterized by limit cycle phenomena. Before 1980, four major
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233
candidates were considered as the cause for oscillations [9]: (i) strong dependence of the activation energy or heat of adsorption on surface coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to slow adsorption of an inert species, and (iv) periodic oxidation and reduction of the surface. After 1980, the work of Sales, Turner, Maple and co-workers provided experimental and theoretical evidence that the origin of oscillations is most likely related to (iv) and to some extend to (iii) [9]. The correct performance of the solid electrolyte cell was verified by introducing into the reactor various air–nitrogen mixtures of known oxygen partial pressure and by measuring open-circuit emfs in agreement with the Nernst equation: RT PO2 E= ln (14) 4F 0.21
The observed SEP and kinetic data allow us to make some conclusions about the mechanism of carbon monoxide oxidation on Fe. We assume a Langmuir type absorption for both atomic oxygen and carbon monoxide. If thermodynamic equilibrium is established between gas-phase and adsorbed carbon monoxide, the coverage of carbon monoxide, θ CO , is given by Eq. (15): KCO PCO θCO = (15) 1 + (KCO PCO )
within ±2 mV. Below 260 ◦ C, the time necessary for the establishment of steady-state emf measurements was quite long and no data could be obtained in this region. Catalyst exposure to air–carbon monoxide mixtures resulted in open-circuit emfs of −50 to −950 mV, indicating that the surface oxygen activity αO given by Eq. (1) was quite low and α2O < PO2 . Therefore, the above under reaction conditions adsorbed oxygen on the Fe surface was not in thermodynamic equilibrium with gaseous oxygen. It was observed that αO increased with increasing PO2 (Fig. 4b), and decreased with increasing PCO (Fig. 6b). Figs. 2–6 show the effect of the reactants concentration and temperature on the carbon dioxide formation rate. It can be seen (Fig. 2), that the reaction rate increased with increasing temperature and the apparent activation energy calculated under the specific reaction conditions was 11.6 kcal/mol. As shown in Figs. 3–6, the production of carbon dioxide was, in general, zero order with respect to oxygen (Figs. 3 and 4a). With respect to carbon monoxide, the reaction order is close to unity at the highest temperature, while this simple first order dependence disappears at lower temperatures (Figs. 5 and 6a) according to Eq. (9). The observed odd behavior of the formation rate of carbon dioxide in Figs. 3 and 4a could be attributed to the change of the catalyst surface. It can be seen that in all cases, the catalyst (iron) was in the form of Fe2 O3 , except under fuel-rich conditions (PO2 < 0.5 kPa) where Fe3 O4 dominated the catalyst surface (Fig. 4b).
where KO is the oxygen adsorption coefficient. The above equation correlates two surface properties and is valid irrespective of whether thermodynamic equilibrium exists between gaseous and surface oxygen. Considering that the reaction takes place between two adsorbed reactants (carbon monoxide, oxygen), the reaction rate could be expressed by the equation:
where KCO is the carbon monoxide adsorption coefficient. For atomic adsorbed oxygen the same equation can be written: K O αO θO = (16) 1 + (KO αO )
r = KR θCO θO2
(17)
According to Eqs. (15) and (16), Eq. (17) can be written as: KCO PCO KO αO r = KR (18) 1 + (KCO PCO ) 1 + (KO αO ) Eqs. (9) and (18) are equal if KO αO 1. From Eq. (11), giving the adsorption coefficient of carbon monoxide, one can calculate an enthalpy and entropy of adsorption using Eq. (19): KCO = e( S/R) e−( H/RT)
(19)
and
HCO ∼ = −22 kJ/mol,
SCO ∼ = −12.3 kJ/mol (20)
Writing down the steady-state mass balance for adsorbed atomic oxygen, the surface oxygen activity dependence on gas-phase composition might be explained: 1/2
KA PO2 (1 − θO ) = (KD θO ) + (KR θCO θO )
(21)
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where KA and KD are the oxygen adsorption and desorption coefficients, respectively. Dividing both terms by KA (1 − θO ) one obtains: 1/2
PO2 =
K D θO KR θCO θO + KA (1 − θO ) KA (1 − θO )
where KD 1 = KA KO
(22)
(23)
and θO = KO α O 1 − θO
(24)
Thus, Eq. (22) now can be written as: 1/2
PO2 − αO =
KR KCO PCO KA 1 + (KCO PCO )
(25)
and dividing by αO and taking the reciprocal of both terms the following equation can be obtained: αO 1/2 PO2
− αO
=
KD 1 KD + KR KCO PCO KR
(26)
Eq. (26) is similar to the Eq. (12) describing the experimental data, where K=
KD KR KCO
and
K1 =
KD KR
(27)
The value of KD can be calculated from the known values of K, KR and KCO and at the temperature of the experiment (T = 400 ◦ C) was found to be: KD = 1.3 × 10−11 mol/s. The observed kinetic data allow us to make some conclusions about the mechanism of carbon monoxide oxidation on Fe. It seems reasonable to assume that the reaction follows a simple Langmuir–Hinselwood mechanism, with two types of adsorption sites one for oxygen and the other for carbon monoxide. The above kinetic model explains both kinetic and SEP results in a satisfactory way. 5.2. Closed-circuit measurements Figs. 8 and 9 depict some typical galvanostatic transients, where at t = 0 a constant current was applied and after steady-state was established, the system returned to open-circuit operation. The selection of reaction conditions was not random. Particularly, in Fig. 8
catalyst-iron is in the form of Fe3 O4 , while in Fig. 9 Fe2 O3 dominates the catalyst surface. It can be seen that in both cases anodic polarizations of the iron electrode (O2− is pumped to the catalyst) enhance the reaction rates with respect to their open-circuit values. The observed changes in the rate formation vs. 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. In the first case, under fuel-rich reaction conditions (PCO = 2 kPa, PO2 = 0.1 kPa and T = 500 ◦ C), where Fe3 O4 dominates the catalytic surface, 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 purely Faradaic (Λ ≈ 0.6–1.5). The same conclusions were obtained in the second case (Fig. 9) where, from the SEP measurements, it was found that, under those reaction conditions (PCO = 3 kPa, PO2 = 1 kPa and T = 500 ◦ C), Fe2 O3 dominated the catalytic surface. In the latter case, the obtained Λ values were higher than in the previous experiment, but still quite low (Λ < 2.5) which implies that the increase in the catalytic rate was nearly Faradaic. The above results are in agreement with those, obtained when Au was used as a working electrode [25]. The different Λ values observed during the above two experiments might be explained by the different electronic conductivity of the two different iron oxidation states. It is well-known that Fe3 O4 exhibits higher electronic conductivity than Fe2 O3 . Therefore, Fe3 O4 , is expected to exhibit higher exchange current density, Io , as electrode than Fe2 O3 . Io 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. That is why, in our case, Fe2 O3 exhibits higher Λ values than Fe3 O4 . On the contrary, in recent studies [21] it has been found that oxygen pumping to Pt, Pd, and Ag electrodes, dramatically enhances the carbon monoxide oxidation rate. The published results showed that the observed behavior was non-Faradaic (Λ ≈ 102 –103 and r/ro ≈ 1.5–3). 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
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chemisorptive bond of oxygen. One might expect that oxygen pumping to Fe (either in the form of Fe3 O4 or Fe2 O3 ) as well as to Pt, Pd, and Ag, would be accompanied by dramatic changes in the rate of carbon monoxide oxidation. However, according to the data obtained in the present study, the catalytic properties of Fe in contrast to Pt, Pd, and Ag electrodes [21] did not alter essentially. An important characteristic of NEMCA is that, for any reaction, the magnitude of |Λ| can be estimated by the equation: |Λ| =
2Fro Io
(28)
where Io is the exchange current density of the catalyst/solid electrolyte interface [21]. In the present study, at the temperature of 500 ◦ C and when carbon monoxide and oxygen partial pressures were equal to 2 and 0.1 kPa, respectively, the open-circuit reaction rate, ro , was found to be equal to 2.75 × 10−8 mol of oxygen per second. From a previous work [35], the magnitude of the exchange current, Io , under similar reaction conditions has been calculated to be about 2–3 mA. Thus, the theoretical |Λ| value is between 1.5 and 2.5, which is very close to the experimental values.
6. Conclusions The reaction of carbon monoxide oxidation on Fe was studied at atmospheric pressure in a CSTR reactor with the simultaneous measurement of the oxygen activity on the catalyst surface. It was found that the dependence of the oxygen activity αO on gas-phase composition could be described quite accurately by the 1/2 expression: (αO /(PO2 − αO )) = (K/PCO ) + K1 . At all temperatures examined the reaction rate increases drastically for low oxygen partial pressures and after going through a maximum decreases to a level off value, indicating that the rate is zero order in oxygen. In the range where the rate becomes independent of PO2 , all the kinetic data could be expressed in a satisfactory way by the equation: r = KR (KCO PCO /(1 + KCO PCO )). Kinetic and SEP measurements were combined in order to elucidate the mechanism of carbon monoxide oxidation on Fe. The results are in agreement with
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a Langmuir–Hinselwood type of adsorption-reaction with two different adsorption sites for carbon monoxide and oxygen. Additionally, the effect of electrochemical oxygen “pumping” was studied in order to enhance the carbon monoxide oxidation rate. Specifically, it was found that between the two different iron oxidation states Fe3 O4 and Fe2 O3 , Fe2 O3 exhibits higher rate enhancement factor values. Nevertheless, the operation of the cell in both cases was almost Faradaic as the (Λ) values measured were close to unity. Contrary, to the present results, in recent studies strong NEMCA effect was observed during oxygen pumping to Pt, Pd, and Ag electrodes [21].
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