YSZ

Applied Catalysis B: Environmental 101 (2010) 31–37

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Electrochemical promotion of methane oxidation on Rh/YSZ A. Nakos, S. Souentie, A. Katsaounis ∗ Department of Environmental Engineering, Technical University of Crete, 73100 Chania, Greece

a r t i c l e

i n f o

Article history: Received 21 May 2010 Received in revised form 14 July 2010 Accepted 31 August 2010 Available online 6 September 2010 Keywords: NEMCA EPOC Permanent EPOC CH4 oxidation CH4 combustion Rh electrodes Rh catalyst

a b s t r a c t The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for the methane oxidation reaction over Rh catalytic films interfaced with YSZ, an oxygen ion conductor, at temperatures from 350 to 550 ◦ C, under reducing, stoichiometric and oxidizing conditions. CO2 is the main reaction product; however, CO is produced in small amounts at high temperatures. The effect of electrochemical promotion of catalysis on the reaction catalytic rate has been found to decrease by increasing partial pressure of oxygen and temperature. Under reducing conditions, at 430 ◦ C, positive current application can cause a 3-fold increase of the catalytic rate, while the apparent Faradaic efficiency is 170. After positive current interruption the catalytic rate reversibly returns to the initial open-circuit state value. Negative current application results in a 57% decrease of the catalytic rate with an apparent Faradaic efficiency equal to 40. After negative current interruption the catalytic rate slowly increases but remains lower than the initial value. This permanent poisoning effect has been interpreted by formation of a surface oxide layer by the strongly adsorbed oxygen from the gas phase upon negative polarization. A poisoning index, ␤, has been defined to quantify the magnitude of the effect. Moreover, under stoichiometric conditions, a periodical oscillation of the rate is observed both under open-circuit and polarization conditions in a narrow temperature window between 480 and 520 ◦ C, which is attributed to catalyst phase transition phenomena. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Methane is the main component of natural gas and is widely used for energy production in gas turbines, solid oxide fuel cells (SOFC) and natural gas fueled vehicles (NGVs). Moreover, it constitutes a promising fuel alternative to the conventional diesel and gasoline, due to its higher H/C ratio which results in lower CO2 emissions. An important drawback of this technology is the release of unburned methane amounts in atmosphere; where, as a major hydrocarbon air pollutant, it causes a 23-fold more detrimental greenhouse effect than carbon dioxide. Catalytic combustion shows promise for significant reduction of CH4 emissions and has been extensively investigated in the past few years for pollution abatement and power generation. For this purpose noble metal (Pd and Pt) supported catalysts are mainly used [1,2], due to their high catalytic activity and sulfur poisoning tolerance (below 500 ◦ C). A parallel approach to the light hydrocarbon catalytic combustion is the use of electrochemical promotion of catalysis effect [3] (EPOC or non-Faradaic electrochemical modification of catalytic activity, NEMCA effect) to electropromote metal catalyst electrodes.

The phenomenon of electrochemical promotion of catalysis has been extensively investigated in the last 30 years for more than 70 catalytic systems [3–5]. In EPOC studies the conductive catalyst-electrode is in contact with an ionic conductor and the catalyst is electrochemically promoted by applying a current or potential between the catalyst film and a counter or reference electrode, respectively. Numerous surface science and electrochemical techniques [3] have shown that EPOC is due to electrochemically controlled migration (reverse spillover or backspillover) of promoting or poisoning ionic species (O2− in the case of YSZ, TiO2 and CeO2 , Na+ or K+ in the case of ␤ -Al2 O3 , protons in the case of Nafion, CZI (CaZr0.9 In0.1 O3−␣ ) and BCN18 (Ba3 Ca1.18 Nb1.82 O9−␣ )) between the ionic or mixed ionic-electronic conductor – support and the gas exposed catalyst surface, through the catalyst–gas–electrolyte three phase boundaries (TPBs) [3]. Two parameters are commonly used to quantify the magnitude of EPOC effect [3], the rate enhancement ratio, , defined from: =

r ro

(1)

where r is the electropromoted catalytic rate and ro the opencircuit, i.e. normal catalytic rate and the apparent Faradaic efficiency, , defined from: ∗ Corresponding author. Tel.: +30 2821037819; fax: +30 2821037847. E-mail address: [email protected] (A. Katsaounis). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.08.030

=

(r − ro ) I/nF

(2)

32

A. Nakos et al. / Applied Catalysis B: Environmental 101 (2010) 31–37

Table 1 Electrochemical promotion studies on the methane deep and partial oxidation. Products

Catalyst

Solid electrolyte

T/◦ C

max a

max a

Ref.

CO2 CO2 CO2 CO2 , CO CO2 CO2 CO2 , C2 H4 , C2 H6 C2 H4 , C2 H6 , CO, CO2 , H2 CO2 , CO CO2 C2 H4 , C2 H6 , CO2 C2 H4 , C2 H6 , CO2

Pd Pd Pt Rh Pd Pd Ag Pt Ag Pt MnOx La0.6 Sr0.4 Co0.8 Fe0.2 O3

YSZ YSZ YSZ TiO2 /YSZ YSZ YSZ, CeO2 /YSZ YSZ YSZ YSZ SrCe0.92 Dy0.08 O3−␣ YSZ YSZ

380–440 400 600–750 550 150–750 470–600 720–850 600–800 700 600–645 800 880

200 150 5 – 258 – 5 4 3 330 – 2

90 70 70 11 2.5 2.6 30 0.1 9 2 4 15

[6] [7] [8] [9] [10] [11] [12] [13] [14] [15,16] [17] [18]

a

In some references there was either direct or additional necessary information for the calculation of  and .

where n is the charge of the ionic species and F is the Faraday’s constant. A reaction exhibits electrochemical promotion when || > 1, while electrocatalysis is limited to || ≤ 1. Previous EPOC studies on CH4 deep oxidation over Pd [6,7] and Pt [8] catalyst electrodes interfaced with YSZ have shown that a significant electropromotion effect (i.e.  and  values up to 90 and 250, respectively) can be obtained. Similar electropromotion effect was observed when Rh catalyst electrodes interfaced with TiO2 /YSZ was used for the partial oxidation of CH4 to synthesis gas [9] at high temperatures under reducing conditions. Table 1 summarizes the majority of the previous reported studies on electrochemical promotion of methane partial and deep oxidation using both single chamber and fuel type reactors, where as shown Rh catalysts have not been widely examined, especially at low temperatures. In this study the effect of electrochemical promotion of catalysis was studied for the first time in the methane deep oxidation reaction over a Rh catalyst electrode interfaced with YSZ at low temperatures, between 350 and 550 ◦ C, under reducing, stoichiometric and oxidizing conditions. Rhodium was selected due to its susceptibility to thin surface oxide layers formation in presence of O2 in the gas phase, which can possibly affect the CH4 adsorption mechanism on the catalytic surface. 2. Experimental 2.1. Sample preparation The solid electrolyte cell consisted of an YSZ (8 mol% Y2 O3 stabilized ZrO2 ) pellet (20 mm diameter and 2 mm thickness, Technox 802 from Dynamic Ceramic), covered on both sides by three electrodes: working, counter and reference. A rhodium polycrystalline thin film (∼10 ␮m), serving as the working electrode, was deposited onto the one side of YSZ pellet by application of metalorganic paste (Engelhard 8826), followed by calcination in air for 30 min at 450 ◦ C and for 60 min at 830 ◦ C. The resulting metal loading was ∼1.3 mgRh cm−2 . Gold thin films were deposited on the other side of the pellet and serve as the reference and counter electrodes, by application of metalorganic paste (Engelhard 8300), followed by calcination in air for 30 min at 450 ◦ C and for 60 min at 750 ◦ C. Since all electrodes were exposed to the gas mixture, blank experiments were carried out using only Au electrodes and confirmed the negligible catalytic activity of Au compared to that of Rh. Furthermore, previous EPOC studies [19] have shown that Au behaves as a good pseudo-reference electrode since only a small variation of its potential (<0.1 V) was observed over the range of gaseous compositions used in the present study. Prior to each experiment, a pretreatment reducing step was performed under pure H2 at 350 ◦ C for 30 min (with total flowrate, FT = 200 cm3 min−1 ) to ensure the same initial catalyst state.

2.2. Reactor operation A single chamber continuous flow quartz reactor (70 cm3 ) was used in this study under atmospheric pressure and has been described in detail previously [3,20]. The reactor was placed into a thermostated furnace and the temperature was measured by a K-type thermocouple placed in proximity to the working electrode surface. Reactants were certified standards of CH4 in He and O2 in He. Pure He (99.99%) was used to further adjust total gas flowrate and inlet gas composition at the desired levels. The CH4 and O2 partial pressures in the feed were held constant at PCH4 /PO2 = 1.8 kPa/2 kPa, PCH4 /PO2 = 1.7 kPa/3.4 kPa, and PCH4 /PO2 = 1.4 kPa/6 kPa, for the reducing, stoichiometric and oxidizing conditions respectively. The total gas flowrate was FT = 200 cm3 (STP) min−1 , which results in HSV = 2.8 min−1 . On-line IR spectroscopy (Fuji Electric) was used for the CO, CO2 and CH4 analysis. Constant currents or potentials were applied using an AMEL 2053 galvanostat–potentiostat. 3. Results and discussion 3.1. Exchange current measurements Fig. 1(a–c) shows the observed dependence of the current on the catalyst overpotential, , for reducing, stoichiometric and oxidizing gaseous compositions at three different temperatures. The overpotential is defined as: o  = UWR − UWR

(3)

o where UWR is the open-circuit (I = 0) value of UWR . The exchange current is estimated by extrapolating the linear ln I vs.  (Tafel) part of the curves to  = 0 [21,22]. As shown in Fig. 1, the obtained values of I0 are almost the same, as it is expected, calculated either by the positive or negative overpotential branch. Furthermore, the observed in each case symmetry in the shape of the two branches is indicative of the similar values of the anodic and cathodic transfer coefficients, ˛a and ˛c , respectively.

3.2. Catalytic and electrocatalytic measurements 3.2.1. Reducing conditions Fig. 2 illustrates the steady-state effect of temperature on the open-circuit Rh–Au potential difference as well as on the CO2 and CO catalytic rates and the current under both open-circuit and polarization conditions for a reducing CH4 /O2 (1.8/2) gas mixture at temperatures from 350 to 550 ◦ C. As shown, the CO2 formation catalytic rate increases monotonically with temperature, under both open-circuit and polarization conditions. Positive polarization results in a significant increase of the CO2 formation rate, while negative polarization results in

A. Nakos et al. / Applied Catalysis B: Environmental 101 (2010) 31–37

0.1

-2000

-1000

10

0 η / mV

1000

2000

0.001

10

(b)

1

1

0.1

I / mΑ 0.0001

= 400o C = 470o C = 550o C

PCH4 = 1.7 kPa PO 2 = 3.4 kPa

-2000

-1000

10

0 η / mV

1000

2000

0.8

0.4 PCH4 = 1.8 kPa PO2 = 2 kPa

0 0.4

0 340

380

CH4 + 2O2 → CO2 + H2 O 0.0001

I / mΑ

o

= 400 C = 470o C = 530o C

PCH4 = 1.4 kPa PO2 = 6 kPa

-2000

-1000

0 η / mV

1000

2000

540

The production of CO2 occurs by the deep oxidation of CH4 via reaction (4).

0.01

0.001

500

0.001

0.1

0.01

460

0.01

1

0.1

420

Temperature / oC

Fig. 2. Steady-state effect of temperature on the open-circuit Rh–Au potential difference, CO2 and CO formation catalytic rates and currents under both open-circuit and polarization conditions. PCH4 /PO2 = 1.8/2.

10

(c)

1

0.0001

1.2

= o.c. = -1 V = +1 V

0.1

0.01

0.001

-2

0.0001

rCO / 10-7 mol O s-1

= 370o C = 450o C = 520o C

PCH4 = 1.8 kPa PO2 = 2 kPa

0

0

-4

rCO2 / 10-6 mol O s-1

I / mΑ 0.0001

0.01

2

40

0.1

0.01

0.001

o

1

1

4

80

I / mA

10

(a)

U WR / mV

10

33

0.001

0.0001

Fig. 1. Dependence of current on the catalyst overpotential, , for reducing (a), stoichiometric (b) and oxidizing (c) gaseous compositions, at three deferent temperatures.

a rate decrease, indicating an electrophobic behavior of the reaction over Rh. However, the effect of electrochemical promotion diminishes at temperatures higher that 530 ◦ C. In fact at elevated temperatures, O2 evolution is favored inhibiting the O2− backspillover on the catalyst surface and the formation of the effective double layer, which modifies the catalytic surface work function [3].

(4)

The activation energy of the CO2 formation reaction has been found to be ∼16 kcal mole−1 under open-circuit state conditions, while it decreases to ∼12 kcal mole−1 upon positive polarization and increases to ∼21 kcal mole−1 upon negative polarization. This agrees with the NEMCA effect theory [3], which predicts a linear dependence of the activation energy on the catalyst work function change, , or equally on the catalyst-electrode overpotential. The CO formation catalytic rate becomes measurable at ∼480 ◦ C. It is more than one order of magnitude lower than that of CO2 and is not affected by polarization. CO production can occur either by the partial oxidation of CH4 (reactions (5a) and (5b)) or, more likely by an autothermal reforming process (ATR) [23] according to reactions (6a) and (6b), towards synthesis gas formation. CH4 + 3/2O2 → CO + 2H2 O

(5a)

CH4 + 1/2O2 → CO + 2H2

(5b)

2CH4 + O2 + CO2 → 3CO + 3H2 + H2 O

(6a)

4CH4 + O2 + 2H2 O → 4CO + 10H2

(6b)

The latter CO production path (reaction (6a) or (6b)) occurs only at high temperatures, where a significant amount of CO2 and H2 O is produced via the deep oxidation of CH4 . This is in agreement with Fig. 2, where CO formation occurs only at T > 480 ◦ C. The case of partial oxidation of CH4 by O2− backspillover species at high temperatures can be neglected, since no effect of the applied current or potential on the CO formation rate was detected. The observed electrophobic behavior of the system can be attributed to the strongly adsorbed electron acceptor species (Oads ) and the weakly adsorbed electron donor species on the catalytic surface [3,24]. Hence, the increase of the CO2 formation rate observed upon positive polarization can be attributed to weakening of the Rh–O bond strength, due to the increase of the catalyst

A. Nakos et al. / Applied Catalysis B: Environmental 101 (2010) 31–37

8

6

4

2

0

rCO2 / 10-6 mol O s-1

CH4 conversion / %

0.8

o.c.

1200

Λ = 170 ρ=3

1000 800

0.6 600

0.4

0

20

40

o.c.

I= -800 μA

0 0.8

200

o

T=430 C PCH4 = 1.8 kPa PO2 = 2 kPa

o.c.

8

400

0.2

0

1

6

4

2

o

T=430 C PCH4 = 1.8 kPa PO2 = 2 kPa

-200

0.6

-400 -600

0.4

-800

0.2 Λ = 40 ρ = 0.4

0

60

80

100

120

0

140

0

Time / min

U WR / mV

I= +600 μA

rCO2 / 10-6 mol O s-1

o.c.

CH4 conversion / %

1

UWR (mV)

34

0

20

40

60

80

-1000 100

Time / min

Fig. 3. Transient effect of a constant applied anodic current (600 ␮A) on the CO2 formation catalytic rate and the Rh–Au potential difference. PCH4 /PO2 = 1.8/2, T = 430 ◦ C.

Fig. 4. Transient effect of a constant applied cathodic current (−800 ␮A) on the CO2 formation catalytic rate and the Rh–Au potential difference. PCH4 /PO2 = 1.8/2, T = 430 ◦ C.

work function and thus, to decrease of the coverage of oxygen of the catalytic surface,  O . Negative polarization results in an increase of the Rh–O bond strength and thus an increase of  O . This agrees with the competitive adsorption of CH4 and O2 model over Pt catalysts, which has been reported in previous studies [25,26]. Although Pt and Rh in general exhibit different catalytic behavior, the competitive adsorption of O2 and CH4 on the catalyst surface could be in common for the two metals. Oxygen is adsorbed on the catalytic surface and weak CH4 adsorption occurs only after an adsorbed oxygen has abstracted a hydrogen atom from CH4 to form CH3 (and other C1 fragments) on the catalytic surface (reaction (7)).

170. This result is in excellent agreement with the predicted value of 160, calculated by the approximate Eq. (9) [3].

CH4 + Oads → CH3,ads + OHads

(7)

adsorption step can also occur by the O2−

This CH4 backspillover species that migrate on the catalyst surface upon anodic polarization, according to reaction (8) [8]. This step accounts for  ≤ 1, i.e. a Faradaic electrocatalytic process. CH4 + O2− → CH3,ads + OHads + 2e−

(8)

The open-circuit potential, as shown in Fig. 2 (top), increases with temperature and becomes positive above 460 ◦ C. This could be due to the Rh–O and/or Rh–CH3 bond strength modification and to enhanced thermal migration of O2− from the solid electrolyte to the catalyst surface in presence of O chemisorbed species [27,28], by temperature increase. The above electrophobic behavior of the system and the high selectivity towards CO2 formation are in contrast to the behavior observed in [9], utilizing a Rh/TiO2 /YSZ/Au solid electrolyte cell at 550 ◦ C under 1 kPa/0.5 kPa CH4 /O2 ratio towards synthesis gas production, where an inverted volcano behavior was observed, i.e. rate increase both by positive and negative polarization with significant CO formation selectivity (∼45%). Moreover, negative polarization was found to result in a highly active steady-state after current interruption, close to the electropromoted state. These differences can be possibly either due to the different working electrodes utilized in the two studies (Rh and Rh/TiO2 ) or to the higher operating temperature in [9]. Fig. 3 illustrates the transient effect of a constant applied anodic current (600 ␮A) on the CO2 formation catalytic rate and the Rh–Au potential difference at 430 ◦ C, where no measurable quantities of CO were detected. Under open-circuit, i.e. normal catalytic conditions the conversion of CH4 is ∼2.7%. Positive current application for 1 h causes an up to 3-fold increase of the catalytic rate, where CH4 conversion reaches ∼8%, while the apparent Faradaic efficiency is

  2Fr0  = I0

(9)

where I0 is the exchange current (350 ␮A) at 430 ◦ C estimated by linear interpolation of the I0 values presented in Fig. 1a. After current interruption, the catalytic rate reversibly returns to its initial steady-state value. Fig. 4 shows a similar catalytic rate transient response upon a constant negative current application (−800 ␮A). Negative polarization results in a decrease of the conversion of CH4 from 2.7% to 1.2%, with  and  values equal to 0.43 (57% rate decrease) and 40, respectively. After current interruption, the CO2 formation catalytic rate does not return to its initial steady-state value, but remains lower. This poisoning phenomenon, which is reported for the first time after negative current interruption, is similar to the “permanent-EPOC” (P-EPOC) effect [29–32], where the catalytic rate remains in a highly active steady-state after positive current interruption and possibly originates from the electrochemically induced storage of oxygen species (e.g. as interfacial metal oxides) in the proximity of the working-catalyst electrode. The irreversibility of the rate after negative current interruption could be rationalized by formation of a catalyst surface oxide by the strongly adsorbed oxygen species (from the gas phase), upon negative polarization. In presence of these surface oxide species, the oxygen chemical potential at the catalyst surface is higher and thus, the chemical potential gradient between the electrolyte support and the catalyst surface is lower. This results in slower thermal diffusion of O2− backspillover species and also, lower O2− coverage (O2− ) of the catalytic surface at the open-circuit steady-state and hence, in lower catalytic activity. Restoration of the catalytic activity to the initial open-circuit steady-state value occurred only by positive current application, supporting the described mechanism. Similar decomposition of thin surface rhodium oxide layers by positive polarization has been reported in several EPOC studies on Rh catalysts interfaced with YSZ [33–36]. It has been attributed to weakening of the Rh–O bond strength via lateral repulsive interactions between the O2− and the strongly adsorbed oxygen species. However, this mechanism is certainly worth further investigation by electrochemical and surface spectroscopic techniques. The difference between this “poisoning permanent-EPOC” effect observed here and the classical permanent-EPOC, observed upon positive polarization [29–32], is the means of supplying oxygen to

A. Nakos et al. / Applied Catalysis B: Environmental 101 (2010) 31–37

1 o.c.

6

4

1000

0.8

rCO2 / 10-6 mol O s-1

CH4 conversion / %

8

2

Λ = 100 ρ=3

800

0.6

600 400

0.4 o

T=420 C PCH4 = 1.7 kPa PO2 = 3.4 kPa

0.2

0

1200

o.c.

I= +700 μA

UWR (mV)

10

35

0

0

20

40

200 0

60

80

100

-200 120

Time / min Fig. 6. Transient effect of a constant applied anodic current (700 ␮A) on the CO2 formation catalytic rate and the Rh–Au potential difference. PCH4 /PO2 = 1.7/3.4, T = 420 ◦ C.

(r0 − rpoisoning ) r0

(10)

where rpoisoning is the catalytic rate in the “poisoned permanentEPOC” steady-state after negative current interruption. In Fig. 4, the poisoning index, ˇ, is 0.17, indicating a 17% lower catalytic activity after negative current interruption. 3.2.2. Stoichiometric conditions Fig. 5 shows the steady-state effect of temperature on the opencircuit Rh–Au potential difference as well as on the CO2 and CO catalytic rate and the current under both open-circuit and polarization conditions for a stoichiometric CH4 /O2 (1.7/3.4) gas mixture at temperatures between 350 and 550 ◦ C. As shown, CO2 and CO formation catalytic rates increase monotonically with temperature, under both open-circuit and polarization conditions, while at T > 500 ◦ C the CO formation rate reaches a plateau. The open-circuit CO2 formation rate value at T < 440 ◦ C is lower than that obtained under reducing conditions (Fig. 2). This can be attributed to higher oxygen coverage of the catalytic surface, inhibiting the competitive adsorption of CH4 , as mentioned above (reactions (7) and (8)). At higher temperatures (T > 440 ◦ C), where the Rh–O bond strength weakens and  O decreases, the CO2 formation rate is higher than that obtained under reducing conditions. This agrees with the positive order dependence of the deep oxidation rate on PO2 for substoichiometric gas mixtures, i.e. PO2 /PCH4 ≤ 2 [8–10]. In a narrow temperature window between 480 and 520 ◦ C, oscillations of the catalytic rate under both open-circuit and polarization conditions as well as of the open-circuit potential were observed. As reported

1

10

I= -1mA

o.c.

0.8

8

6

4

o.c.

0

Λ = 40 ρ = ∼0

-200

0.6

-400 -600

0.4

2

0.2

0

0

o

T=420 C PCH4 = 1.7 kPa PO2 = 3.4 kPa

UWR (mV)

ˇ=

rCO2 / 10-6 mol O s-1

the metal catalyst electrode, i.e. O2 from the gas phase or O2− from the electrolyte support. To quantify the magnitude of the “poisoning permanent-EPOC” effect, a rate poisoning index, ˇ, has been defined (similar to parameter , defined in P-EPOC [29]):

CH4 conversion / %

Fig. 5. Steady-state effect of temperature on the open-circuit Rh–Au potential difference, CO2 and CO formation catalytic rates and currents under both open-circuit and polarization conditions. PCH4 /PO2 = 1.7/3.4.

in previous EPOC studies [3,37], these oscillations are due to a periodic formation-dissociation of the surface oxide species. Positive potential application causes an increase of the CO2 formation rate, which diminishes at high temperatures (T > 460 ◦ C), while a decrease of the rate is observed under negative polarization, similar to the reducing conditions case (Fig. 2). The transient response of the CO2 formation catalytic rate and the Rh–Au potential difference upon a constant positive current imposition (700 ␮A) is presented in Fig. 6, under stoichiometric conditions at 420 ◦ C, where no measurable quantities of CO were detected. As shown, initially the CH4 conversion is ∼2% while upon positive current application it increases to ∼6%, where the rate enhancement ratio, , is 3 and the apparent Faradaic efficiency ∼100. After current interruption, the catalytic rate reversibly returns to its initial value, similar to Fig. 3. The transient effect of a constant negative applied current (−1000 ␮A) on the catalytic rate and Rh–Au potential different at 420 ◦ C is shown in Fig. 7. Negative current application results in a significant decrease of the CO2 formation rate, which totally vanishes after 60 min of polarization ( = 0), while the apparent Faradaic efficiency, , is 40. This behavior is similar to that observed under reducing conditions. However, in this case the effect of nega-

-800 -1000

0

20

40

60

Time / min

80

100

-1200 120

Fig. 7. Transient effect of a constant applied cathodic current (−800 ␮A) on the CO2 formation catalytic rate and the Rh-Au potential difference. PCH4 /PO2 = 1.7/3.4, T = 420 ◦ C.

A. Nakos et al. / Applied Catalysis B: Environmental 101 (2010) 31–37

2

40 20

1

0

0

-20

-1

rCO / 10-7 mol O s-1

rCO2 / 10-6 mol O s-1

-40

4

I / mA

o

U WR / mV

36

-2

= o.c. = -2 V = +2 V

3 2 1

PCH4 = 1.4 kPa PO2 = 6 kPa

0 2 1 0 380

420

460

500

540

Temperature / oC Fig. 8. Steady-state effect of temperature on the open-circuit Rh–Au potential difference, CO2 and CO formation catalytic rates and currents under both open-circuit and polarization conditions. PCH4 /PO2 = 1.4/6.

tive polarization is higher, probably due to higher PO2 which favors the formation of surface rhodium oxide species. After negative current interruption, the catalytic rate increases slowly and stabilizes at a value lower than the initial. This is similar to the result presented in Fig. 4 and attributed to the formation of a catalyst surface oxide by the gaseous oxygen upon negative potential application. In this case, where PO2 is higher, the rate poisoning index is also higher (ˇ = 0.5, i.e. 50% lower catalytic activity). This significant increase of ˇ by the PO2 increase indicates an important effect of PO2 in the poising mechanism, which supports the surface oxide formation mechanism by the gaseous oxygen upon negative polarization. 3.2.3. Oxidizing conditions The steady-state effect of temperature on the open-circuit Rh–Au potential difference as well as on the CO2 and CO catalytic rates and the current under both open-circuit and polarization conditions is shown in Fig. 8, using an oxidizing CH4 /O2 (1.4/6) gas mixture. The CO2 and CO catalytic rates increase with temperature, under both open-circuit and polarization conditions, while at T = 445 ◦ C a sudden increase of the catalytic rates and of the potential is observed. This sudden increase is probably due to thermal decomposition of rhodium oxide species to form a metallic Rh surface, which leads to higher CO2 formation rates. However, the CO formation rate decreases gradually with temperature at T > 445 ◦ C, reaching 1 × 10−7 molO s−1 , which is very close to the plateau observed for stoichiometric conditions and high temperatures in Fig. 5. This may be due to thermodynamic equilibrium limitations of the ATR process (reactions (6a) and (6b)), described in Section 3.2.1. Potential application, both positive and negative, has no effect on the catalytic rate, due to the stable surface rhodium oxide layer formed at low temperatures and to the significant O2 evolution at high temperatures. Although previous studies with Rh catalysts [34–36] reported the decomposition of surface rhodium oxide upon anodic polarization at lower temperatures, which is denoted by a sudden rate increase accompanied by a sudden increase of the current, in this study this behavior was not observed. Similar sudden

rate change was observed in the case of C2 H4 oxidation over Rh/YSZ in [38] and [39], by change of the C2 H4 /O2 ratio or the applied potential value at steady-state conditions.

4. Conclusions The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) was studied for the methane oxidation reaction on Rh catalytic films interfaced with YSZ, an oxygen ion conductor, at temperatures from 350 to 550 ◦ C, under reducing, stoichiometric and oxidizing conditions. CO2 was the main product; however, CO was produced in small amounts at high temperatures. The effect of electrochemical promotion on the reaction rate was found to decrease by increasing PO2 and temperature. Under reducing conditions at 430 ◦ C, positive current application caused a 3-fold increase of the catalytic rate, while the apparent Faradaic efficiency was 170. After positive current interruption the catalytic rate reversibly returned to the initial open-circuit state value. Negative current application led to a 57% decrease of the rate with apparent Faradaic efficiency up to 40. However, after negative current interruption the catalytic rate slowly increased but remained in a lower than the initial value. This permanent poisoning effect was interpreted by formation of a surface oxide layer by the strongly adsorbed oxygen from the gas phase upon negative polarization. Reversibility to the initial steady-state value occurred only by a similar positive potential application. This mechanism is certainly worth further investigation by electrochemical and surface spectroscopic techniques.

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