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Internal carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in electrochemical cell
Applied Catalysis A: General 224 (2002) 111–120
Internal carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in electrochemical cell Taeyoon Kim, Sangjin Moon, Suk-In Hong∗ Department of Chemical Engineering, Korea University, 5-ka Anam-dong, Sungbuk-gu, Seoul 136-701, South Korea Received 11 May 2001; received in revised form 5 July 2001; accepted 16 July 2001
Abstract The carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in an electrochemical cell (CO2 , CH4 , Ni-YSZ-CeO2 | YSZ | La0.79 Sr0.16 Mn0.8 Co0.2 O3 , air) under open- and short-circuit conditions was studied at 800 ◦ C and atmospheric pressure. The microstructure of the catalyst electrode was characterized by SEM. The current–voltage test showed that the electric power generation performance of the electrochemical cell for methane and carbon dioxide was close to that for hydrogen. Under open-circuit condition, the catalyst electrode was deactivated by coke deposition. On the other hand, the catalyst electrode was stable and the selectivity of carbon monoxide was increased under short-circuit condition. The electrochemically pumped oxygen ion was reacted with the surface carbon which formed in the dry reforming on the three boundary phase (Ni/YSZ/gas) to yield carbon monoxide. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Reforming; Methane; Carbon dioxide; Nickel electrode; Electrochemical cell
1. Introduction In recent years, global warming has become a serious world-wide environmental problem. Since CO2 is a greenhouse effect gas and contributes much to global warming, the elimination of CO2 has been attracting interest from an environmental perspective. The CO2 reforming by CH4 (so-called dry reforming) [1–11] is one of the CO2 elimination methods. 0 CH4 + CO2 → 2H2 + 2CO, H298 = 247 kJ/mol
(1) This reaction has an advantage of the production of synthesis gas as well as the elimination of greenhouse gas. And the CO2 reforming by CH4 is of ∗ Corresponding author. Tel.: +82-2-3290-3294; fax: +82-2-926-6102. E-mail address: [email protected] (S.-I. Hong).
special interest from an industrial perspective since it produces synthesis gas with low hydrogen to carbon monoxide ratio, which can be preferentially used for Fischer–Tropsch synthesis. Furthermore, both CH4 and CO2 are the cheapest reactants and most abundant carbon-containing materials. Therefore, this reaction is an important area of current catalytic research. But the CO2 reforming by CH4 has two serious problems. The dry reforming is an intensively en0 dothermic reaction (H298 = 247 kJ/mol), which consumes much energy. And the catalysts used in the dry reforming are inclined to deactivate due to coke deposition on the catalysts surface. So, these problems must be overcome to apply the method for commercial processes. During the past decades, much effort has been focused on the development of catalysts which show high activity and have resistance against coking for long-term operation. Numerous metal catalysts were
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studied for this reaction. Among them, nickel-based catalysts [12–18] and noble metal catalysts (Rh, Ru, Ir, Pd, and Pt) [19–22] have been showing the promising catalytic performance in terms of activity and selectivity to synthesis gas. Noble metal catalysts have been found to have resistance against coking. But it does not seem to be practical because of the high cost of the noble metals. In this work, electrocatalytic dry reforming over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell using solid electrolyte (CO2 , CH4 , Ni-YSZ-CeO2 | YSZ | La0.79 Sr0.16 Mn0.8 Co0.2 O3 , air) was studied in order to overcome the two main problems in the catalytic dry reforming. Electrocatalytic dry reforming in electrochemical cell has some advantages over the catalytic dry reforming. The electro-
chemical cell used in this study is similar to the solid oxide fuel cell that produces electric energy. And catalyst deactivation by coking is suppressed by oxygen ions which are being supplied electrochemically to catalyst electrode through solid electrolyte. This paper presents the effects of electrochemically pumped oxygen ion on the CO2 , CH4 reaction rates and the stability of the catalyst electrode against coking. Also, the power generation performance of the electrochemical cell is discussed.
2. Experimental The structure of electrochemical cell and reactor are shown in Fig. 1. A disk of 8 mol% Y2 O3 doped
Fig. 1. Schematic diagram of the electrochemical cell.
T. Kim et al. / Applied Catalysis A: General 224 (2002) 111–120
ZrO2 (8-YSZ, TOSOH Ceramic Division) was used as the electrolyte. The thickness and diameter of the YSZ disk was 1.2 and 24 mm, respectively. The catalyst electrode and counter electrode were prepared by coating onto each side of the disk. The catalyst electrode material was a mixture of NiO and YSZ (NiO:YSZ = 45:55 v/v). And CeO2 powder (4 wt.%) was added to this mixture. The binder solution for slurry was prepared by adding 0.87 g of methyl cellulose (MC), 1.14 g of carbonyl methyl cellulose (CMC) and 1.74 g of polyethylene oxide (Polyox) as binder and isopropyl alcohol (IPA) 3 ml as dispersing agent in 100 ml deionized water. Mixed powder (100 g) was added slowly to the binder solution with stirring. After all the powder was added, the slurry was milled by zirconia balls for 24 h. Then, the slurry was coated by using a blade on one side of the YSZ disk. The coated disk was dried at 50 ◦ C for 24 h and sintered at 1350 ◦ C for 2 h under air. The thickness and area of the catalyst electrode layer was ca. 20 m and 1.0 cm2 (1.0 cm × 1.0 cm), respectively. La0.79 Sr0.16 Mn0.8 Co0.2 O3 (LSMC) was used as a counter electrode material. The counter electrode was prepared by painting slurry onto the opposite side of the YSZ disk. The coated disk was dried at 50 ◦ C for 24 h and sintered at 1200 ◦ C for 6 h under air. The thickness and surface area of the counter electrode layers were ca. 20 m and 2.25 cm2 (1.5 cm×1.5 cm), respectively. Pt wire (0.5 mm diameter) was used to connect the both electrodes to an electrical circuit for controlling the oxygen flux across the YSZ disk. The electrochemical cell was sealed onto the alumina tube using Pyrex glass powder. A stainless steel tube as the gas inlet was placed inside the alumina tube, where the end of the stainless steel tube was close to the electrochemical cell for good contact between the reactants and the catalyst electrode. The reactor was placed in an electric furnace equipped with a PID controller. And the temperature of the reactor was measured by a thermocouple positioned near the electrochemical cell. The NiO-YSZ-CeO2 electrode was reduced to Ni-YSZ-CeO2 with 40 vol.% H2 in N2 as diluent at 800 ◦ C for 24 h. The open-circuit potential at 800 ◦ C in 40 vol.% H2 was 1.05 V for all experiments. The partial pressure of oxygen inside the reactor can be calculated from the open-circuit potential. This value of open-circuit potential indicates that the oxygen
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partial pressure inside the reactor is below 10−17 atm. Therefore, any leakage of the reactor can be neglected. The currents and voltages of electrochemical cell were measured for the power generation performance of the electrochemical cell. The counter electrode side was left open to air at 1 atm. The operating temperature and pressure were 800 ◦ C and 1 atm. The current and voltage were controlled by changeable resistance. Current was measured by a multimeter (Keithley 169) and voltage was measured by a voltammeter (Keithley 614). The open-circuit potential was increased within a few minutes to a steady value after we first introduced the reactants to the catalyst electrode side of the electrochemical cell. All currents and voltages were measured at the steady state. Oxygen in air was pumped to the catalyst electrode through the YSZ solid-electrolyte disk by applying the electric current. The electric currents were controlled by a galvano-potentiostat (Jisang Electric Co.). The flux of atomic oxygen through the YSZ disk, VO , was calculated by the following equation: VO =
I 2F
(2)
where I is the electric current passing through the electrochemical cell, and F is the Faraday constant. The reactant and product gases were analyzed by two different gas chromatographs which were equipped with a thermal conductivity detector. The first one (Shimadzu 14A), equipped with Porapak Q column and using He as a carrier gas, was used to analyze CH4 , CO2 , and CO. The second one (Donam Co.), equipped with the same column and using N2 as a carrier gas, was used to analyze H2 . The reaction rates of reactants and production rates were calculated by the total flow rates of inlet and exit gas and the partial pressure of each gas measured by gas chromatographs. The reaction rates (ri ) of CH4 and CO2 and formation rate of CO were determined as rCH4 = Fin [CH4 ]in − Fout [CH4 ]out , rCO2 = Fin [CO2 ]in − Fout [CO2 ]out , rCO = Fout [CO]out where the reaction rate per apparent surface area of electrode expressed in mol/cm2 s, [CH4 ], [CO2 ], and [CO] are CH4 , CO2 , and CO concentration, respectively, and F is flow rate of gas mixture (mol/s).
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The production rate of H2 O and the formation rate of deposited carbon were calculated from a mass balance of the hydrogen and the carbon. After each experiment, the deposited carbon was removed by oxidation with air supplied to the catalyst electrode. To evaluate the degradation of the catalyst electrode, a current–voltage measurement using H2 was performed before and after each experiment.
3. Results and discussion 3.1. Characterization of electrode microstructure The microstructure of Ni-YSZ-CeO2 catalyst electrode was characterized by SEM, and it was highly porous, as shown in Fig. 2. The porosity of the electrode is very important, because it is related to the electrochemical charge transfer reaction sites (three-phase site, interface among Ni, YSZ and gases). The sizes of Ni particles and YSZ particles of catalyst electrode were ca. 2–5 and 0.3 m, respectively. The Ni particle was surrounded by YSZ particles which prevent Ni particles from agglomeration and fix them. Ni and YSZ particles were mixed well and formed continuous paths with each other. To maintain the polarization losses at acceptable levels, it is imperative that the Ni and YSZ particles form continuous paths that allow ionic and electronic migration from the electrolyte/catalyst electrode interface through the entire electrode. LSMC used as a counter electrode was prepared by malic acid methods [23]. The sub-stoichiometry of LSMC on the A-site has been found to suppress the formation of insulating SrZrO3 or La2 Zr2 O7 layers between YSZ and the perovskite [24]. The counter electrode was highly porous, and LSMC particles (ca. 1–2 m in size) formed a continuous. The thickness of the catalyst electrode and the counter electrode were both ca. 20 m. 3.2. The power generation performance of the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) Fig. 3 shows the results of the current–voltage and the current–power characteristics for H2 –H2 O and CH4 –CO2 as the reactants at 800 ◦ C. The perfor-
Fig. 2. SEM micrographs of Ni-YSZ-CeO2 catalyst electrode sintered at 1350 ◦ C for 2 h: (a) the surface of the catalyst electrode (2000×); (b) zoomed-up (15,000×).
mance of the electrochemical cell under CH4 –CO2 reactants can be explained with reference to that under H2 reactant. The power outputs for H2 were obtained under excess H2 condition. The maximum power and overall cell resistance were 10.4 mW/cm2 and 27.7 cm2 , respectively, for H2 as a reactant. And those values were 8.6 mW/cm2 and 28.7 cm2 , respectively, in case CH4 –CO2 were supplied to the electrochemical cell as reactants. These results in CH4 –CO2 reactants were obtained within a few minutes after we first introduced the reactants to the catalyst electrode of the electrochemical cell. For H2
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Fig. 3. Current–voltage and current–power performance of the electrochemical cell (Ni-YSZ-CeO2 (4 wt.%)/YSZ/LSMC) when H2 (40%), H2 O (3%), CH4 (5%) and CO2 (5%) was used as the reactant at 800 ◦ C (open symbols: H2 ; closed symbols: CH4 , CO2 ).
as a reactant, electrochemical reaction occurs by the following reaction: H2 + O2− → H2 O + 2e−
(3)
For CH4 –CO2 as reactants, the possible electrochemical reactions occur by the following reactions: CH4 + O2− → CO + 2H2 + 2e−
(4)
CH4 + 2O2− → CO2 + 2H2 O + 4e−
(5)
CO + O2− → CO2 + 2e−
(6)
−
(7)
H2 + O
2−
→ H2 O + 2e
For CH4 –CO2 reactants, the Ni-YSZ catalyst electrode was deactivated by coking within several hours. Therefore, the performance of the electrochemical cell deteriorated rapidly; in the electrochemical cell with Ni-YSZ catalyst electrode, the performance decreased about 80% of the initial value within only 5 h. To enhance the performance of the electrochemical cell, CeO2 was added to the Ni-YSZ catalyst electrode. Adding 2 and 4 wt.% CeO2 , the maximum power was increased from 6.4 to 8.1 and 8.6 mW/cm2 ,
respectively. And the overall cell resistance was decreased from 29.3 to 31.3 and 28.7 cm2 , respectively. From these results, it was concluded that CeO2 significantly improved the electrochemical activity of the catalyst electrode for CH4 –CO2 reactants. For the Ni-YSZ-CeO2 , the current density was decreased slightly during the initial 10–15 h, and then it was stable for 48 h. The CeO2 also significantly enhanced the electrochemical activity of the anode and the stability against coking. 3.3. The dry reforming under open-circuit conditions To avoid significant carbon deposition on the catalyst electrode under open-circuit condition and to make clear the electrochemical reaction at catalyst electrode, CH4 and CO2 diluted at 5 vol.% were supplied to the catalyst electrode. Fig. 4 shows the reaction rates of CH4 and CO2 , and the formation rates of CO and coke at 700–800 ◦ C in an electrochemical cell with a Ni-YSZ-CeO2 catalyst electrode. The reaction rates of CH4 and CO2 and the formation rate of
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formation under conditions where deposition is thermodynamically favorable. 3.4. The dry reforming under short-circuit conditions To investigate the effects of anodic current on dry reforming, CO2 dry reforming by CH4 was performed in an electrochemical cell under short-circuit conditions. The oxygen ions are pumped to the catalyst electrode by passing an anodic current through the electrochemical cell. In the electrochemical cell, oxygen ions (O2− ) are formed on the counter electrode according to the following reaction: 0.5O2 + 2e− → O2−
Fig. 4. The reaction rates of CH4 and CO2 , and the formation rates of CO and coke at various temperatures in the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) under open-circuit condition. CH4 (5 vol.%); CO2 (5 vol.%); total flow rate = 100 ml STP/min.
CO were increased linearly with increasing temperatures. On the other hand, the formation rate of coke was decreased slightly with increasing temperature. The amount of coke was calculated from the carbon balance. The formation of coke during CO2 reforming by CH4 may occur via CH4 decomposition, CH4 ↔ 2H2 + Cs ,
H 0 = 17.9 kcal/mol Cs
(8)
and/or CO disproportion (i.e. the Boudouard reaction), 2CO ↔ CO2 + Cs ,
H 0 = −41.2 kcal/mol Cs (9)
The equilibrium constant of CO disproportion is decreased with increasing temperature and the equilibrium constant of CH4 decomposition is increased with increasing temperature. This would suggest that the main reason for carbon deposition could be CH4 decomposition in the operation of higher temperature in our experiments. Carbon deposition can be avoided by operation at high temperature and with CO2 /CH4 reactant ratios far above unity. However, it may be desirable to operate at lower temperature with CO2 /CH4 ratios near unity from an industrial standpoint. So it is desirable to use reforming catalysts or reaction systems which incorporate a kinetic inhibition of carbon
(10)
These oxygen ions are transferred to the catalyst electrode through the YSZ electrolyte. The possible electrocatalytic reactions with the oxygen ions transferred to the catalyst electrode occur by reactions (6) and (7), and the following reactions: Cs + O2− → CO + 2e−
(11)
Cs + 2O2− → CO2 + 4e−
(12)
Fig. 5 shows the effects of the anodic current (electrochemical pumping of oxygen ions to the catalyst electrode) on the reaction rates of CH4 and CO2 , and the formation rates of CO and coke. The reaction rates of CH4 and CO2 were decreased with time at lower anodic current because coke deposition could not be avoided. To avoid significant coke deposition and to get a steady-state data, the experiment was performed at 20–23 mA. The reaction rates of CH4 and CO2 were constant with anodic currents. However, the formation rate of CO was increased linearly with increasing anodic currents. And the formation rate of coke was decreased linearly with increasing anodic currents. In this range of anodic current, the pumped oxygen ions were reacted with the carbon which is formed in the dry reforming. Therefore, the reaction rates of CH4 and CO2 were not influenced with the applying anodic current. As the applied anodic current was increased from 20.1 to 23 mA (the increase of the electrochemically pumped oxygen ions is 1.5 × 10−2 mol/cm2 s), the increase of the formation rate of CO was ca. 1.6 × 10−2 mol/cm2 s and the decrease of the formation rate of coke was ca. 1.6 × 10−2 mol/cm2 s.
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Fig. 5. The effects of applied current on the reaction rates of CH4 and CO2 (a), and the formation rates of the CO and coke (b) over Ni-YSZ-CeO2 catalyst electrode in electrochemical cell at 800 ◦ C. CH4 (5 vol.%); CO2 (5 vol.%); total flow rate = 100 ml STP/min.
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These results show that all oxygen ions electrochemically pumped through the electrolyte reacted with the atomic carbons formed in dry reforming to CO. Therefore, coke deposition on the electrode surface can be suppressed efficiently by the oxygen ions pumped electrochemically. Galvita et al. [25] and Ishihara et al. [26] suggested that electrocatalytic oxidation of CH4 to synthesis gas over the Ni electrode goes by a direct partial oxidation mechanism. The difference between the complete and partial oxidation mechanism results from different Ni states through the following reactions. Ni catalyst is oxidized partially into nickel oxide if the oxygen is supplied non-uniformly. Nickel oxide is the active catalyst for complete oxidation. In this work, the oxygen ion transferred reacted with a surface carbon through partial oxidation mechanism. The electrochemically pumped oxygen ions are supplied uniformly to the catalyst electrode, which seems to be responsible for the partial oxidation of surface carbon. Ni · · · C + Niδ+ · · · Oδ− → 2Ni0 + CO
(13)
Ni · · · C + 2(Ni2+ –O2− ) → 3Ni0 + CO2
(14)
where Ni · · · C represents carbon on the catalyst surface, Niδ+ · · · Oδ− represents mobile oxygen on the catalyst surface and Ni2+ –O2− represents NiO. Fig. 6 shows the reaction rates of CH4 and CO2 over Ni-YSZ-CeO2 catalyst electrode with time-on-stream under open- and short-circuit conditions. Under open-circuit condition, the reaction rates of CH4 and CO2 were decreased from 3.79 × 10−1 to 1.40 mol/cm2 s and from ca. 4.37 to 1.82 mol/cm2 s, respectively, for 30 h. These decreases in catalytic activity were caused by carbon deposition on catalyst electrode, as seen in Fig. 4. On the other hand, the reaction rates of CH4 and CO2 were stable for 48 h under short-circuit condition. And the maximum current density was decreased slightly with time-on-stream during the initial 10–15 h, and then it became stable with time-on-stream during 48 h. The amount of coke on the catalyst electrode during the dry reforming was measured by TGA. Thermogravimetric oxidation of the used catalyst electrode was performed at 5 ◦ C/min in air. Fig. 7 shows the TGA results of the catalyst electrode used in the dry reforming at 800 ◦ C for 50 h under open- and
Fig. 6. The reaction rates of CH4 and CO2 , and current density in the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) under open- and short-circuit condition at 800 ◦ C with time. CH4 (5 vol.%); CO2 (5 vol.%); total flow rate = 100 ml STP/min.
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Fig. 8. The proposed mechanism of the CO2 reforming by CH4 in the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) under short-circuit condition.
4. Conclusions Fig. 7. TG results of the Ni-YSZ-CeO2 catalyst electrode after reaction in the electrochemical cell at 800 ◦ C during 50 h under open-circuit (a) and short-circuit (b) conditions.
short-circuit conditions. The coke deposition occurred on the catalyst electrode under open-circuit condition, and the amount of coke was ca. 30 mg C/gcat . In contrast to this, no coke deposition occurred under short-circuit condition. The weight of the catalyst electrode was increased in accordance with the amount of Ni was oxidized. From these results, the Ni-YSZ-CeO2 catalyst electrode of the electrochemical cell was stable during the dry reforming under short-circuit condition. In summary, the CO2 reforming by CH4 over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell at 800 ◦ C is schematically shown in Fig. 8. CH4 and CO2 decompose on Ni particles into CHx (0 ≤ x ≤ 3) species and CO + O, respectively. Two reaction pathways exist in which surface carbon is oxidized on the Ni particle surface by atomic oxygen formed from CO2 disproportion and on the three-phase boundary (Ni/YSZ/gas) by oxygen ions pumped electrochemically. In the case that oxygen ion flux is fast enough to remove coke deposition, the catalyst electrode is stable during the reaction. The electric power is generated by electrons released in the oxidation of surface carbon with the oxygen ions.
The CO2 reforming by CH4 was investigated over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell. The electric power generation performance of the electrochemical cell for CO2 and CH4 was close to that of H2 from current–voltage characteristics. Under open-circuit condition, catalyst electrode was deactivated by coke deposition. On the other hand, catalyst electrode was stable and synthesis gas was produced with high yield, in addition to the generation of electric power under short-circuit condition (electrochemical pumping of oxygen ion to the catalyst electrode). The coke formed in dry reforming was oxidized by the oxygen ions pumped electrochemically into carbon monoxide. As such, it is proposed that electrochemical cell is efficient for CO2 reforming by CH4 as regards energy-saving and catalyst stability against coking. Acknowledgements We acknowledge financial support from the R&D Management Center for Energy and Resources, The Korea Energy Management Corporation. References [1] A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, P.D.F. Vernon, Nature 452 (1991) 225.
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[15] Z.I. Zhang, X.E. Verykios, S.M. MacDonald, S.J. Affrossman, Phys. Chem. 100 (1996) 744. [16] Z.I. Zhang, X.E. Verykios, Appl. Catal. A: Gen. 138 (1996) 109. [17] Z. Cheng, Q. Wu, J. Li, Q. Zhu, Catal. Today 30 (1996) 147. [18] V.R. Choudhary, A.M. Rajput, Ind. Eng. Chem. Res. 35 (1996) 3934. [19] F. Solymosi, G. Kutsan, A. Erdohelyi, Catal. Lett. 11 (1991) 149. [20] A. Erdohelyi, J. Cserenyi, F. Solymosi, J. Catal. 141 (1993) 287. [21] D. Qin, J. Lapszewicz, Catal. Today 21 (1994) 551. [22] M.F. Mark, M.F. Maier, J. Catal. 164 (1996) 122. [23] Y. Shimizu, H. Matsuda, N. Miura, N. Yamazoe, Chem. Lett. (1992) 1033. [24] J.W. Erning, T. Hauber, U. Stimming, K. Wippermann, J. Power Sources 61 (1996) 205. [25] V.V. Galvita, V.D. Belyaev, A.K. Demin, V.A. Sobyanin, Appl. Catal. A: Gen. 165 (1997) 301. [26] T. Ishihara, T. Yamada, T. Akbay, Y. Takita, Chem. Eng. Sci. 54 (1999) 1535.
Internal carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in electrochemical cell Taeyoon Kim, Sangjin Moon, Suk-In Hong∗ Department of Chemical Engineering, Korea University, 5-ka Anam-dong, Sungbuk-gu, Seoul 136-701, South Korea Received 11 May 2001; received in revised form 5 July 2001; accepted 16 July 2001
Abstract The carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in an electrochemical cell (CO2 , CH4 , Ni-YSZ-CeO2 | YSZ | La0.79 Sr0.16 Mn0.8 Co0.2 O3 , air) under open- and short-circuit conditions was studied at 800 ◦ C and atmospheric pressure. The microstructure of the catalyst electrode was characterized by SEM. The current–voltage test showed that the electric power generation performance of the electrochemical cell for methane and carbon dioxide was close to that for hydrogen. Under open-circuit condition, the catalyst electrode was deactivated by coke deposition. On the other hand, the catalyst electrode was stable and the selectivity of carbon monoxide was increased under short-circuit condition. The electrochemically pumped oxygen ion was reacted with the surface carbon which formed in the dry reforming on the three boundary phase (Ni/YSZ/gas) to yield carbon monoxide. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Reforming; Methane; Carbon dioxide; Nickel electrode; Electrochemical cell
1. Introduction In recent years, global warming has become a serious world-wide environmental problem. Since CO2 is a greenhouse effect gas and contributes much to global warming, the elimination of CO2 has been attracting interest from an environmental perspective. The CO2 reforming by CH4 (so-called dry reforming) [1–11] is one of the CO2 elimination methods. 0 CH4 + CO2 → 2H2 + 2CO, H298 = 247 kJ/mol
(1) This reaction has an advantage of the production of synthesis gas as well as the elimination of greenhouse gas. And the CO2 reforming by CH4 is of ∗ Corresponding author. Tel.: +82-2-3290-3294; fax: +82-2-926-6102. E-mail address: [email protected] (S.-I. Hong).
special interest from an industrial perspective since it produces synthesis gas with low hydrogen to carbon monoxide ratio, which can be preferentially used for Fischer–Tropsch synthesis. Furthermore, both CH4 and CO2 are the cheapest reactants and most abundant carbon-containing materials. Therefore, this reaction is an important area of current catalytic research. But the CO2 reforming by CH4 has two serious problems. The dry reforming is an intensively en0 dothermic reaction (H298 = 247 kJ/mol), which consumes much energy. And the catalysts used in the dry reforming are inclined to deactivate due to coke deposition on the catalysts surface. So, these problems must be overcome to apply the method for commercial processes. During the past decades, much effort has been focused on the development of catalysts which show high activity and have resistance against coking for long-term operation. Numerous metal catalysts were
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studied for this reaction. Among them, nickel-based catalysts [12–18] and noble metal catalysts (Rh, Ru, Ir, Pd, and Pt) [19–22] have been showing the promising catalytic performance in terms of activity and selectivity to synthesis gas. Noble metal catalysts have been found to have resistance against coking. But it does not seem to be practical because of the high cost of the noble metals. In this work, electrocatalytic dry reforming over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell using solid electrolyte (CO2 , CH4 , Ni-YSZ-CeO2 | YSZ | La0.79 Sr0.16 Mn0.8 Co0.2 O3 , air) was studied in order to overcome the two main problems in the catalytic dry reforming. Electrocatalytic dry reforming in electrochemical cell has some advantages over the catalytic dry reforming. The electro-
chemical cell used in this study is similar to the solid oxide fuel cell that produces electric energy. And catalyst deactivation by coking is suppressed by oxygen ions which are being supplied electrochemically to catalyst electrode through solid electrolyte. This paper presents the effects of electrochemically pumped oxygen ion on the CO2 , CH4 reaction rates and the stability of the catalyst electrode against coking. Also, the power generation performance of the electrochemical cell is discussed.
2. Experimental The structure of electrochemical cell and reactor are shown in Fig. 1. A disk of 8 mol% Y2 O3 doped
Fig. 1. Schematic diagram of the electrochemical cell.
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ZrO2 (8-YSZ, TOSOH Ceramic Division) was used as the electrolyte. The thickness and diameter of the YSZ disk was 1.2 and 24 mm, respectively. The catalyst electrode and counter electrode were prepared by coating onto each side of the disk. The catalyst electrode material was a mixture of NiO and YSZ (NiO:YSZ = 45:55 v/v). And CeO2 powder (4 wt.%) was added to this mixture. The binder solution for slurry was prepared by adding 0.87 g of methyl cellulose (MC), 1.14 g of carbonyl methyl cellulose (CMC) and 1.74 g of polyethylene oxide (Polyox) as binder and isopropyl alcohol (IPA) 3 ml as dispersing agent in 100 ml deionized water. Mixed powder (100 g) was added slowly to the binder solution with stirring. After all the powder was added, the slurry was milled by zirconia balls for 24 h. Then, the slurry was coated by using a blade on one side of the YSZ disk. The coated disk was dried at 50 ◦ C for 24 h and sintered at 1350 ◦ C for 2 h under air. The thickness and area of the catalyst electrode layer was ca. 20 m and 1.0 cm2 (1.0 cm × 1.0 cm), respectively. La0.79 Sr0.16 Mn0.8 Co0.2 O3 (LSMC) was used as a counter electrode material. The counter electrode was prepared by painting slurry onto the opposite side of the YSZ disk. The coated disk was dried at 50 ◦ C for 24 h and sintered at 1200 ◦ C for 6 h under air. The thickness and surface area of the counter electrode layers were ca. 20 m and 2.25 cm2 (1.5 cm×1.5 cm), respectively. Pt wire (0.5 mm diameter) was used to connect the both electrodes to an electrical circuit for controlling the oxygen flux across the YSZ disk. The electrochemical cell was sealed onto the alumina tube using Pyrex glass powder. A stainless steel tube as the gas inlet was placed inside the alumina tube, where the end of the stainless steel tube was close to the electrochemical cell for good contact between the reactants and the catalyst electrode. The reactor was placed in an electric furnace equipped with a PID controller. And the temperature of the reactor was measured by a thermocouple positioned near the electrochemical cell. The NiO-YSZ-CeO2 electrode was reduced to Ni-YSZ-CeO2 with 40 vol.% H2 in N2 as diluent at 800 ◦ C for 24 h. The open-circuit potential at 800 ◦ C in 40 vol.% H2 was 1.05 V for all experiments. The partial pressure of oxygen inside the reactor can be calculated from the open-circuit potential. This value of open-circuit potential indicates that the oxygen
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partial pressure inside the reactor is below 10−17 atm. Therefore, any leakage of the reactor can be neglected. The currents and voltages of electrochemical cell were measured for the power generation performance of the electrochemical cell. The counter electrode side was left open to air at 1 atm. The operating temperature and pressure were 800 ◦ C and 1 atm. The current and voltage were controlled by changeable resistance. Current was measured by a multimeter (Keithley 169) and voltage was measured by a voltammeter (Keithley 614). The open-circuit potential was increased within a few minutes to a steady value after we first introduced the reactants to the catalyst electrode side of the electrochemical cell. All currents and voltages were measured at the steady state. Oxygen in air was pumped to the catalyst electrode through the YSZ solid-electrolyte disk by applying the electric current. The electric currents were controlled by a galvano-potentiostat (Jisang Electric Co.). The flux of atomic oxygen through the YSZ disk, VO , was calculated by the following equation: VO =
I 2F
(2)
where I is the electric current passing through the electrochemical cell, and F is the Faraday constant. The reactant and product gases were analyzed by two different gas chromatographs which were equipped with a thermal conductivity detector. The first one (Shimadzu 14A), equipped with Porapak Q column and using He as a carrier gas, was used to analyze CH4 , CO2 , and CO. The second one (Donam Co.), equipped with the same column and using N2 as a carrier gas, was used to analyze H2 . The reaction rates of reactants and production rates were calculated by the total flow rates of inlet and exit gas and the partial pressure of each gas measured by gas chromatographs. The reaction rates (ri ) of CH4 and CO2 and formation rate of CO were determined as rCH4 = Fin [CH4 ]in − Fout [CH4 ]out , rCO2 = Fin [CO2 ]in − Fout [CO2 ]out , rCO = Fout [CO]out where the reaction rate per apparent surface area of electrode expressed in mol/cm2 s, [CH4 ], [CO2 ], and [CO] are CH4 , CO2 , and CO concentration, respectively, and F is flow rate of gas mixture (mol/s).
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The production rate of H2 O and the formation rate of deposited carbon were calculated from a mass balance of the hydrogen and the carbon. After each experiment, the deposited carbon was removed by oxidation with air supplied to the catalyst electrode. To evaluate the degradation of the catalyst electrode, a current–voltage measurement using H2 was performed before and after each experiment.
3. Results and discussion 3.1. Characterization of electrode microstructure The microstructure of Ni-YSZ-CeO2 catalyst electrode was characterized by SEM, and it was highly porous, as shown in Fig. 2. The porosity of the electrode is very important, because it is related to the electrochemical charge transfer reaction sites (three-phase site, interface among Ni, YSZ and gases). The sizes of Ni particles and YSZ particles of catalyst electrode were ca. 2–5 and 0.3 m, respectively. The Ni particle was surrounded by YSZ particles which prevent Ni particles from agglomeration and fix them. Ni and YSZ particles were mixed well and formed continuous paths with each other. To maintain the polarization losses at acceptable levels, it is imperative that the Ni and YSZ particles form continuous paths that allow ionic and electronic migration from the electrolyte/catalyst electrode interface through the entire electrode. LSMC used as a counter electrode was prepared by malic acid methods [23]. The sub-stoichiometry of LSMC on the A-site has been found to suppress the formation of insulating SrZrO3 or La2 Zr2 O7 layers between YSZ and the perovskite [24]. The counter electrode was highly porous, and LSMC particles (ca. 1–2 m in size) formed a continuous. The thickness of the catalyst electrode and the counter electrode were both ca. 20 m. 3.2. The power generation performance of the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) Fig. 3 shows the results of the current–voltage and the current–power characteristics for H2 –H2 O and CH4 –CO2 as the reactants at 800 ◦ C. The perfor-
Fig. 2. SEM micrographs of Ni-YSZ-CeO2 catalyst electrode sintered at 1350 ◦ C for 2 h: (a) the surface of the catalyst electrode (2000×); (b) zoomed-up (15,000×).
mance of the electrochemical cell under CH4 –CO2 reactants can be explained with reference to that under H2 reactant. The power outputs for H2 were obtained under excess H2 condition. The maximum power and overall cell resistance were 10.4 mW/cm2 and 27.7 cm2 , respectively, for H2 as a reactant. And those values were 8.6 mW/cm2 and 28.7 cm2 , respectively, in case CH4 –CO2 were supplied to the electrochemical cell as reactants. These results in CH4 –CO2 reactants were obtained within a few minutes after we first introduced the reactants to the catalyst electrode of the electrochemical cell. For H2
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Fig. 3. Current–voltage and current–power performance of the electrochemical cell (Ni-YSZ-CeO2 (4 wt.%)/YSZ/LSMC) when H2 (40%), H2 O (3%), CH4 (5%) and CO2 (5%) was used as the reactant at 800 ◦ C (open symbols: H2 ; closed symbols: CH4 , CO2 ).
as a reactant, electrochemical reaction occurs by the following reaction: H2 + O2− → H2 O + 2e−
(3)
For CH4 –CO2 as reactants, the possible electrochemical reactions occur by the following reactions: CH4 + O2− → CO + 2H2 + 2e−
(4)
CH4 + 2O2− → CO2 + 2H2 O + 4e−
(5)
CO + O2− → CO2 + 2e−
(6)
−
(7)
H2 + O
2−
→ H2 O + 2e
For CH4 –CO2 reactants, the Ni-YSZ catalyst electrode was deactivated by coking within several hours. Therefore, the performance of the electrochemical cell deteriorated rapidly; in the electrochemical cell with Ni-YSZ catalyst electrode, the performance decreased about 80% of the initial value within only 5 h. To enhance the performance of the electrochemical cell, CeO2 was added to the Ni-YSZ catalyst electrode. Adding 2 and 4 wt.% CeO2 , the maximum power was increased from 6.4 to 8.1 and 8.6 mW/cm2 ,
respectively. And the overall cell resistance was decreased from 29.3 to 31.3 and 28.7 cm2 , respectively. From these results, it was concluded that CeO2 significantly improved the electrochemical activity of the catalyst electrode for CH4 –CO2 reactants. For the Ni-YSZ-CeO2 , the current density was decreased slightly during the initial 10–15 h, and then it was stable for 48 h. The CeO2 also significantly enhanced the electrochemical activity of the anode and the stability against coking. 3.3. The dry reforming under open-circuit conditions To avoid significant carbon deposition on the catalyst electrode under open-circuit condition and to make clear the electrochemical reaction at catalyst electrode, CH4 and CO2 diluted at 5 vol.% were supplied to the catalyst electrode. Fig. 4 shows the reaction rates of CH4 and CO2 , and the formation rates of CO and coke at 700–800 ◦ C in an electrochemical cell with a Ni-YSZ-CeO2 catalyst electrode. The reaction rates of CH4 and CO2 and the formation rate of
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formation under conditions where deposition is thermodynamically favorable. 3.4. The dry reforming under short-circuit conditions To investigate the effects of anodic current on dry reforming, CO2 dry reforming by CH4 was performed in an electrochemical cell under short-circuit conditions. The oxygen ions are pumped to the catalyst electrode by passing an anodic current through the electrochemical cell. In the electrochemical cell, oxygen ions (O2− ) are formed on the counter electrode according to the following reaction: 0.5O2 + 2e− → O2−
Fig. 4. The reaction rates of CH4 and CO2 , and the formation rates of CO and coke at various temperatures in the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) under open-circuit condition. CH4 (5 vol.%); CO2 (5 vol.%); total flow rate = 100 ml STP/min.
CO were increased linearly with increasing temperatures. On the other hand, the formation rate of coke was decreased slightly with increasing temperature. The amount of coke was calculated from the carbon balance. The formation of coke during CO2 reforming by CH4 may occur via CH4 decomposition, CH4 ↔ 2H2 + Cs ,
H 0 = 17.9 kcal/mol Cs
(8)
and/or CO disproportion (i.e. the Boudouard reaction), 2CO ↔ CO2 + Cs ,
H 0 = −41.2 kcal/mol Cs (9)
The equilibrium constant of CO disproportion is decreased with increasing temperature and the equilibrium constant of CH4 decomposition is increased with increasing temperature. This would suggest that the main reason for carbon deposition could be CH4 decomposition in the operation of higher temperature in our experiments. Carbon deposition can be avoided by operation at high temperature and with CO2 /CH4 reactant ratios far above unity. However, it may be desirable to operate at lower temperature with CO2 /CH4 ratios near unity from an industrial standpoint. So it is desirable to use reforming catalysts or reaction systems which incorporate a kinetic inhibition of carbon
(10)
These oxygen ions are transferred to the catalyst electrode through the YSZ electrolyte. The possible electrocatalytic reactions with the oxygen ions transferred to the catalyst electrode occur by reactions (6) and (7), and the following reactions: Cs + O2− → CO + 2e−
(11)
Cs + 2O2− → CO2 + 4e−
(12)
Fig. 5 shows the effects of the anodic current (electrochemical pumping of oxygen ions to the catalyst electrode) on the reaction rates of CH4 and CO2 , and the formation rates of CO and coke. The reaction rates of CH4 and CO2 were decreased with time at lower anodic current because coke deposition could not be avoided. To avoid significant coke deposition and to get a steady-state data, the experiment was performed at 20–23 mA. The reaction rates of CH4 and CO2 were constant with anodic currents. However, the formation rate of CO was increased linearly with increasing anodic currents. And the formation rate of coke was decreased linearly with increasing anodic currents. In this range of anodic current, the pumped oxygen ions were reacted with the carbon which is formed in the dry reforming. Therefore, the reaction rates of CH4 and CO2 were not influenced with the applying anodic current. As the applied anodic current was increased from 20.1 to 23 mA (the increase of the electrochemically pumped oxygen ions is 1.5 × 10−2 mol/cm2 s), the increase of the formation rate of CO was ca. 1.6 × 10−2 mol/cm2 s and the decrease of the formation rate of coke was ca. 1.6 × 10−2 mol/cm2 s.
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Fig. 5. The effects of applied current on the reaction rates of CH4 and CO2 (a), and the formation rates of the CO and coke (b) over Ni-YSZ-CeO2 catalyst electrode in electrochemical cell at 800 ◦ C. CH4 (5 vol.%); CO2 (5 vol.%); total flow rate = 100 ml STP/min.
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These results show that all oxygen ions electrochemically pumped through the electrolyte reacted with the atomic carbons formed in dry reforming to CO. Therefore, coke deposition on the electrode surface can be suppressed efficiently by the oxygen ions pumped electrochemically. Galvita et al. [25] and Ishihara et al. [26] suggested that electrocatalytic oxidation of CH4 to synthesis gas over the Ni electrode goes by a direct partial oxidation mechanism. The difference between the complete and partial oxidation mechanism results from different Ni states through the following reactions. Ni catalyst is oxidized partially into nickel oxide if the oxygen is supplied non-uniformly. Nickel oxide is the active catalyst for complete oxidation. In this work, the oxygen ion transferred reacted with a surface carbon through partial oxidation mechanism. The electrochemically pumped oxygen ions are supplied uniformly to the catalyst electrode, which seems to be responsible for the partial oxidation of surface carbon. Ni · · · C + Niδ+ · · · Oδ− → 2Ni0 + CO
(13)
Ni · · · C + 2(Ni2+ –O2− ) → 3Ni0 + CO2
(14)
where Ni · · · C represents carbon on the catalyst surface, Niδ+ · · · Oδ− represents mobile oxygen on the catalyst surface and Ni2+ –O2− represents NiO. Fig. 6 shows the reaction rates of CH4 and CO2 over Ni-YSZ-CeO2 catalyst electrode with time-on-stream under open- and short-circuit conditions. Under open-circuit condition, the reaction rates of CH4 and CO2 were decreased from 3.79 × 10−1 to 1.40 mol/cm2 s and from ca. 4.37 to 1.82 mol/cm2 s, respectively, for 30 h. These decreases in catalytic activity were caused by carbon deposition on catalyst electrode, as seen in Fig. 4. On the other hand, the reaction rates of CH4 and CO2 were stable for 48 h under short-circuit condition. And the maximum current density was decreased slightly with time-on-stream during the initial 10–15 h, and then it became stable with time-on-stream during 48 h. The amount of coke on the catalyst electrode during the dry reforming was measured by TGA. Thermogravimetric oxidation of the used catalyst electrode was performed at 5 ◦ C/min in air. Fig. 7 shows the TGA results of the catalyst electrode used in the dry reforming at 800 ◦ C for 50 h under open- and
Fig. 6. The reaction rates of CH4 and CO2 , and current density in the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) under open- and short-circuit condition at 800 ◦ C with time. CH4 (5 vol.%); CO2 (5 vol.%); total flow rate = 100 ml STP/min.
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Fig. 8. The proposed mechanism of the CO2 reforming by CH4 in the electrochemical cell (Ni-YSZ-CeO2 /YSZ/LSMC) under short-circuit condition.
4. Conclusions Fig. 7. TG results of the Ni-YSZ-CeO2 catalyst electrode after reaction in the electrochemical cell at 800 ◦ C during 50 h under open-circuit (a) and short-circuit (b) conditions.
short-circuit conditions. The coke deposition occurred on the catalyst electrode under open-circuit condition, and the amount of coke was ca. 30 mg C/gcat . In contrast to this, no coke deposition occurred under short-circuit condition. The weight of the catalyst electrode was increased in accordance with the amount of Ni was oxidized. From these results, the Ni-YSZ-CeO2 catalyst electrode of the electrochemical cell was stable during the dry reforming under short-circuit condition. In summary, the CO2 reforming by CH4 over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell at 800 ◦ C is schematically shown in Fig. 8. CH4 and CO2 decompose on Ni particles into CHx (0 ≤ x ≤ 3) species and CO + O, respectively. Two reaction pathways exist in which surface carbon is oxidized on the Ni particle surface by atomic oxygen formed from CO2 disproportion and on the three-phase boundary (Ni/YSZ/gas) by oxygen ions pumped electrochemically. In the case that oxygen ion flux is fast enough to remove coke deposition, the catalyst electrode is stable during the reaction. The electric power is generated by electrons released in the oxidation of surface carbon with the oxygen ions.
The CO2 reforming by CH4 was investigated over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell. The electric power generation performance of the electrochemical cell for CO2 and CH4 was close to that of H2 from current–voltage characteristics. Under open-circuit condition, catalyst electrode was deactivated by coke deposition. On the other hand, catalyst electrode was stable and synthesis gas was produced with high yield, in addition to the generation of electric power under short-circuit condition (electrochemical pumping of oxygen ion to the catalyst electrode). The coke formed in dry reforming was oxidized by the oxygen ions pumped electrochemically into carbon monoxide. As such, it is proposed that electrochemical cell is efficient for CO2 reforming by CH4 as regards energy-saving and catalyst stability against coking. Acknowledgements We acknowledge financial support from the R&D Management Center for Energy and Resources, The Korea Energy Management Corporation. References [1] A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, P.D.F. Vernon, Nature 452 (1991) 225.
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