J. Phys. Chem. C 2008, 112, 9943–9949
9943
Propane Dehydrogenation in a Proton-conducting Fuel Cell Yu Feng, Jingli Luo,* and Karl T. Chuang Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed: October 18, 2007; ReVised Manuscript ReceiVed: March 30, 2008
The process and reaction mechanism of propane dehydrogenation in a proton-conducting fuel cell were studied at 600-700 °C using Y-doped BeCeO3 as protonic electrolyte and Pt as electrode catalysts. The electrochemical dehydrogenation of propane was in competition with side reactions caused by the gas species (H2, C2H6, C2H4 etc.), which were the products of nonelectrochemical reactions (e.g., thermal cracking) in the anode chamber. Among those side electrochemical reactions, hydrogen disassociation into hydrogen ions was predominant. At 650 °C electrochemical dehydrogenation of propane has a higher activation energy than hydrogen disassociation over the anode platinum catalyst; however, the former reaction was more readily achieved at 700 °C. Introduction Conversion of propane to propylene in a proton-conducting fuel cell is a process by which propane is dehydrogenated to propylene.1–3 This process has the unique advantage of simultaneously obtaining value-added product propylene and electricity power. Unlike the industrial processes for conversion of propane to propylene, propane dehydrogenation occurring in a fuel cell is not equilibrium limited, and so propane can be completely converted to propylene. In addition, compared to other hydrocarbon fuel cells using oxide ion conductors as electrolytes, there is no greenhouse gas (CO2) or poisonous gas (CO) emissions from this proton-conducting fuel cell, as propane dehydrogenation is the only reaction, and the only products are propylene and water. Although development of this process clearly has economic and environmental attractiveness, there have been few studies directed to realization of the concept. The challenge has been finding suitable proton-conducting electrolytes and compatible electrodes operating at temperatures below 800 °C.4–7 Our fuel cell group at the University of Alberta made a breakthrough in this process and proved the feasibility of conversion of propane to propylene in a proton-conducting fuel cell.1,2 Good fuel cell performance and high selectivity to propylene were achieved using Y-doped BeCeO3 as electrolyte and Pt as electrodes.8–10 In our preliminary work at operating temperatures in the range 600-700 °C we detected hydrogen and other small molecule hydrocarbons in the anode outlet gas, in addition to the desired gas product propylene and unreacted propane. Involvement of these species in the electrochemical reactions in the fuel cell would affect fuel cell performance and propylene selectivity. Consequently, we now have investigated the behaviors of these species to understand the reaction mechanism of propane dehydrogenation in the fuel cell. Experimental Materials. Y-doped BaCeO3 (BCY) was used as the protonconducting electrolyte. According to the data reported in literature, BCY is a mix conductor, especially at high temper* Corresponding author e-mail:
[email protected]; phone: 1-780492-2232; fax: 1-780-492-2881.
ature. Combining some most relevant sources,11,12 it could be concluded that the proton transport numbers are about 1.00, 0.95, and 0.90 at 500, 600, and 700 °C, respectively. The preparation procedure was as previously described.2 Y-doped BaCeO3 powder was prepared by solid-state reactions, and then electrolyte solid disks were made by pressing the powder. Platinum was used for both cathode and anode catalysts, and the electrodes were prepared by screen-printing platinum paste (Heraeus CL11-5100) onto the corresponding surfaces of the BCY electrolyte disk. The membrane electrode assembly (MEA) was dried in air at 120 °C for 30 min and then fired at 1000 °C for 60 min to remove the organic binder and to increase adhesion to the electrolyte. The superficial surface area of each electrode was approximately 0.7 cm2. Fuel Cell System Design. A vertical fuel cell setup was adopted with a glass sealant to achieve a good anode side gas seal,13 as shown in Figure 1. This system comprises anode and cathode compartments, each consisting of two coaxial alumina tubes. The inner tube extended from outside the heated reaction zone to a position close to the respective electrode of the cell. The two tubes were fastened together at the remote end with a tailing gas outlet. The cell was sandwiched between the outer tubes of the anode and cathode compartment. The outer perimeter of anode and cathode outer tubes was sealed to the cell by applying a thin layer of glass and ceramic sealant (Aremco 503), respectively. Platinum gauze (52 mesh) was applied as the current collector for both anode and cathode. The assembled cell was placed in a tubular furnace (Thermolyne F79300). The seals were gradually cured as the cell was heated to the prescribed temperature for testing,13 prior to operation of the cell under reaction conditions. During the heating procedure, nitrogen was passing through the anode chamber and air through the cathode chamber. All the fuel cell experiments were carried out in the temperature range 600-700 °C. After the cell had stabilized at the operating temperature, the anode feed was switched to pure C3H8 (Praxair, grade 2.0). O2 (Praxair, grade 2.6) was supplied to the cathode side as oxidant via the inner tube. The gas flow rates were carefully controlled with mass flow controllers. Electrical Measurements and Gas Analysis. The open circuit voltage and I-V behaviors were measured using a
10.1021/jp710141c CCC: $40.75 2008 American Chemical Society Published on Web 06/06/2008
9944 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Feng et al.
Figure 2. Reaction equilibrium constants of the nonelectrochemical reactions in C3H8-O2 fuel cell at different temperatures.
Figure 1. Schematic of C3H8-O2 fuel cell setup design.
Solartron 1287 electrochemical interface. I-V characteristics were determined potentiodynamically with the electrochemical systems using a scanning rate 5 mV/s. The outlet gases from the anode chamber were analyzed using a Hewlett-Packard model HP5890 GC, using a polypack column at 80 °C and a thermal conductivity detector (TCD) connected to a computer workstation. Based on the inlet and outlet gas concentration, the conversion, selectivity, and yield are defined as:
Conversion )
nfeed,in - nfeed,out nfeed,in
nproduct Cproduct Selectivity ) * nfeed,in - nfeed,out Cpropane Yield )
nproduct Cproduct * nfeed,in Cpropane
other species—CH4, C2H4, C2H6, and H2—were also detected in the anode chamber effluent by gas chromatography because of occurrence of some nonelectrochemical reactions in the anode chamber of the fuel cell.14 This situation became more severe under the open circuit condition because there was no current drawn from the fuel cell, thus there was no electrochemical reaction, and the possible reactions were the thermal reactions occurring in the hollow coaxial fuel cell tube and the chemically catalytic reactions on the anode catalyst Pt. When the fuel cell was discharged, the electrochemical reactions were activated and gas products distribution was improved with enhanced propylene selectivity in contrast to the open circuit condition.2 However, even in this situation, thermal reactions still could not be avoided. The same gas product species were found in the effluent from the anode chamber in different concentrations, whether under open or closed circuit conditions.
(1)
Anode: C3H8 f C3H6 + 2H+ + 2e-
(4)
1 Cathode: 2H+ + O2 + 2e- f H2O 2
(5)
(2)
Based on thermodynamic calculations and anode outlet gas analyses, all the following equations, but not limited to these, are possible nonelectrochemical reactions occurring in the fuel cell system.
(3)
C3H8 f C3H6 + H2
(6)
C3H8 f CH4 + C2H4
(7)
C2H4 + H2 f C2H6
(8)
C2H6 + H2 f 2CH4
(9)
where nfeed,in is the moles of propane feed in, nfeed,out is the moles of propane in the effluent, nproduct is the moles of product in the effluent, Cproduct is the carbon atoms per molecule presented in the product (3 for propylene), and Cpropane is the carbon atoms per molecule of propane (3 for propane). Results and Discussion Nonelectrochemical Reactions in the Fuel Cell System. Ideally, the gas product from the fuel cell system is only propylene if propane dehydrogenation is the only electrochemical reaction (eqs 4 and 5). However, in the experiments some
1 ⁄ nCnHm f C +
m H 2n 2
(10)
All these reactions are thermodynamically favored at the fuel cell operating temperatures, and the reaction equilibrium constants for each reaction are shown in Figure 2. It has been shown that thermal reactions of propane followed a free-radical chain mechanism.15 Equation 11 is the initial step, whereas eqs
Figure 3. Mechanistic diagram of the propane reaction on the platinum catalyst.
Propane Dehydrogenation in a Fuel Cell
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9945
1 C3H8(g) + O2(g) f C3H6(g) + H2O(g) 2 PH2OPC3H6 RT E1 ) E°1(T) ) ln( 2F PC3H8PO21/2
Figure 4. Reaction equilibrium constant for carbon formation reaction 10 from hydrocarbon species in the gas products.
12–18 and 19–24 refer to free radicals’ propagation and termination reactions, respectively. Catalytic reactions of propane on the Pt catalyst primarily involve two pathways: dehydrogenation by breaking of C-H bonds and intramolecular C-C bond cracking.3,16 The two pathways compete with each other, as shown in Figure 3. The gas products distribution in experiments was consistent with these studies, with predominant concentrations of CH4, C2H4, C3H6, and H2 (total selectivity >90%). As predicted, carbon deposition on the anode catalyst was detected14 because, at high temperatures, hydrocarbons easily form carbon in an irreversible process, especially for alkenes,16 which can also be seen from Figure 4. Although some researchers mentioned polymerization reactions might be involved in the reaction network, no hydrocarbons with higher molecular weight were detected in effluent from our fuel cell system.
C3H8 f CH3/ + C2H5/
(11)
CH3/ + C3H8 f C3H7/ + CH4
(12)
C2H5/ + C3H8 f C2H6 + C3H7/
(13)
C3H7/ f C3H6 + H/
(14)
C3H7/ f C2H4 + CH3/
(15)
H/ + C3H8 f C3H7/ + H2
(16)
H/ + C2H6 f C2H5/ + H2
(17)
C2H5/ f C2H4 + H/
(18)
H/ + H/ f H2
(19)
+ CH3/ f CH4
(20)
+ C2H5/ f C2H6
(21)
H/ + C3H7/ f C3H8
(22)
CH3/ + CH3/ f C2H6
(23)
CH3/ + C2H5/ f C3H8
(24)
/
H
/
H
These nonelectrochemical reactions are strongly affected by operating temperatures and propane flow rates, that is, reaction time. Thus the concentrations of the gas species were functions of the operating conditions.2 Open Circuit Voltage. The electromotive force (EMF) for the electrochemical dehydrogenation of propane (eq 25) is expressed by the Nernst equation (eq 26).
(25) (26)
where E°1(T) is the cell standard potential, calculated from thermodynamic data ∆G° at standard conditions, that is, the potential when the partial pressures of all reactants and products are 1 bar. PC3H6, PC3H8, PH2O, and PO2 in the logarithmic term refer to the equilibrium partial pressures of the gas species at the interfaces between electrodes and electrolyte. Usually, the partial pressure at the interface can be replaced by that in the bulk fluid if there is no mass transfer limitation for the gas species. In experiments, the total pressures of anode and cathode compartment were both kept at 1 atm, so the partial pressure of each gas component was not at the standard condition (1 bar). PC3H8and PC3H6 could be calculated from GC analysis of anode outlet gas, but it was not easy to obtain PH2O because the partial pressure of water vapor in the cathode compartment was very low (∼10 ppm) at open circuit condition, 17 which was difficult to accurately measure by GC. To obtain the value of PH2O, experiments were conducted in which oxygen fed to the cathode compartment was saturated with water at room temperature instead of using pure, dry oxygen. Thus, the values of PH2O and PO2 were obtained as 0.031 and 0.969 atm, respectively. Because the partial pressures of the gas species were functions of operating temperatures and propane flow rates, the calculated EMF of propane dehydrogenation from the Nernst equation (Equation 26) also depends on the operating conditions. The calculated EMFs at different temperatures with a propane flow rate 100 mL/min are listed in Table 1, as well as partial pressures of the gas species and standard potential (HSC Chemistry Software, Version 5.1, Outokunpu Research). As predicted from the Nernst equation, the measured open circuit voltage of the C3H8-O2 fuel cell was not only dependent on operating temperature but also on propane flow rate, but the value was not consistent with the calculated EMF. For example, at 600 °C with a propane flow rate of 100 mL/min, the calculated EMF for the electrochemical dehydrogenation of propane was 1.295 V, which was larger than the measured OCV value of 1.085 V. The same phenomena were found during operating the fuel cell at 650 or 700 °C. The apparent discrepancy between the measured OCV values and the calculated EMF values probably was due to involvement of side electrochemical reactions in addition to propane dehydrogenation. However, this speculation was based on the assumption that the measured OCV values of an electrochemical reaction, without interference of other side reactions, was equal or very close to the EMF value calculated from the Nernst equation. This assumption usually was not true because, conceptually, OCV is not same as EMF, and the discrepancy between these two values was even apparent for low-temperature fuel cells (such as PEM fuel cell) due to slow anodic- and/or cathodic-reaction rates where the electrode reactions no longer attain equilibria.18 For hightemperature fuel cells, the OCV value could very closely reach the corresponding EMF value if the electrolyte is a complete proton conductor without electron conduction.19 Therefore, before we safely come to the conclusion that the difference between the EMF and measured OCV values of C3H8-O2 fuel cells was caused by involvements of other side reactions in addition to electrochemical dehydrogenation of propane, the consistency between OCV and EMF values for a single reaction in our fuel cell system must be proven. Apparently, the propane dehydrogenation reaction can never be guaranteed to be the sole electrochemical reaction at the fuel cell operating conditions, even with pure propane feed to the
0.031 0.031 0.031 Propane flow rate: 100 mL/min. a
E3/V
0.011 0.047 0.107 0.969 0.969 0.969
PH2O/atm PC2H4/atm PO2/atm
3.60E-4 0.003 0.009 0.897 0.917 0.938
PC2H6/atm E2/V
0.994 1.029 1.043 0.031 0.031 0.031
PH2O/atm PO2/atm
0.969 0.969 0.969 0.010 0.037 0.072 1.035 1.021 1.006
PH2/atm
anode, as discussed before. So another experiment was conducted with pure hydrogen as anode feed and water-saturated oxygen (saturated at room temperature) fed to the cathode, for which the OCV of the H2-O2 fuel cell is shown in Figure 5. It is obvious that the measured OCV value at 650 °C is 1.158 V, which is almost the same as the calculated value from the Nernst equation for a hydrogen fuel cell (1.160 V). So the experimental results supported the previous deduction that the gas species present in addition to propane took part in the electrochemical reactions. Among the gas species (CH4, C2H4, C3H6, and H2) at the anode outlet, hydrogen is the one most easily disassociated over platinum in the fuel cells (eq 27), because platinum is an active anode catalyst for hydrogen fuel cells.20–24 Another possibility is that ethane may be involved in the electrochemical reactions through reaction 28, similar to propane dehydrogenation. The partial pressures of the gas species involved in reactions 27 and 28 were determined from GC analysis and are summarized in Table 1, as well as the calculated EMF values from the Nernst equations (eqs 29 and 30). From comparison of the measured OCV and calculated EMF values of the three different reactions—eqs 25, 27, and 28—(Table 2), it is apparent that the OCV value was equal to none of the EMF values of any of these three reactions. At 600 and 650 °C, the measured OCV value was less than the EMF value of reaction 25 but was larger than the EMF values of reaction 27 or 28. Thus, measured OCV values may have been a mixed reaction voltage of these three reactions. A difference for data obtained at 700 °C was that the measured OCV value was below all three of these EMF values, which indicated some other side reactions may also have occurred in addition to the three discussed here. It is possible that reaction 10 proceeds, followed by reaction 31, which also affects the OCV.
0.031 0.031 0.031 0.009 0.043 0.085
1.295 1.265 1.255
PH2O /atm PC3H6/atm
E1/V
Figure 5. Open circuit voltage of H2-O2 fuel cell using Y-doped BaCeO3 as electrolyte with pure hydrogen as anode feed and water saturated oxygen as cathode gas at 650 °C.
0.969 0.969 0.969 0.948 0.783 0.521
PO2/atm PC3H8/atm
0.992 1.014 1.036
temperature/°C
600 650 700
C2H6 + 1/2O2 f C2H4 + H2O
E°3/V H2 + 1/2O2 f H2O
E°2/V C3H8 + 1/2O2 f C3H6 + H2O
E°1/V
TABLE 1: The Calculated EMF Values from the Nernst Equation for Reactions 25, 27, and 28, As Well As the Standard Potentials and Partial Pressures of Gas Species at Different Temperaturesa
Feng et al.
0.896 0.937 0.975
9946 J. Phys. Chem. C, Vol. 112, No. 26, 2008
1 H2 + O2 f H2O 2 1 C2H6 + O2 f C2H4 + H2O 2 PH2O RT E2 ) E°2(T) ln 2F PH2PO21/2
(28) (29)
PH2OPC2H4 RT ln 2F PC2H6PO21/2
(30)
m m O f nC + H2O 4 2 2
(31)
E3 ) E°3(T) CnHm +
( ) ( )
(27)
Therefore, the measured OCV of the C3H8-O2 fuel cell was a result of competition between all of the electrochemical
Propane Dehydrogenation in a Fuel Cell
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9947
TABLE 2: Comparison of Measured OCV Values of C3H8-O2 Fuel Cell and the Calculated EMFS for Different Possible Electrochemical Reactions in the C3H8-O2 Fuel Cell at Different Temperatures with A Propane Flow Rate of 100 mL/min temperature/°C
theoretical EMF of reaction 25, E1/V
theoretical EMF of reaction 27, E2/V
theoretical EMF of reaction 28, E3/V
measured OCV, V/V
600 650 700
1.295 1.265 1.255
0.994 1.029 1.043
0.896 0.937 0.975
1.085 1.061 0.892
reactions present under the open circuit condition, such as reactions 25, 27, and 28, and the value of OCV depended on the proportions of each electro-active species present and the reaction potentials. Fuel Cell Performance. The involvements of hydrogen reaction (reaction 27) and other side reactions in the process of propane dehydrogenation in the fuel cell were also reflected in the I-V curves. For example, at 650 °C with a propane flow rate of 100 mL/min, the concentrations of hydrogen and propane were 3.7 and 78.3%, respectively, in the anode chamber under the open circuit conditions. So when the fuel cell was discharged, there could be contributions to the current from all gas species in the anode chamber: 3.7% hydrogen, 78.3% propane and/or other gas components. To analyze the individual contribution by each gas species, especially for the hydrogen in the gas mixture, separate experiments were conducted with propane (100 mL/min) and 3.7% hydrogen (balance with Ar) as the fuels, respectively, at 650 °C (Figure 6). If reactions 25 and 27 were independent, it could be inferred from Figure 6 that the C3H8-O2 fuel cell performance was mainly attributable to hydrogen reaction 27 rather than propane dehydrogenation (eq 25). However, when discharging the fuel cell at low voltage (
