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Ammonia oxidation to nitric oxide in a solid electrolyte fuel cell
Solid State Ionics 5 (1981) 567-570
North-Holland PublishingCompany
AMMONIA OXIDATION TO NITRIC OXIDE IN A SOLID ELECTROLYTE
FUEL CELL
Catherine T. Sigal and Costas G. Vayenas Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, b~ 02139, USA The oxidation of ammonia
to nitric oxide was investigated
in the fuel cell
NH 3,NO,N2,PtlZrO2(8%Y20 ~ IPt,air at temperatures between i000 and 1200 K and at atmospheric pressure. The product selectivity to NO can exceed 95% with simultaneous electrical energy generation. A new design of the cell has resulted in an 800-fold increase in power density over the original design of Farr and Vayenas. In the new design the decreased electrode surface area suppresses the undesirable catalytic side reaction of NH3 and NO to form N2 while the thinner electrolyte (~200 ~m) increases power density.
INTRODUCTION The oxidation of ammonia to nitric oxide is a reaction of great industrial significance, as it is the first step in the production of nitric acid. Typically this reaction is carried out over finely woven Pt-Rh gauzes in the temperature range 1073-1173 K (i). Yields of 94-98% of NH3 converted to NO are achieved. Because this reaction has a large negative free energy change (AG ° at 1073 K = -274 kJ/mole), it is a desirable goal to carry it out in a fuel cell, so that more of the available energy may be recovered as work. Unlike heat engines, fuel cells are not subject to the Carnot limitation and therefore can operate at high efficiencies. The first study of a high temperature NH 3 fuel cell was performed by Farr and Vayenas (2-4). They used a tubular reactor fashioned from yttria stabilized zirconia as the solid electrolyte. Platinum electrodes were deposited on the inside and outside of the tube. The thickness of the zirconia tube was 0.18 cm and the superficial electrode area was 144 cm 2. With this design, high selectivity to NO could be achieved, but only at low power density. For example, at ii00 K with a feed of 2% NH 3 in He and a total flow rate of 5 cc STP/min, a selectivity of 97% was observed with a power density of only 7 pW/ cm 2 . Farr and Vayenas established that the predominant overall reactions in the fuel cell were the following: (i)
2NH3 + 502- ~ 2NO + 3H20 + 10e-
(2)
2NH 3 + 3NO ÷ 5/2N2 + 3H20
They postulated that the2~ate of the first reaction was controlled by 0 diffusion through the solid electrolyte. The second reaction is an undesirable catalytic side reaction, which was assumed to be first order in NH3 and NO. Based on the preceeding assumptions, Farr and Vayenas developed a model (2-4) which showed the selectivity to be a function of two dimensionless
parameters, M and N. fined as follows:
The parameters
were de-
(3)
M = (02 molar flux)/(NH 3 molar flux)
(4)
N = kSYNH 3,f/G
where k is a specific rate constant for the NH3 + NO reaction, S is the catalyst anode surface area, YNH3,f is the mole fraction of NH 3 in the feed and G is the total molar flow rate. According to the model, high values of M favor high selectivity, i.e., high oxygen to NH 3 ratios favor nitric oxide formation. In practice high values of M were achieved by lowering the NH 3 flux into the fuel cell, which caused power density to drop. Hence, increased selectivity was found to correspond to decreased power density. The second parameter, N, is a measure of the extent of the undesirable side reaction. Small values of N therefore enhance selectivity. In this paper we demonstrate that the behavior of the selectivity and power density for the NH 3 fuel cell can be improved dramatically w i t h a new reactor design. In addition, the aforementioned model must be modified to explain the present observations. EXPERIMENTAL The fuel cell was made from an 8 wt.% yttria stabilized zirconia tube provided by Zircoa. The tube was 15.2 cm long with an i.d. of 1.59 cm and a wall thickness of 0.16 cm. It was closed at one end so that the bottom could be ground to a minimal thickness using a diamond powder. A thin electrolyte was desired since it results in a lower resistance. The final thickness was estimated to be <200 pm. Platinum electrodes were applied to the inside and outside of the bottom by depositing a thin coating of Engelhard A3788 fluxed ink. The tube was then calcined at 1173 K for ~5 hrs. One reactor was also made with a 90% Pt-lO% Rh anode using Engelhard 6929 unfluxed ink. A cross sectional view of the fuel cell is shown
0 167 2738/81/0000-0000/$02.75 © North-Holland Publishing Company
C. Z Sigal, (~ G. Vayettas / Nitric oxide in a solid electrolyte jitel cell
568
in Fig. i. Reactants and products were analyzed using Beckman 864 and 865 infrared anelyzers for NH3 and NO, respectively. A complete description of the experimental system is given elsewhere (5).
calculation, at the highest current the rate of mass transfer was at least 4.5 times greater than the observed rate. Hence, mass transfer was assumed to be non-rate limiting in the kinetic treatment of the data.
PLATINUM
LEAD WIRE
PRODUDTS~ (~OOLING COILS
O
.< ~EED
i ~)"
METAL ~UPPORT PYREX-TO-P~ SEAL
FURNACE
l i t rl,PlRAL CONTACT WITH
INSIDE ELECTRODE
Figure i.
Cross sectional view of the fuel cell
Conversion and selectivity are defined in the following way: (i) conversion equals the mo]es of NH 3 reacted per mole of NH~ in the feed and (ii) selectivity equals the moles o[ NO produced per mole of NH 3 reacted. In Fig. 2 it is evident that the conversion is nonzero at open circuit. This is due to NH 3 decomposition. Kinetic studies at this temperature have demonstrated that in the presence of oxygen the rate of NH~ decomposition is much lower than the rate of NH 3 reaction with oxygen or NO (8). Hence, as oxygen is introduced into the fuel cell, these latter reactions become predominant and ammonia decomposition does not take place to any significant extent as evidenced by the very high selectivity to NO at high current (Fig. 4). The steadily rising selectivity with increasing current indicates that NO formation is favored by high oxygen to NH 3 ratios. zl.0
,
Q
r~ Q 0
B •
RESULTS The resistance of the fuel cell was measured by passing air over both electrodes and using a galvanostat to generate a cell potential-current curve. The slope of the resulting straight line gave the resistance. For a cell with electrodes prepared from a fluxed Pt ink, the resistance varied from 2.4~ at 1073 K to 1.5~ at 1173 K. Under operating conditions, however, the resistance of the cell tended to increase with time due to a gradual detachment of the anode from the zirconia. One reason for the decreased adhesion may be the tendency of NH3 to extensively rearrange the catalyst during oxidation (6). This effect limited the useful cell lifetime to ~2 days. Sintering the electrodes at 1373 K for several hours resulted in a longer lifetime, but the cell resistance was increased and the catalyst was less active toward NO formation. A cell was also made with a Pt-Rh anode. This alloy was chosen for its ability to sinter less rapidly than Pt. The cell with the Pt-Rh anode had an increased lifetime of %2 weeks. A typical cell potential-current curve for a cell with unsintered Pt electrodes is shown in Fig. 2. Conversion and selectivity are also shown. There is a large activation overpotential at low currents followed by a region of ohmic overpotential. At high currents evidence exists for concentration overpotential. The mass transfer coefficient for NH 3 in the reactor was estimated from boundary layer theory (7) and the rate of mass transfer compared to the observed rate of reaction. According to this
,
,
,
CELL VOLTAGE o SELECTIVITY + CONVERSION 4 3 . ! CC S T P / M I N 1073 K l . 10~ NH3
.~
O.H t
+
0.2-
L~ 0 ' 0 0 Figure 2.
10
@0 30 'iO 5O CURRENT (mAMP) Cell p o t e n t i a l , s e l e c t l v i t y , and C o N v e r s i o n VS.
current
Two important features of the fuel cell performance, the selectivity and power density, are plotted in Fig. 3 as functions of current. A maximum power density of 4.9 mW/cm ~ was achieved under conditions in which NO was formed with a selectivity of 42% and a conversion of 30%. These same variables are plotted in Fig. 4 at 1131 K. Here a lower NH 3 flux was used, so that higher values of M were achieved. A selectivity of 97% and a conversion of 39% with a power density of 0.61 mW/cm 2 were observed. With the previous fuel cell design (2-4) the power density dropped to only a few microwatts per square centimeter under conditions in which NO was formed. Higher power densities were anticipated in this study due to the thinner electrolyte. However, an unexpected finding was the onset of NO production at lower values of M; specifically, NO
5 69
C 71 Sigal, C G. Vayenas / Nitric oxide in a solid electrolyte fuel cell
formation began at M ~ 0.25 as compared with M = 0.75 in the original study (2-4). This is due to the greatly reduced superficial catalyst surface area in the new fuel cell (2.0 cm 2 vs. 144 cm2). In this way, the new design allowed reaction 1 to proceed but suppressed side reaction 2 by providing less catalyst area on which it could occur. In terms of the model, the reduced surface area caused values of the parameter N to be decreased. 1.0
,
O. B
q 3 . l CC STP/MIN 1073 K 1.10% NH3
,
,
10
,
8
0.6
O Fq
6
R
i
0
0.4
'
z co 4~ -<
cell design resulted in decreased values of the parameter N, thus reducing the extent of the undesirable NH 3 + NO reaction, and causing the onset of NO production to occur at lower oxygen to NH 3 ratios. The new cell design also elucidated another difference in the behavior of the paramter N. When N* = N(G/YNH3, f) = kS was plotted versus M for a particular run, N* was found to be a strongly decreasing function of M, as shown in Fig. 5. This observation was contrary to the original model, since N* = kS was expected to remain constant at constant temperature. The significant decrease in N* with increasing M must be due to the poisoning effect of oxygen on the NH 3 + NO reaction resulting from the occupation of surface sites by oxygen (9). This effect was amplified with the new design, because a given current corresponded to a higher concentration of oxygen on the surface.
N* = kS
6
E ro 0,0
Figure 3.
10
20 CURRENT
Selectivity current
30 (mAMP)
i 40
x 5
5O
and power density vs.
18.3 CC BTP/MIN 1131 K O. 92% NH3
,--,O. B
N3.] CC STP/MIN 30.7 CC 5TP/MIN
I
I
X
1
6~
¢
00. 0
z Figure 5.
i0.4 m
I
0.25 0.50 0.75 M = (O2 FLUX)/(NH 3 FLUX)
.00
Effect of oxygen inhibition
on N*
4-<
\ 2~ E
0.2
0.0
i'o
20
4o
CURRENT
Figure 4.
X
~r ~2
1.0 O.B
L t~ ,% L~ ''3
I
1076°K 1.1% NH3
0.2
Selectivity current
50
&.AMP)
and power density vs.
As discussed earlier, the fuel cell with the Pt Rh anode had an increased lifetime. However, its resistance was greater, and it had to be operated at a higher temperature with low NH3 fluxes in order to achieve values of M high enough for NO formation. A maximum selectivity of 35% corresponding to a power density of 0.29 n~/cm 2and a conversion of 46% was observed. DISCUSSION The decreased
surface area with the new fuel
In order to predict selectivity and conversion in the new fuel cell, it was necessary to account for the effect of oxygen inhibition. This effect was modeled by assuming a LangmuirHinshelwood rate expression for the NH3 + NO reaction in which the surface concentration of oxygen atoms was assumed to be proportional to M. A detailed account of the modified model, which is semiempirical in nature, is presented elsewhere (5). Figure 6 illustrates predicted and experimental selectivity vs. M for two flowrates. Good agreement between the modified model and experiment is obtained. For a given value of M, the higher flowrate corresponds to a higher selectivity. This increase in selectivity occurs because a lower residence time in the fuel cell causes side reaction 2 to proceed to a lesser extent. CONCLUSIONS Ammonia can be converted to NO quantitatively with simultaneous generation of electrical energy. The new fuel cell design resulted in a
570
C.T. Sigal, C G .
~
100
//
BO
I/ '
l~-
V a y e n a s / ,'V"ttrtc " o x i d e in a s o l i d electroh,. te f u e l cell
REFERENCES CTED IELEC.
~
CC STP/MIN
BO
~a 40
1.1 _) w to
2O
~.00
Figure 6.
[i]
Chilton, T.H, "The Manufacture of Nitric Acid by the Oxidation of Ammonia," Chemical Engineering Press Monograph Series No. 3, Vol. 56 (1960).
[2]
Farr, R.D. and Vayenas, C.G., J. Electrochem. Soc. 127 (1980), 1478-1483.
[3]
Vayenas, C.G. and Farr, R.D., Science 208 (1980), 593-594.
[4]
Farr, R.D., M.S. Thesis, Massachusetts Institute of Technology (1979).
[5]
Teague Sigal, C., M.S. Thesis, Massachusett Institute of Technology (1981).
[6]
Satterfield, C.N., Heterogeneous Catalysis in Practice (McGraw-Hill, New York, 1980), 214-221.
[7]
Schlicbting, H., Boundary Layer Theory (McGraw-Hill, New York, 1979), 95-99.
[8]
Pignet, T. and Schmidt, L.D., Chem. Eng. Sci. 29 (1973), 1123-1131. Pignet, T. and Schmidt, L.D., J. Catal. 40 (1975), 212-225.
3 o 7 CCSTP,MzN loz~ K [ , 1 % NH3
0.25 0.50 0.75 1.00 M = (0~ FLUX)/(NH s FLUX)
1.25
Predicted and experimental selectivity vs. M
smaller catalyst surface area and a pronounced oxygen inhibition effect, both of which reduced values of the parameter No The result was the onset of NO formation at lower oxygen to NH3 ratios than in the previous studies (2-4). Since anionic currents of comparable magnitude were produced in both studies, this observation meant that higher NH 3 fluxes could be used in the new fuel cell. This effect coupled with a thinner electrolyte allowed power densities on the order of several milliwatts per square centimeter to be attained under conditions in which NO was produced.
[9]
North-Holland PublishingCompany
AMMONIA OXIDATION TO NITRIC OXIDE IN A SOLID ELECTROLYTE
FUEL CELL
Catherine T. Sigal and Costas G. Vayenas Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, b~ 02139, USA The oxidation of ammonia
to nitric oxide was investigated
in the fuel cell
NH 3,NO,N2,PtlZrO2(8%Y20 ~ IPt,air at temperatures between i000 and 1200 K and at atmospheric pressure. The product selectivity to NO can exceed 95% with simultaneous electrical energy generation. A new design of the cell has resulted in an 800-fold increase in power density over the original design of Farr and Vayenas. In the new design the decreased electrode surface area suppresses the undesirable catalytic side reaction of NH3 and NO to form N2 while the thinner electrolyte (~200 ~m) increases power density.
INTRODUCTION The oxidation of ammonia to nitric oxide is a reaction of great industrial significance, as it is the first step in the production of nitric acid. Typically this reaction is carried out over finely woven Pt-Rh gauzes in the temperature range 1073-1173 K (i). Yields of 94-98% of NH3 converted to NO are achieved. Because this reaction has a large negative free energy change (AG ° at 1073 K = -274 kJ/mole), it is a desirable goal to carry it out in a fuel cell, so that more of the available energy may be recovered as work. Unlike heat engines, fuel cells are not subject to the Carnot limitation and therefore can operate at high efficiencies. The first study of a high temperature NH 3 fuel cell was performed by Farr and Vayenas (2-4). They used a tubular reactor fashioned from yttria stabilized zirconia as the solid electrolyte. Platinum electrodes were deposited on the inside and outside of the tube. The thickness of the zirconia tube was 0.18 cm and the superficial electrode area was 144 cm 2. With this design, high selectivity to NO could be achieved, but only at low power density. For example, at ii00 K with a feed of 2% NH 3 in He and a total flow rate of 5 cc STP/min, a selectivity of 97% was observed with a power density of only 7 pW/ cm 2 . Farr and Vayenas established that the predominant overall reactions in the fuel cell were the following: (i)
2NH3 + 502- ~ 2NO + 3H20 + 10e-
(2)
2NH 3 + 3NO ÷ 5/2N2 + 3H20
They postulated that the2~ate of the first reaction was controlled by 0 diffusion through the solid electrolyte. The second reaction is an undesirable catalytic side reaction, which was assumed to be first order in NH3 and NO. Based on the preceeding assumptions, Farr and Vayenas developed a model (2-4) which showed the selectivity to be a function of two dimensionless
parameters, M and N. fined as follows:
The parameters
were de-
(3)
M = (02 molar flux)/(NH 3 molar flux)
(4)
N = kSYNH 3,f/G
where k is a specific rate constant for the NH3 + NO reaction, S is the catalyst anode surface area, YNH3,f is the mole fraction of NH 3 in the feed and G is the total molar flow rate. According to the model, high values of M favor high selectivity, i.e., high oxygen to NH 3 ratios favor nitric oxide formation. In practice high values of M were achieved by lowering the NH 3 flux into the fuel cell, which caused power density to drop. Hence, increased selectivity was found to correspond to decreased power density. The second parameter, N, is a measure of the extent of the undesirable side reaction. Small values of N therefore enhance selectivity. In this paper we demonstrate that the behavior of the selectivity and power density for the NH 3 fuel cell can be improved dramatically w i t h a new reactor design. In addition, the aforementioned model must be modified to explain the present observations. EXPERIMENTAL The fuel cell was made from an 8 wt.% yttria stabilized zirconia tube provided by Zircoa. The tube was 15.2 cm long with an i.d. of 1.59 cm and a wall thickness of 0.16 cm. It was closed at one end so that the bottom could be ground to a minimal thickness using a diamond powder. A thin electrolyte was desired since it results in a lower resistance. The final thickness was estimated to be <200 pm. Platinum electrodes were applied to the inside and outside of the bottom by depositing a thin coating of Engelhard A3788 fluxed ink. The tube was then calcined at 1173 K for ~5 hrs. One reactor was also made with a 90% Pt-lO% Rh anode using Engelhard 6929 unfluxed ink. A cross sectional view of the fuel cell is shown
0 167 2738/81/0000-0000/$02.75 © North-Holland Publishing Company
C. Z Sigal, (~ G. Vayettas / Nitric oxide in a solid electrolyte jitel cell
568
in Fig. i. Reactants and products were analyzed using Beckman 864 and 865 infrared anelyzers for NH3 and NO, respectively. A complete description of the experimental system is given elsewhere (5).
calculation, at the highest current the rate of mass transfer was at least 4.5 times greater than the observed rate. Hence, mass transfer was assumed to be non-rate limiting in the kinetic treatment of the data.
PLATINUM
LEAD WIRE
PRODUDTS~ (~OOLING COILS
O
.< ~EED
i ~)"
METAL ~UPPORT PYREX-TO-P~ SEAL
FURNACE
l i t rl,PlRAL CONTACT WITH
INSIDE ELECTRODE
Figure i.
Cross sectional view of the fuel cell
Conversion and selectivity are defined in the following way: (i) conversion equals the mo]es of NH 3 reacted per mole of NH~ in the feed and (ii) selectivity equals the moles o[ NO produced per mole of NH 3 reacted. In Fig. 2 it is evident that the conversion is nonzero at open circuit. This is due to NH 3 decomposition. Kinetic studies at this temperature have demonstrated that in the presence of oxygen the rate of NH~ decomposition is much lower than the rate of NH 3 reaction with oxygen or NO (8). Hence, as oxygen is introduced into the fuel cell, these latter reactions become predominant and ammonia decomposition does not take place to any significant extent as evidenced by the very high selectivity to NO at high current (Fig. 4). The steadily rising selectivity with increasing current indicates that NO formation is favored by high oxygen to NH 3 ratios. zl.0
,
Q
r~ Q 0
B •
RESULTS The resistance of the fuel cell was measured by passing air over both electrodes and using a galvanostat to generate a cell potential-current curve. The slope of the resulting straight line gave the resistance. For a cell with electrodes prepared from a fluxed Pt ink, the resistance varied from 2.4~ at 1073 K to 1.5~ at 1173 K. Under operating conditions, however, the resistance of the cell tended to increase with time due to a gradual detachment of the anode from the zirconia. One reason for the decreased adhesion may be the tendency of NH3 to extensively rearrange the catalyst during oxidation (6). This effect limited the useful cell lifetime to ~2 days. Sintering the electrodes at 1373 K for several hours resulted in a longer lifetime, but the cell resistance was increased and the catalyst was less active toward NO formation. A cell was also made with a Pt-Rh anode. This alloy was chosen for its ability to sinter less rapidly than Pt. The cell with the Pt-Rh anode had an increased lifetime of %2 weeks. A typical cell potential-current curve for a cell with unsintered Pt electrodes is shown in Fig. 2. Conversion and selectivity are also shown. There is a large activation overpotential at low currents followed by a region of ohmic overpotential. At high currents evidence exists for concentration overpotential. The mass transfer coefficient for NH 3 in the reactor was estimated from boundary layer theory (7) and the rate of mass transfer compared to the observed rate of reaction. According to this
,
,
,
CELL VOLTAGE o SELECTIVITY + CONVERSION 4 3 . ! CC S T P / M I N 1073 K l . 10~ NH3
.~
O.H t
+
0.2-
L~ 0 ' 0 0 Figure 2.
10
@0 30 'iO 5O CURRENT (mAMP) Cell p o t e n t i a l , s e l e c t l v i t y , and C o N v e r s i o n VS.
current
Two important features of the fuel cell performance, the selectivity and power density, are plotted in Fig. 3 as functions of current. A maximum power density of 4.9 mW/cm ~ was achieved under conditions in which NO was formed with a selectivity of 42% and a conversion of 30%. These same variables are plotted in Fig. 4 at 1131 K. Here a lower NH 3 flux was used, so that higher values of M were achieved. A selectivity of 97% and a conversion of 39% with a power density of 0.61 mW/cm 2 were observed. With the previous fuel cell design (2-4) the power density dropped to only a few microwatts per square centimeter under conditions in which NO was formed. Higher power densities were anticipated in this study due to the thinner electrolyte. However, an unexpected finding was the onset of NO production at lower values of M; specifically, NO
5 69
C 71 Sigal, C G. Vayenas / Nitric oxide in a solid electrolyte fuel cell
formation began at M ~ 0.25 as compared with M = 0.75 in the original study (2-4). This is due to the greatly reduced superficial catalyst surface area in the new fuel cell (2.0 cm 2 vs. 144 cm2). In this way, the new design allowed reaction 1 to proceed but suppressed side reaction 2 by providing less catalyst area on which it could occur. In terms of the model, the reduced surface area caused values of the parameter N to be decreased. 1.0
,
O. B
q 3 . l CC STP/MIN 1073 K 1.10% NH3
,
,
10
,
8
0.6
O Fq
6
R
i
0
0.4
'
z co 4~ -<
cell design resulted in decreased values of the parameter N, thus reducing the extent of the undesirable NH 3 + NO reaction, and causing the onset of NO production to occur at lower oxygen to NH 3 ratios. The new cell design also elucidated another difference in the behavior of the paramter N. When N* = N(G/YNH3, f) = kS was plotted versus M for a particular run, N* was found to be a strongly decreasing function of M, as shown in Fig. 5. This observation was contrary to the original model, since N* = kS was expected to remain constant at constant temperature. The significant decrease in N* with increasing M must be due to the poisoning effect of oxygen on the NH 3 + NO reaction resulting from the occupation of surface sites by oxygen (9). This effect was amplified with the new design, because a given current corresponded to a higher concentration of oxygen on the surface.
N* = kS
6
E ro 0,0
Figure 3.
10
20 CURRENT
Selectivity current
30 (mAMP)
i 40
x 5
5O
and power density vs.
18.3 CC BTP/MIN 1131 K O. 92% NH3
,--,O. B
N3.] CC STP/MIN 30.7 CC 5TP/MIN
I
I
X
1
6~
¢
00. 0
z Figure 5.
i0.4 m
I
0.25 0.50 0.75 M = (O2 FLUX)/(NH 3 FLUX)
.00
Effect of oxygen inhibition
on N*
4-<
\ 2~ E
0.2
0.0
i'o
20
4o
CURRENT
Figure 4.
X
~r ~2
1.0 O.B
L t~ ,% L~ ''3
I
1076°K 1.1% NH3
0.2
Selectivity current
50
&.AMP)
and power density vs.
As discussed earlier, the fuel cell with the Pt Rh anode had an increased lifetime. However, its resistance was greater, and it had to be operated at a higher temperature with low NH3 fluxes in order to achieve values of M high enough for NO formation. A maximum selectivity of 35% corresponding to a power density of 0.29 n~/cm 2and a conversion of 46% was observed. DISCUSSION The decreased
surface area with the new fuel
In order to predict selectivity and conversion in the new fuel cell, it was necessary to account for the effect of oxygen inhibition. This effect was modeled by assuming a LangmuirHinshelwood rate expression for the NH3 + NO reaction in which the surface concentration of oxygen atoms was assumed to be proportional to M. A detailed account of the modified model, which is semiempirical in nature, is presented elsewhere (5). Figure 6 illustrates predicted and experimental selectivity vs. M for two flowrates. Good agreement between the modified model and experiment is obtained. For a given value of M, the higher flowrate corresponds to a higher selectivity. This increase in selectivity occurs because a lower residence time in the fuel cell causes side reaction 2 to proceed to a lesser extent. CONCLUSIONS Ammonia can be converted to NO quantitatively with simultaneous generation of electrical energy. The new fuel cell design resulted in a
570
C.T. Sigal, C G .
~
100
//
BO
I/ '
l~-
V a y e n a s / ,'V"ttrtc " o x i d e in a s o l i d electroh,. te f u e l cell
REFERENCES CTED IELEC.
~
CC STP/MIN
BO
~a 40
1.1 _) w to
2O
~.00
Figure 6.
[i]
Chilton, T.H, "The Manufacture of Nitric Acid by the Oxidation of Ammonia," Chemical Engineering Press Monograph Series No. 3, Vol. 56 (1960).
[2]
Farr, R.D. and Vayenas, C.G., J. Electrochem. Soc. 127 (1980), 1478-1483.
[3]
Vayenas, C.G. and Farr, R.D., Science 208 (1980), 593-594.
[4]
Farr, R.D., M.S. Thesis, Massachusetts Institute of Technology (1979).
[5]
Teague Sigal, C., M.S. Thesis, Massachusett Institute of Technology (1981).
[6]
Satterfield, C.N., Heterogeneous Catalysis in Practice (McGraw-Hill, New York, 1980), 214-221.
[7]
Schlicbting, H., Boundary Layer Theory (McGraw-Hill, New York, 1979), 95-99.
[8]
Pignet, T. and Schmidt, L.D., Chem. Eng. Sci. 29 (1973), 1123-1131. Pignet, T. and Schmidt, L.D., J. Catal. 40 (1975), 212-225.
3 o 7 CCSTP,MzN loz~ K [ , 1 % NH3
0.25 0.50 0.75 1.00 M = (0~ FLUX)/(NH s FLUX)
1.25
Predicted and experimental selectivity vs. M
smaller catalyst surface area and a pronounced oxygen inhibition effect, both of which reduced values of the parameter No The result was the onset of NO formation at lower oxygen to NH3 ratios than in the previous studies (2-4). Since anionic currents of comparable magnitude were produced in both studies, this observation meant that higher NH 3 fluxes could be used in the new fuel cell. This effect coupled with a thinner electrolyte allowed power densities on the order of several milliwatts per square centimeter to be attained under conditions in which NO was produced.
[9]