The Synthesis of Hydrogen Cyanide in a Solid

Vol. 134, No. 8

MOLYBDENUM

L i m i t e d c o r r o s i o n tests w e r e also c a r r i e d out w i t h an e x p e r i m e n t a l Fe-30Ni-15Mo alloy. The objective of these tests was to d e t e r m i n e w h e t h e r m o l y b d e n u m m i g h t prov i d e b e t t e r c o r r o s i o n p r o t e c t i o n f r o m Na2S3 in an ironbased alloy. However, 100h e x p o s u r e tests s h o w e d metal losses greater than those o b s e r v e d with Hastelloy B.

Conclusions M o l y b d e n u m c o r r o d e d v e r y slowly in sulfur and sod i u m polysulfides at 623 K. This indicated that molybdenum, u s e d as an alloying additive with nickel- and ironb a s e d alloys or as a c o a t i n g material, m i g h t be a good c a n d i d a t e for c o n s t r u c t i n g c o n t a i n e r s for s o d i u m / s u l f u r cells. T h e a s s u m e d MoSs c o r r o s i o n scale is an e l e c t r i c a l c o n d u c t o r ; t h e r e f o r e s u c h c o n t a i n e r s w o u l d also m a k e good positive-electrode c u r r e n t collectors. T h e h i g h m o l y b d e n u m c o n t e n t of H a s t e l l o y B results in good corrosion protection w h e n it is e x p o s e d to sulfur at 623 K. H o w e v e r , in Na~S3 v e r y h i g h c o r r o s i o n rates w e r e m e a s u r e d , w i t h only small i m p r o v e m e n t s o v e r rates m e a s u r e d p r e v i o u s l y for p u r e nickel. An e x p er i m e n t a l Fe-Ni-Mo alloy also c o r r o d e d v e r y r a p i d l y in Na2S3 at 623 K. Thus, m o l y b d e n u m is not an effective alloying e l e m e n t for p r o d u c i n g an iron- or nickel-based alloy to use as a sodium/sulfur cell container.

Acknowledgments The co n t i n u al help and e n c o u r a g e m e n t o f J . E. Battles are g r a t e f u l l y a c k n o w l e d g e d . All p o w d e r x-ray diffrac-

AND HASTELLOY

B

1925

tion analyses r e p o r t e d in this p a p e r w e r e e x p e r t l y c a r r i e d out by B. S. Tani of the A n a l y t i c a l C h e m i s t r y L a b o r a t o r y of A r g o n n e N a t i o n a l L a b o r a t o r y . This w o r k was s u p p o r t e d by the U.S. D e p a r t m e n t of Energy, Office of E n e r g y Storage, u n d e r Contract W-31-109-Eng-38. M a n u s c r i p t s u b m i t t e d Sept. 8, 1986; r e v i s e d m a n u script received Jan. 15, 1987.

Argonne National Laboratory assisted in meeting the publication costs of this article. REFERENCES 1. A. P. B r o w n and J. E. Battles, This Journal, 133, 1321 (1986). 2. R. R. Dubin, Mater. Perform., 20(2), 13 (1981). 3. T. L. Markin, A. R. J u n k i s o n , R. J. Bones, and D. A. Teagle, Power Sources, 7, 757 (1979). 4. " S o d i u m - N a k E n g i n e e r i n g H a n d b o o k , " Vol. 5, O. J. Foust, Editor, p. 8, Gordon and Breach Science Publishers, Inc., New York (1979). 5. A. P. B r o w n and J. E. Battles, Synth. React. Inorg. Met.-Org. Chem., 14(7), 945 (1984). 6. F. C. Mrazek, Microscope, 31, 235 (1983). 7. U. R. Evans, "The Corrosion and Oxidation of Metals," St. Martin's Press, New York (1960). 8. P. Kofstad, "H i g h - Tem p er at u r e Oxidation of Metals," J o h n Wiley & Sons, Inc., New York (1966). 9. K. R. K i n s m a n and W. L. Winterbottom, Thin Solid Films, 83, 417 (1981). 10. R . A . Bailey and J. M. Skeaff, J. Chem. Eng. Data, 24, 126 (1979).

The Synthesis of Hydrogen Cyanide in a Solid Electrolyte Fuel Cell Nikolas Kiratzis and Michael Stoukides Department of Chemical Engineering, Tufts University, Medford, Massachusetts 02155 ABSTRACT The overpotential and product selectivity characteristics of the high temperature solid electrolyte fuel cell NH3, CH4, HCN, CO, N~, Pt (Rh)lZrO2 (8% Y203)IPt, air was studied at temperatures 800~176 and atmospheric total pressure. The cell selectivity to HCN could exceed 75% with simultaneous generation of -0.01 W/cm 2 of electrical power. Electrochemical supply of oxygen to the fuel side has a positive effect on the cell selectivity to HCN. The cell is a promising candidate for the cogeneration of electric energy and hydrogen cyanide. T h e d o m i n a n t c o m m e r c i a l p r o c e s s for d i r e c t p r o d u c tion of h y d r o g e n cyanide is based on the A n d r u s s o w reaction (1-3) CH4 + NH3 § 3/2 02 --->H C N + 3H~O The ab o v e reaction is highly e x o t h e r m i c (2, 3). A m i x t u r e of m e t h a n e , a m m o n i a , and air is r e a c t e d by p a s s i n g it t h r o u g h a p l a t i n u m - r h o d i u m gauze at 950~176 (2). In a d d i t i o n to H C N and N2 of the air, t h e p r o d u c t s t r e a m c o n t a i n s a p p r e c i a b l e a m o u n t s o f H~, CO, H20 as well as u n r e a c t e d CH4 and NH3. Selectivities currently a c h i e v e d in i n d u s t r y with r es p e c t to NH3 (moles of HCN p r o d u c e d per m o l e of NH3 reacted) are a b o u t 75-80%. More t h a n 300,000 tons of HC N are p r o d u c e d annually in the U n i t e d States alone (3). Due to the high e x o t h e r m i c i t y of the reactio n large a m o u n t s of t h e r m a l energy are generated. O x y g e n ion c o n d u c t i n g solid e l e c t r o l y t e s can be used in h e t e r o g e n e o u s catalytic p r o c e s s e s in o r d e r to e i t h e r study or influence the rates of catalytic oxidations (4-7). F u r t h e r m o r e , t h e a b o v e m a t e r i a l s can be u s e d as solid e l e c t r o l y t e s in h i g h t e m p e r a t u r e fuel cells. S u c h fuel cells operating with CO, H2, or CH4 as the fuel have been t e s t e d for years (8). One e c o n o m i c d i s a d v a n t a g e of t h e a bov e cells is the very low price of the products, i.e. CO2 and H20. H e n c e it w o u ld be very interesting to e x a m i n e the possibility of c o g e n e r a t i o n of electricity and valuable i n d u s t r i a l c h e m i c a l s . On t h e basis of this idea V a y e n a s and his co-workers have already studied the p r o d u c t i o n of NO and styrene in fuel cells using NH3 and ethylbenzene as fuel, r e s p e c t i v e l y (9-12).

In t h e p r e s e n t s t u d y t h e f easi b i l i t y of a p r o c e s s in w h i c h electrical energy and HCN are p r o d u c e d in a solid e l e c t r o l y t e fuel cell is i n v e s t i g a t e d . A m i x t u r e of m e t h a n e and a m m o n i a is used as the fuel.

Experimental Apparatus A schematic diagram of the e x p e r i m e n t a l apparatus is sh o w n in Fig. 1. It consists of a gas feed system, the fuel cell, and t h e analytical system. Th e fuel cell d e s i g n is s h o w n in Fig. 2. It consists of an 8 mole p e r c e n t (m/o) yttria stabilized zirconia t u b e with an id of 16 m m and 1.8 m m wall t h i c k n e s s . Th e zirconia t u b e is o p e n at b o t h ends and enclosed in a 25 m m od, 22 m m id quartz tube (Fig. 2). The reacting m i x t u r e flows in the annulus space b e t w e e n the quartz and the zirconia tube. A p l a t i n u m e l e c t r o d e was d e p o s i t e d on t h e i n s i d e cyl i n d r i c a l su r f ace of t h e zirconia tube. This e l e c t r o d e catalyst was prepared using En g el h ar d Pt ink A-3788 foll o w e d by air d r y i n g and c a l c i n i n g at 1000~ for 4h. This p r o c e d u r e was repeated twice in order to ach i ev e an elect r o d e r e s i s t a n c e of less t h a n 0.30~. Th e a b o v e p l a t i n u m film was e x p o s e d to t h e a m b i e n t air and s e r v e d as t he cat h o d e of the cell. The anodic electrode, w h i ch was ex~ posed to the reacting mixture, was d ep o si t ed on the outside surface of the zirconia t u b e (Fig. 2) using E n g e l h a r d 8826 Rh and 3788 Pt ink. Th e r h o d i u m ink was first dep o s i t e d on t h e zirconia t u b e f o l l o w e d by t h e p r o c e d u r e d e s c r i b e d a b o v e for t h e c a t h o d e p r e p a r a t i o n . T h e n a Pt film was deposited on top of the Rh film. Electrodes prepared in this m a n n e r have been sh o w n (10) to be e v e n l y

Downloaded 06 Nov 2009 to 78.87.74.26. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

1926

August 1987

J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE A N D T E C H N O L O G Y 9

v

8 .

.

.

,

.

I

. . . . . . . . . . . .

I

II m~

~2

'i-~.-~, I

15 --

/

14/

Fig. 1. Schematic diagram of the experimental apparatus. 1 : Reactant gas cylinders. 2: On-off valves. 3: Fuel cell. 4: Lindberg furnace. 5: Furnace temperature controller. 6: Catalyst-electrade temperature measurements. 7: Voltmeter. 8: AM meter. 9: Resistance box. 10: Sixport valve. 11: Four-port valve. 12: Gas chromatograph. 13: Chart recorder. 14" Carrier gas cylinder. 15: Vent.

l,

l'

z

z

U&,--', Fig. 2. Fuel cell configuration. 1 : Feed gas. 2: Zirconia electrolyte. 3: Quartz tubing. 4: Platinum cathodic electrode. 5: Cooling coils. 6: Platinum-rhodium anodic electrode. 7: Off-gas stream.

a p p l i e d a n d p o r o u s w i t h a n a v e r a g e c r y s t a l l i t e size o f 1-2 ~ m a n d a r e a c t i v e s u r f a c e a r e a o f 100 c m 2 / c m 2 o f superficial catalyst area. The electrode-film thickness w a s e s t i m a t e d to b e 5-10 i~m (13). T h e s u p e r f i c i a l s u r f a c e area of the anodic electrode was -10 cm 2 and that of the c a t h o d e - 6 c m 2. All p o w e r v a l u e s r e p o r t e d h e r e a r e based on the area of the cathode. T h e f u e l cell u n i t w a s l o c a t e d i n s i d e a L i n d b e r g 54232 furnace the temperature of which was controlled within 1~176 b y m e a n s o f a L i n d b e r g 59344 t e m p e r a t u r e c o n t r o l u n i t . Cell v o l t a g e a n d c u r r e n t w e r e m e a s u r e d b y t w o digital multimeters. A decade resistance box was used to v a r y cell load. R e a c t a n t s u s e d w e r e A I R C O c e r t i f i e d s t a n d a r d s o f 5% NH3 i n He, 8% CH4 i n H e . a n d 10% O2 i n He. R e a c t a n t s a n d products were analyzed using on line gas chromatograp h y . A m o l e c u l a r s i e v e 5A c o l u m n w a s u s e d to s e p a r a t e H2, N2, 02, NO, CO, a n d CH4 a n d a C a r b o w a x 600 c o l u m n w a s u s e d t o s e p a r a t e NH3, H C N , CO2, a n d HzO. A d d i t i o n a l d e t a i l s o f t h e e x p e r i m e n t a l a p p a r a t u s a p p e a r elsew h e r e (13).

The possibility of homogeneous gas-phase decomposit i o n o f NH3 a n d CH4 w a s i n v e s t i g a t e d b y u s i n g a r e a c t o r i d e n t i c a l to t h a t u s e d as t h e f u e l cell b u t w i t h o u t d e p o s iting catalyst-electrodes on the zirconia tube. Methane a n d a m m o n i a w e r e p a s s e d i n p r e s e n c e a n d in a b s e n c e o f oxygen through the "blank" reactor at temperatures and gaseous compositions similar to those employed in the fuel cell study. At the highest temperature examined, i.e., 970~ t h e h o m o g e n e o u s r e a c t i o n r a t e d i d n o t e x c e e d 8% o f t h e c a t a l y t i c r a t e o f a m m o n i a c o n s u m p t i o n . F o r m e t h a n e , h o m o g e n e o u s c o n s u m p t i o n a c c o u n t e d f o r 1% a t 970~ A t l o w e r t e m p e r a t u r e s t h e e f f e c t o f h o m o g e n e o u s r e a c t i o n s w a s e v e n less i m p o r t a n t . T h e e f f e c t o f e x t e r n a l d i f f u s i o n , i.e., d i f f u s i o n f r o m t h e b u l k g a s e o u s p h a s e to t h e e l e c t r o d e s u r f a c e , w a s s t u d i e d b y v a r y i n g t h e t o t a l f e e d flow r a t e a t c o n s t a n t o u t l e t methane and ammonia composition and measuring the r e a c t i o n r a t e s . A f o u r f o l d i n c r e a s e i n t h e flow r a t e f r o m 250 to 1000 c m 3 / m i n i n c r e a s e s t h e o b s e r v e d r a t e o f r e a c t i o n o f a m m o n i a o n l y b y 20%. H e n c e e x t e r n a l d i f f u s i o n p r o b l e m s m i g h t h a v e a s m a l l e f f e c t a t l o w flow r a t e s b u t a r e c e r t a i n l y o f m i n o r i m p o r t a n c e a t flow r a t e s as h i g h as 1000 c m 3 / m i n . A l m o s t all o u r e x p e r i m e n t s w e r e d o n e a t s u c h h i g h flow r a t e s . U n d e r t h e s e c o n d i t i o n s t h e c o n t a c t time of the gaseous mixture with the anodic electrodec a t a l y s t w a s o f t h e o r d e r o f 5-15 m s (13). I n t r a p e l l e t d i f f u s i o n a l p r o b l e m s w e r e also c a l c u l a t e d to b e i n s i g n i f i c a n t . T h e g e n e r a l i z e d T h i e l e m o d u l u s c a l c u l a t e d (13) acc o r d i n g to t h e W e i s z - P r a t e r c r i t e r i o n w a s r < < 1, w h i c h indicates insignificant concentration gradients inside t h e p o r o u s e l e c t r o d e film. T h i s is j u s t i f i e d b y t h e s m a l l t h i c k n e s s o f t h e film a n d t h e l a r g e a v e r a g e p o r e size. Typical current-potential plots at constant feed comp o s i t i o n a r e g i v e n i n Fig. 3a a n d b. T h e c e l l v o l t a g e o b t a i n e d u n d e r o p e n - c i r c u i t r e a c h e s 1.4V a n d d e c r e a s e s quasi-linearly with increasing current. Figure 3 indicates t h a t a t t h e s e t e m p e r a t u r e s (1000-1200 K) o h m i c o v e r p o t e n t i a l is t h e d o m i n a n t s o u r c e o f p o l a r i z a t i o n . T a k i n g into account the residual resistance of the connecting w i r e s (0.8~) w e c a n c a l c u l a t e a cell r e s i s t a n c e a t v a r i o u s t e m p e r a t u r e s u s i n g t h e s l o p e s o f t h e l i n e s o f Fig. 3. I t w a s f o u n d t h a t t h e r e s i s t a n c e R o f t h e cell c a n b e a p p r o x imated by the expression R = 9.5 10 -4 e x p (10,900/T), ~t

I

CI. J

1.5

0 9SO~ 9 915~ 9 867~

o

817~

9 0

1.0

m& 9

a 9

a

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9 9

9 9

o

o

a

o

a

0.5

9 il

O

0

9

0

b J 0

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1,5

Results The synthesis of hydrogen cyanide was studied at temp e r a t u r e s b e t w e e n 1000 a n d 1250 K, m e t h a n e p a r t i a l p r e s s u r e s ( o u t l e t ) b e t w e e n 0.005 a n d 0.052 bar, a m m o n i a p a r t i a l p r e s s u r e s b e t w e e n 0.0004 a n d 0.030 bar, a n d oxyg e n p a r t i a l p r e s s u r e s b e t w e e n z e r o a n d 0.0003 b a r . H e l i u m w a s u s e d as a d i l u e n t a n d t h e t o t a l p r e s s u r e w a s 1 bar. U n d e r t h e flow r a t e s e m p l o y e d i n t h i s s t u d y t h e f u e l cell, a l t h o u g h t u b u l a r i n s h a p e , b e h a v e d a l m o s t e x a c t l y as a c o n t i n u o u s flow s t i r r e d t a n k r e a c t o r ( C S T R ) . T h i s w a s v e r i f i e d u s i n g a n I n f r a r e d I n d u s t r i e s 702D CH4 i n f r a red analyzer to obtain the residence time distribution f u n c t i o n o f t h e cell (13).

Q-g90 r PCH&. f=O.040 bar PNH3, f=0.022 bar

i

1.0

~e

o

i

A

a

9

9

Q,-60D cm3/mi'n PCH4, f-0.040 bar

O g60~ a 915~

PNH3, f - 0 . 0 2 2 bar

9 9

9

o a

o

9 9

o &

9

0

0.5

o

0

0 0

e

30

867~ B17~

60 T

90

120

150

mA

Fig. 3. Current-voltage behavior for constant gaseous composition. (a) Flow rate = 990 cm3/mln. (b) Flow rate = 600 cm3/min.

Downloaded 06 Nov 2009 to 78.87.74.26. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

Vol. 134, No. 8 T=870~

SYNTHESIS PNH3, f -

0,027 bar

9

HCN~.

PCH4, f = 0,011 b a r

0

N2

9

CO

0

OF HYDROGEN

4 0

0

0

0

E

2

I

o

t-.

u; 0

(V

o

T=g70Oc

6

L

PNH3. f PCH4, f -

0.011 b a r 0,04 bar

9 0

HcNL.b_ N2

9

CO

c" O

-~

4

L) E~ L.

2

0

0

0

0

0

90

=

50

100

I,

150

mA

Fig. 4. Dependence of reaction rates on current density

T h e t e m p e r a t u r e d e p e n d e n c e f o u n d h e r e is v e r y close to t h a t r e p o r t e d by F a r r a n d V a y e n a s (10) for t h e s a m e solid e l e c t r o l y t e u s e d for an NH3-NO fuel cell. F i g u r e s 4 and 5 c o n t a i n t h e e f f e c t of c u r r e n t d e n s i t y on t h e rates of H C N , N2, a n d CO f o r m a t i o n . T h e i n l e t partial p r e s s u r e o f a m m o n i a v a r i e d b e t w e e n 0.005 a n d 0.05 b a r a n d t h a t o f m e t h a n e b e t w e e n 0.004 a n d 0.045 bar. T h e c o n v e r s i o n o f a m m o n i a w a s 20-60% w h i l e t h a t o f m e t h a n e w a s 10-30%. I n a d d i t i o n to t h e u n r e a c t e d NH3 a n d CH4 t h e e f f l u e n t s t r e a m c o n t a i n e d N2, CO, H C N , a n d H~. H y d r o g e n w a s d e t e c t e d b u t c o u l d n o t b e m e a s u r e d a c c u r a t e l y a n d t h e r e f o r e t h e r a t e o f H2 f o r m a t i o n is n o t r e p o r t e d h e r e . T h e e x p e r i m e n t a l e r r o r in t h e m e a s u r e d r e a c t i o n rates o f N2, CO, a n d H C N did n o t e x c e e d 5%. T h e only oxygenated product detected with current densities as h i g h as 20 m A / c m 2 was c a r b o n m o n o x i d e . P r e l i m i n a r y e x p e r i m e n t s s h o w e d t h a t for h i g h e r o x y g e n c o n c e n t r a t i o n s in t h e r e a c t o r f o r m a t i o n o f H20 o c c u r s (26). N e v e r t h e l e s s , n e i t h e r H20 n o r NO w e r e d e t e c t e d in t h e p r o d u c t

CYANIDE

1927

stream of the experiments presented here. The reaction r a t e s o b t a i n e d w e r e c o m p a r a b l e to t h o s e r e p o r t e d in t h e l i t e r a t u r e (17, 2 2 ) c o n c e r n i n g c a t a l y s t l o a d i n g . F i g u r e 4a s h o w s t h e e f f e c t of c u r r e n t d e n s i t y on t h e rates of H C N , N2, a n d CO f o r m a t i o n . T h e rate of CO form a t i o n rco i n c r e a s e s l i n e a r l y w i t h c u r r e n t . B o t h r a t e s rscN a n d rN2 initially i n c r e a s e w i t h c u r r e n t a n d for I > 100 m A b o t h r a t e s s t a r t d e c r e a s i n g . N o t e t h a t e v e n at I = 0, s i g n i f i c a n t a m o u n t s of H C N a n d N2 are f o r m e d i n d i c a t ing t h a t o x y g e n has o n l y an i n d i r e c t role on t h e H C N synt h e s i s r e a c t i o n . F i g u r e 4b c o n t a i n s a g a i n t h e d e p e n d e n c e of rHCN, rN2, a n d rr on c u r r e n t I b u t at d i f f e r e n t t e m p e r a t u r e a n d g a s e o u s c o m p o s i t i o n . T h e b e h a v i o r of rHcN is s i m i l a r to t h a t s h o w n in Fig. 4a w h i l e rN2 s h o w s initially a w e a k d e c r e a s e w i t h i n c r e a s i n g c u r r e n t . A g a i n t h e rate of CO p r o d u c t i o n i n c r e a s e s l i n e a r l y w i t h I. T h i s c a n b e s h o w n in Fig. 5 w h e r e rco is p l o t t e d v s . I. O n t h e s a m e g r a p h t h e c o n t i n u o u s line c o r r e s p o n d s to CO = I/2F, i.e., t h e r a t e of o x y g e n t r a n s p o r t t h r o u g h t h e e l e c t r o l y t e in g r a m a t o m s of o x y g e n p e r s e c o n d . C l e a r l y o x y g e n is conv e r t e d q u a n t i t a t i v e l y to CO on t h e a n o d e . F i g u r e 6 s h o w s t h e d e p e n d e n c e of t h e r e a c t i o n selectivity on t h e g a s e o u s c o m p o s i t i o n in t h e fuel cell. T h e w h i t e p o i n t s c o r r e s p o n d to d a t a o b t a i n e d u n d e r o p e n c i r c u i t (I = 0) w h i l e t h e d a r k p o i n t s c o r r e s p o n d to closedc i r c u i t c o n d i t i o n s w i t h I = 120 m A . T h e s e l e c t i v i t y increases monotonically with increasing the methane-amm o n i a ratio and in g e n e r a l for t h e s a m e g a s e o u s c o m p o sition, S is h i g h e r u n d e r c l o s e d c i r c u i t (dark points). F i g u r e 7 s h o w s t h e e f f e c t of t e m p e r a t u r e on t h e select i v i t y to H C N at c o n s t a n t i n l e t c o m p o s i t i o n . T h e h i g h e s t v a l u e s ( - 8 0 % ) a r e a t t a i n e d at t e m p e r a t u r e s 1150-1200 K and again closed-circuit operation (dark points) has a p o s i t i v e e f f e c t on t h e s e l e c t i v i t y . F i n a l l y Fig. 8 c o n t a i n s t h e d e p e n d e n c e of t h e c e l l v o l t a g e V, t h e s e l e c t i v i t y to H C N S, a n d t h e f u e l cell p o w e r P on c u r r e n t d e n s i t y I. T h e e l e c t r i c a l p o w e r o u t p u t i n c r e a s e s w i t h I u n t i l it r e a c h e s a p o i n t ( - 7 0 m A ) a b o v e w h i c h an i n c r e a s e in I does not affect the generated power any further. The s a m e figure s h o w s t h a t t h e cell c a n be o p e r a t e d at m a x i m u m p o w e r w i t h o u t loss of t h e s e l e c t i v i t y to t h e d e s i r e d product.

Discussion D u e to its i n d u s t r i a l i m p o r t a n c e t h e H C N s y n t h e s i s has b e e n s t u d i e d b y a l a r g e n u m b e r o f i n v e s t i g a t o r s (14-22). T h e m o s t t h o r o u g h k i n e t i c s t u d y was r e p o r t e d rec e n t l y b y H a s e n b e r g a n d S c h m i d t (17-19). T h e a b o v e aut h o r s s t u d i e d t h e H C N s y n t h e s i s on p l a t i n u m (18) a n d on r h o d i u m (17) in p r e s e n c e a n d a b s e n c e o f o x y g e n in t h e f e e d stream. S c h m i d t and H a s e n b e r g f o u n d t h a t t h e beh a v i o r o n R h is v e r y s i m i l a r to t h a t on P t a n d h e n c e t h e s a m e m e c h a n i s m was p r o p o s e d for b o t h catalysts. I n abs e n c e of o x y g e n t h e rate s t e p s can be w r i t t e n as

10 0.8

Z r

o

O

0,6 o

E o

:=0.5

0 ">0.4

0

L

o

o 0J

O

~0.2

0

Open c i r c u i t

9

Closed c i r c u i t

o

0 50

100

I,

150

mA

Fig. 5. Comparison of CO production rate with the rate of electrochemical oxygen supply. Continuous line: I / 2 F = gram-equivalents of O=-transported through the electrolyte per second.

2

4

PcH6,~/PNH 3 8

10

12

Fig. 6. Dependence of reaction selectivity to HCN on gaseous composition. T = 1150 K, White points: open-circuit operation (I = 0). Dark points: closed-circuit operation (I = 120 mA).

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J. E l e c t r o c h e m . Soc.: E L E C T R O C H E M I C A L

1928

SCIENCE

0,8 9

z c) :3:0. 6

9 0

0

0

0

Open c i r c u i t

9

Closed c i r c u i t

2

">0.4

u~0,2

1000'

1100

1200

1300

T, ~ Fig. 7. Dependence of reaction selectivity to HCN on fuel cell temperature. White points: open-circuit operation (I = 0). Dark points: closed-circuit operation (I = 120 mA). CH4 (gas) -* CHx

[1]

NH3 (gas) -* NH~

[2]

CH~+NH~--*HCN+

x+y2

1 H2

[3]

2NH~ --->N2 + yH~

[4]

w h e r e CH~ a n d N H , s t a n d for c a r b o n c o n t a i n i n g a n d nitrogen containing species adsorbed on the catalyst surf a c e (17). C o n s i d e r i n g t h e a b o v e r e a c t i o n s t e p s a n d ass u m i n g t h a t a d s o r b e d c a r b o n s e r v e s b o t h as a c t i v e s p e c i e s f o r H C N f o r m a t i o n a n d i n h i b i t o r o f NH3 a d s o r p t i o n , H a s e n b e r g a n d S c h m i d t w e r e a b l e t o fit t h e i r e x p e r imental data with the following rate equations for HCN a n d Nz

rHCN = K1PcH4PNn3~ r~2 = K3PN.a

1 + K2PcH4P~R~~ "~ 1+

2 ~pN.3

[5]

)

[6]

A u g u s t 1987

AND TECHNOLOGY

O x y g e n is r e p o r t e d (19) to h a v e a n i n d i r e c t role i n t h e mechanism of HCN synthesis since it can compete for t h e a d s o r b e d c a r b o n o n t h e s u r f a c e . A d s o r b e d c a r b o n is b o t h a p o i s o n a n d a p r o m o t e r a n d h e n c e o x y g e n c a n imp r o v e t h e r a t e o f H C N f o r m a t i o n i f t h e s u r f a c e is poisoned too strongly by adsorbed carbon. Conversely, a d s o r b e d c a r b o n c a n i n h i b i t rHCN if it d e s t r o y s t h e c a r b o n l a y e r n e c e s s a r y for H C N f o r m a t i o n (19). T h e p r e s e n t e x p e r i m e n t a l r e s u l t s s e e m to b e in a g r e e m e n t w i t h t h e H a s e n b e r g - S c h m i d t m o d e l . T h e r e is n o doubt that HCN can be formed in the complete absence o f o x y g e n . W h a t r e m a i n s to b e a n s w e r e d is t h e r o l e o f oxygen and in particular the effect of oxygen when supplied either electrochemically or together with the fuel in the gaseous feed stream. Previous studies have shown t h a t o x y g e n s u p p l i e d e l e c t r o c h e m i c a l l y c a n a l t e r significantly the nature and the properties of heterogeneous c a t a l y s t s (23-25). I n v i e w o f t h e q u a n t i t a t i v e a g r e e m e n t between the rate of CO formation and the rate of O = t r a n s p o r t (Fig. 5) o n e c o u l d c o n c l u d e t h a t t h e o n l y elect r o c h e m i c a l r e a c t i o n t h a t o c c u r s a t t h e a n o d e is CH~ + O = --> CO + x/2 H~ + 2e

I n a d d i t i o n to CO t h e o t h e r o x y g e n a t e d p r o d u c t s t h a t c o u l d b e f o r m e d u n d e r t h e s e c o n d i t i o n s a r e NO, H~O, a n d COs. N o n e o f t h e s e t h r e e p r o d u c t s w a s d e t e c t e d i n t h e o f f g a s s t r e a m . N e v e r t h e l e s s , i t is w e l l e s t a b l i s h e d t h a t N O is t h e m a i n p r o d u c t o f t h e o x i d a t i o n o f NH3 u n der conditions similar to those employed in the present study. Farr and Vayenas used a stabilized zirconia fuel cell with platinum electrodes and obtained selectivities to N O as h i g h as 90% (9, 10). T h e r e f o r e it is h i g h l y l i k e l y t h a t N O is f o r m e d i n o u r s y s t e m . S i n c e it is n o t f o u n d i n t h e e f f l u e n t s t r e a m w e h a v e to a s s u m e t h a t it u n d e r g o e s f a s t r e a c t i o n w i t h a d s o r b e d c a r b o n - c o n t a i n i n g s p e c i e s to p r o d u c e H C N a n d CO. H e n c e a n a l t e r n a t i v e s c h e m e o f the anodic reactions could be CH~ + O = --> CO + x/2 H~ + 2e

[8]

NHy + O =--* N O + y/2 H2 + 2e

[9]

CHx + NH~--) H C N +

x+y-1 2

2CHx + N O - * CO + H C N + o

2NH~--* N2 + yH2

>

[7]

H2

2x - 1 H2 2

[10]

[11] [12]

1.0

The above reaction scheme can provide a better interpretation of the effect of electrochemical oxygen supply o n t h e r a t e s o f N2 a n d H C N f o r m a t i o n (Fig. 4-7) s i n c e cove r a g e s o f b o t h s p e c i e s , CHx a n d NH~, a r e a f f e c t e d b y t h e c h a r g e t r a n s f e r r e a c t i o n s . N e v e r t h e l e s s , a n u m b e r of exp e r i m e n t s is still r e q u i r e d i n o r d e r to v e r i f y t h e v a l i d i t y o f e i t h e r t h e k i n e t i c m o d e l b a s e d o n E q . [1]-[7] or t h a t b a s e d o n Eq. [8]-[12]. W o r k t o w a r d s t h a t g o a l is a l r e a d y i n p r o g r e s s (26).

o

-;

0.5

8

o

1.0

Conclusions

0.8

0.08

Z 0 '1-

,-. 0 . 0 6

0.6

~

0.4

"~

g o 0.04 o @ u

9

0.2

0.02

g~

.L~I

0

0

30

60

90 mA

120

150

I, Fig. 8. Effect of current density on cell voltage, selectivity to HCN, and power output.

Hydrogen cyanide can be produced in a solid electrolyte fuel cell with simultaneous generation of electrical e n e r g y . To o u r k n o w l e d g e t h i s is t h e first a t t e m p t t o s y n t h e s i z e H C N i n a f u e l cell. T h e p r o d u c t s e l e c t i v i t y is a f u n c t i o n o f g a s e o u s c o m p o sition, temperature, and current density. For the curr e n t s t e s t e d so far t h e o n l y o x y g e n a t e d p r o d u c t f o r m e d is CO. I n t h e p r e s e n t e x p e r i m e n t s t h e d o m i n a n t s o u r c e o f pol a r i z a t i o n is o h m i c . A l s o t h e cell p o w e r a n d t h e p r o d u c t selectivity can both be high in a wide range of current density. Considering the above two observations one can c o n c l u d e t h a t t h e p e r f o r m a n c e o f t h i s f u e l cell c a n b e i m p r o v e d s i g n i f i c a n t l y b y a p p r o p r i a t e d e c r e a s e o f t h e zirconia electrolyte resistance. This will increase by at least one order of magnitude the electrical power output while the high selectivity to HCN will hopefully remain unaffected.

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Vol. 134, No. 8

SYNTHESIS

OF HYDROGEN

Acknowledgment We gratefully a c k n o w l e d g e th e U.S. D e p a r t m e n t of Ene r g y for s u p p o r t of this r e s e a r c h u n d e r G r a n t no. DE-FG02-84ER13219. M a n u s c r i p t s u b m i t t e d Aug. 5, 1986; r e v i s e d m a n u script r e c e i v e d Jan. 23, 1987.

T u f t s U n i v e r s i t y assisted in m e e t i n g the p u b l i c a t i o n costs of this article. REFERENCES 1. L. Andrusow, Genie Chim., 86, 39 (1961). 2. C. N. Satterfield, " H e t e r o g e n e o u s Catalysis in Practice," McGraw-Hill, Inc., N e w York (1980). 3. Kirk-Othmer, " E n c y c l o p e d i a of C h e m i c a l Technology," 3rd ed., J o h n Wiley & Sons, Inc., N e w York (1978). 4. T. Etsell and S. N. Flengas, This Journal, 118, 1980 (1971). 5. S. P a n c h a r a t n a m , R. A. Huggins, and D. M. Mason, ibid., 122, 869 (1975). 6. I. S. Metcalfe and S. Sundaresan, Chem. Eng. Sci., 41 (4), 1109 (1986). 7. L.M. Rincon-Rubio, B. C. Nguyen, and D. M. Mason, This Journal, 132, 2219 (1985). 8. T. Etsell and S. Flengas, Chem. Rev., 70, 339 (1970).

1929

CYANIDE

9. C. G. Vayenas and R. D. Farr, Science, 2{}8, 593 (1980). 10. R. D. F a r r and C. G. Vayenas, This Journal, 127, 1478 (1980). 11. J. Michaels and C. G. Vayenas, ibid., 131, 2544 (1984). 12. J. Michaels and C. G. Vayenas, J. Catal., 85, 477 (1984). 13. N. Kiratzis, MS Thesis, Tufts University, Medford, MA (1987). 14. B. Y. K. Pan and R. G. Roth, Ind. Eng. Chem. Proc. Des. Dev., 7 (1), 53 (1968). 15. B. Y. K. Pan, J. Catal., 21, 27 (1971). 16. E. Koberstein, Ind. Eng. Chem. Proc. Des. Dev., 12, 44 (1973). 17. D. H a s e n b e r g and L. D. Schmidt, J. Catal., 91, 116 (1985). 18. D. Hasenberg and L. D. Schmidt, ibid., 97, 156 (1986). 19. D. Hasenberg, Ph.D. Thesis, University of Minnesota, Minneapolis (1985). 20. L. D. S ch m i d t and D. Luss, J. Catat., 22, 169 (1971). 21. B. Y. K. Pan and R. G. Roth, ibid., 21, 27 (1971). 22. M. P. Suarez and D. G. Loftier, ibid., 97, 240 (1986). 23. S. S e i m a n i d e s and M. Stoukides, This Journal, 133, 1536 (1986). 24. K. Otsuka, S. Yokoyama, and A. Morikawa, Bull. Chem. Soc. Jpn., 57, 3286 (1984). 25. K. Otsuka, S. Yokayama, and A. Morikawa, Jpn. Chem. Lett., 3, 319 (1985). 26. N. Kiratzis and M. Stoukides, In preparation.

Multicomponent Phase Diagrams for Battery Applications I. Phase Diagram and Trajectory Calculations T. L. Aselage* and E. E. Hellstrom *'1 Sandia National Laboratories, Albuquerque, New Mexico 87185 ABSTRACT This article describes a simple method for calculating isothermal, isobaric phase diagrams for m-component systems in which the phases present can be considered as stoichiometric compounds. Throughout, it emphasizes battery applications for which it presents equations to determine the following: the equilibrium m-phase region to which a given overall c o m p o s i t i o n belongs, the c o m p o s i t i o n at w h i c h a d d i n g a single c o m p o n e n t m o v e s the system from one e q u i l i b r i u m m-phase region into another, and the voltage of each equilibrium m-phase region in a particular electrochemical cell. Additionally, it describes the concept of trajectories in a phase diagram, which is useful in understanding charge and discharge paths in a battery with an m-component electrode, and chemical pretreatment of electrodes in batteries. In this article we p r e s e n t s i m p l e m e t h o d s for calculat i n g p h a s e r el at i o n s in m - c o m p o n e n t s y s t e m s and t h o s e p r o p e r t i e s o f t h e p h a s e d i a g r a m t h a t are of i n t e r e s t in b a t t e r i e s . As such, our t r e a t m e n t f o c u s e s on p o i n t comp o s i t i o n solid p h a s e s s u c h as are f o u n d in b a t t e r y electrodes, as well as ideal gases with w h i c h they m a y be in e q u i l i b r i u m . Of particular interest are the phases that f o r m in t h e e l e c t r o d e s d u r i n g b a t t e r y d i s c h a r g e (or charge), t h e v o l t a g e of each c o m p o s i t i o n a l r e g i o n that the e l e c t r o d e traverses, and t h e c o m p o s i t i o n at w h i c h p h a s e c h a n g e s o c c u r in th e e l e c t r o d e s d u r i n g d i s c h a r g e (or charge). T h e s e p h a s e c h a n g e s are t y p i c a l l y a c c o m p a n i e d by a s h arp c h a n g e of v o l t a g e in a plot of v o l t a g e vs. c h a r g e d e l i v e r e d . We d e s c r i b e m e t h o d s for calculating each of these quantities as well as the constant voltage theoretical capacity of each region. Finally, we conside r t h e p o s s i b i l i t y of c h a n g i n g the initial p h a s e c o m p o s i t i o n of e l e c t r o d e s by p r e t r e a t m e n t w i t h c h e m i cal additives. T h e o n l y a s s u m p t i o n that we m a k e in th e f o l l o w i n g discussion is that each phase is constant in composition, i.e., the s t o i c h i o m e t r y of solids is not variable and gases are pure. This a s s u m p t i o n allows t h e n u m b e r of moles of e a c h p h a s e p r e s e n t at a g i v e n overall c o m p o s i t i o n , and h e n c e t h e s y s t e m G ib b s free e n e r g y at the o v e r a l l composition to be d e t e r m i n e d by simple linear expressions. While t h e a s s u m p t i o n is r e s t r i c t i v e , in m a n y cases bat-

*Electrochemical Society Active Member.

'Present address: Department of Metallurgical and Mineral Engineering, University of Wisconsin, Madison, Wisconsin 53706.

t e r y e l e c t r o d e s c o r r e s p o n d to such c o n d i t i o n s . F o r syst e m s w h er e multispecies phases are present, such as gas or l i q u i d m i x t u r e s , or w h e r e solid p h a s e s o f v a r i a b l e c o m p o s i t i o n are present, a n u m b e r of a l g o r i t h m s h a v e b e e n d e v e l o p e d for p h ase and r e a c t i o n e q u i l i b r i u m calculations. S m i t h (1) provides a review of the various app r o a c h e s to the p r o b l e m . F o r our p u r p o s e s , it is suffic i e n t to note that each i n v o l v e s an i t e r a t i v e s o l u t i o n to t h e Gibbs free energy m i n i m i z a t i o n p r o b l e m since G for the system is no longer a linear function of m o l e n u m be r .

Phase Diagram Calculations C o n s i d e r o n e m o l e of at o m s in an m - c o m p o n e n t syst e m w i t h specified overall c o m p o s i t i o n xl, x~, x3, 9 9 xm, w h e r e x~ is t h e m o l e f r a c t i o n of c o m p o n e n t A, x2 is t he m o l e fraction of c o m p o n e n t B, etc. In most cases, the ind e p e n d e n t c o m p o n e n t s are t h e e l e m e n t s p r e s e n t in the system. At fixed t e m p e r a t u r e and p r essu r e, t h e G i b b s p h a s e rule tells us that for an m - c o m p o n e n t sy st em, as m a n y as m phases may coexist in equilibrium. Let a part i c u l a r set of m - p h a s e s in an m - c o m p o n e n t s y s t e m be r e p r e s e n t e d by the chemical formulas A~iB~C~ . .. M.~, i = 1, m. For the overall c o m p o s i t i o n specified above, the m o l e fraction of c o m p o n e n t A, for example, is given by mini = xl

[1]

Here ni is the u n k n o w n n u m b e r of moles of phase i, i = 1, m. T h e r e are m of t h e s e e q u a t i o n s , one for each c o m p o nent, that can be written as

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