Carbon Dioxide Reduction with an Electric Field Assisted Hydrogen

Chapter

11

Carbon Dioxide Reduction with an Electric Field Assisted Hydrogen Insertion Reaction W. M . Ayers and M . Farley

Downloaded by UNIV OF GUELPH LIBRARY on May 31, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch011

Electron Transfer Technologies

Inc., Princeton, NJ

08542

A method of chemically synthesizing reduced products including methanol from carbon dioxide and hydrogen has been developed. The method u t i l i z e s a metal hydride f o i l membrane as a continuous source of reactive surface hydrogen atoms and an electrostatic f i e l d to enhance the adsorption of carbon dioxide and bicarbonate onto the hydrogen rich surface. The subsequent chemical(rather than electrochemical) reaction between the adsorbed carbon dioxide and surface hydrogen/metal hydride results in the formation of reduced products. E l e c t r o c a t a l y s i s a t metal e l e c t r o d e s i n aqueous (1,2) a n d n o n - a q u e o u s (^3) s o l v e n t s , p h t h a l o c y a n i n e (4_) a n d r u t h e n i u m (J5) c o a t e d c a r b o n , η - t y p e s e m i c o n d u c t o r s (6,7,8),and p h o t o c a t h o d e s (9,10) have been explored i n an e f f o r t t o d e v e l o p effective catalysts f o r the s y n t h e s i s o f reduced p r o d u c t s from carbon d i o x i d e . The e l e c t r o c a t a l y t i c and p h o t o c a t a l y t i c approaches have high faradaic e f f i c i e n c y of carbon dioxide reduction (1,6), but very low c u r r e n t d e n s i t i e s . Hence t h e r a t e o f p r o d u c t formation i s low. Increasing current densities to p r o v i d e m e a n i n g f u l amounts o f p r o d u c t , s u b s t a n t i a l l y reduces carbon dioxide reduction i n favor of hydrogen evolution . This reduction i n current efficiency i s a d i f f i c u l t problem t o surmount i n l i g h t o f t h e p r o b a b l e e l e c t r o s t a t i c r e p u l s i o n of carbon dioxide, or the aqueous bicarbonate i o n , from a n e g a t i v e l y charged cathode (11,12). In an e f f o r t t o overcome t h e l i m i t a t i o n s o f e l e c t r o c h e m i c a l and p h o t o e l e c t r o c h e m i c a l cathodic r e d u c t i o n o f c a r b o n d i o x i d e , and t o draw upon i n s i g h t s of hydrogen i n s e r t i o n r e a c t i o n s s t u d i e d i n homogeneous

0097-6156/88/0363-0147$06.00/0 €> 1988 American Chemical Society

American Chemical Society Library

1155 16th St., N.W. In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Washington, D.C. Society: 20036Washington, DC, 1988.

148

CATALYTIC ACTIVATION OF CARBON DIOXIDE

c a t a l y s i s ( J J 3 ) , we h a v e d e v e l o p e d a m e t h o d t h a t e l e c t r o s t a t i c a l l y adsorbs carbon dioxide/bicarbonate at a p o s i t i v e l y c h a r g e d s u r f a c e o f a h y d r o g e n atom t r a n s m i s s i v e , m e t a l h y d r i d e f o i l where t h e c a r b o n dioxide undergoes chemical r e d u c t i o n by t h e s u r f a c e a t o m i c h y d r o g e n / m e t a l h y d r i d e (_1J4) .


Experimental

Procedures

A m o d i f i c a t i o n o f a b i p o l a r membrane e l e c t r o d e c o n f i g u r a t i o n , of the type used i n d i f f u s i o n s t u d i e s , i s s h o w n i n F i g u r e 1. On t h e l e f t s i d e o f a m e t a l hydride foil, such as palladium, a constant current electrolysis o f a IN s u l f u r i c a c i d p r o d u c e s a h i g h c o n c e n t r a t i o n o f h y d r o g e n a t t h a t s u r f a c e o f t h e f o i l membrane. Some o f the hydrogen e n t e r s t h e p a l l a d i u m as atomic hydrogen. The atomic hydrogen d i f f u s e s across the f o i l to the o p p o s i t e s i d e due t o t h e hydrogen concentration g r a d i e n t a c r o s s t h e membrane ( 1 5 ) . On t h e r i g h t s i d e o f t h e m e m b r a n e , a 0.1 M s o d i u m bicarbonate s o l u t i o n (saturated with carbon dioxide) i s in contact with the palladium. A p o t e n t i o s t a t holds this s i d e o f t h e membrane a t a c o n s t a n t p o t e n t i a l with respect to a Ag/AgCl r e f e r e n c e e l e c t r o d e . A platinum counter electrode completes the c i r c u i t . The p o t e n t i a l on t h e c a r b o n d i o x i d e s i d e o f t h e m e m b r a n e i s h e l d a t a p o s i t i v e p o t e n t i a l with respect to the counter electrode such that the a d s o r p t i o n of carbon d i o x i d e as bicarbonate a n i o n c a n be e n h a n c e d by t h e e l e c t r i c field b e t w e e n t h e m e t a l h y d r i d e membrane a n d c o u n t e r electrode. The o v e r a l l r e a c t i o n i s e l e c t r o c h e m i c a l i n j e c t i o n h y d r o g e n i n t o t h e l e f t s i d e o f t h e membrane: H+

+ M +

e- =

MH

and r e a c t i o n o f t h e a t o m i c opposite side of the metal products : HC0 ~/C0 (ads)

+

C0

2

+

4MH

HCOH

C0

2

+

6MH

3

2

where

M/MH

= =

of

2MH

CH 0H 3

indicate

= HC00H +

+

hydrogen that d i f f u s e s to the hydride f o i l t o form reduced

+

2 M

H 0 2

H 0 2

the metal/metal

hydride

surface

I d e a l l y , t h e r e s h o u l d be no e l e c t r o c h e m i c a l o x i d a t i o n r e a c t i o n s on t h e c a r b o n d i o x i d e s i d e o f t h e membrane. H e n c e , t h e power r e q u i r e m e n t w o u l d o n l y be t h a t r e q u i r e d to m a i n t a i n a c a p a c i t i v e e l e c t r o s t a t i c f i e l d (no

In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

11.

AYERS AND FARLEY

Hydrogen Insertion Reaction

149


(1a)

(1b)

F i g u r e 1. ( a ) C e l l a r r a n g e m e n t f o r m e t a l h y d r i d e hydrogen insertion reaction. Left side, acid reduction and h y d r o g e n atom i n c o r p o r a t i o n i n p a l l a d i u m ( P d ) f o i l membrane. R i g h t side, electrostatic field for enhancement o f c a r b o n d i o x i d e / b i c a r b o n a t e a d s o r p t i o n on f o i l membrane, ( b . ) Blow-up o f p a l l a d i u m / h y d r i d e foil showing hydrogen i n s e r t i o n i n t o carbon d i o x i d e .


150


c u r r e n t ) . T h e p o l a r i z a t i o n o f t h e s u r f a c e s h o u l d be s u f f i c i e n t to adsorb the carbon dioxide/bicarbonate anion but l e s s than that r e q u i r e d to e l e c t r o c h e m i c a l l y oxidize the surface hydride to protons:


MH

= M + H

+

+ e

Therefore, a d i e l e c t r i c f l u i d with high carbon dioxide s o l u b i l i t y and a n o n - r e a c t i v e s a l t ( e . g . N a C l ) t o p r o v i d e a d o u b l e l a y e r , w o u l d be a b e t t e r c h o i c e o f f l u i d f o r t h i s r e a c t i o n . We f i n d t h a t i n t h e e l e c t r o l y t e u s e d i n t h e s e e x p e r i m e n t s t h e r e i s MH o x i d a t i o n c u r r e n t a s t h e membrane i s b i a s e d t o p o s i t i v e p o t e n t i a l s a b o v e t h e metal hydride r e s t potential. The m e t a l h y d r i d e m a t e r i a l i n t h e s e e x p e r i m e n t s i s a 25 m i c r o n thick palladium f o i l ( 99.999 p u r i t y , Alfa). The d i f f u s i o n c o e f f i c i e n t o f h y d r o g e n t h r o u g h palladium h a s b e e n m e a s u r e d by e l e c t r o c h e m i c a l a n d g g s p h a s e techniques and i s a p p r o x i m a t e l y 1 . 6 e - 7 cm / s e c a t 20 C ( 1 6 ) . Tije e x p o s e d p a l l a d i u m membrane s u r f a c e a r e a i s 2.29 cm . A p l a t i n u m w i r e ( a r e a 0 . 0 8 cm ) i s t h e c o u n t e r e l e c t r o d e f o r t h e c a p a c i t i v e f i e l d on t h e c a r b o n dioxide side. A Nafion p o l y m e r membrane s e p a r a t e s the counter e l e c t r o d e compartment from the p a l l a d i u m foil. The s e p a r a t o r i s n o t n e c e s s a r y f o r t h e p r o c e s s . H o w e v e r , i t was u t i l i z e d i n t h e s e e x p e r i m e n t s a s a p r e c a u t i o n a g a i n s t d i f f u s i o n i n t o t h e r e a c t i o n chamber of p r o d u c t s t h a t c o u l d t h e o r e t i c a l l y be f o r m e d a t t h e p l a t i n u m w i r e c o u n t e r e l e c t r o d e . Only t r a c e amounts c o u l d t h e o r e t i c a l l y be f o r m e d c a t h o d i c a l l y a t t h e counter e l e c t r o d e since hydrogen e v o l u t i o n i s favored over carbon d i o x i d e on2this metal ( J J , and t h e s u r f a c e a r e a i s s m a l l ( 0 . 0 8 cm ) . Results The c o n c e n t r a t i o n o f c a r b o n d i o x i d e r e d u c t i o n products ( f o r m i c a c i d , f o r m a l d e h y d e , and methanol) i n the r e a c t o r a t t h e end o f each r u n i s l i s t e d i n T a b l e 1. T h e m e t h a n o l c o n c e n t r a t i o n was d e t e r m i n e d by gas c h r o m a t o g r a p h y u s i n g a P o r p a c Τ column. F o r m i c acid was d e t e r m i n e d w i t h an D i o n e x i o n c h r o m a t o g r a p h y column, a n d t h e f o r m a l d e h y d e was d e t e r m i n e d with a c o l o r i m e t r i c chromatropic a c i d a n a l y s i s (_17.) . We d i d n o t examine t h e s o l u t i o n or gas phase f o r methane. D u r i n g e a c h r u n , t h e membrane i s e l e c t r o c h e m i c a l l y loaded with hydrogen from the l e f t s i d e with a constant electrolysis c u r r e n t o f 3 0 mA. T h e i n i t i a l p o t e n t i a l o f the p a l l a d i u m / p a l l a d i u m hydride ( v s . Ag/AgCl r e f e r e n c e e l e c t r o d e ) on c a r b o n d i o x i d e r e a c t i o n s i d e a t t h e s t a r t of t h e r u n i s d e f i n e d a s E . T h i s p o t e n t i a l d e p e n d s on t h e h y d r i d e c o n t e n t o f t h e membrane a n d t h e e q u i l i b r i u m between t h e metal h y d r i d e / b i c a r b o n a t e solution. !


11.

AYERS AND FARLEY

151

Hydrogen Insertion Reaction

The p o t e n t i o s t a t on t h e c a r b o n d i o x i d e s i d e i s e i t h e r l e f t i n a f l o a t i n g mode ( E ) o r s e t a some v a l u e p o s i t i v e (more o x i d i z i n g ) t h a n E*. T h i s p o t e n t i a l i s d e f i n e d as E". The d i f f e r e n c e between these two p o t e n t i a l s ( E " - E ) i s c a l l e d d e l t a V i n the t a b l e . T h e r e a c t i o n s i d e s o l u t i o n i s 0.1 M NaHC03 s a t u r a t e d with C02 a t 1 atm. Each run l a s t e d 3 to 4 hours (specific times i n t a b l e ) . T

!

T h e maximum r a t e o f m e t h a n o l f o r m a t i o n i s 3.9e-10 m o l e s / c m - s e c . T h i s o c c u r s a t ( E " - E ) o f 1.07 volts. The formation^rate corresponds to a turnover r a t e of ( a s s u m i n g 10 sites/cm ).


f

2.3

!

The e f f e c t o f t h e membrane p o t e n t i a l d i f f e r e n c e , ( E " - E ) , on t h e r a t e o f m e t h a n o l f o r m a t i o n i s shown i n F i g u r e 2. As t h e p o t e n t i a l i s i n c r e a s e d t o more p o s i t i v e p o t e n t i a l s above the r e s t p o t e n t i a l , Ε , of the palladium/palladium hydride membrane, o x i d a t i o n of the s u r f a c e h y d r o g e n / p a l l a d i u m h y d r i d e to protons, and o x i d a t i o n o f r e d u c e d carbon d i o x i d e i n t e r m e d i a t e s can occur to reduce yields. H e n c e t h e r e i s some o p t i m u m p o t e n t i a l a t w h i c h t o h o l d t h e membrane p o t e n t i a l t o maximize the r a t e of methanol f o r m a t i o n . From t h i s p r e l i m i n a r y data, i t appears t h a t the r a t e of methanol f o r m a t i o n d e c r e a s e s f o r E " m o r e p o s i t i v e t h a n 0.4 vs. Ag/AgCl. 1

Discussion We do n o t k n o w f r o m t h e s e e s p e r i m e n t s i f c a r b o n dioxide or b i c a r b o n a t e i s t h e more r e a c t i v e s p e c i e s w i t h t h e metal hydride. The i n f e r e n c e from the i n c r e a s e i n r a t e with increasing p o t e n t i a l suggests that bicarbonate is the i n t e r m e d i a t e . However, t h e r e i s a chemical e q u i l i b r i u m b e t w e e n c a r b o n d i o x i d e and b i c a r b o n a t e . Hence t h e r a t e d e p e n d e n c e o n p o s i t i v e p o t e n t i a l may just r e f l e c t i n c r e a s e d s u r f a c e coverage of bicarbonate f o l l o w e d by e q u i l i b r a t i o n w i t h c a r b o n d i o x i d e . I t i s i n t e r e s t i n g t o c o n s i d e r a h y d r o g e n mass b a l a n c e in t h e r e a c t i o n . T h e r a t i o o f t h e MH/H+ o x i d a t i o n c u r r e n t ( T a b l e 1) t o t h e h y d r o g e n f e e d current ( 3 0 MA) v a r i e s f r o m no MH o x i d a t i o n t o 9 2 % theoretical o x i d a t i o n ( a s s u m i n g n o n e o f t h e c u r r e n t i s due to reduced product oxidation). The maximum r a t e o f m e t h a n o l formation (3.9e-10 moles/cm -sec) occured i n a run that l a s t e d 10440 s e c o n d s . Hence t h e t o t a l h y d r o g e n i n j e c t e d i n t o t h e m e m b r a n e a t 30mA w o u l d be 3.25 e-3 m o l e s o f a t o m i c h y d r o g e n ( a s s u m i n g no l o s s f r o m h y d r o g e n evolution). I f a l l of t h i s atomic hydrogen reacted w i t h t h e c a r b o n d i o x i d e on t h e o p p o s i t e s i d e o f the m e m b r a n e , t h e CH^OH c o n c e n t r a t i o n w o u l d be 962 ppm. S i n c e t h e r a t i o o f f e e d h y d r o g e n t o MH oxidation in 2


152


Table 1 Rate data for CH30H _

^

^

i n i t i a l applied E"-E' wolts) (volts)


1.2ft NaHC03/CQ2 duration HCOOH

oxid. («Λ)

pH 7.6 HCOH CH3ÛH

(sec) (ppm) ippw)

(ppm)

HCOOH rate

H2C0

CH3GH

rate rate (gwoles/cw2-sec)

-0.41

0.30

1.30

27.5 12300

7. 73

(.11 13.81 1.02E-10

Carbon Dioxide Reduction with an Electric Field Assisted Hydrogen

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