Electrocatalytic Carbon Dioxide Reduction - American Chemical Society

Chapter

6

Downloaded by NORTH CAROLINA STATE UNIV on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch006

Electrocatalytic Carbon Dioxide Reduction B. Patrick Sullivan, Mitchell R. M . Bruce, Terrence R. O'Toole, C. Mark Bolinger, Elise Megehee, Holden Thorp, and Thomas J. Meyer University of North Carolina at Chapel Hill, Chapel Hill, NC 27514

This paper reviews recent work on the development of electrocatalysts for CO reduction. Comparison of our electrocatalysts based on polypyridine complexes of the second and third row transition metals is made with previous work, and both areas are set in the framework of the known chemistry and electrochemistry of both uncoordinated CO and CO -transitior metal complexes. The emphasis of our work has been on mechanistic questions. For example, the family of complexes fac-[Re (bpy)(CO) L]n+ (where bpy is 2

2

2

I

3

-

-

2, 2'-bipyridine and L is Br , Cl or CH CN) are facile stoichiometric or catalytic reagents that reduce CO to CO, formate, or oxalate depending on the external conditions. Synthesis, electrochemical, and kinetic studies implicate the involvement of a minimum of five different pathways for this unusual system. A newly discovered electrocatalyt is the reactive metal hydride, [Os(bpy) (CO)H] , that has been found to reduce CO by an associative mechanism yielding either CO or formate from a common intermediate. Related kinetic studies of fundamental steps in CO activation or reduction have been conducted and their relationship to electrocatalytic CO reduction has been 3

2

-

2

2

2

2

0097-6156/88/0363-0052$ 10.75/0 © 1988 American Chemical Society In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

β.

SULLIVAN ET AL.

Electrocatalytic

53

CO Reduction g

highlighted. Examples include CO insertion into a metal-alkoxide or metal-hydride bond. Finally, chemically modified electrodes have been prepared which allow the transposition of solution electrocatalytic chemistry to electrode surfaces. Although these studies are in their infancy it appears that new products (e.g., oxalate), and therefore new mechanistic pathways, have been found for some of the surface immoblized electrocatalsyts. Downloaded by NORTH CAROLINA STATE UNIV on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch006

2

Photosynthetic r e d u c t i o n of carbon dioxide i s a f a c i l e n a t u r a l p r o c e s s e v e n t h o u g h t h e c h e m i s t r y i s complex a n d m u l t i e l e c t r o n steps are required. R e c e n t work h a s d e m o n s t r a t e d t h a t m e t a l c o m p l e x e s p l a y a c r u c i a l r o l e (1) and i t i s a n t i c i p a t e d t h a t s t u d y o f t h e homogeneous s o l u t i o n c h e m i s t r y o f C 0 and i t s metal complexes w i l l 2

o f f e r valuable clues to the mechanistic steps involved i n biogenic C 0 reduction. I n f a c t , a s i n many p r o b l e m s i n 2

chemical r e a c t i v i t y , a d e t a i l e d understanding of mechanism i s k e y t o t h e d e s i g n o f new c a t a l y t i c s y s t e m s . A m e c h a n i s t i c emphasis s h o u l d a l s o suggest advances i n o t h e r a r e a s s u c h a s r o u t e s t o new e n e r g y s o u r c e s , s y n t h e t i c schemes f o r i n d u s t r i a l c h e m i c a l s , a n d methods f o r the removal o f C 0 as an atmospheric contaminant. 2

Even though d e t a i l e d C 0 uncertain,

and d e s p i t e

2

r e d u c t i o n mechanisms a r e

the fact

that

metal-C0

transition

2

chemistry has developed s l o w l y s i n c e the d i s c o v e r y o f the f i r s t complexes, c a t a l y s t s f o r a b i o g e n i c C 0 r e d u c t i o n , 2

e s p e c i a l l y e l e c t r o c a t a l y s t s , have b e e n f o u n d ( 2 / 1 ) . Our own work i n t h i s a r e a h a s l e d t o t h e d i s c o v e r y o f a s e r i e s of e l e c t r o c a t a l y s t s a c t i v e f o rthe reduction of C0 t o CO, o r f o r m a t e (2â,k,£, s_, wj . More i m p o r t a n t l y , a 2

fundamental grasp o f chemical and e l e c t r o c h e m i c a l p a t h w a y s a n d i n t e r m e d i a t e s f o r a few s e l e c t c a s e s h a s b e e n a c h i e v e d , a n d i n one i n s t a n c e , t h a t o f C 0 insertion 2

i n t o a m e t a l h y d r i d e complex, a d e t a i l e d m e c h a n i s t i c p i c t u r e h a s emerged. These s t u d i e s on C 0 r e d u c t i o n a r e p a r t o f o u r 2

larger

effort

on t h e a c t i v a t i o n and redox c h e m i s t r y o f 2s m a l l molecules and ions such as N 0 , NH^, 0 a n d H 0 (4). Some l o n g r a n g e g o a l s o f t h e work a r e t o d e v e l o p a n understanding o f t h e e s s e n t i a l s y n t h e t i c and m e c h a n i s t i c chemistry leading to reduction of carbon dioxide to formate, formaldehyde, methanol and o x a l a t e and t o u s e t h e r e s u l t s t o d e v e l o p new t r a n s i t i o n m e t a l 3

2

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

2

CATALYTIC ACTIVATION OF CARBON DIOXIDE

54

e l e c t r o c a t a l y s t s which w i l l e x h i b i t high product s e l e c t i v i t y , o p e r a t e c l o s e r t o t h e thermodynamic p o t e n t i a l o f t h e a p p r o p r i a t e redox process, and t o a c h i e v e t h e c a t a l y z e d r e d u c t i o n under c o n d i t i o n s o f h i g h current density. F o l l o w i n g a s h o r t r e v i e w o f t h e thermodynamics o f C0 r e d u c t i o n , C0 r e d u c t i o n a t m e t a l o r c a r b o n 2

2

e l e c t r o d e s , 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 r e d u c t i o n a t s e m i c o n d u c t o r e l e c t r o d e s , C 0 - m e t a l complex Downloaded by NORTH CAROLINA STATE UNIV on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch006

2

r e a c t i v i t y , a n d t h e p r o p e r t i e s o f r e l a t e d homogeneous s o l u t i o n e l e c t r o c a t a l y t i c s y s t e m s , we w i l l d e s c r i b e t h e c u r r e n t s c o p e o f o u r a t t e m p t s t o d e v e l o p new C0 2

e l e c t r o c a t a l y s t s and d e s c r i b e mechanistic studies.

the r e s u l t s o f our

THERMODYNAMICS OF CARBON DIOXIDE REDUCTION. A u s e f u l summary o f t h e t h e r m o d y n a m i c s o f C0 r e d u c t i o n t o 2

o n e - c a r b o n f r a g m e n t s i n aqueous s o l u t i o n u n d e r b a s i c a n d a c i d i c c o n d i t i o n s i s shown i n t h e L a t i m e r - t y p e d i a g r a m i n Scheme 1 ( £ ) . When r e f e r r i n g t o t h e d i a g r a m below, r e c a l l t h a t a n e g a t i v e p o t e n t i a l means t h a t t h e r e d u c e d form o f t h e c o u p l e i s a b e t t e r r e d u c i n g agent than H / and c o n v e r s e l y , a p o s i t i v e v a l u e i n d i c a t e s t h a t t h e o x i d i z e d form i s a b e t t e r o x i d i z i n g agent than t h e p r o t o n a t t h e s p e c i f i e d pH. 2

C0

«J±J£*

2

CO 2

C0 3

C

Q

^ ,-0.01,

H

0

4

^19^

C

H

3

0

H

4

_0 58^

^

1

HC0 " ,- - ° , H C=0 + = 1 ± . CH3OH ^ l i * 7

2

In a c i d i c

2

1.

redox couple,

(Values

in

4

Volts)

o f C0

endergonic with

while

CH

OH ]

solution reduction

a c i d o r CO i s s l i g h t l y 2

=

+

Scheme

H /H

C

[1M H ]

-O.l]

[1M

+

2

reduction

2

to either

respect

formic

to the

t o methane i s

a c t u a l l y spontaneous. Even though t h e v a r i o u s r e d u c t i o n s are a c c e s s i b l e a t reasonable p o t e n t i a l s t h e i r k i n e t i c b a r r i e r s c a n be q u i t e s e v e r e , c o n s e q u e n t l y , s u b s t a n t i a l o v e r p o t e n t i a l s c a n be i n c u r r e d a t t h e e l e c t r o d e s u r f a c e .


β.

SULLIVAN ET AL.

Electrocatalytic

C0

55

Reduction

2

ELECTROCHEMISTRY Of QQ REDUCTION. PRQPVCI? ANP OVfifiPQTSNTIAfr REQUIREMENT? * A number o f e l e c t r o c h e m i c a l 2

s t u d i e s a t m e t a l o r c a r b o n e l e c t r o d e s have documented t h e large overvoltages required f o r C 0 reduction, both f o r 2

a q u e o u s a n d non-aqueous m e d i a (JS) . T y p i c a l r e d u c t i o n p o t e n t i a l s r e q u i r e d a t e i t h e r P t o r Hg w o r k i n g e l e c t r o d e s a r e ; -2.21V f o r d i m e t h y l f o r m a m i d e , -2.16V f o r H 0 a t pH 2

7,

a n d -2.2 t o -2.7V f o r CH.CN ( u s i n g alkylammonium

salts


Ô as s u p p o r t i n g e l e c t r o l y t e s w i t h t h e N a C l s a t u r a t e d s o d i u m c h l o r i d e e l e c t r o d e a s r e f e r e n c e ; SSCE, i . e . , +0.24V v e r s u s NHE) ( £ a ) . Under t h e s e c o n d i t i o n s C 0 ~ i s t h e 2

i n i t i a l r e d u c t i o n p r o d u c t , and once formed, i s exceedingly reactive. The

f a t e o f e l e c t r o c h e m i c a l l y g e n e r a t e d C0 ~" i n 2

w a t e r d e p e n d s upon t h e pH a n d t h e e l e c t r o d e composition. Scheme 2 shows t h e p o t e n t i a l s t h a t have b e e n u s e d i n t h e p r e p a r a t i v e e l e c t r o r e d u c t i o n o f C 0 a t Pb, P t , o r Hg 2

electrodes,

,

along

with

"2.1 t o -2.2 V pH

theultimate

,

(

-°'

Z

6-8

7 t

products

P

V

, CH 0H 3

pH 3-6

1 2

C 0 " 2 4

1 Scheme

2 .

Of p a r t i c u l a r i n t e r e s t i s t h a t i n a c i d i c s o l u t i o n , d i r e c t r e d u c t i o n o f C 0 t o methanol occurs, but t h a t 2

formaldehyde i s apparently bypassed as an intermediate (6&). I n b a s i c s o l u t i o n t h e r e i s no f i r m e v i d e n c e a s t o whether d i s s o l v e d C 0 o r c a r b o n a t e i o n i s reduced t o 2

formate, o r t h a t formate i s reduced t o formaldehyde, even a t p o t e n t i a l s more c a t h o d i c t h a n -2.1V. Formaldehyde, however, i s r e d u c e d a t m o d e r a t e p o t e n t i a l s t o m e t h a n o l , a l t h o u g h i t has been r e p o r t e d t h a t c o m p l i c a t i o n s a r i s e from base promoted f o r m a t i o n o f p o l y o x y m e t h y l e n e g l y c o l s . One c o n c l u s i o n t o be drawn f r o m Scheme 2 i s t h e i m p l i e d k i n e t i c d i f f i c u l t y of reducing carbonate o r bicarbonate, making a c i d s o l u t i o n , where C 0 i s t h e d o m i n a n t f o r m , t h e 2

preferred

medium f o r C 0

o

reduction.


56


F o r m a t i o n o f o x a l a t e i n aqueous s o l u t i o n i s a d i f f e r e n t matter. One b r i e f r e p o r t o f C 0 reduction i n 2

u n b u f f e r e d w a t e r , where c o m p e t i t i v e w a t e r r e d u c t i o n i n c r e a s e d t h e s o l u t i o n pH, gave e v i d e n c e f o r o x a l a t e p r o d u c t i o n (£c_) . By i n f e r e n c e , d i m e r i z a t i o n o f C 0 to 2

oxalate

i s f a v o r e d u n d e r c o n d i t i o n s o f h i g h pH

where

the


+

c o u p l e d H /e"~ s t e p s l e a d i n g t o f o r m a t e , f o r m i c a c i d and methanol are r e l a t i v e l y slow. R e c e n t work on n o n - t r a d i t i o n a l s o l i d e l e c t r o d e s u r f a c e s , s u c h a s Ru c o a t e d c a r b o n , υ) or molybdenum, has b e e n s u c c e s s f u l f o r r e d u c i n g C 0 to 2

e i t h e r methane o r m e t h a n o l . F o r example, w i t h Ru c o a t e d c a r b o n t h e r e d u c t i o n t o methane d e p i c t e d i n Eq. 1 c a n be a c h i e v e d w i t h 24* F a r a d a i c e f f i c i e n c y i n a c i d i c a q u e o u s s o l u t i o n a t c a . -0.38V v e r s u s NHE.

C0

+ 8H

2

+

+

e

•

CH

4

+

2H 0

(1)

2

L i k e w i s e , on a molybedenum e l e c t r o d e m e t h a n o l has b e e n p r o d u c e d i n >50* F a r a d a i c e f f i c i e n c y a t p o t e n t i a l s between -0.7 and -0.8V v s . SCE. A s i n g l e , but t a n t a l i z i n g r e p o r t o f b o t h methane and e t h y l e n e p r o d u c t i o n a t c o p p e r e l e c t r o d e s has b e e n c l a i m e d t o o c c u r between c a . -1.35V v e r s u s NHE . The mechanisms by which these r e d u c t i o n s o c c u r , a l t h o u g h of extreme i m p o r t a n c e i n t h i s f i e l d , a r e unknown. I n nonaqueous s o l v e n t s , s u c h a s DMF o r a c e t o n i t r i l e , e l e c t r o r e d u c t i o n of C0 i s f o l l o w e d by t h e t h r e e 2

p r i n c i p a l r e d u c t i o n pathways a s d e p i c t e d i n Scheme 3. By u s i n g u l t r a f a s t sweep r a t e c y c l i c v o l t a m m e t r y Lamy, N a d j o and S a v e a n t h a v e d e t e r m i n e d t h e s t a n d a r d p o t e n t i a l f o r the C 0 / C 0 ~ 2

2

couple

t o be

-2.21±0.015 V v e r s u s

SCE

in

DMF

w i t h 0.1 M TEAP a s s u p p o r t i n g e l e c t r o l y t e ( 6 g ) . The s u b s e q u e n t r e a c t i v i t y and d i s t r i b u t i o n between f o r m a t e , C 0 ( g ) , o r o x a l a t e as f i n a l p r o d u c t s i s d e p e n d e n t upon f a c t o r s such as the C 0 c o n c e n t r a t i o n , the e l e c t r o d e 2

m a t e r i a l , the type of e l e c t r o l y t e , a d v e n t i t i o u s a c i d (ê,â,y.)·

C0

+

2

e

, k

2C0

C0

l

•

2

k

CO

+

CO

of

2

C 0 2

or the presence

24

2

(carbon-carbon

coupling)

"0 •

χ*

^0

( ccaorubpolni-nogx)y g e n


β.

SULLIVAN ET AL.

Electrocatalytic

,0

57

C0 Reduction 2

CO

+ co„

+

CO

0^


co„

+

H

H-C0 *

+

Scheme

3 .

2

C0

23

HC0

2

r

Amatore a n d S a v e a n t have p r o v i d e d e s t i m a t e s o f t h e r a t e s f o r t h e v a r i o u s pathways shown i n t h e scheme · The f a s t e s t p r o c e s s i s t h e c a r b o n - c a r b o n c o u p l i n g p r o c e s s 7 - l -1 to y i e l d

oxalate,

e.g.,

10

M

sec

, whereas

n e u t r a l i z a t i o n o f C0 ~" b y a d v e n t i t i o u s w a t e r 2

both a n d 0,C

3 —1 —1 c o u p l i n g o c c u r a t c a . 10 M s e c . Of n o t e i s t h a t t h e source o f t h e second e l e c t r o n i n these l a t t e r cases i s p r e d o m i n a n t l y from C 0 ~ r a t h e r t h a n d i f f u s i o n t o t h e electrode surface. Our g o a l s f o r t h e d e v e l o p m e n t o f C 0 2

2

e l e c t r o c a t a l y s t s a r e t w o f o l d ; t h e d e s i g n o f systems which o p e r a t e a t h i g h t u r n o v e r numbers n e a r t h e t h e r m o d y n a m i c p o t e n t i a l f o r c o u p l e s l i k e C0 /HC0 H o r C 0 / H C 0 , and, 2

2

2

2

2

2

t o u n d e r s t a n d m e c h a n i s t i c p a t h w a y s w e l l enough t o d e s i g n and s y n t h e s i z e new e l e c t r o c a t a l y s t s t h a t p o s s e s s a h i g h d e g r e e o f p r o d u c t s p e c i f i c i t y , i . e . , t o be a b l e t o c o n t r o l t h e c o u r s e o f C 0 r e d u c t i o n toward t h e f o r m a t i o n 2

of p r o d u c t s such as formate, methanol r a t h e r t h a n c a r b o n monoxide.

o r even

methane

PH0T0ELECTR0CHEMICAL REDUCTION OF CARBON DIOXIDE USING SEMICONDUCTOR ELECTRODES. Several different strategies f o r c a r b o n d i o x i d e r e d u c t i o n on s e m i c o n d u c t o r s e l e c t r o d e s have b e e n u s e d t o p r o d u c e CO, f o r m i c a c i d , o r e v e n methanol ( ϋ ) . These i n c l u d e : 1. ) Use o f p - t y p e s e m i c o n d u c t o r s u n d e r band g a p i r r a d i a t i o n t o d i r e c t l y reduce C 0 and i t s i n t e r m e d i a t e s . 2

2. ) known C 0

2

Use o f p - t y p e s e m i c o n d u c t o r s reduction catalysts.

Direct

t o photoreduce

photoassisted reduction of C0

2

t o formic acid

o c c u r s w i t h Zn d o p e d - p - t y p e GaP a s a p h o t o c a t h o d e i n aqueous p h o s p h a t e b u f f e r (pH 6.8) u s i n g 365nm l i g h t a t a c e l l b i a s p o t e n t i a l o f -1.0V ( S C E ) . It i s significant t h a t s m a l l e r amounts o f b o t h f o r m a l d e h y d e a n d m e t h a n o l were a l s o o b s e r v e d u n d e r t h e s e c o n d i t i o n s ( 1 4 f ) .


58

CATALYTIC ACTIVATION OF CARBON DIOXIDE Chemically

c a t a l y z e d r e d u c t i o n of C0

using

2

soluble

r e d o x c a t a l y s t s a s e l e c t r o n a c c e p t o r s from a p - t y p e S i p h o t o c a t h o d e has been r e p o r t e d by B r a d l e y and c o - w o r k e r s . II 2 + F o r example, by u s i n g [ ( M e _ [ 1 4 ] a n e N „ ) N i ] (vide infra) in CH CN/[n-Bu N][C10 ] s o l u t i o n , C0 c o u l d be r e d u c e d t o


3

4

4

2

CO w i t h 752nm l i g h t a t an a p p l i e d p o t e n t i a l o f -1.0V (SCE) ( H â ) . A r e l a t e d , b u t more i n t e r e s t i n g a p p r o a c h i s to c h e m i c a l l y modify a semiconductor e l e c t r o d e w i t h the c a t a l y t i c s p e c i e s of i n t e r e s t . T h i s a p p r o a c h has b e e n t a k e n by C a b r e r a and A b r u n a u s i n g p-WSe m o d i f i e d w i t h 2

poly-[Re(vbpy)(C0) C1] 3

t o p r o d u c e CO

at

-0.65V

(SCE)

u n d e r i r r a d i a t i o n w i t h a He/Ne l a s e r ( v i d e i n f r a ) ( 1 4 k ) . U n d o u b t e d l y t h e a p p r o a c h embodied by t h i s work w i l l be f e r t i l e ground f o r p h o t o e l e c t r o c h e m i c a l c e l l r e s e a r c h i n the near f u t u r e . REACTIVITY OF existence metal-C0

2

TRANSITION METAL COMPLEXES TOWARD C 0 .

The

2

of a we11-developed c h e m i s t r y of t r a n s i t i o n complexes would a i d i n the d e s i g n of

e l e c t r o c a t a l y s t s , but u n f o r t u n a t e l y t h i s c h e m i s t r y remains obscure. At l e a s t f o u r a r e a s o f s i g n i f i c a n c e c a n be i d e n t i f i e d where more i n f o r m a t i o n w o u l d e n h a n c e o u r a b i l i t y to d e f i n e p o s s i b l e r e d u c t i o n pathways: 1. ) The s t u d y o f C 0 b o n d i n g modes and t h e i r 2

c h a r a c t e r i s i t i c r e a c t i v i t y p r o p e r t i e s , e s p e c i a l l y as a f u n c t i o n o f t h e c e n t r a l m e t a l and c o o r d i n a t i o n number. 2. ) I n t e r c o n v e r s i o n o f s i d e - b o u n d and c a r b o n - b o u n d

co 2 .

3. ) E l e c t r o p h i l i c and coordinated C0 ligand.

n u c l e o p h i l i c attack at

the

2

4. )

The

redox p r o p e r t i e s of

coordinated

C0 2

D e s p i t e the absence of s y s t e m a t i c i n f o r m a t i o n i n t h e s e a r e a s , t h e r e h a v e b e e n s y n t h e t i c , s t r u c t u r a l , and r e a c t i v i t y s t u d i e s which p r o v i d e a u s e f u l background to the C0 r e d u c t i o n p r o b l e m , a s has b e e n d e s c r i b e d i n 2

recent

reviews

(£).

B o n d i n g Modes and T h e i r R e a c t i v i t i e s . Many m e t a l c o m p l e x e s i n low ( e . g . , N i ( 0 ) ) o r i n t e r m e d i a t e (e.g. Ru(II)) o x i d a t i o n states react with C0 i n s o l u t i o n , or 2

i n t h e s o l i d s t a t e , a l t h o u g h t h e i s o l a t i o n and c o m p l e t e c h a r a c t e r i z a t i o n of the r e s u l t i n g m e t a l - C 0 complexes i s 2

difficult. From t h e r e s u l t s o f x - r a y c r y s t a l l o g r a p h y , s e v e n d i f f e r e n t s t r u c t u r a l t y p e s have b e e n i d e n t i f i e d :


6.

Electrocatalytic

SULLIVAN ET AL. 1. ) cyclohexyl 2. )

C0

g

Reduction

59 2

Tetrahedral Ni(0);

( C y P ) N i ( i 7 - C 0 ) , Cy i s 3

2

2

(2£,sJ· 2

O c t a h e d r a l Mo(0);

(PMe ) (CNR)Mo(/7 -C0 ) 3

3

2

2

,


where R i s Me, i - P r o r t - B u ( 7 n , q ) . 5

2

3. )

Nb(/7 -C H Me) (CH SiMe )(/7 -C0 )

4. )

Formai

5. )

Octahedral

5

4

2

Mo ( I I ) ;

2

3

[ (η^-Ο^Ά^

2

(2p) .

2

*[ο(η -00

2

)]

2

n

(2v) .

1

R h ( I I I ) ; d i a r s ) ( C l )Rh(/7 -C0 ) , 2

2

where d i a r s i s 1 , 2 - ( b i s - d i m e t h y l a s i n o ) e t h a n e ( 2 1 ) . 6. ) [Co(Salen)(A/-C0 )(K)(THF)] (££) . 2

7. )

n

[((CO) Re) (^-C0 ) (Re(CO) ) 4

2

2

2

5

(2b).

2

The f i r s t f o u r c a s e s i n v o l v e s i d e - b o u n d c o o r d i n a t i o n w h i c h i s r e m i n i s c e n t o f m e t a l - a l k e n e complexe a s shown i n s t r u c t u r e I , w h i l e c a s e s 4-6 i n v o l v e a t l e a s t some carbon-metal interaction.

^0 (I)

The c a r b o n - b o u n d c o m p l e x e s show more d i v e r s e s t r u c t u r a l b e h a v i o r t h a n t h e s i d e - b o u n d s p e c i e s , f o r example, t h e Rh complex i s s t r i c t l y monohapto w i t h r e s p e c t t o t h e m e t a l a s shown i n I I , w h i l e t h e Co complex h a s b o t h o x y g e n atoms

+

"supported"

by c o o r d i n a t i o n t o K .

0_K

co—cT

+

(ID

0—κ (where t h e K

+

ions are chemically inequivalent)

The Re c l u s t e r d i s c o v e r e d by B e c k a n d c o w o r k e r s ( H i ) i s 1

s i m i l a r i n t h a t R e i s c o o r d i n a t e d t o b o t h C and 0 i n t h e manner d e p i c t e d i n I I I .

2

2C0

[M(C0)r

•

2

2

C

0

+

20H

(2a)

2

(

3 "

i n t r a m o l e c u l a r example o f C 0

a c t i n g as

2

a

2

b

)

Lewis

a c i d , o x y g e n a c c e p t o r , has b e e n d e m o n s t r a t e d by t h e x - r a y c r y s t a l s t r u c t u r e o f t h e r e a c t i o n p r o d u c t between I r C l ( C H ) ( P M e ) and C 0 i n b e n z e n e (21) The complex, 8

1 4

3

3

IrCl(C 0 )(PMe ) , 2

4

3

3

2

contains

t h e m e t a l l o c y c l e shown i n

IV,

w h i c h i s an i s o m e r o f o x a l a t e , and c a n be v i e w e d as a r i s i n g from an i n t r a m o l e c u l a r a c i d - b a s e i n t e r a c t i o n o— between C 0 and CO. o

if—h

(iv)

I n t e r m o l e c u l a r examples a r e known i n m e t a l c a r b o n y l - a n i o n c h e m i s t r y where r e d u c t i v e o x y g e n t r a n s f e r o c c u r s , a p p a r e n t l y between f r e e and c o m p l e x e d C 0 . An 2

example

i s shown i n Eq.

3,

although

the

intermediate

complex i s a p p a r e n t l y t o o r e a c t i v e t o a p p e a r a s observable i n t e r m e d i a t e (£â). Li [W(C0) ] 2

5

+

2C0

2

•

W(C0)

6

+

Li C0 2

2

an

3

O t h e r o x y g e n a c c e p t o r s have been i d e n t i f i e d , most PR , which r e s u l t s i n the c o r r e s p o n d i n g phosphine 3

and CO; a r e a c t i o n t y p e t h a t c o u l d be e l e c t r o c a t a l y t i c c y c l e s (âd) .

C0

(3)

notably oxide

exploited i n future


β.

Electrocatalytic

SULLIVAN ET AL. Of g r e a t e r coordinated C0

C0

61

Reduction

2

p o t e n t i a l i n t e r e s t i s the r e a c t i o n of with with reactants that attack a t

2

carbon. A l t h o u g h t h e r e a r e few e x a m p l e s , t h e a c r y l i c a c i d formation that r e s u l t s v i a ethylene-C0 coupling 2

shown i n E q . 4 i s n o t e w o r t h y


2

UajlS-Mo(/7 -C H ) (PMe ) 2

4

2

3

(In).

+

4

C0

>

2

(4) 1 /2 [ Mo ( H C=CHC0 H ) (η - C ^ ) ( P M e ) ] 2

2

2

3

2

2

Whether t h e r e a c t i o n p r o c e e d s i n a n i n t e r - o r i n t r a m o l e c u l a r f a s h i o n i s n o t known. Protonation (or e l e c t r o p h i l i c attack) a t carbon t o g i v e formate o r formate p r e c u r s o r s i s another p o s s i b l e ο r e a c t i o n i f η - C 0 e x h i b i t s r e a c t i v i t y modes s i m i l a r t o dihapto alkenes. L a t e r we w i l l d i s c u s s r e c e n t k i n e t i c evidence of t h i s type of r e a c t i v i t y i n the 2

electrocatalytic

reduction

of C0

2

by [ O s ( b p y ) ( C 0 ) H ] . 2

A n o t h e r i m p o r t a n t r e a c t i o n i s t h e c o u p l i n g o f two c a r b o n d i o x i d e m o l e c u l e s a t t h e c a r b o n atoms t o g i v e oxalate. T h i s p r o c e s s a p p e a r s t o be a s i d e r e a c t i o n i n some e l e c t r o c a t a l y t i c r e d u c t i o n s (3j,Q,l yi) y e t i t has n e v e r been o b s e r v e d a s a r e a c t i o n pathway f r o m C 0 f

2

complexes. However, a model o f t h e r e a c t i o n e x i s t s i n t h e example o f t h e c a r b o n - c a r b o n c o u p l i n g o f d i m e t h y l malonate by (rç^-C^H^) Ti(CO) ( 2 ) . 2

Insertion of C 0

2

i n t o Metal-Liaand

2

o f s y n t h e t i c r e a c t i o n s where C 0

2

Bonds.

Examples

i n s e r t s i n t o metal

h y d r i d e , a l k y l , a r y l , a l k o x i d e , h y d r o x i d e , a n d amide bonds a r e w e l l known (1£)· O n l y r e c e n t l y h a v e k i n e t i c and mechanism s t u d i e s b e e n c o n d u c t e d w h i c h r e v e a l t h e d e t a i l s o f the i n s e r t i o n process on the molecular level. N o t a b l e i s t h e work o f D a r e n s b o u r g a n d c o w o r k e r s ( lOçr. 1 ,o) o n t h e W - a l k y l i n s e r t i o n shown i n E q . 5: [ ( P R ) (CO) WMe]"~ + C 0 3

4

2

—

—

• [ ( P R ( CO) W-0^-Me] " 3

4

(5)

R e c e n t l y we have r e p o r t e d (lûfl,r)the f i r s t d e t a i l e d k i n e t i c s t u d i e s of C 0 i n s e r t i o n i n t o both a metal 2

h y d r i d e bond a n d a r e l a t e d m e t a l a l k o x i d e t h a t o f f a c - R e ( b p y ) ( C O ) H (Eq. 6 ) , a n d

bond, i . e . ,

3

Ph Îâ£-Re(bpy)(CO) Oj-H 3

(Eq. 7 ) .


62


lâ£-Re(bpy)(CO) H + C0 3

i a ^ - R e ( bpy ) ( C O ) 0 C H

2

3

(6)

2

CH CN 3

Îâ£-Re(bpy)(C0) 0£-H +

C0

3

2

Îâ£-Re(bpy)(C0) 0 Downloaded by NORTH CAROLINA STATE UNIV on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch006

3

Both

t h e h y d r i d e and a l k o x i d e e x h i b i t

order k i n e t i c s

i n t h e p r e s e n c e o f c a . 10

i n s o l v e n t s s u c h a s THF,

psuedo-first M t o 0.33M

a c e t o n e , o r CH CN.

C0

2

Figure 1

3

'3

θ /

Me THF s o l u t i o n ; t h e s p e c t r a l c h a n g e s e x h i b i t i s o b e s t i c b e h a v i o r a t 346 nm and 411 nm t h a t c o r r e s p o n d s t o t h e d e c a y and a p p e a r a n c e , r e s p e c t i v e l y , o f t h e m e t a l - t o - l i g a n d c h a r g e t r a n s f e r a b s o r p t i o n bands o f t h e r e a c t a n t a l k o x i d e and t h e p r o d u c t m e t a l l o c a r b o x y e s t e r complexes. Concentration studies e s t a b l i s h a f i r s t order d e p e n d e n c e f o r C 0 , and t h e s i m p l e r a t e law, 2

-d[Re]/dt=k [Re][C0 ], i

The process

2

temperature

holds f o r both

dependencies

f o r both i n s e r t i o n s

insertions.

s u p p o r t an AH* =

( f o r Re-H;

associative 12.8

k c a l / m o l e , 4 S = -33.0

eu;

f o r Re-0CH(Me)Ph; ΔΗ*=

k c a l / m o l e , AS* = -27.6

eu)

and

f

the a c t i v a t i o n

compare f a v o r a b l y w i t h t h e c a s e o f

10.9

parameters

[(MeO) P(C0) WMe)] 3

4

{AH* = 10.2 k c a l / m o l e , AS* = -43.3 e u ) . That s i g n i f i c a n t charge t r a n s f e r c h a r a c t e r i s i n v o l v e d i n b o t h mechanisms i s i n d i c a t e d by t h e l a r g e , g e n e r a l d i e l e c t r i c e f f e c t on t h e i n s e r t i o n r a t e , a phenomenon t h a t i s l a r g e r f o r t h e h y d r i d e r e a c t i o n t h a n the a l k o x i d e . F o r example, t h e s e c o n d o r d e r r a t e constant, k ^ 1

M" sec~

1

but

for lac-Re(bpy)(CO) H

i n THF

3

i n CHgCN i s 5 . 4 4 χ 1 θ "

2

1

i s 1.97χ1θ" 1

M" sec" , while for

h

l l a £ - R e ( b p y ) ( C 0 ) 0 - C - H i n THF Me 3

1

M~ sec"

1

and

4

the r a t e

i n CHgCN i t i s 1 . 6 2 x 1 ο "

1

i s 2.2x10 M

_ 1

-2

1

sec" .

One p o s s i b l e mechanism f o r t h e m e t a l - a l k o x i d e i n s e r t i o n r e a c t i o n i s a water or a l c o h o l c a t a l y z e d c h a i n


β.

SULLIVAN ET AL.

Electrocatalytic

63

CO Reduction g


-1.9,

0,-4

1

350

1

450

μ-

550

WAVELENGTH(NM)

FIGURE 1. S p e c t r a l c h a n g e s w h i c h o c c u r i n t h e v i s i b l e r e g i o n o f t h e s p e c t r u m upon i n s e r t i o n o f C 0 i n t o t h e 2

metal-alkoxide

bond o f f a ç - R e ( b p y ) ( C O ) O C ( H ) ( M e ) P h . 3

Inset i s t y p i c a l p s e u d o - f i r s t order decay m o n i t o r e d a t 450nm i n THF s o l u t i o n .

kinetics


64


l i k e t h a t p r o p o s e d by C h i s o l m and c o w o r k e r s f o r amide a l k o x i d e c o m p l e x e s o f t h e mid- and e a r l y t r a n s i t i o n e l e m e n t s ( E q s . 8-9) (10s,t).

and

R-OR

(8)

+

C0

,

2

R0C0 H 2

[R


M-0R

1

+

R0C0 H

>

2

is aryl

or

M-0-C-0R

+

alkyl]

R'OH

(9)

To r u l e o u t t h i s p o s s i b i l i t y we have f o l l o w e d t h e c o u r s e o f t h e i n s e r t i o n r e a c t i o n i n t h e p r e s e n c e o f D^O using

Nmr

spectroscopy.

F i g u r e 2a shows t h e p r o t o n Ph s p e c t r u m o f faj£-Re(bpy) (CO) -0Ç-H i n CD.CN e m p h a s i z i n g t h e 3

3

Me

a s s i g n m e n t s f o r t h e bpy and p h e n e t h y l a l k o x i d e groups. The s p e c t r u m shows t h a t t h e l a c k o f symmetry a t t h e a l k o x i d e carbon i s d r a m a t i c a l l y f e l t at the bpy. A d d i t i o n o f 1.36M Ό^Ο t o t h e s o l u t i o n c a u s e s o n l y s l i g h t s h i f t s , p r e s u m a b l y due t o s p e c i f i c s o l v a t i o n e f f e c t s l i k e hydrogen bonding ( F i g . 2b). From t h i s r e s u l t we c a n be s u r e t h a t h y d r o l y s i s (Eq. 10) d o e s n o t o c c u r on t h e t i m e s c a l e r e q u i r e d t o o b t a i n t h e s p e c t r u m , i . e . , c a . 30 min, s i n c e any s u b s t i t u t i o n p r o c e s s t h a t p r o d u c e s t h e t h e r m o d y n a m i c a l l y f a v o r e d f a c i a l isomer would r e s u l t i n f o r m a t i o n o f a Re complex w i t h a symmetry p l a n e .

faj£-Re(bpy) ( C 0 ) 0 f c - H + 3

D0 2

ÎâÊ-Re(bpy)(C0) 0D 3

+ (10)

Ph DOC-H Me Figure

2c

shows t h e

same e x p e r i m e n t

i n C0

saturated

2

2

s o l u t i o n where i t i s c l e a r t h a t

h

l fac-Re(bpy)(CO)-0-C-Oy-H 3

Me

i s c l e a n l y formed. N o t e t h a t t h e s p e c t r u m o f t h e bpy r e g i o n s t i l l shows t h e e f f e c t o f t h e c h i r a l c a r b o n o f t h e metallocarboxyester, a l t h o u g h i t i s a t t e n u a t e d by t h e g r e a t e r d i s t a n c e between t h e bpy l i g a n d and t h e c h i r a l center. From t h e d a t a a t hand i t i s p o s s i b l e t o p r o p o s e a common t y p e o f c h a r g e - s e p a r a t e d s t a t e f o r b o t h h y d r i d e and a l k o x i d e i n s e r t i o n s ; t h e s e a r e d e p i c t e d i n s t r u c t u r e s V and V I .


β.

65

Electrocatalytic CO Reduction

SULLIVAN ET AL.

t

CH3CN

Me


6(6')

3(3') 4(4')

5(5")

JJLËLxJjLil 9.0

I

7.0

5.0

CH CN With 1.36 M H 0 3

2

λ

JULJÎJUU 9.0

7.0

5.0

CH CN With 1.36 M H 0 and C0 3

_

i

_

m

_

2

1

j

FIGURE 2. P r o t o n NMR s p e c t r a l e x p e r i m e n t s demonstrating the h y d r o l y t i c s t a b i l i t y of f â £ - R e ( b p y ) ( C 0 ) 0 C ( H ) ( M e ) P h i n CD CN s o l u t i o n . 3

are for

2

3

r e a l t i v e t o TMS a s a n e x t e r n a l s t a n d a r d ; e x p l a n a t i o n o f t h e s p e c t r a l changes.

Shifts

see text


66


δ-

o-—

0

Re

Η

Η

+

(VI)

(V)


δ-

Involvement of V as the t r a n s i t i o n s t a t e of the h y d r i d e r e a c t i o n i s f u r t h e r s u p p o r t e d by t h e a p p e a r a n c e o f a n i n v e r s e i s o t o p e e f f e c t o f c a . 1/2 ( e . g . , f o r CH CN, q

1

s e r i e s o f 4, 4 - d i s u b s t i t u t e d bpy c o m p l e x e s . The a s s o c i a t i v e , a l k o x i d e a t t a c k on t h e C 0

carbon

2

d e p i c t e d i n V I b e a r s f o r m a l r e s e m b l a n c e t o t h a t o f V, however, a n a l t e r n a t i v e mechanism i s a c a r b o n i u m i o n m i g r a t i o n from t h e a l k o x i d e oxygen t o the C 0 carbon. 2

T h i s i n t e r e s t i n g a l t e r n a t i v e i s t h e c o n c e r n o f some o f our c u r r e n t s t u d i e s . Photochemical Reduction of C0 . The r e a c t i o n o f C 0 2

w i t h fajç-Re(bpy) ( C 0 ) H , 3

2

a s shown i n E q . 6, i s a l s o

photocatalyzed i n s o l v e n t s l i k e THF o r b e n z e n e where t h e thermal i n s e r t i o n i s i n h e r e n t l y slow (12b). Another example o f a p h o t o c h e m i c a l l y driven i n s e r t i o n i s that of Re(diphos) H (diphos i s Ph PCH CH PPh ) which e l i m i n a t e s 2

H

2

3

2

on UV i r r a d i a t i o n

unsaturated that

2

2

to generate

complex R e ( d i p h o s ) H

(or a solvated

2

reacts with C0

2

the coordinately form)

2

2

t o f o r m R e ( d i p h o s ) (/7 -0 CH) 2

2

(122.) .

A n o t h e r a p p r o a c h h a s b e e n t a k e n by Z i e s s e l , L e h n a n d c o w o r k w e r s , who i n a s e r i e s p a p e r s , b o t h i n t e r e s t i n g a n d 2+ rich,

use the r e d u c t i v e

Re(bpy)(C0) C1 3

quenching of Ru(bpy)

t o produce

3

ana

f o r m a t e a n d CO, r e s p e c t i v e l y

(JL2â, [M Lj + CO + ~0H • 2NaHC0

2

A direct

a

2

+ e" "

[M

t

f

1 ) +

LJL

\• I

+

(15) (16)

3

model o f t h e C 0

2

complex i n E q . 12 i s

o f f e r e d by t h e x - r a y s t r u c t u r e o f [Co(pr-salen)K(C0 )THF] w h i c h shows a C-bound C 0 2

n

2

+

with

the oxygen c o o r d i n a t e d t o K ( i t ) . Employing a very d i f f e r e n t s t r a t e g y , Kapusta and Hackerman (3k) have e x a m i n e d t h e e l e c t r o c a t a l y t i c behavior of Co-phthalocyanine f i l m s d e p o s i t e d on c a r b o n e l e c t r o d e s a n d f i n d i n a q u e o u s s o l u t i o n , o v e r a w i d e pH



68

r a n g e , good c u r r e n t y i e l d s o f f o r m a t e . A t low pH v a l u e s , however, up t o 5* m e t h a n o l i s p r o d u c e d . If confirmed these r e s u l t s imply t h a t the m u l t i e l e c t r o n , stepwise r e d u c t i o n of C0 to methanol v i a metal e l e c t r o c a t a l y s t s 2


ia a viable

strategy.

OTHER SQUARE-PLANAR COMPLEXES. The m a j o r i t y o f t h e m a c r o c y l i c systems, l i k e t h o s e d i s c u s s e d above, a r e s q u a r e p l a n a r and c o n t a i n n e u t r a l o r n e g a t i v e l y c h a r g e d n i t r o g e n o r m i x e d n i t r o g e n and o x y g e n d o n o r atoms. An e l e c t r o n i c a l l y and s t e r i c a l l y d i s t i n c t c l a s s o f complex is

represented

by

Rh(diphos)

shown t o r e d u c e C 0

+ 2

,

w h i c h has

to formate.

2

Under

r e c e n t l y been

electrocatalytic

c o n d i t i o n s , S l a t e r and Wagenknecht (2g) have s u g g e s t e d t h e i n t e r m e d i a t e f o r m a t i o n o f R h ( d i p h o s ) H by r a d i c a l 2

abstraction the

from

reduction,

1

[Rh (diphos) ] 2

the

CH^CN s o l v e n t

i . e . , Eqs.

+

+ e"

In

their

and

.

step

(17)

2

• Rh(diphos) H

3

in

18.

2

p r o p o s e d mechanism, s u b s e q u e n t

i n t o t h e Rh-H completes the

first

[Rh(diphos) ]°

[ R h ( d i p h o s ) ] ° + CH CN 2

17

i s the

+

'CHgCN

i n s e r t i o n of

bond and d i s s o c i a t i o n o f p r o d u c t catalytic cycle.

(18)

C0

2

formate

METAL CLUSTERS. A l t h o u g h as a c l a s s c l u s t e r s a r e p r o m i s i n g as e l e c t r o c a t l y s t s o n l y two r e l e v a n t s t u d i e s have a p p e a r e d , one c o n c e r n i n g t h e e l e c t r o r e d u c t i o n o f Fe-S c l u s t e r s w h i c h r e d u c e C 0 to a v a r i e t y of products 2

i n c l u d i n g CO and f o r m a t e ( ϋ ) , and t h e o t h e r i n v o l v i n g the c a t a l y t i c chemical r e d u c t i o n of C0 t o CO by Ru 2

carbonyl

c l u s t e r s (8eJ

mechanisitic systems.

4Ru^CO),,, . No 4 12 i s a v a i l a b l e f o r e i t h e r of

s u c h as

information

these

METAL-PQLYPYRIDINE COMPLEXES. The m a j o r i t y o f t h e m e c h a n i s t i c d a t a has come f r o m s t u d i e s on c o m p l e x e s containing polypyridyl ligands. Among t h e a p p e a l i n g p r o p e r t i e s o f l i g a n d s l i k e 2, 2 - b i p y r i d i n e (bpy) and 1, 1 0 - p h e n a n t h r o l i n e (phen) i s t h a t t h e y s t a b i l i z e m e t a l s i n a l a r g e number o f o x i d a t i o n s t a t e s w h i l e a t t h e same t i m e they are " e l e c t r o n r e s e r v o i r s " capable of s t o r i n g e l e c t r o n s a t p o t e n t i a l s between c a . -0.7 and -1.7V by 1

utilizing

vacant π

orbitals.


6.

Electrocatalytic

SULLIVAN ET AL.

C0

£

Reduction

69

ÇQfralt S l P Y r i f l i n e c o m p l e x e s . The work o f Keene, C r u e t z , and S u t i n (££) on t h e s t o i c h i o m e t r i c r e d u c t i o n o f C0

2

I

by

[Co (bpy) ]

demonstrates metal-based,


sense

+

3

i n b u f f e r e d aqueous

t h a t two e l e c t r o n s , b o t h o f w h i c h a r e c a n be u s e d t o r e d u c e C 0 t o CO and i n

is similar

to C0

reduction with

2

remain

whether C 0

or HC0 ~ i s the s u b s t r a t e .

their

kinetic

with C0 , 2

Co-macrocycles.

c o n c e r n i n g t h e mechanism, The

3

studies i s consistent with

Co(bpy) (H 0)H 2

this

2

Ambiguities 2

solutions

2 +

2

result

of

Co(bpy) (H 0)

3

f a v o r the

end

either

r e a c t i n g with HC0 ",or

although they

especially

2

former

2

+ 2

interpretation.

Of p a r t i c u l a r i n t e r e s t i s t h e d e a c t i v a t i o n pathway shown i n Eq. 19 where t h e CO p r o d u c t i n t e r c e p t s t h e Co-bpy r e a g e n t p r e c i p i t a t i n g a Co d i m e r .

Co(bpy)

+ 3

+ 2C0

• l/2[Co(bpy)(CO) ] 2

11

1

foMtrpy) (dppenelL " " Complexes 2 , 2 , ^ " - t e r o v r i d i n e : dppene i s 1,2-bis(diphenvlphosphinoMethylene:

2

+ 2bpy

(19)

( t;rpy i s L i s C l - , n = l , CH CN 3

and C0.n=2). B a s e d on t h e r e s u l t s o f c y c l i c v o l t a m m e t r y and b u l k e l e c t r o l y s i s s t u d i e s we have f o u n d t h a t t h e above c o m p l e x e s c o m p l e x e s u n d e r g o a two e l e c t r o n r e d u c t i o n p r o c e s s a t p o t e n t i a l s between -1.06 and -1.30V ( v e r s u s SCE) t o g e n e r a t e a h i g h l y r e a c t i v e r e d u c e d i n t e r m e d i a t e , R u ( t r p y ) ( d p p e n e ) , which w h i l e not i s o l a b l e , i s r e a c t i v e toward C 0 t o g i v e CO (£cj . From t h e r e s u l t s 2

o f c h e m i c a l r e a c t i v i t y and c y c l i c v o l t a m m e t r y ( s e e F i g . 3) t h e mechanism shown i n E q s . 20-25 c a n be p r o p o s e d t o a c c o u n t f o r t h e f o r m a t i o n o f CO i n t h i s s y s t e m . Although t h e v a r i o u s s t e p s i n t h e mechanism a r e r e a s o n a b l e b a s e d on t h e p r o p o s e d c h e m i s t r y and t h e o b s e v e d p r o d u c t s , d i r e c t e v i d e n c e i s a v a i l a b l e f o r o n l y t h e s t e p s shown i n E q s . 20 and 25. The e q u i l i b r i u m shown i n Eq. 24, however, has b e e n i n f e r r e d f r o m c h e m i c a l s t u d i e s .

[Ru(trpy)(dppene)L]

+ 2e

_ CH CN — 1

Ru(trpy)(dppene) Ru(trpy)(dppene)

+ C0

2

+ 2[L]

n +

• Ru(trpy)(dppene)(C0 ) 2


(20) (21)


70 Ru(trpy)(dppene)(C0 )

+ [N(n-Bu) ]

2

+

•

4

[(typy)(dppene)Ru-E-OH]

+

+ N(n-Bu)

3

+ CH CH CH=CH 3

2

(22)

2

[(trpy)(dppene)Ru-C-OH]


[(trpy)(dppene)Ru(CO)] [(trpy)(dppene)Ru(CO)]

"OH

+

C0

2 +

+ 2e~ • Ru(trpy)(dppene)

2 +

+ ~OH

(23)

+

(24)

CO

• Hob-0

2

(25)

The R u - t r p y s y s t e m c a n o n l y a c h i e v e r a t e s o f 0.15 t u r n o v e r s / m i n w i t h 70-90% F a r a d a i c e f f i c i e n c y a t a p o t e n t i a l o f between -1.3 a n d -1.4V u s i n g a P t g a u z e electrode. Catalyst d e a c t i v a t i o n occurs slowly i n the presence o f C 0 , b u t experiments i n the absence o f C 0 2

2

show t h e r a p i d d e c o m p o s i t i o n o f R u ( t r p y ) ( d p p e n e ) . C a r e f u l i n e r t a t m o s p h e r e e x p e r i m e n t s show t h a t t h i s p u t a t i v e intermediate i s not i s o l a b l e using our present techniques. S e v e r a l i m p o r t a n t m e c h a n i s t i c p o i n t s have emerged from t h e above s t u d i e s . F i r s t , c o m p l e x e s e x h i b i t i n g

JRu^y)(dpp#nt)NCCH^(PF ) e

0.1 M TBAPFe Pt button

2

In CH CN 3

r— •H.0

•2.0

FIGURE 3.

Cyclic

"T"

T"

0.0

-1.0

-2OWS. SCE

voltammogram o f

Ru(trpy)(dppene)(CH CN) 3

+

showing t h e near

two e l e c t r o n r e d u c t i o n p r o c e s s t y p e o f complex.

simultaneous

characteristic

of this


β.

Electrocatalytic

SULLIVAN ET AL.

CO

g

71

Reduction

c l o s e l y spaced metal-based ( i . e . R u ( I I ) / R u ( I ) ) and l i g a n d - b a s e d ( t r p y / t r p y ( - ) ) c o u p l e s c a n be p r e p a r e d their reducing equivalents transferred i n v i r t u a l l y electron

fashion.

And s e c o n d ,

the proposed

C0

2

and a two

complex

shown i n E q . 22 must be e x t r e m e l y b a s i c s i n c e i t c a n e f f e c t t h e Hofmann d e g r a d a t i o n o f N ( n - b u ) under m i l d 4

conditions. fifr ( bpy ) & * Complexes (l?py £s g , Ζ ' - b j p y r j f l i n e a n d X Downloaded by NORTH CAROLINA STATE UNIV on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch006

2

is

CI

or

2

0-i-CF ).

Our work on 2, 2'- bpy c o m p l e x e s o f

3

Rh h a s d e m o n s t r a t e d methods f o r t h e c a t a l y t i c r e d u c t i o n of C 0 t o f o r m a t e (3jc_) . F i g u r e 4 and E q s . 26-31 p o r t r a y 2

a s e r i e s of molecular steps that a r e c o n s i s t e n t w i t h our c h e m i c a l and e l e c t r o c h e m i c a l r e s u l t s . The s t e p s i n E q s . 26-28 a r e b a s e d on e l e c t r o c h e m i c a l r e s u l t s o f Hanck a n d

10 μΑ

Τ

FIGURE

4.

Cyclic

voltammogram o f

[cJs-Rh(bpy) (0 SCF ) ] 2

3

-2.0V ν*. SCE

-1.0

0.0

• 1.1

3

2

+

taken

i n CH CN/0.1M TBAH w i t h 3

a g l a s s y c a r b o n b u t t o n w o r k i n g e l e c t r o d e . The most p o s i t i v e r e d u c t i o n wave c o r r e s p o n d s t o a two e l e c t r o n process

coupled +

[Rh

(bpy) ] . 2

t o l o s s o f Cl~~ and f o r m a t i o n o f The two s e q u e n t i a l one

r e d u c t i o n s a r e bpy-based p r o c e s s e s I

elelectron

involving

+

[Rh (bpy) ] . 2


72


DeArmond and t h e i r c o w o r k e r s (JUL)· F u r t h e r m o r e , e x i s t e n c e o f an i n t e r m e d i a t e CO complex i n Eq.

[Rh(bpy) X r 2

[Rh(bpy) ]

+

2

+

2

[Rh(bpy) ]

2e


2

2

•

+

-

[N(n-bu) ]

[Rh(bpy) ]""

(28)

2

+

> Rh(bpy) (C0 H)

i m p l i c a t e d by

3

2

2

+

2

the

species

+ (30)

3

+ HC-0

t h a t ÛO_ r e a c t i o n

fact

between t h e m u l t i p l e - r e d u c e d

2

+ H C=CHCH CH

• [Rh(bpy) ]

2

(29)

2

2

[ R h ( b p y ) ( C 0 H ) ] + 2e

strongly

(27)

[Rh(bpy) (C0 )]~

4

N(n-bu)

2

[Rh (bpy) (bp?)] 2

+ CO

2

(26)

I

+ e " r=

[Rh(bpy) (C0 )]"

+ 2X~

2

[Rh^bpYMbpy)] [Rh(bpy) ]~

+

the 29 i s

[Rh(bpy) ] 2

(31)

occurs and

N(n-bu) * 4 For these c a t a l y s t s , formate p r o d u c t i o n i s found t o o c c u r a t a r a t e o f c a . 0.2 t u r n o v e r s / r a i n a t -1.55V u s i n g a c a r b o n c l o t h e l e c t r o d e w i t h an i n i t i a l c u r r e n t e f f i c i e n c y o f >80*. The s y s t e m s l o w l y d e g r a d e s by two r o u t e s , one w h i c h l e a d s t o t h e p r o d u c t i o n o f an i n t e r m e d i a t e t h a t i s c a t a l y t i c toward H e v o l u t i o n from t h e medium, and t h e o t h e r w h i c h r e s u l t s i n t h e d e p o s i t i o n o f an i n s o l u b l e complex. N e i t h e r of the decomposition p r o c e s s e s have b e e n s t u d i e d a l t h o u g h t h e H producing 2

2

reaction i s intriguing since i t apparently involves a Hofmann d e g r a d a t i o n pathway t h a t g i v e s H a t the expense 2

o f q u a t e r n a r y ammonium s a l t s . The s t u d i e s o f t h e Rh c a t a l y s t s r e v e a l t h a t bpy/bpy redox couples can a c t as i n t e r n a l e l e c t r o n t r a n s f e r s i t e s f o r t h e u l t i m a t e d e l i v e r y o f two e l e c t r o n s t o a coordinated C0 molecule. I n a d d i t i o n , t h e r e s u l t s show 2

that a

feeble acid

like

N(n-bu)

+ 4

can

a c t as

the

oxygen

s i n k f o r CO f o r m a t i o n ( l i k e t h e R u - t r p y s y s t e m s ) and, i n a d d i t i o n , c a n e f f e c t i v e l y a c t as a p r o t o n s o u r c e f o r formate p r o d u c t i o n . Rhenium P o l v p v r i d i n e C o m p l e x e s . The e a r l y s t u d i e s of L e h n and c o w o r k e r s (3m) d e s c r i b e d t h e f o r m a t i o n o f CO and t r a c e f o r m a t e f r o m t h e b u l k e l e c t r o l y s i s o f fajç-Re(bpy) (CO) Χ (X i s B r o r CI) i n CO saturated DMF.


β.

Electrocatalytic

SULLIVAN ET AL.

73

CO Reduction g

T h e s e o b s e r v a t i o n s have been c o n f i r m e d by u s , a n d by B r e i k s s and Abruna ( 5 1 ) . F o r f a c - R e ( b o v ) ( C O ) C l , Z i e s s e l 3

( l q ) h a s s u g g e s t e d a mechanism f o r CO p r o d u c t i o n t h a t appears t o invoke a contra-thermodynamic step, i . e . , t h e production of the strong oxidant ca.

+1.3V) a s shown below


E Re(bpy) ( C 0 ) C 1

+ e~

3

Re

i n Eqs.

*

[Re(bpy)(C0) C1]~ + C0 3

+

Q

2

2

[ Re ( bpy ) ( CO ) C 1 ] " 3

+ 2H

+

• [Re

1 1

(bpy)(CO) C1] 3

(bpy)(C0) C1]

+

+ 2e"

3

+

(31)

+

(32)

2

I I

.

x

/

CO + H 0

[Re

(E

= -1.35V

X /

K

(bpy)(CO) C1 ο 31-33.

• Re(bpy)(CO) C1

(33)

3

In t h i s sequence an i n t e r m e d i a t e t h a t i n v o l v e s a s e s q u i - b p y l i g a n d , t h a t i s , a b i p y r i d i n e w i t h one arm n o t c o o r d i n a t e d t o t h e Re, h a s b e e n s u g g e s t e d . Such i n t e r m e d i a t e s have been p o s t u l a t e d i n s u b s t i t u t i o n r e a c t i o n s o f m e t a l - b p y c o m p l e x e s , b u t w o u l d be e x p e c t e d t o l e a d t o r a p i d bpy l o s s a n d s u b s e q u e n t , r a p i d , l o s s o f c a t a l y t i c a c t i v i t y f o r the system. Future experiments s h o u l d be d e s i g n e d t o i l l u m i n a t e t h i s somewhat counterintuitive suggestion. From t h e r e s u l t s o f o u r work w i t h t h e Re s y s t e m CO o r f o r m a t e i s formed f r o m f o u r c o m p e t i t i v e p a t h w a y s . A l s o , a n o t h e r pathway must e x i s t w h i c h i s r e s p o n s i b l e f o r t h e p r o d u c t i o n o f s m a l l amounts o f o x a l a t e . E q s . 34-39 r e p r e s e n t t h e s e r i e s o f s t e p s w h i c h we b e l i e v e a r e r e s p o n s i b l e f o r t h e p r o d u c t i o n o f CO f r o m h i g h l y reducing, electrogenerated Re(bpy)(C0) r a d i c a l s (or the 3

solvated

form

Re(bpy)(CO) (CH CN)). 3

3

CH CN lac-Re(bpy)(CO) X + e 3

[Re(bpy) ( C O ) X ] " + C0

3

(34)

3

3

+ X"

* Re(bpy)(CO) (C0 )

2

3

2Re(bpy)(CO) (C0 ) 3

[Re(bpy)(CO) X] , Re(bpy)(CO)

3

Re(bpy)(CO)

ν

2

(35) (36)

,

2

(CO) (bpy)Re-0-§-0-Re(bpy)(C0) 3

3

+ CO


(37)

74


(CO) (bpy)Re-O-C-O-Re(bpy)(CO) 3

Re(bpy)(CO)

Re(bpy) ( C O ) C 0


3

+ e

3

+

3

o§o-Re(bpy)(CO)

+

Q

e

• Re(bpy)(CO)

+ C0

3

(38) 2-

(39)

3

I n t h e above scheme, t h e e x i s t e n c e o f t h e i n t e r m e d i a t e C0 complex c a n be i n f e r r e d from c o m p e t i t i o n s t u d i e s , a n d 2

from t h e e x i s t e n c e o f a c a r b o n a t o b r i d g e d dimer (CO) (bpy)Re-0-§-0-Re(bpy)(C0) , 3

analog.

which has r e c e n t l y

3

been

d e t e c t e d by i n s i t u FT-IR methods f o l l o w i n g t h e p h o t o l y s i s o f t h e s u b s t i t u t e d Re d i m e r shown i n E q . 40.

[Re(bpy')(C0)

T

1 + C0 3 2 2 2

J

r

H

F

2

5

' A>450nm

C

1

(bpy )(CO) ReOCORe(CO) (bpy') 3

3

+

CO

(40)

1

(where bpy' i s ( 4 . 4 ' - d i - t e r t - b u t v l ) - 2 . 2 - b i p y r i d i n e )

A non-destructive deactivation route f o r this pathway i n v o l v e s t h e f o r m a t i o n a n d p r e c i p i t a t i o n o f t h e b i c a r b o n a t e complex shown i n Eq 41, where t h e s o u r c e o f p r o t o n s i s e i t h e r a d v e n t i t i o u s , o r d e l i b e r a t e l y added H 0. 2

lac-Re(bpy)(CO) C0 3

3

+ H 0

•

2

Îâ£-Re(bpy)(C0) C0 H 3

+ 0H~

3

(41)

The n o v e l t y o f t h e Re-bpy c a t a l y t i c s y s t e m i s f u r t h e r demonstrated by t h e involvement o f a t w o - e l e c t r o n pathway b a s e d o n t h e r e d - p u r p l e a n i o n R e ( b p y ) ( C 0 )

3

.

T h i s e x c e e d i n g l y r e a c t i v e s p e c i e s c a n be g e n e r a t e d e i t h e r from a second r e d u c t i o n o f f a c - R e ( b p y ) ( C O ) X complexes, 3

o r from t h e m e t a l - m e t a l bonded d i m e r i c s p e c i e s [fac-Re(bpy)(CO) ] ( E q s . 42 a n d 4 3 ) . 3

[Re(bpy)(C0) X] 3

2

+ e

f

CH CN ^ > [Re(bpy)(CO) ] a

t

3

+ X


(42)

β.

Electrocatalytic

SULLIVAN ET AL.

[fac-Re(bpy)(CO) ] 3


L i k e t h e Re electrocatalytic

2

D

+ 2e"

M

CO

Reduction

g

F

75

. 2[Re(bpy)(CO) J"

(43)

3

r a d i c a l , the a n i o n d r i v e s the reduction of C0 t o CO i n CHgCN 2

s o l u t i o n , but because o f i n s t a b i l i t y of the c a t a l y t i c s y s t e m a t t h e h i g h p o t e n t i a l s n e c e s s a r y (-1.7 t o - 1 . 8 V ) , and t h e r a p i d d e c o m p o s i t i o n o f t h e a n i o n , o n l y a t e n t a t i v e r e d u c t i o n mechanism c a n be p r o p o s e d (See E q s . 44-45) .

CH Re(bpy)(CO) " 3

+ C0

Re(bpy) ( C O ) ( C 0 ) " + C 0 3

CN -

2

2

2

• [Re(bpy)(CO) (C0 )] 3

+ 2e" +

[A]

>

[ R e ( b p y ) ( C 0 ) ] " + CO 3

The

nature of

recent believe from

t h e o x y g e n a c c e p t o r , A,

experiments that

the a n i o n i s a c e s s i b l e 1 1

*

+

[A-0]"

(45)

i s unknown, b u t

w i t h fâ£-[Re(bpy)(CO)

o t h e r f a c - f Re (boy) ( C O ^ L I

(44)

2

at

(CH CN)]

we

3

lower

in

potentials

derivatives

and

t h e r e f o r e i n t e n d t o c h a r a c t e r i z e t h i s pathway more completely i n l a t e r studies. A t h i r d m e c h a n i s t i c path which l e a d s to the p r o d u c t i o n o f formate a p p e a r s t o a r i s e from the i n s e r t i o n of C0 i n t o t h e m e t a l - h y d r i d e bond o f f a c - R e ( P P V ) ( C O ) H 2

(Eq.

3

46).

Îâ£-Re(bpy) (CO) H + C 0 3

2

CH CN — • iaç_-Re(bpy) ( C O ) C 0 H 3

2

(46)

Under o u r c o n d i t i o n s o f b u l k e l e c t r o l y s i s t h e f o r m a t e complex c a n be l a b i l i z e d a s i n t h e c h l o r o c a s e , by e i t h e r one o r two e l e c t r o n r e d u c t i o n a s shown i n E q s . 47-49, t h u s c o m p l e t i n g a c a t a l y t i c c y c l e c a p a b l e o f producing f r e e formate. t

Re(bpy)(CO) C0 H 3

2

+ e

> [Re(bpy)(CO) 0 CH] 3

S

[Re(bpy)(CO) OC H]" 3

2

[Re(bpy)(CO) OC H]" 3

2

+ e"

l

°

W

. Re(bpy)(CO)

3

2

(47)

+ "0 CH

(48)

0 CH

(49)

2

> [Re(bpy)(C0) ] 3

+

2


76

the

CATALYTIC ACTIVATION OF CARBON DIOXIDE Our r e c e n t d e t a i l e d k i n e t i c s t u d y ( v i d e s u p r a ) o n i n t i m a t e mechanism o f C 0 i n s e r t i o n shown i n E q . 47 2

demonstates t h a t i t i s an a s s o c i a t i v e p r o c e s s e x h i b i t i n g a h i g h d e g r e e o f bond b r e a k i n g o f t h e R e - Η bond c o n c u r r e n t w i t h bond f o r m i n g between t h e h y d r i d e l i g a n d and t h e c a r b o n o f t h e C 0 . The i m p o r t a n c e o f t h i s


2

f i n d i n g i s t h a t t h e r e s t o f t h e Re c o o r d i n a t i o n s p h e r e i s i n e r t during the formal hydride t r a n s f e r process. This observation c a r r i e s with i t the i m p l i c a t i o n that such a mechanism c o u l d l e a d t o e l e c t r o c a t a l y s t s t a b i l i t y d u r i n g the i n s e r t i o n a c t . A q u e s t i o n w h i c h we a r e a t t e m p t i n g t o answer a t t h e p r e s e n t i s w h i c h o f t h e a v a i l a b l e C 0 2

r e d u c t i o n pathways i s d o m i n a n t a t a g i v e n e l e c t r o l y s i s potential. As d i s c u s s e d a b o v e , t h e p o s s i b l i t i e s i n c l u d e the r a d i c a l " o n e - e l e c t r o n " , the a n i o n " t w o - e l e c t r o n " o r , the h y d r i d e i n s e r t i o n pathways. There i s the l i k l i h o o d , however, o f a f o u r t h pathway i n v o l v i n g t h e d i r e c t interaction of C0

2

with

(Re(bpy)(CO)^X]~.

The o n l y

e v i d e n c e o f t h i s h a s b e e n p r o v i d e d by t h e f l a s h p h o t o l y s i s s t u d i e s o f K u t a l and coworkers f 1 2 a ) . C u r r e n t l y , we a r e i n v e s t i g a t i n g t h e c y c l i c voltammetry o f fac-Re(bpy)(CO) Br i n an attempt t o ο determine the r e l a t i v e c o n t r i b u t i o n s of these v a r i o u s p a t h w a y s (13) · F i g 5 shows a c y c l i c voltammogram o f t h e complex i n t h e p r e s e n c e a n d a b s e n c e o f C 0 . P e r h a p s t h e most s t a r t l i n g f i n d i n g i s t h e d i r e c t e v i d e n c e t h a t 2

[fac-Re(bpy)(CO) Br]~is only a catalyst 3

precursor (Fig.

5b), and t h a t t h e r a t e l i m i t i n g s t e p i s t h e f u r t h e r r e d u c t i o n o f a n i n t e r m e d i a t e complex. We a r e c u r r e n t l y p r o b i n g t h e e v e n t s t h a t o c c u r between t h e f o r m a t i o n o f t h e r e d u c e d complex a n d t h e f o r m a t i o n o f t h e i n t e r m e d i a t e by m o n i t o r i n g t h e d i s a p p e a r a n c e o f t h e i n t i a l complex by single-sweep techniques. Thus f a r , t h e f o l l o w i n g p o i n t s have been e s t a b l i s h e d c o n c e r n i n g t h e d i s a p p e a r a n c e o f [Re(bpy)(CO) Br]": 3

1. ) both

In the absence of C 0

( i n TBAH/CH CN s o l u t i o n )

2

3

R e ( b p y ) ( C O ) ( C H C N ) and [ f â £ - R e ( b p y ) ( C O ) ] 3

3

3

formed a n d t h e a d d i t i o n o f TBABr s u p p r e s s e s of both. 2. )

In the presence

of C 0

2

3

(0.14M i n TBAH/CH CN 3

3

[fac-Re(bpy)(CO) ]

the r a t e o f d i s a p p e a r a n c e enhanced. A d d i t i o n o f Br disappearance.

are

the formation

s o l u t i o n ) R e ( b p y ) ( C O ) ( C H C N ) and t h e e x p e c t e d coupling product

2

3

2

radical

a r e n o t formed b u t

of the [Re(bpy)(CO) Br]" i s 3

however, d e c r e a s e s

the r a t e of


β.

Electrocatalytic

SULLIVAN ET AL. 3. )

CO

I n TBAH/CHgCN s o l u t i o n w i t h 0.14M

increasing

the c o n c e n t r a t i o n of a n i o n

[Re(bpy)(CO) Br]~,

i n the presence

3

electron transfer-catalyzed s o l v e n t o c c u r s , most l i k e l y 50-51 :

2

3

to

produce

of excess

Br",

rapid

s u b s t i t u t i o n o f Br for by t h e r o u t e shown i n E q s .

fast

R e ( b p y ) ( C 0 ) ( C H C N ) + Br 3

C0

[Re(bpy)(CO) Br]

increases i t s disappearance rate. 4. ) A t t h e p o t e n t i a l s n e c e s s a r y


77

Reduction

g

3

+ CH CN

(50)

Re(bpy)(C0) (CH CN)

(51)

[Re(bpy)(C0) Br] 3

[Re(bpy)(C0) Br] 3

+

3

[Re(bpy)(CO) (CH CN)] 3

fâc-Re(bpy)(CO) Br 3

+

3

3

3

Our d a t a a p p e a r s t o show b o t h a d i s s o c i a t i v e X pathway and an a s s o c i a t i v e pathway where C 0 reacts

loss

2

d i r e c t l y with Re(bpy)(CO) X~. 3

From t h e r a p i d

scan

data

i t appears that both r o u t e s r e s u l t i n the p r o d u c t i o n of t h e i n t e r m e d i a t e shown i n F i g . 5b. Bulk e l e c t r o l y s i s of fac-Re(bov)(CO) C1 a t -1.5V i n 3

CHgCN/TBAH medium u s i n g e i t h e r

a carbon

c l o t h or Pt

guaze

e l e c t r o d e g i v e s CO i n 92-99* c u r r e n t y i e l d . The a c t i v i t y a t t h i s p o t e n t i a l c a n be up t o s e v e r a l t u r n o v e r s / m i n a l t h o u g h t h e l o n g term s t a b i l i t y has n o t b e e n tested. However, t h e s y s t e m has been o p e r a t e d between 100-1000 t u r n o v e r s w i t h o u t l o s s o f c a t a l y t i c a c t i v i t y . B u l k e l e c t r o l y s i s a t -1.8V u s i n g t h e same c o n d i t i o n s a s a b o v e g i v e s CO i n 80-90* c u r r e n t y i e l d w i t h r a t e s i n t h e 1-10 t u r n o v e r / m i n r a n g e , a l t h o u g h i n t h i s c a s e , c a t a l y s t deactivation occurs rapidly, t y p i c a l l y w i t h i n 20-40 t u r n o v e r s . I n b o t h c a s e s t h e main d e a c t i v a t i o n pathway a p p e a r s t o be t h e p r e c i p i t a t i o n o f f â £ - R e ( b p y ) ( C O ) 0 - ? - 0 H from 3

the r e a c t i o n

mixture.

The Re s y s t e m has l e d t o s e v e r a l , p o s s i b l y g e n e r a l c o n c l u s i o n s c o n c e r n i n g the d e s i g n of f u t u r e c a t a l y t i c reactions : 1. ) One e l e c t r o n r e d u c t i o n o f a c a t a l y s t p r e c u r s o r c a n l e a d t o e f f i c i e n t n e t two e l e c t r o n r e d u c t i o n o f C 0 2

2. ) E l e c t r o c h e m i c a l g e n e r a l o f m e t a l h y d r i d e complexes which a r e c a p a b l e of i n s e r t i n g C 0 to g i v e 9


78


f o r m a t e v i a a s s o c i a t i v e mechanisms c o u l d be v i a b l e , h i g h s t a b i l i t y c a t a l y t i c routes. 3. ) A m u l t i p l i c i t y o f f i n e l y b a l a n c e d pathways c a n c o - e x i s t f o r a c a t a l y s t p r e c u r s o r some o f w h i c h c a n r e s u l t i n t h e same n e t c h e m i s t r y , s u c h a s t h e r e d u c t i o n of C 0 t o CO. 2

4. ) A c a t a l y s t d e a c t i v a t i o n pathway c a n be p r e c i p i t a t i o n of h i g h l y p o l a r , i n s o l u b l e bicarbonate complexes. Downloaded by NORTH CAROLINA STATE UNIV on December 12, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch006

1

Osfbpv^fCOlH" ".

Electrocatalytic

r e d u c t i o n of

C0

2

to g i v e s u b s t a n t i a l y i e l d s o f b o t h CO and f o r m a t e has b e e n a c h i e v e d w i t h t h i s complex, and b e c a u s e o f i t s s t a b i l i t y , m e c h a n i s t i c s t u d i e s have p r o v i d e d t h e u n p r e c e d e n t e d o p p o r t u n i t y t o e x p l o r e what f a c t o r s d e t e r m i n e f o r m a t e f o r m a t i o n a t t h e e x p e n s e o f CO (14w). I n F i g . 6 i s shown a s e r i e s o f c y c l i c voltammograms which demonstrate t h a t the c a t a l y t i c p r o p e r t i e s of the t h e complex a r e due t o c h e m i s t r y t h a t o r i g i n a t e s f r o m t h e s e c o n d b p y - b a s e d r e d u c t i o n wave. Using bulk e l e c t r o l y s i s and c y c l i c v o l t a m m e t r y t e c h n i q u e s combined w i t h d i g i t a l s i m u l a t i o n methods, t h e f o l l o w i n g mechanism c a n be p r o p o s e d f o r e l e c t r o c a t a l y t i c CO p r o d u c t i o n i n CH CN 3

s o l u t i o n u s i n g TBAH a s electrode surfaces:

[Os(bpy) (CO)H]

+

+ e"

2

supporting e l e c t r o l y t e

, .^35^

O s ( b p y ) ( b p y ) ( C O ) H + e" * _

[0s(bpy) (C0)H]

+ C0

2

[ I

C0

3 2

~

+

C 0

2

+

e

f

"

a

S

>

5

5

k =40 i

2

t

1

'

V

— C0

fast

1

1

2 0 s

C

- O s ( b p y ) (bpy) (CO)H

(52)

» [Os(bpy) (CO)H]"

(53)

2

1

1

M~ s~ > [I

C

Q

]

[Os(bpy)(bpy)(CO)H]

co + 2 [

a t Pt or

— ( PY)(bPY)(C0)H

(54) +

c o

3

2

"

95* r e c o v e r y o f t h e s t a r t i n g complex. I n t h e p r e s e n c e o f H 0 as a p r o t o n d o n o r , k i n e t i c 2

and p r o d u c t e v i d e n c e i s c o n s i s t e n t w i t h a f o r m a t e p r o d u c i n g pathway t h a t f u n c t i o n s i n c o m p e t i t i o n w i t h formation. The p r o p o s e d mechanism i s shown b e l o w i n 57 and 58 where t h e k step i s rate l i m i t i n g .

[0s(bpy) (C0)(C0 )H] o

I

+

2

+ HO

9

e-

>

a Bulk e l e c t r o l y s i s

I

+

OH

CO Eqs.

(57)

[0s(bpy) (C0)H] + 0-C-H (58) £, e x p e r i m e n t s w i t h added H 0 give o

2

f o r m a t e w i t h a F a r a d a i c e f f i c i e n c y o f up t o c a . 25*. In o u r c u r r e n t e f f o r t s , w h i c h a r e f o c u s s e d on understanding the k i n e t i c b r a n c h which produces formate a t the expense o f CO, s e v e r a l p o i n t s a r e a l r e a d y a p p a r e n t , t h e s e i n c l u d e : 1.) t h e o x a l a t e pathway w h i c h i s r a p i d f o r t h e d i m e r i z a t i o n of C0 " 2

w h i c h has

been e l e c t r o c h e m i c a l l y

generated i s nearly t o t a l l y suppressed at e x p e n s e o f CO and f o r m a t e p r o d u c t i o n , and, 2.) H 0 as a p r o t o n source i m p l i e s a C - p r o t o n a t i o n 2

followed

by

the the use route

of

electron transfer.

MODIFIED ELECTRODES BASED ON E l e c t r o p o l y m e r i z a t i o n of

POLYMERIC ELECTROCATALYSTS.

fac-Re(vbpy)(CO) C1 3

(vbpy i s

1

4 - m e t h y l , 4 ' - v i n y l - 2 , 2 - b i p y r i d i n e ) on P t o r g l a s s y carbon surfaces y i e l d s a chemically modified e l e c t r o d e


β.

CO

e

81

Reduction

w i t h a h i g h s p e c i f i c a c t i v i t y f o r CO p r o d u c t i o n (.ââ) . A t y p i c a l e l e c t r o d e response f o r a modified s u r f a c e i s shown i n F i g . 7. I n t i t i a l r a t e s o b t a i n e d by b u l k e l e c t r o l y t i c e x p e r i m e n t s a t -1.5V g i v e 20 turnovers/min, b u t a f t e r c a . 350 t u r n o v e r s t h e s y s t e m d e a c t i v a t e s . C o p o l y m e r i z a t i o n o f t h e Re complex w i t h , f o r example, t h e c a t a l y t i c a l l y i n a c t i v e complex 2+ [Ru(bpy) (vpy) ] (vpy i s 4 - v i n y l - p y r i d i n e ) , c a n y i e l d m o d i f i e d e l e c t r o d e s r a t i o s o f Re t o Ru o f up t o 1:3. In t h e s e s y s t e m s c o n s i d e r a b l e improvement b o t h i n r a t e s and i n s t a b i l i t y c a n be a c h i e v e d , a s e x e m p l i f i e d by up t o c a . 3 4x10 t u r n o v e r s a t an i n i t i a l r a t e o f 100-200 turnovers/min. Of p a r t i c u l a r i n t e r e s t i s t h a t e v e n when the c a t a l y t i c a c t i v i t y of the system i s l o s t the metal complex i s s t i l l on t h e e l e c t r o d e s u r f a c e , a p p a r e n t l y i n an i n a c t i v e f o r m . We have a l s o p r o d u c e d up t o c a . 6% o x a l a t e d u r i n g t h e c a t a l y t i c r e a c t i o n , and i t i s t e m p t i n g t o s p e c u l a t e t h a t t h e p r o x i m i t y e f f e c t o f two bound C 0 2


Electrocatalytic

SULLIVAN ET AL.

2

2

m o l e c u l e s i s t o enhance t h e C-C c o u p l i n g r e a c t i o n . R e c e n t l y D e r o n z i e r and c o w o r k e r s (3p) have o b t a i n e d r e s u l t s s i m i l a r to ours u s i n g o x i d a t i v e l y polymerized p y r r o l e s u b s t i t u t e d b i p y r i d i n e complexes. A n o t h e r f i l m - b a s e d e l e c t r o c a t a l y s t w h i c h we have investigated involves oxidatively electropolymerized N i ( T A P ) (where TAP i s tetrakis-(o-aminophenyl)-tetraphenylporphine) on P t e l e c t r o d e s u r f a c e s (là). For t h i s m o d i f i e d e l e c t r o d e the m a j o r p r o d u c t i s f o r m a t e (50% c u r r e n t y i e l d ) , w h i l e minor products include H (35%) and CO (2%). The c o n d i t i o n s 2

for added

the

reduction

water

as

a

include using proton

CH CN as 3

source.

By

the

solvent

electrolyzing

at

with a

p o t e n t i a l o f -1.35V i n i t i a l r a t e s o f 200-300 t u r n o v e r s / m i n c a n be a c h i e v e d w i t h h i g h s t a b i l i t y . B o t h t h e Re-vbpy and t h e N i ( T A P ) e l e c t r o d e s have s e v e r a l o r d e r s o f m a g n i t u d e g r e a t e r s t a b i l i t y and a c t i v i t y t h a n t h e i r s o l u t i o n a n a l o g s and a r e w i t h i n an o r d e r o f m a g n i t u d e o f t h e CO p r o d u c i n g C o - p h t h a l o c y a n i n e m o d i f i e d e l e c t r o d e r e p o r t e d by L i e b e r and L e w i s ( a n ) . E f f o r t s a r e underway t o m a x i m i z e t h e p e r f o r m a n c e o f t h e s e novel film-based electrodes. DESIGN OF

FUTURE C 0

2

REDUCTION CATALYSTS.

Our m e c h a n i s t i c work has r e s u l t e d i n s e v e r a l i n s i g h t s w h i c h may be o f v a l u e i n t h e d e s i g n o f f u t u r e c a t a l y s t systems f o r the r e d u c t i o n of C 0 p a s t t h e f o r m a t e o r CO 2

stage. They i n c l u d e t h e f o l l o w i n g p o i n t s : 1.) t h e u s e o f " e l e c t r o n r e s e r v o i r " c o m p l e x e s a c t i n g as c a t a l y s t s i n w h i c h more t h a n one e l e c t r o n i s h e l d on a n c i l l a r y l i g a n d s , t h e c e n t r a l m e t a l atom, o r b o t h , 2.) t h e f a c t




82

OO

-Q2

-OA

-06

-08

-1.0

-1.2

-14

-16

Volts vs SSCE FIGURE 7. C y c l i c voltammograms o f c h e m i c a l l y m o d i f i e d e l e c t r o d e s p r e p a r e d by e l e c t r o p o l y m e r i z a t i o n o f f a c - R e ( v b p y ) ( C 0 ) C 1 i n CH CN/0.1M TBAH w i t h a P t 3

3

button working e l e c t r o d e . a. ) F o r m a t i o n o f t h e s u r f a c e l a y e r d u r i n g s i n g l e scanning. b. ) The c a t a l y t i c c u r r e n t ( d a s h e d l i n e ) o b s e r v e d i n a C 0 s a t u r a t e d CH^CN s o l u t i o n . o


6.

Electrocatalytic

SULLIVAN ET AL.

83

CO Reduction g


t h a t complex s t a b i l i t y c a n be m a x i m i z e d by u s i n g c h e l a t e t y p e l i g a n d s , o r m o n o d e n t a t e l i g a n d s ( e . g . CO) t h a t have e x c e p t i o n a l l y s t r o n g bond e n e r g i e s , 3.) a s s o c i a t i v e mechanisms w h i c h i n v o l v e bond making a n d b r e a k i n g a t one s i t e i n t h e m o l e c u l e may m i n i m i z e d e a c t i v a t i o n r o u t e s , 4.) b i m o l e c u l a r o n e - e l e c t r o n s t e p s t o make CO a r e f a c i l i t a t e d by m e t a l c o m p l e x e s a t t h e e x p e n s e o f o x a l a t e f o r m a t i o n , 5.) f o r m a t e c a n be e l e c t r o c a t a l y t i c a l l y p r o d u c e d e i t h e r by d i r e c t i n s e r t i o n mechanisms, o r by C 0

2

c o o r d i n a t i o n f o l l o w e d by e l e c t r o n - p r o t o n a t i o n s t e p s , although i n the l a t t e r case the subsequent m e c h a n i s t i c d e t a i l s s t i l l r e m a i n o b s c u r e , 6.) f o r f u r t h e r r e d u c t i o n of C 0 t o methanol, c a t a l y t i c i n t e r m e d i a t e s which 2

c u r r e n t l y p r o d u c e f o r m a t e a r e l i k e l y c a n d i d a t e s . 7.) polymeric systems o f f e r t h e promise o f h i g h turnover numbers a n d added s t a b i l i t y t o w a r d d e g r a d a t i v e p a t h w a y s . Complexes w h i c h c o n t a i n e l e c t r o n s w h i c h a r e l o c a l i z e d a t chemical s i t e s p o s s e s s i n g redox p o t e n t i a l s necessary f o r C 0 reduction (electron r e s e r v o i r s ) can 2

clearly

a c t as c a t a l y s t s

f o rC0

2

reduction.

At t h i s

e a r l y s t a g e , however, a d i s t i n c t i o n between t h e e f f e c t i v e n e s s o f d i f f e r e n t e l e c t r o n r e s e r v o i r complexes c a n n o t be made s t r i c t l y o n t h e b a s i s o f t h e number o r l o c a t i o n of the reducing equivalents w i t h i n the molecule, r a t h e r such c h o i c e s depend on t h e a v a i l a b i l t y o f a c o o r d i n a t i o n s i t e f o r t h e C 0 l i g a n d and t h e s u b s e q u e n t 2

m e c h a n i s t i c paths t h a t form p r o d u c t s , o r , t h a t r e s u l t i n catalyst deactivation. We have shown i n s e v e r a l c a s e s , e . g . t h a t o f Os(bpy) (CO)H 2

+

and f a c - R e ( b p y ) ( C O ) H , 3

that a s s o c i a t i v e

mechanisms r e s u l t i n no d e t e c t a b l e d e g r a d a t i v e p a t h w a y s . T h i s i n d i c a t e s t h a t a s s o c i a t i v e mechanisms w h i c h h a v e a h i g h degree o f s p e c i f i c i t y f o r a s i n g l e c o o r d i n a t i o n (and h e n c e r e a c t i o n ) s i t e w i t h i n t h e m o l e c u l e s h o u l d be f u t u r e targets i n the design of highly stable c a t a l y s t s . The c h o i c e o f s e c o n d a n d t h i r d row t r a n s i t i o n m e t a l s i s a l o g i c a l one h e r e , s i n c e d e g r a d a t i v e e x c h a n g e o f t h e a n c i l l a r y ligands with solvent or with other p o t e n t i a l l i g a n d s , l i k e CO, c a n be m i n i m i z e d compared w i t h t h e i r l i g h t e r congeners. I t may be t h a t c o m p l e x e s l i k e t h e Os a n d Re e x a m p l e s c i t e d h e r e w h i c h p r o d u c e f o r m a t e a r e good c a n d i d a t e s f o r f u r t h e r r e d u c t i o n chemistry that can occur i n formal two-electron steps. In a r e a c t i v i t y sense the a n a l o g i e s shown below i n s t r u c t u r e s a l - a 8 s u g g e s t t h e p o s s i b l e e x i s t e n c e o f r e l a t e d pathways w o r t h p u r s u i n g .

[Os-0-C-H]

+

( p r o t o n a t i o n a t carbon)

(al)


84

CATALYTIC ACTIVATION OF CARBON DIOXIDE 2H

[Os-0=:C

Electrocatalytic Carbon Dioxide Reduction - American Chemical Society

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