Chapter
6
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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 .
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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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.
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
56
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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
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+
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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
β.
SULLIVAN ET AL.
Electrocatalytic
,0
57
C0 Reduction 2
CO
+ co„
+
CO
0^
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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 ) .
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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
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 :
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
,
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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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 뉣-Re(bpy)(CO) Oj-H 3
(Eq. 7 ) .
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
62
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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
뉣-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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
β.
SULLIVAN ET AL.
Electrocatalytic
63
CO Reduction g
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-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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
64
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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
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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 .
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
β.
65
Electrocatalytic CO Reduction
SULLIVAN ET AL.
t
CH3CN
Me
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
66
CATALYTIC ACTIVATION OF CARBON DIOXIDE
δ-
o-—
0
Re
Η
Η
+
(VI)
(V)
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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
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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.
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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,
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
(20) (21)
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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]
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[(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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
72
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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
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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.
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
(37)
74
CATALYTIC ACTIVATION OF CARBON DIOXIDE
(CO) (bpy)Re-O-C-O-Re(bpy)(CO) 3
Re(bpy)(CO)
Re(bpy) ( C O ) C 0
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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
뉣-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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
(42)
β.
Electrocatalytic
SULLIVAN ET AL.
[fac-Re(bpy)(CO) ] 3
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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).
뉣-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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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
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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
78
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
CATALYTIC ACTIVATION OF CARBON DIOXIDE
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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
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
6.
Electrocatalytic
SULLIVAN ET AL.
83
CO Reduction g
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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)
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
84
CATALYTIC ACTIVATION OF CARBON DIOXIDE 2H
[Os-0=:C