Gaseous Evolution of Molecular Hydrogen and Oxygen in

18 Gaseous Evolution of Molecular Hydrogen and Oxygen in Photochemical Splitting of Water Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

by Platinized Chlorophyll a Dihydrate Polycrystals Laboratory Simulation of the Primary Light Reaction in Plant Photosynthesis

L.

GALLOWAY,

D . R.

FRUGE,

and F. K .

FONG

Department of Chemistry, Purdue University, West Lafayette I N 47907

The proposal that (Chl a•2H O) is the photoreaction center Chl a aggregate for the water splitting reaction in plant photosynthesis led us to the belief that chlorophyll dihydrate polycrystals can be used in efficient water photolysis in vitro. In this chapter, the observation of gaseous evolution under white light illumination of platinized chlorophyll dihydrate polycrystals is described. The photoelectrolytic products were determined to be molecular hydrogen and oxygen by diffusion pyrolysis and mass spectrometry. The photochemi cal activity of (Chl a∙2H O) is attributed to the presence of water in Chl a aggregation via the C9 keto C = O • • • H(H)O • • • Mg bonding interaction. The demonstration of the decomposition of water by (Chl a∙2H O) lends support to the suggestion that a single photosystem in plant photo synthesis may be capable of splitting water in vivo. 2

2

2

n

2

n

" R e c e n t l y w e r e p o r t e d C h i a p h o t o g a l v a n i c effects a t t r i b u t a b l e t o w a t e r s p l i t t i n g reactions that result f r o m i l l u m i n a t i o n of t h e c h l o r o p h y l l a d i h y d r a t e aggregate ( C h i a - 2 H 0 ) ( I ) . T h e photooxidation of ( C h i a-H 0) > b y w a t e r w a s s u b s e q u e n t l y o b s e r v e d i n E S R experiments 0-8412-0429-2/79/33-173-210$05.00/0 © 1979 American Chemical Society v

2

2

w

w

2

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

18.

(2).

GALLOWAY

ET AL.

Photochemical

Splitting

of

211

H0 2

H o w e v e r , the q u a n t u m efficiency of the o b s e r v e d effects w a s l o w ,

a n d w e w e r e u n a b l e to detect the d i s c h a r g e of h y d r o g e n b y

direct

a n a l y t i c a l means. I n this chapter, w e describe the c o n d i t i o n s u n d e r w h i c h w e w e r e successful i n b r i n g i n g a b o u t gaseous h y d r o g e n e v o l u t i o n a t t r i b u t a b l e to w a t e r s p l i t t i n g i n a p h o t o e l e c t r o l y t i c c e l l c o n t a i n i n g p l a t i n i z e d chlorophyll polycrystals ( C h i a - 2 H 0 ) . 2

n

T h e d e m o n s t r a t i o n of

water

p h o t o e l e c t r o l y s i s b y the c h l o r o p h y l l is o f interest because of the role of Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

C h i a i n p l a n t photosynthesis.

It is also t o p i c a l i n v i e w of the c u r r e n t

search f o r a d i r e c t process f o r h a r v e s t i n g solar energy to p r o d u c e gaseous h y d r o g e n for f u e l . C o n s i d e r a b l e attention has b e e n f o c u s e d o n n - t y p e s e m i c o n d u c t i n g p h o t o a n o d e s s u c h as T i 0

2

and S r T i 0

3

(3,4,5).

However,

these materials operate i n t h e n e a r U V w a v e l e n g t h r e g i o n w h e r e

the

solar r a d i a n t energy d e n s i t y is l o w . I n contrast, the a c t i o n s p e c t r u m of t h e p h o t o r e a c t i v i t y of ( C h i a - 2 H 0 ) 2

f a r r e d w a v e l e n g t h regions ( J ) .

w i t h w a t e r spans the v i s i b l e a n d

n

T h e p o l y c r y s t a l s of c h l o r o p h y l l a d i -

h y d r a t e thus b e c a m e o u r p r i m e target f o r i n v e s t i g a t i o n . I n o u r earlier p h o t o c h e m i c a l c o n v e r s i o n experiments, the c h l o r o p h y l l was p l a t e d o n a s h i n y p l a t i n u m electrode ( I ) .

T h e p h o t o l y t i c reactions

w e r e detected b y m e a s u r i n g t h e e l e c t r o n f l o w i n a n external c i r c u i t of a l i q u i d j u n c t i o n p h o t o v o l t a i c c e l l c o n s i s t i n g of the P t - C h l a h a l f c e l l a n d a C h i a-free h a l f c e l l . It o c c u r r e d to us that the q u a n t u m efficiency of this assembly c a n b e l i m i t e d b y the p o o r contact b e t w e e n the s m o o t h m e t a l surface a n d the c h l o r o p h y l l . It appears that o n l y those

Chi a

m o l e c u l e s i n d i r e c t contact w i t h p l a t i n u m are effectively e n g a g e d i n the p h o t o c h e m i c a l process.

I n this context, J a c k i R o e t t g e r of this l a b o r a t o r y

has d e m o n s t r a t e d r e c e n t l y that a n increase i n the C h i a thickness of the P t - C h l a electrode has no systematic effect o n the c e l l p e r f o r m a n c e .

We

a c c o r d i n g l y sought to o v e r c o m e the p r o b l e m of l o w q u a n t u m efficiency b y f i l l i n g i n the crevices that separate the p o l y c r y s t a l l i n e C h i a aggregates f r o m each other a n d f r o m the s m o o t h m e t a l electrode surface b y

finely

d i v i d e d p l a t i n u m particles.

Preparation of Platinized (Chi a • 2H 0) 2

Oxygen on the Photovoltaic

n

Electrode.

The Effect of

Activity

A s h i n y P t f o i l w a s p l a t i n i z e d b y p a s s i n g a 30 m A c u r r e n t f o r 10 m i n through a 7 X

1 0 " M chloroplatinic acid solution containing 6 X 2

10" M

l e a d acetate. A l a y e r o f p o l y c r y s t a l l i n e c h l o r o p h y l l , c o n t a i n i n g 1.5 X

4

10

17

C h i a m o l e c u l e s , w a s d e p o s i t e d o n the p l a t i n i z e d electrode surface, u s i n g the p r o c e d u r e d e s c r i b e d b y T a n g a n d A l b r e c h t (6).

T h e C h i a-plated

electrode t h e n w a s p l a t i n i z e d a g a i n i n t h e same c h l o r o p l a t i n i c a c i d s o l u t i o n , except that the 30 m A c u r r e n t was p a s s e d f o r o n l y 15 seconds.


212

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

II

T h e a c t i o n spectra of t h e p h o t o v o l t a i c response (1) of t h e p l a t i n i z e d C h i a electrode at p H =

7, m e a s u r e d i n a c e l l (see

F i g u r e 1) u s i n g as

t h e s e c o n d h a l f - c e l l a p l a t i n i z e d electrode n o t c o v e r e d w i t h C h i a, a r e g i v e n i n F i g u r e 2. T h e 740-nm m a x i m u m of the s p e c t r a l response s h o w n i n F i g u r e 2 confirms that ( C h i a - 2 H 0 ) 2

f o r the o b s e r v e d p h o t o v o l t a i c effects.

n

(7,8)

is p r i m a r i l y r e s p o n s i b l e

U n d e r an argon atmosphere the

o b s e r v e d response of the C h i a c e l l is p h o t o c a t h o d i c .

A remarkable

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

c h a n g e w a s o b s e r v e d w h e n the electrolyte s o l u t i o n w a s saturated w i t h oxygen.

T h e photogalvanic current reversed i n sign.

O n p u r g i n g the

o x y g e n - s a t u r a t e d s o l u t i o n w i t h a r g o n , t h e o r i g i n a l p h o t o c a t h o d i c response w a s restored. T o e n h a n c e t h e p h o t o v o l t a i c response, t h e p H values ( I ) of the C h i a a n d C h i a-free h a l f cells w e r e m a i n t a i n e d at 3 a n d 11, r e s p e c t i v e l y , i n another e x p e r i m e n t .

TO

A f t e r t h e h a l f cells w e r e degassed b y t h e passage

MASS SPECTROMETER

t

t

Ar Figure 1. The platinized Chi a cell. The Chi a-free electrode is used as a half cell in a liquid-junction photovoltaic cell. In photolytic reaction, only the platinized Chi a electrode is used in the production of molecular hydrogen and oxygen from water.


18.

Photochemical

GALLOWAY E T AL.

Splitting

of

213

H0 2

2.40-

2.00


1.60 1.20

0.80

UJ

6

0.40

CM b

8

w

O

-0.40

in UJ *

-0.80

o

0

0

0

° *

o

O

o oooooooo° o u

0

a 0 o

a

°a

oo

0 0

-1.20 400

450

500

550

600

650

700

750

WAVELENGTH (nm) Figure 2. Effect of oxygen in the photovoltaic response of the platinized Chi a electrode, (a) /\, Electrolyte purged with argon gas, measurements made under a positive pressure of Ar; (b) O , electrolyte saturated with oxygen; (c) •, oxygen removed by passage of argon gas through the electrolyte solution for 30 min. The ordinate units are given in 10" electrons per incident photon. The efficiency of the liquid junction cell was calculated by dividing the photovoltaic response (electrons sec' ) by the incident photoresponse (photons sec' ). 2

1

1

of a r g o n gas f o r a b o u t 30 m i n u t e s , t h e p h o t o v o l t a i c response of t h e c e l l w a s m o n i t o r e d w i t h t h e entire o u t p u t f r o m t h e 1000 W t u n g s t e n - h a l o g e n l a m p f o c u s e d o n t h e p l a t i n i z e d C h i a electrode.

A n initial photocurrent

of a b o u t 1.5 /AA w a s o b t a i n e d . A f t e r t w o h o u r s of c o n t i n u o u s i l l u m i n a t i o n , a r e v e r s a l i n s i g n of t h e p h o t o c u r r e n t w a s o b s e r v e d , p o s s i b l y i n d i c a t i v e of a b u i l d u p i n t h e o x y g e n content of t h e e l e c t r o l y t e s o l u t i o n . T h e p o w e r of t h e r a d i a t i o n i n c i d e n t o n t h e s a m p l e w a s d e t e r m i n e d , u s i n g a L a s e r P r e c i s i o n r a d i o m e t e r , to b e 1.7 W . A p p r o x i m a t e l y one h a l f of this p o w e r occurs b e y o n d 800 n m , o u t of t h e r e a c h of C h i a p h o t o chemistry.

T h e o b s e r v e d p h o t o c u r r e n t , 1.5 ^ A , a m o u n t s t o a c o n v e r s i o n


214

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

efficiency of a b o u t 10~ , a s s u m i n g that t h e average e n e r g y of the p h o t o 6

c h e m i c a l l y a c t i v e p h o t o n s is 2 V .

T h e peak monochromatic q u a n t u m

efficiency at 740 n m is 4 X 10"

F i g u r e 2 ) . I n the " g r e e n g a p " w h e r e

3

(see

t h e C h i a a b s o r p t i o n is w e a k , the average m o n o c h r o m a t i c c o n v e r s i o n efficiency is a b o u t 8

X

10" .

The

4

average

q u a n t u m efficiency u s i n g

m o n o c h r o m a t i c l i g h t is thus three orders of m a g n i t u d e greater t h a n that o b t a i n e d u s i n g w h i t e l i g h t . It w a s f o u n d (10)

that at a n i n c i d e n t flux


c o r r e s p o n d i n g to the 1.7 W i n c i d e n t p o w e r , the photoresponse of the P t - C h l a c e l l observes a s e m i l i n e a r flux d e p e n d e n c e . T h e m o n o c h r o m a t i c incident ^ 10

14

fluxes

photons

u s e d i n the e x p e r i m e n t r e p r e s e n t e d sec cm" , _1

2

at w h i c h

i n Figure 2 were

t h e photoresponse

varies l i n e a r l y

w i t h flux. It is e v i d e n t f r o m F i g u r e 2 of R e f . 10 that a l i n e a r extension of the l o w - f l u x p h o t o r e s p o n s e d a t a at the m a x i m u m a v a i l a b l e p o w e r of the source w o u l d extrapolate to a q u a n t u m efficiency a b o u t 1 0

times

3

h i g h e r t h a n that o b s e r v e d . Gaseous Evolution of Molecular Hydrogen and Oxygen. Mass Spectrometric and Pyrolytic

Analyses C o n t i n u e d i l l u m i n a t i o n of the c e l l w i t h

Qualitative Observations.

a n o p e n e x t e r n a l c i r c u i t l e d to the o b s e r v a t i o n of gaseous e v o l u t i o n f r o m the P t - C h l

a electrode,

n o t a b l y the f o r m a t i o n of gas

bubbles

i n the i l l u m i n a t e d area f o l l o w e d instantaneously t h e r e p o s i t i o n i n g of the i l l u m i n a t e d area.

T o e l i m i n a t e the p o s s i b i l i t y that the gas b u b b l e s m a y

h a v e r e s u l t e d f r o m the d e g a s s i n g of a r g o n b e c a u s e of h e a t i n g b y t h e l i g h t source, the c e l l w a s p u r g e d w i t h h e l i u m a n d the e x p e r i m e n t w a s r e p e a t e d u n d e r a p o s i t i v e pressure of h e l i u m . i n w a t e r increases

T h e s o l u b i l i t y of h e l i u m

w i t h increasing temperature,

b e i n g 0.94

m L / 1 0 0 m L w a t e r at 2 5 ° a n d 7 5 ° C , r e s p e c t i v e l y .

and

1.21

Gaseous e v o l u t i o n

w a s a g a i n o b s e r v e d u n d e r i d e n t i c a l i l l u m i n a t i o n c o n d i t i o n s . W h e n the C h i a-free p l a t i n i z e d p l a t i n u m electrode w a s i l l u m i n a t e d u n d e r these c o n d i t i o n s , no signs of b u b b l i n g w e r e detected. Mass Spectrometric Analysis.

A f t e r t h e p l a t i n i z e d C h i a electrode

i n the c e l l s h o w n i n F i g u r e 1 was i l l u m i n a t e d f o r 30 m i n u t e s , the gaseous content o v e r the electrolyte s o l u t i o n w a s e v a c u a t e d d i r e c t l y i n t o

the

s a m p l e c h a m b e r of a C o n s o l i d a t e d E l e c t r o d y n a m i c s C o r p . 21-110-B mass spectrometer.

T h e r e s u l t i n g mass s p e c t r u m is s h o w n i n F i g u r e 3.

a d d i t i o n to the e x p e c t e d H e

+

In

l i n e at mass 4, a s t r o n g p e a k at mass 2

w i t h a n attendant trace p e a k at mass 3 w a s o b s e r v e d . T h e latter peaks are r e s p e c t i v e l y a t t r i b u t e d to H

2

+

a n d to t h e t r i a t o m i c i o n H

3

+

(11,12).

These identifications were confirmed b y using pure h y d r o g e n a n d h e l i u m as source. T h e gaseous content a b o v e the electrolyte s o l u t i o n i n the C h i a-free c e l l also w a s a n a l y z e d b y mass s p e c t r o m e t r y after a s i m i l a r l i g h t


18.

GALLOWAY

4

0

Photochemical

E T AL.

8

12

Sample

Splitting

of

20

24

16

215

H0 2

28

32

• o z o Xo xZCM

+ CV, O

+

a> X

+


(M

CO z UJ

+

I

+

X • I +

I X

•2SL

.

o

+ + CVJ ro

CM O

O I

2

12

16

MASS

20

24

NUMBER

Figure 3. Mass spectrometric determination of the gaseous products of the photoelectrolytic process, (a) Gaseous sample collected from illuminated platinized Chi a cell; (b) gaseous content over a similarly irradiated, platinized electrode in the absence of Chi a. Molecular hydrogen and oxygen, the two main products of the water splitting reaction, are readily manifested by the lines observed at masses 2 and 32 in (a). Helium gas was used to purge the cell prior to the light reaction. See text for a more detailed analysis. t r e a t m e n t of the C h i a-free p l a t i n i z e d electrode. a n d 3 w e r e d e t e c t e d (see H

2

+

N o lines at masses 2

c o m p a r i s o n i n F i g u r e 3 ) . S m a l l quantities of

are k n o w n to a c c o m p a n y h y d r o c a r b o n fragments at masses 13, 15, 25,

26, 27, a n d 29. T h e s e fragments a r e f o u n d t o o c c u r i n s i m i l a r i n t e n s i t y ratios i n b o t h the s a m p l e a n d b l a n k d e t e r m i n a t i o n s s h o w n i n F i g u r e 3. T h e possibility that the observed H

2

+

line m a y have originated from

h y d r o c a r b o n f r a g m e n t a t i o n is thus r u l e d o u t . T h e intense mass 32 l i n e i n t h e s a m p l e s p e c t r u m is a t t r i b u t e d to a i r leakage t h r o u g h the e x t e r n a l leads a d m i t t e d i n t o the s a m p l e c e l l v i a e p o x i e d m e t a l - g l a s s

joints f o r

t h e m e a s u r e m e n t of the p h o t o v o l t a i c response. B y e l i m i n a t i n g these joints w e w e r e able to r e d u c e t h e mass 32 l i n e to a l e v e l at w h i c h a q u a n t i t a t i v e e v a l u a t i o n o f the o x y g e n e v o l v e d c a n b e m a d e i n terms o f a statistical d i s t r i b u t i o n of i s o t o p i c o x y g e n ( 1 3 ) .


216

INORGANIC

COMPOUNDS WITH UNUSUAL

PROPERTIES

II

M a s s spectrometric analyses of t h e p h o t o l y t i c p r o d u c t s f r o m v a r i o u s m i x t u r e s of H

2

1 6

0, D

2

i e

O , andH

2

1 8

0 are compared w i t h the corresponding

mass spectra o f e l e c t r o l y z e d samples c o n t a i n i n g i d e n t i c a l i s o t o p i c m i x t u r e s i n F i g u r e s 4, 5, a n d 6. T h e results of a n e x p e r i m e n t u s i n g 1:1 D 0 - H 0 2

2

are s h o w n i n F i g u r e 4. I n this e x p e r i m e n t a r g o n gas w a s u s e d t o p u r g e the c e l l p r i o r to t h e l i g h t r e a c t i o n . the mass 4 ( D

2

+

If w e assume that t h e o b s e r v a t i o n of

) l i n e has r e s u l t e d f r o m w a t e r s p l i t t i n g a n d use t h e mass

20 ( D 0 ) l i n e as a n i n t e r n a l r e f e r e n c e f o r c a l i b r a t i o n , w e estimate, f r o m Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

2

+

a c o m p a r i s o n of the relative intensity ratios o f lines at masses 20 ( D 0 ) 2

I 'I 0

1

4

i

i

i

D

8 i

i

2

0 - H

12 i

i

2

0

16 i

Photolysis

i

20 i

i

22

H 0,0D 2

3h;

HDCtHjO*

Htf Ort

>-

Nf 0

- —10 1

+

CO LU

Electrolysis

UJ

cr

Blank

4

8

12

16

20

22

MASS NUMBER Figure 4. Comparison of the mass spectrometric determination of the photolytic products of 1:1 DiO-HtO with that of conventional electrolysis. Under comparable instrumental settings, the lines at masses 3 and 4 are not observed in the blank. Argon gas was used to purge the cell prior to the light reaction. Note changes in scale in the low-mass region.


+

18.

GALLOWAY

Photochemical

E T A L . ?

D

of

217

H0 2

$

?

3:|

Splitting

D 0-H 0 2

+

2

Photolysis


HD* ?2

Ha

+

Electrolysis HD

+

?2

D

Blank

+

H+ HP 2

2

MASS and 4 ( D

2

+

3

4

Figure 5. Comparison of the mass spectra of the products of the photolysis and electrolysis of 3:1 D 0-H 0. Argon gas was used to purge the cell prior to the light reaction.

D+ 2

4

2

NUMBER

2

) observed i n the photolytic a n d electrolytic runs, that water

p h o t o l y s i s occurs at a rate o f 9 X 10" m o l h r " , c o r r e s p o n d i n g to a gaseous 6

1

( h y d r o g e n a n d o x y g e n ) e v o l u t i o n rate o f 0.3 m L h r . A significant l i n e - 1

at mass 2 is present i n t h e b l a n k . W e a t t r i b u t e this t o t h e f r a g m e n t a t i o n of D

2

0 t o D a n d O D , w h i c h is consistent w i t h t h e o b s e r v a t i o n o f w a t e r +

+

f r a g m e n t s at masses 17, 18, a n d 19 i n t h e p h o t o l y t i c , e l e c t r o l y t i c , a n d b l a n k runs. I n t h e b l a n k , t h e lines at masses 3 a n d 4 a r e n o t o b s e r v e d u n d e r settings s i m i l a r t o those o f t h e p h o t o l y t i c a n d e l e c t r o l y t i c e x p e r i ments. W e w e r e u n a b l e t o detect t h e H s i g n a l at mass 1 o n a c c o u n t o f +

instrumental limitations. T h e a b o v e considerations suggested t h e use of h i g h e r i n s t r u m e n t a l r e s o l u t i o n so that t h e interference could be differentiated f r o m the H

at mass 2 f r o m D 2

+

2

0 fragmentation

l i n e . T h e results o f a h i g h e r r e s o l u -


218

INORGANIC

COMPOUNDS

34

35

I

0 0* 33.994


, 6

WITH

UNUSUAL

PROPERTIES

II

36

I —

Photolysis

, 8

34

35 MASS

36

NUMBER

Figure 6. Comparison of mass spectrometric analyses of the photolytic and electrolytic products using 5:1 H2 0-Hi 0. The occurrence of the Cl* and H Cl lines is attributable to the presence of KCl in the aqueous electrolyte. Helium gas was used to purge the cell prior to the light reaction. 16

35

1

35

18

+

t i o n d e t e r m i n a t i o n of the m o l e c u l a r species H

2

+

, H D , and D +

2

+

obtained

i n the p h o t o l y s i s a n d electrolysis 3 : 1 D 0 - H 0 are s h o w n i n F i g u r e 5. 2

2

T h e D l i n e w a s s i g n i f i c a n t l y r e d u c e d o n f r e e z i n g the s a m p l e p r i o r to t h e +

e v a c u a t i o n of the gaseous contents of the c e l l into t h e mass This

o b s e r v a t i o n corroborates

the supposition that

o b s e r v e d as a result of w a t e r f r a g m e n t a t i o n .

spectrometer.

the D

+

line was

I n a separate e x p e r i m e n t ,

i n w h i c h h e l i u m gas w a s u s e d to p u r g e t h e c e l l p r i o r to the l i g h t r e a c t i o n ,


18.

GALLOWAY

ET

AL.

Photochemical

Splitting

of

219

H0 2

the presence of m o l e c u l a r o x y g e n i n the p h o t o c h e m i c a l s p l i t t i n g of w a t e r is ascertained b y u s i n g 5 : 1 H of the lines at masses 34 (

1 6

0

2 1 8

1 6

0-H

2

1 8

0 (see

Figure 6).

0 ) a n d 35.998 ( +

1 8

0

2

+

The

occurrence

) compares w e l l w i t h

the c o r r e s p o n d i n g lines o b s e r v e d f o r t h e e l e c t r o l y t i c s a m p l e .

B o t h lines

are absent i n a b l a n k r u n i n w h i c h the C h i a-free electrode i n F i g u r e 1 was

treated

under

conditions

identical

to

those

of

the

photolytic

experiment. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

Quantitative Determination by Diffusion Pyrolysis. I n a n

attempt

to q u a n t i f y t h e rate of the p h o t o c h e m i c a l w a t e r - s p l i t t i n g process,

we

d e s i g n e d a p y r o l y t i c apparatus, s h o w n s c h e m a t i c a l l y i n F i g u r e 7, t h a t a l l o w e d us to d e t e r m i n e t h e q u a n t i t y of the h y d r o g e n gas p r o d u c e d b y p h o t o l y s i s b y b u r n i n g it i n a stream of o x y g e n gas c a r r i e d b y gaseous h e l i u m . T h e l e v e l of b a c k g r o u n d o x y g e n i n the h e l i u m flow was registered as a c u r r e n t u s i n g the H e r s c h o x y g e n i n d i c a t o r ( 1 5 ) .

Gaseous h e l i u m

w a s a d m i t t e d t h r o u g h the entire apparatus at a constant flow rate u n t i l the o x y g e n c u r r e n t r e a c h e d a steady-state v a l u e . A t this p o i n t the valves, d e n o t e d b y o p e n circles i n F i g u r e 7, w e r e a d j u s t e d so that the h e l i u m flow w a s d i r e c t e d a l o n g a p a t h w a y ( m a r k e d b y arrows i n F i g u r e 7) that b y passed the s a m p l e c e l l . W i t h b o t h valves a b o v e a n d b e l o w the p l a t i n i z e d C h i a

electrode

c l o s e d , the p h o t o l y t i c r e a c t i o n w a s a l l o w e d to c a r r y o n f o r a k n o w n d u r a t i o n of t i m e .

After a brief period ( ^

1 m i n ) of c o o l i n g , the v a l v e

at ( a ) w a s o p e n e d . T h e distance b e t w e e n ( a )

and (b)

(at w h i c h p o i n t

the gas l i n e r e a c h e d the h y d r o g e n a n d o x y g e n r e a c t o r ) w a s 45 c m .

The

h y d r o g e n a n d o x y g e n p r o d u c e d b y w a t e r p h o t o l y s i s d i f f u s e d a l o n g the

Figure 7. The diffusion pyrolysis apparatus. The arrows indicate the of helium flow during operation.


direction

220

INORGANIC

helium

flow.

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

M o l e c u l a r h y d r o g e n , b e i n g 16 times l i g h t e r t h a n

II

oxygen,

a r r i v e d at ( b ) a p p r o x i m a t e l y f o u r times as q u i c k l y as t h e p h o t o l y t i c o x y g e n , c a u s i n g a r e d u c t i o n i n t h e b a c k g r o u n d o x y g e n c u r r e n t l e v e l as it w a s b u r n e d b y t h e p l a t i n u m h e a t i n g c o i l i n the h y d r o g e n a n d o x y g e n r e a c t o r (see F i g u r e 8 ) . O n d e p l e t i o n o f h y d r o g e n t h r o u g h c o m b u s t i o n , t h e o x y g e n c u r r e n t l e v e l w a s restored t o t h e steady-state b a c k g r o u n d l e v e l . S u b s e q u e n t l y , t h e s l o w e r d i f f u s i n g o x y g e n f r o m the p l a t i n i z e d C h i Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

a c e l l r e a c h e d t h e o x y g e n i n d i c a t o r , r e s u l t i n g i n a n increase i n t h e o x y g e n current. I n F i g u r e 8, t h e p y r o l y t i c analyses o f t w o p h o t o l y s i s experiments i n w h i c h t h e p h o t o c h e m i c a l r e a c t i o n w a s c a r r i e d o u t f o r five a n d 15 m i n u t e s ( r e s p e c t i v e l y s h o w n i n F i g u r e s 8 a a n d 8 b ) a r e c o m p a r e d w i t h a correI

1—I—I—I—I—I—I—I—I—I—I

1

a

T I M E (min) Figure 8. Comparison of pyrolytic analyses of the photolysis and electrolysis of water at pH = 3. (aJo) Illumination of the Pt-Chl a sample with the entire output of a 1000 W tungsten-halogen lamp for 5 and 15 min, respectively, (c) L o w e r , hydrogen generated by passing 10 mA for 20 sec; m i d dle, oxygen generated by passing a current of 10 mA for 20 sec; upper, sum of middle and lower curves.


18.

GALLOWAY

E T AL.

Photochemical

Splitting

of

221

H0 2

s p o n d i n g analysis of h y d r o g e n a n d o x y g e n generated b y electrolysis ( F i g u r e 8 c ) i n w h i c h t h e C h i a-free ( b l a n k ) h a l f c e l l i n F i g u r e 1 a c t e d as a c o u n t e r electrode.

F i r s t , t h e b l a n k electrode was u s e d as t h e a n o d e i n t h e

electrolysis, so that h y d r o g e n w a s p r o d u c e d i n t h e a b s e n c e of l i g h t i n t h e enclosed s a m p l e c e l l w i t h t h e P t - C h l a electrode o p e r a t i n g as t h e cathode. A f t e r a 10 m A c u r r e n t w a s passed f o r 20 seconds, t h e s a m p l e c e l l w a s o p e n e d so that the h y d r o g e n gas w a s a d m i t t e d i n t o t h e h e l i u m flow Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch018

t o w a r d t h e h y d r o g e n a n d o x y g e n reactor.

T h e expected reduction i n the

o x y g e n c u r r e n t w a s o b s e r v e d after a t i m e l a g s i m i l a r t o that o b s e r v e d i n t h e p h o t o l y t i c experiments. O n c o m p l e t i o n of the e l e c t r o l y t i c h y d r o g e n measurement, t h e s a m p l e c e l l w a s flushed w i t h h e l i u m to get r i d of a n y h y d r o g e n that r e m a i n e d i n t h e electrolyte. W i t h the b l a n k electrode o p e r a t i n g as a c a t h o d e " a n d the P t - C h l a e l e c t r o d e as the anode, t h e entire p r o c e d u r e d e s c r i b e d i n t h e p r e c e d i n g p a r a g r a p h was r e p e a t e d f o r a n a l y z i n g t h e o x y g e n p r o d u c e d b y electrolysis i n t h e s a m p l e c e l l . T h e s u m m a t i o n of these t w o analyses r e s u l t e d i n the c u r r e n t - t i m e p l o t g i v e n i n F i g u r e 8c, w h i c h is c o m p a r e d w i t h t h e c o r r e s p o n d i n g plots o b t a i n e d i n t h e p h o t o l y t i c experiments. T h e results s h o w n i n F i g u r e 8 w e r e o b t a i n e d at a h e l i u m flow rate o f 28 b u b b l e s p e r m i n u t e t h r o u g h 0.25 i n . i . d . glass t u b i n g s i n the flow i n d i c a t o r . T h e observed H : 0 2

photolysis

(Figure

2

ratio appears to b e greater i n t h e p r o d u c t f r o m 8a, 8 b )

than

f r o m electrolysis

(Figure

8c).

In

e x t e n d e d w o r k c o m p l e t e d after t h e o r a l p r e s e n t a t i o n o f this p a p e r , w e o b t a i n e d e v i d e n c e that p l a t i n u m itself c a n b e r e s p o n s i b l e f o r the d e s c r i p t i o n of w a t e r i n a t h e r m a l r e a c t i o n of t h e p l a t i n u m w i t h w a t e r , i n w h i c h the p r o d u c t s are h y d r o g e n a n d some o x i d a t i o n c o m p o u n d of p l a t i n u m (14). Discussion O u r m o d e l for the C h i a - H 0 photoelectrolytic cell was based 2

o n the p-type semiconductor properties (8,9).

(17,18)

(16)

of ( C h i a - 2 H 0 ) 2

w

T h e e x p e r i m e n t a l b e h a v i o r d e s c r i b e d above c a n b e e x p l a i n e d b y

c o n s i d e r i n g t h e p o l y c r y s t a l l i n e c h l o r o p h y l l as a s e m i c o n d u c t i n g c a t h o d e a n d t h e finely d i s p e r s e d p l a t i n u m p a r t i c l e s as a m e t a l l i c a n o d e i m m e r s e d i n a n a c i d i c aqueous electrolyte a c c o r d i n g to t h e d e s c r i p t i o n of W r i g h t o n (4), M a v r o i d e s ( 5 ) , O h n i s h i (19),

a n d their co-workers. I n dark equili-

b r a t i o n , there is a n energy b a n d b e n d i n g at t h e C h i a surface, p r o d u c i n g a S c h o t t k y b a r r i e r at t h e C h i a - e l e c t r o l y t e interface.

Electron-hole pairs

generated at t h e s e m i c o n d u c t o r surface b y l i g h t a b s o r p t i o n across t h e b a n d g a p leads to a s e p a r a t i o n b y t h e electric e l i m i n a t i n g w a s t e f u l r e c o m b i n a t i o n b a c k reactions.

field

of the barrier,

E l e c t r o n s are trans-


222

INORGANIC

COMPOUNDS WITH UNUSUAL

f e r r e d f r o m t h e c a t h o d e surface to the H / H +

PROPERTIES

II

free energy l e v e l o f t h e

2

aqueous phase, l i b e r a t i n g h y d r o g e n i n t h e r e a c t i o n : 2 C h l a + 2 H 0 -> 2 C h l a + H + 2 0 H " +

2

2

T h e holes m o v e i n t o t h e b u l k o f ( C h i a • 2 H 0 ) 2

t h r o u g h t h e p l a t i n u m anode, to t h e H 0 / 0 2

2

a n d are t h e n t r a n s m i t t e d

2

l e v e l of t h e electrolyte, g i v i n g


off o x y g e n i n t h e r e a c t i o n :

2Chl a + H 0 +

2Chl a + ^ 0

2

T h e photooxidation of ( C h i a - 2 H 0 ) > 2

n

2

+ 2H

+

b y w a t e r r e c e n t l y has b e e n

2

e s t a b l i s h e d b y E S R observations ( 2 ) . R has b e e n n o t e d r e c e n t l y (2,7,16) of c h l o r o p h y l l (8,9,20-28)

( C h ia - H 0 ) 2

that a m o n g k n o w n aggregates ( A ) and ( C h i a - 2 H 0 ) >

2

2

n

2

( B ) are d i s t i n g u i s h e d b y t h e i r a b i l i t y to b e p h o t o o x i d i z e d i n t h e p r e s e n c e of w a t e r , g i v i n g rise t o w e l l - c h a r a c t e r i z e d p h o t o v o l t a i c b e h a v i o r

(1,7,16).

S i g n i f i c a n t l y , i t w a s suggested that t h e p h o t o o x i d i z e d d i h y d r a t e aggregate, (Chi a • 2H 0) 2

n

+

, not ( C h i a • H 0 ) 2

2

+

, is sufficiently strong a n o x i d a n t t o

b e r e a d i l y r e d u c e d b y w a t e r ( 2 ) . T h e p h o t o c h e m i c a l a c t i v i t y of C h i a H 0 aggregates has b e e n a t t r i b u t e d t o p h o t o a c t i v a t e d p r o t o n shifts 2

32)

(31,

between the magnesium-bound water molecule a n d the c a r b o n y l

g r o u p t o w h i c h i t is h y d r o g e n b o n d e d , r e s u l t i n g i n t h e a c q u i s i t i o n b y the m a g n e s i u m a t o m of a n e g a t i v e charge (31,32).

T h e magnesium atom

b e i n g conjugated to r i n g V , t h e negative c h a r g e it acquires d u r i n g p h o t o a c t i v a t i o n is expected to b e r e a d i l y t r a n s m i t t e d v i a vr-electron resonance to t h e C 9 keto g r o u p , w h i c h is p r e s u m a b l y t h e site at w h i c h t h e e l e c t r o n leaves b o t h aggregates ( A ) a n d ( B ) d u r i n g t h e p r i m a r y l i g h t r e a c t i o n . W e c a n thus r a t i o n a l i z e the a p p a r e n t l y s y m b i o t i c roles of t h e m a g n e s i u m atom a n d the ring V cyclopentanone r i n g i n the photochemical activity of ( A ) a n d ( B ) . W e note that the charge transfer m e c h a n i s m d e s c r i b e d here appears to b e feasible o n l y i n ( A ) a n d ( B ) . I n C

2

C h i a dimers

i n w h i c h the C = 0 • • • H ( H ) O • • • M g interactions i n v o l v e t h e C 9 k e t o g r o u p (22, 23, 26, 27, 28) as i n t h e d i m e r of C h i a p o l y h y d r a t e (26) c o v a l e n t l y l i n k e d d i m e r s (22,23)

or i n

i n w h i c h the C I O C = O • • • H ( H ) -

O - • • M g interactions are s t e r i c a l l y f o r b i d d e n (26,27),

d e r e a l i z a t i o n of

the n e g a t i v e charge a c q u i r e d b y the m a g n e s i u m a t o m i n p h o t o a c t i v a t i o n to t h e C 9 keto c a r b o n y l is e x p e c t e d to b e s t a b i l i z e d b y t h e w a t e r p r o t o n to w h i c h the c a r b o n y l g r o u p is h y d r o g e n b o n d e d . It has b e e n r e p o r t e d that, u n l i k e ( C h i a - H 0 ) 2

2

and (Chi a - 2 H 0 ) , 2

n

the C 9 - l i n k e d C

2

sym-

m e t r i c a l d i m e r c a n o n l y b e p h o t o o x i d i z e d i n t h e p r e s e n c e of a n a d d e d e l e c t r o n acceptor, s u c h as t e t r a n i t r o m e t h a n e

(28).


18.

GALLOWAY E T A L .

223

Photochemical Splitting of H 0 2

T h e present d e m o n s t r a t i o n o f t h e p h o t o c h e m i c a l s p l i t t i n g o f w a t e r b y ( C h i a • 21120)^ lends s u p p o r t t o t h e r e c e n t l y p r o p o s e d photosynthesis m o d e l i n w h i c h a single p h o t o s y s t e m m a y b e c a p a b l e of t h e i n v i v o w a t e r splitting reaction v i a a two-photon activation mechanism ( 1 ) . It was at o n e t i m e c o m m o n l y b e l i e v e d that t w o C h i a photosystems are r e q u i r e d i n b r i n g i n g a b o u t w a t e r s p l i t t i n g a c c o r d i n g to the so-called "series s c h e m e " or " Z s c h e m e " o f photosynthesis.

Attempts to achieve

photochemical


s p l i t t i n g of w a t e r i n v i t r o b a s e d (see, f o r e x a m p l e , R e f . 33) o n t h e series scheme have n o t thus f a r b e e n b r o u g h t t o f r u i t i o n . C u r r e n t efforts i n this l a b o r a t o r y a r e c o n c e r n e d w i t h d e t e r m i n i n g the q u a n t u m r e q u i r e m e n t f o r t h e i n v i t r o w a t e r - s p l i t t i n g r e a c t i o n a n d the p o w e r efficiency of t h e p h o t o c h e m i c a l c o n v e r s i o n .

T h e effects o f

o x y g e n o n t h e p h o t o v o l t a i c a c t i v i t y of t h e p l a t i n i z e d C h i a electrode s h o w n i n F i g u r e 2 c a n b e r e l a t e d t o s i m i l a r effects o b s e r v e d earlier ( 1 ) . M o l e c u l a r o x y g e n appears to exert a n i n h i b i t i v e role i n C h i a • H 0 l i g h t 2

reactions.

T h i s role c a n b e responsible, i n p a r t at least, f o r t h e o b s e r v e d

a c c e l e r a t i o n o f t h e a p p a r e n t p h o t o l y t i c rate a t temperatures o x y g e n is i n s o l u b l e i n w a t e r .

at w h i c h

I t is k n o w n that the manganese i o n p l a y s

a p a r t i n the e v o l u t i o n of o x y g e n i n p l a n t photosynthesis (34). I t m a y b e p o s s i b l e to find t h e i n v i t r o a n a l o g f o r t h e i n v i v o manganese i o n effect. It appears reasonable to suppose that success i n a c h i e v i n g h i g h l y efficient p h o t o c h e m i c a l s p l i t t i n g of w a t e r u l t i m a t e l y c a n d e p e n d o n t h e extent t o w h i c h w e are c a p a b l e of u n d e r s t a n d i n g a n d d u p l i c a t i n g t h e i n v i v o l i g h t reaction i n the laboratory. A chnowledgmen t T h i s research w a s s u p p o r t e d b y t h e A l c o a F o u n d a t i o n a n d b y t h e N a t i o n a l Science F o u n d a t i o n . Assistance b y R . G . C o o k s a n d A . B . C o d d i n g t o n i n t h e m e a s u r e m e n t of mass spectra is g r a t e f u l l y a c k n o w l e d g e d . Literature Cited 1. Fong, F. K., Polles, J. S., Galloway, L., Fruge, D. R., J. Am. Chem. Soc. (1977) 99, 5802. 2. Fong, F. K., Hoff, A. J., Brinkman, F. A., J. Am. Chem. Soc. (1978) 100, 619. 3. Fujishima, A., Honda, K., Nature (1972) 238, 37. 4. Wrighton, M. S., Wolczansdi, P. T., Ellis, A. B., J. Solid State Chem. (1977) 22, 17. 5. Mavroides, J. G., Katalas, J. A., Kolesar, D. F., Appl. Phys. Lett. (1976) 28, 241. 6. Tang, C. W., Albrecht, A.C.,Mol.Cryst. Liq. Cryst. (1974) 25, 53. 7. Fetterman, L. M., Galloway, L., Winograd, N., Fong, F. K., J. Am. Chem. Soc. (1977) 99, 653. 8. Fong, F. K., Koester, V. J., J. Am. Chem. Soc. (1975) 97, 6888. 9. Fong, F. K., Koester, V. J., Biochim. Biophys. Acta (1976) 423, 52.



224

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

II

10. Galloway, L., Roettger, J. D., Fruge, D. R., Fong, F. K., J. Am. Chem. Soc. (1978) 100, 4635. 11. Beynon, J. H., Cooks, R.G.,Caprioli, R. M., Proc. R. Soc. London, Ser. A (1972) 327, 1. 12. Papp, N., Kerwin, L., Phys. Rev. Lett. (1969) 22, 1343. 13. Fong, F. K., Galloway, L., J. Am. Chem. Soc. (1978) 100, 3594. 14. Galloway, L., Fruge, D. R., Fong, F. K., J. Am. Chem. Soc. unpublished data. 15. Hersch, P., Proc. Int. Symp. Microchem. 1958 (1959) 141-150. 16. Fong, F. K., Winograd, N., J. Am. Chem. Soc. (1976) 98, 2287. 17. Tang, C. W., Albrecht, A. C., J. Chem. Phys. (1975) 62, 2139. 18. Ibid. (1975) 63, 953. 19. Ohnishi, T., Nakato, Y., Tsubomura, H., Ber. Bunsenges. Phys. Chem. (1975) 79, 523. 20. Strouse, C. E., Proc. Nat. Acad. Sci. USA (1974) 71, 325. 21. Chow, H.-C., Serlin, R., Strouse, C. E., J. Am. Chem. Soc. (1975) 97, 7230. 22. Boxer, S.G.,Closs, G. L.,J.Am. Chem. Soc. (1976) 98, 5406. 23. Boxer, S. G., Ph.D. dissertation, Department of Chemistry, University of Chicago (1976). 24. Winograd, N., Shepard, A., Karweik, D. H., Koester, V. J., Fong, F. K., J. Am. Chem. Soc. (1976) 98, 2369. 25. Fong, F. K., Koester, V. J., Polles, J. S., J. Am. Chem. Soc. (1976) 98, 6406. 26. Fong, F. K., Koester, V. J., Galloway, L., J. Am. Chem. Soc. (1977) 99, 2372. 27. Shipman, L. L., Cotton, T. M., Norris, J. R., Katz, J. J., Proc. Nat. Acad. Sci. USA (1976) 73, 1791. 28. Wasielewski, M. R., Studier, M. H., Katz, J. J., Proc. Nat. Acad. Sci. USA (1976) 73, 4282. 29. Cotton, T. M., Ph.D. dissertation, Department of Chemistry, Northwestern University (1976). 30. Clarke, R. H., Hobart, D. R., FEBS Lett. (1977) 82, 155. 31. Fong, F. K., Proc. Nat. Acad. Sci. USA (1974) 71, 3692. 32. Fong, F. K., Appl. Phys. (1975) 6, 151. 33. Bolton, J. R., J. Solid State Chem. (1977) 22, 4. 34. Cheniae, G. M., Annu. Rev. Plant Physiol. (1970) 21, 467. RECEIVED February 22, 1978.


Gaseous Evolution of Molecular Hydrogen and Oxygen in

Recommend Documents