The Chemistry of Acid Rain - ACS Publications - American Chemical

Chapter 10

Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 11, 2015 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch010

Photocatalytic Formation of Hydrogen Peroxide 1

Detlef W. Hahnemann , Michael R. Hoffmann, Andrew P. Hong, and Claudius Kormann W. M. Keck Laboratories, California Institute of Technology, Pasadena, CA 91125

The two-electron reduction of molecular oxygen to hydrogen peroxide can be catalyzed by metal oxide semiconductor p a r t i c l e s i n the presence of visible and near-UV l i g h t . Even though very high quantum y i e l d s for t h i s process are observed, rather low steady-state concentrations of HO are reached. D e t a i l e d mechanisms are presented to e x p l a i n these experimental f i n d i n g s . Metal oxide p a r t i c l e s are found i n atmospheric and surface waters. The environmental significance of photocatalytic formation of H O on these p a r t i c l e s i n n a t u r a l systems i s discussed. 2

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Hydrogen p e r o x i d e , H 0 , i s one o f t h e most p o w e r f u l o x i d a n t s i n haze a e r o s o l s , c l o u d s , a n d hydrometeors a t low pH ( I ) . A major pathway f o r t h e f o r m a t i o n o f s u l f u r i c a c i d i n humid atmospheres below pH 5 seems t o be t h e o x i d a t i o n o f s u l f u r d i o x i d e b y H 0 (2-41. Hydrometeor c o n c e n t r a t i o n s o f hydrogen p e r o x i d e a s h i g h a s 50 μΜ have r e c e n t l y been r e p o r t e d b y Kok a n d co-workers ( 5 - 7 ) , w h i l e t y p i c a l atmospheric H 0 l e v e l s up t o 5 ppb have been p r e d i c t e d from Henry's law c a l c u l a t i o n s ( 8 , 9 ) . Various sources f o r t h e p r o d u c t i o n o f H 0 i n t h e atmosphere have been proposed: i t c a n be g e n e r a t e d i n t h e g a s phase b y t h e c o m b i n a t i o n o f h y d r o p e r o x y l r a d i c a l s , Η 0 · ( 1 0 ) , a t t h e a i r - w a t e r i n t e r f a c e due t o p h o t o i n d u c e d redox p r o c e s s e s (XX), and i n t h e aqueous phase v i a photo-catalyzed r e a c t i o n s w i t h h u m i c / f u l v i c a c i d and green algae as m e d i a t o r s (Zepp, R. G. , EPA E n v i r o n m e n t a l R e s e a r c h L a b o r a t o r y a t A t h e n s , p e r s o n a l communication ). F u r t h e r m o r e , Η 0 · r a d i c a l s 2

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'Permanent address: Hahn-Meitner Institut, Bereich Strahlenchemie, Glienicker StraBe 100, D 1000 Berlin 39, Federal Republic of Germany

0097-6156/87/0349-0120$06.00/0 © 1987 American Chemical Society

In The Chemistry of Acid Rain; Johnson, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Photocatalytic Formation of Hydrogen Peroxide

w h i c h a r e produced i n the gas phase can be scavenged by the water d r o p l e t s where they then form H 0 . This i n s i t u generated h y d r o g e n p e r o x i d e t o g e t h e r w i t h the H 0 scavenged from the gas phase i s thought t o be the main source f o r the H 0 a c c u m u l a t e d i n c l o u d w a t e r d r o p l e t s (10.12.13). Another v e r y l i k e l y p o s s i b i l i t y f o r H 0 p r o d u c t i o n , however, has so f a r been n e g l e c t e d i n the d e s i g n of models f o r a e r o s o l - , f o g - , and c l o u d w a t e r c h e m i s t r y . V a r i o u s m e t a l o x i d e s , some of w h i c h a r e v e r y abundant i n n a t u r a l environments, have been shown t o a c t as p h o t o c a t a l y s t s f o r a l a r g e v a r i e t y o f r e a c t i o n s ( 1 4 ) , e.g., the p h o t o l y s i s o f d e s e r t sands r e s u l t e d i n the p r o d u c t i o n of ammonia from d i n i t r o g e n ( 1 5 ) . Atmospheric p a r t i c u l a t e m a t t e r o r i g i n a t e s m a i n l y from n a t u r a l s o u r c e s (721-1850 Tg y r " i n 1979 g l o b a l l y ) b u t a l s o from man-made emissions (125-385 Tg y r " i n 1982 i n the USA) with iron, t i t a n i u m , and z i n c b e i n g some of the most abundant t r a n s i t i o n m e t a l s d e t e c t e d i n ambient samples ( 1 6 ) . These m e t a l s a r e p r e s e n t as h y d r o x i d e s b u t a l s o f r e q u e n t l y as the r e s p e c t i v e o x i d e s , depending upon the e n v i r o n m e n t a l c o n d i t i o n s ( 1 7 ) . Hence the p r e s e n t work a d d r e s s e s the q u e s t i o n of hydrogen p e r o x i d e f o r m a t i o n i n aqueous s o l u t i o n s as c a t a l y z e d by m e t a l o x i d e s upon i r r a d i a t i o n w i t h v i s i b l e and near-UV l i g h t . 2

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P h o t o c a t a l v s i s w i t h Semiconductors Many m e t a l c h a l c o g e n i d e s and o x i d e s a r e known t o be semiconduc tors. These m a t e r i a l s can a c t as s e n s i t i z e r s f o r l i g h t - i n d u c e d r e d o x p r o c e s s e s due to t h e i r e l e c t r o n i c s t r u c t u r e c o n s i s t i n g of a valence band w i t h f i l l e d molecular orbitals (MO's) and a c o n d u c t i o n band w i t h empty MO's. A b s o r p t i o n of a p h o t o n w i t h an energy above the bandgap energy Eg g e n e r a l l y l e a d s t o the forma t i o n of an e l e c t r o n / h o l e p a i r i n the semiconductor p a r t i c l e ( 1 8 ) : semiconductor

^

e~(cb)

+

+

h (vb)

(1)

I n the absence of s u i t a b l e scavengers, r e c o m b i n a t i o n o c c u r s w i t h i n a few nanoseconds ( 1 9 ) . V a l e n c e band h o l e s ( h ( v b ) ) have been shown t o be p o w e r f u l o x i d a n t s (20-23) whereas c o n d u c t i o n band e l e c t r o n s ( e " ( c b ) ) can a c t as r e d u c t a n t s (24,25). The redox p o t e n t i a l s of b o t h , e~ and h , a r e d e t e r m i n e d by the r e l a t i v e p o s i t i o n of the c o n d u c t i o n and v a l e n c e band, respectively. Bandgap p o s i t i o n s a r e material constants which have been determined f o r a wide v a r i e t y of s e m i c o n d u c t o r s (26). Most m a t e r i a l s show " N e r n s t i a n " b e h a v i o r w h i c h r e s u l t s i n a s h i f t of the s u r f a c e p o t e n t i a l by 59 mV i n the n e g a t i v e d i r e c t i o n w i t h a pH i n c r e a s e of ΔρΗ = 1. C o n s e q u e n t l y e l e c t r o n s a r e b e t t e r r e d u c t a n t s in alkaline s o l u t i o n s w h i l e h o l e s have a h i g h e r o x i d a t i o n p o t e n t i a l i n the a c i d pH-range ( 2 6 ) . Thus, w i t h the r i g h t c h o i c e of semiconductor and pH, the redox p o t e n t i a l of the e ~ ( c b ) can be v a r i e d from +0.5 t o -1.5 V ( v s . NHE) and t h a t of the h ( v b ) from +1.0 to more than +3.5 V. T h i s s u f f i c i e n t l y c o v e r s the f u l l range of r e d o x c h e m i s t r y o f the H 0 / 0 system ( 2 7 ) . +

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THE CHEMISTRY OF ACID RAIN

H 0 c a n t h e r e f o r e be formed v i a two d i f f e r e n t pathways i n a n a e r a t e d aqueous s o l u t i o n p r o v i d e d e ~ ( c b ) a n d h ( v b ) a r e g e n e r a t e d *· 2

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R e a c t i o n 2 has been s t u d i e d i n g r e a t d e t a i l (28-44) s i n c e Baur and N e u w e i l e r ( 2 8 ) i n 1927 observed t h e f o r m a t i o n o f H 0 when they i l l u m i n a t e d aqueous z i n c o x i d e s u s p e n s i o n s i n the presence of g l y c e r i n a n d g l u c o s e w h i c h i n t u r n were o x i d i z e d . Appreciable y i e l d s o f hydrogen p e r o x i d e a r e d e t e c t e d o n l y when a p p r o p r i a t e e l e c t r o n d o n o r s , D, a r e added p r i o r t o i l l u m i n a t i o n . This s t r o n g l y i n d i c a t e s t h a t i t i s t h e e l e c t r o n donor, D, w h i c h i s a d s o r b e d on t h e c a t a l y s t ' s s u r f a c e and hence s a c r i f i c e d v i a R e a c t i o n 4. 2

D

+

+

h (vb)

-»

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+

ϋ ·

(4) +

The presence of s u r f i c i a l D interferes with the e~/h r e c o m b i n a t i o n l e a v i n g e " ( c b ) ( c o n d u c t i o n band e l e c t r o n s ) b e h i n d w h i c h then r e a c t w i t h d i o x y g e n v i a R e a c t i o n 2. S m a l l q u a n t i t i e s of H 0 d e t e c t e d i n t h e absence o f added donors c o n t a i n e d o n l y oxygen atoms from 0 a s shown b y l a b e l l i n g s t u d i e s ( 3 4 ) w h i c h i n d i c a t e s the presence o f contaminants i n the semiconductor that are able to f i l l the h ( v b ) . Cadmium s u l f i d e , CdSe, and ZnO showed t h e h i g h e s t c a t a l y t i c a c t i v i t y f o r d i o x y g e n reduction (35,38). T i t a n i u m d i o x i d e , on t h e o t h e r hand, w h i c h seems t o be the m a t e r i a l w i t h t h e g r e a t e s t p o t e n t i a l f o r w a t e r s p l i t t i n g i s r e p o r t e d t o have n e g l i g a b l e a c t i v i t y (35,41-43). The o x i d a t i o n o f w a t e r v i a R e a c t i o n 3, however, h a s n o t y e t been demonstrated unambiguously. W h i l e Rao e t αϊ. ( 4 5 ) r e p o r t e d t h e f o r m a t i o n o f H 0 i n w a t e r s p l i t t i n g e x p e r i m e n t s on ZnO and T i 0 , S a l v a d o r and Decker ( 4 6 ) c o u l d n o t v e r i f y t h i s o b s e r v a t i o n . The i n t e r m e d i a t e production of H 0 as the f i r s t molecular step of the four-hole o x i d a t i o n o f water t o d i o x y g e n has been p r e d i c t e d (46,47) a n d a v a r i e t y o f f r e e r a d i c a l i n t e r m e d i a t e s have a l r e a d y been d e t e c t e d (48-50). Whenever t h e o x i d a t i o n o f H 0 i s found t o p r o c e e d w i t h h i g h y i e l d s t h e r e i s no i n d i c a t i o n o f hydrogen p e r o x i d e f o r m a t i o n (51-54), but the dioxygen production seems t o p r o c e e d v i a d i f f e r e n t i n t e r m e d i a t e p e r o x i d e s a s shown by Baur and P e r r e t (51.52): they s t u d i e d t h e p h o t o c a t a l y t i c f o r m a t i o n o f 0 on ZnO i n the p r e s e n c e o f s i l v e r n i t r a t e and observed the intermediate production of s i l v e r peroxides. I n t h e c a s e o f T i 0 , t h e absence of detectable amounts of H 0 and 0 during water photoelectrolysis experiments has been explained w i t h the we11-documented a d s o r p t i o n and p h o t o - u p t a k e o f these o x o - s p e c i e s by t h e m a t e r i a l (55-71) and t h e i r i n c o r p o r a t i o n i n t o t h e m o l e c u l a r s t r u c t u r e a s p e r o x y t i t a n a t e s (68.72.73). The above d i s c u s s i o n i n d i c a t e s t h a t a d e t a i l e d s t u d y o f t h e mechanism o f t h e l i g h t - i n d u c e d hydrogen p e r o x i d e p r o d u c t i o n on 2

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various oxide surfaces i s warranted i n order t o judge i t s p o t e n t i a l environmental implications. U s i n g a newly d e v e l o p e d p o l a r o g r a p h i c technique f o r t h e k i n e t i c a n a l y s i s o f hydrogen p e r o x i d e c o n c e n t r a t i o n s w i t h a s e n s i t i v i t y o f 1 0 ~ M, we p r e s e n t f u r t h e r i n s i g h t i n t o t h e p h o t o c a t a l y t i c f o r m a t i o n and d e s t r u c t i o n of H 0 . 7

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Details

Several c a t a l y s t s were p r e p a r e d from c h e m i c a l s of highest a v a i l a b l e grade. Mechanistic s t u d i e s were p e r f o r m e d using colloidal dispersions of z i n c oxide, ZnO, w h i c h contained p a r t i c l e s w i t h a mean d i a m e t e r o f 5 nm (Bahnemann, D. W. ; Kormann, C; Hoffmann, M. R. .1. Phvs. Chem. submitted 11/3/86). S u s p e n s i o n s o f such s m a l l a g g r e g a t e s e x h i b i t n e g l i g i b l e light s c a t t e r i n g p r o p e r t i e s and c o n v e n t i o n a l s p e c t r o s c o p i c techniques can be u s e d t o study o n g o i n g r e a c t i o n s ( 7 4 ) . The p r e p a r a t i o n o f t i t a n i u m d i o x i d e p a r t i c l e s , T i 0 , coated w i t h c o b a l t ( I I ) t e t r a s u l f o p h t h a l o c y a n i n e , C o ( I I ) T S P , has r e c e n t l y been r e p o r t e d (Hong, A. P.; Bahnemann, D. W. ; Hoffmann, M. R. J . Phvs. Chem. a c c e p t e d f o r p u b l i c a t i o n 12/12/86). These p a r t i c l e s have a mean d i a m e t e r of 30 nm w i t h 3 0 % o f t h e s u r f a c e a r e a c o v e r e d w i t h C o ( I I ) T S P . D e s e r t sand o b t a i n e d from D e a t h V a l l e y h a s been t r e a t e d w i t h s e v e r a l a c i d washing c y c l e s and s e l e c t e d by s i z e and w e i g h t (Kormann, C. ; Bahnemann, D. W. ; Hoffmann, M. R. u n p u b l i s h e d d a t a ) . The H 0 c o n c e n t r a t i o n was measured c o n t i n u o u s l y w i t h a Y S I - C l a r k 2510 O x i d a s e Probe c o n n e c t e d t o a Y S I model 25 o x i d a s e meter. The s u r f a c e o f t h e e l e c t r o d e was c o v e r e d w i t h a d i a l y s i s membrane (molecular w e i g h t c u t o f f 12,000 - 14,000) t o p r e v e n t any i n t e r f e r e n c e caused by t h e c a t a l y s t p a r t i c l e s . Equilibration times between 30 and 60 minutes were a l l o w e d t o e n s u r e a s t a b l e s i g n a l once t h e e l e c t r o d e was immersed i n t o t h e r e a c t i o n s o l u t i o n . F o l l o w i n g e q u i l i b r a t i o n , changes i n H 0 c o n t e n t were m o n i t o r e d a t a s e n s i t i v i t y o f 1 0 " M and a time c o n s t a n t o f 1 s. The e l e c t r o d e was c a l i b r a t e d f o l l o w i n g each k i n e t i c e x p e r i m e n t u s i n g a s t a n d a r d a d d i t i o n method; l i n e a r response was o b t a i n e d between 1 0 ~ and 2·10~ M H 0 . S i n c e t h i s p o l a r o g r a p h i c method i s s e n s i t i v e t o any s p e c i e s w i t h a redox p o t e n t i a l o f 700 mV, d i f f e r e n t methods were used t o v e r i f y t h e f o r m a t i o n o f hydrogen p e r o x i d e . Following the i l l u m i n a t i o n a n a l i q u o t o f t h e s o l u t i o n was t a k e n a n d t i t r a t e d w i t h i o d i d e i n t h e p r e s e n c e o f a c a t a l y s t (75,76) t o form t h e I ~ a n i o n w h i c h was q u a n t i t a t i v e l y measured by s p e c t r o p h o t o m e t r y (fe(352nm) = 26400 M" cm" ) ( 7 7 ) . As a second check and when v e r y s m a l l c o n c e n t r a t i o n s o f H 0 were d e t e c t e d w i t h t h e p o l a r o g r a p h i c method a v e r y s e n s i t i v e f l u o r o m e t r i c d e t e r m i n a t i o n ( d e t e c t i o n l i m i t : 1.2·10" M H 0 ) was employed. This l a t t e r procedure i n v o l v e s t h e r e a c t i o n o f H 0 w i t h p-hydroxypheny l a c e t i c a c i d i n the p r e s e n c e o f h o r s e r a d i s h p e r o x i d a s e t o y i e l d a p r o d u c t w h i c h f l u o r e s c e s a t 400 nm ( X ( e x ) = 320 nm) (78.79). Even though t h e p o l a r o g r a p h i c method c a n n o t d i f f e r e n t i a t e between H 0 and o t h e r species w i t h a s i m i l a r e l e c t r o c h e m i c a l half-wave p o t e n t i a l , the use o f two a l t e r n a t i v e c h e m i c a l a n a l y s i s t e c h n i q u e s e l i m i n a t e s such a r t i f a c t s . I n t e r f e r e n c e s by o t h e r c h e m i c a l s s u c h a s ozone 2

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c a n a l s o be e x c l u d e d i n t h i s s t u d y s i n c e i t i s e x t r e m e l y u n l i k e l y t h a t they s h o u l d o c c u r w i t h a l l t h r e e methods t o the same e x t e n d . F i g u r e 1 shows the expermental s e t - u p f o r the p h o t o l y s i s studies. White l i g h t of a 450 W Xe lamp (Osram XBO) passes through a water f i l t e r , a monochromator (PRA B102) and an a p p r o p r i a t e U V - f i l t e r b e f o r e the c o n t i n u o u s l y s t i r r e d sample i s irradiated. The a n a l o g s i g n a l of the o x i d a s e meter i s a m p l i f i e d to g i v e the r i g h t a m p l i t u d e f o r the i n p u t o f the A/D converter (MBC Dash 8) w h i c h d i g i t i z e s the d a t a and feeds them i n t o an IBM PC/AT f o r k i n e t i c a n a l y s i s . Aberchrome 540 i s used f o r the d e t e r m i n a t i o n of the l i g h t f l u x t o e n a b l e an a b s o l u t e measurement of the quantum y i e l d s ( 8 0 ) . R e s u l t s and D i s c u s s i o n The f o r m a t i o n of hydrogen p e r o x i d e upon the i l l u m i n a t i o n of an a i r - s a t u r a t e d aqueous c o l l o i d a l s u s p e n s i o n o f z i n c o x i d e p a r t i c l e s i s shown i n F i g u r e 2. No H 0 i s p r o d u c e d when l i g h t e n e r g i e s below the bandgap energy of ZnO where employed (Eg(ZnO)= 3.4 eV or X ( e x ) = 365 nm), e.g., X ( e x ) = 400 nm. As the i r r a d i a t i o n was p e r f o r m e d a t lower w a v e l e n g t h s , e.g., X ( e x ) = 366 and 320 nm, the sudden i n c r e a s e of the s i g n a l from the p o l a r o g r a p h i c a n a l y z e r indicated H 0 f o r m a t i o n w i t h a w a v e l e n g t h - i n d e p e n d e n t quantum y i e l d of 6%. No hydrogen p e r o x i d e i s formed o r d e p l e t e d i n the absence of l i g h t . P r o l o n g e d i l l u m i n a t i o n l e a d s t o the f o r m a t i o n of a s t e a d y - s t a t e i n [ H 0 ] of 1.2·10~ M i n i r r a d i a t e d samples. The q u a n t u m - y i e l d s φ d e t e r m i n e d from the i n i t i a l s l o p e of the [ H 0 ] v s . time p l o t s depended s t r o n g l y on the oxygen-content of the s o l u t i o n : φ i n c r e a s e d t o 10% i n 0 - s a t u r a t e d samples and d e c r e a s e d s h a r p l y below 2·10~ M 0 . No H 0 i s p r o d u c e d when anoxic s o l u t i o n s were i r r a d i a t e d . The f o l l o w i n g mechanism e x p l a i n s these o b s e r v a t i o n s . E l e c t r o n s and h o l e s a r e formed i n the z i n c o x i d e p a r t i c l e s under bandgap i r r a d i a t i o n ( R e a c t i o n 1 ) . T h e i r r e c o m b i n a t i o n i s p r e v e n t e d by the p r e s e n c e of 2 mM a c e t a t e i o n s w h i c h a r e s t r o n g l y a d s o r b e d onto the ZnO s u r f a c e and a r e oxidized v i a : 2

2

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CH C00~ 3

+

+

(CH C00)*

h (vb)

-»

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#

->

CH C00"/H* 2

2

(CH C00)

2

#

(5)

3

q

and/or

#

CH /C0 3

2

(6)

The f a t e of the r a d i c a l s produced v i a R e a c t i o n 6 i s c u r r e n t l y being investigated. I n i t i a l r e s u l t s i n d i c a t e the f o r m a t i o n of o r g a n i c p e r o x i d e s and suggest the i n t e r m e d i a t e f o r m a t i o n of p e r o x y - r a d i c a l s (Kormann, C. ; Bahnemann, D. W. ; Hoffmann, M. R. u n p u b l i s h e d r e s u l t s ) . W i t h the i n h i b i t i o n of charge r e c o m b i n a t i o n c o n d u c t i o n band e l e c t r o n s a r e now a v a i l a b l e t o reduce m o l e c u l a r oxygen y i e l d i n g H 0 (Reaction 2). The observed steady-state c o n c e n t r a t i o n o f hydrogen p e r o x i d e can be u n d e r s t o o d as a c o m p e t i t i o n between R e a c t i o n s 2 and 7. 2

2


10.


BAHNKMANN ET AL.

125

H 0 ELECTRODE 2

2

1cm


CELL LIGHT IR-FILTER M0N0CHR0- UV- Γ FILTER MATOR SOURCE (HoO) MAGNETIC (450 W) STIRRER

IBM PC/AT

A/DCONVERTER (MBC)

OXIDASE METER (YSI)

F i g u r e 1. E x p e r i m e n t a l c o n f i g u r a t i o n u s e d f o r i r r a d i a t i o n s and hydrogen p e r o x i d e d e t e c t i o n .

T I M E / MIN.

Figure 2. H 0 f o r m a t i o n upon i l l u m i n a t i o n o f a n aqueous c o l l o i d a l s u s p e n s i o n o f z i n c o x i d e i n t h e p r e s e n c e o f 2 mM a c e t a t e ( o t h e r exp. cond. a r e g i v e n i n t h e f i g u r e ) . 2

2



126

H 0 2

2e"(cb)

2

+

+

2H aq

-*

2H 0

(7)

2

4

The value [ H 0 ] ( s s ) = 1.2·10" M measured i n air-saturated s o l u t i o n s ( [ 0 ] ( s s ) = 2.4·10" M) s u g g e s t s t h a t R e a c t i o n 7 i s about t w i c e a s e f f i c i e n t a s t h e i n i t i a l d i o x y g e n r e d u c t i o n . H 0 i s a l s o produced i n t h e absence o f h o l e scavengers p r o v i d e d t h e r i g h t c a t a l y s t i s used. F i g u r e 3 shows t h e f o r m a t i o n of hydrogen p e r o x i d e upon bandgap i l l u m i n a t i o n o f a n oxygenated aqueous s u s p e n s i o n o f T i 0 c o a t e d w i t h C o ( I I ) T S P w h i c h a c t s a s a n e l e c t r o n r e l a y t o t r a n s f e r e " ( c b ) o n t o 0 (Hong, A. P.; Bahnemann, D. W. ; Hoffmann, M. R. J . Phvs. Chem. a c c e p t e d f o r p u b l i c a t i o n 12/12/86). I t i s c l e a r l y o b v i o u s from F i g u r e 3 t h a t t h e same s t e a d y - s t a t e c o n c e n t r a t i o n o f H 0 i s reached once t h e system i s p e r t u r b e d by t h e a d d i t i o n o f H 0 d u r i n g p r o l o n g e d i r r a d i a t i o n . As i n t h e case o f ZnO no f o r m a t i o n o r d e p l e t i o n o f hydrogen p e r o x i d e i s o b s e r v e d i n t h e absence o f l i g h t . An o c t a h e d r a l l y c o o r d i n a t e d s u r f a c e complex, e.g., T i - 0 ~ - C o ( I I I ) T S P - 0 ~ * , has been identified as the c a t a l y t i c a l l y active species i n the T i 0 - C o ( I I ) T S P system. W i t h d i o x y g e n b e i n g bound i n t h e form o f s u p e r o x i d e , 0 ~ · , t h i s complex p r o v e d t o be e x t r e m e l y s t a b l e b u t a c t s a s a n e f f e c t i v e a c c e p t o r f o r c o n d u c t i o n band e l e c t r o n s p r o d u c e d upon i r r a d i a t i o n o f t h e b u l k T i 0 : 2

2

4

2


2

2

2

2

2

2

2

2

2

2

2

2

T i - 0 " - C o ( I I I ) T S P - 0 " - + e " ( c b ) -» T i - 0 " - C o ( I I I ) T S P - H 0 2

+ H 0

2

The aquo complex formed e " ( c b ) t o form a reduced with 0

2

2

(8)

i n R e a c t i o n 8 r e a d i l y accepts another m e t a l c e n t e r w h i c h then q u i c k l y r e a c t s

2

Ti-0"-Co(II)TSP-H 0

+

2

0

-»

2

Ti-0~-Co(III)TSP-0 "*

(9)

2

y i e l d i n g a g a i n t h e s t a b l e 0 ~· complex. H 0 i s formed i n quantum y i e l d s c l o s e t o 5 0 % b y t h i s r e a c t i o n sequence. The observed n e t f o r m a t i o n o f H 0 i n d i c a t e s t h a t t h e v a l e n c e band h o l e s , h ( v b ) , a r e a b l e t o o x i d i z e water v i a R e a c t i o n 3. The low s t e a d y - s t a t e c o n c e n t r a t i o n s [ H 0 ] ( s s ) w h i c h a r e reached d u r i n g these experiments ( 5 - 25 μΜ depending on t h e n a t u r e o f t h e c a t a l y s t ) suggest t h a t o x i d a t i o n o f H 0 v i a 2

2

2

2

2

+

2

2

H 0 2

2

+

+

2h (vb)

-*

0

2

+

2

2

+

2H aq

(10)

e f f i c i e n t l y competes w i t h R e a c t i o n 3. When p r e s e n t a t h i g h c o n c e n t r a t i o n s , hydrogen p e r o x i d e c a n a l s o compete w i t h m o l e c u l a r oxygen ( R e a c t i o n 9) f o r t h e open c o o r d i n a t i o n s i t e on C o ( I I ) T S P : Ti-0"-Co(II)TSP-H 0 2

+ H 0 2

2

-» T i - 0 " - C o ( I I I ) T S P - 0 H

+ OH"

(11)

R e a c t i o n 11 i s a n a l o g o u s t o R e a c t i o n 8 i n t h a t i t l e a d s t o t h e o v e r a l l r e d u c t i o n o f H 0 b y two e " ( c b ) . The major d i f f e r e n c e 2

2


10.


BAHNEMANN ET AL.

Yll

between t h e z i n c o x i d e and t h e T i 0 - C o ( I I ) T S P systems i s t h a t t h e s u r f a c e complex ( C o ( I I ) T S P ) a c t s a s a n e l e c t r o n r e l a y i n one c a s e while sacrificial e l e c t r o n donors such a s a c e t a t e are a p r e r e q u i s i t e f o r i n t e r r u p t i o n of the e"/h recombination i n the other case. F o l l o w i n g t h e s t u d i e s o f t h e s e r a t h e r w e l l - d e f i n e d model systems a e r a t e d aqueous s u s p e n s i o n s o f d e s e r t sand p a r t i c l e s were i r r a d i a t e d ( X ( e x ) = 350 nm) i n t h e p r e s e n c e o f sodium a c e t a t e . The v e r y r a p i d f o r m a t i o n o f a s t e a d y - s t a t e c o n c e n t r a t i o n o f 0.2 uM H 0 upon i l l u m i n a t i o n i s shown i n F i g u r e 4. T h i s , however, decays v e r y q u i c k l y when t h e l i g h t i s t u r n e d o f f . A similar d e p l e t i o n i s a p p a r e n t when H 0 i s added i n t h e d a r k . I f the l i g h t i s t u r n e d on d u r i n g t h i s decay p e r i o d a s t e a d y - s t a t e o f 0.2 uM i s a g a i n e s t a b l i s h e d . These c y c l e s c a n be r e p e a t e d many t i m e s w i t h o u t any a p p a r e n t l o s s i n a c t i v i t y . I t has a l r e a d y been demonstrated ( 1 5 ) t h a t d e s e r t sand c o n t a i n s m i n e r a l s such a s hematite, a-Fe 0 , and a n a t a s e , Ti0 , and c a n thus be photocatalytically active. We e n v i s i o n a mechanism s i m i l a r t o t h a t p r o p o s e d f o r t h e model systems above t o a c c o u n t f o r t h e observed formation of H 0 on d e s e r t sands. I n t h i s case the e"/h p a i r ( R e a c t i o n 1) i s i n t e r c e p t e d b y t h e donor a c e t a t e ( R e a c t i o n s 5 a n d 6 ) l e a v i n g e ~ ( c b ) b e h i n d t o reduce 0 v i a R e a c t i o n 2. The e f f i c i e n t d e s t r u c t i o n o f H 0 , on t h e o t h e r hand, might be caused b y m e t a l i o n c o n t a m i n a n t s (M ) a d s o r b e d on t h e sand p a r t i c l e s (81-85). Hydrogen p e r o x i d e c a n thus r e a c t w i t h M i n a F e n t o n type r e a c t i o n ( 8 6 ) , 2


+

2

2

2

2

2

3

2

2

2

+

2

2

2

x+

x +

M

x +

+

H 0 2

2

-»

i x + 1 ) +

M

+ OH"

f o l l o w e d by t h e f r e e r a d i c a l R e a c t i o n s H 0 2

+

2

13 t o 17

(12)

(87.88). (13)

->

Η 0

Η0 ·

τ-

ο ~·

+

Η aq

(14)

-»

Η 0

+

ο

(15)

-»

Η 0

+

Η 0 · + 0 "· + Η aq 2

0Η·

0Η· 2

2

2

+

+

2

2

2

2

Η0 · 2

+

2

2

Η0 ·

+

0Η·

0Η·

+

0Η·

2

+

2

Η 0 2

ο

2

2

(16) (17)

overal1 the in results This reaction sequence disproportionation of H 0 and thus explains i t s rapid disappearance i n the dark. F i n a l l y , a word o f c a u t i o n s h o u l d be added r e g a r d i n g o t h e r p o s s i b i l i t i e s t o p h o t o g e n e r a t e hydrogen p e r o x i d e i n s u c h a n i l l - d e f i n e d n a t u r a l system. I n s p i t e of the r i g o r o u s p r e - t r e a t m e n t i t i s p o s s i b l e t h a t t r a c e amounts o f o r g a n i c a d s o r b a t e s a r e s t i l l p r e s e n t on t h e d e s e r t sand samples employed i n t h i s study. On t h e o t h e r hand, i t i s w e l l documented t h a t n a t u r a l l y o c c u r r i n g humic type m a t e r i a l s c a n a l s o be photochemical l y a c t i v e (89). Thus we cannot exclude the p o s s i b i l i t y t h a t p a r t o f t h e o b s e r v e d r a t h e r low s t e a d y - s t a t e 2

2



128


hi> on

' -5 mM H 02 o ô b e d 2

0 0

30

60

75 165

180

210

240

255

Time (min)

F i g u r e 3. F o r m a t i o n and d e p l e t i o n o f H 0 upon i r r a d i a t i o n ( X ( e x ) = 366 nm) o f a n oxygenated aqueous s u s p e n s i o n o f 0.3 g T i 0 - C o ( I I ) T S P / l a t pH 12. 2

2

2

time/min F i g u r e 4. F o r m a t i o n and d e p l e t i o n o f H 0 upon i l l u m i n a t i o n a n d i n t h e d a r k o b s e r v e d i n a n a e r a t e d aqueous s u s p e n s i o n o f Death V a l l e y d e s e r t sand ( o t h e r exp. cond. a r e g i v e n i n t h e f i g u r e ) . 2

2


10.

129


BAHNEMANN ET AL.

c o n c e n t r a t i o n o f H 0 i s formed v i a t h e d i r e c t e x c i t a t i o n o f such molecules. Further experiments a r e c u r r e n t l y being performed to d i f f e r e n t i a t e between these pathways. 2

2


C o n c l u d i n g Remarks We have shown that hydrogen peroxide c a n be produced photocatalytically i n the presence of semiconductor p a r t i c l e s . Quantum y i e l d s up t o 5 0 % and s t e a d y - s t a t e c o n c e n t r a t i o n s between 0.2 and 120 μΜ H 0 have been observed. While the reduction of m o l e c u l a r oxygen by e ~ ( c b ) seems t o be t h e most l i k e l y p r o c e s s f o r H 0 - f o r m a t i o n , o x i d a t i o n o f water by h ( v b ) s h o u l d i n p r i n c i p l e y i e l d t h e same p r o d u c t s . I t i s t h e o b s e r v a t i o n o f hydrogen p e r o x i d e p r o d u c t i o n upon t h e i l l u m i n a t i o n o f a n a e r a t e d aqueous s u s p e n s i o n o f d e s e r t sand p a r t i c l e s t h a t p r e s e n t s t h e most i n t r i g u i n g r e s u l t o f t h i s s t u d y . Submicron sand p a r t i c l e s a r e v e r y abundant i n t h e atmosphere where they a c t a s c o n d e n s a t i o n n u c l e i ( 9 0 ) . T h e i r involvement a s catalysts and/or photocatalysts i n chemical transformations o c c u r r i n g i n n a t u r a l environments h a s so f a r been n e g l e c t e d even though they a r e r a t h e r p e r s i s t e n t i n t h e atmosphere with h a l f - l i v e s o f s e v e r a l days b e f o r e p r e c i p i t a t i o n t a k e s p l a c e . We p r o p o s e t h e s u r f a c e r e a c t i o n o f p h o t o c a t a l y t i c a l l y formed H 0 with sulfur d i o x i d e ( S 0 ) and n i t r o u s o x i d e s (N0 ) as an a d d i t i o n a l pathway f o r t h e f o r m a t i o n o f a c i d r a i n . Even though due to i t s metal-catalyzed dismutation the steady-state c o n c e n t r a t i o n o f H 0 observed i n t h e d e s e r t sand e x p e r i m e n t was r a t h e r low, i t may n e v e r t h e l e s s be h i g h enough t o y i e l d r e a s o n a b l e q u a n t i t i e s o f o x i d a t i o n p r o d u c t s l i k e H S 0 o r HN0 . Further experiments a r e i n progress t o study the p h o t o c a t a l y t i c a c t i v i t y of n a t u r a l systems. 2

2

+

2

2

2

2

2

2

X

2

2

4

3

Acknowledgment We g r a t e f u l l y acknowledge t h e f i n a n c i a l s u p p o r t o f t h e U.S. EPA (CR812356-01-0 a n d R811612-01-0) a n d i n p a r t i c u l a r we want t o thank D r s . D o n a l d C a r e y and M a r c i a Dodge f o r t h e i r s u p p o r t .

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36. 37. 38. 39. 40. 41.


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