Photocatalysis by Inorganic Components of Natural Water Systems

Chapter 17

Photocatalysis by Inorganic Components of Natural Water Systems

Downloaded by UNIV OF MISSOURI COLUMBIA on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch017

Cooper H. Langford and John H. Carey Department of Chemistry, Concordia University, Montreal, Quebec, Canada, and National Water Research Laboratory, Canada Center for Inland Waters, Burlington, Ontario, Canada

A brief inventory of aquatic components potentially active as photocatalysts includes colloidal and sedimentary oxides and sulfides, metal complexes of organic ligands, and certain metal ions in clays. The reactions are to be understood in terms of the redox reactions following ligand to metal charge transfer and its extension to solid lattices, reactions of photogenerated electron hole pairs according to the "microcorrosion cell" model. Studies, many of which were directed to development of waste treatment techniques, have suggested probable pathways. Inventory

o f Chromophores

The f i r s t i n o r g a n i c s p e c i e s t o b e c o n s i d e r e d a s c a n d i d a tes f o r photoreactions important i n natural waters were c a r b o x y l i c a c i d complexes o f Fe+ andC u [ e . g . t h e NTA c o m p l e x e s I (1, 2). In these complexes, photooxidation of the organic ligand i s i n i t i a t e d b y an e x c i t a t i o n process i n which a Jigand l o c a l i z e d e l e c t r o n i s promoted into the partly f i l l e d d s h e l l o f the metal ion. This results i n reduction o f t h e metal ion. Since the reduced metal i o n i s o f t e n r e a d i l y r e o x i d i z e d by oxygen, the r e a c t i o n s can be rendered c a t a l y t i c . The i m p l i c a tions o f t h e model studies have been v a l i d a t e d by results obtained f o r both marine (3) and fresh (4) waters. In considering the extensions o f these preced e n t s , we m u s t consider t h e range o f p o s s i b l e metal organic complexes s a t i s f y i n g the c o n d i t i o n s f o r charge transfer within t h e wavelength range available i n i l l u m i n a t e d waters, r e d u c i b i l i t y o f the metal ion, and an i r r e v e r s i b l e f i r s t step f o r the oxidation of the 3

+ 2

0097-6156/87/0327-0225$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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226

o r g a n i c l i g a n d t h a t can e f f e c t i v e l y b l o c k back r e a c t i o n . The m e t a l ions are few [ n o t a b l y F e ( I I I ) , ΜηίΙΙΙ, I V ) , perhaps U0+ , a n d C u ( I I ) i n t h e uv l i m i t ) . The organic ligands that give i r r e v e r s i b l e o x i d a t i o n are predomina t e l y t h o s e w h i c h c o n t a i n a c a r b o x y l a t e and can decompose r a p i d l y a f t e r one e l e c t r o n o x i d a t i o n t o r e l e a s e CO2 and escape back r e a c t i o n or those containing f u n c t i o n a l i t y l i k e phenol where the r e s u l t of one e l e c t r o n o x i d a t i o n is a relatively stable r a d i c a l w h i c h may survive. The o v e r a l l impression i s that the range of r e a c t i o n s w i l l be l i m i t e d and t h a t the evidence to date i s c o n s i s t e n t with such a notion. Indeed, since a major source of organic ligands i n f r e s h w a t e r s i s humic m a t e r i a l which has s t r o n g e r l i g h t a b s o r p t i o n at long wavelength than most m e t a l c o m p l e x e s and has p h o t o i n i t i a t i o n p a t h w a y s o f i t s own ( 5 , 6) w h i c h may be metal ion quenched, the i n t e r a c t i o n o f m e t a l i o n s w i t h o r g a n i c l i g a n d s may do as much t o quench p h o t o c h e m i s t r y in natural w a t e r s as t o initiate i t ! A more promising p o s s i b i l i t y f o r r e a c t i o n s of ge n e r a l s i g n i f i c a n c e began t o emerge w i t h the r e c o g n i t i o n of the i m p l i c a t i o n s f o r n a t u r a l water photochemistry of the d e v e l o p i n g subject of initiation of photoreaction f o l l o w i n g b a n d gap i r r a d i a t i o n o f s e m i c o n d u c t o r s . It is i n t e r e s t i n g to r e f l e c t , i n the face of current torrent of papers on t h i s subject (7), t h a t t h e i d e a has been a r o u n d f o r a r a t h e r l o n g t i m e (8) and t h a t some of the earliest thoughts on its utilization for solution r e a c t i o n s were connected to waste water treatment. A report from the Robert A. T a f t C e n t e r a p p e a r e d i n 1969 ( 9 ) a n d CCIW w o r k e r s r e p o r t e d t h e d e c h l o r i n a t i o n o f P C B s u s i n g a n a t a s e i n 1975 (.10) . There are i m p o r t a n t p a r a l l e l s b e t w e e n t h e way that i l l u m i n a t i o n of a semiconductor can produce r e a c t i v e species in s o l u t i o n and the r e d u c t i o n of the metal ion and o x i d a t i o n o f t h e l i g a n d c h a r a c t e r i z i n g t h e l i g a n d t o metal charge transfer photochemistry described above. But, aspects of the behaviour of the s e m i c o n d u c t o r solution interface can overcome s e v e r a l of the limita t i o n s mentioned f o r the homogeneous c o m p l e x e s . When a semiconductor is illuminated with light of energy greater than i t s optical band gap, an electron is t r a n s f e r r e d to the c o n d u c t i o n band l e a v i n g a h o l e i n the v a l e n c e band. At the i n t e r f a c e , e l e c t r o n t r a n s f e r can o c c u r e i t h e r f r o m t h e c o n d u c t i o n b a n d t o an a c c e p t o r , A, i n s o l u t i o n or from a donor, D, in solution to the v a l e n c e band. These processes are s h o w n i n F I G U R E 1. They compete with the fruitless recombination of electron and hole to produce thermal energy. In the case of the metal complexes above, the r e c o m b i n a t i o n c o u l d o n l y be a v o i d e d b y p r o d u c t i o n o f " s t a b l e " c h e m i c a l intermediates. In this case, there are two other p o s s i b i l i t i e s for r e s i s t a n c e to the back r e a c t i o n . The


2



17.

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Photocatalysis by Inorganic Components

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first is essentially thermodynamic and t h e s e c o n d i s essentially kinetic. When a p a r t i c l e o f s u f f i c i e n t s i z e e l e c t r o c h e m i c a l l y e q u i l i b r a t e s w i t h t h e Eh o f t h e s u r r o u n d i n g s o l u t i o n , the Fermi l e v e l i n the semiconductor b u l k comes to the p o t e n t i a l o f t h e e n v i r o n m e n t a l Eh. This usually results i n band b e n d i n g at the i n t e r f a c e and a depletion layer at the surface of the semiconductor w i t h a p o t e n t i a l g r a d i e n t (1_1) f a v o u r i n g the s e p a r a t i o n of the photogenerated charge c a r r i e r s as i s shown i n FIGURE 2. This means t h a t t h e b a n d b e n d i n g can promote charge s e p a r a tion in a way u n a v a i l a b l e to a molecular system. The e x a c t e n e r g e t i c s w i l l d e p e n d on t h e r e l a t i o n b e t w e e n t h e v a l e n c e and c o n d u c t i o n band e n e r g i e s a n d t h e Eh o f t h e solution. One o f the e l e c t r o n t r a n s f e r processes, the one r e s u l t i n g from the transfer of the c a r r i e r toward the particle solution interface, i s sometimes c h a r a c t e r i z e d as a photoreaction. The o t h e r , t h e o n e r e s u l t i n g f r o m t h e t r a n s f e r o f t h e c a r r i e r t r a n s f e r r e d by t h e band bending toward the i n t e r i o r of the p a r t i c l e , i s thought of c o r r e s p o n d i n g l y as a t h e r m a l r e a c t i o n . But, the rate of each r e a c t i o n w i l l n o r m a l l y be g o v e r n e d by t h e r u l e s of i n t e r f a c i a l e l e c t r o n t r a n s f e r . The two most i m p o r tant factors are the degree of e x e r g o n i c i t y of the i n t e r f a c i a l e l e c t r o n t r a n s f e r and t h e e x t e n t of overlap of the d i s t r i b u t i o n of energy l e v e l s of s o l u t e species a n d t h e b a n d e d g e s (.LL) . In n a t u r a l w a t e r s , many interesting semiconducting p a r t i c l e s , e s p e c i a l l y those of the w e l l hydrated hydrous o x i d e s which have l a r g e "interior" surface, w i l l have particle sizes much s m a l l e r that the t y p i c a l thickness of t h e d e p l e t i o n layer at the solution semiconductor interface. I t might be a s s u m e d t h a t s u c h s m a l l p a r t i c l e s would c a r r y no advantage over molecular excited states in the c o m p e t i t i o n of separation of charge c a r r i e r s ys b a c k r e a c t i o n . This i s not e n t i r e l y the case. E v e n a s m a l l p a r t i c l e may h a v e many a t o m s a n d t h e m o b i l i t y of a c a r r i e r , once g e n e r a t e d , is sufficiently large to carry i t away from the s i t e of g e n e r a t i o n . Moreover, a t y p i c a l semiconductor will be capable of b e i n g "doped" with localized s i t e s of one t y p e o r t h e other. These l o c a l i z e d s i t e s a r e t r a p s t h a t can c a p t u r e photogenerated carriers in times of the order of a n a n o s e c o n d o r two. E v i d e n c e f o r t h i s t r a p p i n g mechanism has r e c e n t l y been d e v e l o p e d i n a s t u d y o f t h e p i c o s e c o n d t r a n s f e r f r o m an e x c i t e d d y e t o T 1 O 2 w h e r e the trapping p r o c e s s can be r e s o l v e d and t h e back r e a c t i o n i s slow (12). The p i c o s e c o n d s p e c t r a s h o w t h a t t h e t i m e c o n s t a n t s for trapping o f an excess c a r r i e r from a semiconductor band to the l o c a l i z e d s i t e s which correspond to doping is a process i n the nanosecond time domain. However, the trapped site spectra d i d not appear under a l l c i r c u m s t a n c e s when l i g h t was a b s o r b e d a t t h e




228

F I G U R E 1. The P h o t o c a t a l y t i c Semiconductor P a r t i c l e " C o r r o s i o n " C e l l I l l u s t r a t e d by T i 0 . Note that t h e Redox C o u p l e s may n e e d t o b e A d s o r b e d i f R e a c t i o n i s to Compete w i t h R e c o m b i n a t i o n . 2

SEMICONDUCTOR

SOLUTION

Fermi Level

F I G U R E 2. Energetics of Oxidation tor Electrolyte Interface.

at the

Semiconduc-



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semiconductor surface. Analysis of the circumstances indicated that the c r u c i a l factor was the competition between recombination of holes and e l e c t r o n s a t t h e s u r f a c e and the escape o f c a r r i e r s i n t o t h e i n t e r i o r . A key f a c t o r f o r the e f f i c i e n t escape o f a c a r r i e r from the s u r f a c e to the i n t e r i o r was the presence of a reactant on the surface that could react with either electrons or holes. At high light intensity, these initial steps must be i n t h e p i c o s e c o n d time domain. Therefore, efficient photochemistry will depend very s t r o n g l y on the presence of adsorbed reactants at the surface. With adsorbed reactants, even the small p a r t i c l e c a n c o n s i d e r a b l y f a v o u r c h a r g e s e p a r a t i o n b y an e s s e n t i a l l y k i n e t i c mechanism. We s h o u l d a d d p a r e n t h e t i c a l l y t h a t t h e o b s e r v a t i o n s just described may provide the r e s o l u t i o n o f a minor controversy. T h e r e h a s b e e n some d i s p u t e o v e r the role of l i g h t a b s o r p t i o n by t h e s e m i c o n d u c t o r and by s u r f a c e complexes. The g r e a t e r a b s o r p t i v i t y o f t h e s e m i c o n d u c tor suggests that light i s absorbed by the solid. However, the requirement of a surface scavenger t o prevent rapid recombination will give the overall kinetics the characteristics indicating a surface complex. C o l l o i d a l p h o t o c a t a l y s t s can form i n n a t u r a l waters i n a v a r i e t y o f ways. In general, precipitation of a solution species initiates. The p r o c e s s e s include at least the following p o s s i b i l i t i e s : i. Iron and manganese oxides a r e p r e c i p i t a t e d by oxidation of l o w Eh s o l u t i o n s i n e n v i r o n m e n t s such as those which occur when ground water emerges at the s u r f a c e o r r e d u c i n g water flows from a wetland. i i . U, V, Cu, Se, a n d Ag a r e p r e c i p i t a t e d as low valency oxides (or metals) when w a t e r s at high Eh mix with reducing waters or encounter organic reducing matter. i i i . F e , C u , A g , Z n , P b , Hg, N i , Co, A s , a n d Mo a r e p r e c i p i t a t e d as s u l f i d e s by r e d u c t i o n o f s u l f a t e w a t e r s , u s u a l l y by s u l f a t e r e d u c i n g b a c t e r i a . The f i n a l category o f chromophore that we w i l l consider here i s the coloured inorganic i o n adsorbed ( o r i o n e x c h a n g e d i n t o ) a s i t e on a c l a y p a r t i c l e . In such cases, the light absorption i s l o c a l i z e d and charge separation does not occur because of migration of carriers i n the solid. Rather, moderately e f f i c i e n t r e a c t i o n c a n a r i s e when t a r g e t m o l e c u l e s c a n b e c o a d s o r bed and t h e i n i t i a t i n g chromophore r e a c t s e f f i c i e n t l y with the coadsorbed molecule. An a p p r o p r i a t e e x a m p l e i s the uranyl exchanged c l a y p h o t o c a t a l y s t which accomplis h e s t h e o x i d a t i o n o f a l c o h o l s t o k e t o n e s (13.) .


229


230


Photocatalysts

i n the

Naj^j~al

Setting

Especially in the c a s e o f the p a r t i c u l a t e s e m i c o n d u c t o r s , t h e r e i s now an e x t e n s i v e l a b o r a t o r y chemistry of the catalytic reactions which may be accomplished. However, t h e s i t u a t i o n i n a n a t u r a l s e t t i n g i s much more complex t h a n i s c h a r a c t e r i s t i c o f model s t u d i e s . Before we b e g i n a r e v i e w o f i m p o r t a n t r e a c t i o n s now known, i t i s u s e f u l t o s e t out some comments on the f a c t o r s , w h i c h differentiate the laboratory study from the n a t u r a l system. F i r s t , we must r e m a i n aware t h a t n a t u r a l w a t e r s do not receive extensive illumination at wavelengths s h o r t e r than about 350 nm. Thus, some o f t h e p r o c e s s e s involving high band gap oxide photocatalysts which attract a great deal of l a b o r a t o r y a t t e n t i o n do not figure importantly in natural systems. Much o f our a t t e n t i o n should be d i r e c t e d t o w a r d p r o c e s s e s u t i l i z i n g longer wavelengths. It is true that, at least for shallow waters, models of the i l l u m i n a t i o n a v a i l a b l e have p r o v e d a d e q u a t e t o a c c o u n t f o r the o v e r a l l r a t e s o f photoreaction (14.) so t h a t this factor i s w e l l understood. On the other hand, we do not require large quantum yields to produce significant effects. Half l i v e s of d i r e c t p h o t o l y s i s of organic c h r o m o p h o r e s have been e s t i m a t e d between a few h o u r s and t h i r t y days ( 1 4 ) . I f the shortest reflect quantum y i e l d s n e a r one, then values near 0.001 are still i n t e r e s t i n g on t h e t i m e s c a l e o f one month. With c o l l o i d a l p h o t o c a t a l y s t s , i n c r e a s e d d i f f u s i v e ness of l i g h t c a u s e d by s c a t t e r i n g i n t u r b i d w a t e r s can l e a d t o enhanced p h o t o l y s i s r a t e s . For example, a s t u d y of the e f f e c t s of c l a y s on the r a t e o f a uv p h o t o l y s i s showed an i n i t i a l i n c r e a s e in the photolysis r a t e on a d d i t i o n of clay followed by a d e c r e a s e when the c l a y c o n c e n t r a t i o n f i n a l l y produced o f f s e t t i n g l i g h t attenuation (15)· The light available for photochemistry s h o u l d be measured by t h e r m a l l e n s i n g and photoacoustic measurements. T h i s has been d e m o n s t r a t e d t o be e f f e c t i v e f o r w a t e r s c o n t a i n i n g humic c o l l o i d s and suspended f i n e sands (UB). A very important difference between laboratory photocatalysts and corresponding materials in the natural setting is relative purity. In t h e l a b o r a t o r y , most work has been c o n d u c t e d u s i n g pure m i n e r a l phases. In n a t u r a l w a t e r s , many m i n e r a l s have c o l o u r s character i s t i c of e x c i t a t i o n of e l e c t r o n s from impurity l e v e l s within the bandgap. In t h i s c a s e , the h o l e energies w i l l be l e s s f a v o r a b l e for oxidative p r o c e s s e s than i n the case of excitation at w a v e l e n g t h s h o r t enough t o promote e l e c t r o n s from the d e e p e r l y i n g c o n d u c t i o n band. This has been well documented by l a b o r a t o r y s t u d i e s d i r e c t e d toward the shifting of the water o x i d a t i o n reaction catalyzed by anatase toward t h e v i s i b l e by



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doping the titanium dioxide l a t t i c e with metal ions with partly filled d shells. The c a s e o f d o p i n g w i t h C r ( I I I ) is an e s p e c i a l l y interesting e x a m p l e (1_7). Hydrogen e v o l u t i o n f r o m w a t e r was o b s e r v e d a t C r ( I I I ) d o p e d T 1 O 2 irradiated with light passed t h r o u g h a 4 1 5 nm c u t o f f filter. The e l e c t r o n promoted t o t h e c o n d u c t i o n band has t h e r e d u c i n g power o f t h e c o n d u c t i o n band edge. I n t h i s case, the Cr(IV) p r o d u c e d has t h e o x i d i z i n g power (locally) necessary t oo x i d i z e water. I t appeared that the C r i s l o c a t e d i n the s u r f a c e r e g i o n o fthe particle which was p r o b a b l y necessary t o the oxidation of a solution species. A typical coloured internal ion i na m i n e r a l might not be as s t r o n g l y o x i d i z i n g as C r ( I V ) b u t i f i twere, t h e r e would be a d i f f i c u l t y i n i t s m i g r a t i o n to t h e s u r f a c e t oo x i d i z e a s o l u t e . An additional relevant p r o b l e m was i d e n t i f i e d i n t h i s study o f C r doping o fanatase. The p h o t o r e a c t i v i t y c u t o f f a t a d o p i n g l e v e l above 0.4% w h i c h c o r r e s p o n d s to the s o l u b i l i t y l i m i t i n anatase. The a u t h o r s b e l i e v e that a chromic oxide surface layer forms which b l o c k s reaction. T h i s c a l l s a t t e n t i o n t o a common s t a g e i n t h e aqueous o x i d a t i v e weathering o f mineral p a r t i c l e s ,t h e f o r m a t i o n on t h e s u r f a c e o f a f i l m o f an o x i d e i n the higher oxidation state of the ion. This i s seen, especially, i n t h e i n c r e a s e o f Fe2Û3 i n the oxide analysis o f the products of weathering. Such s u r f a c e f i l m s may i n t e r d i c t t h e e x p e c t e d p h o t o r e a c t i v i t y o f the u n d e r l y i n g pure m i n e r a l a t , as t h e example s u g g e s t s , l o w total levels. On t h e o t h e r h a n d , s u c h f i l m s c a n p r o d u c e oxide semiconductors on u n d e r l y i n g m i n e r a l s t h a t might o t h e r w i s e seem u n p r o m i s i n g . Another aspect o fthe impurity doping which w i l l be common i n m i n e r a l p a r t i c l e s i s i l l u s t r a t e d b y t h e e f f e c t o f Nb(V) on a n a t a s e . This i o n substitutes isomorphically f o rTi(IV) and as an η dopant c r e a t e s a S h o t t k y barrier atthe interface. This assists i n j e c t i o n o f an e l e c t r o n from a r e d u c i n g s o l u t e i n t o t h e c o n d u c t i o n band (13.)The f l a t b a n d i s a l s o s h i f t e d cathodically. I t has r e c e n t l y been c l a i m e d t h a t t h e r e l a t i v e inefficiency of haematite as a photoanode i s a function o f low mobility of carriers and that this problem may b e overcome by doping w i t h S i ( I V ) ( 1 9 ) . I n t h i s we s e e t h e last effect o f doping, the modification of carrier mobi1ity. Having r a i s e d the question o f surface f i l m s on mineral particles, i t i s t e m p t i n g t o a l s o comment h e r e on t h e t e n d e n c y o f p a r t i c l e s t o acquire organic coa tings. Again, t h e photochemical consequences a r e complex. The o r g a n i c m a t t e r may a b s o r b l i g h t . These excited states may " s e n s i t i z e " the underlying minerals by e l e c t r o n o r h o l e i n j e c t i o n . Some e v i d e n c e f o r s u c h a process h a s been obtained i n studies o ff u l v i c acid a d s o r b e d o n a n a t a s e (20.). On t h e o t h e r hand, hydrous f e r r i c oxides have r e c e n t l y been o b s e r v e d t oquench t h e



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photochemistry i n i t i a t e d by t h e f u l v i c a c i d . As a t h i r d e f f e c t , t h e o r g a n i c f i l m may be t h e r e d o x species which r e a c t s w i t h t h e phot©generated c a r r i e r s f r o m t h e m i n e r a l part icle.


Εathways o f

reaction -

reduction

As m e n t i o n e d a b o v e , b o t h electrons and holes m u s t be e f f i c i e n t l y separated and b o t h must r e a c h the p a r t i c l e surface. E v e n when both reach the s u r f a c e , recombina t i o n may s t i l l predominate i f b o t h are not consumed i n appropriate fast reactions. It i s worth r e i t e r a t i n g that the requirements f o r f a s t r e a c t i o n s are t w o f o l d . The f i r s t is favorable energetics. The r e d o x c o u p l e s m u s t be i n c l u d e d w i t h i n t h e b a n d gap as shown i n FIGURE 2. But, i t i s a l s o necessary to maximize the r a t e of interfacial electron transfer. This i s e s s e n t i a l l y a standard problem of electrochemical kinetics and, as such, one for which there are e x t e n s i v e precedents. Above, we mentioned the general theoretical rules. H e r e , we w i l l g i v e more a t t e n t i o n t o s p e c i f i c e x a m p l e s . In most c a s e s , e x p e r i m e n t a l r e s u l t s can p r o v i d e precedents. Since the main p o i n t w i l l be t o c o m p a r e t h e r a t e s of competing processes, i t is useful to d i v i d e the d i s c u s s i o n i n t o r e d u c t i o n and o x i d a t i o n r e a c t i o n s . In o r d e r f o r an oxidant to react r a p i d l y , i t w i l l be m o s t a d v a n t a g e o u s f o r i t to be p r e s e n t adsorbed on the s u r f a c e so t h a t mass t r a n s p o r t w i l l not l i m i t the rate. T h i s means that we should first c o n s i d e r the solvent which i s always at the i n t e r f a c e . The possibi l i t y o f w a t e r a c t i n g a s an a c c e p t o r t o produce hydrogen has a t t r a c t e d a great deal of a t t e n t i o n because of the p o s s i b i l i t y of s t o r i n g s o l a r energy i n a chemical fuel by t h i s process. U n f o r t u n a t e l y , water i s not t h a t easy to reduce. In every case where t h e r e has been s u c c e s s , a known catalyst for hydrogen e v o l u t i o n has been required. Naturally occurring materials do not o f f e r many o f t h e known catalysts. The o n l y e x a m p l e u s i n g a m a t e r i a l which might occur i n n a t u r a l waters i s t h e use o f ZnS as a catalyst for H2 e v o l u t i o n (21). Here the special property of ZnS to photodegrade partially leaving a deposit of Zn m e t a l on t h e s u r f a c e l i n k s t h e example to other cases. Hydrogen e v o l u t i o n most o f t e n is achieved by the mediation of a metal catalystcommonly t h e noble metals known f o r t h e i r lower overvoltages for hydrogen. T h i s " u n r e a c t i v i t y " can prove b e n e f i c i a l i n some e n v i r o n m e n t a l c o n t e x t s . I f water i s not r a p i d l y reduced there is a better opportunity for direct reaction with a solute (including xenobiotics). The n e x t most likely acceptor to occur at the surface of a mineral p a r t i c l e i s the oxygen molecule which w i l l generally be present at reasonable levels throughout the p h o t i c zone. Oxygen i s s t r o n g l y adsorbed



17. LANGFORD AND CAREY

233


on a number o f s u r f a c e s and e s p e c i a l l y on o x i d e s u r f a ces. The g e n e r a t i o n o f the superoxide ion, O 2 " , from one e l e c t r o n r e d u c t i o n o f the oxygen m o l e c u l e i s a w e l l known r e a c t i o n t h a t has r e c e n t l y been shown t o occur in humic w a t e r s (22.). Many s t u d i e s of p h o t o c a t a l y s i s at semiconductor surfaces i n c l u d i n g e a r l y work in one o f our l a b o r a t o r i e s on t h e p h o t o l y s i s of PCBs i n a n a t a s e s l u r r i e s (9) have n o t e d t h a t no r e a c t i o n occurs i n the absence of oxygen. Superoxide i s a good n u c l e o p h i l e . A l s o , a r e a c t i o n o f the f o l l o w i n g t y p e i s known:

V

cl

Cr —

° 0.1. In t h e s o l u t i o n p h a s e p h o t o l y s i s o f f e r r i c p e r c h l o rate i n the presence of tertiary alcohols w i t h more carbons than t - b u t a n o l , we observed that H a b s t r a c t i o n b y OH radicals produced some radicals that d i d not reduce F e ( I I I ) to Fe(II). The a d d i t i o n o f C u ( I I ) i o n s caused these r a d i c a l s to be oxidized and resulted in additional ferrous ion production. For example, the r a t i o of Fe(II) quantum yields for 3-ethy1-3-pentanol w i t h o u t and w i t h 0.0050M a d d e d C u ( I I ) was 0 . 8 4 4 . When the anatase d i s p e r s i o n was irradiated under s i m i l a r c o n d i t i o n s , t h e r a t i o was 0 . 8 6 1 . These r e s u l t s i n d i c a t e fairly directly that the more r e a c t i v e o x i d a n t a t t h e a n a t a s e s u r f a c e i s k i n e t i c a l l y e q u i v a l e n t t o OH r a d i c a l . Several s t u d i e s of the hydroxylated anatase surface are a v a i l a b l e i n the l i t e r a t u r e . Two t y p e s of surface OH g r o u p s h a v e b e e n d i s t i n g u i s h e d , t h o s e b o u n d t o o n e T i and t h o s e w h i c h b r i d g e two. The s i n g l y bound h y d r o x y l s are thought to be r e a d i l y exchangeable, the b r i d g i n g ones n o t . F o r most c r y s t a l f a c e s , t h e s e two t y p e s o c c u r i n a l t e r n a t e rows and a r e t h u s a v a i l a b l e i n a p p r o x i m a t e l y e q u a l numbers. A p l a u s i b l e e x p l a n a t i o n f o r the above results i s that either type o f s u r f a c e h y d r o x y l can f u n c t i o n as a h o l e t r a p . I f the hole i s trapped at a singly bound hydroxyl, a species similar t o t h e OH r a d i c a l i s produced. But, i f the hole i s trapped at a bridging group, a weaker oxidant capable only of the d i r e c t o x i d a t i o n r e a c t i o n pathway, i s produced. This model has p r e c e d e n t i n t h e homogeneous s o l u t i o n c h e m i s try of Fe(III) where the bridged dimer i s inactive t o w a r d p h o t o p r o d u c t i o n o f OH r a d i c a l . FIGURE 4 shows t h a t there i s a drop i n the F e ( I I ) y i e l d as a c i d c o n c e n t r a t i o n i s i n c r e a s e d . This probably r e f l e c t s the increase of p o s i t i v e charge at the surface a n d an e l e c t r o s t a t i c b a r r i e r t o h o l e t r a n s f e r a c r o s s t h e surface. The s u r f a c e a l s o w i l l c h a n g e i t s p r o c l i v i t y t o adsorb v a r i o u s potential reactants as a function of surface charge. T h i s w i l l be a g e n e r a l p r o b l e m . It is important t o keep track of the relation between the point of z e r o c h a r g e o f t h e p a r t i c l e s a n d t h e pH o f t h e s o l u t i o n i n c o n s i d e r i n g any s p e c i f i c r e a c t a n t . Anatase i s not a unique system. Many of the features of anatase were, a f t e r a l l , " a n t i c i p a t e d " by t h e homogeneous c h e m i s t r y o f F e ( I I I ) . This leads to the suggestion that the mechanistic studies of anatase should provide c o n s i d e r a b l e guidance to the behaviour of other oxide systems. For t h i s reason, i t i s i n t e r e s t i n g to r e c o r d h e r e a number of reactions that have been



17.

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237

r e p o r t e d u s i n g l i g h t a b s o r p t i o n by a n a t a s e as initiating step. Many o f t h e s e were explored for their possible u t i l i t y i n photochemical waste water treatment. Donors of a p p r o p r i a t e redox p o t e n t i a l to r e a c t w i t h holes at the anatase surface include organic acids, carbohydrates, fats, CN~, and halides (24). (The c y a n i d e r e a c t i o n has been studied for i t s u t i l i t y in treatment of the waste streams from gold mining operations in the Canadian Northwest Territories.) More immediately r e l e v a n t to n a t u r a l water i s the observation t h a t an a n a t a s e s l u r r y c o u l d e f f e c t the d e c o l o r a t i o n of a chlorinated bleach plant e f f l u e n t . A s a m p l e o f amber c o l o u r , pH = 1.8, a n d l o w r e s i d u a l c h l o r i n e was irradiated in the presence o f 0.5% (wt) a n a t a s e w i t h l i g h t o f 350 nm f o r p e r i o d s up t o 18 h r . The o p t i c a l absorbance decreased by half in 1080 min. Small amounts of c h l o r i d e and formaldehyde were detected (25). This r e a c t i o n may provide a precedent for o b s e r v a t i o n of a r e l a t i o n between p h o t o b l e a c h i n g of humics i n w a t e r and metal ions. If s o , we are brought to the question of the r e a c t i v i t y of c o l l o i d a l i r o n oxides. As m e n t i o n e d a b o v e , some laboratory s t u d i e s have indicated that anatase i s s u p e r i o r to a l l other oxides tested with reference reactions such as the a l c o h o l o x i d a t i o n s (2_5) land hydrogen e v o l u t i o n as w e l l ( 2 4 ) ] . H o w e v e r , r e c e n t work has a l s o s u g g e s t e d t h a t a l l t h a t i s required to increase the efficiency of f e r r i c oxide photoanodes i s s u i t a b l e doping w h i c h may occur r e a d i l y in nature. Recently two i m p o r t a n t s t u d i e s have given s p e c i f i c evidence for the photooxidative a c t i v i t y of f e r r i c oxides. The first is the report of Faust and H o f f m a n (2JS) t h a t h a e m a t i t e c a n c a t a l y z e t h e photooxidation of S0z to sulfate. The s c a v e n g i n g c o m p o u n d was present in large concentration in these e x p e r i m e n t s so t h a t t h e r e w e r e no s e r i o u s m a s s t r a n s p o r t l i m i t a t i o n s on the d e l i v e r y of the substrate directly to the s u r f a c e . The s e c o n d i s the r e p o r t of the o x i d a t i o n of o x a l a t e at a hydrous f e r r i c oxide surface (27). There has been some c o n c e r n that t h i s l a s t r e a c t i o n i s the r e a c t i o n of a specific molecular species since the homogeneous ferric oxalate complex does undergo p h o t o o x i d a t i o n at w a v e l e n g t h s b e l o w 520 nm. The distinction is not too important. I f the i r o n l i g a n d centered hole reaches the s u r f a c e f r o m t h e i n t e r i o r i t m u s t be t r a n s f e r r e d to the ligand. If i t i s g e n e r a t e d by l i g h t a b s o r p t i o n a t t h e s u r f a c e , t h e same local structure will result. The difference will be that the s e c o n d c a s e w i l l show a small quantum yield which we will characterize by a t t r i b u t i n g i t t o t h e c o n s e q u e n c e s o f low h o l e m o b i l i t y in the i n t e r i o r . I f that p r o b l e m can be o v e r c o m e , t h e mechanisms merge. The p r e l i m i n a r y i n d i c a t i o n i s t h a t the r e a c t i o n i s e f f i c i e n t enough to suggest the semicond u c t o r mechanism. Thus, we s e e t h a t some e v i d e n c e f o r f e r r i c oxides reacting with organic acids i s i n .


238



Relationships

to B i o l o g i c a l

Activity

One l a s t p o i n t i s i m p o r t a n t . The pathways i n i t i a t e d by i n o r g a n i c p h o t o c a t a l y s t s may n o t t e r m i n a t e w i t h t h e immediate r e a c t i o n s t h e m s e l v e s . The e f f e c t o f p h o t o d e c h l o r i n a t i o n o f PCBs r e n d e r s t h e o r g a n i c p r o d u c t s a c c e s s i b l e t o biodégradation. A s i m i l a r r e s u l t was o b t a i n e d i n a s t u d y o f a l i g n i n model compound, v e r a t r y l a l c o h o l (2i> ) . I r r a d i a t i o n o f an a n a t a s e d i s p e r s i o n o f a s o l u t i o n gave v a n i l l y l a l c o h o l w h i c h i s b i o d e g r a d e d by a c o l o n y o f o r g a n i s m s c a p a b l e o f d e g r a d i n g a r o m a t i c s more than t h r e e times f a s t e r . I t i s q u i t e p o s s i b l e t h a t one o f t h e most i m p o r t a n t a r e a s f o r f u t u r e i n v e s t i g a t i o n w i l l be t h e r e l a t i o n o f i n o r g a n i c p h o t o c h e m i s t r y t o t h e increase of biodegradabi1ity of refractory organic matter. References

1. T. Trott, R.W. Henwood, C.H. Langford, 1972, Envir. Sci. Technol., 6, 367. 2. C.H. Langford, M. Wingham, V.S. Sastri, 1973, Envir. Sci. Technol., 7, 820. 3. R.H. Collienne, 1983, Limnol. Oceanog., 28, 83. 4. C.T, Miles, P.L. Brezonik, 1981, Envir. Sci. Technol., 15, 1089. 5. R.G. Zepp, P.F. Scholtzauer, R.M. Sink, 1985, Envir. Sci. Technol., 19, 74. 6. J.F. Power, C.H. Langford, D.K. Sharma, R. Bonneau, J. Joussot-Dubien, 1985, this symposium. 7. M. Graetzel, ed., 1983, "Energy Resources Through Photochemistry and Catalysis", Academic Press, New York. 8. For a Review of Earlier Work See: 1974, "Photoeffects in Adsorbed Species", Discuss. Faraday Soc., 58. 9. L.C. Kinney, V.R. Ivanuski, 1969, Robert A. Taft Research Center Report Number TWRC-13 Cincinnati, Ohio. 10. J.H. Carey, J. Lawrence, H.M. Tosine, 1976, Bull. Envir. Contam. Toxicol., 16, 697. 11. H. Gerischer, 1979, Topics Appl. Phys., 31, 115. 12. A.D. Kirk, C.H. Langford, C. St-Joly, D.K. Sharma, R. Lesage, 1984, JCS Chem. Communs. 13. S.L. Suib, K.A. Carrado, 1985, Inorg. Chem., 24, 863. 14. T. Mill, et al, 1981, Chemosphere, 10, 1281. 15. G.C. Miller, R.G. Zepp, 1979, Envir. Sci.Technol., 13, 453. 16. J.F. Power, C.H. Langford, Unpublished. 17. E. Borgarello, J. Kiwi, M. Graetzel, E. Pellizetti, M. Visca, 1982, J. Am. Chem. Soc., 104, 2996. 18. P. Salvador, 1980, Solar Energy Material's, 2, 413. 19. C. Leygraf, M. Hendewerk, G.H. Somorjai, 1982, J. Catal., 78, 341.


17.

LANGFORD

AND

CAREY

Photocatalysis

by

Inorganic

Components


20.

239

C.H. Langford, 1982, Proc. ENERGEX, 1982, Solar Energy Society of Canada, Regina, Saskatchewan, August 1982, p. 928. 21. N. Zeug, J. Buchler, H. Kisch, 1985, J. Am. Chem. Soc., 107, 1459. 22. R.M. Baxter, J.H. Carey, 1984, Nature, 206, 575. 23. C.H. Langford, J.H. Carey, 1973, Can. J. Chem., 51. 24. T. Sakata, T. Kawai, 1983, Reference 7, Chapter 10. 25. J.H. Carey, B.G. Oliver, 1980, Water Poll. Res. J. of Canada, 15, 157. 26. B.C. Faust, M.R. Hoffman, Abstracts Environmental Division, 186th ACS National Meeting, Washington, DC, August 28th, 1983 - September 2nd, 1983. 27. M.C. Goldberg, K.M. Cunningham, Abstracts Environmental Division, 185th ACS National Meeting, Seattle, Washington, March 27th, 1983 - April 1st, 1983. RECEIVED July

15, 1986


Photocatalysis by Inorganic Components of Natural Water Systems

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