Room Temperature Oxidations, Isotopic Exchanges, and

2

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Room Temperature Oxidations, Isotopic Exchanges, and Dehydrogenations over Illuminated Neat or Metal-Supporting Semiconductor Catalysts PIERRE PICHAT Institut de Recherches sur la Catalyse, CNRS, 69626 Villeurbanne, Cédex, France

The possibilities of using photonic excitation of large area powder semiconductor catalysts in organic synthesis are surveyed principally by considering the t i t l e types of reactions. It is first underlined thatthe effects of parameters, such as texture, defects, impurities, on the creation and separation of charges within ghe semiconductor should be taken into account, in addition to their catalytic influence, when choosing or modifying a s o l i d , which increases the difficulties. In presence of O2 ( a i r ) , alkanes, alkenes and alcohols are oxidized to aldehydes or ketones. The s e l e c t i v i t y depends on the molecule, the photocatal yst and the conditions, since unspecific attack and cleavages can occur. Aromatic rings withstand oxidation and alkyltoluenes are converted to alkylbenzaldehydes. At least in gas phase, these oxidations over semiconductor oxides involve dissociated adsorbed/surface oxygen species activated by photoproduced holes as inferred from photoconductance and oxygen isotope exchange measurements and from the replacement of O2 by NO. Group VIII metal deposition on semiconductors allows the extension of heterogeneous photocatalysis to reactions involving H2, either endergonic (dehydrogenations) or exergonic (acid decarboxylations), as well as hydrogen isotopic exchange. However, charge recombination caused by these deposits determines an optimal metal amount for each metal-semiconductor system. Various liquid alcohols, saturated or unsaturated ( a l l y l , cinnamyl, citronellol, geraniol), are dehydrogenated without over-oxidation and with quantum yields in the 0.025-0.8 range for Pt/TiO2 Current address Cedex, F r a n c e .

: E c o l e C e n t r a l e de L y o n ,

Β . P . 163, 69131

0097-6156/85/0278-0021$06.50/0 © 1985 American Chemical Society

Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Ecully


22

ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS MEDIA

samples. A C = C bond is partially reduced only when adjacent to the hydroxy. In situ hydrogenation with an alcohol as reductant can be performed. In solution high chemical yields are reached. Cyclopentane-deuterium exchange only gives rise to the monodeuterated compound, whereas polydeuteration occurs by thermally activated catalysis. In conclusion, practical applications of heterogeneous photocatalysis for organic syntheses pose a challenge. In view of the background reviewed in this Symposium, concerted efforts of specialists in organic chemistry, chemical engineers and industry, in connection with scientists already involved in this f i e l d , are now needed. O v e r t h e l a s t d e c a d e , i n t e n s i v e r e s e a r c h has appeared on t h e use o f i l l u m i n a t e d s o l i d s e m i c o n d u c t o r s and redox r e a c t i o n s t o c o n v e r t l i g h t energy ( s p e c i a l l y s o l a r energy) to e l e c t r i c a l (photoregenerative cells) or chemical energy (1-7). Unlike the conversion to e l e c t r i c i t y , t h e g e n e r a t i o n o f v a l u a b l e c h e m i c a l compounds through p h o t o n i c e x c i t a t i o n o f s e m i c o n d u c t o r s does n o t r e q u i r e e l e c t r o d e s ; a c c o r d i n g l y , n u m e r o u s s t u d i e s h a v e b e e n concerned w i t h the use o f powdered o r c o l l o i d a l s e m i c o n d u c t o r s ( 6 _ , 7). Because o f t h e i r l a r g e s u r f a c e a r e a , t h e s e d i v i d e d s o l i d s enhance the photon c a p t u r e and t h e c o n t a c t w i t h t h e r e a c t a n t s . T h e i r s u r f a c e p r o p e r t i e s can be m o d i f i e d b y t h e methods u s u a l l y employed i n c a t a l y s i s , and s e v e r a l t e c h n i q u e s e x i s t to i n v e s t i g a t e e l e c t r o n t r a n s f e r processes and r e a c t i o n i n t e r m e d i a t e s a t t h e i r i n t e r f a c e s w i t h gases o r l i q u i d s . In a d d i t i o n , t h e s e d i v i d e d s e m i c o n d u c t o r s c a n be m a n u f a c t u r e d b y s i m p l e r and t h e r e f o r e much l e s s e x p e n s i v e means than s e m i c o n d u c t o r e l e c t r o d e s . Many r e p o r t s have been w r i t t e n on e n d e r g o n i c r e a c t i o n s (AG > 0 ) , i n v o l v i n g an abundant r e a c t a n t , such as water d e c o m p o s i t i o n , a n d , t o a l e s s e r extent, n i t r o g e n or carbon dioxide r e d u c t i o n s . These r e a c t i o n s a r e o u t s i d e t h e s c o p e o f t h i s Symposium. Moreover, t h e i r y i e l d s , i n the absence o f e l e c t r i c a l a s s i s t a n c e o r o f a s a c r i f i c i a l compound a r e , as y e t , v e r y low. Dehydrogenations o f o r g a n i c compounds are a l s o e n d e r g o n i c a t room temperature b u t w i t h a s m a l l e r c h a n g e i n free energy, and c o n s e q u e n t l y t h e y a r e e a s i e r t o p e r f o r m . F o r e x a m p l e , t h e h y d r o g e n p r o d u c t i o n f r o m C 1 - C 4 p r i m a r y or s e c o n d a r y a l i p h a t i c a l c o h o l s h a s been s t u d i e d ( 8 - l l ) . I f more complex a l c o h o l s o r o t h e r dehydrogenable o r g a n i c compounds o f i n t e r e s t were c o n s i d e r e d and s e l e c t i v e l y t r a n s f o r m e d , t h i s d e h y d r o g e n a t i o n m e t h o d m i g h t h a v e an impact i n o r g a n i c s y n t h e s i s . On t h e o t h e r h a n d , o x i d a t i o n s o f o r g a n i c r e a c t a n t s a t room temperature m i g h t a l s o be i n t e r e s t i n g ( a l t h o u g h i n t h a t c a s e t h e r a d i a n t e n e r g y o n l y s e r v e s t o o v e r c o m e t h e energy o f a c t i v a t i o n ) , p r o v i d e d one p r o d u c t h a s a h i g h a d d e d v a l u e , and p r o v i d e d a r e a s o n a b l e quantum y i e l d a n d , above a l l , a h i g h c h e m i c a l y i e l d c a n be r e a c h e d . Indeed s t u d i e s o f p h o t o c a t a l y t i c o x i d a t i o n s h a v e p r e c e d e d those on the s t o r a g e o f l i g h t energy, but more s y s t e m a t i c r e s e a r c h e s , which w i l l b e n e f i t from the improved u n d e r s t a n d i n g o f p h o t o c a t a l y t i c p r o c e s s e s , s h o u l d be u n d e r t a k e n . A l a r g e v a r i e t y o f o r g a n i c compounds have r e d o x p o t e n t i a l s a l l o w i n g t h e i r o x i d a t i o n b y s e m i c o n d u c t o r o x i d e s i l l u m i n a t e d w i t h photons c o r r e s p o n d i n g t o an energy a t l e a s t e q u a l t o t h e i r b a n d gap ( 1 2 ) . B e s i d e s , t h e u s e o f a i r and o f



2.

PICHAT

Illuminated

Semiconductor

Catalysts

23

i n e x p e n s i v e m a t e r i a l s , s u c h as t i t a n i u m d i o x i d e , as w e l l as the absence o f p o l l u t i n g s p i l l s , c o n s t i t u t e advantages o f t h i s o x i d a t i o n m e t h o d . The e x p l o r a t i o n o f i t s p o s s i b i l i t i e s s h o u l d be e n v i s a g e d i n c o n n e c t i o n with other p o t e n t i a l developments for heterogeneous p h o t o c a t a l y s i s , such as the p h o t o c a t a l y t i c d e g r a d a t i o n o f p o l l u t a n t s (13) and t h e r e c o v e r y o f p r e c i o u s a n d / o r t o x i c m e t a l s i n d i l u t e d solutions (14). After some general remarks on the relations between s e m i c o n d u c t o r p r o p e r t i e s and t h e i r use as p h o t o c a t a l y s t s , t h i s t e x t w i l l f i r s t d e a l w i t h o x i d a t i o n s o f o r g a n i c compounds.The i n t e r a c t i o n s o f i l l u m i n a t e d s e m i c o n d u c t o r s w i t h gaseous O2 (and, f o r c o m p a r i s o n , w i t h gaseous NO) w i l l be then p r e s e n t e d , whereas t h e l a s t p a r t w i l l c o n s i d e r m e t a l / s e m i c o n d u c t o r p h o t o c a t a l y s t s and the o r g a n i c r e a c t i o n s they a l l o w . In t h i s p r e s e n t a t i o n , the r e s u l t s o f t h i s l a b o r a t o r y w i l l be h i g h l i g h t e d . G e n e r a l remarks reactions

on t h e η - t y p e s e m i c o n d u c t o r s used i n p h o t o c a t a l y t i c

The n a t u r e o f the s e m i c o n d u c t o r i n t e r v e n e s i n v a r i o u s r e s p e c t s . From the e n e r g e t i c v i e w p o i n t , the l o c a t i o n s o f the v a l e n c e band and o f the c o n d u c t i o n band r e s p e c t i v e l y determine the o x i d a t i o n s and r e d u c t i o n s which are t h e r m o d y n a m i c a l l y a l l o w e d , and the band gap i n d i c a t e s t h e l i g h t f r e q u e n c i e s r e q u i r e d f o r a c t i v a t i n g the s e m i c o n d u c t o r , i . e . f o r g e n e r a t i n g h o l e s i n t h e v a l e n c e b a n d . As i n t h e r m a l c a t a l y s i s , s u r f a c e / c a t a l y t i c p r o p e r t i e s , w h i c h c o n t r o l t h e s t r u c t u r e o f the adsorbed s p e c i e s , depend on the c h e m i c a l n a t u r e o f t h e s o l i d and on the presence o f v a r i o u s d e f e c t s . For i n s t a n c e , acid-base surface s i t e s ( s p e c i a l l y t h e c o v e r a g e i n OH g r o u p s ) a r e c r i t i c a l f o r t h e a d s o r p t i o n o f o r g a n i c m o l e c u l e s which c o n t a i n a c i d i c or b a s i c f u n c t i o n a l g r o u p s . D e f e c t s , such as i m p u r i t i e s ( w h i c h c a n b e h a v e as s u b s t i t u t i o n a l d o p i n g l e v e l s ) and oxygen v a c a n c i e s , can have the same r o l e as i n t h e r m a l c a t a l y s i s i n f o r m i n g a d s o r p t i o n s i t e s , but f u r t h e r m o r e t h e y c a n a c t as r e c o m b i n a t i o n c e n t e r s o f photoproduced charges and/or they can change the m o b i l i t y of the charge c a r r i e r s . P r o f o u n d c h a n g e s i n p h o t o c a t a l y t i c a c t i v i t y can thus be observed according to the stoichiometry and purity of the semiconductor o x i d e . The t e x t u r e o f t h e s e m i c o n d u c t o r powder i s a l s o very i m p o r t a n t . The r o l e o f the s u r f a c e a r e a i n d e t e r m i n i n g the e x t e n t o f c o n t a c t o f the s o l i d w i t h the gaseous or l i q u i d or dissolved reactants is e a s i l y understandable. P o r e s s h o u l d be o f a s i z e c o m p t a b i l e w i t h easy p e n e t r a t i o n o f the r e a c t a n t s and r e c o v e r y o f the p r o d u c t s . However, b o t h t h e s e f a c t o r s have s u p p l e m e n t a r y e f f e c t s in t h e c a s e o f i l l u m i n a t e d s o l i d g r a i n s . F o r porous m a t e r i a l s , at l e a s t p a r t o f the i n t e r n a l s u r f a c e o f the pores cannot be d i r e c t l y reached b y t h e l i g h t . F o r n o n p o r o u s m a t e r i a l s , a t a g i v e n wavelength the depth o f p e n e t r a t i o n o f the p h o t o n s , which i s r e l a t e d to the a b s o r p t i o n c o e f f i c i e n t a , s h o u l d be compared w i t h the average s i z e o f the g r a i n s . I f α v a r i e s i n the ΙΟ^-ΙΟ^πΓ* r a n g e , the Beer-Lambert law i n d i c a t e s t h a t 99 °l o f t h e i n c i d e n t f l u x i s a b s o r b e d w i t h i n 20 t o 200 nm t h i c k n e s s . F o r anatase ( d e n s i t y 3.85 g cm"^) t h e s e p a r t i c l e s d i a m e t e r s c o r r e s p o n d t o s u r f a c e a r e a o f a b o u t 78 o r 7 . 8 m ^ g " * , r e s p e c t i v e l y ; i n o t h e r words, i n n e r r e g i o n s of the p a r t i c l e s of s a m p l e s o f l o w e r s p e c i f i c a r e a a r e n o t i l l u m i n a t e d ( 1 5 ) . In t h i s r e s p e c t , the type o f photoreactor plays a r o l e (fixed-bed c a t a l y s t , suspension, i l l u m i n a t i o n f r o m one s i d e o r a n n u l a r i l l u m i n a t i o n



24


e t c . . . ) . In a d d i t i o n , the r a d i u s o f the s e m i c o n d u c t o r p a r t i c l e can be s m a l l e r t h a n t h e w i d t h o f t h e s p a c e c h a r g e l a y e r w h i c h w o u l d be formed i n an i n f i n i t e sample o f the same s e m i c o n d u c t o r , and t h i s w i l l a f f e c t the charge s e p a r a t i o n . These l a t t e r e f f e c t s o f p a r t i c l e s i z e o n l y b e g i n t o be t h e o r e t i c a l l y c o n s i d e r e d ( 1 6 ) . One s h o u l d n o t i c e t h a t they can cause d i f f e r e n c e s i n p h o t o c a t a l y t i c b e h a v i o r . As a r e s u l t , though comparisons between v a r i o u s semiconductors f o r a g i v e n p h o t o c a t a l y t i c r e a c t i o n are u s e f u l , the c l a s s i f i c a t i o n s thus d e r i v e d must not be r e g a r d e d as d e f i n i t i v e , s i n c e the e f f e c t s o f t h e t e x t u r e , o f t h e i m p u r i t i e s and o f o t h e r s t r u c t u r a l d e f e c t s , are even more c r u c i a l than i n t h e r m a l c a t a l y s i s . A l t h o u g h e c o n o m i c a l f a c t o r s are not o f p r i m a r y importance i n the e a r l y s t a g e s o f fundamental r e s e a r c h e s i f r e s u l t s o b t a i n e d w i t h r a r e m a t e r i a l s a l l o w our knowledges to p r o g r e s s , i t i s nevertheless of i n t e r e s t to m e n t i o n t h a t i n d u s t r y produces l a r g e q u a n t i t i e s o f some powder s e m i c o n d u c t o r s , e i t h e r pure o f m o d i f i e d , and, among them, the c h e a p e s t a r e Έβ2θ^ and T1O2. Many s t u d i e s on p h o t o c a t a l y t i c o r g a n i c c o n v e r s i o n s have been c a r r i e d out w i t h t h i s l a t t e r c a t a l y s t because of i t s higher e f f i c i e n c y . Note t h a t when p r e p a r e d i n a flame r e a c t o r i t i s nonporous. Photocatalytic oxidations

of

organic

compounds

The r e v i e w w i l l be l i m i t e d t o r e l a t i v e l y s i m p l e m o l e c u l e s . O t h e r , and g e n e r a l l y more complex examples, w i l l be found i n M - A . F o x ' s and K. Tokumaru's papers i n t h i s Symposium. Alkanes . D e t a i l e d s t u d i e s o f the p h o t o c a t a l y t i c o x i d a t i o n o f C2-Cg a l k a n e s , l i n e a r o r r a m i f i e d , i n gas phase u s i n g a d i f f e r e n t i a l - f l o w p h o t o r e a c t o r w i t h a f i x e d bed o f T i 0 2 i have been r e p o r t e d (17-18). Under such c o n d i t i o n s , which i m p l i e s low c o n v e r s i o n s ( 0 . 6 - 2.5 J ) , a h i g h s e l e c t i v i t y (55 - 9 5 i n a l d e h y d e ( s ) a n d / o r k e t o n e ( s ) as c o m p a r e d w i t h t o t a l o x i d a t i o n to CO21 was found, and a quantum y i e l d (number o f m o l e c u l e s o x i d i z e d f o r each quantum o f r a d i a t i o n absorbed b y t h e s e m i c o n d u c t o r ) o f 0 . 1 was i n d i c a t e d . T h i s s e l e c t i v i t y t o p a r t i a l o x i d a t i o n products i s r e m a r k a b l e w i t h r e g a r d to c a t a l y t i c a l k a n e o x i d a t i o n s at h i g h t e m p e r a t u r e s . The r e a c t i v i t y o f the c a r b o n atoms f o l l o w e d the sequence C > Cq > C > C p i , the c a r b o n atom which was p r e f e r e n t i a l l y , but n o t e x c l u s i v e l y , a t t a c k e d b e i n g t h a t w i t h the h i g h e s t e l e c t r o n d e n s i t y t o g e t h e r w i t h the l e a s t s t e r i c o b s t r u c t i o n . I t ensued t h a t , u n f o r t u n a t e l y , a v a r i e t y o f aldehydes and k e t o n e s was o b t a i n e d i f the alkane s t r u c t u r e a l l o w e d i t (scheme I). To t h e a u t h o r ' s k n o w l e d g e , no e q u i v a l e n t s t u d i e s h a v e b e e n h i t h e r t o published for n e a t - l i q u i d alkanes. t

e

r

t

u a t

s

e

c

r

r a

A l k y l t o l u e n e s . We have s t u d i e d the o x i d a t i o n o f gaseous a l k y l t o l u e n e s RC6H4CH3 (R - C H , (CH3) CH, (0113)30 i n a differential-flow p h o t o r e a c t o r w i t h a f i x e d bed o f T1O2 ( 1 9 ) . The r a t i o 0 2 / i y d r o c a r b o n s h o u l d n o t be too g r e a t , otherwise the surface coverage in h y d r o c a r b o n l i m i t e d the c o n v e r s i o n . In a l l c a s e s , the s e l e c t i v i t y i n RC5H4CHO was h i g h . F o r R = (0113)30, no CO2 was d e t e c t e d . Under the same c o n d i t i o n s , t o l u e n e y i e l d e d o n l y t r a c e s o f b e n z a l d e h y d e . This shows t h e s t a b i l i t y o f t h e a r o m a t i c r i n g and o f t h e m e t h y l group d i r e c t l y a t t a c h e d to i t . 2

5

2



2

3

2

3

3

2

2

2

CH -CO-CH -CH3 +C0 +H 0

CH =C-CH -CH

I

CH

3

3

CH3-CO-CH3 +CH3-CHO

)

CH -C=CH-CH

I

CH

3

3

Scheme I

2

+ H 0

(17)

CH3-CH-CO-CH3

I

CH

0

3

2

I

CH3

3

2

2

2

2

2

+ H 0

CH -CH-CH -CHO

I

CH3

CH -CH-CHO+C0 +H 0

\^

CH3-CH-CH=CH

I

CH3

2

CH3-CH-CH -CH OH



26

These oxidations are not c l a s s i c a l , since, usually, the s e c o n d a r y o r t e r t i a r y hydrogen atomsare p r e f e r e n t i a l l y e l i m i n a t e d to g i v e r i s e t o h y d r o p e r o x i d e s , w h i c h c o u l d mean t h a t t h e s e latter i n t e r m e d i a t e s are not formed i n t h e p h o t o c a t a l y t i c p r o c e s s . These o x i d a t i o n s c a n a l s o be p e r f o r m e d w i t h t h e n e a t - l i q u i d i n a s t a t i c p h o t o r e a c t o r . The m e t h y l group i s o x i d i z e d , whereas t h e o t h e r a l k y l g r o u p w i t h s t a n d s o x i d a t i o n . However, p a r t o f the photons r e q u i r e d t o e x c i t e T i 0 i s a l s o absorbed by the a l k y l b e n z a l d e h y d e , p r o d u c i n g f r e e radicals i n the p r e s e n c e o f 0 . T h i s a c c e l e r a t e s the aldehyde t r a n s f o r m a t i o n i n t o the c o r r e s p o n d i n g a c i d . I f a h i g h s e l e c t i v i t y i n a l d e h y d e i s s o u g h t f o r , t h i s imposes to o p e r a t e e i t h e r w i t h d i l u t e s o l u t i o n s o r w i t h a r e a c t o r a v o i d i n g t h a t the photons pass t h r o u g h a t h i c k l a y e r o f t h e l i q u i d . The a c i d f o r m a t i o n i s a l s o enhanced by r a i s i n g the t e m p e r a t u r e ; room t e m p e r a t u r e a p p e a r s as an optimum c o m p r o m i s e b e t w e e n k i n e t i c s and s e l e c t i v i t y . F o r n e a t - l i q u i d s , i n i t i a l quantum y i e l d s o f about 0 . 2 were f o u n d . 2


2

A l k e n e s . Gas p h a s e propene o x i d a t i o n has been s t u d i e d at 320 Κ o v e r T i 0 , Z r 0 , V 0 5 , ZnO, S n 0 , S b 0 4 , C e 0 , WO3 and a mixed Sn-O-Sb sample ( 2 0 ) . T h e s e s o l i d s were n o t a c t i v e i n t h e d a r k a t t h i s t e m p e r a t u r e . U n d e r b a n d - g a p i l l u m i n a t i o n , o n l y the V 0 5 sample was found i n a c t i v e . U n f o r t u n a t e l y , o n l y one anatase sample had a quantum y i e l d s u f f i c i e n t f o r a p o s s i b l e p r a c t i c a l u s e . The s e l e c t i v i t y g r e a t l y depended on the p a r t i c u l a r specimen u s e d . Among t h e p r o d u c t s o f p a r t i a l o x i d a t i o n , e t h a n a l was g e n e r a l l y formed i n h i g h e r amounts than a c r o l e i n , the p e r c e n t a g e o f acetone b e i n g s m a l l . P r o p e n e oxide was d e t e c t e d , p a r t i c u l a r l y a t low c o n v e r s i o n s , and, i n the case o f T i 0 , c o u l d be t h e p r i m a r y p r o d u c t r e s u l t i n g from t h e a t t a c k o f adsorbed propene by the a c t i v e oxygen s p e c i e s . U n d e r s i m i l a r c o n d i t i o n s , the p h o t o c a t a l y t i c o x i d a t i o n o f 2 - o r 3 - m e t h y l - l - b u t e n e and o f 2 - m e t h y l - 2 - b u t e n e o v e r T i 0 y i e l d e d c a r b o n y l compounds as partial oxidation products (IS). However, the s e l e c t i v i t y t o a p a r t i c u l a r a l d e h y d e o r k e t o n e was r e d u c e d b y c l e a v a g e s not o n l y o f the double bond but a l s o o f the C^-Cy bond. 2

2

2

2

2

2

2

2

2

Alcohols. The o x i d a t i o n o f 2 - p r o p a n o l p r i n c i p a l l y to acetone has been chosen t o t e s t the p h o t o s t a b i l i t y o f p i g m e n t s . The mechanism has been s t u d i e d i n d e t a i l , however m a i n l y by p e r f o r m i n g t h i s r e a c t i o n i n gas p h a s e , e s s e n t i a l l y o v e r T i 0 ( 2 1 - 2 4 , ). Samples o f o t h e r o x i d e s l i k e V 0 5 , C r 0 3 , M n 0 , F e 0 3 , C 0 3 O 4 , NiO and CuO do not e x h i b i t a p h o t o c a t a l y t i c a c t i v i t y ( 2 3 ) . T h i s gas (23, 25) or l i q u i d (24) phase o x i d a t i o n was extended t o C 4 - C 5 a l i p h a t i c a l c o h o l s . In gas phase, the ease o f o x i d a t i o n f o l l o w e d t h e s e q u e n c e : s e c o n d a r y > t e r t i a r y > p r i m a r y and o x i d a t i v e 0 - 0 β s c i s s i o n was f o u n d f o r 2-propanol, 2 - b u t a n o l , and 2 and 3 - m e t h y l - 2 - b u t a n o l ( s c h e m e II) . Whether or not t h i s s c i s s i o n i s r e l a t e d to the f o r m a t i o n o f an o l e f i n i n t e r m e d i a t e has been d i s p u t e d (23, 2_5). A l s o , t h o u g h 3 - m e t h y 1 - 1 - b u t a n o l is oxidized chiefly to the corresponding aldehyde, acetone, 2-methylpropanal and e t h a n a l a r e supposed to r e s u l t from t h e subsequent c l e a v a g e o f t h i s aldehyde (25). O x i d a t i o n o f e t h a n o l , 1- and 2 - p r o p a n o l was i n v e s t i g a t e d i n l i q u i d p h a s e and i n aqueous s o l u t i o n s ( 2 4 ) . The same apparent energy o f a c t i v a t i o n was f o u n d r e g a r d l e s s o f the a l c o h o l . However, the p r i m a r y a l c o h o l s were found t o r e a c t p r e f e r e n t i a l l y t o 2 - p r o p a n o l when m i x e d t o i t . Quantum y i e l d s i n the range 0 . 1 - 0 . 5 at near room temperature have been r e p o r t e d . 2

2

2

2

2

α


2.

PICHAT

Illuminated

Semiconductor

Catalysts

27

NO, w h i c h s u p p l i e s oxygen atoms to T i 0 on forming N and N 0 , (26) can r e p l a c e 0 i n s u c h o x i d a t i o n s as a l r e a d y m e n t i o n e d . F o r example, 1-butanol was o x i d i z e d to b u t a n a l , 2 - b u t a n o l t o butanone and 2-methyl-2-propanol to acetone (2-methyl propene being also detected), w i t h t h e e x p e c t e d o r d e r i n the ease o f o x i d a t i o n , and a average quantum y i e l d o f about 0.1 (27, 28). 2

2

2

2

Oxalic acid. I t s o x i d a t i o n to C 0 i n aqueous s o l u t i o n was used as a t e s t r e a c t i o n to examine the p h o t o s e n s i t i v e p r o p e r t i e s o f T i 0 , Fe 03, 0| 0 2 » S b 0 4 , C e 0 and WO3 powder samples i n the v i s i b l e as w e l l as i n t h e UV r e g i o n ( 2 9 ) . In f a c t , a l l o f these specimens, e x c e p t S b 0 4 and WO3, were p a r t i a l l y d e g r a d e d i n t h e d a r k i n an o x y g e n a t e d 5 χ 1 0 " ^ M s o l u t i o n (pH : 2.34) and C 0 was e v o l v e d . The slight p h o t o s e n s i t i v i t i e s observed i n the v i s i b l e present no p r a c t i c a l i n t e r e s t . Under U V - i l l u r a i n a t i o n , a quantum y i e l d o f ~ 0.35 was found f o r the anatase sample which was t h e most e f f i c i e n t . The p r e s e n c e o f d i s s o l v e d o x y g e n was n e e d e d t o a l l o w t h e C - C b o n d cleavage. 2


2

2

Z n

Z r

2

2

2

2

Conclusion. As p o i n t e d o u t i n t h e i n t r o d u c t i o n , heterogeneous p h o t o c a t a l y s i s o f f e r s e c o n o m i c a l a d v a n t a g e s f o r o x i d a t i o n s . The p r e c e d i n g p a r a g r a p h s , by showing t h a t a t room t e m p e r a t u r e , alkanes c a n be o x i d i z e d and a l k y l t o l u e n e s transformed t o a l k y l b e n z a l d e h y d e s , g i v e an i d e a o f the c a p a b i l i t i e s o f t h i s method. G e n e r a l l y , aldehydes o r k e t o n e s a r e t h e f i n a l d e g r e e o f o x i d a t i o n , p r o v i d e d t h a t the c o n d i t i o n s a v o i d p h o t o c h e m i c a l o x i d a t i o n s i n the presence o f 0 ; t h i s u n d e r l i n e s the n e c e s s i t y o f r e s e a r c h e s on p h o t o c a t a l y t i c r e a c t o r s . H o w e v e r , d e p e n d i n g on t h e m o l e c u l a r s t r u c t u r e o f t h e oxidizable compound, the oxidation can be unspecific and, f u r t h e r m o r e , u n d e s i r e d c l e a v a g e s can o c c u r as a r e s u l t o f the method e f f i c i e n c y . In t h i s r e s p e c t , no s y s t e m a t i c s t u d y o f t h e e f f e c t of t e m p e r a t u r e , n a m e l y b e l o w t h e ambient, on the s e l e c t i v i t y has been r e p o r t e d . F u r t h e r m o r e , a deeper u n d e r s t a n d i n g o f o x i d a t i o n c a t a l y s t s might h e l p to d e s i g n more s e l e c t i v e p h o t o c a t a l y s t s , w i t h n e v e r t h e l e s s the added d i f f i c u l t y o f o b t a i n i n g a h i g h p h o t o s e n s i t i v i t y . F i n a l l y , b y c o n s i d e r i n g m o l e c u l e s more complex t h a n those c i t e d h e r e i n (see M.A. F o x ' s and K. Tokumaru's p r e s e n t a t i o n s i n t h i s Symposium) and by changing the c o n d i t i o n s , a p p l i c a t i o n s might be u n c o v e r e d . 2

Interaction of

i l l u m i n a t e d η-type semiconductor oxides

w i t h 0? and NO

Oxygen a d s o r p t i o n on s e m i c o n d u c t o r o x i d e s has g i v e n r i s e to a l a r g e number o f s t u d i e s , m a i n l y by ESR ( 3 0 ) . Various ionosorbed species have been f o u n d d e p e n d i n g upon t h e c o n d i t i o n s and t h e sample p r e t r e a t m e n t . Continuous i l l u m i n a t i o n w i t h photons o f energy at l e a s t e q u a l t o t h e s e m i c o n d u c t o r b a n d gap s h o u l d change the e q u i l i b r i u m between gaseous o x y g e n and n e g a t i v e l y c h a r g e d c h e m i s o r b e d o x y g e n s p e c i e s , s i n c e t h e c o n c e n t r a t i o n s o f b o t h e l e c t r o n and h o l e s are m o d i f i e d . T h i s e f f e c t i s v e r y marked f o r the m i n o r i t y c a r r i e r s Q ). As a r e s u l t , p h o t o d e s o r p t i o n and p h o t o a d s o r p t i o n phenomena have been r e p o r t e d ( 3_1_). In t h e f o l l o w i n g p a r a g r a p h s we b r i e f l y show how we h a v e t r i e d t o g a i n i n f o r m a t i o n on the oxygen a c t i v e s p e c i e s at the surface of various i l l u m i n a t e d semiconductor oxides, by u s i n g i s o t o p i c exchange experiments and photoconductance measurements.




28

02 and NO i s o t o p i c e x c h a n g e o v e r i l l u m i n a t e d semiconductor o x i d e . Oxygenisotopeexchange(OIE),îîî thedark andattemperatures g e n e r a l l y l a r g e l y above the ambient, has been e m p l o y e d t o d e t e r m i n e t h e l a b i l i t y o f a d s o r b e d / s u r f a c e oxygen o f v a r i o u s m e t a l o x i d e s i n c o n n e c t i o n w i t h the c a t a l y t i c a c t i v i t y o f these s o l i d s i n o x i d a t i o n s (32^). F o r i l l u m i n a t e d m e t a l o x i d e s , t h e s e OIE e x p e r i m e n t s are p a r t i c u l a r l y r e l e v a n t , s i n c e OIE and o x i d a t i o n r e a c t i o n can b o t h o c c u r a t room t e m p e r a t u r e , b y c o n t r a s t w i t h t h e r m a l l y a c t i v a t e d e x c h a n g e s w h i c h o f t e n r e q u i r e h i g h e r temperatures than o x i d a t i o n s . The t y p e o f OIE c a n be d e r i v e d from t h e c h a n g e s i n t h e v a r i o u s i s o t o p i c s p e c i e s i n the gas phase and the c o m p e t i t i o n between a g i v e n o x i d a t i o n and OIE e n a b l e s one to know whether the same oxygen s p e c i e s are i n v o l v e d i n both r e a c t i o n s . F o r i n s t a n c e , i l l u m i n a t i o n o f p r e o x i d i z e d ( t r e a t m e n t i n 16Q at 723 K) s a m p l e s o f T i 0 (33), S n 0 , ZnO and Z r 0 (34), previously e x p o s e d t o 4 Pa * 0 ^ " d a r k , c a u s e d immediatê~DIE, p r o v i d e d photons o f an energy s u f f i c i e n t to c r e a t e e l e c t r o n - h o l e p a i r s were employed, whereas a V 0 5 specimen was i n a c t i v e ( 3 4 ) . The e v o l u t i o n o f the gas isotopic composition perfectly corresponded to an e x c h a n e e - t y p e r e p r e s e n t e d by the e q u a t i o n 0 ( g ) + 1δο(β) £± 0 0(g) + 0(s) (1) where one a d s o r b e d / s u r f a c e oxygen atom i s exchanged at a t i m e , w h i l e t h e t y p e i n v o l v i n g two s u c h atoms f o r e a c h e x c h a n g e a c t d i d not i n t e r v e n e , i n c o n t r a s t w i t h most t h e r m a l l y a c t i v a t e d OIE ( 3 2 ) . On the o t h e r h a n d , t h e same o r d e r o f a c t i v i t i e s was observed for s e v e r a l Ti0 specimens (33), as w e l l as f o r the S n 0 , ZnO and Z r 0 samples ( 3 4 ) i n b o t h OIE and i s o b u t a n e o x i d a t i o n , chosen as an example o f a l k a n e o x i d a t i o n . I n a d d i t i o n , c o m p e t i t i o n b e t w e e n OIE and t h i s r e a c t i o n was examined by i n t r o d u c i n g a m i x t u r e o f * 0 j isobutane over T i 0 i n t h e d a r k ( 3 3 ) . On i l l u m i n a t i n g , a d e c r e a s e i n gaseous i s o b u t a n e was o b s e r v e d , as e x p e c t e d , whereas OIE o c c u r r e d o n l y a f t e r the d i s a p p e a r a n c e o f the a l k a n e i n t h e gas p h a s e ( F i g u r e 1 ) . From t h e s e r e s u l t s i t was c o n c l u d e d t h a t t h e l a b i l e adsorbed/surface atomic oxygen s p e c i e s which t o o k p a r t i n O I E , a l s o p a r t i c i p a t e i n isobutane o x i d a t i o n . M o r e o v e r , t h e i s o t o p i c heteroexchange o f oxygen is a very s e n s i t i v e p h o t o a s s i s t e d g a s - s o l i d r e a c t i o n . F o r e x a m p l e , i t i s much more a f f e c t e d b y t h e t e x t u r e ( 3 3 ) o r b y t h e d o p i n g (3^5) o f T i 0 s a m p l e s t h a n t h e o x i d a t i o n o f d i f f e r e n t o r g a n i c compounds i n gas ( 3 3 , 35 ), l i q u i d or aqueous s o l u t i o n s ( 3 5 ) . I t can t h e r e f o r e be used to e v a l u a t e more p r e c i s e l y the e f f e c t o f v a r i o u s t r e a t m e n t s . A n a l o g o u s r e s u l t s were found w i t h N * 0 ( 2 6 ) . On U V - i l l u m i n a t e d T i 0 , h e t e r o e x c h a n g e o c c u r r e d at room temperature w i t h a much h i g h e r i n i t i a l r a t e than i n the case o f * 0 (Figure 2). Instantaneous exchange with T i 0 s u r f a c e s p r e r e d u c e d i n H at 723 Κ showed t h a t i l l u m i n a t i o n r e n d e r s l a b i l e s u r f a c e oxygen atoms which have n o t b e e n removed by such a t r e a t m e n t . However, a h i g h e r i n i t i a l e f f i c i e n c y f o r p r e o x i d i z e d samples i n d i c a t e d t h a t a d s o r b e d a t o m i c o x y g e n s p e c i e s a l s o p a r t i c i p a t e d . F i n a l l y , i n the presence o f 2 - b u t a n o l the exchange of N* 0 s u p p r e s s e d , w h i l e butanone was formed. T h i s phenomenon was s i m i l a r to the s u p p r e s s i o n o f OIE by i s o b u t a n e . The r o l e o f NO i n t h e o x i d a t i o n o f 2 - b u t a n o l and o t h e r b u t a n o l s ( v i d e supra) i s to r e p l e n i s h the coverage o f T i 0 i n r e m o v a b l e a t o m i c s p e c i e s as e v i d e n c e d by t h e f o r m a t i o n o f N 0 and N ( 2 6 ) . T h i s c o n f i r m s the i m p o r t a n c e o f a t o m i c oxygen s p e c i e s i n the p h o t o c a t a l y t i c r e a c t i o n s performed w i t h gaseous oxygen. 9

2

8

2

l

2

n

n

2

e

2

l 8

1 8

2

1 6

1 8

2

2

2

8

2

a r l (

2

2

8

2

8

2

2

8

w

a

2

s

2

2

2


2.

PICHAT

Illuminated

Semiconductor

Catalysts

29

C-C-CHO+C0 +H 0 2

C-C-C-C

C C-C=C-C


2

C-C=0 + C-CHO

Scheme I I

(25)

t/min

Figure

1.

C h a n g e s i n t h e 1 0 ^ θ / ^ 0 r a t i o and i n the i s o b u t a n e c o n c e n t r a t i o n i n g a s phase o v e r T i 0 as a f u n c t i o n o f i l l u m i n a t i o n time 8

8

2

2

1001

30 t i m e / m in

Figure

2.

45

Changes i n t h e i s o t o p i c c o m p o s i t i o n o f gas phase o x y g e n and d e c l i n e s o f ^ 0 i s o t o p e c o n t e n t o f gaseous oxygen or n i t r i c o x i d e as a f u n c t i o n o f i l l u m i n a t i o n time f o r preoxidized T i 0 samples. 2



30

^°2/^°2 e q u i l i b r a t e d o r n o n - e q u i l i b r a t e d m i x t u r e s have been used by o t h e r r e s e a r c h g r o u p s t o i n v e s t i g a t e t h e e f f e c t s of i l l u m i n a t i o n p r i n c i p a l l y on ZnO and T i 0 (36, 3 7 ) . In p a r t i c u l a r , i l l u m i n a t i o n c a n p r o d u c e horaoexchange o f g a s e o u s ^ 0 and * 0 m o l e c u l e s . For i n s t a n c e , o v e r p r e o x i d i z e d ZnO t h i s e x c h a n g e , which does not i n v o l v e a d s o r b e d or l a b i l e s u r f a c e oxygen s p e c i e s , was i n d u c e d b y l o w - i n t e n s i t y i l l u m i n a t i o n w i t h w a v e l e n g t h s £ 460 nm (i.e. < b a n d - g a p e n e r g y ) , so t h a t an o r i g i n a l l y non-equilibrated m i x t u r e became e q u i l i b r a t e d . T h i s phenomenon was a t t r i b u t e d t o t h e l o c a t i o n o f e l e c t r o n s on c o o r d i n a t i v e l y u n s a t u r a t e d Zn c a t i o n and subsequent f o r m a t i o n o f O4 s p e c i e s on these s i t e s . F o r as long as the e l e c t r o n s remain so l o c a t e d , c o n d i t i o n s e x i s t f o r t u r n o v e r o f s e v e r a l m o l e c u l e s o f the gaseous m i x t u r e to ^ 0 1^0 on the same s i t e (_7, 3 7 ) . T h i s e x a m p l e f u r t h e r d e m o n s t r a t e s the i n t e r e s t o f OIE for s t u d y i n g the r e a c t i v i t y o f v a r i o u s l y p r e t r e a t e d o x i d e s u r f a c e s under d i f f e r e n t illumination conditions. 2

8


2

2

P h o t o c o n d u c t a n c e o f η - t y p e semiconductor o x i d e s exposed to 0? o r NO. P h o t o c a t a l y t i c r e a c t i o n s i n v o l v e the exchange o f e l e c t r o n s between an e x c i t e d semiconductor and at l e a s t one adsorbed r e a c t a n t o r surface s p e c i e s . C o n s e q u e n t l y , p h o t o c o n d u c t a n c e m e a s u r e m e n t s appear as a d i s c r i m i n a t i n g method to i n v e s t i g a t e these r e a c t i o n s . E l e c t r o p h i l i c g a s e s , s u c h as 0 (38) and NO ( 2 7 , 2 8 ) , d e c r e a s e t h e p h o t o c o n d u c t a n c e o f p r e e v a c u a t e d η - t y p e semiconductor o x i d e s . With 0 t h i s e f f e c t was found f o r T i 0 , ZnO, Z r 0 , C e 0 , S b 0 4 , S n 0 and WO3 samples ( 3 8 ) . By c o n t r a s t , a V 0 5 specimen was i n s e n s i t i v e to e x p o s u r e t o 0 , " T n a g r e e m e n t w i t h i n a c t i v i t y f o r 0 I E and propene o x i d a t i o n . A l s o , an a n a t a s e s a m p l e , homogeneously doped w i t h C r ^ ions (0.85 at.Jf), h a d a much l o w e r p h o t o c o n d u c t a n c e t h a n an e q u i v a l e n t pure anatase sample, and furthermore was almost u n a f f e c t e d by v a r y i n g the wavelength a t c o n s t a n t p h o t o n i c f l u x i n an 0 a t m o s p h e r e ( 3 5 ) . T h i s C r - d o p e d s o l i d e x h i b i t e d p o o r a p t i t u d e s as oxidation photocatalyst, and, above a l l , for OIE as already m e n t i o n e d . U n d o u b t e d l y , the photoconductance v a r i a t i o n s i n 0 allow one to p r e d i c t t h e e f f i c i e n c y o f η - t y p e s e m i c o n d u c t o r o x i d e s for p h o t o c a t a l y t i c oxidations with 0 . In a d d i t i o n , from the s l o p e o f these v a r i a t i o n s as a f u n c t i o n o f 0 p r e s s u r e i n a l o g - l o g p l o t , the n a t u r e o f the oxygen s p e c i e s c o n t r o l l i n g the a d s o r p t i o n e q u i l i b r i u m between the semiconductor f r e e e l e c t r o n s and t h e gas ( f o r g i v e n i l l u m i n a t i o n , t e m p e r a t u r e and p r e s s u r e range) can be d e d u c e d , p r o v i d e d t h e ways o f f o r m a t i o n o f t h e s e o x y g e n s p e c i e s f r o m g a s e o u s 0 are assumed. F o r example, the predominance o f 2

2

2

2

2

2

2

2

2

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2

2

2

2

2

0 (g)

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(ads)

2

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0(ads)

0 (ads) + e~ j ? 0 (ads). O t h e r p o s s i b i l i t i e s e x i s t and s h o u l d be d i s c u s s e d when i n t e r p r e t i n g the photoconductance data (38). A l s o , n o t e t h a t the f a c t t h a t one s p e c i e s g o v e r n s t h e a d s o r p t i o n e q u i l i b r i u m do not n e c e s s a r i l y i m p l y T



2.

PICHAT

Illuminated

Semiconductor

Catalysts

31

t h a t o t h e r s p e c i e s a r e not p r e s e n t , but can mean t h a t t h e y s a t u r a t e the s u r f a c e . Moreover, i f the s e m i c o n d u c t o r samples previously e v a c u a t e d u n d e r i l l u m i n a t i o n t o remove t h e l a b i l e oxygen s p e c i e s under t h e s e c o n d i t i o n s , are exposed to o r g a n i c compounds i n v o l v e d i n oxidations, as reactants or products, the effect on the photoconductance i n d i c a t e s w h e t h e r o r n o t t h e s e compounds c o m p e t e w i t h O2 f o r e l e c t r o n c a p t u r e . S i m u l t a n e o u s measurements o f the p h o t o c a t a l y t i c a c t i v i t y A and of the photoconductance σ i n a s p e c i a l l y d e s i g n e d cell yield i n f o r m a t i o n on t h e p a r t i c i p a t i o n o f t h e o x y g e n s p e c i e s i n t h e o x i d a t i o n p r o c e s s . For i n s t a n c e , i n the case of isobutane (IS) oxidation over T i 0 2 i the f o l l o w i n g r e l a t i o n s were found (39) : a = k

0

PQI P ?

S

A = k

A

P8

2

P?

S

3 5

The independence o f σ on IS p r e s s u r e c o n f i r m s t h a t IS d o e s n o t c a p t u r e n o r r e l e a s e e l e c t r o n s , whereas the f r a c t i o n a l k i n e t i c o r d e r 0.35 shows t h a t IS r e a c t s i n an adsorbed phase, s i n c e t h i s v a l u e is v e r y c l o s e t o t h e a p p a r e n t o r d e r o f a d s o r p t i o n 0 . 3 found f o r the s u r f a c e coverage i n IS a c c o r d i n g to a Langmuir model i n t h e p r e s s u r e r a n g e i n v e s t i g a t e d ( 1 3 - 6 0 k P a ) . The or-Pç^ r e l a t i o n s h i p c o r r o b o r a t e s t h a t OJ s p e c i e s c o n t r o l the a d s o r p t i o n e q u i l i b r i u m f o r the p r e s s u r e s chosen, while 0 s i t e s a r e s a t u r a t e d . S i n c e A i s u n a f f e c t e d by oxygen p r e s s u r e , i t i s deduced t h a t the a c t i v e oxygen s p e c i e s are a s s o c i a t e d with 0 ion-radicals. T

T

Conclusion. The photoconductance measurements are thus i n agreement w i t h t h e OIE e x p e r i m e n t s c o n c e r n i n g the r o l e o f d i s s o c i a t e d oxygen s p e c i e s i n p h o t o c a t a l y t i c o x i d a t i o n s i n gas p h a s e . In the a b s e n c e o f other e l e c t r o p h i l i c substances, t h i s c a n a l s o be t h e c a s e for o x i d a t i o n s o f o r g a n i c compounds e i t h e r as n e a t - l i q u i d s o r d i l u t e d i n an o r g a n i c s o l v e n t . The r o l e o f atomic oxygen s p e c i e s has a l s o been s t r e s s e d i n s t u d i e s by C u n n i n g h a m e t a l . ( 2 1 ) d e a l i n g w i t h the i n t e r a c t i o n s of vapors of C3-C4 a l i p h a t i c a l c o h o l s with v a r i o u s m e t a l o x i d e s ( p r i n c i p a l l y ZnO and r u t i l e ) . T h i s a c t i v e s p e c i e s w o u l d r e s u l t f r o m h o l e t r a p p i n g at c o o r d i n a t i v e l y u n s a t u r a t e d 0 ^ " s u r f a c e anions. The formation of 0Η· , Ηθ£ species and o f H2O2 for s e m i c o n d u c t o r o x i d e s i n c o n t a c t w i t h aqueous s o l u t i o n s o r e x p o s e d t o w a t e r v a p o r h a s o f t e n b e e n p r o p o s e d ( 2 2 , 2 3 , 4 0 - 4 2 ) . As m e t a l o x i d e s u r f a c e s c a r r y OH g r o u p s and as the o x i d a t i o n o f o r g a n i c compounds produces w a t e r , the f o r m a t i o n o f the above s p e c i e s cannot be e x c l u d e d even i n the absence o f added w a t e r .

Reactions

over m e t a l / s e m i c o n d u c t o r

photocatalysts

P r e p a r a t i o n and c h a r a c t e r i z a t i o n o f the m e t a l d e p o s i t s . Pt d e p o s i t was made by i m p r e g n a t i o n w i t h ^ P t C l ^ and r e d u c t i o n i n R^at 753 K. The Pt p a r t i c l e s i z e d i s t r i b u t i o n was d e t e r m i n e d by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) (8, 9) ( F i g u r e 3) and H2, O2 c h e m i s o r p t i o n s and t i t r a t i o n s ( 4 3 ) . The P t p a r t i c l e s i z e d i s t r i b u t i o n was narrow w i t h a s u r f a c e weighted mean d i a m e t e r o f c a . 2 mn, almost independent of t h e Pt c o n t e n t b e t w e e n 0 . 5 and 10 wt % (9), provided the p r e p a r a t i o n m e t h o d , w h i c h i n c l u d e s a t r e a t m e n t i n O2 b e f o r e the r e d u c t i o n , was t h o r o u g h l y followed.


32


N i was d e p o s i t e d by i m p r e g n a t i o n w i t h n i c k e l hexamine n i t r a t e and r e d u c t i o n i n H at 753 K. The Ni p a r t i c l e s i z e was a s s e s s e d from magnetic measurements. For N i c o n t e n t s from 0.1 to 14 wt %, t h e mean d i a m e t e r v a r i e d f r o m c a 6 . 5 t o 15 nm, w h i c h e x p l a i n s t h a t the N i p a r t i c l e s were d i f f i c u l t t o d i s t i n g u i s h from the T i 0 g r a i n s (Degussa P - 2 5 , 1 5 - 3 0 nm d i a . ) by TEM ( 1 0 ) . However, on employing another T i 0 s p e c i m e n (170 nm d i a . ) i t was p o s s i b l e t o c o n f i r m by TEM the v a l u e s d e d u c e d from magnetic measurements ( 4 4 ) . N i c k e l was chosen above a l l f o r e c o n o m i c a l r e a s o n s , but a l s o for i t s d i f f e r e n t c h a r a c t e r i s t i c s (ease o f o x i d a t i o n , l a r g e r c r y s t a l l i t e s ) . P h o t o c o n d u c t a n c e measurements were used to determine the e f f e c t o f t h e m e t a l d e p o s i t s on t h e d e n s i t y o f t h e s e m i c o n d u c t o r f r e e e l e c t r o n s . A t 295 K, a f t e r an o v e r n i g h t e v a c u a t i o n i n the d a r k , a l l M/TÎ0 samples had a h i g h e l e c t r i c a l r e s i s t a n c e . Ultraviolet i l l u m i n a t i o n under vacuum caused i m p o r t a n t d e c r e a s e s (10, 45).Figure 4 shows t h e v a l u e s o f the t i t a n i a photoconductance σ at e q u i l i b r i u m . M e t a l d e p o s i t s d e c r e a s e d σ, w h i c h r e f l e c t s t h e d e c r e a s e i n t h e d e n s i t y o f f r e e e l e c t r o n n o f the η - t y p e s e m i c o n d u c t o r . T h i s e f f e c t c a n be e x p l a i n e d by the a l i g n m e n t o f the F e r m i l e v e l s o f T i 0 and o f the d e p o s i t e d m e t a l . The works f u n c t i o n s o f Pt and o f N i h a v e v a l u e s o f a b o u t 5 . 3 6 and 5 . 0 3 eV, r e s p e c t i v e l y . Note t h a t the former v a l u e c a n h o w e v e r be s l i g h t l y lower f o r s m a l l p a r t i c l e s (~ 280 Pt atoms i n the p r e s e n t c a s e ) . V a l u e s o f 4 . 6 o r 5 . 5 eV h a v e b e e n f o u n d f o r a ( 1 1 0 ) r u t i l e s u r f a c e i n two d i f f e r e n t s t a t e s : argon bombarded o r w e l l a n n e a l e d , r e s p e c t i v e l y ( 4 6 ) . From a d i f f e r e n c e i n conductance u n d e r vacuum a t 298 Κ o f a l e a s t 3 o r d e r s o f magnitude f o r a pure Ti0 s a m p l e e i t h e r p r e o x i d i z e d i n 0 at 723 Κ or e v a c u a t e d at 423 K, i t can be i n f e r r e d t h a t t h e work f u n c t i o n o f t h e s e s a m p l e s v a r i e d over a s i m i l a r range o f v a l u e s than t h a t of the r u t i l e single c r y s t a l . T h e r e f o r e f o r n o n - p r e o x i d i z e d samples, an e l e c t r o n t r a n s f e r from t h e s e m i c o n d u c t o r to the m e t a l p a r t i c l e s was p o s s i b l e ( 4 7 ) . But the s i t u a t i o n changed i n the presence o f H . Whereas H d i d not a f f e c t σ o f n e a t T i 0 , σ o f the P t / T i 0 samples i n c r e a s e d when t h e y were e x p o s e d to H at 295 Κ ( 4 5 ) . T h i s i s c o n s i s t e n t w i t h a d e c r e a s e i n t h e work f u n c t i o n o f Pt Γ 4 8 ) . A s i m i l a r e f f e c t o f H f o r r u t i l e s i n g l e c r y s t a l s o n t o w h i c h d o t s o f Pt (or Rh or Ru) f i l m s had been e v a p o r a t e d h a s b e e n r e c e n t l y f o u n d . The r e c t i f y i n g air-exposed m e t a l - s e m i c o n d u c t o r e l e c t r i c a l c o n t a c t became ohmic on f l o o d i n g w i t h H a t 1 atm p r e s s u r e (5j 4 9 ) . More g e n e r a l l y , t h e i m p o r t a n c e o f surface preparation of T i 0 and t r e a t m e n t o f t h e r u t i l e - s i n g l e c r y s t a l / P t e v a p o r a t e d f i l m f o r the b e h a v i o r o f the e l e c t r i c a l c o n t a c t has been emphasized (50). In c h e m i s t r y t e r m s , the half-order d e p e n d e n c e o f a on H p r e s s u r e we o b s e r v e d c a n be c o n s i d e r e d t o r e s u l t from 2

2


2

2

s

2

2

2

2

2

2

2

2

2

2

2

2

Pt -H + 0 " s

2

->

Pt

s

+ OH" +

e"

which s y m b o l i z e s the m i g r a t i o n ( s p i l l o v e r ) o f H atoms from Pt to T i 0 (45). T h e s e m e a s u r e m e n t s c a n n o t be u s e d t o q u a n t i f y t h e e l e c t r o n t r a n s f e r from the s e m i c o n d u c t o r to the m e t a l d e p o s i t , but an e s t i m a t e h a s b e e n drawn from s t u d i e s o f o x y g e n p h o t o a d s o r p t i o n on P t / T i 0 samples i n a p r e s s u r e range such t h a t n e a r l y a l l o f the free e l e c t r o n s a r e c a p t u r e d t o form adsorbed 0 i o n - r a d i c a l s . I n c r e a s i n g Pt c o n t e n t s c o r r e s p o n d e d t o d e c r e a s i n g amounts o f photoadsorbed o x y g e n , w h i c h c o r r o b o r a t e s t h e e f f e c t o f d e p o s i t e d Pt on the T i 0 free e l e c t r o n d e n s i t y . For T i 0 s a m p l e s e v a c u a t e d a t 423 Κ and 2

2

2

2

2



2.

PICHAT

Figure

Illuminated

Semiconductor

3. T r a n s m i s s i o n

F i g u r e 4.

33

Catalysts

e l e c t r o n micrograph of sample.

a 10 wt °/ P t / T i 0

C o r r e l a t i o n s between the P t ( A , C) o r N i ( Β ) c o n t e n t and (i) the logarithm of the photoconductance a at e q u i l i b r i u m under vacuum (A, B) o r ( i i ) the amounts o f oxygen photo-adsorbed (C).


2

34

O R G A N I C P H O T O T R A N S F O R M A T I O N S IN N O N H O M O G E N E O U S M E D I A

e x p o s e d t o 0 at 295 Κ up t o a f i n a l p r e s s u r e o f about 2.66 χ 1 0 P a , average e l e c t r o n e n r i c h m e n t s o f c a . 0 . 7 , 0.1 and 0.05 e l e c t r o n per Pt atom were c a l c u l a t e d f o r t h e samples c o n t a i n i n g 0 . 5 , 5 and 10 wt °j[ Pt, respectively (44). I n c o n c l u s i o n , t h e e l e c t r o n t r a n s f e r from the s e m i c o n d u c t o r to the m e t a l depends on the s t a t e o f b o t h components, i . e . , n a m e l y , on the medium s u r r o u n d i n g the sample and t h i s i s c e r t a i n l y an element t o c o n s i d e r when u s i n g these s o l i d s as p h o t o c a t a l y s t s . 2


2

C y c l o p e n t a n e - d e u t e r i u m i s o t o p i c exchange i n gaseous phase (43). W i t h o u t m e t a l d e p o s i t and u n d e r U V - i l l u m i n a t i o n , the exchange was l i m i t e d and o c c u r r e d o n l y i f the sample was p r e v i o u s l y c o v e r e d w i t h OD g r o u p s . In o t h e r words, i t was not p h o t o c a t a l y t i c and c o r r e s p o n d e d to the consumption o f OD g r o u p s . With d e p o s i t e d Pt and i n t h e d a r k , the exchange required temperatures > 273 Κ and polydeuterated c y c l o p e n t a n e m o l e c u l e s were o b t a i n e d ( F i g u r e 5 ) . I t was inferred, n a m e l y by r e p l a c i n g T i 0 b y o t h e r s u p p o r t s , t h a t the dark r e a c t i o n took p l a c e on the m e t a l p a r t i c l e s where t h e r e s i d e n c e t i m e o f t h e c h e m i s o r b e d c y c l o p e n t a n e m o l e c u l e s enabled a m u l t i p l e exchange. With d e p o s i t e d Pt and u n d e r U V - i l l u m i n a t i o n , t h e e x c h a n g e o c c u r r e d a t l o w e r t e m p e r a t u r e s , y i e l d i n g C5H9D ( a f t e r an i n d u c t i o n p e r i o d , weak amounts o f m u l t i p l y exchanged m o l e c u l e s p r o g r e s s i v e l y a p p e a r ) a t a c o n s t a n t r a t e ( F i g u r e 5) . A n experiment c a r r i e d out w i t h the same c a t a l y s t sample showed t h a t , i n s u c c e s s i v e r u n s , a l a r g e amount o f C5H1Q was e x c h a n g e d w i t h o u t d e c r e a s e i n a c t i v i t y . I t was c o n c l u d e d t h a t t h e e x c h a n g e was p h o t o c a t a l y t i c and t h a t i t took p l a c e on T i 0 where the s h o r t r e s i d e n c e time a l l o w e d o n l y s i m p l e exchange f o r e a c h a d s o r p t i o n a c t . I n a d d i t i o n , an o p t i m a l Pt c o n t e n t , which w i l l be d i s c u s s e d l a t e r o n , was found ( F i g u r e 6 ) . A l t h o u g h t h i s r e a c t i o n was chosen for m e c h a n i s t i c reasons, i t s e x t e n s i o n to appropriate m o l e c u l e s might be u s e f u l when i n c o m p l e t e d e u t e r a t i o n i s d e s i r e d . 2

2

A l c o h o l d e h y d r o g e n a t i o n i n l i q u i d phase (8-11, 51). H evolution from aqueous m e t h a n o l s o l u t i o n s w i t h P t / T i 0 (or Pt and T i 0 ) had been r e p o r t e d (52). M o r e o v e r , i t was c l a i m e d t h a t H a l s o r e s u l t e d from water d e c o m p o s i t i o n ( 5 2 ) . From a s e t o f e x p e r i m e n t s we h a v e established t h a t i n t h a t case the d e h y d r o g e n a t i o n o f m e t h a n o l a c c o u n t s f o r the H produced (8). In the absence o f d e p o s i t e d m e t a l , the r a t e of H generation f r o m an a l c o h o l p r o g e s s i v e l y d e c r e a s e d , w h i l e T i 0 t u r n e d to a b l u e c o l o r . By c o n t r a s t , i n the p r e s e n c e o f a M/TÎ0 sample (M = P t , Rh, Ni), t h e d e h y d r o g e n a t i o n was p h o t o c a t a l y t i c ( 8 - 1 0 ) . An o p t i m a l Pt c o n t e n t o f a b o u t 0 . 5 wt % ( c a . 1 Pt p a r t i c l e p e r T i 0 g r a i n ) was found (8) ( F i g u r e 6) and has been c o n f i r m e d ( 1 1 ) . F o r N i , the o p t i m a l v a l u e i s about 5 wt Jf and the maximum i n i t i a l a c t i v i t y i s c a . 7 times l o w e r t h a n t h a t o b t a i n e d w i t h P t / T i 0 ( 1 0 ) . T h i s emphasizes the f a c t t h a t d i r e c t c o m p a r i s o n o f t h e e f f e c t s o f v a r i o u s m e t a l s s h o u l d be c o n s i d e r e d w i t h c a r e i f the m e t a l - c o n t e n t a c t i v i t y dependence has not b e e n d e t e r m i n e d . The d i f f e r e n c e s b e t w e e n N i and Pt can a r i s e from d i s t i n c t c a t a l y t i c p r o p e r t i e s . Most p r o b a b l y , they a l s o r e l a t e d to the m e t a l p a r t i c l e sizes. The r e v e r s e r e a c t i o n e x i s t s and as e x p e c t e d , i n c r e a s e s w i t h the m e t a l l o a d i n g (9). The N i / T i 0 samples make c l e a r t h a t z e r o v a l e n t m e t a l atoms are r e q u i r e d ( 1 0 ) . A s t u d y o f t h e t e m p e r a t u r e i n f l u e n c e i n d i c a t e d t h a t the r e a c t i o n r a t e can be c o n t r o l l e d by H d e s o r p t i o n below room temperature (9). 2

2

2

2

2

2

2

2

2

2

2

2


2.

PICHAT

Illuminated

Semiconductor

Catalysts


4

D-i

Figure

D

2

D3

D

4

D

5

D

6

D

7

De

D

9

D-JO

5. Number o f d e u t e r i u m atoms e x c h a n g e d p e r c y c l o p e n t a n e m o l e c u l e o v e r a 5 wt Jf P t / T i 0 2 sample : d o t t e d l i n e s , a f t e r i l l u m i n a t i n g f o r 15 m i n a t 263 K , s o l i d l i n e s a f t e r 2 5 min i n the d a r k at 343 K. #

Pt/wt% F i g u r e 6.

I n i t i a l r a t e s v s Pt c o n t e n t s o f the P t / T i 0 2 specimens for : liquid methanol (A) or 1-propanol (B) dehydrogenation a t 298 Κ ; cyclopentane-deuterium exchange i n gas p h a s e a t 263 Κ (C) ; oxygen isotope h e t e r o e x c h a n g e a t 298 Κ o v e r n o n - p r e o x i d i z e d (D) or p r e o x i d i z e d (E) samples.




36

F o r n e a t - l i q u i d i t has been e s t a b l i s h e d t h a t the d e h y d r o g e n a t i o n r e p r e s e n t s a l m o s t c o m p l e t e l y t h e phenomena o b s e r v e d , s i n c e w i t h e t h a n o l and 1 - p r o p a n o l , 1.1 v a l u e s were f o u n d f o r t h e ratios H 2 / ( a l d e h y d e + a c e t a l ) , which i s w i t h i n the e x p e r i m e n t a l e r r o r s , and t h e f o r m a t i o n o f CO2 m o l e c u l e s o n l y amounted t o 1 °l ( e t h a n o l ) or 4 % ( 1 - p r o p a n o l ) o f t h e aldehyde + a c e t a l m o l e c u l e s ( 8 ) . Depending upon t h e c a t a l y s t and t h e a l c o h o l , quantum y i e l d s i n the range 0 . 1 - 0 . 8 were found for the f o l l o w i n g a l c o h o l s whose o r d e r o f r e a c t i v i t y was MeOH > EtOH > 1-PrOH, 2 - P r 0 H , 1-BuOH H o w e v e r , about t h r e e times more o f p r o p a n a l than o f acetone was o b t a i n e d from an e q u i m o l e c u l a r m i x t u r e o f 1 and 2 - p r o p a n o l ; t h i s d i f f e r e n c e m i g h t a r i s e f r o m c o m p e t i t i v e a d s o r p t i o n s ; i t seems t o i n d i c a t e t h a t f r e e r a d i c a l s are not i n v o l v e d under these c o n d i t i o n s .

Table

I.

Mean quantum y i e l d s and p r o d u c t d i s t r i b u t i o n f o r the dehydrogenation of the alcohols indicated (catalyst : 0 . 5 wt °l P t / T i 0 2 ï i l l u m i n a t i o n time : 6 h ) .

Φ

Products Unsat. a l l y l alcohol cinnamyl a l c o h o l citronellol tetrahydrogeraniol geraniol

0.025 0.065 0.14 0.14 0.35

aid .

79 77 100

-75 (citral)

of

Sat.

Sat.

ale.

4 17 0

-

24 (citronellol)

aid.

18 6 0 100 1 (citronellol)

The mechanism s u g g e s t e d (8) i n c l u d e s the d i s s o c i a t i v e the a l c o h o l on a n a t a s e b a s i c s i t e s : ^CHOH ->^CH0~ + H

adsorption

+

and t h e abstract

i n t e r v e n t i o n o f h o l e s ( p o s s i b l y as a H atom ( h e t e r o l y t i c b r e a k i n g ) :

0H°

or

0

species)

to

would be f a v o r e d

by

T

a

^CHO" + p

+

- » ^ C O + H°

The f o r m a t i o n o f H2, w h i c h the m e t a l c r y s t a l l i t e s H° + H

+

+ e~

->

H

involves

electrons,

2

T h i s d e h y d r o g e n a t i o n m e t h o d was r e c e n t l y extended to v a r i o u s liquid unsaturated alcohols (allyl and cinnamyl alcohols, c i t r o n e l l o l , g e r a n i o l and, f o r c o m p a r i s o n , t e t r a h y d r o g e r a n i o l , chosen as model m o l e c u l e s ) (53) ( F i g u r e 7 ) . The quantum y i e l d s are i n d i c a t e d i n T a b l e I. For c i t r o n e l l o l (and t e t r a h y d r o g e r a n i o l ) , i t was i n f e r r e d f r o m t h e ^ / a l d e h y d e r a t i o s , e q u a l to 1 w i t h i n e x p e r i m e n t a l e r r o r s , t h a t the dehydrogenat i o n was by f a r the dominant p h e n o m e n o n . I n t h e p r e s e n c e o f a c o n j u g a t e d d o u b l e b o n d , T a b l e I shows t h a t the c o r r e s p o n d i n g s a t u r a t e d a l c o h o l and t h e s a t u r a t e d a l d e h y d e ( i s s u e d f r o m i s o m e r i z a t i o n ) were a l s o d e t e c t e d , a l t h o u g h t h e amount o f



2.

PICHAT

Figure

Illuminated

Semiconductor

Catalysts

7. H2 g e n e r a t e d from s u s p e n s i o n s o f 70 mg 0 . 5 wt Jf P t / T i 0 2 i n 10 cm^ (A) a l l y l a l c o h o l , (B) c i n n a m y l a l c o h o l , (C) c i t r o n e l l o l , (D) t e t r a h y d r o g e r a n i o l , (E) g e r a n i o l as a function of i l l u m i n a t i o n time.


37


38


u n s a t u r a t e d aldehyde predominated m a r k e d l y . On the c o n t r a r y , a double bond remote from the a l c o h o l f u n c t i o n and i n a d d i t i o n h i n d e r e d w i t h two m e t h y l g r o u p s , s u c h as i n c i t r o n e l l o l or i n g e r a n i o l , was not r e d u c e d . E x p e r i m e n t s w i t h naked T1O2 showed t h a t the Pt d e p o s i t was not i n v o l v e d i n the i s o m e r i z a t i o n . U n d e r t h e s e c o n d i t i o n s i n v o l v i n g t h e a b s e n c e o f o x y g e n , no s i g n i f i c a n t o v e r - o x i d a t i o n was d e t e c t e d . Because o f the low i n c i d e n t r a d i a n t f l u x (some 10"^ E i n s t e i n h " * c m ~ O , c o n v e r s i o n s below 1 % were o b t a i n e d w i t h pure a l c o h o l s a f t e r i l l u m i n a t i n g f o r 10 h , d e s p i t e r e a s o n a b l e quantum y i e l d s . However, h i g h c h e m i c a l y i e l d s c o u l d be o b t a i n e d f o r c o m p a r a b l e numbers o f photons and o f m o l e c u l e s to t r a n s f o r m e d , i . e . w i t h d i l u t e d s o l u t i o n s . F o r example, c o n v e r s i o n s o f ~ 50 o r - 80 were found a f t e r 2 o r 6 h , r e s p e c t i v e l y , w i t h 6 χ 1 0 " ^ m o l c i t r o n e l l o l i n 10 cm^ n - h e p t a n e , u s i n g a 125 W Hg-lamp. These a l c o h o l d e h y d r o g e n a t i o n s c a n be u s e d for in situ h y d r o g é n a t i o n o f u n s a t u r a t e d compounds. Only a s m a l l q u a n t i t y o f H was found to e v o l v e compared w i t h t h e amount i n c o r p o r a t e d i n t o t h e unsaturated molecule. Moreover, when u s i n g an a l c o h o l w i t h a c o n j u g a t e d double bond, the presence o f a r e d u c i b l e compound a l l o w e d t h e s e l e c t i v i t y t o t h e u n s a t u r a t e d a l d e h y d e t o be i n c r e a s e d ( f o r instance, increase i n a c r o l e i n percentage for allyl alcohol d e h y d r o g e n a t i o n i n the presence o f d i p h e n y l a c e t y l e n e ) . U n f o r t u n a t e l y , a t l e a s t f o r the P t / T i 0 2 samples employed, the hydrogénation was not s e l e c t i v e ; f o r i n s t a n c e , both the C-C double bond and t h e c a r b o n y l g r o u p o f c i n n a m a l d e h y d e were s a t u r a t e d by hydrogen a b s t r a c t e d from 2 - p r o p a n o l . B e s i d e s the a d d i t i o n o f an u n s a t u r a t e d compound d i d n o t r e n d e r c a t a l y t i c the a l c o h o l d e h y d r o g e n a t i o n over naked T1O2. In conclusion, this dehydrogenation method, beyond its fundamental o b j e c t i v e s , c o u l d be o f i n t e r e s t i n o r g a n i c s y n t h e s i s . 2

I n t e r p r e t a t i o n o f t h e optimum m e t a l c o n t e n t f o r these r e a c t i o n s . As a l r e a d y mentioned an optimum Pt c o n t e n t was found f o r d e h y d r o g e n a t i o n o f l i q u i d a l c o h o l s and c y c l o p e n t a n e - d e u t e r i u m exchange i n gas p h a s e . A l s o , w i t h P t / T i 0 2 samples which had not been p r e o x i d i z e d and which were a c c o r d i n g l y n o n - s t o i c h i o m e t r i c a c c o r d i n g t o c o n d u c t i v i t y measurements, the same optimum c o n t e n t was found f o r the i n i t i a l r a t e o f OIE, whereas t h i s r a t e d e c r e a s e d as a f u n c t i o n o f Pt c o n t e n t f o r p r e o x i d i z e d samples (44). I f t h e m e t a l had o n l y a b e n e f i c i a l c a t a l y t i c r o l e n e c e s s a r y (i) t o e v o l v e H2 ( o r H D ) , and ( i i ) t o r e g e n e r a t e OD groups ( C 5 H i o ~ 2 exchange), the r e a c t i o n r a t e s s h o u l d i n c r e a s e w i t h i n c r e a s i n g m e t a l c o n t e n t s , p o s s i b l y up to a l i m i t , and no i n c r e a s e s h o u l d be e x p e c t e d i n the case o f oxygen i s o t o p e exchange. The maximum r a t e s o b s e r v e d f o r a Pt c o n t e n t o f about 0 . 5 wt % f o r i n s t a n c e , r e g a r d l e s s o f the r e a c t i o n ( F i g u r e 6 ) , show t h a t Pt has a l s o a d e t r i m e n t a l e f f e c t . T h i s c a n n o t a r i s e from b a c k - r e a c t i o n s , s i n c e i n i t i a l r e a c t i o n r a t e s have been c o n s i d e r e d . T h i s cannot r e s u l t from g e o m e t r i c a l f e a t u r e s e i t h e r , s i n c e ( i ) o n l y about 6 % o f the anatase s u r f a c e was o c c u l t e d by Pt f o r the h i g h e s t c o n t e n t (10 wt , (ii) t h e T1O2 a r e a s b e t w e e n t h e Pt p a r t i c l e s r e m a i n e d largely s u f f i c i e n t t o a l l o w t h e a d s o r p t i o n o f s e v e r a l C5H10 o r a l c o h o l m o l e c u l e s , as shown by TEM ( F i g u r e 3 ) , and ( i i i ) the metal p a r t i c l e s i z e d i d not v a r y f o r the c o n t e n t s employed. The OIE experiments show t h a t an optimum Pt c o n t e n t e x i s t s o n l y i f T1O2 is sufficiently n o n - s t o i c h i o m e t r i c to a l l o w an e l e c t r o n t r a n s f e r to P t . T h i s tends to i n d i c a t e t h a t t h e e l e c t r o n i c r o l e o f Pt i s a t t h e o r i g i n o f the D


2.

PICHAT

Illuminated

Semiconductor

39

Catalysts

optimum c o n t e n t . F o r low Pt c o n t e n t s , the d e c r e a s e d e l e c t r o n d e n s i t y of Ti0 i n the p r e s e n c e of 0 can reduce the electron-hole r e c o m b i n a t i o n i n t h i s m a t e r i a l , whereas for h i g h Pt c o n t e n t s (5 and 10 wt the r e c o m b i n a t i o n at the much more numerous m e t a l p a r t i c l e s can s i g n i f i c a n t l y compete w i t h the r e a c t i o n s d r i v e n by t h e m i n o r i t y c h a r g e c a r r i e r s . O n l y t h i s l a t t e r e f f e c t can o c c u r f o r p r e o x i d i z e d Ti0 w h i c h h a s a F e r m i l e v e l p r o b a b l y v e r y c l o s e to t h a t o f Pt and t h i s w o u l d e x p l a i n t h e d e c l i n e i n OIE r a t e f o r a l l t h e P t / T i 0 p r e o x i d i z e d samples. The f a c t t h a t o n l y a low amount o f m e t a l d e p o s i t i s r e q u i r e d to r e n d e r c a t a l y t i c r e a c t i o n s i n v o l v i n g hydrogen over i l l u m i n a t e d T i 0 i s advantageous to l i m i t back r e a c t i o n s c a t a l y z e d by t h e m e t a l , as w e l l as f o r e c o n o m i c a l r e a s o n s . C o n v e r s e l y , t h i s r e s u l t i l l u s t r a t e s the l i m i t a t i o n s encountered when t r y i n g t o m o d i f y t h e s u r f a c e o f a semiconductor. Organic acid d e c a r b o x y l a t i o n . B a r d e t a l . ( 5 4 - 5 7 ) i n i t i a t e d the i n v e s t i g a t i o n o f t h e e x e r g o n i c d e c a r b o x y l a t i o n o f v a r i o u s R-COOH a c i d s (R = CH3, C H , C 3 H , C4H9, C 3 H C ( C H 3 ) ) and o f a d i a c i d ( h e x a n e d i o i c ) over T i 0 ( o r WO3) w i t h and w i t h o u t p h o t o d e p o s i t e d P t ( l - 5 wt i n v a r i o u s m e d i a ( a q u e o u s and m i x e d aqueous/organic s o l u t i o n s ( 5 4 - 5 7 ) , gas phase ( 5 8 ) ) . The main r e a c t i o n was 2

2

2


2

2

2

5

7

7

2

2

RC0 H -» C 0 + RH, the f o r m a t i o n s o f the alkane R-R and o f H 2

while

2

2RC0 H

-»

2

2C0

2

+ R - R + H

2

were a l s o

detected

2

I n a d d i t i o n , l i t t l e amounts o f i n t e r m e d i a t e , o x y g e n - c o n t a i n i n g compounds, such as e t h a n a l , m e t h a n o l and e t h a n o l , were p r o d u c e d , w h i c h was n o t u n e x p e c t e d s i n c e a l k a n e s a r e o x i d i z e d u n d e r t h e s e c o n d i t i o n s ( v i d e s u p r a ) . The maximum quantum y i e l d was a b o u t 0.1. Much h i g h e r c o n v e r s i o n s were o b t a i n e d i n the p r e s e n c e o f 0 . In t h i s c a s e , the Pt d e p o s i t seemed to p l a y a minor r o l e , whereas i t was more i m p o r t a n t for deoxygenated s o l u t i o n s . T h i s b e n e f i c i a l e f f e c t was a t t r i b u t e d to r a t e enhancements o f r e d u c t i o n p r o c e s s e s . With b e n z o i c a c i d , t o t a l d e g r a d a t i o n o f the r i n g to C 0 o c c u r r e d and t h e d e t e c t i o n o f s a l y c i l i c a c i d suggested the i n t e r v e n t i o n o f 0H° r a d i c a l s ( 5 6 ) . These r a d i c a l s were a l s o proposed to e x p l a i n the o x i d a t i o n o f n - C a l k a n e s (x = 6 , 7 , 9 , 1 0 ) and o f c y c l o h e x a n e i n 1:1 v o l . water/4iydrocarbon two-phase m i x t u r e s o v e r 10 wt °j[ P t / T i 0 . T r a c e s o f a l c o h o l s ( a n d o f 2 - , 4 - , 5-decanone w i t h d e c a n e ) were d e t e c t e d . No t r a n s f o r m a t i o n o c c u r r e d without 0 and i n t h e a b s e n c e o f H 0 the r a t e was s u b s t a n t i a l l y d e c r e a s e d . The r o l e o f Pt was a t t r i b u t e d to a g r e a t e r ease o f oxygen r e d u c t i o n ; however the o x i d a t i o n r a t e was o n l y d e c r e a s e d by a f a c t o r o f about 1.5 w i t h o u t Pt (59). S i m i l a r l y , w i t h t h e same t y p e o f p h o t o c a t a l y s t ( P t / T i 0 or 2°3^ decomposition of l e v u l i n i c (4-oxopentanoic) acid i n o x y g e n - f r e e aqueous s o l u t i o n has been i n v e s t i g a t e d i n d e t a i l ( 6 0 ) . In a d d i t i o n to the d e c a r b o x y l a t i o n r e a c t i o n , o x i d a t i v e C-C s c i s s i o n s l e d to p r o p i o n i c and a c e t i c a c i d s ( f u r t h e r c o n v e r t e d i n t o methane and ethane) and r e d u c t i v e c l e a v a g e s to acetone and e t h a n a l . The f o r m a t i o n o f acetone was a p p a r e n t l y f a v o u r e d by h i g h e r Pt c o n t e n t s ( h o w e v e r p r o d u c t d i s t r i b u t i o n s r e f e r r e d to e q u a l i l l u m i n a t i o n d u r a t i o n s and not t o e q u a l c o n v e r s i o n s ) . I t was s u g g e s t e d t h a t t h e v a r i e t y o f p r o d u c t s r e s u l t e d from t h e p r e s e n c e o f two f u n c t i o n a l g r o u p s i n l e v u l i n i c a c i d . The quantum y i e l d was p r o b a b l y o f t h e o r d e r o f 5 χ ΙΟ" . 2

2

x

2

2

2

2

F e

t

n

e

3


40


In s h o r t , t h e p r o p o s e d mechanism was based on the f o r m a t i o n o f R° r a d i c a l s ( C H Q was d e t e c t e d by ESR (54) b y c a p t u r e o f photoproduced h o l e s (RCOJ + h * - » R° + CO2), w h i l e 0 was c o n s i d e r e d t o i n t e r v e n e i n e l e c t r o n c a p t u r e , and P t i n t h e f o r m a t i o n o f H2 from H° as i n the reactions previously discussed i n this text.


2

Conclusion. As e x p e c t e d , the d e p o s i t i o n o f a group V I I I t r a n s i t i o n m e t a l o n t o an a p p r o p r i a t e semiconductor presents a very great i n t e r e s t , s i n c e i t a l l o w s one t o extend the domain o f h e t e r o g e n e o u s p h o t o c a t a l y s i s t o r e a c t i o n s i n v o l v i n g H2, such as d e h y d r o g e n a t i o n s and i s o t o p i c exchanges, because o f the c a t a l y t i c p r o p e r t i e s o f t h e s e m e t a l s f o r e i t h e r r e c o m b i n i n g o r d i s s o c i a t i n g H2. The e x i s t e n c e o f the s p i l l o v e r phenomenon e n a b l e s a r e v e r s i b l e t r a n s f e r o f h y d r o g e n atoms o r p r o t o n s between b o t h p h o t o c a t a l y s t components ( 6 1 ) . However, t h e s e d e p o s i t s a c t as r e c o m b i n a t i o n c e n t e r s f o r the photoproduced c h a r g e s . The r e s u l t i n g o p t i m a l m e t a l a m o u n t , w h i c h d e p e n d s on t h e m e t a l , on t h e s e m i c o n d u c t o r and on t h e i r r e s p e c t i v e p a r t i c l e s i z e s , s h o u l d be determined f o r each c a s e . Acknow1e dgmen t s The a u t h o r i s i n d e b t e d t o h i s CNRS c o - w o r k e r s Dr J . - M . Herrmann, Mr H. Courbon, Mr J . D i s d i e r , Mrs M . - N . Mozzanega whose c o n t r i b u t i o n s appear i n the l i t e r a t u r e c i t e d .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15.

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