Raman Spectroscopic Studies of Surface Species - American

7 Raman Spectroscopic Studies of Surface Species B. A. MORROW

Downloaded by UNIV OF MONTANA on September 30, 2014 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch007

Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 9B4, Canada

As this Volume illustrates, surface vibrational data can be obtained using a wide variety of experimental techniques. Raman spectroscopy is a particularly useful method insofar as there are virtually no restrictions as to the type of surface which can be studied (oxides, oxide supported catalysts and bulk metals), the accessible frequency range (50 - 4000 ) or the ambient gas pressure (one atmosphere to UHV). By way of comparison, although transmission infrared spectroscopy is widely used to study the adsorption of gases on high area oxides and oxide supported metals (1,2,3), (i) i t cannot be used to study adsorption on single crystal metal surfaces, (ii) i t sometimes cannot be used to investigate low frequency modes because most oxides are opaque to infrared radiation in certain spectral regions below ~ 1000 cm , and ( i i i ) different and sometimes expensive optical materials are required for a complete spectral analysis and more than one spectrometer might be required. Other techniques can overcome some of these difficulties and can be used over a wide spectral range (electron energy loss, IR reflectance, IR ellipsometry, diffuse reflectance) but there are sometimes restrictions on the type of material which can be studied (either metals or oxides), the ambient gas pressure, and the spectral resolution. -1

-1

In c o n t r a s t ,

recent work (4-12) has shown t h a t Raman

spectroscopy can be used to studyTi) adsorption on oxides, oxide supported metals and on bulk metals [ i n c l u d i n g an unusual e f f e c t sometimes termed "enhanced Raman s c a t t e r i n g " wherein s i g n a l s o f the order o f 1 0 - 106 more intense than a n t i c i p a t e d have been reported f o r c e r t a i n molecules adsorbed on s i l v e r ] , ( i i ) c a t a l y t i c processes on z e o l i t e s , and ( i i i ) the surface p r o p e r t i e s o f supported molybdenum oxide d e s u l f u r i z a t i o n catalysts. Further, the technique i s unique i n i t s a b i l i t y to o b t a i n v i b r a t i o n a l data for adsorbed species at the w a t e r - s o l i d i n t e r f a c e . I t i s to these t o p i c s t h a t we w i l l turn our a t t e n t i o n . We w i l l mainly confine our d i s c u s s i o n to work s i n c e 1977 ( i n c l u d i n g unpublished work from our l a b o r a t o r y ) because two e a r l y reviews (13., 14) have covered work before 1974 and two short recent reviews have d i s cussed work up to 1977 (1_5,16j. 4

0-8412-0585-X/80/47-137-119$05.50/0 © 1980 American Chemical Society

In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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Experimental Considerations The b a s i c requirement f o r studying Raman s c a t t e r i n g a r e , (a) a monochromatic l i g h t course [ u s u a l l y a l a s e r operating i n the v i s i b l e r e g i o n } , (b) a spectrograph to d i s p e r s e the compon ents o f the s c a t t e r e d l i g h t [ u s u a l l y a double o r t r i p l e monochromator i n order to reduce s t r a y l i g h t ] , and (c) a photom u l t i p l i e r and photon counting d e t e c t i o n system, sometimes used w i t h a computer f o r data manipulation and s i g n a l averaging. For bulk s o l i d s or l i q u i d s the s c a t t e r e d l i g h t i s g e n e r a l l y c o l l e c t e d at 90° w i t h respect to angle o f incidence o f the l a s e r , although sometimes a 180° "back s c a t t e r i n g " arrangement i s used. In studying molecules adsorbed on s u r f a c e s , a wide v a r i e t y of l a s e r i l l u m i n a t i o n and l i g h t c o l l e c t i o n geometries have been employed (13-16), the exact geometry sometimes being d i c t a t e d by the n e c e s s i t y o f c o n t a i n i n g the sample to be s t u d i e d i n a s u i t able c e l l f o r pretreatment. The l a t t e r may range from a simple Pyrex tube c o n t a i n i n g an o p t i c a l f l a t (4) to complex UHV chambers which permit use o f other surface c h a r a c t e r i z a t i o n techniques (5^,J_7,V8). F u r t h e r , i n many adsorption s t u d i e s one must be c a r e f u l t h a t the l a s e r i t s e l f w i l l not induce desorption due to l o c a l heating and, i n l i e u o f simply using low l a s e r power l e v e l s (20-200 mW), some groups have used c y l i n d r i c a l lens focus techniques i n order to spread out the l a s e r beam i n t o a l i n e image on the sample, (8,13-16) or e l s e the sample has been rotated so that the energy f l u x o f a focused l a s e r spot i s de creased (9,10,19,20). Powdered oxides o r oxide supported samples have g e n e r a l l y been studied as pressed d i s c s o r extrudates and we have found (21) t h a t optimum s i g n a l i n t e n s i t y has g e n e r a l l y been achieved w i t h geometries s i m i l a r to those i l l u s t r a t e d i n Figure 1, p a r t i c u l a r l y the near 180° back s c a t t e r i n g geometry (Figure 1A) where the l a s e r beam s t r i k e s the sample a t about 10° from the normal such t h a t the " r e f l e c t e d " and scattered l i g h t i s d i r e c t e d w i t h near normal incidence i n t o the spectrometer. For bulk metals or evaporated metal f i l m s , a 90° i l l u m i n a t i o n - s c a t t e r i n g geometry has g e n e r a l l y been used (as i n Figure IB) but the angle 3 has been v a r i e d c o n s i d e r a b l y depending on the nature or purpose o f the study ( 2 2 ) . Greenler and S l a g e r (23) have discussed the question o f the optimum i l l u m i n a t i o n - s c a t t e r i n g geometry f o r studying the Raman spectra o f adsorbed molecules on m e t a l s . A major problem which i s endemic to Raman studies o f high surface area oxides i s background f l u o r e s c e n c e , a problem so severe i n some cases t h a t weak s i g n a l s due to Raman s c a t t e r i n g are simply l o s t i n the background (13-16). A t y p i c a l background spectrum o f a s i l i c a sample i s shown i n Figure 2 , ( t h i s a l s o shows the r e l a t i v e l y weak features due to Si02 i t s e l f below 1000 cm-1) where a very broad maximum near 2500 α ι Η i s observed using 488.0 nm i r r a d i a t i o n ( i . e . , a t 18500 c n H a b s o l u t e ) . This fluorescence maximum, which s h i f t s i n terms o f Raman displacement but remains at the same absolute frequency i f the l a s e r f r e quency i s changed ( 2 1 ) , i s sometimes g r e a t l y diminished i f the



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Raman Spectroscopic Studies

A

Β

C

Figure 1. Schematics of various laser illumination geometries. The laser beam strikes either a prism (p) or a mirror (m) and is directed upward toward the sample. The "lens" is the collection lens of the spectrometer (21).

Figure 2. (A) Raman spectrum of silica; (B) Raman spectrum of methylated silica, new bands are marked with vertical arrows; (C) spectrum of the weak feature at 1465 cm with 10-fold wavenumber and ordinate expansion. Laser power 0.6 W for (A) and (B), 1.7 W for (C) (21). 1



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sample i s heated i n an oxygen atmosphere at 700°C, a r e s u l t which has been a t t r i b u t e d by many to the o x i d a t i o n o f t r a c e q u a n t i t i e s of hydrocarbon i m p u r i t i e s ( 4 , 1 8 , 1 3 , 1 4 , 1 5 , 1 6 ) . With z e o l i t e s the presence o f t r a c e q u a n t i t i e s o f i o n i c i r o n i m p u r i t i e s has a l s o been discussed as a c o n t r i b u t i n g f a c t o r ( 2 4 ) , and J e z i o r o w s k i and Knozinger (25) have presented evidence which i n d i c a t e s that surface hydroxide ions may a l s o be a source o f p a r t o f the fluorescence. Thus, i n s t u d i e s o f oxide supported molybdenum oxide c a t a l y s t s [see below] p a r t o f the e l i m i n a t i o n o f strong f l u o r e s c e n t background has been a t t r i b u t e d to removal o f OH during c a t a l y s t processing ( 1 0 ) , although other t h e o r i e s have been advanced i n these casesTl9) · F i n a l l y , S a p o r s t e i n and Rein (26) have r e c e n t l y reported t h a t the f l u o r e s c e n t background from a 4A z e o l i t e can be g r e a t l y diminished by washing i n 0.2 Ν NaOH and c a l c i n i n g at 400°C. This should prove useful i n future s t u d i e s o f z e o l i t e s where, because o f p o s s i b l e c r y s t a l s t r u c t u r e c o l l a p s e , i t may not be d e s i r a b l e to subject the sample to high temperatures. In the d i s c u s s i o n which f o l l o w s we w i l l f r e q u e n t l y make reference to the " q u a l i t y " o f observed Raman s p e c t r a . Raman i n t e n s i t i e s are u s u a l l y reported i n terms o f photon counts per second ( c . p . s . ) and whereas pure l i q u i d s frequently e x h i b i t Raman spectra i n which the s i g n a l t o noise r a t i o i s greater than 1000 and maximum peak i n t e n s i t i e s are i n the range 1 0 - 1 0 c . p . s . (depending on l a s e r power, u s u a l l y i n the range 20-4000 mW, and on the s l i t width o f the spectrometer), the s e n s i t i v i t y o f the technique f o r studying surfaces i s often f a r l e s s than t h i s and wherever p o s s i b l e we have quoted s i g n a l i n t e n s i t i e s f o r comparative purposes. F u r t h e r , i n cases where the background fluorescence cannot be e n t i r e l y e l i m i n a t e d , the Raman spectrum o f an adsorbed species appears as a superimposed s i g n a l . Figure 2B shows the Raman spectrum o f a methylated s i l i c a , a sample where a l l surface SiOH groups have been r e p l a c e d by SiOCH3 groups. The sharp features near 3000 cm" due to v(CH) modes w i l l be discussed f u r t h e r l a t e r , but the s i g n a l to fluorescence i n t e n s i t y can be estimated from the o r d i n a t e s c a l e . 3

6

1

Adsorption on Z e o l i t e s The a d s o r p t i o n o f carbon monoxide on metal c a t a l y s t s (3) has been the model system o f choice f o r the development o f many v i b r a t i o n a l s p e c t r o s c o p i c techniques ( I R , EELS and r e f l e c t i o n absorption) where energy exchange depends on a change i n d i p o l e moment during v i b r a t i o n . P y r i d i n e (13-16) has h i s t o r i c a l l y been used f o r the development o f the Raman technique f o r studying surfaces because energy exchange ( i n e l a s t i c s c a t t e r i n g ) occurs when a molecule undergoes a change i n p o l a r i z a b i l i t y during v i b r a t i o n . Thus, whereas the CO s t r e t c h i n g mode o f adsorbed CO y i e l d s an intense band i n the i n f r a r e d spectrum, the i n - p l a n e symmetric (A]) r i n g deformation modes o f p y r i d i n e have a l a r g e Raman s c a t t e r i n g cross s e c t i o n . Figure 3A shows a p o r t i o n o f



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Figure 3. Raman spectrum of pyridine in the region of the v (991-1016 cm' ) 1

t

and v

12

1

(1030-1036

cm' ) fundamentals:

(A) liquid pyridine, (B) pyridine adsorbed on a NaX zeolite, and (C) pyridine adsorbed on a Zn * exchanged (78% exchange) NaX zeolite. The vertical bar +

'

1

—i33— cm" 1

1

'—SE

-

represents: (A) 2000 Hz, (B) 100 Hz, (C) 50

Hz.


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the Raman spectrum of l i q u i d p y r i d i n e i n the region o f the p a r t i c u l a r l y intense v-| (991 cm" ) and v i 2 (1030 cm-') bands. The symmetric v] r i n g breathing mode i s most useful from the surface d i a g n o s t i c p o i n t o f view because i t has been shown to s h i f t i n a p r e d i c t a b l e way when p y r i d i n e i n t e r a c t s with v a r i o u s s i t e s on an oxide o r z e o l i t e surface ( 8 , 1 3 , 1 4 , 1 5 , 1 6 , 2 7 ) . E a r l y studies (13, 14,24,27) using model systems and w e l l c h a r a c t e r i z e d oxides have shown t h a t there are f a i r l y c l e a r domains f o r t h i s mode depending on whether adsorption occurs v i a p h y s i c a l adsorption (991 c m ) , hydrogen-bonding (994-1008 c n r T J , proton t r a n s f e r due to Brtfnsted a c i d i t y (1009-1012 c m ) or c o o r d i n a t i o n to e l e c t r o n d e f i c i e n t A l atoms due to Lewis a c i d i t y (1016-1025 c m " ) . A c c o r d i n g l y , the Raman spectra o f adsorbed p y r i d i n e provides a useful probe f o r the determination o f the types o f adsorption s i t e s which are present on oxide s u r f a c e s , and much o f the e a r l i e r work has been 1

- 1

-1


1

reviewed

{U ]± }^ ]6). 9

9

9

The extension o f these s t u d i e s to z e o l i t e s has y i e l d e d some interesting results. In an e a r l y study Egerton, Hardin and Sheppard (27) showed, i n agreement w i t h Ward's previous i n f r a r e d r e s u l t s ( 2 8 , 2 9 ) , that p y r i d i n e adsorbs mainly to the c a t i o n i n a s e r i e s o f c a t i o n exchanged Y z e o l i t e s but that there i s a l i n e a r s h i f t o f v i to higher frequency w i t h the e l e c t r o s t a t i c p o t e n t i a l (or charge to radius r a t i o , e / r , o f the exchange c a t i o n ) . We r e c e n t l y c a r r i e d out a d e t a i l e d study (8) o f the X - z e o l i t e system (NaX and 10 c a t i o n exchanged samples) and found a s i m i l a r v i v s . e / r c o r r e l a t i o n f o r adsorbed p y r i d i n e and f o r some metal n i t r a t e - p y r i d i n e - n i t r o m e t h a n e s o l u t i o n s . T y p i c a l spectra i n the v i s p e c t r a l region showing the a t t a i n a b l e s i g n a l - t o - n o i s e r a t i o for adsorption on the NaX z e o l i t e , and on a ZnNaX z e o l i t e f o r which 78% o f the N a had been exchanged with Z n , are shown i n Figures 3B and 3C r e s p e c t i v e l y . We concluded t h a t adsorption occurred e x c l u s i v e l y v i a a d i r e c t i n t e r a c t i o n between the p y r i d i n e lone p a i r e l e c t r o n s and the p o s i t i v e l y charged c a t i o n . Two v i bands were g e n e r a l l y observed when there was r e s i d u a l unexchanged Na as can be seen i n Figure 3C where the Z n - p y r i d i n e band i s at 1016 c m and t h a t due to N a - p y r i d i n e i s unchanged at 996 c m " . We f u r t h e r postulated t h a t where i o n i c s i z e i s not a c o n s i d e r a t i o n , the exchange c a t i o n s (monovalent and b i v a l e n t ) are d i s t r i buted a t l e a s t among s i t e s Sj and SJI· In the case of the Cs+ exchanged z e o l i t e some unusual frequency s h i f t s were observed for other p y r i d i n e modes r e l a t i v e to the other exchanged z e o l i t e s , the sample was f a i n t l y pink at high p y r i d i n e coverage and we suggested t h a t some photo reduction might have occurred r e s u l t i n g i n the a d d i t i o n a l formation o f a C s ^ * " ) - p y species. A d d i t i o n a l recent Raman work has shown t h a t other h i g h l y p o l a r i z a b l e molecules can a l s o be used to probe the p r o p e r t i e s o f z e o l i t e s . Freeman and Uni and (7) s t u d i e d the adsorption o f benzene on a s e r i e s o f a l k a l i exchanged X and Y z e o l i t e s and a l s o used the measured s h i f t o f the v-j r i n g breathing mode (992 cm" f o r l i q u i d benzene) as a d i a g n o s t i c measure o f the i n t e r a c t i o n w i t h the exchange c a t i o n s . The spectra were comparable i n q u a l i t y to those shown i n Figure 3 B , C . In a l l cases the V] band s h i f t e d +

+ +

+

+ +

-1

+

1

1

+ 1

1



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to lower frequency r e l a t i v e t o l i q u i d benzene, the s h i f t being g r e a t e r f o r l a r g e r i o n i c r a d i u s , which the authors i n t e r p r e t e d as being evidence f o r an i n t e r a c t i o n between the π - e l e c t r o n s o f benzene and the exchange c a t i o n . They a l s o found ( u n l i k e p y r i dine) t h a t the exact band p o s i t i o n v a r i e d w i t h benzene dosage and concluded that the excess c a t i o n s i n X v £ . Y z e o l i t e s r e s u l t e d i n higher f i e l d s at the aromatic nucleus o f benzene. These trends were c o r r e l a t e d w i t h the s e l e c t i v i t y o f these c a t a l y s t s f o r the a l k y l a t i o n o f toluene to g i v e e t h y l benzene and s t y r e n e . In an e a r l i e r study Tarn et a l . ( 3 0 , 3 1 , 3 2 ) i n v e s t i g a t e d the adsorption o f a c e t y l e n e , dimethyl acetylene and pyrazine ( 1 , 4 - d i a z i n e ) on a s e r i e s o f a l k a l i and a l k a l i n e earth exchanged X zeo l i t e s , and o f acetylene on A type z e o l i t e s . A complete v i b r a t i o n al a n a l y s i s o f the f i v e acetylene fundamentals was p o s s i b l e i n the case o f NaA and CaA z e o l i t e s ( 3 0 ) , demonstrating the v e r s a t i l i t y o f the Raman technique as a method o f o b t a i n i n g v i b r a t i o n a l fundamentals over a wide frequency range. For pyrazine ( 3 2 ) , the high frequency s h i f t o f the r i n g breathing mode r e l a t i v e to l i q u i d pyrazine was p r o p o r t i o n a l to the p o l a r i z i n g power o f the c a t i o n [ i . e . , a s m a l l e r s h i f t f o r i n c r e a s i n g r ] and independent o f the c a t i o n charge, a r e s u l t which c o n t r a s t s w i t h the e / r c o r r e l a t i o n found by Ward ( 3 3 ) , and by us ( 8 ) , f o r p y r i d i n e adsorption [although q u a l i t a t i v e l y the r dependence i s the same f o r equal c a t i o n c h a r g e ] . Tarn and Cooney (32) noted t h a t p y r a z i n e i s a much weaker base than p y r i d i n e s i n c e i t has no d i p o l e moment and therefore would be l e s s able to accommodate p o s i t i v e charge than p y r i d i n e . For acetylene adsorbed on exchanged A and X z e o l i t e s , Tarn et al_. (30,31) found t h a t the d i a g n o s t i c C E C s t r e t c h i n g mode TT974 c m ^ T T ô r gaseous C2H2] s h i f t e d downward upon a d s o r p t i o n , the magnitude o f the s h i f t now being i n v e r s e l y p r o p o r t i o n a l to the c a t i o n p o l a r i z i n g power o f the c a t i o n , i . e . , a l a r g e r s h i f t for l a r g e r c a t i o n r a d i u s [as was found f o r Unland et^ a L {7) f o r benzene]. A complex model was discussed i n terms o f adsorbateadsorbate i n t e r a c t i o n s , cation-adsorbate a t t r a c t i o n s and o x i d e adsorbate r e p u l s i o n s . I t i s worth noting t h a t q u a l i t a t i v e l y the n i t r o g e n heteroc y c l i c systems (8,27^,32) behave s i m i l a r l y i n showing a high frequency s h i f t which decreases with i n c r e a s i n g c a t i o n r a d i u s , whereas benzene (7) and acetylene (30,31) show a low frequency s h i f t which increases w i t h r . This probably a r i s e s because the l a t t e r can o n l y i n t e r a c t v i a a π - e l e c t r o n mechanism, whereas i n t e r a c t i o n w i t h the n i t r o g e n lone p a i r e l e c t r o n s i s probably the dominant f a c t o r with p y r i d i n e and p y r a z i n e . In a l a t e r study of the adsorption o f C2H2 on γ - a l u m i n a and KX z e o l i t e , Heaviside et aj_. (34) concluded t h a t acetylene i s physisorbed via^ i t s π - e l e c t r o n system i n a " s i d e - o n " o r i e n t a t i o n , perhaps to surface OH groups or t r a c e s o f r e t a i n e d water. In a d i f f e r e n t a p p l i c a t i o n o f Raman spectroscopy to s t u d i e s of z e o l i t e s , Cooney and Tsai (6) have i n v e s t i g a t e d the adsorption of bromine on a l k a l i exchanged z e o l i t e s X , NaY and NaA, and the


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subsequent r e a c t i o n with benzene on NaX and CsX ( 3 5 ) . On z e o l i t e Χ, Β Γ 2 adsorbed on the more exposed I I I s i t e s (vBr?=280-314 c n H ) and on the l e s s exposed I I s i t e s (vBr2=240-280 cm"*), the l a t t e r a l s o having an v ( 0 . . . B r ) band a t ^ 160 c n H i n d i c a t i v e o f an oxide-bromine e l e c t r o n t r a n s f e r i n t e r a c t i o n . No s i t e I I I c a t i o n s e x i s t f o r NaY and o n l y the s i t e II NaY-Br2 band was observed 271 cm" f o r NaX and NaY) w h i l e f o r NaA, both s i t e II (281 c n H ) and I (259 c n H ) bands were observed, but i n n e i t h e r case was a low frequency band observed near 160 c n H i n d i c a t i n g , i n agreement w i t h o t h e r s , the apparent l a c k o f charge t r a n s f e r interactions. The subsequent r e a c t i o n o f bromine t r e a t e d NaX and CsX with benzene revealed two types o f behavior ( 3 5 ) . At s a t u r a t i o n Br2 coverage surface donor complexes were formed on s i t e s I I I and I I , whereas at l e s s than Br2 s a t u r a t i o n ( o n l y s i t e I I occupied) benzene reacted r a p i d l y to form a d d i t i o n products c o n t a i n i n g carbon-bromine bonds. The unique a b i l i t y to use Raman s p e c t r o scopy i n general f o r o b t a i n i n g low frequency s p e c t r a l data i n s t u d i e s o f i n s i t u c a t a l y t i c process was discussed by the a u t h o r s . F i n a l l y , i n a b r i e f communication S a p e r s t a i n and Rein (26) have reported t h a t Raman spectroscopy can even be used to d e t e c t the presence o f p h y s i c a l l y adsorbed N2 and O2 on a 4A z e o l i t e , and the r e s u l t s are discussed i n terms o f the chromatographic a b i l i t y o f the A z e o l i t e s to separate N2 from other gases. This a b i l i t y to detect adsorbed gases on z e o l i t e might prove useful for i n s i t u c a t a l y t i c s t u d i e s , but thus f a r no a d d i t i o n a l uses of the technique have been r e p o r t e d .


1

Chemisorption on Oxides The work described i n the previous s e c t i o n was e s s e n t i a l l y concerned w i t h the p h y s i c a l r a t h e r than chemical adsorption o f some h i g h l y p o l a r i z a b l e molecules on z e o l i t e s . With p y r i d i n e , such information can sometimes be obtained as e a s i l y using i n f r a red spectroscopy. However, t r a n s m i s s i o n IR spectroscopy cannot so e a s i l y be used to study chemisorption on oxides i f i t i s e s s e n t i a l to o b t a i n low frequency s p e c t r a l data ( e . g . , adsorbent-adsorbate s t r e t c h i n g modes) because o f the o p a c i t y o f most oxides over much o f the low frequency s p e c t r a l r e g i o n . Recent work has shown t h a t the Raman technique can be extremely useful i n t h i s context ( 4 ) . Surface hydroxy1 groups on an oxide can u s u a l l y be replaced by other organic f u n c t i o n a l groups, thereby a l t e r i n g the p o l a r i t y or hydrophobic!*ty o f the s u r f a c e . One simple process i n v o l v e s the d i s s o c i a t i v e chemisorption o f methanol on s i l i c a . ESiOH + CH3OH + =SiOCH + H 0 3

2

The methoxylation can be c a r r i e d out by r e a c t i n g s i l i c a with methanol vapor a t 300-400°C, or by r e f l u x i n g s i l i c a i n methanol ( 2 1 , 3 6 ) . Because the i n f r a r e d spectrum o f the modified surface i s well understood (36) we chose to use t h i s system as a model to t e s t the f e a s i b i l i t y o f using Raman spectroscopy (21_) f o r studying such surface m o d i f i c a t i o n procedures.


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Raman spectra obtained i n the s p e c t r a l region above 1200 cm" were o f a q u a l i t y which was comparable to t h a t which could be obtained using i n f r a r e d spectroscopy, although longer scan times and l a r g e r s p e c t r a l s l i t widths were required i n order to achieve a comparable s i g n a l - t o - n o i s e r a t i o (Figure 4 ) . However, we were unable to observe low frequency Raman bands (Figure 2 ) , p a r t i c u l a r l y those a s s o c i a t e d w i t h the v ( S i - O - C ) modes, a most d i s a p p o i n t i n g r e s u l t because i t i s i n t h i s low frequency region (< 1200 cm" ) where the Raman technique should prove so advantageous over the i n f r a r e d . A p o s s i b l e cause f o r our f a i l u r e to d e t e c t these modes w i l l be discussed f u r t h e r below. Surface hydroxyl groups a l s o react w i t h a c l a s s o f molecules sometimes r e f e r r e d to as hydrogen sequestering (HS) agents, examples o f which are T i C l 4 , AlMe3 [Me = CH3], Me3SiNHSiMe3 [HMDS o r h e x a m e t h y l d i s i l a z a n e ] , MCl4- Me [n = 0 - 3 , M = S i o r Ge] and BX3 [X = F , C I , B r ] . Not o n l y are some o f these molecules used to modify the surface p r o p e r t i e s o f an adsorbent o r to impart new r e a c t i v e s i t e s on a c a t a l y s t , they have a l s o been widely used as probe molecules to study the c o n f i g u r a t i o n s o f surface hydroxyl groups (37^ - 4 3 ) . For example, T i C l 4 might be expected to r e a c t d i f f e r e n t l y w i t h s i n g l e surface OH groups o r with "paired" OH groups as f o l l o w s (M represents a metal atom o f an o x i d e ) : single MOH + T i C l ( g ) -> M 0 T i C l + HCl(g) I 1


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These r e a c t i o n s have been e x t e n s i v e l y s t u d i e d using c l a s s i c a l a n a l y t i c a l techniques and using IR spectroscopy (37-43). In the l a t t e r case, because of the low frequency o p a c i t y o f the oxide one can g e n e r a l l y only observe spectra associated w i t h the d i s appearance o f v(0H) modes during r e a c t i o n , o r the formation o f CH s t r e t c h i n g modes i f a methyl group i s present, and i n some cases i t i s d i f f i c u l t to d i s t i n g u i s h between r e a c t i o n s i l l u s t r a t e d i n schemes I I and I I I . We have used Raman spectroscopy (4) i n order to see i f low frequency data could be obtained when the above HS agents d i s s o c i a t i v e l y chemisorb on the i s o l a t e d hydroxyl groups o f s i l i c a (scheme I ) . As was found using methanol, very good q u a l i t y Raman spectra i n the CH s t r e t c h i n g and deformation regions could be obtained when a methyl group was present i n the HS agent, and over the e n t i r e s p e c t r a l region (^ 50-4000 c n r ) f o r the adsorbed germanium compounds, HMDS, AlMe3, and T i C l 4 . The spectra were analyzed i n terms o f the surface product p r e d i c t e d i n scheme I . For example, Figure 5 shows the low frequency p o r t i o n o f the Raman spectrum o f adsorbed GeClMe3(A), 6eCl2Me2(B) and GeCl3Me(C) where the chemisorbed species are r e s p e c t i v e l y ESiOGeMe3(A), , =Si0GeClMe2(B) and ESiOGeCl Me(C). The bands between 600-700 cm" 1

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Figure 4. Raman spectra of methylated silica in the CH stretching region using 1J W of laser power at 488.0 nm. The scan rate and response time constant (seconds) were as follows: (A) 500 cm' / min, 0.1 s; (B) 50 cm' /min, 1 s; (C) 10 cm /min, 10 s. Curve D shows the IR spectrum recorded with a scan time of 83 cm' /min and a time constant of 1 s. The vertical bar represents 3000 Hz for the Raman spectra and a transmittance of 0.20 for the IR spectrum (21). 1

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Figure 5. Raman spectra of chemisorbed (A) GeClMe , (B) GeCl Me , and (C) GeCl Me. Using 0.7 W of laser power (488 nm), the maximum intensity is as follows: (A) 4600 Hz at 573 cm' ; (B) 1000 Hz at 595 cm ; (C) 3000 Hz at 410 cm' (4). s

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are due to GeC s t r e t c h i n g modes, those between 400-450 c n H due to GeCl s t r e t c h i n g modes, the broad band between 100-200 cm"" i s associated w i t h an o v e r l a p p i n g system o f complex GeC and GeCl deformation modes, and the accentuated peak near 480-490 c n H (which i s a l s o observed weakly i n the background spectrum o f s i l i c a ) has been assigned to the symmetric SiOGe s t r e t c h i n g mode. Other spectra and a complete d i s c u s s i o n o f the assignments a r e given i n reference ( 4 ) . Very s u r p r i s i n g l y , no Raman bands a t a l l were observed f o r the adsorbed boron compounds, i n s p i t e o f the f a c t t h a t the band p o s i t i o n f o r some o f these were known from IR s t u d i e s ( 4 0 , 4 3 ) . This was a l s o the case f o r the low frequency modes o f chemisorbed methanol (2]_). We have speculated t h a t t h i s e f f e c t may a r i s e when the mass o f the atom M attached to the surface oxygen forming the ESiOM l i n k a g e i s l e s s than t h a t o f s i l i c o n , such t h a t the low frequency modes are s t r o n g l y coupled to the l a t t i c e o r phonon modes o f the s u b s t r a t e r e s u l t i n g i n a broad smeared-out p r o f i l e , which i s e s s e n t i a l l y undetectable i n the general background a t low frequency. As i s w e l l known, v i b r a t i o n a l modes which are intense i n the Raman e f f e c t are often weak i n the i n f r a r e d , and v i c e v e r s a . This was true i n the above study, where the antisymmetric MC and MCI bands were almost undetectable i n some cases, and i n order to c a r r y out a complete v i b r a t i o n a l a n a l y s i s i t would a l s o be d e s i r a b l e to o b t a i n the i n f r a r e d spectrum. I t has been pointed out t h a t t h i s i s d i f f i c u l t using t r a n s m i s s i o n techniques because oxides are such strong IR absorbers a t low frequency. However, the e x t r a c t i o n o f weak s i g n a l s which are superimposed on an i n t e n s e l y absorbing background can sometimes be r e a l i z e d using a h i g h l y s e n s i t i v e d i s p e r s i v e o r F o u r i e r transform spectrometer. Figure 6A shows a p o r t i o n o f the Raman spectrum o f chemisorbed hexamethyldisilazane on s i l i c a , a r e a c t i o n i n which the surface hydroxyl groups are replaced by =SiOSiMe3 groups. The strong and weak features a t 600 and 690 c n H a r e due to the symm e t r i c and anti-symmetric v ( S i C ) modes r e s p e c t i v e l y whereas the weaker features a t 755 and 850 cm" are due to methyl r o c k i n g modes. Figure 6B shows the corresponding i n f r a r e d spectrum. S i l i c a i s t o t a l l y opaque from 1200-1000 cm-" and the transmittance i s o n l y 2.5% from 850-800 c n H , but w i t h i n the "window" o f p a r t i a l transparency to high frequency o f the 850-800 r e g i o n one can c l e a r l y d i s c e r n an e x t r a band due to a methyl t w i s t i n g mode which i s absent i n the Raman spectrum. The methyl r o c k i n g and t w i s t i n g modes are expected to be weak i n the Raman e f f e c t so the combinat i o n o f the two spectroscopies s h o u l d , i n f a v o r a b l e cases, go a long way towards enabling one to c a r r y o u t a f u l l v i b r a t i o n a l a n a l y s i s . Although t h i s type o f a p p l i c a t i o n o f IR spectroscopy i s a new development, and the technique i s tedious s i n c e extremely slow scan times are r e q u i r e d , we have r e c e n t l y been able to observe s i m i l a r modes f o r the chemisorbed Ge compounds. In some cases we have a l s o observed the pseudoantisymmetric SiOGe s t r e t c h i n g mode, a mode which was not observed i n the Raman


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Figure 6. (A) Raman spectrum and (B) IR spectrum of chemisorbed hexamethyldisilazane on silica: ( ) the back ground spectrum in each case; ( ) new features after formation of surface ==SiOCH groups. These are downward relative to background in the IR spectrum Β and upward in the Raman Spectrum A, and the horizontal line for Curve Β indi cates total absorption for the IR spec trum. The band marked (*) does not appear in the Raman spectrum.

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spectrum, probably because the change i n p o l a r i z a b i l i t y i s so low. The i n f r a r e d spectrum shown i n Figure 6B serves to show why the Raman technique i s so useful i n the low frequency s p e c t r a l regions. However, i n the higher frequency s p e c t r a l regions where most oxides are r e l a t i v e l y t r a n s p a r e n t to IR r a d i a t i o n , the combined i n f r a r e d and Raman technique has been very s u c c e s s f u l l y used by others to study chemisorption on z i n c o x i d e ( 4 4 , 4 5 , 4 6 ) . One a l s o makes use o f the f a c t that the s e l e c t i o n r u l e s d i f f e r f o r each technique, and hence the r e l a t i v e i n t e n s i t i e s v a r y . Nguyen and Sheppard (44) found t h a t propylene gave a chemisorbed a l l y l species on t h i s oxide whereas Nguyen (45) found t h a t e t h y n y l benzene (phenylacetylene) d i s s o c i a t i v e T y chemisorbed to y i e l d an a c e t y l i d e s p e c i e s , probably Z n . . . C E C C S ^ , accompanied by d i m e r i c products which were generated o n l y a f t e r a d s o r p t i o n and l a s e r irradiation. In a more d e t a i l e d work Nguyen, L a v a l l e y , Saussey and Sheppard (46) i n v e s t i g a t e d the chemisorption o f methyl phenyl a c e t y l e n e (C6H5C=CCH3) and benzyl acetylene ( C ç H s C H ^ C H ) on z i n c o x i d e . With e i t h e r adsorbate, two types o f d i s s o c i a t i v e chemis o r p t i o n o c c u r r e d , g i v i n g r i s e to a c e t y l i d e species (CçH5CH CEC...Zn) and the p r o p a r g y l l i c s p e c i e s [ C H C H — C H = C H ] ~ and I Ç H 5 C — C — C H 2 ] " , and the r e s u l t s were d i s c u s s e d i n terms of an i s o m e r i z a t i o n mechanism i n v o l v i n g the formation o f the other isomer, p h e n y l a l l e n e (C6H5CH=C=CH2). C u r i o u s l y , however, s p e c t r a due to some o f the intermediates were not observed using both s p e c t r o s c o p i c t e c h n i q u e s , and the authors d i s c u s s p o s s i b l e causes f o r t h i s . As w i t h the p r e v i o u s l y d i s c u s s e d a c e t y l e n e - z e o l i t e systems ( 3 0 , 3 1 ) , very strong Raman bands were observed f o r the ν ( 0 Ξ 0 ) modes which are very weak i n the i n f r a r e d (and forbidden i n C2H2). F i n a l l y , i n another i n t e r e s t i n g a p p l i c a t i o n o f the Raman technique, Murray and Graytak (5) r e c e n t l y observed the spectrum o f ammonia adsorbed on s i l i c a (porous g l a s s ) under UHV c o n d i t i o n s at coverages as low as 0.01 monolayer. Spectra were obtained o f Η-bonded NH3, coordinated NH3, and o f surface SiNH2 groups produced from the d i s s o c i a t i v e chemisorption o f ammonia. In a d d i t i o n , they s t u d i e d the r a t e o f exchange o f NH3 and ND3 w i t h surface SiOD and SiOH groups r e s p e c t i v e l y , and were able f o r the f i r s t time t o use the Raman technique t o monitor changes i n the SiO s t r e t c h i n g modes of ESiOH groups (^ 980 c n r ) as a f u n c t i o n of coverage w i t h NH3. Although t h i s study was more concerned w i t h p h y s i c a l than chemical a d s o r p t i o n , i t d i d show t h a t extremely good q u a l i t y Raman spectra can be obtained f o r a simple molecule under very c a r e f u l l y c o n t r o l l e d UHV experimental c o n d i t i o n s . T h i s p o t e n t i a l can p o s s i b l y g i v e the Raman technique the same s t a t u s f o r studying oxides as the UHV e l e c t r o n techniques have f o r studying m e t a l s . 2

6

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The Surface S t r u c t u r e o f Oxide C a t a l y s t s The work described i n the previous two s e c t i o n s has been concerned w i t h o b t a i n i n g the Raman spectrum o f molecules adsorbed


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on oxide surfaces without much c o n s i d e r a t i o n o f how the adsorbent modes may themselves be perturbed. These i n v e s t i g a t i o n s were f e a s i b l e because a l u m i n o s i l i c a t e oxides are r e l a t i v e l y poor Raman s c a t t e r e r s and 1t has been a simple matter to observe the Raman spectrum o f an adsorbed molecule. However, the low i n t e n s i t y o f the background s c a t t e r i n g has proven to be useful i n recent Raman s t u d i e s o f s i l i c a and alumina supported molybdenum oxides (9,10,]9_ 20,47,48) s i n c e the s c a t t e r i n g cross s e c t i o n o f the l a t t e r i s h i g h . A c c o r d i n g l y , several groups have used the Raman technique to probe the nature o f these c a t a l y s t surfaces themselves, work which i s important s i n c e these m a t e r i a l s , p a r t i c u l a r l y when promoted by adding oxides o f c o b a l t o r n i c k e l , are w i d e l y used as h y d r o s u l f u r i z a t i o n (HDS) c a t a l y s t s . I t i s not p o s s i b l e to review i n d e t a i l the Raman work which has been undertaken on these c a t a l y s t systems because a f u l l account would r e q u i r e a d e t a i l e d d e s c r i p t i o n o f the c a t a l y t i c p r o p e r t i e s o f these substances, the chemistry o f the molybdenum oxides i n g e n e r a l , and a f u l l a n a l y s i s o f v i b r a t i o n a l modes o f the ( M o 0 y ) " a n i o n s . B r i e f l y one can say t h a t e x c e p t i o n a l l y good q u a l i t y but complex Raman s p e c t r a , an example o f which i s shown i n Figure 7, have been obtained i n the frequency range from 501100 c m . Acknowledging t h a t the "group frequency" concept may be d i f f i c u l t to apply to the molybdenum o x i d e s , J e z i o r o w s k i and Knozinger (10) have concluded t h a t Raman bands i n t h i s r e g i o n may be assigned as f o l l o w s : 310-370 cm" (Mo=0 bend), 900-1000 cm" (Mo-0 s t r e t c h ) , 200-250 cm" (Mo-0-Μο d e f o r m a t i o n ) , 400-600 c n H (symmetric Mo-0-Μο s t r e t c h ) and 700-850 cm" (anti-symmetric Mo-0-Μο s t r e t c h ) . On t h i s b a s i s several groups ( 9 , 1 0 , 1 ^ , 2 0 , 4 7 , 4 8 ) have c a r r i e d out d e t a i l e d Raman s t u d i e s varying sucfTparameters as ( i ) the method o f c a t a l y s t p r e p a r a t i o n (pH d u r i n g wet impregnation, dry impregnation, c a l c i n i n g c o n d i t i o n s ) , ( i i ) the c a t a l y s t support, ( i i i ) the r a t i o o f molybdena to support, and ( i v ) the e f f e c t o f promoters, the o b j e c t i v e being to understand how these v a r i a b l e s i n f l u e n c e the s t r u c t u r e o f the molybdena s u r f a c e , sometimes under c a t a l y t i c HDS c o n d i t i o n s . P a r t i c u l a r a t t e n t i o n has been devoted to o b t a i n i n g an understanding o f the c o n d i t i o n s which r e s u l t i n the formation of such s p e c i e s as s o l i d M0O3, polymeric oxyanions, surface i n t e r a c t i o n s ( e . g . , the formation of Al2 (^004)3 and o f surface Al-0-Mo bonds), mixed Mo-Co-Oxide species when a promoter i s used, and s u l f i d e species ( e . g . , M0S2) under d e s u l f u r i z a t i o n conditions. The s p e c t r a o b t a i n e d , although complex, have provided a v a l u a b l e i n s i g h t i n t o the c a t a l y t i c chemistry o f these m a t e r i a l s , information which i s p a r t i c u l a r l y d i f f i c u l t to o b t a i n using other techniques. More important, the Raman technique can a l s o be used under r e a l r e a c t i o n c o n d i t i o n s , and i t has been used to examine a c t u a l " r e a c t o r " c a t a l y s t samples. I t i s a l s o apparent t h a t these techniques can be used to study other types o f c a t a l y s t s . For example, Kerkhoff et a l . (49) have obtained the Raman spectrum o f an alumina-supported FFTenium oxide metathesis c a t a l y s t prepared by c a l c i n i n g (NH4)ReÛ4 on Y-A12Û3 a t 830 K. An extremely simple


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spectrum was obtained (50-1200 cm" ) which revealed t h a t a s i n g l e rhenium species was present, probably a t e t r a h e d r a l l y d i s t o r t e d ReÛ4" i o n . As y e t no attempt has been made to study t h i s surface under r e a c t i o n c o n d i t i o n s but i t i s apparent that these techniques should r e c e i v e much f u r t h e r a p p l i c a t i o n i n the f u t u r e . F i n a l l y , we have not discussed cases where Raman spectroscopy can be used to study c a t a l y s t s i n d i r e c t l y , as f o r example, by e x t r a c t i n g a sample from a r e a c t o r and preparing a KBr d i s c f o r IR o r Raman i n v e s t i g a t i o n . Such techniques may be useful i n s p e c i a l circumstances (50) but have l i m i t e d a p p l i c a b i l i t y w i t h regard to the d i r e c t examination o f surfaces under r e a c t i o n c o n d i tions. A d s o r p t i o n on Metals Ten years ago one would have p r e d i c t e d that Raman s p e c t r o scopy could never be used to study monolayer a d s o r p t i o n on metals because e i t h e r (a) the s e n s i t i v i t y o f the technique would be too low to permit d e t e c t i o n o f s i g n a l s using a s i n g l e r e f l e c t i o n from a smooth metal s u r f a c e , o r (b) oxide supported metal surfaces are black i f the metal l o a d i n g i s high and therefore the l a s e r l i g h t would be absorbed. Both o f the above o b j e c t i o n s have been shown to be f a u l t y i n s o f a r as the technique has now been used to study absorption on s i l i c a - s u p p o r t e d n i c k e l (51_,52,53) and on s i n g l e c r y s t a l n i c k e l ( 5 4 ) . Moreover i n the s p e c i a l case o f s i l v e r , Raman spectra have been observed wherein the s i g n a l s obtained are of the order o f 104-10^ g r e a t e r than one would p r e d i c t on the basis o f the normal Raman s c a t t e r i n g cross s e c t i o n o f bulk molecules ( 1 1 , 1 2 ) . The l a t t e r e f f e c t has sometimes been termed "enhanced Raman s c a t t e r i n g " and has been the subject o f i n t e n s i v e t h e o r e t i c a l and experimental i n v e s t i g a t i o n . We w i l l f i r s t d i s c u s s the normal o r general s i t u a t i o n . ( i ) Normal Raman S c a t t e r i n g . Because o f the o b j e c t i o n s l i s t e d above i t has proven d i f f i c u l t to use the Raman technique to study m e t a l s . Krasser e t a]_. have had some success i n o b t a i n i n g spectra o f carbon monoxide adsorbed on s i l i c a - s u p p o r t e d Ni and on Raney Ni (51_), and o f hydrogen (52,53) and benzene (53) on s i l i c a - s u p p o r t e d N i . The spectra are very weak f o r adsorbed CO ( s i g n a l - t o - n o i s e r a t i o ^ 4 f o r the s t r o n g e s t band, but the absolute i n t e n s i t y was not s t a t e d ) and the peak p o s i t i o n s and i n t e n s i t i e s v a r i e d w i t h CO p r e s s u r e , the s t a t e o f focus o f the l a s e r beam, and on the length o f exposure o f the sample to the laser. In a d d i t i o n to ν ( θ Ξ θ ) bands assigned as usual to l i n e a r (> 2000 cm" ) and bridged (< 2000 c n r ) CO ( 3 ) , a weak low f r e quency band a t 340 c n r ' was assigned to an v[NiC) mode, and the authors claimed t h a t adsorbed Ni(C0)4 was a l s o present. There was l i t t l e d i s c u s s i o n o f how some o f the above v a r i a b l e s affected the spectrum and i t would appear t h a t the technique has not been fully exploited. 1

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Figure 8. Raman spectra of: (a) a freshly polished A g sample exposed to air, (b) a A g sample in air after exposure for 30 min to a 3.7% KCN solution. Redrawn from Ref. 63.


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For adsorbed H2 (and D2) Krasser and Renouprez (52) found s e v e r a l extremely weak Raman bands i n the range 600-2100 cm"] which were assigned to l i n e a r NiH species (2028 and 1999 cm"') and multibonded N i H species at 1600 and 950 cm-1 and again no s t u d i e s of the s p e c t r a l changes w i t h r e s p e c t to r e a c t i v i t y o f the NiH species w i t h other molecules were c a r r i e d o u t . Stencel and Bradley (54) c a r r i e d out an elegant study o f the adsorption o f CO on N i ( 1 0 0 , 110 and 111) surfaces under UHV c o n d i t i o n s although s i g n a l i n t e n s i t i e s were o n l y o f the order o f 2-20 c . p . s . (compared w i t h ^ 1 χ 1 0 f o r chemisorption on high area o x i d e s ) . Spectra were obtained o n l y i n the r e g i o n a s s o c i a t e d w i t h ν ( 0 Ξ θ ) modes and although the authors acknowledged t h a t t h e i r c r y s t a l s may not have been t o t a l l y free o f o x i d e , no bands normally a s s o c i a t e d w i t h surface carbonates (950-1700 cm"') were detected. F u r t h e r , no Raman bands were observed i n the 1940-2000 cm" r e g i o n a s s o c i a t e d w i t h the formation o f bridged CO s p e c i e s . D i f f e r e n t v(NiC0) s p e c t r a (2000-2050 cm" ) were obtained f o r each c r y s t a l plane but the data was not d i s c u s s e d i n d e t a i l i n t h i s p r e l i m i n a r y communication because of doubts a s s o c i a t e d w i t h the s t a t e o f c l e a n l i n e s s o f the s u r f a c e . F i n a l l y , two papers have been concerned w i t h a study o f a d s o r p t i o n on platinum e l e c t r o d e s i n aqueous s o l u t i o n . One was concerned w i t h the a d s o r p t i o n o f CO (55) and the other w i t h 12 ( 5 6 ) , and although again very weak s i g n a l s were detected [v(PtC0) at 2096 and 2081 cm-T, and v ( I - I ) at 174 c m " ] , the experiments do i l l u s t r a t e one o f the major advantages o f the Raman technique, v i z . the a b i l i t y to study a d s o r p t i o n from aqueous media. x


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(ii) Enhanced Raman S c a t t e r i n g . Raman s p e c t r a which were about 10* - 10b more i n t e n s e than a n t i c i p a t e d were f i r s t observed (57-62) f o r p y r i d i n e adsorbed on s i l v e r e l e c t r o d e s i n an e l e c t r o chemical s i t u a t i o n and subsequently f o r other h e t e r o c y c l i c mole c u l e s (60) and i o n s such as CN" (63,64,65_,66) CO3-, (60) C I " (1JJ and SCN" ( 6 7 ) . I t was o r i g i n a l l y thought t h a t t h i s e f f e c t was an a r t e f a c t due to the a p p l i e d f i e l d generated i n the e l e c t r o chemical d e p o s i t i o n but s i m i l a r enhanced spectra have been observed f o r CO (18) and p y r i d i n e (17) deposited on s i l v e r under UHV c o n d i t i o n s . Although Raman enhancement has now been demon s t r a t e d many times f o r a d s o r p t i o n on s i l v e r , o n l y r e c e n t l y has i t a l s o been shown to occur f o r e l e c t r o c h e m i c a l l y deposited p y r i d i n e on copper and gold e l e c t r o d e s (68^69). To t h i s a u t h o r ' s knowledge Raman enhancement has not been achieved using other m e t a l s . The spectrum shown i n F i g u r e 8 f o r adsorbed CN serves to i l l u s t r a t e the q u a l i t y o f the spectra obtained w i t h respect to peak i n t e n s i t y and s i g n a l to n o i s e r a t i o . In s t u d i e s using p y r i d i n e , spectra o f the q u a l i t y shown i n Figure 3A f o r l i q u i d p y r i d i n e have been observed. I t i s too e a r l y to say whether surface enhanced Raman spectroscopy w i l l become a w i d e l y a p p l i c a b l e technique because, i n s p i t e o f several t h e o r e t i c a l i n v e s t i g a t i o n s , no general theory o f



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the e f f e c t has been forthcoming which would p o i n t to the c o n d i t i o n s necessary i n order to study a d s o r p t i o n on other m e t a l s . Recent reviews o f t h e o r i e s o f the e f f e c t have been given by Hexter (11) and by Efrima and Metiu (1_2) and, s i n c e several new papers are c u r r e n t l y appearing each month, i t i s not a p p r o p r i a t e i n a short a r t i c l e t h a t we d i s c u s s t h i s work i n d e t a i l . B r i e f l y , most t h e o r i e s assume t h a t a resonance Raman s c a t t e r i n g e f f e c t i s r e s p o n s i b l e f o r the enhancement and models have been developed i n terms o f ( i ) c l a s s i c a l (12,64,70-73) and quantum mechanical (74) d e s c r i p t i o n s o f l i g h t s c a t t e r i n g as r e l a t e d to the o p t i c a l p r o p e r t i e s o f m e t a l s , ( i i ) the r o l e o f surface roughness i n p r o moting a resonant e x c i t a t i o n o f conduction e l e c t r o n resonances i n an adsorbate ( 7 5 ) , o r i n promoting a breakdown i n c e r t a i n momentum c o n s e r v a t i o n r u l e s ( 6 5 ) , ( i i i ) a mixing o f surface plasmons with adsorbate s t a t e s (11,68,76,77) and ( i v ) resonance e x c i t a t i o n o f free r a d i c a l species at an e l e c t r o d e surface ( 7 8 ) . These t h e o r e t i c a l developments are hampered by c o n f l i c t i n g experimental evidence w i t h r e s p e c t to the c o n d i t i o n s necessary to generate the e f f e c t . For example, there i s no c l e a r consensus on the e f f e c t o f changing the wavelength o f the e x c i t i n g r a d i a t i o n ( 1 1 , 1 2 ) , and some groups c l a i m t h a t e l e c t r o c h e m i c a l "roughening" i s a necessary precursor to observing the e f f e c t ( 6 3 , 6 5 , 6 6 , 6 8 , 7 9 , 80,81). However, the e f f e c t has c l e a r l y been demonstrated unclêr UHV c o n d i t i o n s f o r CO adsorbed on evaporated s i l v e r f i l m s (18) and for p y r i d i n e adsorbed on i o n sputtered bulk s i l v e r (1_7) and i t remains to be seen whether the necessary surface "roughening" c o n d i t i o n may have been met. F i n a l l y , an adequate theory must account f o r the f o l l o w i n g experimental o b s e r v a t i o n s ; ( i ) o n l y a l i m i t e d number o f adsorbed molecules on s i l v e r g i v e r i s e to the enhancement e f f e c t , ( i i ) enhancement f o r p y r i d i n e i n s o l u t i o n apparently does not occur w i t h non-aqueous s o l v e n t s ( 6 1 ) , ( i i i ) using s i l v e r e l e c t r o d e s the s i g n a l i n t e n s i t y i s s t r o n g l y dependent on the a n o d i z a t i o n time (63,66,68,69,79,80,81) and ( i v ) f o r much l e s s than a monolayer of p y r i d i n e on an Ag e l e c t r o d e the i n t e n s i t y o f the s c a t t e r e d l i g h t ( f o r a constant 90° i l l u m i n a t i o n - s c a t t e r i n g system) s t r o n g l y depends on the angle o f i n c i d e n c e ( 6 3 ° , h a l f width 5° f o r a s i l v e r f i l m on g l a s s ) which P e t t i n g e r et al_. (68,77) c l a i m provides strong evidence f o r the surface plasmon enhanced model f o r the e f f e c t . Very r e c e n t l y a s i m i l a r angular dependence on the s c a t t e r i n g i n t e n s i t y has a l s o been found by Wenning, P e t t i n g e r and Wetzel (69) f o r p y r i d i n e adsorbed on copper and gold e l e c t r o d e s . Most i n t e r e s t i n g l y however, Raman enhancement f o r these metals was o n l y achieved using red (647 nm) l a s e r e x c i t a t i o n whereas f o r s i l v e r , enhancement occured w i t h red or blue (457 nm) l i g h t . The r e s o l u t i o n o f the problems o u t l i n e d above w i l l o b v i o u s l y r e q u i r e much more t h e o r e t i c a l and experimental work before the Raman technique w i l l become u n i v e r s a l l y a p p l i c a b l e to the study o f adsorption a t metal s u r f a c e s . I t w i l l be i n t e r e s t i n g to see i f tuneable l a s e r s capable o f o p e r a t i n g i n the near i n f r a r e d w i l l be


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the key to extending the use o f t h i s technique to other m e t a l s . Indeed, i f spectra o f the q u a l i t y shown i n Figure 8 could be obtained f o r any m e t a l , other v i b r a t i o n a l techniques, a t l e a s t as a p p l i e d to the study o f adsorption on m e t a l s , could be rendered o b s o l e t e . E q u a l l y important, the technique can be used to study adsorption from aqueous s o l u t i o n where i t i s v i r t u a l l y impossible to o b t a i n v i b r a t i o n a l data f o r adsorbed species using any other technique.


Summary We have demonstrated t h a t Raman spectroscopy i s a v e r s a t i l e technique f o r studying p h y s i c a l and chemical adsorption on o x i d e s , the surface s t r u c t u r e o f c e r t a i n types o f supported oxide c a t a l y s t s , and i n s p e c i a l circumstances, f o r studying adsorption on m e t a l s . Of a l l v i b r a t i o n a l techniques f o r surface s t u d i e s i t i s unique i n that ( i ) there are v i r t u a l l y no r e s t r i c t i o n s on the type o f sample t h a t can be studied ( i i ) f o r g a s - s o l i d s t u d i e s there are no r e s t r i c t i o n s on the c o n d i t i o n s o f ambient gas pressure, ( i i i ) the f u l l range o f v i b r a t i o n a l fundamentals (^ 50-4000 cm"') i s a c c e s s i b l e under r e l a t i v e l y high r e s o l u t i o n c o n d i t i o n s and ( i v ) adsorption can be s t u d i e d from aqueous s o l u t i o n s on to both oxide and metal s u r f a c e s . Although the s e n s i t i v i t y o f the technique i s not always as high as one would l i k e , [with some exceptions ( 5 ) , and p a r t i c u l a r l y the enhanced e f f e c t f o r s i l v e r ] t h i s can be p a r t i a l l y overcome by the use o f a data a c q u i s i t i o n computer which permits s i g n a l a v e r a g i n g , and w i t h the use o f several l a s e r s o f a tuneable l a s e r i n order to minimize background f l u o r e s c e n c e . In the f u t u r e , the technique w i l l undoubtedly be f u r t h e r e x p l o i t e d along the l i n e s d e s c r i b e d , p a r t i c u l a r l y f o r the study o f i n s i t u c a t a l y t i c processes and, i f the experimental and t h e o r e t i c a l problems can be s o l v e d , f o r the study o f adsorption on w e l l defined m e t a l s . As y e t the technique has not been f u l l y used f o r study o f adsorption from aqueous s o l u t i o n and, f o r example, there i s c o n s i d e r a b l e p o t e n t i a l f o r studying the r o l e o f sediments i n transporting pollutants. Raman spectroscopy has a l s o shown c o n s i d e r a b l e promise as a means o f studying the generation o f defects i n z e o l i t e s as a r e s u l t o f v a r i o u s thermal o r chemical treatments ( 8 2 ) . Cooney and Tsai (6) have c o r r e c t l y pointed out t h a t although X-ray d i f f r a c t i o n i s useful f o r studying ordered systems, i t i s not g e n e r a l l y capable o f d e t e c t i n g disordered domains. On the other hand, the Raman technique i s capable o f s t u d y i n g both ordered and d i s o r d e r e d domains and s i n c e h i g h l y c r y s t a l l i n e z e o l i t e s often e x h i b i t sharp c h a r a c t e r i s t i c bands i n the low frequency r e g i o n , small changes due to t h e r m a l l y induced surface defects are r e a d i l y d e t e c t e d , as has been found i n the i n f r a r e d spectrum o f amorphous s i l i c a ( 8 3 ) . Murray and Greytak (5) have a l s o discussed s i m i l a r s i t u a t i o n s i n the case o f the Raman spectrum o f porous g l a s s . The a b i l i t y to


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use Raman spectroscopy to study z e o l i t e s under various r e a c t i o n c o n d i t i o n s should prove useful i n future s t u d i e s o f these e f f e c t s . Acknowledgement F i n a n c i a l support f o r p a r t o f t h i s work was provided by N . S . E . R . C . o f Canada and Imperial O i l L i m i t e d i n c o l l a b o r a t i o n with Dr. A . H . H a r d i n , D r . W.N. Sont, Mr. A . S t . Onge and Mrs. M. Klernes. Literature Cited Downloaded by UNIV OF MONTANA on September 30, 2014 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch007

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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Raman Spectroscopic Studies of Surface Species - American

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