IR Spectroscopic Characterization of Adsorbed ... - ACS Publications

35 Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch035

IR Spectroscopic Characterization of Adsorbed Species and Processes on Surfaces John T. Yates, Jr., PatrickGelin1,and Thomas Beebe Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260

The use of infrared spectroscopy as a probe of chemical and physical phenomena on surfaces extends back to the early work of Terenin in Russia(1) who first employed near-IR spectroscopy as a tool to observe surface OH groups on SiO2, working in the region of the second harmonic OH stretching mode. This work was extended dramatically in the 1950's by R. P. Eischens and coworkers, who first applied transmission IR spectroscopy to the study of species chemisorbed on supported metals of catalytic interest(2). Eischens found that the use of group vibrational frequency assignments was a powerful method for deducing general structural information about adsorbed surface species, building on spectra of compounds of known structure. Results of this body of work are summarized in three monographs (3-5). The infrared method is widely applied for surface studies in industry and academia today, a testament to the wide ranging utility of the method even after almost 30 years of use. In addition, a wide range of other types of surface vibrational spectroscopic methods have now been developed and are widely employed. IR spectroscopy has several distinct advantages as a probe of surface species character, as listed below: • Ability to work under high gas densities to study catalytic surfaces under working conditions. • Ability to use high resolution to accurately characterize small shifts in oscillator frequencies, as well as to perform lineshape analysis. • High sensitivity of species. vibrational frequency to bonding modes at surfaces. • Strong correlation of the spectra of surface species with vibrational spectroscopy of molecules of known structure. The first two advantages listed above allow an optical method like transmission or reflection IR spectroscopy to be used for studies which would be impossible for a widely used competitive technique, electron energy loss spectroscopy (EELS). EELS must 1

Current address: CNRS Laboratories, Lyon, France 0097-6156/85/0288-O404$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch035

35.

YATES E T A L .

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Adsorbed Species and Processes on Surfaces

o p e r a t e under u l t r a h i g h vacuum c o n d i t i o n s and does not possess the r e s o l u t i o n i n h e r e n t i n the o p t i c a l methods(6). Thus the examples s e l e c t e d f o r p r e s e n t a t i o n i n t h i s paper c o u l d not have been done u s i n g the EELS t e c h n i q u e . Two examples of the a p p l i c a t i o n of t r a n s m i s s i o n IR methods w i l l be p r e s e n t e d . The f i r s t , d e a l i n g w i t h the c h e m i s o r p t i o n of CO on a Pd/Si02 c a t a l y s t s u r f a c e , i l l u s t r a t e s the f i r s t o b s e r v a t i o n of a l o c a l s t o i c h i o m e t r i c s u r f a c e s p e c i e s i n t e r c o n v e r s i o n process w h i c h o c c u r s among chemisorbed CO s p e c i e s at h i g h CO coverages. Evidence f o r the o p e r a t i o n o f the l o c a l s t o i c h i o m e t r i c process has been o b t a i n e d on 75Â Pd p a r t i c l e s . These p a r t i c l e s seem t o show c l o s e s i m i l a r i t i e s as w e l l as to d i f f e r i n some r e s p e c t s from a P d ( l l l ) s i n g l e c r y s t a l s u r f a c e i n s o f a r as t h e i r i n t e r a c t i o n w i t h CO i s concerned. The second example d e a l s w i t h the use of IR s p e c t r o s c o p y t o s t u d y bonding d e t a i l s w i t h i n a l a y e r of p h y s i s o r b e d CO s p e c i e s on a S i 0 2 s u r f a c e . Here s p e c i f i c bonding o f CO to p o l a r 8 1 0 Η surface groups i s observed; i n a d d i t i o n , CO m o l e c u l e s h a v i n g r o t a t i o n a l freedom are produced a t h i g h e r CO coverages, and the degree of r o t a t i o n a l freedom a l l o w e d seems to be determined by the magnitude of s h i e l d i n g of the p o l a r c e n t e r s on the Si02 s u r f a c e by CO s p e c i e s w h i c h bond to SiOH, and a l s o p a r t i c i p a t e as a d i e l e c t r i c s c r e e n i n g medium. δ+

Experimental

Methods

Commercial IR Spectrometer Developments. For the study of the IR spectrum of s p e c i e s p r e s e n t on h i g h a r e a s u r f a c e s , both g r a t i n g and F o u r i e r t r a n s f o r m i n s t r u m e n t s are commonly employed. B o t h types of i n s t r u m e n t s now f e a t u r e computer d a t a a c q u i s i t i o n t e c h n i q u e s w h i c h p e r m i t enhancement of s i g n a l / n o i s e r a t i o s by m u l t i p l e scan a v e r a g i n g methods. T h i s f e a t u r e , coupled w i t h smoothing r o u t i n e s and background f i t t i n g p r o c e d u r e s , has l e d to a s i g n i f i c a n t enhancement o f the q u a n t i t a t i v e a s p e c t s of IR s u r f a c e s p e c t r o s c o p y . The d a t a shown i n t h i s paper have been o b t a i n e d a t a r e s o l u t i o n of 3-4 cm"l w i t h a d a t a a c q u i s i t i o n time of from 0.4 - 1 sec/cm""!. Under these c o n d i t i o n s , a background n o i s e l e v e l o f about 0.001 absorbance u n i t s i s r e a d i l y a c h i e v e d , p e r m i t t i n g the o b s e r v a t i o n of weak a b s o r p t i o n bands due t o s u r f a c e s p e c i e s . IR C e l l Developments. The d e s i g n of c e l l s f o r IR s p e c t r o s c o p y has taken many c o u r s e s , depending upon the o b j e c t i v e s of the r e s e a r c h e n v i s i o n e d . For example, i n the case of combining IR s p e c t r o s c o p y w i t h s u r f a c e k i n e t i c s measurements, the d e s i g n of V a n n i c e ( 7 ) i s i d e a l ; here, r e a c t a n t gases f l o w through the porous pressed d i s k sample d u r i n g IR measurements. S i m i l a r arrangements are employed by B e l l u s i n g FTIR methods under r e a c t i o n c o n d i t i o n s ( 8 ) . Another v a r i a t i o n i n IR c e l l d e s i g n w h i c h we have developed i s shown i n F i g u r e 1. I n t h i s c e l l , t h r e e s u r f a c e measurement o b j e c t i v e s are r e a d i l y a c h i e v e d : • S t a b i l i z a t i o n of s u r f a c e s p e c i e s and r e a c t i o n i n t e r m e d i a t e s at c r y o g e n i c temperatures. • Simultaneous a d s o r p t i o n s t u d i e s on supported m e t a l s and on t h e i r s u p p o r t , where both have been t r e a t e d w i t h r e a c t a n t s i n e x a c t l y the same manner.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N


406

Windows

SCIENCE

System QMS

F i g u r e 1.

UHV c e l l

for

IR s p e c t r o s c o p y o f adsorbed s p e c i e s .



35.

YATES E T A L .


407

• Measurement o f s u r f a c e s p e c i e s under h i g h gas p r e s s u r e s , by means o f complete c a n c e l l a t i o n of i n t e r f e r i n g gas phase spectra. The u l t r a h i g h - v a c u u m i n f r a r e d c e l l , F i g u r e 1, c o n s i s t s of a s t a i n l e s s s t e e l body made from a d o u b l e - s i d e d confiât f l a n g e , 2-3/4" d i a m e t e r , s e a l e d by two CaF2 windows i n confiât f l a n g e s ( a v a i l a b l e from Harshaw C h e m i c a l Co., C r y s t a l and E l e c t r o n i c P r o d u c t s Dept., 6801 S o l o n , Ohio, 44139). T h i s c e l l may be c o n v e n i e n t l y heated t o 500K by b e i n g p l a c e d i n s i d e a s m a l l oven. Other c e l l s , f e a t u r i n g i n t e r n a l e l e c t r i c a l h e a t i n g , may be o p e r a t e d a t h i g h e r temperatures(9)· The c e n t r a l body o f the c e l l i n F i g u r e 1 c o n t a i n s a Cu support r i n g w h i c h may be c o o l e d by c i r c u l a t i o n o f l i q u i d N2 t h r o u g h t u b i n g b r a z e d to the copper. The temperature o f the support r i n g i s monitored by means o f a 0.003" diameter chrome1-alumel t h e r mocouple. The temperature o f the sample below room temperature may be c o n t r o l l e d to ± IK by adjustment of the l i q u i d - N 2 flow rate t h r o u g h the a p p a r a t u s u s i n g the thermocouple s i g n a l f o r c o n t r o l o f an e l e c t r o n i c s e r v o s y s t e m . We have shown e x p e r i m e n t a l l y t h a t the a d s o r b a n t sample, supported on a c i r c u l a r CaF2 p l a t e c l i p p e d i n s i d e the Cu r i n g , r e a c h e s the temperature of the r i n g a t a l l p o i n t s to w i t h i n about 1K(10). Samples of h i g h a r e a powders and o f supported m e t a l s may be a p p l i e d t o the CaF2 support p l a t e by a s p r a y i n g t e c h n i q u e , p r e v i o u s l y described i n d e t a i l ( l l ) . In F i g u r e 1, we show a " h a l f p l a t e " d e s i g n i n w h i c h a supported m e t a l d e p o s i t , produced by H2 r e d u c t i o n of m e t a l i o n s h e l d on the s u p p o r t , o c c u p i e s one h a l f o f the p l a t e w h i l e the pure support o c c u p i e s the o t h e r h a l f . T r a n s l a t i o n o f the c e l l l e f t and r i g h t p e r m i t s the achievement of each of the t h r e e o b j e c t i v e s l i s t e d above, u s i n g a p p r o p r i a t e d a t a s u b t r a c t i o n p r o c e d u r e s t o remove c o n t r i b u t i o n s from gas phase spec i e s i f p r e s e n t d u r i n g measurements. The c a n c e l l a t i o n of gas phase s p e c t r a l f e a t u r e s u s i n g the " h a l f p l a t e " d e s i g n i s f a r s u p e r i o r t o methods i n v o l v i n g a second gas c e l l p l a c e d i n the r e f e r e n c e beam. T h i s i s because the gas d e n s i t y and i t s r o t a t i o n a l s t a t e p o p u l a t i o n w i l l d i f f e r i n the two c e l l s f o r d i f f e r e n t sample (and t h e r e f o r e gas) t e m p e r a t u r e s . For h i g h sens i t i v i t y measurements, these e f f e c t s can be d i f f i c u l t t o handle u s i n g two c e l l s . Experimental

Results

The C h e m i s o r p t i o n o f CO on Pd/Si02 - O b s e r v a t i o n of S t o i c h i o m e t r i c S p e c i e s I n t e r c o n v e r s i o n E f f e c t s i n the Chemisorbed CO Layer a t H i g h Coverages ( 1 2 , 1 3 ) . I t has l o n g been r e c o g n i z e d t h a t b r i d g i n g - C O r e a d i l y forms upon c h e m i s o r p t i o n on b o t h supported Pd(14-16) and on many Pd s i n g l e c r y s t a l s u r f a c e s ( 1 7 - 2 1 ) . At temperatures below 300K, o r a t h i g h CO p r e s s u r e s , a t e r m i n a l form of chemisorbed CO may a l s o be p o p u l a t e d , e x h i b i t i n g a c a r b o n y l s t r e t c h i n g f r e q u e n c y near 2100 cm".. T h i s i s shown i n F i g u r e 2, where spectrum a^ r e p r e s e n t s the s t a b l e IR s p e c i e s w h i c h remain on Pd/Si02 f o l l o w i n g CO s a t u r a t i o n and e v a c u a t i o n a t 300K. We see t h a t s e v e r a l bridged-CO s p e c i e s e x i s t t o g e t h e r i n the 1700 c m - 2000 cm". r e g i o n i n c l u d i n g a sharp h i g h frequency b r i d g e d c a r b o n y l band a t 1979 cm" (B^) w h i c h i s most p r o m i n e n t . Upon c o o l i n g t h i s s u r f a c e to 80K and adding s m a l l quan-1

A



408

τ

1

1

SCIENCE

Γ


1995 Cm"'

2200

2100

2000 WAVENUMBER

F i g u r e 2.

Additional

1900

1800

1700

(cm" ) 1

CO a d s o r p t i o n on P d / S i O

?

at low t e m p e r a t u r e .


35.

YATES E T

AL.

409

Adsorbed Species and Processes on Surfaces 1

t i t l e s of CO ( s p e c t r a b-Ji) a t e r m i n a l CO s p e c i e s (L) a t 2103 cm"" d e v e l o p s ; as the t e r m i n a l 2103 cm" CO species develops, systematic changes o c c u r i n the 1979-1995 cm" r e g i o n . In a d d i t i o n , a s m a l l band a t 1883 cm" i s e v i d e n t at the h i g h e s t CO c o v e r a g e s . A l t h o u g h the s p e c t r a l developments shown i n F i g u r e 2 have been o b t a i n e d by c o o l i n g the Pd/Si02 s u r f a c e to 80K, we have shown t h a t s p e c i e s L may a l s o be formed a t 300K a t CO p r e s s u r e s of s e v e r a l hundred T o r r . T h i s b e h a v i o r i s c o n s i s t e n t w i t h s p e c i e s L having a f a i r l y low heat of a d s o r p t i o n . S u b t r a c t i o n o f the s p e c t r a i n F i g . 2 y i e l d s a s e t of d i f f e r e n c e s p e c t r a shown i n F i g . 3. I t may be seen t h a t as the 2103 cm" band d e v e l o p s a t 80K, t h e r e i s a l o s s of i n t e n s i t y at 1979 cm" ( B j ) and a concomitant g a i n i n i n t e n s i t y a t 1995 cm" ( B 2 ) . These changes, as judged by the peak a b s o r b a n c e s , are a c c u r a t e l y l i n e a r f u n c t i o n s of each o t h e r ( F i g . 4 ) , s u g g e s t i n g t h a t a s i m p l e s t o i c h i o m e t r i c p r o cess occurs. I t i s a l s o o b s e r v e d , F i g . 5, t h a t the Β χ ·.- B2 c o n v e r s i o n i n v o l v e s an i s o s b e s t i c p o i n t . T h i s s t r o n g l y s u g g e s t s t h a t a s i m p l e s t o i c h i o m e t r i c p r o c e s s i s i n v o l v e d , r a t h e r than some s o r t of f r e q u e n c y s h i f t as a consequence of i n c r e a s i n g CO coverage. F u r t h e r i n s i g h t i n t o the nature of the s t o i c h i o m e t r i c p r o c e s s has been o b t a i n e d u s i n g i s o t o p i c a l l y l a b e l e d ^CO ( d e s i g n a t e d .) as the s p e c i e s used t o p o p u l a t e L. - CO a t 80K on top of a C 0 l a y e r produced a t 300K. The r e s u l t s of t h i s experiment are shown i n F i g . 6, and a s u r p r i s i n g f e a t u r e i s n o t e d . The a d s o r p t i o n of L. - CO i n d u c e s the f o r m a t i o n of L - CO. The C 0 - L s p e c i e s produced by a d s o r p t i o n can o n l y a r i s e from preadsorbed C 0 - B j s p e c i e s . D e t a i l e d i n v e s t i g a t i o n of t h i s B^ L conversion suggests that about 2Bi -»· 2L o c c u r f o r each L. - CO adsorbed. The s t o i c h i o m e t r i c r e l a t i o n s h i p induced by L - CO a d s o r p t i o n c a u s i n g 2Βχ .• 2L i s shown i n F i g u r e 7, where i s o t o p i c a l l y l a b e l e d CO has been employed i n s e p a r a t e e x p e r i m e n t s as B^ or as L. A f t e r a s m a l l c o r r e c t i o n i s made f o r the e x p e r i m e n t a l l y determined d i f f e r e n c e i n e x t i n c t i o n c o e f f i c i e n t f o r C 0 ( a d s ) compared to C 0 ( a d s ) , i t may be seen t h a t the s t o i c h i o m e t r y 2Β^ + L ->· 3L i s c l o s e l y v e r i f i e d . A s c h e m a t i c o n e - d i m e n s i o n a l model of the p r o c e s s i s i l l u s t r a t e d i n F i g . 8. Here i t i s e n v i s i o n e d t h a t the p r o c e s s 4Bi+L > 3L+2B2 i s o c c u r r i n g . An e l e c t r o n i c model suggests t h a t t h i s s t o i c h i o m e t r i c s p e c i e s c o n v e r s i o n p r o c e s s may be due t o the f a c t t h a t the e l e c t r o n a c c e p t o r c a p a c i t y of an ensemble of 2Bj + L exceeds the donor capa c i t y of the 3 Pd s i t e s ; a c o n v e r s i o n to 3L reduces the t o t a l a c c e p t o r c a p a c i t y of the CO ensemble to a p o i n t where the donor c a p a c i t y o f the 3 Pd s i t e s i s not exceeded. These p r o c e s s e s are e n t i r e l y r e v e r s i b l e upon warming the Pd s u r f a c e to 300K. The average Pd c r y s t a l l i t e s i z e , measured by CO c h e m i s o r p t i o n uptake i s about 75Â. The s t o i c h i o m e t r i c e f f e c t seen here i s s u r p r i s i n g f o r c r y s t a l s of t h i s s i z e , and must i n d i c a t e t h a t a l o c a l p i c t u r e of CO c h e m i s o r p t i o n a p p l i e s to these s m a l l Pd c r y s t a l s , i n agreement w i t h m e t a l ensemble i d e a s of S a c h t l e r ( 2 2 ) . I t i s very i n s t r u c t i v e to compare the r e s u l t s shown above f o r Pd/Si02 s u r f a c e s w i t h s i m i l a r measurements made on P d ( l l l ) u s i n g r e f l e c t i o n IR methods(20). As shown i n F i g . 9, a c l o s e s i m i l a r i t y of CO s p e c i e s development o c c u r s i n comparing r e s u l t s w i t h F i g . 2 f o r Pd/Si02« In b o t h c a s e s , f o u r c a r b o n y l s t r e t c h i n g bands are seen a t s a t u r a t i o n c o v e r a g e s , d i f f e r i n g by o n l y a few cm" i n frequency i n comparing 1

1

1


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1 2

1 3

1 2

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410


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2000

1900

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1800

1700

(cm"')

s p e c t r a f o r CO c h e m i s o r p t i o n on P d / S ^

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(.ΔΑ F i g u r e 4. Evidence f o r adsorbed CO s p e c i e s .

CO, |995

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412


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DIFFERENCE SPECTRA

|T = 80K|

,3

2100

2000

1900

1800

1700 2200

WAVENUMBER

2100

2000

1900

1800

(cm"')

Figure 6. D i r e c t evidence f o r b r i d g e d - t o - l i n e a r i s o t o p i c CO e x p e r i m e n t s .

CO c o n v e r s i o n

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IR Spectroscopic Characterization of Adsorbed ... - ACS Publications

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