Investigations of Adsorption Centers, Molecules, Surface Complexes

8 Investigations of Adsorption Centers, Molecules, Surface Complexes, and Interactions Among Catalyst Components by Diffuse Reflectance Spectroscopy

Downloaded by CORNELL UNIV on August 29, 2016 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch008

K. KLIER Lehigh University, Sinclair Laboratory, Bethlehem,PA18015 D i f f u s e r e f l e c t a n c e spectroscopy (DRS) i s a method f o r o b t a i n i n g l i g h t i n t e n s i t y l o s s spectra i n absorbing and s c a t t e r ing specimens and, with the use of appropriate t h e o r i e s , f o r e x t r a c t i n g information on the molar a b s o r p t i v i t i e s of species involved i n the l i g h t absorption process. Extensive monograph (1-5) and review ( £ - 8 ) l i t e r a t u r e i s a v a i l a b l e to cover both the t e c h n i c a l d e t a i l s and a p p l i c a t i o n s . The v a r i o u s c o n f i g u r a t i o n s of spectrometers f o r DRS have been thoroughly discussed i n Kortum's book ( 1 ) , and c e l l s f o r a i r - s e n s i t i v e samples such as c a t a l y s t s have been reviewed i n d e t a i l i n references 5 and 8. The wavelengths of the r a d i a t i o n probe range from i n f r a r e d to u l t r a v i o l e t and both v i b r a t i o n a l and e l e c t r o n i c s p e c t r a l measurements of powdered s o l i d s can be r e a d i l y accomplished i n combinat i o n with standard techniques of sample p r e p a r a t i o n and p r o t e c tion. Among the recent instrumental advances the most s i g n i f i cant appears to be computerization of r e f l e c t a n c e spectrometers which a l l o w s , i n a d d i t i o n to an easy q u a n t i t a t i v e data processing, small q u a n t i t i e s of adsorbed species to be detected and r e f l e c tance measurements to be made on very dark samples. We w i l l not review i n d e t a i l these instrumental arrangements, which c o n s i s t at the present stage of i n d i v i d u a l i n t e r f a c i n g of v a r i o u s spectrophotometers with m i c r o - , m i n i - , and l a r g e computers and c e r t a i n l y have not reached a mature commercial stage of development. Rather, a f t e r an exposure of the b a s i c model and r a d i a t i v e t r a n s f e r theory of the a b s o r b i n g - s c a t t e r i n g media, we s h a l l proceed to those a p p l i c a t i o n s which have s u c c e s s f u l l y c o n t r i b u t e d to the r e s o l u t i o n of s t r u c t u r e s , dynamics, and e l e c t r o n i c s t a t e s of adsorbates and c a t a l y s t s . Theory A comprehensive theory of r a d i a t i v e t r a n s f e r , which governs the r a d i a t i o n f i e l d i n a medium that absorbs, emits, and s c a t t e r s

0-8412-0585-X/80/47-137-141$05.50/0 © 1980 American Chemical Society

Bell and Hair; Vibrational Spectroscopies for Adsorbed Species ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

142

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r a d i a t i o n , has been formulated and elaborated by S. Chandrasekhar (9) . The fundamental Chandrasekhar s equation of r a d i a t i v e t r a n s f e r , which i s a p p l i c a b l e to s c a t t e r i n g - a b s o r b i n g - e m i t t i n g media of a l l p o s s i b l e geometries and s p a t i a l d i s t r i b u t i o n s of absorbers, s c a t t e r e r s , and e m i t t e r s , reads as f o l l o w s : 1

Here 1^ i s the i r r a d i a n c e (or i n t e n s i t y ) at a point ( x , y , z ) and i n a d i r e c t i o n s of frequency between V and V + dv, ρ i s the l o c a l d e n s i t y of the medium and ds i s elemental pathlength i n the d i r e c t i o n s. The mass scattering-mass absorption c o e f f i c i e n t K d e s c r i b e s the r e l a t i v e l o s s of i n t e n s i t y upon passage of l i g h t through ds due to s c a t t e r i n g away from the d i r e c t i o n s and attenu a t i o n of l i g h t by absorption along g; the emission c o e f f i c i e n t j describes the gain of i n t e n s i t y i n the d i r e c t i o n s due to scatter ing from a l l other d i r e c t i o n s and emission from i n t e r n a l sources. The angular d i s t r i b u t i o n of the s c a t t e r e d r a d i a t i o n i s s p e c i f i e d by the so c a l l e d phase function ρ ( c o s Θ) which i s p r o p o r t i o n a l to the r a t e at which l i g h t i s being s c a t t e r e d from the d i r e c t i o n s i n t o s such that s s ' = cos Θ. I s o t r o p i c s c a t t e r i n g has a phase f u n c t i o n ρ ( c o s Θ) = ώ , where ω i s an angle-independent albedo for s i n g l e s c a t t e r i n g and assumes values between 0 and 1. In plane p a r a l l e l i s o t r o p i c a l l y s c a t t e r i n g media without i n t e r n a l sources, i n which no unscattered l i g h t propagates, equation (1) assumes the simple form


v

v

f

e

0

di

0

(z, y )

,

1

-1 where μ i s the cosine of the angle of propagation with respect to the inward normal to the i l l u m i n a t e d surface and ζ i s the a x i s of that normal with p o s i t i v e d i r e c t i o n inward. S o l u t i o n s of the i n t e g r o - d i f f e r e n t i a l equations (1) and (2) have been found by Chandrasekhar f o r a v a r i e t y of phase functions and boundary con ditions. For each p h y s i c a l problem s p e c i f i e d by the boundary c o n d i t i o n s the angular d i s t r i b u t i o n of the l i g h t i n t e n s i t y becomes known at any p o i n t of the medium. In d i f f u s e r e f l e c t a n c e spectroscopy the t o t a l f l u x e s , r a t h e r than angular d i s t r i b u t i o n s of i n t e n s i t i e s , are measured. The + ι forward f l u x F ( ζ ) = / μ I ( ζ , μ) d μ and the backward f l u x _

ο

v

ο

v

F~(z) = - / μ I ( ζ , μ ) dp define the r e f l e c t a n c e R from the ι ( i l l u m i n a t e d ) surface of a plane p a r a l l e l specimen as R = F f ( 0 ) / F +(0) v

v


front

8.

Diffuse Reflectance Spectroscopy

KLiER

143

and the transmittance Τ a t a thickness Ζ as +

+

Τ = F (Z)/F (0) v

v

where ζ = 0 f o r the i l l u m i n a t e d surface and ζ = Ζ f o r the noni l l u m i n a t e d surface of the absorbing and s c a t t e r i n g plane p a r a l l e l medium. Equation (2) has been solved by Chandrasekhar f o r the case of s e m i - i n f i n i t e plane p a r a l l e l medium and the expressions for R and Τ have been obtained by K l i e r (10) i n the form 1 + R (b coth Y-a)


S

(3)

a + b coth Y - R g Τ = b cosh Y + a s i n h Y

(4) '

v

Here Υ = ξ κ ^ ρ Ζ , where ξ i s the p o s i t i v e r e a l root of the charac t e r i s t i c equation

_2L

ω = ο λη[(1+ξ)/(1-ξ)1 2

a - -

(1 + φ ) 2 φ ~

. , a

n

-

and R i s the r e f l e c t a n c e i l l u m i n a t e d surface R

Π~ - '

.

Λ

d b

/

a

1

w

h

e

r

. ξ+£η(1-ξ) * - ξ-£η(1+ξ) >

e

of the background placed at the non-

=

g

V

( Z ) / F

v

+ ( Z )

-

Equations (3) and (4) are formally i d e n t i c a l with the e a r l i e r Kubelka's h y p e r b o l i c s o l u t i o n s of d i f f e r e n t i a l equations f o r f o r ward and backward fluxes (11), although the Chandrasekhar-Klier and Kubelka's t h e o r i e s s t a r t from d i f f e r e n t sets of assumptions and employ d i f f e r e n t d e f i n i t i o n s of constants c h a r a c t e r i z i n g the s c a t t e r i n g and absorption p r o p e r t i e s of the medium. In Kubelka's theory, the constants a , b , and Y are r e l a t e d to the SchusterKubelka-Munk (SKI) absorption Κ and s c a t t e r i n g S c o e f f i c i e n t s as K/S = a-1,

SbZ = Y

f

In Chandrasekhar s theory, the true absorption c o e f f i c i e n t = K p ( l - û J ) and the true s c a t t e r i n g c o e f f i c i e n t σ = κ ρ ώ . K l i e r has found the r e l a t i o n s between the Chandrasekhar and SIM coefficients v

0

ν

α

ν

= ηΚ and σ

ν

= xS


ν

0

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VIBRATIONAL

SPECTROSCOPIES

and tabulated the " c o r r e c t i o n f a c t o r s " η and χ as a f u n c t i o n of albedo ω ( 1 0 ) . The r a t i o ( χ / η ) i s f a i r l y constant and e q u a l , w i t h i n l?5%,to (8/3) f o r values of K/S between 0 and 0 . 3 . The r a t i o K/S i s most e a s i l y experimentally determined from the r e f l e c t a n c e 1^, from s e m i - i n f i n i t e specimen (Z-*») as ο

— = (1~R») S

/ tr \ V

2Roo

'

which i s the w e l l known SKM e q u a t i o n . K l i e r ' s c o n t r i b u t i o n c o n s i s t s of demonstrating that the r a t i o of the true a b s o r p t i o n to s c a t t e r i n g c o e f f i c i e n t s ( / ) be determined as α

σ


ν

a

n

2

^

σ

c

ν

=

ν

(

η , d-Rx>) 2R»

(

Y

6

)

where η and χ are known numerical f a c t o r s f o r each value of Roo (or K / S ) . These c o r r e c t i o n s become s i g n i f i c a n t f o r s t r o n g l y absorbing specimens with (K/S) > 0.3 or < 0.5. I t should be emphasized at t h i s p o i n t that the expressions (5) and (6) are v a l i d i f the medium i s a homogeneous i s o t r o p i c s c a t t e r e r with no unscattered l i g h t propagating i n i t . There are numerous experimental t e s t s to determine whether these c o n d i t i o n s are s a t i s f i e d (1); i n the case they are not s a t i s f i e d , more com plex s o l u t i o n s of the r a d i a t i v e t r a n s f e r equation have to be employed (9), u s u a l l y r e s o r t i n g to numerical methods. Experimen t a l input i n t o the theory then i n v o l v e s the phase f u n c t i o n s , which have to be determined by a number of tedious measurements of the angular d i s t r i b u t i o n of l i g h t i n t e n s i t i e s . To take advantage of the s i m p l i c i t y of expression (6) f o r determining ( & / ) > i n e r t white i s o t r o p i c s c a t t e r e r of high s c a t t e r i n g power may be added to those specimens which themselves do not s c a t t e r i s o t r o pically. I s o t r o p i c s c a t t e r i n g i s u s u a l l y produced by randomly o r i e n t e d i r r e g u l a r p a r t i c l e s l o o s e l y packed i n t o a powder l a y e r i n which the average d i s t a n c e s between p a r t i c l e s are smaller than the average p a r t i c l e s i z e . As the s c a t t e r i n g c o e f f i c i e n t s S or σ do not depend on or are only a s l i g h t monotonous f u n c t i o n of the l i g h t frequency V (1), the v a r i a t i o n with V of the true a b s o r p t i o n c o e f f i c i e n t Κ or a determines the s p e c t r a l s t r u c t u r e of (K/S) or ( ο ^ / ^ ) and i s , except f o r a m u l t i p l i c a t i v e constant A. or _ L , a true r e p r e s e n t a S Oy t i o n of the energy absorption spectrum of the s p e c i e s contained i n the medium. Because c a t a l y s t p a r t i c l e s of i n t e r e s t u s u a l l y have a l a r g e surface to volume r a t i o , a s u b s t a n t i a l p a r t of the observed spectrum may come from surface molecules, complexes, and surface band s t r u c t u r e . When the adsorbed species are f a i r l y i s o l a t e d , the a b s o r p t i o n c o e f f i c i e n t s Κ or σ are expected to r i s e σ

ν

v

ν

ν

ν


a

ν

n

8.

KLIER

145


l i n e a r l y with t h e i r c o n c e n t r a t i o n , which can be v e r i f i e d by l i n e a r dependence of the experimentally observed

2

F(Roo) = (1 - Foo) /(2Roo),


c o r r e c t e d i f necessary at h i g h a b s o r p t i o n according to equation ( 6 ) , against concentration. An example i s given i n F i g . 1 f o r the n e a r - i n f r a r e d combination band 1 ^ 0 ( V + ό ) of water adsorbed on silica. The i n t e g r a t e d i n t e n s i t y ΣΙ of the a b s o r p t i o n band i s an i n t e g r a l of F ( R » ) over a l l frequencies V under the band. Numerous other examples are given i n Kortum's book (1) and some a d d i t i o n a l examples are shown below f o r h i g h l y absorbing media. V i b r a t i o n a l Spectra 1

DRS has been used i n both the fundamentals ( 8 0 0 - 4 , 0 0 0 cm" ) and the overtone-combination ( 4 , 0 0 0 - 1 5 , 0 0 0 cm" ) r e g i o n s . In the f i r s t r e g i o n , combination of DRS w i t h r a p i d scanning F o u r i e r transform spectrometry with r e f l e c t e d l i g h t c o l l e c t i o n by an e l l i p s o i d a l m i r r o r proved to provide good q u a l i t y and q u a n t i t a t i v e l y accurate i n f r a r e d s p e c t r a of organic m a t e r i a l s mixed with KBr ( 1 2 ) . The SKM i n t e n s i t i e s were l i n e a r with the c o n c e n t r a t i o n of the absorber up to F(Roo) = 0 . 6 and the SKM theory was considered v a l i d f o r F(Roo) ranging from 0 to 0 . 6 . A direct o b s e r v a t i o n by DRS of adsorbates and elementary r e a c t i o n s on c a t a l y s t s was reported by Kortum and D e l f t s ( 1 3 ) , who s t u d i e d the i n t e r a c t i o n of ethylene with the A ^ O ^ - S i C ^ c r a c k i n g c a t a l y s t s and concluded that a ^"^5 surface species i s formed from adsorbed ethylene and hydrogen donated by the surface OH groups. The sen s i t i v i t y of DRS i n i n f r a r e d fundamentals r e g i o n was s u f f i c i e n t to detect the decay of OH i n t e n s i t i e s upon ethylene a d s o r p t i o n and to contest e a r l i e r t r a n s m i s s i o n IR r e s u l t s (14) which d i d not i n d i c a t e that OH groups were the source of hydrogen f o r the buildup of C 2 H 5 residues from e t h y l e n e . I n t e r e s t i n g r e s u l t s were a l s o obtained by Kortum and D e l f t s concerning the adsorbates and p r o ducts of HCN i n t e r a c t i o n w i t h v a r i o u s oxide surfaces and w i t h the cracking catalyst A l 2 0 3 - S i 0 2 « The adsorbed monomer, the products of i t s decomposition, polymer buildup and dicyan formation were r e a d i l y detected by DRS i n the IR r e g i o n . The s p e c t r a were not q u a n t i t a t i v e l y e v a l u a t e d , however, as much as most IR s p e c t r a obtained by transmission through p e l l e t s are not q u a n t i t a t i v e l y evaluated e i t h e r . Without doubt the i n t e r f a c i n g of both the r e f l e c t a n c e and t r a n s m i s s i o n IR spectrometers w i t h computers w i l l y i e l d a wealth of q u a n t i t a t i v e i n f o r m a t i o n concerning adsorbates and t h e i r products on c a t a l y s t surfaces i n the near f u t u r e . 1

An example of such a q u a n t i t a t i v e study i s an i n v e s t i g a t i o n of the i n t e r a c t i o n s of surface hydroxyls w i t h adsorbed water on a v a r i e t y of s i l i c a s ( 1 5 , 1 6 ) . I t was e s t a b l i s h e d that the surface hydroxyl groups are hydrogen bond donors, that they form a 1:1 complex w i t h water on non-porous H i S i l s i l i c a s (15) and 2 : 1 com-



146

VIBRATIONAL

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Journal of Physical Chemistry

Figure 1.

The integrated intensities 51 = f

F(R ,v)dv of the H 0 (v + S) bands x

2

band

(O, Q ) on Na-HiSil(650) and ( 0 ) on HiSil(700) vs. water coverage, v . The temperatures of heat treatments of these silicas are given in parentheses in degrees Celsius (\ 5). a


8.


KLiER

147

plex on porous amorphous s i l i c a s (16). Since the 1:1 0H...0H complex has a b i n d i n g energy of approximately 6 k c a l / m o l (17), which i s lower than the heat of l i q u e f a c t i o n of water, 10.6 k c a l / mol, and the 2:1 O H . . . O H 2 complex has a binding energy l a r g e r than the heat of l i q u e f a c t i o n of water, the non-porous s i l i c a s appear hydrophobic whereas the porous s i l i c a s are h y d r o p h i l i c . These p r o p e r t i e s have important consequences i n water c l u s t e r i n g and n u c l e a t i o n (18). Surface hydroxyls were a l s o found on aluminumexchanged f l u o r i n e micas (19), the mechanism suggested f o r t h e i r formation being 2

3

2

+

+

2κΛ. + A l , * . + H 0 -> [ A £ ( 0 H ) ] " \ + 2 K , + H, . (s) (aq) 2 (s) (aq) (aq) Downloaded by CORNELL UNIV on August 29, 2016 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch008

o

λ

where (s) denotes a c a t i o n s i t e i n the oxygen s i x - r i n g windows of the cleavage plane of mica. These "aluminols" bind s t r o n g l y and s e l e c t i v e l y organic phosphates such as n u c l e o t i d e s and n u c l e i c acids by a mechanism depicted i n F i g . 2. This property and the f a c t that the strong A l - O - P bond can be r e a d i l y cleaved by aqueous f l u o r i d e s l e d to the design of e f f i c i e n t separation methods f o r messenger RNA's (20). I t i s p o s s i b l e to devise s i m i l a r methods for anchoring of homogeneous c a t a l y s t s . The above r e s u l t s concerning the surface water, OH groups, and t h e i r r e a c t i v i t y have been obtained i n the overtone and com b i n a t i o n bands r e g i o n , rather than i n the fundamentals i n f r a r e d . The reasons f o r t h i s are ( i ) a p a r t i c u l a r l y "clean" d i s t i n c t i o n between the v a r i o u s v i b r a t i o n a l modes of water and of the OH groups (21), and ( i i ) employment of quartz windows and PbS d e t e c t o r s which permits an easy sample handling and a r e l a t i v e l y f a s t multiscanning over the wavelength range 4,000-15,000 cm" . The same region proved to be u s e f u l f o r the d e t e c t i o n and a n a l y s i s of organic molecules and f u n c t i o n a l groups, N 0 and C0 adsorbed on surfaces. A near i n f r a r e d spectrum of ethylene adsorbed on Mn-^A z e o l i t e i s shown i n F i g . 3. The f i r s t two bands at 4450 cm" and 4660 cm" are e a s i l y i d e n t i f i e d as the (V5 + V ) (2 9^ modes by comparison with the e a r l i e r observed overtone spectra of ethylene (22), both bands being s h i f t e d by -65 cm" from t h e i r gas phase analogues. The second observed set of bands at 5860 cm" and 6040 cm" have not been reported e a r l i e r but t h e i r frequencies are c l o s e to double the frequency of the V-Q(CH-b3 ) and V 9 ( C H - b ) fundamentals and are therefore assigned the overtone labels 2v (5860 cm"" ) and 2v (6040 cm" ). The s t r u c t u r e of the e t h y l e n e - M n complex i s expected to be s i m i l a r to one determined by Seff f o r the a c e t y l e n e - M n complex (23). That ethylene i s π - b o n d e d to the d i v a l e n t ions placed i n the oxygen s i x - r i n g win dows of the z e o l i t e has been shown by i n v e s t i g a t i o n s of the ana logous ethylene-Co^A system (24) . The b i n d i n g energy of ethylene was determined to be 17 k c a l / m o l (24). E t h y l e n e - i o n b i n d i n g i n Type A z e o l i t e , and probably i n a l l a l u m i n o s i l i c a t e s i n which c a t i o n s are placed i n oxygen s i x - r i n g windows, i s thus r e l a t i v e 1

2

2

1

1

a

n

d

v

+

v

1 2

1

1

1

u

2u

1

1

1 1

9

11

11

American Chemical Society Library 1155 16th St., N.W. Bell and Hair; Vibrational Spectroscopies Washington, D.C.for Adsorbed 20036 Species ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

148

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BINDING OF POLY - A BY THYMIDYLIC ACID ANCHORED ON Al-MICA

M

Λ


/

A

A

Τ

f

/

A Ρ

/

A\ A /

A

A

A

A

A

f

Τ

f

f

f

A Τ

_L

-ribose

ribose —

Polyadenylic acid

Thymidylic acid

mica

Figure 2. Thymidylic acid bound through its phosphate group to aluminum-ex changed mica. The thymidylic acid further adsorbs a poly-A tail of messenger RNA by hydrogen bonding, represented on top. The necessary active center for thymi dylic acid are surface aluminol groups as discussed in text.


KLIER

I


149


I

ι

Λ

/ / / / / / / / / / /

-

•3 -

/ /

/

/ / /

-

f

/ /

-

/

-

/

/ Λ / \

/ /

I

•1

/Μ

\

/

λ \/ Ji V/

IV—^ Γ , I

w

y

y

/

\

\ \ \

\

\

\

•

l

I

cmMO"' Figure 3. Overtone and combination band spectrum of ethylene and water ad sorbed on Mn A zeolite. The ethylene bands lie close to the gaseous (ν-, + v ), (v + ν*)* 2vn, and 2v vibrational modes, indicating that the ethylene molecule has retained its chemical composition and structural integrity: ( , 1) MnA + ethyl ene; ( ,2) MnA hydrated; (U) £* >> (s) bands. u

12

2

9

Η


150

VIBRATIONAL SPECTROSCOPIES

l y weak but s p e c i f i c and gives r i s e to w e l l defined s t r u c t u r e of the surface complex. Because of the small mass of hydrogen, the n e a r - i n f r a r e d bands so f a r discussed were f i r s t overtones or simple combination bands of hydrogen.containing s p e c i e s . However, higher overtones such as the ( ν ν V ) = (0 4 ° 1) band of COo at £ 8 3 5 cm"" and more complex combination bands such as the (V-jV V^) = (1 2 ° 1) of C 0 at 4970 cm"" and the ( V - j V ^ V o ) = (2 0 ° 1) of C 0 at 5090 cm" and of N 0 at 4775 cm"" were observed by DRS i n z e o l i t e s where C 0 and N 0 formed weak ligands to a C r ( I I ) surface i o n (25). Although DRS has so f a r y i e l d e d smaller volume of data on v i b r a t i o n a l spectra of adsorbates than transmission spectroscopy of p e l l e t s , there are c l e a r l y cases where DRS has advantages. These are summarized as f o l l o w s : ( i ) DRS i s a good q u a n t i t a t i v e technique; ( i i ) DRS i n the n e a r - i n f r a r e d often allows to r e s o l v e species whose bands overlap i n the fundamentals r e g i o n ; ( i i i ) the sample p r e p a r a t i o n , h a n d l i n g , and o p t i c s e n c l o s i n g the sample v e s s e l s are often simpler and more expedient than when using conventional transmission techniques. 1

χ

2

3

2

1

2

2

1

1

2


2

2

C o r r e l a t i o n Motion Given the true v i b r a t i o n a l bandshape, the time development of the molecular r e o r i e n t a t i o n can be determined from F o u r i e r t r a n s form of the band i n t e n s i t i e s onto the time base (26). The ensuing " c o r r e l a t i o n f u n c t i o n " has the s i g n i f i c a n c e of the time develop ment of the t r a n s i t i o n d i p o l e of the i n f r a r e d a c t i v e mode (27). Where adsorbed molecules are of i n t e r e s t , one can determine whether they are "fixed", r o t a t e f r e e l y , or are hindered i n r o t a t i o n by neighboring ad-molecules or atoms of the s u r f a c e . A com p l e t e temporal d e s c r i p t i o n of the molecular motion i s i n p r i n c i p l e p o s s i b l e by choosing two i n f r a r e d bands with n o n - c o l i n e a r t r a n s i t i o n d i p o l e s f o r the determination of the time development of the two d i p o l e v e c t o r s t r a v e l l i n g with the molecule. As a matter of example, the deformation v i b r a t i o n V 3 of water has a t r a n s i t i o n d i p o l e along the molecular a x i s whereas the antisymmetric s t r e t c h V p e r p e n d i c u l a r to i t ; the time c o r r e l a t i o n functions of these two bands c o n t a i n a complete information about the o r i e n t a t i o n of the water molecule at any moment of i t s tumbling at s u r f a c e s , i n gaseous or condensed phases, wherever the v i b r a t i o n a l spectrum i s measured. A n a l y s i s of c o r r e l a t i o n motion about one s i n g l e mole c u l a r a x i s i s not as complete but s t i l l i s of value f o r determin ing the r o t a t i o n a l m o b i l i t y of adsorbed molecules. The c o r r e l a t i o n functions f o r water i n v a r i o u s adsorbates such as on s i l i c a s , micas, and z e o l i t e s have been reviewed i n r e f . 18. The c o r r e l a t i o n (or r e o r i e n t a t i o n a l ) times can be determined from the c o r r e l a t i o n f u n c t i o n s , o r , when the bandshape i s known, from the band width. For Gaussian bands, f o r example, the c o r r e l a t i o n time i s given as 4/£n2/Δω^-, where Δωϊ- i s the h a l f - w i d t h of the band. Table I l i s t s some c o r r e l a t i o n times f o r water i n z e o l i t e s , on 2


8.

151


KLiER

s i l i c a and mica s u r f a c e s , and compares them with those f o r water i n bulk l i q u i d and i n the c r i t i c a l s t a t e (28). TABLE I R e o r i e n t a t i o n a l times τ f o r water molecules i n v a r i o u s adsorbates and i n bulk water. Sorbent (line) NaY


NaY NaY NaA

τ

1

(5320 cm" ) 1

(5240 cm" ) (5120 (5130

Hi-Sil

1

cm" ) 1

cm" )

(V+6)

Method f o r determining τ

(sec)

n" 2.7 χ i10

1 2

n" χ i10

1 2

n" 5.3 χ i10

1 3

1.7

A A

1 3

A

l(f

1

4

Β Β

(8.2-9.5) χ Ι Ο " 4.0 χ

A

F l u o r i n e mica (\Η-δ)

3.5 χ

l(f

1 4

Bulk water

5.0 χ Ι Ο "

1 4

Β

7.0 χ Ι Ο "

1 4

Β

(v+6)

C r i t i c a l water ( ν + δ )

A - From h a l f - w i d t h s of Gaussian peaks Β - From c o r r e l a t i o n f u n c t i o n s C ( t ) ; C ( T ) = C ( 0 ) * e The r e l a t i v e l y long c o r r e l a t i o n time of the NaY and NaA water species i n d i c a t e s that i n t r a z e o l i t i c water i s r o t a t i o n a l l y pinned compared to that on s i l i c a and mica surfaces and i n bulk water. On s i l i c a s a l o n e , water bound to two surface hydroxyls appears r o t a t i o n a l l y l e s s mobile than water bound to a s i n g l e h y d r o x y l (16). On mica s u r f a c e s , the r e o r i e n t a t i o n was found f a s t but i r r e v e r s i b l e , i n d i c a t i n g angular t r a p p i n g of t h i s surface spe c i e s (18). Many d e t a i l s of fundamental and p r a c t i c a l i n t e r e s t can be obtained from the time c o r r e l a t i o n f u n c t i o n s of adsorbates on c a t a l y s t s , s i m i l a r to the r e s u l t s of s t u d i e s of adsorbed water and surface hydroxyls discussed above. To our knowledge, how ever, no s t u d i e s of c o r r e l a t i o n f u n c t i o n of adsorbates other than water have been r e p o r t e d . Perhaps the reason f o r t h i s i s a j u s t i f i e d l a c k of confidence concerning the true handshape as obtained by t r a d i t i o n a l methods employing ( d i f f u s e ) t r a n s m i s s i o n through pellets. That i s not to say that DRS i s without problems i n the respect of true bandshapes, i n v o l v i n g the p o s s i b l e n o n - i s o t r o p i c s c a t t e r i n g where i s o t r o p i c i s assumed, but experimental evidence i s mounting t h a t , with c a r e f u l a p p l i c a t i o n of the r a d i a t i v e t r a n s f e r theory, the true lineshapes can indeed be obtained by spectroscopy of s c a t t e r i n g media.


152

VIBRATIONAL

SPECTROSCOPIES


E l e c t r o n i c Spectra of Adsorption Centers and Complexes. While the purpose of v i b r a t i o n a l spectroscopy i n mechanistic s t u d i e s of adsorption and c a t a l y s i s i s mainly to gain information about the i d e n t i t y and s t r u c t u r e of adsorbed s p e c i e s , electronic spectra are f i n g e r p r i n t s of the s t a t e s of valence e l e c t r o n s i n the c a t a l y s t and c a t a l y s t - a d s o r b a t e complexes. Since most c a t a l y s t s are o p e n - s h e l l systems, they have e l e c t r o n i c s p e c t r a i n the UVv i s i b l e - n e a r i n f r a r e d r e g i o n ; changes of these spectra are r e a d i l y observed upon chemisorption, i f the c a t a l y s t ' s surface-to-volume ratio is large. In a number of cases the e l e c t r o n i c motion i n t e r a c t s with phonons and v i b r a t i o n s and vibronic spectra or v i b r o n i c s p l i t t i n g of e l e c t r o n i c bands i s observed i n the s p e c t r a l r e g i o n between 5,000 cm and 40,000 cm" . V i b r o n i c i n t e r a c t i o n s are of importance i n determining the s t a b i l i t y of molecular species and complexes, and may thus i n i t i a t e or a l t e r the chemical, i n c l u d i n g c a t a l y z e d , pathways. I t i s f o r t h i s reason that e l e c t r o n i c spect r a bear perhaps the most d i r e c t r e l a t i o n to chemical r e a c t i v i t y of s u r f a c e s . The task to i n t e r p r e t these spectra i s formidable, however, because i t i n v o l v e s advanced quantum mechanical a n a l y s i s of s i z e a b l e m o l e c u l a r - c r y s t a l or amorphous c l u s t e r systems the s t r u c t u r e of which i s often e l u s i v e . A notable exception are chemisorbed complexes i n z e o l i t e s , which have been c h a r a c t e r i z e d both s t r u c t u r a l l y and s p e c t r o s c o p i cally, and f o r which the i n t e r p r e t a t i o n of e l e c t r o n i c s p e c t r a has met with a considerable success. The reason f o r the former i s the w e l l - d e f i n e d , although complex, s t r u c t u r e of the z e o l i t e framework i n which the c a t i o n s are d i s t r i b u t e d among a few types of a v a i l able s i t e s ; the fortunate circumstance of the l a t t e r i s that the i n t e r a c t i o n between the c a t i o n s , which act as s e l e c t i v e chemisorpt i o n c e n t e r s , and the z e o l i t e framework i s p r i m a r i l y only e l e c t r o static. The theory that a p p l i e s f o r t h i s case i s the l i g a n d f i e l d theory of the ion-molecule complexes u s u a l l y placed i n t r i g o n a l f i e l d s of the z e o l i t e c a t i o n s i t e s (29). Quantum mechanical exchange i n t e r a c t i o n s with the z e o l i t e framework are j u s t i f i a b l y neglected except f o r very small e f f e c t s i n resonance energy t r a n s f e r (30). A l l of the s p e c t r a l data concerning z e o l i t e s have so f a r been obtained by DRS. A review of e l e c t r o n i c spectroscopy of t r a n s i t i o n metal i o n complexes i n Type A z e o l i t e s up to 1974 (7) summarizes r e s u l t s concerning the energy l e v e l s of the bare s i t e s occupied by the ions o n l y , and of t h e i r 1:1 complexes with water, o l e f i n s , and cyclopropane. Both e a r l i e r (8) and more recent (31) s t u d i e s a l s o showed that the adsorption process may not stop at the 1:1 ion-molecule complex but may continue to form i n t r a c a v i t a l , m u l t i - l i g a n d e d complexes, many of which have p r o p e r t i e s simi l a r to those e x i s t i n g i n s o l u t i o n s . The 1:1 complexes have s t r u c t u r e s (32) u n p a r a l l e l l e d i n s o l u t i o n s , however, i n which the c o o r d i n a t i o n of the metal i o n i s low and i t s energy s p e c t r a and other p h y s i c a l p r o p e r t i e s such as magnetic moments are pronounced-


8.

KLiER

153


l y d i f f e r e n t from any p r e v i o u s l y observed i n compounds and com plexes e x i s t i n g i n s o l u t i o n s and c r y s t a l s . A p a r t i c u l a r l y d e t a i l e d d e s c r i p t i o n was obtained for complex es of Co**A z e o l i t e with mono-olefins (24). S t e r i c e f f e c t s due to methyl groups adjacent to the double bond r e s u l t e d i n the l i g a n d strength spectrochemical s e r i e s ethene > propene > cis-butene-2

> trans-butene-2

i n which the π - b o n d e d o l e f i n s , i n the above o r d e r , produced decreasing s p e c t r a l s p l i t t i n g of the 14,000-20,000 c m electronic band of the o l e f i n - C o A complex, as p r e d i c t e d by the l i g a n d f i e l d theory f o r the s t a t e of t h i s low symmetry (C2 ) complex (24). The b a n d - s p l i t t i n g i n the o l e f i n - C o ^ A complexes i s temperature independent and r e s u l t s from the permanently present low symmetry components of the adsorbed molecules. In r e l a t e d t e t r a h e d r a l com p l e x e s , i n which the C o ions are bound to three s k e l e t a l oxygens and one water molecule at the v e r t i c e s of a t e t r a h e d r o n , the p r e d i c t e d s p e c t r a l s p l i t t i n g of the ^P(^T^) s t a t e gives r i s e to a symmetric and temperature dependent t r i p l e t . Such a behavior i s c h a r a c t e r i s t i c of the dynamic J a h n - T e l l e r e f f e c t w i t h i n the excited T-^ s t a t e . The t r a n s i t i o n s and t h e i r parameters are shown i n F i g . 4. The temperature dependence of the s p e c t r a l s p l i t t i n g s thus allows a d i s t i n c t i o n to be made between s t a t i c f i e l d s and dynamic d i s t o r t i o n s and f u r t h e r c h a r a c t e r i z e s the nature of the s t u d i e d surface complex. A t h e o r e t i c a l study of the J a h n - T e l l e r e f f e c t i n t r a n s i t i o n metal i o n exchanged z e o l i t e s showed that offa x i a l d i s t o r t i o n s w i l l occur i n the bare s i t e s of ions with degen erate grounds s t a t e s ( i . e . C u , C r , Co , T i of the f i r s t t r a n s i t i o n s e r i e s ) (29). Strong anharmonicities may amplify the e f f e c t s of v i b r o n i c c o u p l i n g and d r i v e the ions to the proximal framework oxygen, which may r e s u l t i n the d e s t r u c t i o n of the com p l e x , i . e . i n an i o n - s k e l e t a l chemical r e a c t i o n . Such an i n s t a b i l i t y - r e a c t i o n i s b e l i e v e d to be the cause of the notorious chemical i n s t a b i l i t y of the Cu A z e o l i t e s . - 1


V

1 1

1 1

1 1

1

1

1

A chemical r e a c t i o n can a l s o occur i n the adsorbed l i g a n d , and where the o r i g i n a l bare i o n s i t e has been regenerated by a sequence of r e a c t i o n s , the i o n s i t e had performed a f u n c t i o n of a catalyst. An example of such a process i s the o x i d a t i o n of CO over the C r A z e o l i t e (25) . The s i g n i f i c a n c e of the C r A c a t a l y s t r e s t s not i n any of i t s p r a c t i c a l or commercial value f o r CO o x i d a t i o n but i n the f a c t that every step of oxygen a c t i v a t i o n for t h i s c a t a l y z e d r e a c t i o n has been c h a r a c t e r i z e d s p e c t r o s c o p i c a l l y , g r a v i m e t r i c a l l y , and by magnetic moment measurements. A mechanism I ] :

Cr

i : t

I 3 :

A + 0 , . 2(g) blue 0

2 5

o

C r c

m

A - o : 2 gray

> 1

5

0

O

c

Cr

I V

2

A-0 " + \ θ , . 2 2(g) red


0

154

VIBRATIONAL

ENERGY SURFACES Α ι—• T,

FOR DYNAMIC

TRANSITIONS IN

JAHN-TELLER

TETRAHEORAL

SPECTROSCOPIES

SPLITTING

IN THE

Co(ll)


COUPLING

CONSTANTS

STATES

T.

.1 / c

e

K/z

11

Figure 4. Dynamic Jahn-Teller effect in the excited states of tetrahedral Co species. Vibronic interactions of the electronic 4 T 2 states with normal modes (left,) result in the splitting of the triply degenerate Tj state into three intersecting wells fright). Optical transitions are symmetric triplets, the separation of which is tem perature dependent: c is the coupling constant for the state with the T normal modes; b the coupling constant of T, with the Ε normal modes. The resulting "wob bling" motion is indicated in one of the T normal modes on the left. In zeolites, near-tetrahedral complexes are realized by the four vertices of the tetrahedron being occupied by three skeletal oxygens and a water molecule. 2

2


8.

155


KLiER

Cr

I V

A-0

2

>

C r ^ A + CO

.

( C

CO,300°C red

g

;

blue

has been w e l l e s t a b l i s h e d (25), i n conjunction w i t h the e a r l i e r reported r e v e r s i b l e oxygen chemisorption on C r A , (33), depicted above as the f i r s t s t e p . The CO o x i d a t i o n over C r - ^ A was a l s o followed i n a continuous flow r e a c t o r and r e a c t i o n observed at 3 0 0 ° C but not at 2 5 ° C . I t may be concluded that the chemisorbed oxygen molecular i o n 0^ i s not s u f f i c i e n t l y a c t i v a t e d for o x i d i z ing CO but the 0 " species i n the C r A - 0 ~ complex i s . The r o l e of the v a r i o u s oxygen species observed i n Cr A i n m i l d o x i d a t i o n r e a c t i o n s has yet to be i n v e s t i g a t e d . There are t r a n s i t i o n metal ions on which the molecular species i s formed but the subse quent 0 ~ i s not formed; an example of such a system i s the Cu Y which i s o x i d i z e d by molecular oxygen to C u Y - 0 2 ~ . It i s be l i e v e d that the p o t e n t i a l c o n t r o l of c a t a l y z e d s e l e c t i v e o x i d a t i o n s by the s e l e c t i o n of c a t i o n and the z e o l i t e matrix has not been tapped, and although the progress of r a t i o n a l approach to c a t a l y s t s e l e c t i o n i s slower than that of an e m p i r i c a l approach, the future advantages of " t a i l o r e d " over e m p i r i c a l c a t a l y s t s may r e s u l t i n a higher s e l e c t i v i t y of the former. For c a t a l y s t t a i l o r i n g a d e t a i l e d s p e c t r a l d e s c r i p t i o n such as one i n the few examples given above i s of s u b s t a n t i a l b e n e f i t because each p i e c e of information on the e l e c t r o n i c s t r u c t u r e of adsorbed complexes generates new ideas f o r preparations of new e f f e c t i v e c a t a l y s t s guiding a given r e a c t i o n to d e s i r e d p r o d u c t s . I I


2

I V

2

2

I I

Photoluminescence L i g h t energy absorbed i n c a t a l y s t p a r t i c l e s and surfaces may be r e - e m i t t e d , i n p a r t , at a d i f f e r e n t wavelength (or d i f f e r e n t wavelengths). When t h i s occurs i n s c a t t e r i n g media, the emission coefficient j of equation (2) w i l l have two c o n t r i b u t i o n s : l i g h t j ^ ^ s c a t t e r e d i n t o the d i r e c t i o n μ from a l l other d i r e c t i o n s without a change of frequency V , and l i g h t j^> emitted i n to the d i r e c t i o n μ by photoluminescence. In previous paragraphs were discussed cases i n which j ^ ^ i s equal to z e r o . The com p l e t e r a d i a t i v e t r a n s f e r equation that i n c l u d e s photoluminescence reads as f o l l o w s : s

v

e

—

I

(j

( s )

+ j

(

e

)

)

(7)

Here j^J takes any of the forms discussed e a r l i e r and j , in the case that emission i s i s o t r o p i c and p r o p o r t i o n a l to e x c i t a t i o n l i g h t i n t e n s i t y at some other frequency V * , becomes v


156

VIBRATIONAL

SPECTROSCOPIES

An a d d i t i o n a l r a d i a t i v e t r a n s f e r equation .(s)

(9)

K ,pds v

holds f o r the d i s t r i b u t i o n of the e x c i t a t i o n i n t e n s i t y I , . Equations (7) - (9) s p e c i f y the r a d i a t i v e f i e l d i n the presence of photoluminescence, p r o v i d i n g that the phase f u n c t i o n p t and the boundary c o n d i t i o n s are known. In the case of i s o t r o p i c s c a t t e r ing i n plane p a r a l l e l media, p , = ω ( » ) , equation (9) has the h y p e r b o l i c s o l u t i o n s o u t l i n e d i n r e f . 10 and the d i s t r i b u t i o n of the "emitted" i n t e n s i t y I ( ] i ) becomes known by s o l v i n g equations (7) and (8). An undetermined constant i s the quantum e f f i c i e n c y c , of energy t r a n s f e r from the e x c i t a t i o n frequency V to the emission frequency v . A s t r a i g h t f o r w a r d method f o r determining c ^ i i s the use of i n t e r n a l luminescent standard i n the s c a t t e r ing medium under i n v e s t i g a t i o n . Thus the p h o t ο l u m i n e s c e n c e quan tum e f f i c i e n c i e s can be determined i n t u r b i d media of known s c a t t e r i n g p r o p e r t i e s as a c c u r a t e l y as i n o p t i c a l l y homogeneous media such as s o l u t i o n s and c r y s t a l s . The frequency (v) dependence of κ i s known as absorption spectrum, of j ^ ) as emission spectrum, and of as excitation spectrum of emission at frequency V . v

v

0

ν


v

1

v v

e

ν

1

In a d d i t i o n to these three kinds of s p e c t r a , the ρ h o t o l u m i n e s c e n c e decay time can be measured a f t e r pulsed or step i l l u m i n a t i o n . Measurements of a b s o r p t i o n , e x c i t a t i o n , and emission s p e c t r a as w e l l as of decay times have been made f o r z e o l i t e specimens c o n t a i n i n g u n i v a l e n t copper i o n s . A f t e r i n i t i a l r e p o r t s of Cu* photoluminescence i n Type Y z e o l i t e by Maxwell and Drent ( 3 4 ) , Texter et a l . Q 5 ) o f f e r e d a f i r s t i n t e r p r e t a t i o n of the t r a n s i t i o n i n v o l v e d i n the emission as a E ~ [ 3 d 4(sp)] ^ D d ] , where the E s t a t e i s the lowest l y i n g , J a h n - T e l l e r s p l i t t r i p l e t s t a t e of the e x c i t e d c o n f i g u r a t i o n 3d 4(sp) of the C u i o n i n the trigonal (C ) zeolite f i e l d . Strome and K l i e r (36) demonstrated that new emission l i n e s were produced by CO a d s o r p t i o n - d e s o r p t i o n and i n t e r p r e t e d t h i s r e s u l t as a m i g r a t i o n of the Cu ions from the o r i g i n a l S I p o s i t i o n s to the surface SII and S I I positions. Strome (30) c a r r i e d out a d e t a i l e d study of luminescence l i f e times and of energy t r a n s f e r i n Cu*Y z e o l i t e s co-exchanged with N i , C o , and M n ions and determined the quantum e f f i c i e n c i e s of the green Cu^Y emission i n the presence of the other c o l o r e d ions. The experimental quantum e f f i c i e n c i e s are compared i n F i g . 5 w i t h those d e r i v e d from a model using s t a t i s t i c a l i o n d i s t r i b u t i o n and the F o e r s t e r - D e x t e r theory of resonance t r a n s f e r . The agreement between the experimental and t h e o r e t i c a l values i s e x c e l l e n t and shows that photoemission data obtained by DRS are amenable to v a l i d and r i g o r o u s t h e o r e t i c a l i n t e r p r e t a t i o n which can be used f o r the determination of e l e c t r o n i c i n t e r a c t i o n s be tween the emitters and acceptors across r e l a t i v e l y l a r g e d i s t a n ces. The b a s i c component of the t r a n s i t i o n p r o b a b i l i t y f o r e x c i 3

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Predicted 77 l

Figure 5. Relative quantum efficiencies of photoluminescence in Cu Y zeolites containing co-exchanged ions. The predicted quantum efficiencies were calculated from overlaps of the emission band of Cu with the absorption band of the coexchanged ion using the Foerster-Dexter resonance transfer theory. 1


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t a t i o n energy t r a n s f e r between the emitter (Cu ) and the acceptor center i s the overlap of the emission band with the absorption spectrum of the acceptor ( N i , C o , Mn** i n t h i s case) (37). S i m i l a r energy t r a n s f e r was observed between the luminescing Cu* ions and non-luminescing C u ions that were produced i n the Cu*Y z e o l i t e by p a r t i a l oxygen chemisorption C ^ Y + 0 ( ) + C u ^ Y - O ^ . The C u ions have a weak absorption spectrum that p a r t i a l l y overlaps with the emission band of C u , r e s u l t i n g i n resonant energy transfer. In f a c t the time course of oxygen chemisorption could be followed by monitoring the Cu photoluminescence quantum e f f i ciency with the time of exposure of Cu*Y to oxygen. Although a p p l i c a t i o n s of photoemission techniques i n surface chemistry and c a t a l y s i s are but a few, t h e i r s e n s i t i v i t y , which i s orders of magnitude higher than that of adsorption measurements, may lead to future i n v e s t i g a t i o n s of very small surfaces or adsorbed complexes i n very small c o n c e n t r a t i o n s . The technique i s not u n i v e r s a l , however, because r e l a t i v e l y few surface species w i l l d i s p l a y photoluminescence. 1 1

1 1

1 1

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1

E l e c t r o n i c Spectra of Bulk C a t a l y s t P a r t i c l e s In two instances are the e l e c t r o n i c spectra of the bulk of the c a t a l y s t p a r t i c l e s of i n t e r e s t i n c a t a l y s i s r e s e a r c h : first, when chemisorption gives r i s e to e l e c t r o n exchange that extends to l a r g e d i s t a n c e s i n t o the s o l i d and second, when v a r i o u s components of a multiphase c a t a l y s t i n t e r a c t so as to dope one phase with the chemical elements of another, r e s u l t i n g i n new, enhanced, or reduced a c t i v i t y of the c a t a l y s t . An example of the former e f f e c t i s the well-known "blackening" of n i c k e l oxide upon oxygen chemisorption: the pure green s t o i c h i o m e t r i c NiO has a DRS spectrum c h a r a c t e r i s t i c of N i * * ions i n the octahedral environment of the nearest neighbor oxygen anions of the r o c k - s a l t l a t t i c e of NiO while the oxygen-covered NiO d i s p l a y s an a d d i t i o n a l intense charge t r a n s f e r band at 550 nm which e v e n t u a l l y overlaps the whole v i s i b l e spectrum (38). This s p e c t r a l change r e s u l t s from a t r a n s f e r of e l e c t r o n s from the surface n i c k e l ions to chemisorbed oxygen, °2(g) ^( ds) -Ni***, and a subsequent exchange of the excess p o s i t i v e charge of N i * * * with N i * * of the b u l k . The l a t t e r process can be observed as an increase of the p - c o n d u c t i v i t y of the NiO upon oxygen exposure (39) and i t i s the Ni^JJ 4- N i * ) £ " [ ï ) + Î2^ t r a n s i t i o n that gives r i s e to the o p t i c a l charge t r a n s f e r band at 550 nm. The process can be reversed by a r e a c t i o n of the chemisorbed oxygen with o x i d i z e a b l e gases. For example, CO w i l l react at room temperature as C 0 / ) + 0~-Ni*** •*· C02(ads) (40), r e s t o r i n g the spectrum ana green c o l o r of the s t o i c h i o m e t r i c n i c k e l oxide and lowering the p - c o n d u c t i v i t y to the o r i g i n a l level. The ease with which n i c k e l oxide chemisorbs oxygen and 0~-Ni*** r e a c t s with CO would make n i c k e l oxide an e x c e l l e n t low temperature c a t a l y s t f o r CO o x i d a t i o n , were the whole process +

a

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not l i m i t e d by the desorption of (X^. The merit of s p e c t r o s c o p i c techniques i n t h i s case i s to resolve the r e a c t i o n mechanism encompassing the e l e c t r o n exchange with the c a t a l y s t and to f i n d the r a t e determining step of the r e a c t i o n . A good example of an a p p l i c a t i o n of DRS to the determination of the a c t i v e component i n a multiphase c a t a l y s t i s the recent study of the C u / Z n O / A ^ O ^ and C u / Z n O / C ^ C ^ methanol synthesis c a t a l y s t s (41-43). While none of the components alone i s a c a t a l y s t f o r the s y n t h e s i s at low ( l e s s than 100 atm) p r e s s u r e s , the ternary systems are h i g h l y a c t i v e and s e l e c t i v e c a t a l y s t s at those p r e s s u r e s . The a c t i v i t y r e s i d e s i n the bi-phase Cu/ZnO system (41) which was studied by DRS i n some d e t a i l (43). I t was e s t a b l i s h e d t h a t , although z i n c oxide was present i n i t s o r d i n a r y w u r t z i t e c r y s t a l form, i t s c h a r a c t e r i s t i c o p t i c a l absorption edge at 25,800 cm""^- was completely missing i n the most a c t i v e c a t a l y s t s , and a new band appeared i n the v i s i b l e at 17,000 cnT* due to copper d i s s o l v e d i n the z i n c oxide l a t t i c e . These f i n d i n g s were confirmed by a n a l y t i c a l e l e c t r o n microscopy which determined the amount of d i s s o l v e d copper to be up to 16% i n the z i n c oxide phase (42). The genesis of the a c t i v e methanol c a t a l y s t was followed from p r e c i p i t a t e precursors through the c a l c i n a t i o n and r e d u c t i o n stages, with subsequent a n a l y s i s by v a r i o u s methods i n c l u d i n g DRS, Auger/XPS (X-ray photoelectron spectroscopy), STEM (scanning transmission e l e c t r o n microscopy), X-ray d i f f r a c t i o n , TEM (transmission e l e c t r o n microscopy), surface area and chemis o r p t i o n methods, and pore d i s t r i b u t i o n determinations. A d e s c r i p t i o n of the c a t a l y s t as complete as obtained i n these s t u d i e s would not have been p o s s i b l e without employing a great many techniques of c a t a l y s t c h a r a c t e r i z a t i o n . Among these, however, DRS stands out as l i t t l e time consuming, inexpensive, sens i t i v e to e l e c t r o n i c i n t e r a c t i o n s among the c a t a l y s t components, and having high s p e c t r a l r e s o l u t i o n . For these reasons, DRS i s l i k e l y to remain an e s t a b l i s h e d technique f o r c h a r a c t e r i z a t i o n of dispersed c a t a l y s t s and, as was shown i n e a r l i e r d i s c u s s i o n , a l s o their surfaces. The present l i m i t a t i o n of the l a t t e r stems from the l i m i t a t i o n s of the theory of s u r f a c e s . I t i s hoped, however, that w i t h the advances i n s t r u c t u r a l information on a molecular s c a l e , such as from high r e s o l u t i o n STEM, the theory w i l l anchor i t s e l f on r e l i a b l e and r e a l i s t i c models and provide background f o r i n t e r p r e t a t i o n of DRS i n h i g h l y i n t e r a c t i n g systems. Abstract

Diffuse Reflectance Spectroscopy(DRS)is suited for the study of real catalysts as it measures and interprets light intensity loss spectra in absorbing and scattering specimens. DRS has been applied both to the analysis of vibrational spectra of surface species in the fundamental, overtone, and combination band regions,and to the determination of time correlation motion of adsorbed molecules by Fourier inversion of the spectra onto



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the time base. In addition, electronic spectra of the chemisorp tion centers have been studied prior and during chemisorption which, in combination with the vibrational spectra, resulted in a nearly complete determination of the structure of adsorbed com plexes between hydrocarbons, oxygen, carbon monoxide, water, metal ions, and aluminosilicate surfaces. As the recent research results indicated the importance of interactions among the cata lyst components in determining its selectivity and activity, DRS has proved a powerful tool assisting the analysis of chemical and physical interactions in multicomponent catalysts by other tech niques such as scanning transmission electron microscopy. Exam ples are given from the analysis of the copper-zinc oxide based methanol and water gas shift catalysts. Combined with photoemission, DRS provides quantitative data on excitation-luminescence behavior of powdered specimens which can be used to determine photοluminescence quantum efficiencies and the extent of resonant energy transfer among the bulk and surface activators and sensitizers. Literature Cited

1. Kortüm, G., "Reflectance Spectroscopy. Principles, Methods, and Applications". Springer-Verlag, Berlin and New York, 1969. 2. Wendlandt, W.W., and Hecht, H.G., "Reflectance Spectroscopy". Wiley (Interscience), New York, 1966. 3. Wendlandt, W.W., ed., "Modern Aspects of Reflectance Spectroscopy", Plenum, New York, 1968. 4. Frei, R.W., and MacNeil, J.D., "Diffuse Reflectance Spectroscopy in Environmental Problem Solving". CRC Press, Cleveland, Ohio, 1973. 5. Delgass, W.N., Haller, G.L., Kellerman, R., and Lunsford, J.H., "Spectroscopy in Heterogeneous Catalysis". Academic Press, New York, 1979. 6. Jones, C.E., and Klier, Κ., Annual Revs. Mater. Sci. 2, 1 (1972). 7. Kellerman, R., and Klier, Κ., Surface and Defect Properties of Solids (Chem. Soc. London) 4, 1 (1975). 8. Klier, Κ., Catalysis Revs. 1, 207 (1967). 9. Chandrasekhar, S., "Radiative Transfer". Dover, New York, 1960. 10. Klier, Κ., J. Opt. Soc. Am. 62, 882 (1972). 11. Kubelka, P., J. Opt. Soc. Am. 38, 448 (1948). 12. Fuller, M.P., and Griffiths, P.R., Anal. Chem. 50, 1906 (1978). 13. Kortüm, G., and Delfts, H., Spectrochimica Acta 20, 405 (1964). 14. Lucchesi, P.J., Charter, J.L., and Yates, D.J.C., J. Phys. Chem. 77, 1457 (1962).



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15. Klier, Κ., Shen, J.H., and Zettlemoyer, A.C., J. Phys. Chem. 77, 1458 (1973). 16. Shen, J.H., and Klier, K., J. Colloid Interface Sci., in press. 17. Bassett, D.R., Boucher, E.A., and Zettlemoyer, A.C., J. Colloid Interface Sci. 34, 3 (1970). 18. Klier, Κ., and Zettlemoyer, A.C., J. Colloid Interface Sci. 58, 216 (1977). 19. Huang, S.-D., Pulkrabek, P., and Klier, K., J. Colloid Inter face Sci. 65, 583 (1978). 20. Pulkrabek, P., Klier, K., and Grunberger, D., Anal. Biochem. 68, 26 (1975). 21. The following IR species of water and surface hydroxyls have been observed: H 0(V + V ) around 5300cm ,H 0(V +V )at7150cm , SiOH(2v) at 7300 cm , and SiOH(2v +δ)at 8100 cm . 22. Herzberg, G., "Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra of Polyatomic Molecules". Van Nostrand Reinhold Co., New York 1945, p. 326. 23. Riley, P.E., and Seff, K., J. Amer. Chem. Soc. 95, 8180 (1973). 24. Klier, K., Kellerman, R., and Hutta, P.J., J. Chem. Phys. 61, 4224 (1974). 25. Kellerman, R., and Klier, Κ., Molecular Sieves-II, ACS Symposium Series 40, 120 (1977). 26. Gordon, R.G., J. Chem. Phys. 43, 1307 (1965). 27. Klier, K., J. Chem. Phys. 58, 737 (1973). 28. Shen, J.H., Zettlemoyer, A.C., and Klier, K., J. Phys. Chem., in press. 29. Klier, K., Hutta, P.H., and Kellerman, R., Molecular SievesII, ACS Symposium Series 40, 108 (1977). 30. Strome, D.H., Dissertation, Lehigh University, 1977. 31. Lunsford, J.H., Molecular Sieves-II, ACS Symposium Series 40, 473 (1977). 32. Seff, K., Accounts of Chemical Research 9, 121 (1976). 33. Kellerman, R., Hutta, P.J., and Klier, K., J. Am. Chem. Soc. 96, 5946 (1974). 34. Maxwell, I.E., and Drent, E., J. Catal. 41, 412 (1976). 35. Texter, J . , Strome, D.H., Herman, R.G., and Klier, Κ., J. Phys. Chem. 81, 333 (1977). 36. Strome, D.H., and Klier, K., J. Phys. Chem., in press. 37. Dexter, D.L., J. Chem. Phys. 21, 836 (1953). Förster, Th., Ann. Phys. 2, 55 (1948). Dexter, D.L., Förster, Th., and Knox, R.S., Phys. Stat. Sol. 34, K159 (1969). 38. Klier, Κ., Kinetics and Catalysis, 3, 65 (1962). 39. Kuchynka, Κ., and Klier, K., Coll. Czech. Chem. Communica tions 28, 148 (1963). -1

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40. Klier, Κ., and Jiratova, Μ., Proceedings of the Third Inter national Congress on Catalysis, North-Holland Pub. Co., Amsterdam 1965, p. 763. 41. Herman, R.G., Klier, K., Simmons, G.W., Finn, B.P., Bulko, J.B., and Kobylinski, T.P., J. Catal. 56, 407 (1979). 42. Mehta, S., Simmons, G.W., Klier, K., and Herman, R.G., J. Catal. 57, 339 (1979). 43. Bulko, J.B., Herman, R.G., Klier, K., and Simmons, G.W., J. Phys. Chem., 83, 3118 (1979). June 3,

1980.


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Investigations of Adsorption Centers, Molecules, Surface Complexes

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