Factors Affecting X-ray Diffraction Characteristics of Catalyst Materials

Oct 3, 1989 - Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, NJ 08066-0480. Characterization and Catalyst Devel...
0 downloads 12 Views 2MB Size
Chapter 27

Factors Affecting X-ray Diffraction Characteristics of Catalyst Materials Wayne J. Rohrbaugh and Ellen L . W u

Downloaded by GEORGETOWN UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch027

Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, N J 08066-0480

X-ray diffraction (XRD) is a major tool employed to identify and characterize catalytic materials. A brief review of the diffraction phenomenon and the effect of crystallite size is presented. Appli­ cations of XRD to catalyst characterization are illustrated, including correlation of XRD powder patterns to molecular structural features, deter­ mination of Pt crystallite size and others. Factors that affect the appearance of XRD powder patterns, such as framework structure perturbations, extra­ framework material, crystal morphology, impurities, sample preparation, instrument configurations, and x-ray sources, are discussed.

X-ray d i f f r a c t i o n (XRD), i n p a r t i c u l a r x - r a y powder d i f f r a c t i o n , has been used e x t e n s i v e l y t o i d e n t i f y and c h a r a c t e r i z e m a t e r i a l s used as c a t a l y s t s i n t h e p e t r o l e u m r e f i n i n g and p e t r o c h e m i c a l i n d u s t r i e s . T h i s i s n o t s u r p r i s i n g , s i n c e many o f t h e m a t e r i a l s are c r y s t a l l i n e ; i . e . , t h e atoms c a n be a r r a n g e d i n t h r e e d i m e n s i o n a l p e r i o d i c a r r a y s . E v e r y atom i n a c r y s t a l l i n e s o l i d c o n t r i b u t e s t o t h e o b s e r v e d XRD p a t t e r n , t h u s making t h e p a t t e r n c h a r a c t e r i s t i c o f t h e m a t e r i a l - a v i r t u a l f i n g e r p r i n t . The d i f f r a c t i o n d a t a o b t a i n e d f o r c a t a l y t i c m a t e r i a l s can t e l l us many t h i n g s about t h e s e m a t e r i a l s , r a n g i n g from t h e s t r u c t u r a l n a t u r e o f the c r y s t a l l i n e components t o t h e average P t c r y s t a l l i t e s i z e on a s u p p o r t e d - m e t a l c a t a l y s t . I t can be used t o m o n i t o r and c o r r e l a t e m o l e c u l a r s t r u c t u r a l f e a t u r e s t o c a t a l y t i c p r o c e s s and r e a c t i o n v a r i a b l e s , o r t o specify composition f o r patent d e f i n i t i o n . I n s h o r t , XRD i s an i n v a l u a b l e t e c h n i q u e i n c a t a l y s t r e s e a r c h and development. T h i s d i s c u s s i o n w i l l i l l u s t r a t e some o f t h e a p p l i c a t i o n s o f XRD t o c a t a l y s t c h a r a c t e r i z a t i o n and p o i n t o u t some of t h e f a c t o r s t h a t a f f e c t t h e appearance o f x - r a y powder d i f f r a c t i o n patterns of these m a t e r i a l s . 0097-6156/89/0411-0279$07.00/0 © 1989 American Chemical Society

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

280

Downloaded by GEORGETOWN UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch027

XRD

CHARACTERIZATION AND CATALYST DEVELOPMENT

Fundamentals

X - r a y s have b o t h e l e c t r i c f i e l d and magnetic f i e l d components a s s o c i a t e d w i t h them. C l a s s i c a l l y , the o s c i l l a t i n g e l e c t r i c f i e l d can a c c e l e r a t e e l e c t r o n s , and t h e s e e l e c t r o n s , i n t u r n , can emit new x - r a y s u s u a l l y o f the same energy ( e l a s t i c s c a t t e r i n g ) , b u t i n a r b i t r a r y d i r e c t i o n s i n space. S i n c e the d i m e n s i o n o f an atones e l e c t r o n c l o u d i s a p p r o x i m a t e l y the same as an x - r a y w a v e l e n g t h , i n t e r f e r e n c e occurs. D i f f r a c t i o n i s an i n t e r f e r e n c e phenomenon t h a t produces s c a t t e r e d x - r a y i n t e n s i t y maxima and minima dependent upon the arrangement o f the s c a t t e r e r s (1,2). I f the s c a t t e r e r s are somewhat randomly o r i e n t e d , d i f f r a c t i o n maxima are d i f f u s e and i l l - d e f i n e d , e.g., as seen i n XRD powder p a t t e r n s o f c l a y s and z e o l i t e s w i t h d i s o r d e r e d s t r u c t u r e s o r stacking faults. I f the s c a t t e r e r s are w e l l o r d e r e d and p e r i o d i c i n r e a l space, then so are the d i f f r a c t i o n maxima i n " d i f f r a c t i o n " space. I d e a l l y , by measuring the p o s i t i o n ( s c a t t e r i n g a n g l e ) and the a m p l i t u d e ( c o n t a i n i n g the s i g n o r phase i n f o r m a t i o n ) o f e l a s t i c a l l y d i f f r a c t e d x - r a y s , i t i s p o s s i b l e t o determine t h e p o s i t i o n i n space o f a l l the atoms which comprise the m a t e r i a l and, t h e r e f o r e , unambiguously c h a r a c t e r i z e i t . The c a l c u l a t i o n i s s t r a i g h t f o r w a r d i n terms o f an e l e c t r o n d e n s i t y d i s t r i b u t i o n function, •

> ^

r

r

.

r

2W\(hx + ky+ I 2)



/9(x,y,z) a EEE F

h k |

e

hki where the e l e c t r o n d e n s i t y , p a t any p o s i t i o n i n r e a l space ( x , y , z ) i s r e l a t e d t o the summation over d i f f r a c t i o n space (h,k, L) of the F o u r i e r t r a n s f o r m o f the d i f f r a c t e d wave a m p l i t u d e and phase, F , known as a " s t r u c t u r e f a c t o r " . Each s t r u c t u r e f a c t o r , f o r a p a r t i c u l a r r e c i p r o c a l l a t t i c e v e c t o r (h,k,£), i s a summation of the s c a t t e r i n g d e n s i t y over a l l atoms. However, t h e r e a r e s e v e r a l p r o p e r t i e s o f both the m a t e r i a l s and the d i f f r a c t i o n experiment which make the c a l c u l a t i o n s somewhat more c o m p l i c a t e d . F i r s t , a l t h o u g h the d i f f r a c t i o n angle can be measured q u i t e a c c u r a t e l y , the phase and magnitude o f the d i f f r a c t e d x - r a y s cannot. The measurement o f d i f f r a c t e d - w a v e i n t e n s i t y p r o v i d e s a q u a n t i t y t h a t i s r e l a t e d t o the square o f the a m p l i t u d e , c f . F i g u r e 1, which c o n s e q u e n t l y l a c k s the phase i n f o r m a t i o n n e c e s s a r y f o r t h e c a l c u l a t i o n o f the e l e c t r o n d e n s i t y d i s t r i b u t i o n . I n a d d i t i o n , s p e c t r a l , i n s t r u m e n t a l and sample f a c t o r s can c o n t r i b u t e t o l i n e b r o a d e n i n g , thus making i t more d i f f i c u l t t o r e s o l v e i n d i v i d u a l d i f f r a c t i o n peaks a t n e a r l y e q u a l s c a t t e r i n g a n g l e s . Indeed, c r y s t a l l i t e s i z e broadening can have a profound e f f e c t on the q u a n t i t y o f i n f o r m a t i o n t h a t can be o b t a i n e d u s i n g x - r a y diffraction. y

h k |

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

X-ray Diffraction Characteristics of Catalysts

ROHRBAUGH & WU

'hkl

['• «£>'] [i^vJ

1+ COS

Downloaded by GEORGETOWN UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch027

2

Pt

26

-2Bsin 6/\ 2

2

hkl

s i n 6 cos 6 2

2

e

*'