Expert System Applications in Chemistry - ACS Publications

Chapter 15 CACSS Computer-Aided Characterization of Solids and Surfaces D. P. de Bruijn and H. P. C. E. Kuipers

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: September 1, 1989 | doi: 10.1021/bk-1989-0408.ch015

Koninlijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), P.O. Box 3003, 1003 AA Amsterdam, Netherlands

The Computer-Aided Characterization of Solids and Surfaces (CACSS) project i s aimed at streamlining and improving the multi-technique analytical approach. Together with conventional computer programs (for example, those used to simulate an experiment), expert systems and 3d graphics will be combined to assist i n analyzing the problem under study, choosing the most suitable set of analytical techniques and, f i n a l l y , simultaneously interpreting the various experimental results. A discussion of the analytical strategy, the general outline of the resulting project and the implementation of the first parts are presented. The characterization of complex solids, such as heterogeneous catalysts, composites or ceramics, involves the determination of many parameters. Several techniques are always needed to obtain a l l the necessary data to yield these parameters. For example, to characterize a catalyst surface, one might apply a combination of XPS (X-ray Photoelectron Spectroscopy), various forms of electron microscopy and LEIS (Low Energy Ion-scattering Spectroscopy) to produce a correct model. These techniques involve separate areas of expertise, the coordination of which, essential to the success of the project, requires a dedicated effort. The Matrix Discussion Group. To this end, a discussion group was established i n the analytical department of our laboratory about four years ago. This 'Structural Analysis of Solids' (SAS) group brings together the experts i n the relevant techniques i n order to study, improve, streamline and stimulate the multidisciplinary approach. Case studies are the principle means of doing this. Our experience has taught us that only the simultaneous presence of a l l the required s k i l l s can guarantee both a balanced strategy and optimal performance of the characterization process. Besides the definite strengths of a 'matrix discussion' team, several of i t s shortcomings have become obvious. F i r s t , because of 0097-6156/89/0408-0190$06.00/0 o 1989 American Chemical Society

Hohne and Pierce; Expert System Applications in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

15. DE BRUUN & KUIPERS

CACSS: Computer-Aided Characterization

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i t s time-consuming n a t u r e , such an a p p r o a c h c a n o n l y h a n d l e a moder a t e number o f s e l e c t e d c a s e s . Second, d u r i n g t h e d i s c u s s i o n s , t h e r e l e v a n t d a t a (whether t h e y be t h e r e s u l t s o f s i m u l a t i o n s o r b a s i c d a t a ) cannot a l l be p r o d u c e d on t h e s p o t , a t l e a s t n o t q u a n t i t a t i v e l y . T h i r d , the r e l e v a n c e o f a v a i l a b l e extramural f a c i l i t i e s (e.g. a s p a l l a t i o n s o u r c e f o r n e u t r o n d i f f r a c t i o n ) i s sometimes d i f f i c u l t t o j u d g e due t o t h e l a c k o f s u f f i c i e n t i n - d e p t h e x p e r t i s e . A f o u r t h p r o b l e m e n c o u n t e r e d by t h e p a n e l members, namely t h e l a c k o f a b a s i c u n d e r s t a n d i n g o f e a c h o t h e r ' s t e c h n i q u e s , was g r a d u a l l y overcome by t h e p e r f o r m a n c e o f t h e s e e x e r c i s e s . The p r o b l e m o f communicating s t r u c t u r a l a s p e c t s o f s o l i d s and s u r f a c e s , however, remained. The CACSS P r o j e c t . The SAS a p p r o a c h has p r o v e d t o be most u s e f u l f o r s t u d y i n g and i m p r o v i n g t h e m u l t i - t e c h n i q u e approach, b u t much l e s s so i n d a y - t o - d a y p r a c t i c e . C l e a r l y , a v e h i c l e w h i c h makes a l a r g e r number o f problems amenable t o t h e same s y n e r g i s t i c b e n e f i t s as p r o d u c e d by t h e d i s c u s s i o n s has t o be c o n s t r u c t e d . F o r t h i s p u r pose, t h e SAS group c o n t i n u e s t o make t h e m u l t i d i s c i p l i n a r y s t r a t e g y b e h i n d c h a r a c t e r i z i n g complex m a t e r i a l s e x p l i c i t . T h i s r e q u i r e s an i n - d e p t h s t u d y o f t h e complementary n a t u r e o f a l l t h e t e c h n i q u e s i n v o l v e d . F u r t h e r , a new r e s e a r c h p r o j e c t was s t a r t e d two y e a r s ago, the 'Computer-Aided C h a r a c t e r i z a t i o n o f S o l i d s and S u r f a c e s ' (CACSS) p r o j e c t . The f i n a l g o a l o f t h i s p r o j e c t i s t o p r o d u c e a computerb a s e d system w h i c h makes t h e m u l t i d i s c i p l i n a r y a p p r o a c h p r a c t i c a l f o r d a y - t o - d a y u s e . F o r i n d i v i d u a l a n a l y t i c a l e x e r c i s e s CACSS must aid in: 1. d e v e l o p i n g a w o r k i n g h y p o t h e s i s ( a model o f t h e m a t e r i a l ) , 2. p l a n n i n g t h e m u l t i - t e c h n i q u e a n a l y t i c a l s t r a t e g y , and 3. i n t e r p r e t i n g j o i n t l y t h e a n a l y t i c a l r e s u l t s . The R o l e o f E x p e r t Systems. S e v e r a l developments i n computer s c i e n c e have made us b e l i e v e t h a t t h e CACSS p r o j e c t i s f e a s i b l e , a l though i t w i l l be a major e f f o r t . These developments a r e t h e f a s t growth i n CPU power, 3d computer g r a p h i c s ( t o model a s t r u c t u r e ) , d a t a b a s e t e c h n o l o g y , and l a s t b u t n o t l e a s t : a r t i f i c i a l i n t e l l i gence. To a c h i e v e t h e r e q u i r e d f u n c t i o n a l i t y , t h e CACSS system has t o p r o v i d e , among o t h e r t h i n g s , u s e r - f r i e n d l y a c c e s s t o : - b a s i c e x p e r t i s e i n f i e l d s a l i e n to the user - s t a t e - o f - t h e - a r t p h y s i c a l s i m u l a t i o n software - i n t e r a c t i v e c r y s t a l l o g r a p h i c and m o l e c u l a r m o d e l l i n g - m a t h e m a t i c a l r e f i n e m e n t o f models u s i n g a n a l y t i c a l r e s u l t s - c h e m i c a l , p h y s i c a l and s p e c t r o s c o p i c d a t a b a s e s . The f i r s t g e n e r a t i o n o f e x p e r t systems, f o r w h i c h r e a s o n a b l y developed commercial ' s h e l l s ' a r e a v a i l a b l e , i s capable o f c a p t u r i n g t o some degree t h e b e h a v i o r o f one e x p e r t i n a w e l l - u n d e r s t o o d a r e a o f e x p e r t i s e . I n t h i s r e s p e c t our a p p l i c a t i o n i s a t y p i c a l because i t w i l l d e r i v e i t s s t r e n g t h from combining t h e e x p e r t i s e o f a group o f s p e c i a l i s t s . Moreover, p a r t o f t h e knowledge w i l l a c t u a l l y be g a i n e d t h r o u g h t h e p r o j e c t . T h e r e f o r e , we have c h o s e n t o f i r s t c r e a t e ' t o o l b o x e s ' , m a i n l y f i l l e d w i t h c o n v e n t i o n a l programs b u t w i t h easy a c c e s s t o a l l t h e t o o l s needed, t o t e s t and o p t i m i z e p o s s i b l e a n a l y t i c a l s t r a t e g i e s manually. A t a l a t e r stage, the experience gained w i l l be t r a n s l a t e d i n t o an e x p e r t system t o g u i d e t h e a n a l y s t t h r o u g h t h e system, e n a b l i n g h i m t o a p p l y t h e b e s t p o s s i b l e s t r a t e -



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g i e s . S u b s e q u e n t l y , e x p e r t systems w i l l be u s e d t o a c t u a l l y s o l v e a p p r o p r i a t e p a r t s o f the problem. A r t i f i c i a l i n t e l l i g e n c e t e c h n o l o g y i s r e g u l a r l y used, e s p e c i a l l y i n r e s e a r c h e n v i r o n m e n t s , t o s o l v e ' p u z z l e s ' i n v o l v i n g many p i e c e s o f d a t a . Programs o f t h i s type a r e u s u a l l y w r i t t e n d i r e c t l y i n a computer language: P r o l o g , L i s p o r i n a c o n v e n t i o n a l language l i k e F o r t r a n . Good examples i n the f i e l d o f c h e m i s t r y can be found i n r e f e r e n c e s (1) and ( 2 ) . The l a t t e r c o n c e r n s a program by Bremser f o r the combined i n t e r p r e t a t i o n o f NMR ( N u c l e a r M a g n e t i c Resonance), IR ( I n f r a r e d S p e c t r o s c o p y ) and MS (Mass S p e c t r o m e t r y ) r e s u l t s t o f i n d the s t r u c t u r e o f an o r g a n i c m o l e c u l e by s e a r c h i n g l a r g e s p e c t r o s c o p i c d a t a b a s e s . I n the CACSS p r o j e c t , t h i s t y p e o f program c o u l d be used, f o r example, t o d e r i v e a c r y s t a l l o g r a p h i c s t r u c t u r e from powder e x p e r i m e n t s u s i n g s e v e r a l t e c h n i q u e s . A n o t h e r advantage i n t h i s case i s t h a t d i f f r a c t i o n e x p e r i m e n t s ( w i t h X - r a y s , n e u t r o n s o r e l e c t r o n s ) can be f u l l y s i m u l a t e d . A Case Study To i l l u s t r a t e the s t r e n g t h o f a m u l t i - t e c h n i q u e a n a l y t i c a l approach and the k i n d o f s t r a t e g y w h i c h can be chosen, we w i l l b r i e f l y desc r i b e the c h a r a c t e r i z a t i o n o f a t i t a n i u m / v a n a d i u m d e n o x i n g c a t a l y s t . T h i s work was c a r r i e d o u t i n our l a b o r a t o r y and was u s e d as a case s t u d y by the SAS group ( 3 ) . The T i V O C a t a l y s t . I n d u s t r i a l waste gases f r e q u e n t l y c o n t a i n n i t r o u s o x i d e e m i s s i o n s ( N 0 ) . T h i s p o l l u t a n t can be removed by c o p r o p o r t i o n a t i o n w i t h ammonia i n the p r e s e n c e o f a s u i t a b l e c a t a l y s t . V a n a d i a (vanadium o x i d e ) i s an a c t i v e m a t e r i a l t o t h i s end and t i t a n i a ( t i t a n i u m o x i d e ) i s o f t e n used as a s u p p o r t because i t a l s o has a p r o m o t i n g e f f e c t . To improve t h i s c a t a l y s t , and t o make i t cheaper, t i t a n i a and v a n a d i a were d e p o s i t e d on a h i g h - s p e c i f i c - s u r f a c e - a r e a s i l i c a . The most s u c c e s s f u l v e r s i o n was o b t a i n e d by c o i m p r e g n a t i o n o f the a c t i v e compounds. To g a i n i n s i g h t i n t o the w o r k i n g o f the c a t a l y s t i n o r d e r t o f u r t h e r improve i t , an a n a l y t i c a l c h a r a c t e r i z a t i o n was c a r r i e d o u t . The e x p e r i m e n t s f o c u s e d on the s u r f a c e o f the c a t a l y s t where the c a t a l y t i c r e a c t i o n t a k e s p l a c e . Some s u r f a c e - s e n s i t i v e t e c h n i q u e s t o c o n s i d e r , depending on the m a t e r i a l and the k i n d o f r e s u l t s d e s i r e d , a r e AES (Auger E l e c t r o n S p e c t r o s c o p y ) , EELS ( E l e c t r o n Energy Loss S p e c t r o s c o p y ) , L E I S , SAXRD ( S m a l l A n g l e X-Ray D i f f r a c t i o n ) , SEXAFS ( S u r f a c e Extended X - r a y A b s o r p t i o n F i n e S t r u c t u r e ) , SIMS (Secondary I o n Mass S p e c t r o m e t r y ) , STM ( S c a n n i n g T u n n e l i n g M i c r o s c o p y ) , UPS ( U l t r a v i o l e t P h o t o e l e c t r o n S p e c t r o s c o p y ) and XPS. The models d e r i v e d from the s u r f a c e t e c h n i q u e ( s ) s u b s e q u e n t l y have t o be r e c o n c i l e d w i t h the r e s u l t s o b t a i n e d w i t h the b u l k s o l i d - s t a t e t e c h n i q u e s . x

X

S u r f a c e C h a r a c t e r i z a t i o n . I n the p r e s e n t case we s t a r t e d w i t h XPS w h i c h , due t o the low energy o f the e j e c t e d e l e c t r o n s , y i e l d s r e s u l t s on the top 5-15 a t o m i c l a y e r s . S i n c e the energy o f t h e e l e c t r o n s i s c h a r a c t e r i s t i c o f the element found, and even o f i t s v a l e n c e s t a t e , a s e m i - q u a n t i t a t i v e p i c t u r e o f the s u r f a c e coverage o f elements can be deduced. I f a c c u r a t e r e s u l t s o f an o v e r a l l e l e m e n t a l a n a l y s i s a r e a v a i l a b l e and a s e t o f models i s a p p l i e d : f o r example, one o f the models s t a t e s t h a t v a n a d i a i s o n l y found mixed w i t h t i t a n i a , t h e n the a n a l y t i c a l r e s u l t s can be q u a n t i f i e d and





p a r t i c l e s i z e s e s t i m a t e d ( 4 ) . A l l t h e s e models p r e d i c t e d low d i s p e r s i o n s f o r the a c t i v e m a t e r i a l s . The XPS s i g n a l r a t i o s r e l a t i v e t o s i l i c a were r a t h e r low, and c o n s e q u e n t l y r a t h e r l a r g e p a r t i c l e s (20-30 nm f o r the mixed phase model) were c a l c u l a t e d . F u r t h e r m o r e , no i n d i c a t i o n s were f o u n d o f T i and V b e i n g i n a v a l e n c e s t a t e d i f f e r e n t from 4+ and 5+ r e s p e c t i v e l y . U s i n g a complementary t e c h n i q u e , T r a n s m i s s i o n E l e c t r o n M i c r o scopy (TEM), the p a r t i c l e s i z e s were measured i n o r d e r t o choose the b e s t m o d e l ( s ) . I n t h i s way, the l a t e r a l r e s o l u t i o n o f one t e c h n i q u e (TEM) i s combined w i t h t h e s u r f a c e s e n s i t i v i t y o f the o t h e r (XPS) t o y i e l d some p a r a m e t e r s . I n t h i s c a s e , however, the r e s u l t s appeared c o n t r a d i c t o r y . M e a s u r i n g w i t h a r e s o l u t i o n o f 1 nm, TEM c o u l d n o t f i n d t i t a n i a o r v a n a d i a p a r t i c l e s and w i t h EDX ( E n e r g y D i s p e r s i v e X - r a y a n a l y s i s ) i t was c o r r o b o r a t e d t h a t , even on a s m a l l e r s c a l e , the m e t a l s were e v e n l y s p r e a d t h r o u g h the c a t a l y s t . Hence a t l e a s t one o f the b a s i c assumptions u s e d i n the s e t o f models f o r q u a n t i t a t i v e XPS h a d t o be i n c o r r e c t i n t h e p r e s e n t c a s e . Most l i k e l y the XPS s i g n a l s f o r T i and V a r e a t t e n u a t e d by a t h i r d compound. The o n l y o t h e r m a t e r i a l p r e s e n t i s s i l i c a from the supp o r t . No model c o v e r i n g t h i s c o m b i n a t i o n o f compounds i s i n c l u d e d i n t h e s t a n d a r d s e t . The new T i / V / S i model h a d t o be p r o v e n w i t h a n o t h e r a n a l y t i c a l t e c h n i q u e . S p u t t e r i n g o f the p o s s i b l e s i l i c a o v e r l a y e r i n a LEIS e x p e r i m e n t was chosen. T h i s t e c h n i q u e has a s u r f a c e s e n s i t i v i t y s u p e r i o r t o XPS, i s s e n s i t i v e t o e s s e n t i a l l y o n l y t h e topmost atomic l a y e r , b u t i s n o t c a p a b l e o f s e p a r a t i n g t h e T i and V s i g n a l s . LEIS p r o v e d t h a t the main f r a c t i o n o f T i / V was b u r i e d under a t h i n l a y e r o f s i l i c a . S i m i l a r l y , a c o m b i n a t i o n o f powder XRD (X-Ray D i f f r a c t i o n ) and EXAFS ( E x t e n d e d X - r a y A b s o r p t i o n F i n e S t r u c t u r e ) was u s e d f o r the s t r u c t u r a l c h a r a c t e r i z a t i o n o f t h e phases o f t i t a n i a and v a n a d i a . A r e l a t i v e l y r a r e , mixed phase was found, w h i c h may e x p l a i n t h e h i g h catalytic activity. C o n c l u s i o n s from the Case Study. E x e r c i s e s such as t h e s e a r e q u i t e common i n the c h a r a c t e r i z a t i o n o f complex s o l i d s and do i n d e e d r e q u i r e t h e combined e x p e r t i s e o f a group o f s p e c i a l i s t s . The s e t o f t e c h n i q u e s r e q u i r e d v a r i e s from c a s e t o c a s e , b u t t h e more o r l e s s s t a n d a r d c o m b i n a t i o n o f two o r more complementary t e c h n i q u e s as p a r t o f the a r s e n a l i s v e r y u s e f u l . I n r e t r o s p e c t , we were a b l e t o i d e n t i f y t h e t e c h n i q u e s w h i c h were c r u c i a l t o s o l v i n g t h i s problem: XPS/TEM, LEIS, XRD and EXAFS. A number o f o t h e r s ( M a g i c - A n g l e S p i n n i n g NMR (MAS-NMR), Raman S p e c t r o s c o p y and FTIR ( F o u r i e r T r a n s f o r m I n f r a r e d S p e c t r o s c o p y ) were a p p l i e d , b u t d i d n o t add s i g n i f i c a n t l y t o the f i n a l r e s u l t . The s t u d y o f v a r i o u s samples w h i c h were s y n t h e s i z e d i n d i f f e r e n t ways and w h i c h showed d i f f e r e n t c a t a l y t i c a c t i v i t i e s d i d prove r e l e v a n t , but i s not d e s c r i b e d i n d e t a i l here. From c a s e s t u d i e s l i k e t h i s we l e a r n how t o use complementary t e c h n i q u e s and how t o choose the most p r o m i s i n g s t a r t i n g s e t . I t i s c l e a r t h a t i n the p r e s e n t c a s e the use o f o n l y one t e c h n i q u e (XPS, TEM o r L E I S ) would have l e d t o e r r o n e o u s , o r a t l e a s t v e r y incomp l e t e , r e s u l t s on the s u r f a c e t o p o l o g y o f t h e c a t a l y s t . O n l y the use o f a complementary s e t o f t e c h n i q u e s and the combined i n t e r p r e t a t i o n o f t h e i r outcomes y i e l d e d a t r u s t w o r t h y model.


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O u t l i n e o f the CACSS P r o j e c t The p r o c e s s o f c h a r a c t e r i z i n g a complex s o l i d v i a a t r u l y m u l t i t e c h n i q u e a n a l y t i c a l approach has been examined, case s t u d i e s l i k e t h e one d e s c r i b e d above p r o v i d i n g v a l u a b l e i n f o r m a t i o n . The r e s u l t i s a g e n e r a l o u t l i n e f o r the CACSS p r o j e c t w h i c h we w i l l d e s c r i b e i n some d e t a i l h e r e . I t r e f l e c t s our b a s i c i d e a s about how t o p r o c e e d t h r o u g h the a n a l y t i c a l p r o c e s s i n the optimum way, g u i d e d by an e x p e r t system when a p p r o p r i a t e . I n F i g u r e 1 the d i f f e r e n t s t a g e s o f an a n a l y t i c a l e x e r c i s e and the n e c e s s a r y u t i l i t i e s a r e i l l u s t r a t e d s c h e m a t i c a l l y . The l e f t - h a n d sequence o f a c t i o n s resembles the p r o c e d u r e d e s c r i b e d above. The l a r g e box on the r i g h t s y m b o l i z e s the most i m p o r t a n t c o m p u t a t i o n a l f a c i l i t i e s the a n a l y s t ( s ) may i n v o k e d u r i n g a l l s t a g e s o f an a n a l y t i c a l p r o c e s s . I n the f o l l o w i n g pages we s h a l l d e s c r i b e the s y m b o l i c framework o f CACSS, d i s c u s s i n g the d i f f e r e n t s t e p s and the d e c i s i o n s to be made. The C h o i c e o f the O p t i m a l S t a r t i n g Set o f A n a l y t i c a l Techniques. Seldom i s a s o l i d s and s u r f a c e s problem p r e s e n t e d i n pure a n a l y t i c a l terms. T h e r e f o r e , the f i r s t s t e p i s t o d e r i v e a w o r k i n g h y p o t h e s i s w h i c h can r e l a t e the o b s e r v e d b e h a v i o r o f a m a t e r i a l t o i t s p o s s i b l e c o m p o s i t i o n and s t r u c t u r e . T h i s s h o u l d r e s u l t i n a ' m i c r o s c o p i c model', w h i c h can range from a complete, q u a n t i t a t i v e p r o p o s i t i o n t o a mere c l a s s i f i c a t i o n o f the problem a r e a . The d e t a i l o f the model, i n g e n e r a l , r e f l e c t s the d e t a i l o f the d e s i r e d a n a l y t i c a l r e s u l t . T h i s s t e p , an a p r i o r i d e f i n i t i o n o f a w o r k i n g h y p o t h e s i s , i s an imp o r t a n t f a c e t o f the CACSS approach. I t i s an e f f i c i e n t way t o make e x p l i c i t the e x i s t i n g knowledge o f the m a t e r i a l and the e x a c t n a t u r e o f the a n a l y t i c a l problem. A t the same time t h i s model i s conveyed to the computer, e n a b l i n g an automated s e a r c h f o r an o p t i m a l s e t o f a n a l y t i c a l techniques. Even i n t h i s f i r s t s t e p the CACSS system can be u s e d b e n e f i c i a l l y . The m o d e l l i n g o f s t r u c t u r a l p r o p o s a l s ( i n the case o f a newl y s y n t h e s i z e d c r y s t a l l i n e m a t e r i a l , f o r example) can be p e r f o r m e d w i t h t h e a i d o f p o w e r f u l 3d g r a p h i c s . To f o l l o w the example f u r t h e r , one m i g h t choose a known s t r u c t u r e from a c r y s t a l l o g r a p h i c database as a f i r s t model, t o be r e f i n e d d u r i n g the p r o c e s s . However, a n t i c i p a t e d s t r u c t u r a l changes can be made a t the m o d e l l i n g s t a g e and can be combined w i t h t h e o r y ( 5 ) . I n the n e x t s t e p , the most e f f i c i e n t s t a r t i n g s e t o f a n a l y t i c a l t e c h n i q u e s has t o be c o n s t r u c t e d . T h i s s e t s h o u l d be a b l e t o produce i n f o r m a t i o n on the most p r o m i n e n t c h a r a c t e r i s t i c s o f the f i r s t model. T h i s can be done by f o r m a l i z i n g the h e u r i s t i c r e a s o n i n g p r o c e s s o f an ( i d e a l ) a n a l y s t , making the problem a c l e a r - c u t case f o r a f i r s t g e n e r a t i o n ( r u l e - b a s e d ) e x p e r t system. The r e a s o n i n g must i n c l u d e the s e t o f parameters w h i c h has t o be d e t e r m i n e d and the r e q u i r e d p r e c i s i o n , g i v e n the g e n e r a l model as ' f a c t s ' ( e . g . the e l e m e n t a l c o m p o s i t i o n ) . The system s h o u l d c o n t a i n b a s i c i n f o r m a t i o n on most e x i s t i n g t e c h n i q u e s and how they complement each o t h e r . T h i s e n a b l e s the u s e r t o s e l e c t the most a p p r o p r i a t e s e t o f c a n d i d a t e s ( i n c l u d i n g ones u n f a m i l i a r t o h i m / h e r ) . Other p r a c t i c a l c o n s i d e r a t i o n s l i k e the a v a i l a b i l i t y o f the t e c h n i q u e , can be e a s i l y i n c o r p o r a t e d . A f t e r h a v i n g found one o r more p r o m i s i n g s e t ( s ) o f t e c h n i q u e s a t the end o f t h i s h e u r i s t i c p r o c e s s , the u s e r can d e c i d e whether t o


15.

DE BRUUN & KUIPERS


195


check t h e p r o p o s e d s t r a t e g y . To t h i s end, CACSS w i l l p r e d i c t t h e outcomes o f t h e c a n d i d a t e e x p e r i m e n t s . The degree o f p h y s i c a l u n d e r s t a n d i n g u n d e r l y i n g t h e v a r i o u s t e c h n i q u e s w i l l determine whether the f u l l s p e c t r a o r o n l y some p r o p e r t i e s , w h i c h c a n be d e r i v e d from the s p e c t r a i n a d a t a r e d u c t i o n s t e p , a r e s i m u l a t e d . F o r some t e c h n i q u e s a s p e c t r o s c o p i c database c a n be u t i l i z e d t o p r e d i c t a s p e c trum. The r e s u l t may be t h a t t h e s e l e c t e d s e t o f t e c h n i q u e s p r o v i d e s i n s u f f i c i e n t i n f o r m a t i o n , n e c e s s i t a t i n g the r e t u r n t o the h e u r i s t i c search step. The I n t e r p r e t a t i o n o f E x p e r i m e n t a l R e s u l t s . Once t h e u s e r has deduced t h e s e t o f t e c h n i q u e s w h i c h c a n s e r v e as a c r u c i a l t e s t f o r the f i r s t model, t h e a c t u a l experiments have t o be performed. The s p e c t r a o f those t e c h n i q u e s w h i c h c a n be f u l l y s i m u l a t e d have t o be t r a n s f e r r e d t o CACSS v i a t h e l a b o r a t o r y computer network. T h i s e n a b l e s a combined i n t e r p r e t a t i o n , i n i t s s i m p l e s t form by u s i n g t h e r e s u l t s o f one t e c h n i q u e as c o n s t r a i n t s f o r t h e i n t e r p r e t a t i o n o f the o t h e r . F o r t h e r e m a i n i n g t e c h n i q u e s a s t a n d a r d format has t o be d e f i n e d i n w h i c h t h e reduced d a t a a r e p r e s e n t e d t o CACSS. These d a t a a r e produced by s p e c i a l i z e d s o f t w a r e on t h e d e d i c a t e d computers o f the a n a l y t i c a l i n s t r u m e n t s . T h i s approach i s p r e f e r r e d t o i n c o r p o r a t i n g a l l t h e d a t a r e d u c t i o n s t e p s i n t o t h e CACSS system because i n t e r p r e t a t i o n o f t h e raw d a t a r e q u i r e s t h e e x p e r i e n c e o f the e x p e r t i n t h e i n d i v i d u a l t e c h n i q u e . I f , however, a s t a g e o f i n t e r p r e t a t i o n has been r e a c h e d where presumptions about t h e model o f t h e m a t e r i a l have t o be made, t h e r e s u l t s have t o be t r a n s f e r r e d t o CACSS. The e x p e r i m e n t a l r e s u l t s c a n now be u s e d f o r c o n f i r m i n g o r r e j e c t i n g t h e f i r s t model. T h i s i n v o l v e s c h e c k i n g t h e match between p r e d i c t e d and measured d a t a . T h i s s t e p i s c r i t i c a l because r e j e c t i o n i m p l i e s t h a t an e n t i r e l y new model and s t r a t e g y be developed. I f t h e r e j e c t i o n o f t h e o r i g i n a l model was n o t j u s t i f i e d , one c o u l d spend a l o n g time f o l l o w i n g t h e wrong t r a c k b e f o r e r e t u r n i n g t o t h e o r i g i n a l model. However, i f one has t h e e v i d e n c e and t h e m u t u a l i m p l i c a t i o n s o f s e v e r a l t e c h n i q u e s a t hand, t h e r i s k o f making such a m i s t a k e i s g r e a t l y reduced. The c r i t e r i o n f o r r e j e c t i o n i s t h a t t h e f r e e parameters o f a model cannot be a d j u s t e d such t h a t a l l t h e e x p e r i m e n t a l d a t a a r e r e produced. I f t h e model cannot be r e j e c t e d , o r f u l l y c o n f i r m e d , o t h e r d e c i s i v e experiments have t o be i d e n t i f i e d . S i n c e , however, some o f the o r i g i n a l l y f r e e parameters a r e now f i x e d by t h e q u a n t i t a t i v e r e quirements o f t h e e x p e r i m e n t a l r e s u l t s , t h i s has become e a s i e r . Once a model i s r e g a r d e d as b e i n g b o t h s u f f i c i e n t l y t r u s t w o r t h y and det a i l e d , t h e problem has been s o l v e d . The chance t h a t a d i f f e r e n t model a l s o f i t s t h e e x p e r i m e n t a l d a t a c a n never be f u l l y e x c l u d e d . The m u l t i - t e c h n i q u e approach, howe v e r , reduces t h i s chance c o n s i d e r a b l y . F o r some c a s e s , i t i s p o s s i b l e t o use r e f i n e m e n t t e c h n i q u e s (6) to o b t a i n very accurate s t r u c t u r a l data. This i s e s p e c i a l l y true f o r the w e l l u n d e r s t o o d d i f f r a c t i o n t e c h n i q u e s and, t h e r e f o r e , f o r c h a r a c t e r i z a t i o n s o f p o l y c r y s t a l l i n e m a t e r i a l . W i t h an i n c r e a s i n g l e v e l o f p h y s i c a l u n d e r s t a n d i n g t h e number o f t e c h n i q u e s w h i c h a r e s u i t a b l e f o r use i n r e f i n e m e n t i s bound t o grow. Because o f t h e ' l o c a l minimum problem', t h e s t a r t i n g model s h o u l d a l r e a d y be c l o s e t o r e a l i t y f o r s i n g l e - t e c h n i q u e r e f i n e m e n t s . We e x p e c t t h a t t h e


196


i m p l e m e n t a t i o n o f a s i m u l t a n e o u s , m u l t i - t e c h n i q u e r e f i n e m e n t (2) w i l l p a r t i a l l y s o l v e t h i s problem because o f a s t r o n g e r convergence i n the i t e r a t i o n p r o c e s s .


Implementation

o f the P r o j e c t

The i m p l e m e n t a t i o n o f the f u l l CACSS p r o j e c t i s a major e f f o r t , r e q u i r i n g q u i t e a number o f man-years. T h e r e f o r e , i t was d e c i d e d t o f i r s t d e v e l o p a u s e f u l s u b s e t o f the system. The s t r u c t u r a l e l u c i d a t i o n o f p o l y c r y s t a l l i n e m a t e r i a l was chosen, because t h i s t a s k can be w e l l s e p a r a t e d and because t h i s s u b s e t w o u l d be v e r y u s e f u l t o us now. F u r t h e r , t h i s s m a l l e r p r o j e c t w i l l p r o v e the f e a s i b i l i t y o f the o v e r a l l p r o j e c t . I n t h i s r e s p e c t , i t can be c o n s i d e r e d as a case s t u d y f o r the f u l l CACSS system. The p r o j e c t has been c h r i s t e n e d NUT, a f t e r the E g y p t i a n goddess o f the sky. NUT w i l l have a modular c o n s t r u c t i o n l i k e CACSS i t s e l f ( F i g u r e 1 ) . S t r u c t u r a l m o d e l l i n g w i t h the a i d o f 3d g r a p h i c s w i l l p l a y a more c e n t r a l r o l e h e r e . The c r e a t i o n o f a d e d i c a t e d computer program f o r t r u l y c r y s t a l l o g r a p h i c m o d e l l i n g was the f i r s t subp r o j e c t we embarked upon two y e a r s ago. Work i s a l s o under way t o d e s i g n and implement a framework t o i n t e g r a t e t h i s program, and e x i s t i n g c o n v e n t i o n a l programs used f o r the i n t e r p r e t a t i o n o f exper i m e n t a l r e s u l t s , i n t o a ' t o o l b o x ' f o r the a n a l y s t . By u s i n g t h i s system we hope t o a c q u i r e s u f f i c i e n t e x p e r i e n c e on the most e f f i c i e n t way t o use the m u l t i - t e c h n i q u e approach i n t h i s f i e l d . T h i s e x p e r i e n c e w i l l e n a b l e us t o b u i l d an a d v i s o r y e x p e r t system t o g u i d e the a n a l y s t t h r o u g h the t o o l b o x . An e x p e r t system t o f i n d the o p t i m a l s t a r t i n g s e t o f t e c h n i q u e s i s l e s s i m p o r t a n t h e r e because d i f f r a c t i o n t e c h n i q u e s always p l a y a c e n t r a l r o l e . To e n a b l e us t o g a i n e x p e r i e n c e i n the use o f e x p e r t systems i n a n a l y t i c a l c h e m i s t r y , b e t t e r u n d e r s t o o d a r e a s have been chosen. An e x p e r t system s h e l l i s b e i n g used t o d e v e l o p a p r o t o t y p e o f an e x p e r t system f o r s e l e c t i n g a n a l y t i c a l t e c h n i q u e s i n the f i e l d o f e l e m e n t a l a n a l y s i s . I n a d d i t i o n , a P r o l o g program i s b e i n g developed to a s s i s t i n the p l a n n i n g and i n t e r p r e t a t i o n o f combined XPS and TEM e x p e r i m e n t s o f heterogeneous c a t a l y s t s , as d e s c r i b e d i n the case s t u d y . T h i s program t e s t s the t o p o l o g i c a l models, and c o m b i n a t i o n s t h e r e o f , u s i n g the e x p e r i m e n t a l r e s u l t s o b t a i n e d by XPS, TEM and EDX. There a r e a l s o r u l e s implemented t o check f o r v i o l a t i o n s o f the b a s i c assumptions made i n the s e t o f models. F i n a l l y , i f r e q u i r e d , a d v i c e i s g i v e n on a d d i t i o n a l e x p e r i m e n t s . S t r u c t u r e E l u c i d a t i o n from C r y s t a l Powders. For many p r a c t i c a l mat e r i a l s , such as polymers and z e o l i t e c a t a l y s t s , i t i s i m p o s s i b l e t o s y n t h e s i z e l a r g e c r y s t a l s . T h e r e f o r e the s t r u c t u r e has t o be found from powders. Powder XRD ( p r e f e r a b l y u s i n g s y n c h r o t r o n r a d i a t i o n ) and n e u t r o n d i f f r a c t i o n a r e the most i m p o r t a n t t e c h n i q u e s , b u t exper i m e n t s u s i n g o t h e r a n a l y s i s methods l i k e H i g h R e s o l u t i o n E l e c t r o n M i c r o s c o p y (HREM) and E l e c t r o n D i f f r a c t i o n (ED), MAS-NMR and EXAFS can add v a l u a b l e i n f o r m a t i o n ( 8 ) . The f i r s t s t e p i n the c h a r a c t e r i z a t i o n i s i n d e x i n g o f powder d i f f r a c t o g r a m ( s ) , i f p o s s i b l e a l s o u s i n g ED d a t a . T h i s y i e l d s , i f s u c c e s s f u l , the parameters o f the u n i t c e l l and, t h r o u g h e x t i n g u i s h e d d i f f r a c t i o n peaks, i n d i c a t i o n s o f the p o s s i b l e symmetry groups. I n the n e x t s t e p , the p o s i t i o n s o f the atoms i n the


15. D E B R U U N & KUIPERS


PROBLEM

MICROSCOPIC MODEL


CHOICE OF ANALYTICAL TECHNIQUES

COMPUTER GRAPHICS EXPERIMENT SIMULATIONS DATABASES MODEL REFINEMENTS

INTERPRETATION

(BEST) SOLUTION Figure 1. Schematic outline of the Computer-Aided Characterization of Solids and Surfaces project.

asymmetric u n i t have t o be found. S e v e r a l approaches e x i s t f o r t h i s . I n f a v o r a b l e c a s e s (9), w i t h a l a r g e number o f w e l l - r e s o l v e d peaks, ' d i r e c t ' methods c a n be a p p l i e d (10). Such methods, w h i c h i t e r a t i v e l y seek t h e phases o f t h e d i f f r a c t e d beams, have been d e v e l o p e d f o r s i n g l e c r y s t a l XRD. The r e s u l t c a n be improved w i t h R i e t v e l d r e f i n e m e n t (6), a l e a s t squares r e f i n e m e n t o f t h e s t r u c t u r a l model a g a i n s t t h e f u l l d i f f r a c t o g r a m . T h i s s t e p c a n be b e s t c o n d u c t e d on a n e u t r o n d i f f r a c t o g r a m , because n e u t r o n s m a i n l y 'see' t h e atomic nuclei. I n many c a s e s , however, one has t o r e l y on t r i a l and e r r o r methods t o f i n d t h e atom p o s i t i o n s from t h e i n t e n s i t i e s o f t h e d i f -


197

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f r a c t i o n l i n e s . Through s i m u l a t i o n s o f t h e d i f f r a c t o g r a m s one has t o t r y t o f i n d the atom c o o r d i n a t e s . I n t h i s p r o c e s s , d a t a o b t a i n e d by o t h e r t e c h n i q u e s c a n be u s e d as i m p o r t a n t c o n s t r a i n t s . D i s t a n c e geometry (11) o r o t h e r c a l c u l a t i o n s (5) c a n be u s e d t o o p t i m i z e s t r u c t u r a l p r o p o s a l s and d a t a b a s e s c a n be u s e d t o f i n d comparable s t r u c t u r e s ( e . g . o n l y d i f f e r i n g i n atom t y p e s and u n i t c e l l l e n g t h s ) . To f a c i l i t a t e the whole e x e r c i s e , b r i d g i n g the gap between i n d e x i n g and automated r e f i n e m e n t , we a p p l y a 3d c r y s t a l l o g r a p h i c m o d e l l i n g program w h i c h i s c a p a b l e o f c o n s e r v i n g t h e p a r a m e t e r s ( u n i t c e l l d i m e n s i o n s and symmetry) w h i c h a r e f i x e d by the i n d e x i n g . Crystallographic Modelling. In c o l l a b o r a t i o n w i t h the c r y s t a l l o graphy group a t the U n i v e r s i t y o f Amsterdam ( P r o f . H. Schenk), the program PLUVA f o r the m o d e l l i n g o f c r y s t a l s t r u c t u r e s has b e e n w r i t t e n ( 1 2 ) . PLUVA runs on an Evans & S u t h e r l a n d PS390 3d g r a p h i c s comp u t e r . I n c o n t r a s t t o m o l e c u l a r m o d e l l i n g programs, w h i c h c o n s i d e r a ( m o l e c u l a r ) s t r u c t u r e as an e n t i t y , PLUVA u s e s the symmetry o f the s t r u c t u r e i n the d i s p l a y i t s e l f . I f an atom, o r a group o f atoms, i n one asymmetric u n i t i s moved i n t e r a c t i v e l y , a l l symmetry e q u i v a l e n t atoms move a t the same t i m e . I n t h a t way, t h e symmetry o f t h e s t r u c t u r e i s p r e s e r v e d . The f a c i l i t i e s t o v i e w t h e s t r u c t u r e i n t h r e e d i m e n s i o n s , m a n i p u l a t e i t i n r e a l time, and d i s p l a y i t i n a c o n v e n i e n t s t y l e ( s t i c k , s t i c k and b a l l , e t c . ) p r o v i d e a v e r y p o w e r f u l a i d f o r s t r u c t u r e e l u c i d a t i o n from powders. I n f a c t , t h e a v a i l a b l e s e a r c h space has b e e n made v i s i b l e and d i r e c t l y a c c e s s i b l e . A powder XRD s i m u l a t i o n program i s c o u p l e d i n t e r a c t i v e l y t o PLUVA, a l l o w i n g a l m o s t r e a l time s i m u l a t i o n o f a d i f f r a c t o g r a m from the s t r u c t u r e on the s c r e e n and c o m p a r i s o n w i t h the e x p e r i m e n t a l d i f f r a c t o g r a m . I n t h i s way a f a s t 'manual' r e f i n e m e n t c a n be p e r formed, w i t h d a t a i n t r o d u c e d from o t h e r t e c h n i q u e s s e r v i n g as c o n s t r a i n t s . One p o s s i b l e c o n s t r a i n t would be k e e p i n g two atoms t o g e t h e r whose i n t e r n u c l e a r d i s t a n c e has b e e n measured. I n a d d i t i o n t h e a n a l y s t c a n a p p l y h i s e x p e r i e n c e and i n t u i t i o n . F o r example, two atoms cannot occupy the same space ( t h e t a s k i s i n f a c t t o f i l l a f i x e d volume w i t h a f i x e d s e t o f s p h e r e s ) , a f a c t w h i c h i s v e r y h a r d to t a k e advantage o f i n d i r e c t methods. A framework f o r NUT. To make t h e a f o r e m e n t i o n e d ' t o o l b o x ' o p e r a t i o n a l , a u s e r - f r i e n d l y environment i s needed. F o r t h i s p u r p o s e a framework i s now under c o n s t r u c t i o n t h r o u g h w h i c h t h e u s e r c a n model a c r y s t a l s t r u c t u r e and i n v o k e the v a r i o u s programs and u t i l i t i e s . T h i s c a n be done w i t h m i n i m a l knowledge about t h e way t h e s e programs a r e t o be o p e r a t e d and w i t h o u t the d i s t r a c t i n g and time-consuming t a s k s o f r e f o r m a t t i n g d a t a and m a i n t a i n i n g e x t e n s i v e f i l e management. The d a t a on a p a r t i a l l y s o l v e d s t r u c t u r e a r e s t o r e d i n a d a t a b a s e i n a s t a n d a r d way, the h i s t o r y o f the c a s e b e i n g i n c l u d e d . A l l k i n d s o f f a c i l i t i e s , from i n d e x i n g programs t o r e f i n e m e n t , a r e i n v o k e d t h r o u g h p r e - and p o s t - p r o c e s s i n g r o u t i n e s w h i c h g a t h e r and t r a n s f o r m a l l n e c e s s a r y data, o n l y q u e r y i n g back to the u s e r f o r u n i q u e d a t a . The programs a r e c a l l e d from menus and a t e v e r y s t a g e o f the e l u c i d a t i o n p r o c e s s the s t r u c t u r e c a n be v i e w e d and f u r t h e r m o d e l l e d w i t h PLUVA, i n c l u d i n g s e m i - r e a l - t i m e s i m u l a t i o n s o f s e v e r a l d i f f r a c t i o n t e c h n i q u e s . The p o s s i b i l i t y o f i m p l e m e n t i n g an a d v i s o r y e x p e r t system has been e x p l i c i t l y t a k e n i n t o a c c o u n t i n the d e s i g n o f the framework.





Conclusions A n a l y s i s o f t h e m u l t i - t e c h n i q u e approach t o t h e c h a r a c t e r i z a t i o n o f complex s o l i d s and s u r f a c e s has d e m o n s t r a t e d t h e need f o r f a r r e a c h i n g computer a s s i s t a n c e and, a t t h e same t i m e , t h a t t h i s need c a n l a r g e l y be met by a p p r o p r i a t e i n t e g r a t i o n o f systems a v a i l a b l e t o d a y . E x p e r t systems and c o n v e n t i o n a l programs w i l l be combined i n t h e Computer-Aided C h a r a c t e r i z a t i o n o f S o l i d s and S u r f a c e s (CACSS) p r o j e c t . The s t r a t e g i e s r e q u i r e d t o p e r f o r m t h e a n a l y t i c a l t a s k i n an o p t i m a l way a r e g r a d u a l l y b e i n g d i s c o v e r e d d u r i n g t h e p r o j e c t . I m p l e m e n t a t i o n has s t a r t e d w i t h NUT, w h i c h i s t h e p a r t o f CACSS c o n c e r n i n g t h e s t r u c t u r a l e l u c i d a t i o n o f p o l y c r y s t a l l i n e powders. A framework t o c o n n e c t a l a r g e number o f e x i s t i n g programs needed d u r i n g t h e e l u c i d a t i o n p r o c e s s i s under c o n s t r u c t i o n . A t r u l y c r y s t a l l o g r a p h i c m o d e l l i n g program has been c r e a t e d w h i c h a l l o w s f a s t n a v i g a t i o n t h r o u g h t h e r e m a i n i n g s e a r c h space. I n a d d i t i o n , a P r o l o g program i s under development w h i c h i s a b l e t o i n t e r p r e t , i n c o m b i n a t i o n , XPS, TEM and EDX r e s u l t s f o r s u r f a c e c h a r a c t e r i z a t i o n .

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

Bremser, W.; Angewandte Chemie 1988, 27, 247. Liebman, S. A. In Artificial Intelligence Applications in Chemistry: Hohne and Pierce, Eds.; ACS Symposium Series No. 306; American Chemical Society: Washington, DC, 1986. Kuipers, H. P. C. E . , Boxhoorn, G., De Bruijn, D. P., Brongersma, H. H. et al.; submitted for publication in Applied Catalysis. Kuipers, H. P. C. E . , Van Leuven, H. C. E . , Visser, W. M.; Surf. and Interf. Analysis 1986. 8, 235. Catlow, C. R. A . , Mackrodt, W. C. Computer Simulations of Solids: Lecture Notes in Physics No. 166; Springer-Verlag: Berlin, 1982. Rietveld, H. M.; J. Appl. Cryst. 1969, 2, 65. Maichle, J. K . , Ihringer, J., Prandl, W.; J. Appl. Crystallogr. 1988, 21, 22. Cheetham, A. K.; Proc. Indian Natn. Sci. Acad. 1986, 52, A, No. 1, 25. Christensen, A. N . , Lehmann, M. S., Nielsen, M.; Aust. J. Phys. 1985, 38, 497. Woolfson, M. M.; Acta Cryst. 1987, A43, 593. Barrer, R. M., V i l l i g e r , H . ; Z. Kristallogr. 1969, 128, 352. Driessen, R. A. J.; Loopstra, B. O., De Bruijn, D. P., Kuipers, H. P. C. E . , Schenk, H . ; J. Comput.-Aided Molec. Design 1988, 2, No. 3, 225.

RECEIVED June 9, 1989


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