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INDUSTRIAL APPLICATIONS OF EXAFS/XANES
R. Oldman
To cite this version:
R. Oldman. INDUSTRIAL APPLICATIONS OF EXAFS/XANES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-321-C8-325. �10.1051/jphyscol:1986863�. �jpa-00226185�
JOURNAL DE P H Y S I Q U E
Colloque C 8 , suppl6ment a u n o 12, Tome 47, d6cembre 1986
INDUSTRIAL APPLICATIONS OF EXAFS/XANES
R . J . OLDMAN
Technical Department, ICI Mond Division, The Heath, GB-Runcorn, Cheshire, Great-Britain
Abstract
Direct industrial application of x-ray absorption spectroscopy is demonstrated by research in catalysis (CO/H2 and naphtha reforming) and corrosion
protection.
1 Introduction
X-ray absorption spectroscopy (XAS) has rapidly become the major technique associated with real industrial synchrotron applications. This stems from the very particular advantages offered. Structural information can be derived in a wide range of non-crystalline situations including wet materials or solutions and it is a resonance technique so that a specific kind of atom centre can be investigated. Using fairly hard x-rays material is accessible at interfaces, in pores or in reqlistic sample environments.
Finally the technique can be made surfacelinterface specific under conditions for total reflection. These points are illustrated by examples drawn from CO/H2 and naphtha reforming catalysis and corrosion protection carried out experimentally at Daresbury SRS.
Porous Oxide Supported Rhodium Catalysts
It has been shown that selectivity in the conversion of CO/H2 to ethanol depends markedly on catalyst preparation procedures 1 1 3 and in particular on the rhodium precursor. The existence of small particles on very porous and sometimes heavy supports prevented techniques such as XPS and TEM from giving a complete structural picture. Fig 1 shows Rh K edge EXAFS ?or rhodium foil and a series of such catalysts following reduction in situ in flowing Hz at 300°C in a treatment rig [2]. For 1%Rh on SiOZ measurements were carried out in transmission but for heavy oxide supports (W03) the rig has been modified to allow fluorescence detection.
Table 1 lists the first shell co-ordination numbers and bond radii derived from the EXAFS by curve fitting using the spherical wave scattering model contained in the program EXCURVE at Daresbury 131. In this program
calculated phase shifts are refined against standard materials. Also shown in table 1 are Rh crystallite sizes estimated from the co-ordination numbers using published calculations for ideal cuboctahedral crystals [ 4 1 .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986863
The size information correlates nicely with changes in selectivity. In order to understand more fully the differences in fig 1 data for Rh foil was first examined in detail. Fig 2a shows the computer fit for four shells of the structure using standard crystallographic data. The fit is good but the splitting at 10.21-l(395 ev) is not reproduced and comparison
Fig 1 Rh K edge EXAFS (filtered) Fig 2 Comparison of experimental for (a) Rh foil (b)-(d) Reduced (-) and theoretical (---) k3 EXAFS 1% catalysts on Si02 derived for the first 4 shells of Rh in foil from (b) Rh(N03)3 (c) Rh203 (a) without and (b) with multiple
(d) RhC13 scattering
of the fourier transforms indicates that the fourth shell contribution is substantially underestimated. Fig 2b shows the fit including the multiple scattering term due to the linear configuration involving the central atom, first shell and fourth shell and demonstrates that the feature at 10.211-I provides a guide to the growth of crystallites in terms of the fraction of atoms which have a complete ordered fcc environment. Catalysts can therefore be quickly differentiated. We have not yet completed
multishell modelling for the catalysts but the fourier transforms show the development of higher shells with size and confirms the importance of crystallite size and surface geometry in controlling catalyst selectivity.
TABLE 1
EXAFS results for reduced oxide supported 1% Rh catalysts
Rh Rh-Rh (1) 2e2(112)* Crystallite diameter (1)
Rh Foil 12 2.69 0.007
* 2m2 is a Debye-Waller factor including static and thermal disorder.
)cooled with liquid N2. Otherwise a11 measurements at room temperature.
This in turn depends on precursor and support. We have seen no evidence for support/metal interactions C51 but this is not surprising since even for 20E particles it is estimated that as few as 4% of the bonds could involve oxygen.
3 Alumina SuDDorted Pt/Re Na~htha reform in^: Catalvsts
The preparation of reforming catalysts involves a number of classic but poorly understood basic steps such as impregnation, calcination and reduction followed by modifying steps such as sulphiding. This paper concentrates on sulphiding because it illustrates the importance of XAS as a resonance technique. An important part of the work too has been the use of edge data for interpretation. Fig 3 shows Pt L3 edges for different oxidation states. At the edge absorption occurs via a fully allowed pseudo-atomic transition, 2p+5d, with intensity depending on the number of unoccupied valence levels [a]. There is an excellent correlation between the normalised white line amplitude and oxidation state. A similar corre- lation exists for ReL3.
Fig 3 Pt L3 edge XAS (displaced for clarity) for some standard materials showing the correlation between normalised white line intensity (A /A ) and oxidation state
E Pt
(a) Pt foil (b) PtCl2 (c) Pt02.2H20 (d) & I ~ P ~ ( O H ) ~ (e) H2PtC16 (f) PtS
Fig 4 shows Pt and Re Lg XAS data for 2.5% Pt and Re on Condea A1203 for two sulphiding procedures giving widely different catalyst performance.
Using the edge data (fig 4) with the fourier transforms (fig 5) and detailed computer fitting shows that poor performance is associated with complete conversion of Pt to PtS (4s atoms at 2.318) while Re is not sulphided at all. The revised sulphiding procedure gives partial conversion of both Pt and Re with PtS ( 50%) forming a coating on Pt preventing air oxidation while Re is clearly not protected in this way.
Under in situ reduction conditions at 450°C PtS is completely reduced to Pt fcc crystallites while Re S2 remains. Detailed computer fitting (fig 6) gives 42% KeS2/58% Re.
Fig 4 Pt and Re L3 XAS for 2.5%
Pt/2.5% Re on alumina
(a) original sulphiding procedure followed by air exposure
(b) revised procedure and air exposure (c) (b) reduced in Hz at 450°C 11.2 11.6 12.0 10.2 10.6 11.0
KeV
JOURNAL DE PHYSIQUE
F i g 5 k3 weighted f o u r i e r t r a n s f o r m s w i t h o u t phase s h i f t s f o r 2.5% Pt/2.5% Re on alumina
( a ) , ( b ) o r i g i n a l s u l p h i d i n g p r o c e d u r e followed by a i r exposure
( c ) r e v i s e d procedure and a i r exposed
( d ) ( c ) reduced i n H2 a t 450°C
F i g 6 Computer f i t t o t h e f i l t e r e d Re L j EXAFS f o r reduced s u l p h i d e d c a t a l y s t
( e x p t -, t h e o r y ---) g i v i n g a v e r a g e c o - o r d i n a t i o n numbers of 2.5s atom3 a t 2.308 and 7.1 Re a t 2.748 i e 42%
o c t a h e d r a l ReS2 w i t h 58% Re.
4 S u r f a c e Coatings and 2 Dimensional X-ray Techniques
Coatings f o r c o r r o s i o n p r o t e c t i o n o r a d h e s i v e s can r e a d i l y be t e s t e d by c o n v e n t i o n a l s a l t s p r a y o r mechanical t e s t s . I n f o r m a t i o n on t h e
i n t e r f a c i a l c h e m i s t r y r e s p o n s i b l e f o r t h e i m p o r t a n t b a r r i e r o r a d h e s i o n p r o p e r t i e s is less e a s i l y derived. F l u o r e s c e n c e EXAFS p r o v i d e s a n
o p p o r t u n i t y t o l o o k a t i n t e r f a c e s n o n - d e s t r u c t i v e l y i n s i t u . To t e s t t h i s t h i n l a y e r s of m e t a l s (7-100nm) were d e p o s i t e d on g l a s s and coated.
Thicknesses of m e t a l s were chosen s o t h a t t h e e n t i r e l a y e r was consumed d u r i n g t h e e n s u i n g i n t e r f a c i a l r e a c t i o n . F i g 7 shows r e s u l t s f o r
' H a l o f l e x ' , a n I C I water based v i n y l c h l o r i d e / v i n y l i d e n e d i c h l o r i d e polymer emulsion w i t h i m p r e s s i v e c o r r o s i o n p r o t e c t i o n p r o p e r t i e s . A l l t h e o r i g i n a l Fe r e a c t s and t h e r e a c t i o n product depends on c o a t i n g pH.
Comparison of t h e XANES r e g i o n t o d e f i n e pre-edge f e a t u r e s and edge p o s i t i o n ( f i g 7 ) w i t h a s e r i e s of Fe ( 1 1 ) and Fe ( I I I ) compounds and d a t a i n r e f e r e n c e 161 g i v e s Fe (11) a t pH 2 and Fe (111) a t pH 4.5 w i t h a s e p a r a t i o n of 3ev. The pre-edge spectrum, EXAFS, f o u r i e r t r a n s f o r m s and computer f i t t i n g i n d i c a t e o c t a h e d r a l f i r s t s h e l l c o - o r d i n a t i o n w i t h 40 a t 2.16x p l u s 2C1 a t 2.61x and 60 a t 2.09A r e s p e c t i v e l y approximating t o t h e environment round Fe i n FeC12.4H20 and Fe (111) o x i d e systems.
F i g 7 Fe K edge XAS, edge s p e c t r a and f o u r i e r t r a n s f o r m s (without phase s h i f t s ) f o r ( a ) 6 0 ' H a l o f l e x ' a t pH2 on 300A Fe supported on g l a s s ( b ) pH ~ 4.5 c o a t i n g ( c ) 3001 Fe on g l a s s
The above r e s e a r c h has r e l i e d on c a r e f u l l y c o n t r o l l e d t h i c k n e s s e s of m e t a l f i l m s . However by working a t t h e a p p r o p r i a t e c r i t i c a l a n g l e of i n c i d e n c e X-rays w i l l t o t a l l y r e f l e c t a t an i n t e r f a c e . The r e s u l t a n t r e f l e c t e d (REFLEXAFS), s c a t t e r e d and f l u o r e s c e n c e (FREXAFS) beams a l l c o n t a i n s t r u c t u r a l i n f o r m a t i o n about t h e i n t e r f a c i a l r e g i o n L71. These t e c h n i q u e s a r e being developed j o i n t l y with t h e U n i v e r s i t y of S t r a t h c l y d e t o examine s o l i d / s o l i d and l i q u i d j s o l i d i n t e r f a c e s of importance t o t h e chemical i n d u s t r y , f o r example, c o r r o s i o n p r o t e c t i o n .
So f a r we have been a b l e t o d e t e c t t h e oxide f i l m s on Fe, Ag and Cu, t h u s confirming s u r f a c e s p e c i f i c i t y , and a metal s u r f a c e under l i q u i d and i t s subsequent m o d i f i c a t i o n f o l l o w i n g chemical r e a c t i o n . This is i l l u s t r a t e d by t h e f o r m a t i o n of s i l v e r s u l p h i d e on Ag under s a t u r a t e d aqueous H2S [81.
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