SURFACE STRUCTURE AND COMPOSITION CHANGES ON PLATINUM - RHODIUM ALLOY CATALYSTS

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SURFACE STRUCTURE AND COMPOSITION CHANGES ON PLATINUM - RHODIUM ALLOY

CATALYSTS

A. Mccabe, G. Smith

To cite this version:

A. Mccabe, G. Smith. SURFACE STRUCTURE AND COMPOSITION CHANGES ON PLATINUM - RHODIUM ALLOY CATALYSTS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-483-C9-488.

�10.1051/jphyscol:1984980�. �jpa-00224469�

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Colloque C9, supplément au n012, Tome 45, décembre 1984 page C9-483

SURFACE STRUCTURE AND C O M P O S I T I O N CHANGES ON P L A T I N U M

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RHODIUM A L L O Y C A T A L Y S T S

A.R. McCabe and G.D.W. Smith

Department of MetaZZurgy and Science of Materiazs, University of Oxford, Parks Road, Oxford 0x1 3PH, U. K.

Résumé - Les catalyseurs en toile de platine-rhodium utilisés au cours de la fabrication de l'acide nitrique sont l'objet de processus de reconstruction de surface étendus. Ceci a été examiné à l'aide d'un réacteur catalytique miniature et de techniques telles que la sonde à atomes (AF-PIM), la microscopie électronique, et les rayons X. Un mé- canisme impliquant le transport en phase vapeur est proposé pour expliquer les principaux traits de la variation du comportement du catalyseur dans les conditions opératoires.

Abstract - Platinum-rhodium gauze catalysts used in the manufacture of nitric acid undergo an extensive surface reconstruction process. This has been investigated using a miniature catalytic reactor, FIM atom probe, electron microscopy and X-ray techniques. A mechanism involving vapour transport is proposed to explain the main features of the variation in catalyst behaviour with operating conditions.

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INTRODUCTION In the manufacture of nitric acid, ammonia is oxidised over a platinum

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rhodium gauze catalyst, the reaction being both strongly oxidising and highly exothermic. The remarkable surface reconstruction process occurring during operation has long been known to exist [l-4

1,

but no detailed understanding of the mechanism of, or limitations on, this reconstruction are known. We report an investigation of this problem, using a range of microscopical and microanalytical techniques.

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EXPERIMENTAL PROCEDURE A miniature catalytic reactor was constructed to simulate the reconstruction taking place, the pressure and flow rates being as for a commercial plant. The apparatus allowed the reaction to be stopped at various stages during the reconstruction process, the gauzes being studied by SEM and EPMA. FIM samples could also be placed in the reactor directly behind the rear gauze, these being examined later using both FIM and atom probe techniques. The gas mixture in the reactor was also varied to investigate the range of conditions over which the reconstruction process occurred.

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AIR OXIDATION Initially FIM samples made of the catalyst material were oxidised in air for 1 hour at 800°C giving a 200

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3008 thick oxide film. A concentration depth profile through this film showed substantial enrichment of the oxide with rhodium. However a similar profile for 600°C indicated that the outer region of the oxide was not as enriched in rhodium as the bulk oxide. The loss of platinum as PtOg vapour was suggested (this recondensing on the outer surface under certain cooling conditions), and the possibility that the surface

reconstruction on the catalysts was also due to a vapour transport process was suggested. A detailed account of this work has been given previously [5,6].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984980

C9-484 JOURNAL DE PHYSIQUE

Fig. 1

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Scanning e l e c t r o n micrographs of gauze s u r f a c e s .

( a ) Hydrogen flamed, showing r e c r y s t a l l i s a t i o n and thermal grooving a t g r a i n boundries.

( b ) Exposed t o 10% ammonia-air mixture f o r 2 hours i n c a t a l y t i c r e a c t o r , showing development of deep c a v i t i e s a l o n g g r a i n boundaries.

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AMMONIA OXIDATION

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GAUZES A gauze i n i t s as-received s t a t e shows t y p i c a l w i r e drawing marks on i t s s u r f a c e . However, a f t e r p r e t r e a t m e n t (which i n v o l v e s f l a m i n g w i t h a hydrogen t o r c h ) , r e c r y s t a l l i s a t i o n occurs and g r a i n s become c l e a r l y v i s i b l e , w i t h grooves along t h e g r a i n boundaries ( f i g u r e l a ) . A f t e r only 2 hours i n t h e c a t a l y t i c r e a c t o r t h e s u r f a c e has a l r e a d y s t a r t e d t o r e c o n s t r u c t ( f i g u r e l b ) . The development of deep c a v i t i e s a l o n g t h e g r a i n boundaries i s c l e a r l y seen.

A mechanism which e x p l a i n s t h e f o r m a t i o n of t h e s e c a v i t i e s i s a s f o l l o w s .

Consider t h e boundary between two g r a i n s ( f i g u r e 2). The f e e d gas c o n t a i n s e x c e s s oxygen ( t h e mix used i s always l e s s than s t o i c h i o m e t r y ) , s o most of t h e ammonia w i l l be o x i d i s e d on i n i t i a l c o n t a c t w i t h t h e o u t e r s u r f a c e . The gas r e a c h i n g t h e bottom of t h e grain-boundary groove w i l l t h e r e f o r e be s t r o n g l y o x i d i s i n g .

P l a t i n u m oxide w i l l form h e r e , and w i l l m i g r a t e outwards, where i t meets a more r e d u c i n g atmosphere, and s o p l a t i n u m i s d e p o s i t e d on t h e o u t e r s u r f a c e . Thus t h e grooves deepen, and t h e s u r f a c e b u i l d s up, r e s u l t i n g i n t h e hollow cage-like growths s e e n i n r e c o n s t r u c t e d gauzes.

The b u i l d up of t h e s e s t r u c t u r e s w i t h time i s shown i n f i g u r e 3a (0.5 h o u r s , some g r a i n s s t a r t t o r e c o n s t r u c t ) , 3b (encroachment of t h e growth from t h e

r e c o n s t r u c t i n g g r a i n s ) , and 3c ( t h e e n t i r e s u r f a c e covered). The r e c o n s t r u c t e d s u r f a c e i s shown a t h i g h e r m a g n i f i c a t i o n i n f i g u r e 3d.

A m m o n i a a n d o x y g e n

O x i d e r e d u c e d a n d d e p o s i t e d o n t 0 s u r f a c e ( p r o d u c i n g f a c e t s

\ I

t c s u r f a c e R E G I O N 1

Fig. 2

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Schematic diagram i l l u s t r a t i n g vapour t r a n s - p o r t mechanism of s u r f a c e r e c o n s t r u c t i o n , i n v o l v i n g p r o g r e s s i v e deepening of p r e - e x i s t i n g grooves and c a v i t i e s on t h e s u r f a c e of t h e c a t a l y s t .

a m m o n i a

YC

R E G I O N 2

o x i d i s e d - s u r p l u s o x y g e n

- o l a t i n u r n o x i d e s f o r m e d

Fig. 3

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S u r f a c e s t r u c t u r e of gauzes exposed t o an 8% ammonia-air f o r v a r y i n g l e n g t h s of time. ( a ) 0.5 hours, ( b ) 2 hours and ( c ) 8 hours. 3 ( d ) shows a h i g h e r m a g n i f i c a t i o n view of t h e r e c o n s t r u c t e d s u r f a c e ( a f t e r 15 h o u r s ) .

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I f t h e t e s t s a r e c a r r i e d o u t a t d i f f e r e n t gas m i x t u r e s , i t i s found t h a t a t lower ammonia l e v e l s t h e c a t a l y s t c e a s e s t o work, and a t h i g h e r ammonia l e v e l s t h e s u r f a c e smooths out due t o d i f f u s i o n [7]. (As t h e r e a c t i o n i s exothermic, i n c r e a s i n g t h e ammonia c o n t e n t a l s o i n c r e a s e s t h e gauze t e m p e r a t u r e very markedly).

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AMMONIA OXIDATION

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X-RAY ANALYSIS. E l e c t r o n microprobe a n a l y s i s (uSing a ZOkeV i n c i d e n t e l e c t r o n beam) shows t h a t t h e s t r o n g rhodium enrichment s e e n i n a i r o x i d a t i o n does n o t occur i n ammonia o x i d a t i o n , and t h a t i n some c a s e s s l i g h t p l a t i n u m enrichment i s even observed [7]. A f i n e w i r e placed behind t h e gauzes a l s o shows s l i g h t p l a t i n u m enrichment, t h i s i n d i c a t i n g vapour d e p o s i t i o n from t h e gauzes. ( I t n u s t be noted, however, t h a t t h e microprobe a n a l y s i s samples n o t only t h e e n t i r e s u r f a c e f i l m but a l s o some of t h e bulk m e t a l beneath).

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AMMONIA OXIDATION

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FIM ATOM PROBE ANALYSIS FIM samples made of t h e c a t a l y s t m a t e r i a l were i n s e r t e d d i r e c t l y behind t h e gauzes, and s o were exposed t o t h e r e a c t i o n environment. An i n t e r e s t i n g c o n t r a s t i s s e e n between samples exposed f o r 10 minutes and t h o s e f o r 30 minutes.

A sequence of FIM images f o r a sample t r e a t e d f o r 10 minutes i n t h e r i g i s shown i n f i g u r e 4. A t h i c k oxide f i l m i s formed, and both t h e o x i d e and t h e s u b s t r a t e a r e s t r o n g l y f a c e t t e d . A c o n c e n t r a t i o n d e p t h ' p r o f i l e ( f i g u r e 5) shows t h a t t h e oxide i s c l e a r l y e n r i c h e d i n rhodium compared t o t h e bulk, a l t h o u g h t h e r e i s an i n d i c a t i o n of platinum d e p o s i t i o n on t h e very o u t e r s u r f a c e .

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Fig. 4

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Sequence of f i e l d i o n micrographs showing p r o g r e s s i v e removal of o x i d e from a platinum-rhodium specimen exposed f o r 10 minutes t o a 10% ammonia-air mixture i n t h e c a t a l y t i c r e a c t o r . Note t h e heavy oxide l a y e r , and t h e s t r o n g l y f a c e t t e d n a t u r e of t h e oxide s u r f a c e , a s w e l l a s t h a t of t h e metal s u b s t r a t e . Images ( a ) , ( b ) and ( c ) show only t h e oxide f i l m , while i n images ( d ) , ( e ) and ( f ) t h e metal s u b s t r a t e i s p r o g r e s s i v e l y r e v e a l e d .

Gl?+

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^Fig. c o n c e n t r a t i o n 5

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Atom probe depth p r o f i l e through f i l m formed a f t e r exposure i n t h e r e a c t o r f o r 10 minutes. Note t h e s t r o n g enrichment i n rhodium c o n t e n t - - - of t h e o x i d e f i l m .

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A sample t r e a t e d f o r 30 minutes l o o k s v e r y d i f f e r e n t ( f g g u r e 6 ) . The s u r f a c e r e g i o n is now m e t a l l i c , any oxide formed i n i t i a l l y having a p p a r e n t l y been reduced.

A depth p r o f i l e through t h i s f i l m ( f i g u r e 7 ) shows no s u b s t a n t i a l change i n t h e Pt/Rh r a t i o from t h a t of t h e bulk, but l a r g e amounts of hydrogen and n i t r o g e n a r e d e t e c t e d b e f o r e t h e bulk m e t a l s u b s t r a t e i s reached. This shows t h a t t h e FIM sample i s now a c t i n g a s a c a t a l y s t i n t h e same way a s t h e gauzes.

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CONCLUSION AND DISCUSSION The r e s u l t s confirm t h a t t h e r e c o n s t r u c t i o n p r o c e s s t a k e s p l a c e by vapour t r a n s p o r t a s f i r s t s u g g e s t e d by McCabe and Schmidt

[a].

D e t a i l s of e v e n t s o c c u r r i n g on t h e p l a t i n u m

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rhodium c a t a l y s t s u r f a c e can now be examined. S i x main p r o c e s s e s need t o be c o n s i d e r e d i n t h e r e c o n s t r u c t i o n :- 1) P t 0 2 vapour f o r m a t i o n , m i g r a t i o n and decomposition; 2) The s e l e c t i v e o x i d a t i o n of t h e rhodium s p e c i e s ; 3) Thermal f a c e t t i n g ; 4 ) The f o r m a t i o n of g r a i n boundary grooves; 5) S u r f a c e d i f f u s i o n ; 6) Bulk d i f f u s i o n i n both m e t a l and t h e oxide.

The e x t e n t of each p r o c e s s i s dependent on t h e r e a c t i o n c o n d i t i o n s , t h e vapour t r a n s p o r t p r o c e s s o p e r a t i n g over only a l i m i t e d range of t h e s e c o n d i t i o n s . I f t h e t e m p e r a t u r e i s t o o low (low ammonia c o n t e n t i n t h e gas s t r e a m ) , t h e r e i s i n s u f f i c i e n t P t 0 2 vapour p r e s s u r e s o t h e dominant p r o c e s s is t h e s e l e c t i v e o x i d a t i o n of rhodium t o form RhpOg which i s n o t c a t a l y t i c a l l y a c t i v e f o r n i t r i c o x i d e formation. I f t h e t e m p e r a t u r e i s t o o high ( h i g h ammonia c o n t e n t ) , t h e n s u r f a c e and bulk d i f f u s i o n dominate, l e a d i n g t o a r e d u c t i o n i n s u r f a c e a r e a and a drop i n c a t a l y s t e f f i c i e n c y .

Fig. 6

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F i e l d i o n micrographs of a sample exposed f o r 30 minutes i n t h e c a t a l y t i c r e a c t o r . (a) The s u r f a c e l a y e r i s m e t a l l i c i n n a t u r e , any oxide formed having been s u b s e q u e n t l y reduced. ( b ) The f u l l y o r d e r e d s u b s t r a t e .

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3 0 MINUTES, AIR - 1 0 % AMMONIA CONCENTRATION D E P T H P R O F I L E

G a s l o n s ion b l o c k )

1 5 0

R R

R R R R

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- ^{3 3 %} - - k R -

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R C o m p o s c t t o n R Bulk

R

Number of l o n s C o l l e c t e d

Fig. 7

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Atom probe c o n c e n t r a t i o n d e p t h p r o f i l e through t h e s u r f a c e l a y e r formed a f t e r exposure f o r 30 minutes i n t h e r e a c t o r . The composition of t h e m e t a l i n t h e s u r f a c e r e g i o n does not d i f f e r g r e a t l y f rom t h a t of t h e bulk; but s u b s t a n t i a l q u a n t i t i e s of hydrogen and n i t r o g e n a r e d e t e c t e d b e f o r e t h e s u b s t r a t e m e t a l i s reached.

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The a b i l i t y t o make FIM samples from c a t a l y s t m a t e r i a l , and t o expose them t o r e a l r e a c t i o n c o n d i t i o n s s o t h a t t h e FIM sample a c t s a s a n a c t u a l c a t a l y s t , opens up a l a r g e a r e a of new work i n t h e s t u d y of t h i n s u r f a c e f i l m s . The use of l a s e r p u l s i n g i n t h e atom probe a l l o w s s t u d y of t h e s e f i l m s even i f they a r e n o t v e r y conducting. The e x t e n s i o n of t h e p r e s e n t work i n t o o t h e r c a t a l y s t systems s u c h a s t h o s e used i n a u t o m o b i l e e m i s s i o n c o n t r o l , chernicals and p e t r o c h e m i c a l s i s now f e a s i b l e .

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ACKNOWLEDGEMENTS T h i s work was c a r r i e d o u t i n c o l l a b o r a t i o n w i t h Johnson Matthey Group Research Centre. We a r e p a r t i c u l a r l y i n d e b t e d t o Dr.D.Thompson and Mr.A.Pratt f o r a d v i c e and a s s i s t a n c e and Mr.D.Hepburn f o r e x t e n s i v e h e l p w i t h SEM and e l e c t r o n microprobe work. Dr.C.Grovenor and Mr.A.Cerezo a s s i s t e d i n t h e l a s e r o p e r a t i o n of t h e atom probe. A.R.McCabe acknowledges t h e award of a n SERC Case s t u d e n t s h i p . We thank Verlag-Chemie f o r p e r m i s s i o n t o reproduce i l l u s t r a t i o n s .

REFERENCES 1'

2 '3' '4 '

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Handforth S.L. and T i l l e y J.N., Ind. Eng. Chem. (12) (1934) 1287.

Busby J.A., Knapton A.G. and Budd A.E.R., Proc. F e r t i l i s e r Soc. (1978) 169.

Pszonicka M. and Dymkowski T., P o l i s h J. Chem. 52 (1978) 121.

P i e l a s z e k J., P l a t i n u m Metals Rev;

2

⁽¹⁹⁸⁴⁾ 10x

McCabe A.R. and Smith G.D.W., Proc. 2 9 t h I n t . F i e l d Emission Symposium Gothenburg, Sweden, Eds. Andren 11-0 and Norden H, Publ. Almqvist and W i k s e l l , Stockholm (1982), p.201.

F; 1

McCabe A.R. and Smith G.D.W., P l a t i n u m Metals Rev. (1983) 19.

McCabe A.R. and Smith G.D.W., Proc. 8 t h I n t . C a t a l y s i s Congress, B e r l i n , Publ. Verlag-Chemie, B e r l i n (1984) vo1.4, p.73.

[ 8 ] McCabe R.W., P i g n e t T. and Schmidt L.D., J. C a t a l . (1974) 114.