Dy-Cu Alloy Films: Catalytic Activity, Composition, Structure

GUni, L d al. (Editom), New Frontiers in Catalysis Proccedigs of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest,Hungary 0 1993 Elsevier Science PublishersB.V. All righfs reserved

Dy-Cu A U O Y FILMS: CATALYTICACTIVITY, COMPOSITION, STRUCTURE

K N.Zhavoronkova and 0.A. Boeva Mendelejev Institute of Chemical Technology, Moscow 125190, Miusskaja sq. 9, Russia

ABSTRACT T h e model reaction H2+D2i22HD has been used for investigation of specific catalytic activity ( K ) of alloy D y - C u films, which were found to be much more a c t i v z than pure D y , copper being absolutely inactive. T h e influence of the method of preparation of films on K was investigated, three different method being used. Results obtained were compared with the data of X - r a y s electron spectroscopy investigation of alloy films composition. Conclusions were made that the active component of all condensed films is intermetalic compound DyCu, and K s d e process pends on the depth of interaction of Dy and Cu in the of film condensation and heating. This conclusion is supported by the results of electronographical phase analysis of f i l m s . 1.INTRODUCTlON In our previous investigations C 1 1 we showed that the allying of rare earth metals (REM) with copper results in the formation of very active catalysts for the model reaction H 2 + D F 2 2 M , REW having quite low activity and Cu being absolutely inactive. In searching the reason of this effect the d e pendence of specific catalytic activity ( K ) o f alloy films o n S the method of preparation and composition was investigated in system Dy-Cu, components being able to form a t least 4 intermetalic compounds (IMC): DyCu, DyCuz, DyCus and DyCu7C21. 2 . EXPER IHENTAL

Kinetics of model reaction was investigated in high vacuum in temperature range of glass apparatus described in C1.31 7 7 - 2 0 3 K a t pressure 0 , 5 Torr. K was calculated by the equation: c -cI, -2 -1 Ks- -. In molecules cm s , S.Z

c -cz

where Cg, C , C a r e fractions of HD present a t time z e r o , Z and at equiyibrium, N - the number of molecules in reaction chemisorption volume, S - the area of the film measured by H2 a t 7 7 K. T h e following intermetalic compounds were prepared by alloying and DyCus with the exin atmosphere of pure Ar : DycU, D-2 in which the atomic ratio C u / D y - G , let us call it cess of -, D-.. All samples for investigation were prepared as filmd 10condensed o n the inner walls of glass reactor in vacuum

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1818 Torr. We used three methods of films preparation: 1 . A r a p i d ( 1 - 7 m i n ) a n d c o m p l e t e e v a p o r a t i o n o f 5-25 m g o f t h e s a m p l e o f IMC D W , D W P or D S u a f r o m W - s p i r a l . D u r i n g f i l m deposition reactor walls were not thermostated, but after this t h e a n n e a l i n g o f t h e f i l m a t 423-473 K w a s d o n e f o r 30-60 m i n . 2. S e p a r a t e a n d s u c c e s s i v e e v a p o r a t i o n f r o m t w o d i f f e r e n t s o u r Dy a n d Cu, r e a c t o r w a l l s d u r i n g d e p o s i t i o n b e i n g k e p t a t ces 77 K . A f t e r t h i s b i l a y e r f i l m s w e r e a n n e a l e d a t d i f f e r e n t temperatures for different periods of time. After each operation of annealing surface area and catalytic activity w e r e measurod. 3. S u c c e s s i v e e v a p o r a t i o n and c o n d e n s a t i o n of 5-6 f i l m s from thermostated o n e s a m p l e ( 2 0 - 5 0 m g ) of INC o n the reactor walls finia t 77 K. C h e m i s o r p t i o n o f Hr w a s m e a s u r e d a t o n c e a f t e r the possibility s h i n g c o n d e n s a t i o n ( n o m o r e 1-2 m i n ) to a v o i d of film contamination. After all catalytic measurements were f i n i s h e d h y d r o g e n w a s p u m p e d f r o m t h e f i l m a t 473 K and the next portion was evaporated f r o m the s a m e sample and condensed above the previous film. The degree of the evaporation Q was estimated as: Zi'Ii

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-

rli

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CZi.Ii

'

100%

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where r . t i m e of t h e e v a p o r a t i o n o f i - p o r t i o n (min), 1 I. c u r r e n t p o w e r in t h e s o u r c e , A . for M e l h o d o f X - r a y s e l e c t r o n s p e c t r o s c o p y (XBS) w a s u s e d the e s t i m a t i o n of t h e s u r f a c e c o m p o s i t i o n , a n d being combined the w i t h i o n Ar' e t c h i n g it a l l o w e d u s t o f i n d t h e p r o f i l e o f *etching curves" C41. c o n c e n t r a t i o n in t h e d e p t h o f t h e f i l m MQ ? - r a d i a t i o n w a s u s e d , t h e power of X-rays source was 10 k .20 m A . F i l m s w e r e p r e p a r e d b y t h e f i r s t m e t h o d in t h e Xray e l e c t r o n s p e c t r o m e t e r A D E S - 4 0 0 . R a t e o f e t c h i n g w a s @bout 40 R / m i n . E l e c t r o g r a p h i c a l p h a s e a n a l y s i s w a s m a d e with films prepared by the first method o n electrograph ER-100 (accelerat i n g t e n s i o n 75 kV).

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3. R E S U L T S A N D D I S C U S S I O N .

F i g u r e 1 s h o w s d e p e n d e n c e K o n T in A r r h e n i u s c o o r d i n a t e s f o r by D y - f i l m s C31, ( c u r v e l ) ,a n 3 a l s o f o r t h e f i l m s , p r e p a r e d ), DyCur t h e 1 - s t m e t h o d f r o m IHC DyCum ( c u r v e 2, p o i n t s 0 ( c u r v e 3) a n d DyCu ( c u r v e 4, p o i n t s ) . The type of the temperature dependence of K with abrupt break, has been discussed alloys 1s m a c h h i g h e r . b e f o r e [ l l . It m a y b e E e e n t h a t K s o f then % o f Dy, but it d e p e n d s o n c o m p o s i t i o n of e v a p o r a t e d IHC. M a x i m u m i n c r e a s e o f K L a t 77 K is 1 4 0 t i m e s . B u t we h a v e no is the r i g h t t o s a y t h a t t h i s K S b e l o n g s t o D $ h e b e c a u s e it whole composition of film but not the composition o f its surfa c e . In f i g u r e 1 r e s u l t s o f i n v e s t i g a t i o n o f f i l m 1 p r e p a r e d b y t h e s e c o n d m e t h o d ( p o i n t s o ) a r e s h o w n , w h e n Dy f i l m w a s d e p o (point 0-1). sited first and i t s Ks a t 1 4 3 K w a s m e a s u r e d C u - f i l m w a s d e p o s i t e d a b o v e Dy a t 77 K , a t o m i c r a t i o C u / D y - l .

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1819

44

2

15

14

A

13

12

F i g u r e 1 . D e p e n d e n c e of Kb of f i l m s d e p o s i t e d f r o m Dy ( l ) , DyCu. ( 2 ) . DyCuo (3) a n d D e (4) on t e m p e r a t u r e ( d e t a i l e d in the text).

94L14

13

1

F i g u r e 2. of f i l m 1 p r e p a r e d the second method in d e p e n d e n c e o n t i m e o f s i n t e r i n g a t 473 K .

!;

M s a t once increased in 1 4 times, and points 2 and also 3 and 4 situated near the c u r v e 4. Figure 2 shows the change of K of this film in dependence of the time of its sintering at 493K : K s increased maximum in 90

times and riched the same values a s K of films prepared by the first me’thod from (fig 1). In tie second experiment (film 2 ) C u - f i l m was condensated a t first and D y - f i l m - a b o v e , atomic ratio C u l D y - 5 . A f t e r measuring K film were sintered a t 373, 473 and 73:3K during 40, 30 and 90 m f n , correspondently. In fig.1 of this points X with correspondent numbers show values of K film a t 77K. K s increased totally in 400 times.After ’sintering in wide temperature interthe opportunity arisad to measure K val (alloying with Cu decreases the’dissolution of H, in D y , which made difficult K S measuring on pure REN-film a t high T 11,31). In fig 1 i t is seen that points X of this film situated straight o n the curve 2 . Thus it is evidently that interaction of D y and Cu leads t o the formation of active composition, intaking place even after the interaction a t 77K. creasing of K In sintering the degree of interaction increases a n d K reaches maximum values independently on full bulk concentratio: of c o m ponents. Profiles of concentrattons of films prepared by the first method from the samples of IHC DyCum,D y C u t , DyCu and alloys DytCu and Dy&u were found by X E S . I t appeared that character of curves depends o n the initial composition of the sample. First portion condensed on the support a r e enriched with that component which prevails in the evaporating sample. Than its concentration decreased and a t the end of evaporation the a t o mic ratio on the surface of film (on the depth o r 40A) approaches 1 lndependently,,onthe initial composition of the sample t 4 l . A ~ it’s shown in fig 3a (curve l), in evaporation from I M C DyCu t h e concentration in all depth o f films remains constant the evaporating INC, and equal t o the initial composition of except for the surface itself which is enriched by D y in the result of its interaction with the gases of vacuum when the uas support Is not cooled. Such *oxidizing segregation- of D y observed o n the surface of all films prepared from alloys of all composition. B u t if support was cooled by liquid Nt segregation of REM did not take place a s i t was found in the case of alloys of Cu wlth Tm t4l. In the same f i g 3aresults of measuthe ring K s a t 77K of films prepared from 2 sample of D W by third method (curve 2 ) a r e shown. A s i t could be supposed KS was does not depends on the degree of sample evaporation. K+ measured in the lnterval of 77-203K and middle values situated straight on the curve 4 o f fig l.(points 4 ) U e see coincidence of results for films prepared by the flrst and third methods added by points 2 , 3 and 4 of film, prepared by the second m e thod.[t must be remembered that in the first and second methods (i.e.the the ’oxisizing segregationm of Dy can’t be avoided contamlnatlon of the surface), b u t in the third method all e f forts were made to avoid it. T h e conclusion m a y , be d o n e that the oxidized surface layers I s permeable for hydrogen molecul e s , and in the case of its formation t h e under surface layer

1821 o f m e t a l t a k e s p a r t in c a t a l y s i s . E x p e r i m e n t s w i t h f i l m s p r e p a red by t h e t h i r d m e t h o d a l s o c o n f i r m t h e f a c t t h a t t h e * a c t i v e c o m p o s i t i o n * (DyCu) f o r m a t i o n t a k e s p l a c e e v e n a t 77K. In f i g u r e 3 b t h e p r o f i l e o f c o n c e n t r a t i o n s ( c u r v e s 1) of f i l m s p r e p a r e d f r o m DyCua a n d a l s o r e s u l t s o f m e a s u r i n g K at S 77K o f f i l m s p r e p a r e d by t h e t h i r d m e t h o d f r o m 3 s a m p l e s of O w . ( c u r v e 2) a r e s h o w n . I t a l l o w s t o c o m p a r e ( i f n o t q u i t e e x a c t ) K s w i t h t h e cornposition o f t h e s u r f a c e . I t . is s e e n t h a t in t h e r e g i o n o f h i g h c o n c e n t r a t i o n o f Cu K is q u i t e l o w , b u t it i n c r e a s e s a b r u p t l y w h e n C u - c o n c e n t r a t i o n S d e c r e a s e s r e a c h i n g m a x i m u m w h e n Cu/Dy is a b o u t 4 a n d r e m a i n s c o n s t a n t a t its d e c r e a s i n g u p t o C u / D y - l . V a l u e s Ks o f t h e m o s t a c t i v e f i l m ( t h e t h i r d e v a p o r a t i o n o f t h e f i r s t s a m p l e ) m e a s u r e d in i n t e r v a l o f 77-203 K s i t u a t e d o n t h e c u r v e 2 o f f i g . 1 ( p o i n t s v ) . B u t K s o f films deposited during the last evaporation of samples turned o u t to b e m u c h l o w e r a p p r o a c h i n g to t h a t o f 0 or lower. These films adsorbed much more hydrogen than the previous ones, which s h o w s t h e p r e s e n c e of f r e e Dy.

F i g u r e 3. D e p e n d e n c e o f Cu/Dy o n t h e t i m e o f ion e t c h i n g of f i l m s e v a p o r a t e d f r o m 0( a ) a n d 0-a (b) ( curves 1 ), and d e p e n d e n c e of K o n t h e d e g r e e of e v a p o r a t i o n of t h e s a m p l e for f i l m s p r e p a r e d %y 3 r d m e t h o d ( c u r v e s 2 1 f r o m 0(a) and DycUa (b). 4. C O N C L U S I O N S

T h e a n a l y s i s o f a l l d a t a lead u s t o the conclusion that t h e a c t i v e c o m p o s i t i o n in t h e a l l o y f i l m in 0 y - m s y s t e m is t h e intermetallc compound O m , and the value of K depends o n the d e g r e e o f i n t e r a c t i o n Cu+DyrDyCu, w h i c h in i t s t u r n d e p e n d s o n

1822 C u - c o n c e n t r a t i o n a n d t e m p e r a t u r e . W h e n f i l m f o r m r a t 77K (3-rd method) the first factor plays the main role. At the big excess o f C u t h e r e a c t i o n p r o c e e d s in t h e m a x i m u m d e g r e e d u r i n g the p r o c e s s of c o n d e n s a t i o n . But t h e e x c e s s o f Cu m a k e s KS d e c r e a s e because o f the "cluster effect",i.e.too large dilution of activ e c o m p o n e n t by i n a c t i v e o n e . W h e n 4>Cu/Dyrl t h e degree of i n t e r a c t i o n s e e m s to b e q u i t e l a r g e , t h e n e g a t i v e i n f l u e n c e of 'cluster e f f e c t " b e i n g a v o i d e d and K reached the maximum vaproceed completely a t lues. W h e n C u / D y - 1 t h e r e a c t i o n does'not l o w T, f r e e c o m p o n e n t s b e i n g n o t i n t e r a c t e d a n d ' 4 3 0 K r e s u l t s in t h e s h i f t of the e q u i l i b r i u m to t h e r i g h t to t h e f o r m a t i o n of IHC D W and g i v e s a c c e l e r a t i o n of t h i s p r o c e s s . T h i s r e s u l t s in i n c r e a s i n g of KS w h i c h w e o b s e r v e d o n f i l m s p r e p a r e d by t h e f i r s t a n d s e c o n d m e t h o d s . For f i l m s d e p o s i t e d by t h e f i r s t m e t h o d f r o m DyClurr h a ving the maximum activity the both factors were important, the which e x c e s s of Cu and s i n t e r i n g f o r 1 hour at 4 7 3 K a s w e l l , formation. of r e s u l t e d in t h e c o m p l e t e i n t e r a c t i o n o f Dy w i t h D W . T h e e x a c t c o n f i r m a t i o n of:this w a s f o u n d in researching the s t r u c t u r e of f i l m s by e l e c t r o n d i f f r a c t i o n . In f i l m s d e p o sited f r o m DyCu., DyCu and Cu p h a s e s w e r e f o u n d , and in f i l m s d e p o s i t e d f r o m DyCuz-DyCu, (=u and D y - p h a s e s . 5.ACKNOULEDGEHENTS who Ye gratefully thanks prof. V.Nefedov and dr. A.Vinogradov helped us in X E S - s t u d y i n g , a n d d r . E . L a z a r e v a n d d r . A . G o r d e e v , who p e r f o r m e d e l e c t r o n o g r a p h i c a l r e s e a r c h .

6.REFERENCES B o e v a O.A., Proc. of 9th I n t e r n a t . C o n g r . o n C a t a l y s i s , C a l g a r y , 3, ( 1 9 8 8 1 , 1330. 2. S a v i t s k y E . H . , T e r e h o v a V.F., Metalovedeniye redkozernelnyh m e t a l o v , H., N a u k a , 1 9 7 5 , p . 1 0 1 . 3. Z h a v o r o n k o v a K . N . , B o e v a O . A . at a l l , K i n e t i c a i C a t a l i z , 23, 4, ( 1 3 8 2 ) , 8 8 1 . 4. N e f e d o v V . I . , V i n o g r a d o v A . R . , Z h a v o r o n k o v a K.N., B o e v a O . A . , P o v e r k h n o s t , 1 0 , (1987). 123. 1 . Z h a v o r o n k o v a K.N., P e s h k o v A . V . ,