- Email: [email protected]
A Comparison of Hydrogen Sorption by Tungsten and Molybdenum Sulphides
1424
A COMPARISON OF HYDROGEN SORPTION BY TUNGSTEN AND MOLYBDENUM SULPHIDES D. FRASER, R. B . MOYESa) a n d P. B . WELLSa) D e p a r t m e n t o f C h e m i s t r y , T h e U n i v e r s i t y , H u l l , H U 6 7RX, E n g l a n d a n d C . J . WRIGHT')
a n d C . F. SAWSON
AERE, H a r w e l l , Didcot, O x f o r d s h i r e , OX11 ORA, E n g l a n d SUMMARY:
I n e l a s t i c n e u t r o n s c a t t e r i n g s p e c t r o s c o p y (INSS) h a s
been used t o i n v e s t i g a t e t h e n a t u r e o f s o r b e d hydrogen c o n t a i n e d i n p o w d e r e d t u n g s t e n d i s u l p h i d e o f f o r m u l a H0.05WS2.
The h i g h 1
f r e q u e n c y v i b r a t i o n s p e c t r u m i n t h e r a n g e 550 t o 1100 cm-
contains t w o e x c i t a t i o n s , one of high i n t e n s i t y centred at 730 c m - l
a n d o n e o f lower i n t e n s i t y c e n t r e d a t 8 8 0 cm-',
This
s p e c t r u m c l o s e l y resembles t h a t r e p o r t e d b y u s e l s e w h e r e f o r h y d r o g e n s o r b e d b y molybdenum d i s u l p h i d e l ) INTRODUCTION:
.
The s u r f a c e c h e m i s t r y o f t u n g s t e n a n d molybdenum
s u l p h i d e s c o n t i n u e s t o a t t r a c t a t t e n t i o n b e c a u s e of t h e i m p o r t a n c e o f t h e h y d r o d e s u l p h u r i s a t i o n c a t a l y s t s of which t h e s e materials are t h e p r i n c i p a l c o m p o n e n t s . We recently reported INS s p e c t r a o f h y d r o g e n s o r b e d on molybdenum d i s u l p h i d e ' ) . E x c i t a t i o n s i n H0.07MoS2 o c c u r r e d a t 662 a n d 8 6 4 c m - l w i t h i n t h e range p r e s e n t l y o f i n t e r e s t ( c u r v e ( b ) , F i g . l ) , and also a t 1 3 4 8 a n d 1977 cm-', from which w e concluded t h a t t h e hydrogen was atomic i n n a t u r e a n d b o n d e d t o s u l p h u r . We r e p o r t h e r e t h e INS s p e c t r u m o f h y d r o g e n s o r b e d by t u n g s t e n d i s u l p h i d e . EXPERIMENTAL:
T u n g s t e n d i s u l p h i d e w a s p r e p a r e d f r o m diammonium
t h i o t u n g s t a t e by d e c o m p o s i t i o n o f t h e l a t t e r i n a h y d r o g e n
stream a t 6 7 3 K .
Evacuation at
T o r r for 15 h a t 6 7 3 K
provided a m a t e r i a l s u b s t a n t i a l l y f r e e o f hydrogen ( a s judged b y a n INS s p e c t r u m a n d by H2/D2 e x c h a n g e ) .
0.05 mol H p e r m o l
WS2 was t h e n s o r b e d by e x p o s u r e o f t h e p o w d e r t o 6 7 0 T o r r
hydrogen a t 5 7 3 K.
T h e I N S s p e c t r u m of t h i s h y d r o g e n - c o n t a i n i n g
m a t e r i a l w a s o b t a i n e d b y u s e o f t h e AERE b e r y l l i u m f i l t e r spectrometer. A f t e r e x p o s u r e t o a i r t h e material e x h i b i t e d a s u r f a c e area ( N 2 p h y s i s o r p t i o n , BET m e t h o d ) o f 1 2 m2 g - ' , and a c r y s t a l l i t e dimension o f 47
8
in the C-direction.
a ) t o whom c o r r e s p o n d e n c e may b e a d d r e s s e d .
1425 RESULTS AND DISCUSSION:
F i g u r e 1 s h o w s a p o r t i o n of t h e h i g h
f r e q u e n c y INS s p e c t r u m o f H0.05WS2 ( c u r v e ( a ) ) t o g e t h e r w i t h t h a t r e p o r t e d e a r l i e r f o r Ho.07"loS2 Grenoble, France')). t h i s r e g i o n (Ho.05;S2, H0,07f;S2, 20 em
).
6 6 2 cm-
( c u r v e ( b ) - o b t a i n e d a t t h e ILL,
Each s p e c t r u m c o n t a i n s t w o e x c i t a t i o n s i n 7 3 0 cm-' ( s t r o n g ) a n d 880 cm-' ( w e a k ) ; ( s t r o n g ) a n d 864 c m - l
(weak);
e a c h -f10 t o
From a d e t a i l e d a s s e s s m e n t o f t h e s p e c t r a o f H0.07MoS2
w e c o n c l u d e d t h a t t h e h y d r o g e n was a t o m i c i n n a t u r e a n d b o n d e d t o o n e o r more s u l p h u r atoms') a n d n o t t o metal atoms ( a s is t h e case i n , f o r example, H0,4TaS22)). From t h e s i m i l a r i t y of t h e s p e c t r a i n F i g u r e 1 w e c o n c l u d e t h a t t h e e n v i r o n m e n t of h y d r o g e n i n H0.09\W2 i s v e r y s i m i l a r t o t h a t i n H 0 . 0 7 M ~ 2S'
The e x t e n t of h y d r o g e n s o r p t i o n b y WS2, u n l i k e t h a t b y MoS2, is
c l o s e t o t h a t tobe a t t r i b u t e d t o chemisorption a t t h a t s u r f a c e which
is m e a s u r e d by t h e p h y s i s o r p t i o n o f n i t r o g e n .
Since H2/D2
e x c h a n g e i s f a s t e r t h a n h y d r o g e n s o r p t i o n w e e n v i s a g e t h a t simple c h e m i s o r p t i o n o c c u r s i n t h e f i r s t i n s t a n c e a t m e t a l atom s i t e s , a n d t h a t h y d r o g e n atoms s u b s e q u e n t l y m i g r a t e i n a slow a c t i v a t e d p r o c e s s t o become b o n d e d t o s u l p h u r i n t h e b u l k .
Such i n t e r c a l a -
t i o n o f h y d r o g e n c a n a c c o u n t f o r t h e gross amount s o r b e d .
This
p r o c e s s c o u l d e v e n t u a l l y c a u s e t h e b r e a k - u p o f t h e WS2 o r hloSz s t r u c t u r e i n t o s e p a r a t e d l a y e r s by e x f o l i a t i o n 3 ) ,
a n d t h i s model
w o u l d i n t e r p r e t t h e h i g h c a t a l y t i c h y d r o g e n a t i o n a c t i v i t y of t h e s e materials.
C o a n d N i p r o m o t e r s may a l s o a s s i s t h y d r o g e n d i s -
s o c i a t i o n and i n t e r c a l a t i o n . P i g . 1. H i g h f r e q u e n c y I N S s p e c t r a o f ( a ) HO.OpWSZ a n d ( b ) H0.07MoS2 Ordinate = counts per incident neutron flux i n arbitrary units. Abscissa = n e u t r o n e n e r g y transfer.
c m -1 REFERENCES I r i g h t , D . F r a s e r , R.B. Moyes, C . R i e k e l , C . Sampson a n d P . B . W e l l s , J . Chem. S O C . F a r a d a y T r a n s . I , i n p r e s s . 2.C. R i e k e l , H.G. Reznik, R. Schullhorn and C . J . Wright, J. Chem. P h y s . , 70, 5 2 0 3 ( 1 9 7 9 ) . 3 . R . R . C h i a n e l l i , E.B. P r e s t r i d g e , T.A. P e c o r a r o , a n d J . P . D e N e u f v i l l e , S c i e n c e , 203, 1 1 0 5 ( 1 9 7 9 ) . 4 . M . R . B l a k e , M . E y r e , R . B . Moyes a n d P . B . Wells, t h i s C o n g r e s s l.C.J.
A COMPARISON OF HYDROGEN SORPTION BY TUNGSTEN AND MOLYBDENUM SULPHIDES D. FRASER, R. B . MOYESa) a n d P. B . WELLSa) D e p a r t m e n t o f C h e m i s t r y , T h e U n i v e r s i t y , H u l l , H U 6 7RX, E n g l a n d a n d C . J . WRIGHT')
a n d C . F. SAWSON
AERE, H a r w e l l , Didcot, O x f o r d s h i r e , OX11 ORA, E n g l a n d SUMMARY:
I n e l a s t i c n e u t r o n s c a t t e r i n g s p e c t r o s c o p y (INSS) h a s
been used t o i n v e s t i g a t e t h e n a t u r e o f s o r b e d hydrogen c o n t a i n e d i n p o w d e r e d t u n g s t e n d i s u l p h i d e o f f o r m u l a H0.05WS2.
The h i g h 1
f r e q u e n c y v i b r a t i o n s p e c t r u m i n t h e r a n g e 550 t o 1100 cm-
contains t w o e x c i t a t i o n s , one of high i n t e n s i t y centred at 730 c m - l
a n d o n e o f lower i n t e n s i t y c e n t r e d a t 8 8 0 cm-',
This
s p e c t r u m c l o s e l y resembles t h a t r e p o r t e d b y u s e l s e w h e r e f o r h y d r o g e n s o r b e d b y molybdenum d i s u l p h i d e l ) INTRODUCTION:
.
The s u r f a c e c h e m i s t r y o f t u n g s t e n a n d molybdenum
s u l p h i d e s c o n t i n u e s t o a t t r a c t a t t e n t i o n b e c a u s e of t h e i m p o r t a n c e o f t h e h y d r o d e s u l p h u r i s a t i o n c a t a l y s t s of which t h e s e materials are t h e p r i n c i p a l c o m p o n e n t s . We recently reported INS s p e c t r a o f h y d r o g e n s o r b e d on molybdenum d i s u l p h i d e ' ) . E x c i t a t i o n s i n H0.07MoS2 o c c u r r e d a t 662 a n d 8 6 4 c m - l w i t h i n t h e range p r e s e n t l y o f i n t e r e s t ( c u r v e ( b ) , F i g . l ) , and also a t 1 3 4 8 a n d 1977 cm-', from which w e concluded t h a t t h e hydrogen was atomic i n n a t u r e a n d b o n d e d t o s u l p h u r . We r e p o r t h e r e t h e INS s p e c t r u m o f h y d r o g e n s o r b e d by t u n g s t e n d i s u l p h i d e . EXPERIMENTAL:
T u n g s t e n d i s u l p h i d e w a s p r e p a r e d f r o m diammonium
t h i o t u n g s t a t e by d e c o m p o s i t i o n o f t h e l a t t e r i n a h y d r o g e n
stream a t 6 7 3 K .
Evacuation at
T o r r for 15 h a t 6 7 3 K
provided a m a t e r i a l s u b s t a n t i a l l y f r e e o f hydrogen ( a s judged b y a n INS s p e c t r u m a n d by H2/D2 e x c h a n g e ) .
0.05 mol H p e r m o l
WS2 was t h e n s o r b e d by e x p o s u r e o f t h e p o w d e r t o 6 7 0 T o r r
hydrogen a t 5 7 3 K.
T h e I N S s p e c t r u m of t h i s h y d r o g e n - c o n t a i n i n g
m a t e r i a l w a s o b t a i n e d b y u s e o f t h e AERE b e r y l l i u m f i l t e r spectrometer. A f t e r e x p o s u r e t o a i r t h e material e x h i b i t e d a s u r f a c e area ( N 2 p h y s i s o r p t i o n , BET m e t h o d ) o f 1 2 m2 g - ' , and a c r y s t a l l i t e dimension o f 47
8
in the C-direction.
a ) t o whom c o r r e s p o n d e n c e may b e a d d r e s s e d .
1425 RESULTS AND DISCUSSION:
F i g u r e 1 s h o w s a p o r t i o n of t h e h i g h
f r e q u e n c y INS s p e c t r u m o f H0.05WS2 ( c u r v e ( a ) ) t o g e t h e r w i t h t h a t r e p o r t e d e a r l i e r f o r Ho.07"loS2 Grenoble, France')). t h i s r e g i o n (Ho.05;S2, H0,07f;S2, 20 em
).
6 6 2 cm-
( c u r v e ( b ) - o b t a i n e d a t t h e ILL,
Each s p e c t r u m c o n t a i n s t w o e x c i t a t i o n s i n 7 3 0 cm-' ( s t r o n g ) a n d 880 cm-' ( w e a k ) ; ( s t r o n g ) a n d 864 c m - l
(weak);
e a c h -f10 t o
From a d e t a i l e d a s s e s s m e n t o f t h e s p e c t r a o f H0.07MoS2
w e c o n c l u d e d t h a t t h e h y d r o g e n was a t o m i c i n n a t u r e a n d b o n d e d t o o n e o r more s u l p h u r atoms') a n d n o t t o metal atoms ( a s is t h e case i n , f o r example, H0,4TaS22)). From t h e s i m i l a r i t y of t h e s p e c t r a i n F i g u r e 1 w e c o n c l u d e t h a t t h e e n v i r o n m e n t of h y d r o g e n i n H0.09\W2 i s v e r y s i m i l a r t o t h a t i n H 0 . 0 7 M ~ 2S'
The e x t e n t of h y d r o g e n s o r p t i o n b y WS2, u n l i k e t h a t b y MoS2, is
c l o s e t o t h a t tobe a t t r i b u t e d t o chemisorption a t t h a t s u r f a c e which
is m e a s u r e d by t h e p h y s i s o r p t i o n o f n i t r o g e n .
Since H2/D2
e x c h a n g e i s f a s t e r t h a n h y d r o g e n s o r p t i o n w e e n v i s a g e t h a t simple c h e m i s o r p t i o n o c c u r s i n t h e f i r s t i n s t a n c e a t m e t a l atom s i t e s , a n d t h a t h y d r o g e n atoms s u b s e q u e n t l y m i g r a t e i n a slow a c t i v a t e d p r o c e s s t o become b o n d e d t o s u l p h u r i n t h e b u l k .
Such i n t e r c a l a -
t i o n o f h y d r o g e n c a n a c c o u n t f o r t h e gross amount s o r b e d .
This
p r o c e s s c o u l d e v e n t u a l l y c a u s e t h e b r e a k - u p o f t h e WS2 o r hloSz s t r u c t u r e i n t o s e p a r a t e d l a y e r s by e x f o l i a t i o n 3 ) ,
a n d t h i s model
w o u l d i n t e r p r e t t h e h i g h c a t a l y t i c h y d r o g e n a t i o n a c t i v i t y of t h e s e materials.
C o a n d N i p r o m o t e r s may a l s o a s s i s t h y d r o g e n d i s -
s o c i a t i o n and i n t e r c a l a t i o n . P i g . 1. H i g h f r e q u e n c y I N S s p e c t r a o f ( a ) HO.OpWSZ a n d ( b ) H0.07MoS2 Ordinate = counts per incident neutron flux i n arbitrary units. Abscissa = n e u t r o n e n e r g y transfer.
c m -1 REFERENCES I r i g h t , D . F r a s e r , R.B. Moyes, C . R i e k e l , C . Sampson a n d P . B . W e l l s , J . Chem. S O C . F a r a d a y T r a n s . I , i n p r e s s . 2.C. R i e k e l , H.G. Reznik, R. Schullhorn and C . J . Wright, J. Chem. P h y s . , 70, 5 2 0 3 ( 1 9 7 9 ) . 3 . R . R . C h i a n e l l i , E.B. P r e s t r i d g e , T.A. P e c o r a r o , a n d J . P . D e N e u f v i l l e , S c i e n c e , 203, 1 1 0 5 ( 1 9 7 9 ) . 4 . M . R . B l a k e , M . E y r e , R . B . Moyes a n d P . B . Wells, t h i s C o n g r e s s l.C.J.