Chapter 19
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Laser-Driven Synthesis of Transition-Metal Carbides, Sulfides, and Oxynitrides Gary W. Rice
1
Exxon R&E Company, Annandale, NJ 08801
Laser pyrolysis of transition metal (Cr, Mo, W, Mn, Fe, Co) carbonyls and substituted carbonyls yielded high surface area powders of the t i t l e species. A cw CO laser beam intersected a stream of reactant gas, with absorption of energy by ethylene or ammonia as co-reactants, or ethylene as a sensitizer. Powder particle diameters ranged from 10 to 100 nm, decreasing as the melting point of the major product increased. Syntheses of bimetallic products and selectivity toward carbon, nitrogen, oxygen and sulfur incorporation are also discussed. 2
L a s e r s , a s h i g h p r e c i s i o n p r o b e s , c a n g i v e deep i n s i g h t i n t o the microscopic d e t a i l s of chemical reactions. Lasers are ideal f o rcontrolled i n i t i a t i o n of reactions, and a l l o w h i g h s e n s i t i v i t y f o r d e t e c t i o n o f t r a n s i e n t i n t e r m e d i a t e s and s t a b l e p r o d u c t s t h a t i s s e l e c t i v e i n b o t h space and t i m e . However, l a s e r s a s h i g h power i n d u s t r i a l t o o l s a r e mainly found i n non-chemical a p p l i c a t i o n s such as c u t t i n g , d r i l l i n g , w e l d i n g and surface heat t r e a t i n g . The p o t e n t i a l o f l a s e r s t o d r i v e o r c o n t r o l l a r g e s c a l e c h e m i c a l p r o c e s s i n g has been reviewed p e r i o d i c a l l y ( 1 - 3 ) , with the r e c u r r i n g c o n c l u s i o n t h a t t h e c o s t r e q u i r e s h i g h v a l u e added p r o d u c t s f o r economic v i a b i l i t y . However, i n C O 2 l a s e r s y n t h e s i s o f u l t r a f i n e c e r a m i c powders f r o m s i l a n e ( 4 ) , a p r o c e s s e v a l u a t i o n s u g g e s t e d t h a t r e a g e n t c o s t was dominant. T h a t work a l s o showed t h a t l a s e r synthesized p o w d e r s y i e l d e d b u l k c e r a m i c s whose p r o p e r t i e s s u r p a s s e d t h o s e o f m a t e r i a l s from c o n v e n t i o n a l precursors. Current address: Engelhard Corporation, 2655 Route 22 West, Union, NJ 07083
0097-6156/93/0530-0273$06.00/0 © 1993 American Chemical Society
Chaiken; Laser Chemistry of Organometallics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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274
LASER CHEMISTRY OF ORGANOMETALLICS
The work d e s c r i b e d h e r e was u n d e r t a k e n t o e x p l o r e p y r o l y t i c l a s e r s y n t h e s i s o f f i n e powders f r o m t r a n s i t i o n metal organometallics. Compared t o s l o w e r t h e r m a l syntheses, l a s e r driven r e a c t i o n s give high surface areas w i t h o u t c o n t a m i n a t i o n , and some c h e m i c a l a n d p h y s i c a l p r o p e r t i e s t h a t a p p e a r t o be u n i q u e . I t i s very d i f f i c u l t t o o b t a i n r e f r a c t o r y m a t e r i a l s t h a t combine h i g h s u r f a c e a r e a and s u r f a c e p u r i t y by c o n v e n t i o n a l techniques. Post s y n t h e s i s p r o c e s s i n g , such as g r i n d i n g , i n e v i t a b l y introduces contaminants t h a t bind s t r o n g l y t o surfaces. Laser synthesis y i e l d s high surface area powder i n a s i n g l e s t e p , w i t h o u t c o n t a c t between t h e r e a c t i o n zone and t h e r e a c t o r w a l l . The r e a c t i o n s employ g a s e o u s o r g a n o m e t a l l i c s , so t r a n s f e r i n t o t h e r e a c t i o n zone i s i t s e l f a p u r i f y i n g d i s t i l l a t i o n s t e p . Laser s y n t h e s i z e d powders h a v e h i g h p u r i t y s u r f a c e s w i t h r e s p e c t t o unwanted a l k a l i and a l k a l i n e e a r t h m e t a l s , f o r example, a l t h o u g h a c h i e v i n g p r e c i s e s t o i c h i o m e t r y a n d high c r y s t a l l i n i t y i n the bulk m a t e r i a l s are d i f f i c u l t . The l a s e r s y n t h e s i s t e c h n i q u e h a s b e e n d e s c r i b e d elsewhere ( 5 ) . B r i e f l y , a gaseous r e a c t a n t stream c r o s s e d t h e beam o f a C O 2 l a s e r . A luminous flame i n i t i a t e d at the crossing point yielded fine p a r t i c l e s w h i c h were c o l l e c t e d on a membrane f i l t e r . Additional g a s f l o w s were p r o v i d e d t o p r e v e n t s p r e a d i n g o f t h e r e a c t a n t s t r e a m and a c c u m u l a t i o n o f powder on c e l l windows. The need f o r r e a c t a n t v a p o r p r e s s u r e s o f a b o u t 1 0 0 t o r r was n o t u n d u l y r e s t r i c t i v e . Volatile o r g a n o m e t a l l i c compounds e x i s t f o r a l l t h e f i r s t row t r a n s i t i o n m e t a l s from t i t a n i u m t o n i c k e l , as w e l l as f o r many o f t h e s e c o n d and t h i r d row m e t a l s . R e a c t i o n s were d r i v e n w i t h a cw C O 2 l a s e r b u i l t f o r the purpose with o p t i o n a l l i n e - t u n a b i l i t y . I t produced 2 0 0 W a s a f r e e - r u n n i n g l a s e r on t h e 1 0 Ρ ( 2 0 ) l i n e a t 9 4 4 cm" , o r up t o 1 2 0 W when l i n e turned. Studies of metal c a r b i d e s y n t h e s i s employed C 2 H 4 a s b o t h a r e a c t a n t a n d photon absorber. E q u i v a l e n t r e s u l t s were o b t a i n e d w i t h 2 0 0 W l a s e r power a t 9 4 4 cm" , and w i t h 1 2 0 W a t t h e 1
1
absorption
maximum o f
C2H4.
TRANSITION METAL CARBIDE
SYNTHESIS
P y r o l y s i s of F e ( C 0 ) and e x c e s s C H y i e l d e d f i n e p o w d e r s w h i c h were a n a l y z e d i n d e t a i l . TEM showed t h a t t h e p a r t i c l e s h a d d i a m e t e r s l e s s t h a n one m i c r o n , w h i l e XRD i n d i c a t e d F e 3 C was t h e m a j o r p h a s e . XRD l i n e b r o a d e n i n g s u g g e s t e d c r y s t a l l i t e s were s u b s t a n t i a l l y s m a l l e r t h a n the p a r t i c l e s . BET s u r f a c e a r e a a n a l y s i s i n d i c a t e d t h a t t h e p a r t i c l e s were n o t p o r o u s . XPS showed t h a t s u r f a c e s were c a r b i d i c i r o n and e x c e s s c a r b o n , f r e e o f o x i d e phases. However, M ô s s b a u e r showed t h a t up t o 2 0% o f t h e i r o n was p r e s e n t a s a - and
