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Low Temperature Oxidation of Hydrogen Enhanced by Spillover on A Nickel-Based Composite Catalyst
181
G.M.Pajonk, S.J. Teichner and J.E. Germain (Editors), Spillouer of Adsorbed Species 0
1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
LOW TEMPERATURE OXIDATION OF HYDROGEN ENHANCED BY SPILLOVER ON A NICKEL-BASED COMPOSITE CATALYST
T. I N U I , Y. MIYAMOTO, and Y . TAKEGAMI Department o f Hydrocarbon Chemistry, F a c u l t y o f E n g i n e e r i n g , Kyoto U n i v e r s i t y , Sakyo-ku,
Kyoto 606 (Japan)
ABSTRACT The removal o f a small c o n c e n t r a t i o n o f 0 i n hydrogen gas by c a t a l y t i c combustion o f hydrogen has been s t u d i e d . A tiree-component composite c a t a l y s t , Ni-CesO-j-Pt, s u p p o r t e d on a t h i n seramic f i b e r p l a t e ( F i b e r f r a x ) e x h i b i t e d e x c e l t e n t performance. Using t h i s c a t a l y s t hydrogen combustion o c c u r r e d a t room temperature, and 1.3% 02 i n H2 was e l i m i n a t e d t o below 1 ppm even w i t h a gaseous h o u r l y space v e l o c i t y o f 410,000. On t h e b a s i s o f comparison among t h e a c t i v i t i e s o f v a r i o u s c a t a l y s t s and o x i d a t i o n s t a t e s o f t h e c a t a l y s t metal d u r i n g t h e r e a c t i o n t h e s y n e r g i s t i c e f f e c t due t o hydrogen s p i l l o v e r was c o n f i r m e d . INTRODUCTION I t has been w i d e l y f o u n d t h a t when s m a l l c o n c e n t r a t i o n o f p l a t i n u m - g r o u p
m e t a l s such as P t o r Pd were mixed w i t h t r a n s i t i o n - m e t a l o x i d e s , t h e necessary temperature f o r hydrogen r e d u c t i o n lowered d r a m a t i c a l l y . T h i s phenomenon has been i n t e r p r e t e d as a hydrogen s p i l l o v e r e f f e c t . R e c e n t l y much e f f o r t has been c o n c e n t r a t e d on i t , s i n c e t h e s p i l l o v e r e f f e c t i s c o n s i d e r e d t o be one o f t h e p r i n c i p a l d i r e c t i o n s o f c a t a l y s t design f o r the preparation o f a h i g h l y a c t i v e catalyst. We have a l r e a d y r e p o r t e d t h a t composite c a t a l y s t s such as N i - L a 2 0 3 - R ~ supp o r t e d on s i l i c a , alumina, o r a c t i v e carbon e x e r t e d h i g h a c t i v i t y f o r methane s y n t h e s i s by h y d r o g e n a t i o n o f C O Y C02 ( r e f s . 1 and 2 ) , and a c t i v e carbon ( r e f . 3 ) . C o r r e l a t i o n between a d s o r p t i o n c a p a c i t i e s o f CO, COP, and H2 on t h e three-compon e n t c a t a l y s t system and t h e i r c a t a l y t i c a c t i v i t i e s were observed ( r e f . 4), and we have proposed t h a t these h i g h a c t i v i t i e s a r e caused by a hydrogen s p i l l o v e r supposing t h e Ru p a r t as t h e t r a n s p o r t agent f o r t h e r a p i d hydrogen t r a n s m i s s i o n w h i l s t t h e Ni-La203 p a r t a c t s as t h e hydrogen a c c e p t o r .
Furthermore, when t h i s
c a t a l y s t system was adopted f o r c a t a l y t i c r e a c t i o n s between a c t i v e carbon and some o x i d a t i v e gases, such as NO ( r e f . 5 ) and 02 ( r e f . 6 ) , t h e s y n e r g i s t i c e f f e c t o f c a t a l y t i c a c t i v i t y was a l s o observed, where t h e a c t i v i t y o f t h e t h r e e component c a t a l y s t i s f a r beyond t h e sum o f t h e a c t i v i t i e s o f t h e c o n s t i t u e n t c a t a l y s t s . The s u p p o r t i n g procedure o f t h e three-component c a t a l y s t on t h e supp o r t a f f e c t e d t h e o x i d a t i o n a c t i v i t y t o a marked degree ( r e f . 7 ) , as w e l l as on t h e h y d r o g e n a t i o n a c t i v i t y ( r e f . 1 ) . These f a c t s i n d i c a t e t h a t t h e r e i s enhancement o f oxygen t r a n s m i s s i o n which resembles hydrogen s p i l l o v e r , a l t h o u g h such
182
enhancement with oxygen has seldom been reported. We will present here a new important application of t h i s c a t a l y s t system f o r complete removal of oxygen contamination in hydrogen gas with high e f f i c i e n c y , since the demand f o r pure hydrogen production i s increasing i n various f i e l d s . EXPERIMENTAL Catalysts Fiberfrax produced by Carborundum Co. L t d was used as the base material f o r c a t a l y s t support. I t i s made of f i n e ceramic f i b e r s in a p l a t e form of 1 mm thickness. I t was impregnated with colloidal s i l i c a and then dried. The s i l i c a content was 17.8 w t % and t h e voidness was 88%. This support was impregnated with an aqueous chloride or n i t r a t e solution of a platinum-group metal. I t was dried a n d heated t o 350°C, and then reduced by heating t o 400°C f o r 2 hr in a nitrogen stream containing 10% hydrogen. This was then impregnated with a s o l u t i o n of mixed n i t r a t e s a l t s of iron-group and lanthanide-group elements, The reduction procedure was then repeated. The compositions of t h e c a t a l y s t s a r e presented in Table 1 . TABLE I Composition of c a t a l y s t s
Catalyst number 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Compos i t i on 13.9% Ni- 3.4% La2O313.9% Ni- 3.4% LapO313.9% Ni- 3.4% LapO313.9% Ni- 3.4% Ce2O33.9% Ni- 3.4% Ce2O33.9% Co- 3.4% La2O33.9% Co- 3.4% La9O.l3.9% Co- 3.4% La;Oi4.6% Ni 4.0% Ce2O3 0.5% P t 4.1% Ni- 3.4% Ce2O3 4.5% Ni- 0.4% P t 4.0% Ce2O3- 0.5% P t
0.4% Rh 0.4% P t 0.4% Pd 0.4% Rh 0.4% P t 0.4% P t 0.4% Ru 0.4% Rh
Apparatus and reaction method An ordinary flow-reaction apparatus was used a t atmospheric pressure, however as shown in Fig. l the r e a c t o r was not e l e c t r i c a l l y heated and t o avoid the e f f e c t of convection of theahlosphere on the c a t a l y s t temperature the r e a c t o r was covered by a quartz tube of 1 mm thickness and 13 mm inner diameter. The c a t a l y s t was shaped in a disk form of 10 mm diameter, and usually two pieces o f the c a t a l y s t were s e t in a quartz tube of 1 mm thickness and 10 mm inner diameter as shown in Fig. 2. I n order t o d i s t r i b u t e the reaction gas uniformly, two sheets of 40 mesh s t a i n l e s s s t e e l net were inserted between two
183
u
u
Fig. 1. Schematic diagram of flow-reaction apparatus Product gas
I
Stainless s t e e l net
0 "
Reactor
fl
Reaction gas Fig. 2. S t a t e of the c a t a l y s t packing
I
Flow meter
Si Gas
Micro-TG Apparatus
cvlinder
Fig. 3. Apparatus f o r t h e oxidation s t a t e and hydrogen adsorption measurements
184
pieces of the c a t a l y s t . When heating was necessary, the reaction gas was heated before t h e i n l e t p a r t o f r e a c t o r tube by an e l e c t r i c a l heater. The catalyst-bed temperature was measured by attaching the t o p of a thermocouple t o the center of the c a t a l y s t disk located a t the i n l e t side. The reaction gas was composed of hydrogen containing 1.3 3.5% 02 (usually
-
1.3%). The space velocity ( S V ) was changed within the range from 33,000 t o 480,000 hr-'. Usually the reaction gas was introduced t o the c a t a l y s t bed a t room temperature. The reaction gas was analyzed by a n on-line gas chromatograph. The oxidation s t a t e of t h e c a t a l y s t metal during t h e reaction was measured by a micro thermo-gravimetric analyzer equipped with a gas flow control and a gas p u r i f i c a t i o n systems. RESULTS AND DISCUSSION Performance of various nickel-based three-component c a t a l y s t s Reaction gas composed of 1.3% 02 and 98.7% Hz was allowed t o flow over the c a t a l y s t bed a t room temperature f o r varying SV from 50,900 t o 4 9 0 , 0 0 0 hr-l. 02 conversion f o r various c a t a l y s t s with d i f f e r e n t SV i s shown in Fig. 4. Catalyst 1 (Ni-LazO3-Rh) maintained 100%02-conversion with SV below 200,000 hr-I, however, above t h i s SV, 02 conversion decreased suddenly and no reaction occurred except f o r heated gas. Catalysts 2 and 4 (Ni-LazO-j-Pt a n d Ni-CezO3-Rh) showed the same tendency. Catalyst 3 (Ni-LapO3-Pd) showed a t l e a s t 70% 02-conve r s i o n even with a low SV, such a s 50,000 hr-1. On t h e o t h e r hand, c a t a l y s t 5
100 h
be
Y
z
0 U
z
I
0 Ni -La203-Pt Ni-La203-Rh D Ni -Ce203-Rh
0
I
0 Ni-La203-Pd
2 W
50
I
I
w
0 c\1
0
0
5
10
15
sv
20
25
30
( 1o
- hr-I ~
35
40
45
50
Fig. 4. Effect of space velocity on 02 conversion f o r various Ni-based composite c a t a l y s t s
185
z
0 c (
m
“>
I I I
50
I
I I I I
I
W
z
I
0 V
0 Co-Laz03-Pt A Co-Laz03-Rh
1
cu
0 Co-Laz03-Ru
I
0
0
5
10
15
I
I
20
25
F i g . 5. E f f e c t o f space v e l o c i t y on O z c o n v e r s i o n f o r v a r i o u s Co-based composite c a t a l y s t s
-
300
300 0 : Ni-La203-Pt
‘I
II 11
A: Ni-Ce203-Rh
u
v
200
0 : Ni-La203-Pd /
W
0: 3 I-
h
:d II II f
a
200
I f I
w
i i
p:
z
I
a
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p.
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100
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v)
v)
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2 I-
2 4 I-
5
P
0:Co-La203-Pt A: Co-La203-Rh
0 : Co-La203-Ru
5 0
5
10
sv
15
20
25
0
(10-~1,,--1)
F i g . 6. C a t a l y s t t e m p $ r a t u r e r e q u i r e d t o a c h i e v e 100% OZ-conversion(brocken 1 ine), (a)The Ni-based c a t a l y s t s .
( b ) The Co-based c a t a l y s t s .
186 (Ni-CezO3-Pt) c o n v e r t e d O 2 c o m p l e t e l y b e l o w SV o f 410,000 h r - 1 and f i r s t showed a t e n d e n c y t o d e c r e a s e 02 c o n v e r s i o n w i t h S V o f 480,000 h r - 1 . As shown i n F i g . 3 t h e a c t i v i t y o f Co-based c o m p o s i t e c a t a l y s t s were s m a l l e r t h a n t h o s e o f N i - b a s e d catalysts. Complete 02 c o n v e r s i o n on m o d e r a t e l y a c t i v e c a t a l y s t s b y h e a t i n g t h e r e a c t i o n
gas
As shown i n F i g . 6, f o r t h e m o d e r a t e l y a c t i v e N i - b a s e d
and Co-based c a t a -
l y s t s , when t h e r e a c t i o n gas was h e a t e d b e f o r e e n t r a n c e i n t o t h e c a t a l y s t bed
100% 02 c o n v e r s i o n was a c h i e v e d a l t h o u g h t h e t e m p e r a t u r e was d i f f e r e n t w i t h SV Change i n c a t a l y s t - b e d t e m p e r a t u r e w i t h c h a n g i n g 02 c o n c e n t r a t i o n The e f f e c t o f 02 c o n c e n t r a t i o n on t h e c a t a l y s t - b e d t e m p e r a t u r e was i n v e s t i g a t e d u s i n g c a t a l y s t 2 (Ni-La203-Pt)
u n d e r t h e c o n d i t i o n s o f SV 50,000 h r - 1 a t
r o o m - t e m p e r a t u r e f e e d . I n t h e w h o l e r a n g e o f 02 c o n c e n t r a t i o n (0.4 - 3 . 4 % )
02
c o n v e r t e d c o m p l e t e l y and a f t e r a b o u t 1 5 w i n f r o m b e g i n i n g o f t h e r e a c t i o n gas f l o w t h e c a t a l y s t - b e d t e m p e r a t u r e a t t a i n e d each s t e a d y - s t a t e l e v e l as shown i n F i g . 7.
3.4 %
1
1.2 %
50
0
5
TIME
10
15
20
(min)
F i g . 7. Change i n c a t a l y s t t e m p e r a t u r e w i t h t i m e on s t r e a m f o r each i n l e t 02-concentration
187
The r e l a t i o n between t h i s s t e a d y - s t a t e temperature ( t ; " C ) and 02 c o n c e n t r a t i o n i n f e e d gas ( C t = 10
1.8co2
O2
; % ) was expressed as t h e e q u a t i o n ,
0.55
Thus, t h e ' h i g h l y a c t i v e c a t a l y s t s such as 5 and 2 can c o n v e r t 02 c o m p l e t e l y i r r e s p e c t i v e o f 02 c o n c e n t r a t i o n even s u p p l y i n g t h e f e e d gas a t room temperature. However, as shown by t h e d a t a i n Table 2 f o r t h e m o d e r a t e l y a c t i v e c a t a l y s t s such as 6 and 8 , i n s p i t e o f i n c r e a s i n g 02 c o n c e n t r a t i o n t o more t h a n 3%, no enhancement o f t h e r e a c t i o n was observed. These f a c t s i n d i c a t e t h a t t h e c o n t r o l l i n g f a c t o r o f O2 c o n v e r s i o n a t room temperature would be i n t h e o x i d a t i o n s t a t e o f t h e c a t a l y s t m e t a l d u r i n g t h e r e a c t i o n r a t h e r t h a n t h e k i n e t i c c o n d i t i o n such as 02 c o n c e n t r a t i o n . TABLE 2 E f f e c t o f 02 c o n c e n t r a t i o n on 02 c o n v e r s i o n f o r m o d e r a t e l y a c t i v e c a t a l y s t s
02 c o n c e n t r a t i o n 02 c o n v e r s i o n 02 space-time conv.
C a t a l y s t number
Flow r a t e (l/hr)
8 8 8 8 8
10.0 17.6 28.5 30.0 29.8
1.3 1.3 1.3 2.0 3.1
6 6 6 6 6
9.0 21 .o 18.9 19.8 9.3
1.3 1.3 2.0 3.5 2.2
(%I
(%I
100 100 0 0 0
86.0 91 .a 97.0 96.6 96.8
(mol/l. hr 36.9 65.0 0 0 0 28.6 70.6 104 190 56.3
S y n e r g i s t i c e f f e c t o f c a t a l y s t c o m b i n a t i o n on t h e c a t a l y t i c a c t i v i t y I n o r d e r t o c o n f i r m t h e s y n e r g i s t i c e f f e c t o f combining t h r e e c a t a l y s t components, performance o f t h e s i n g l e and two-component c a t a l y s t s were compared w i t h t h a t o f t h e three-component c a t a l y s t , Ni-CepO3-Pt.
As shown i n F i g . 8,
w i t h SV 50,000 h r - l , f o r c a t a l y s t s o f N i , Ce2O3, and Ni-CezOg no 02 c o n v e r s i o n was observed. On t h e o t h e r hand, P t , N i - P t , 98.3,
and Ce203-Pt c o n v e r t e d O2 96.6,
and 99.1%, r e s p e c t i v e l y , b u t c o u l d n o t be achieved complete 02 c o n v e r s i o n .
A t c o n d i t i o n s o f h i g h e r space v e l o c i t y t h a n 50,000 h r - l , t h e d i f f e r e n c e o f t h e performance between P t and Ni-CepO3-Pt was more e v i d e n t . These r e s u l t s i n d i c a t e t h a t , f o r t h e s u p p l y o f t h e f e e d gas a t room temperature, t h e r e a c t i o n occurs on t h e s i t e o f t h e P t p a r t . The c a t a l y s t - b e d temperature i s r a i s e d by t h e h e a t o f r e a c t i o n as mentioned b e f o r e and t h e a c t i v i t y o f Ni-Ce203 p a r t j o i n s i n t h e r e a c t i o n . However, t h e pronounced enhancement i n t h e c a t a l y t i c a c t i v i t y o f N i Ce2Oj-Pt composite c a t a l y s t s s t i l l cannot be e x p l a i n e d by t h e s i m p l e temperature factor.
188
- -
1
w h
M
v
z
0 u
v, cz W >
50
-
Ni-CezOg-Pt 0 Ni-Pt
0 z
0 CeZ03-Pt
V
0 Pt
(v
0
0
d
I
I
I
5
10
15
20
25
I
I
I
I
30
35
40
45
50
sv Fig. 8. Performance of various c a t a l y s t s o f Ni-Ce&-Pt 02 i n l e t concentration: 1.3%, i n l e t temperature: 20°C.
system.
Oxidation s t a t e of c a t a l y s t metal during the reaction Three typical examples which had the d i f f e r e n t c a t a l y t i c a c t i v i t i e s were employed f o r the comparison o f t h e oxidation s t a t e o f the metal during the reaction. A piece of the c a t a l y s t sample of 20 mg was placed on t h e sample pan of the micro thermogravimetric analyzer and was reduced by hydrogen flow up t o 400°C. I t was cooled t o the same temperature as the steady s t a t e temperature of
0.2 Ni
Ni-La203-Pd
0.1
I
0
Ni-Ce203-Pt
10 TIME (min)
Fig. 9. Weight increase o f t h e c a t a l y s t s during t h e oxidation of hydrogen
189 t h e f l o w r e a c t i o n w i t h room-temperature f e e d as shown i n F i g . 7. Hydroaen c o n t a i n i n g 1.3% O 2 was t h e n a l l o w e d t o f l o w t h r o u g h t h e c a t a l y s t sample a t each s t e a d y - s t a t e temperature. R e s u l t s a r e shown i n F i g . 9. The l e s s a c t i v e c a t a l y s t ( N i , 02 c o n v e r s i o n was 0% a t room t e m p e r a t u r e ) was o x i d i z e d e a s i l y by Oxygen i n t h e r e a c t i o n gas. The w e i g h t i n c r e a s e corresponds t o 59.4% o f t h a t i n t h e o x i d a t i o n a t 100°C w i t h 1.3% 02 d i l u t e d w i t h He. For t h e m o d e r a t e l y a c t i v e c a t a l y s t (Ni-La203-Pd, O 2 c o n v e r s i o n was 69%), t h e degree o f o x i d a t i o n d u r i n g t h e r e a c t i o n was l e s s t h a n t h a t o f t h e N i c a t a l y s t . On t h e o t h e r hand, c a t a l y s t 5 (Ni-Ce203-Pt) which have t h e a c t i v i t y o f 100% 02 c o n v e r s i o n showed n e g l i g i b l e w e i g h t i n c r e a s e d u r i n g t h e r e a c t i o n i n d i c a t i n g t h a t almost a complete reduced s t a t e o f t h e c a t a l y s t m e t a l s was m a i n t a i n e d . A d s o r p t i o n c a p a c i t y o f hydrogen
A
20 mg p o r t i o n
of t h e c a t a l y s t sample i n t h e sample pan o f t h e thermo-
g r a v i m e t r i c a n a l y z e r was reduced in s i t u w i t h hydrogen a t 400°C f o r 20 min, and t h e n c o o l e d t o room temperature i n hydrogen f l o w . To t h i s sample 1.3% 02 d i l u t e d w i t h He was a l l o w e d t o f l o w w i t h a f l o w r a t e o f 50 ml/min. The w e i g h t i n c r e a s e i n t h e e a r l y stage was g r a d u a l l y decreased w i t h t i m e on stream and a t t a i n e d a c o n s t a n t l e v e l f o r each c a t a l y s t . The excess w e i g h t i n c r e a s e o t h e r t h a n o x i d a t i o n i s a t t r i b u t e d t h e adsorbed H20 which i s formed by t h e o x i d a t i o n o f absorbed hydrogen. The hydrogen amounts measured by t h i s method a r e l i s t e d i n T a b l e 3 f o r v a r i o u s c a t a l y s t s o f N i - C e e O y P t system. As can be seen i n t h i s d a t a hydrogen a d s o r p t i o n c a p a c i t y corresponds t o t h e c a t a l y t i c a c t i v i t y . TABLE 3 Uptakes o f oxygen and hydrogen f o r v a r i o u s c a t a l y s t s Catalyst Catalyst number constituent
9 11 13 12 14 5
Ni Pt Ni-Pt Ni-Ce203 CepO3-Pt Ni-Ce707-Pt
Oxygen u p t a k e o f Amount o f H20 formed Amount of absorbed t h e reduced by t h e room-tempera- hydrogen c a t a l y s t a t 100°C t u r e o x i d a t i o n o f adsorbed hydrogen (mg/g-catalyst) (mg/g-catalyst) (mg/g-catalyst) 13.30 0 0 4.77 0.53 0 8.75 1.93 0.21 0 0 11.15 10.75 1.20 0 11.90 25.45 2.83
CONCLUSION On t h e b a s i s o f e x p e r i m e n t a l r e s u l t s t h e s y n e r g i s t i c e f f e c t s o f t h e t h r e e component c a t a l y s t were i n t e r p r e t e d as f o l l o w s , u s i n g t h e example o f t h e N i Ce203-Pt c a t a l y s t . P t , which is c l o s e t o b u t s e p a r a t e f r o m Ni-Ce203,
a c t s as
t h e t r a n s p o r t a g e n t o f hydrogen a d s o r p t i o n , and t h e adsorbed hydrogen d i f f u s e s
by s p i l l o v e r t o Ni-Ce203 p a r t which a c t s as t h e hydrogen a c c e p t o r . Consequently,
190
t h e Ni m a i n t a i n s t h e reduced s t a t e and i t s c a t a l y t i c a c t i v i t y j o i n s t o t h e a c t i v i t y o f P t p a r t . The c a t a l y s t t e m p e r a t u r e i s r a i s e d by t h e r e a c t i o n h e a t and t h e pronounced c a t a l y t i c a c t i v i t y i s e x e r t e d .
REFERENCES T. I n u i , M. F u n a b i k i , M . S u e h i r o , And T. Sezume, J . C . S . Faraday I , 75 ( 1 9 7 9 ) 787-802. 2 T. I n u i , M. F u n a b i k i , and Y . Takegami, Ind. Eng. Chem. Prod. Res. Dev., 19 ( 1 980) 385-388. 3 T. I n u i , K. Ueno, M. F u n a b i k i , M. S u e h i r o , T. Sezume, and Y. Takegami, J . C . S. Faraday I , 75 (1979) 1495-1506. 4 T. I n u i , M. F u n a b i k i , and Y. Takegami, J . C. S. Faraday I , 76 (1980) 22372250. 5 T. I n u i , T. Otowa, and Y. Takegami, Ind. Eng. Chem. Prod. Res. Dev., 21 (1 9 8 2 ) 56-59. 6 T. I n u i , T. Otowa, K. T s u c h i h a s h i , and Y . Takegami, Carbon, 20 (1982) 213217. 7 T. I n u i , T. Otowa, and Y. Takegami, J . C a t a l . , 76 ( 1 9 8 2 ) 84-92.
1
G.M.Pajonk, S.J. Teichner and J.E. Germain (Editors), Spillouer of Adsorbed Species 0
1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
LOW TEMPERATURE OXIDATION OF HYDROGEN ENHANCED BY SPILLOVER ON A NICKEL-BASED COMPOSITE CATALYST
T. I N U I , Y. MIYAMOTO, and Y . TAKEGAMI Department o f Hydrocarbon Chemistry, F a c u l t y o f E n g i n e e r i n g , Kyoto U n i v e r s i t y , Sakyo-ku,
Kyoto 606 (Japan)
ABSTRACT The removal o f a small c o n c e n t r a t i o n o f 0 i n hydrogen gas by c a t a l y t i c combustion o f hydrogen has been s t u d i e d . A tiree-component composite c a t a l y s t , Ni-CesO-j-Pt, s u p p o r t e d on a t h i n seramic f i b e r p l a t e ( F i b e r f r a x ) e x h i b i t e d e x c e l t e n t performance. Using t h i s c a t a l y s t hydrogen combustion o c c u r r e d a t room temperature, and 1.3% 02 i n H2 was e l i m i n a t e d t o below 1 ppm even w i t h a gaseous h o u r l y space v e l o c i t y o f 410,000. On t h e b a s i s o f comparison among t h e a c t i v i t i e s o f v a r i o u s c a t a l y s t s and o x i d a t i o n s t a t e s o f t h e c a t a l y s t metal d u r i n g t h e r e a c t i o n t h e s y n e r g i s t i c e f f e c t due t o hydrogen s p i l l o v e r was c o n f i r m e d . INTRODUCTION I t has been w i d e l y f o u n d t h a t when s m a l l c o n c e n t r a t i o n o f p l a t i n u m - g r o u p
m e t a l s such as P t o r Pd were mixed w i t h t r a n s i t i o n - m e t a l o x i d e s , t h e necessary temperature f o r hydrogen r e d u c t i o n lowered d r a m a t i c a l l y . T h i s phenomenon has been i n t e r p r e t e d as a hydrogen s p i l l o v e r e f f e c t . R e c e n t l y much e f f o r t has been c o n c e n t r a t e d on i t , s i n c e t h e s p i l l o v e r e f f e c t i s c o n s i d e r e d t o be one o f t h e p r i n c i p a l d i r e c t i o n s o f c a t a l y s t design f o r the preparation o f a h i g h l y a c t i v e catalyst. We have a l r e a d y r e p o r t e d t h a t composite c a t a l y s t s such as N i - L a 2 0 3 - R ~ supp o r t e d on s i l i c a , alumina, o r a c t i v e carbon e x e r t e d h i g h a c t i v i t y f o r methane s y n t h e s i s by h y d r o g e n a t i o n o f C O Y C02 ( r e f s . 1 and 2 ) , and a c t i v e carbon ( r e f . 3 ) . C o r r e l a t i o n between a d s o r p t i o n c a p a c i t i e s o f CO, COP, and H2 on t h e three-compon e n t c a t a l y s t system and t h e i r c a t a l y t i c a c t i v i t i e s were observed ( r e f . 4), and we have proposed t h a t these h i g h a c t i v i t i e s a r e caused by a hydrogen s p i l l o v e r supposing t h e Ru p a r t as t h e t r a n s p o r t agent f o r t h e r a p i d hydrogen t r a n s m i s s i o n w h i l s t t h e Ni-La203 p a r t a c t s as t h e hydrogen a c c e p t o r .
Furthermore, when t h i s
c a t a l y s t system was adopted f o r c a t a l y t i c r e a c t i o n s between a c t i v e carbon and some o x i d a t i v e gases, such as NO ( r e f . 5 ) and 02 ( r e f . 6 ) , t h e s y n e r g i s t i c e f f e c t o f c a t a l y t i c a c t i v i t y was a l s o observed, where t h e a c t i v i t y o f t h e t h r e e component c a t a l y s t i s f a r beyond t h e sum o f t h e a c t i v i t i e s o f t h e c o n s t i t u e n t c a t a l y s t s . The s u p p o r t i n g procedure o f t h e three-component c a t a l y s t on t h e supp o r t a f f e c t e d t h e o x i d a t i o n a c t i v i t y t o a marked degree ( r e f . 7 ) , as w e l l as on t h e h y d r o g e n a t i o n a c t i v i t y ( r e f . 1 ) . These f a c t s i n d i c a t e t h a t t h e r e i s enhancement o f oxygen t r a n s m i s s i o n which resembles hydrogen s p i l l o v e r , a l t h o u g h such
182
enhancement with oxygen has seldom been reported. We will present here a new important application of t h i s c a t a l y s t system f o r complete removal of oxygen contamination in hydrogen gas with high e f f i c i e n c y , since the demand f o r pure hydrogen production i s increasing i n various f i e l d s . EXPERIMENTAL Catalysts Fiberfrax produced by Carborundum Co. L t d was used as the base material f o r c a t a l y s t support. I t i s made of f i n e ceramic f i b e r s in a p l a t e form of 1 mm thickness. I t was impregnated with colloidal s i l i c a and then dried. The s i l i c a content was 17.8 w t % and t h e voidness was 88%. This support was impregnated with an aqueous chloride or n i t r a t e solution of a platinum-group metal. I t was dried a n d heated t o 350°C, and then reduced by heating t o 400°C f o r 2 hr in a nitrogen stream containing 10% hydrogen. This was then impregnated with a s o l u t i o n of mixed n i t r a t e s a l t s of iron-group and lanthanide-group elements, The reduction procedure was then repeated. The compositions of t h e c a t a l y s t s a r e presented in Table 1 . TABLE I Composition of c a t a l y s t s
Catalyst number 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Compos i t i on 13.9% Ni- 3.4% La2O313.9% Ni- 3.4% LapO313.9% Ni- 3.4% LapO313.9% Ni- 3.4% Ce2O33.9% Ni- 3.4% Ce2O33.9% Co- 3.4% La2O33.9% Co- 3.4% La9O.l3.9% Co- 3.4% La;Oi4.6% Ni 4.0% Ce2O3 0.5% P t 4.1% Ni- 3.4% Ce2O3 4.5% Ni- 0.4% P t 4.0% Ce2O3- 0.5% P t
0.4% Rh 0.4% P t 0.4% Pd 0.4% Rh 0.4% P t 0.4% P t 0.4% Ru 0.4% Rh
Apparatus and reaction method An ordinary flow-reaction apparatus was used a t atmospheric pressure, however as shown in Fig. l the r e a c t o r was not e l e c t r i c a l l y heated and t o avoid the e f f e c t of convection of theahlosphere on the c a t a l y s t temperature the r e a c t o r was covered by a quartz tube of 1 mm thickness and 13 mm inner diameter. The c a t a l y s t was shaped in a disk form of 10 mm diameter, and usually two pieces o f the c a t a l y s t were s e t in a quartz tube of 1 mm thickness and 10 mm inner diameter as shown in Fig. 2. I n order t o d i s t r i b u t e the reaction gas uniformly, two sheets of 40 mesh s t a i n l e s s s t e e l net were inserted between two
183
u
u
Fig. 1. Schematic diagram of flow-reaction apparatus Product gas
I
Stainless s t e e l net
0 "
Reactor
fl
Reaction gas Fig. 2. S t a t e of the c a t a l y s t packing
I
Flow meter
Si Gas
Micro-TG Apparatus
cvlinder
Fig. 3. Apparatus f o r t h e oxidation s t a t e and hydrogen adsorption measurements
184
pieces of the c a t a l y s t . When heating was necessary, the reaction gas was heated before t h e i n l e t p a r t o f r e a c t o r tube by an e l e c t r i c a l heater. The catalyst-bed temperature was measured by attaching the t o p of a thermocouple t o the center of the c a t a l y s t disk located a t the i n l e t side. The reaction gas was composed of hydrogen containing 1.3 3.5% 02 (usually
-
1.3%). The space velocity ( S V ) was changed within the range from 33,000 t o 480,000 hr-'. Usually the reaction gas was introduced t o the c a t a l y s t bed a t room temperature. The reaction gas was analyzed by a n on-line gas chromatograph. The oxidation s t a t e of t h e c a t a l y s t metal during t h e reaction was measured by a micro thermo-gravimetric analyzer equipped with a gas flow control and a gas p u r i f i c a t i o n systems. RESULTS AND DISCUSSION Performance of various nickel-based three-component c a t a l y s t s Reaction gas composed of 1.3% 02 and 98.7% Hz was allowed t o flow over the c a t a l y s t bed a t room temperature f o r varying SV from 50,900 t o 4 9 0 , 0 0 0 hr-l. 02 conversion f o r various c a t a l y s t s with d i f f e r e n t SV i s shown in Fig. 4. Catalyst 1 (Ni-LazO3-Rh) maintained 100%02-conversion with SV below 200,000 hr-I, however, above t h i s SV, 02 conversion decreased suddenly and no reaction occurred except f o r heated gas. Catalysts 2 and 4 (Ni-LazO-j-Pt a n d Ni-CezO3-Rh) showed the same tendency. Catalyst 3 (Ni-LapO3-Pd) showed a t l e a s t 70% 02-conve r s i o n even with a low SV, such a s 50,000 hr-1. On t h e o t h e r hand, c a t a l y s t 5
100 h
be
Y
z
0 U
z
I
0 Ni -La203-Pt Ni-La203-Rh D Ni -Ce203-Rh
0
I
0 Ni-La203-Pd
2 W
50
I
I
w
0 c\1
0
0
5
10
15
sv
20
25
30
( 1o
- hr-I ~
35
40
45
50
Fig. 4. Effect of space velocity on 02 conversion f o r various Ni-based composite c a t a l y s t s
185
z
0 c (
m
“>
I I I
50
I
I I I I
I
W
z
I
0 V
0 Co-Laz03-Pt A Co-Laz03-Rh
1
cu
0 Co-Laz03-Ru
I
0
0
5
10
15
I
I
20
25
F i g . 5. E f f e c t o f space v e l o c i t y on O z c o n v e r s i o n f o r v a r i o u s Co-based composite c a t a l y s t s
-
300
300 0 : Ni-La203-Pt
‘I
II 11
A: Ni-Ce203-Rh
u
v
200
0 : Ni-La203-Pd /
W
0: 3 I-
h
:d II II f
a
200
I f I
w
i i
p:
z
I
a
dW
n E
E
u
0
1
/’
w
p.
z
p
100
I-
100
I-
*
v)
v)
>
2 I-
2 4 I-
5
P
0:Co-La203-Pt A: Co-La203-Rh
0 : Co-La203-Ru
5 0
5
10
sv
15
20
25
0
(10-~1,,--1)
F i g . 6. C a t a l y s t t e m p $ r a t u r e r e q u i r e d t o a c h i e v e 100% OZ-conversion(brocken 1 ine), (a)The Ni-based c a t a l y s t s .
( b ) The Co-based c a t a l y s t s .
186 (Ni-CezO3-Pt) c o n v e r t e d O 2 c o m p l e t e l y b e l o w SV o f 410,000 h r - 1 and f i r s t showed a t e n d e n c y t o d e c r e a s e 02 c o n v e r s i o n w i t h S V o f 480,000 h r - 1 . As shown i n F i g . 3 t h e a c t i v i t y o f Co-based c o m p o s i t e c a t a l y s t s were s m a l l e r t h a n t h o s e o f N i - b a s e d catalysts. Complete 02 c o n v e r s i o n on m o d e r a t e l y a c t i v e c a t a l y s t s b y h e a t i n g t h e r e a c t i o n
gas
As shown i n F i g . 6, f o r t h e m o d e r a t e l y a c t i v e N i - b a s e d
and Co-based c a t a -
l y s t s , when t h e r e a c t i o n gas was h e a t e d b e f o r e e n t r a n c e i n t o t h e c a t a l y s t bed
100% 02 c o n v e r s i o n was a c h i e v e d a l t h o u g h t h e t e m p e r a t u r e was d i f f e r e n t w i t h SV Change i n c a t a l y s t - b e d t e m p e r a t u r e w i t h c h a n g i n g 02 c o n c e n t r a t i o n The e f f e c t o f 02 c o n c e n t r a t i o n on t h e c a t a l y s t - b e d t e m p e r a t u r e was i n v e s t i g a t e d u s i n g c a t a l y s t 2 (Ni-La203-Pt)
u n d e r t h e c o n d i t i o n s o f SV 50,000 h r - 1 a t
r o o m - t e m p e r a t u r e f e e d . I n t h e w h o l e r a n g e o f 02 c o n c e n t r a t i o n (0.4 - 3 . 4 % )
02
c o n v e r t e d c o m p l e t e l y and a f t e r a b o u t 1 5 w i n f r o m b e g i n i n g o f t h e r e a c t i o n gas f l o w t h e c a t a l y s t - b e d t e m p e r a t u r e a t t a i n e d each s t e a d y - s t a t e l e v e l as shown i n F i g . 7.
3.4 %
1
1.2 %
50
0
5
TIME
10
15
20
(min)
F i g . 7. Change i n c a t a l y s t t e m p e r a t u r e w i t h t i m e on s t r e a m f o r each i n l e t 02-concentration
187
The r e l a t i o n between t h i s s t e a d y - s t a t e temperature ( t ; " C ) and 02 c o n c e n t r a t i o n i n f e e d gas ( C t = 10
1.8co2
O2
; % ) was expressed as t h e e q u a t i o n ,
0.55
Thus, t h e ' h i g h l y a c t i v e c a t a l y s t s such as 5 and 2 can c o n v e r t 02 c o m p l e t e l y i r r e s p e c t i v e o f 02 c o n c e n t r a t i o n even s u p p l y i n g t h e f e e d gas a t room temperature. However, as shown by t h e d a t a i n Table 2 f o r t h e m o d e r a t e l y a c t i v e c a t a l y s t s such as 6 and 8 , i n s p i t e o f i n c r e a s i n g 02 c o n c e n t r a t i o n t o more t h a n 3%, no enhancement o f t h e r e a c t i o n was observed. These f a c t s i n d i c a t e t h a t t h e c o n t r o l l i n g f a c t o r o f O2 c o n v e r s i o n a t room temperature would be i n t h e o x i d a t i o n s t a t e o f t h e c a t a l y s t m e t a l d u r i n g t h e r e a c t i o n r a t h e r t h a n t h e k i n e t i c c o n d i t i o n such as 02 c o n c e n t r a t i o n . TABLE 2 E f f e c t o f 02 c o n c e n t r a t i o n on 02 c o n v e r s i o n f o r m o d e r a t e l y a c t i v e c a t a l y s t s
02 c o n c e n t r a t i o n 02 c o n v e r s i o n 02 space-time conv.
C a t a l y s t number
Flow r a t e (l/hr)
8 8 8 8 8
10.0 17.6 28.5 30.0 29.8
1.3 1.3 1.3 2.0 3.1
6 6 6 6 6
9.0 21 .o 18.9 19.8 9.3
1.3 1.3 2.0 3.5 2.2
(%I
(%I
100 100 0 0 0
86.0 91 .a 97.0 96.6 96.8
(mol/l. hr 36.9 65.0 0 0 0 28.6 70.6 104 190 56.3
S y n e r g i s t i c e f f e c t o f c a t a l y s t c o m b i n a t i o n on t h e c a t a l y t i c a c t i v i t y I n o r d e r t o c o n f i r m t h e s y n e r g i s t i c e f f e c t o f combining t h r e e c a t a l y s t components, performance o f t h e s i n g l e and two-component c a t a l y s t s were compared w i t h t h a t o f t h e three-component c a t a l y s t , Ni-CepO3-Pt.
As shown i n F i g . 8,
w i t h SV 50,000 h r - l , f o r c a t a l y s t s o f N i , Ce2O3, and Ni-CezOg no 02 c o n v e r s i o n was observed. On t h e o t h e r hand, P t , N i - P t , 98.3,
and Ce203-Pt c o n v e r t e d O2 96.6,
and 99.1%, r e s p e c t i v e l y , b u t c o u l d n o t be achieved complete 02 c o n v e r s i o n .
A t c o n d i t i o n s o f h i g h e r space v e l o c i t y t h a n 50,000 h r - l , t h e d i f f e r e n c e o f t h e performance between P t and Ni-CepO3-Pt was more e v i d e n t . These r e s u l t s i n d i c a t e t h a t , f o r t h e s u p p l y o f t h e f e e d gas a t room temperature, t h e r e a c t i o n occurs on t h e s i t e o f t h e P t p a r t . The c a t a l y s t - b e d temperature i s r a i s e d by t h e h e a t o f r e a c t i o n as mentioned b e f o r e and t h e a c t i v i t y o f Ni-Ce203 p a r t j o i n s i n t h e r e a c t i o n . However, t h e pronounced enhancement i n t h e c a t a l y t i c a c t i v i t y o f N i Ce2Oj-Pt composite c a t a l y s t s s t i l l cannot be e x p l a i n e d by t h e s i m p l e temperature factor.
188
- -
1
w h
M
v
z
0 u
v, cz W >
50
-
Ni-CezOg-Pt 0 Ni-Pt
0 z
0 CeZ03-Pt
V
0 Pt
(v
0
0
d
I
I
I
5
10
15
20
25
I
I
I
I
30
35
40
45
50
sv Fig. 8. Performance of various c a t a l y s t s o f Ni-Ce&-Pt 02 i n l e t concentration: 1.3%, i n l e t temperature: 20°C.
system.
Oxidation s t a t e of c a t a l y s t metal during the reaction Three typical examples which had the d i f f e r e n t c a t a l y t i c a c t i v i t i e s were employed f o r the comparison o f t h e oxidation s t a t e o f the metal during the reaction. A piece of the c a t a l y s t sample of 20 mg was placed on t h e sample pan of the micro thermogravimetric analyzer and was reduced by hydrogen flow up t o 400°C. I t was cooled t o the same temperature as the steady s t a t e temperature of
0.2 Ni
Ni-La203-Pd
0.1
I
0
Ni-Ce203-Pt
10 TIME (min)
Fig. 9. Weight increase o f t h e c a t a l y s t s during t h e oxidation of hydrogen
189 t h e f l o w r e a c t i o n w i t h room-temperature f e e d as shown i n F i g . 7. Hydroaen c o n t a i n i n g 1.3% O 2 was t h e n a l l o w e d t o f l o w t h r o u g h t h e c a t a l y s t sample a t each s t e a d y - s t a t e temperature. R e s u l t s a r e shown i n F i g . 9. The l e s s a c t i v e c a t a l y s t ( N i , 02 c o n v e r s i o n was 0% a t room t e m p e r a t u r e ) was o x i d i z e d e a s i l y by Oxygen i n t h e r e a c t i o n gas. The w e i g h t i n c r e a s e corresponds t o 59.4% o f t h a t i n t h e o x i d a t i o n a t 100°C w i t h 1.3% 02 d i l u t e d w i t h He. For t h e m o d e r a t e l y a c t i v e c a t a l y s t (Ni-La203-Pd, O 2 c o n v e r s i o n was 69%), t h e degree o f o x i d a t i o n d u r i n g t h e r e a c t i o n was l e s s t h a n t h a t o f t h e N i c a t a l y s t . On t h e o t h e r hand, c a t a l y s t 5 (Ni-Ce203-Pt) which have t h e a c t i v i t y o f 100% 02 c o n v e r s i o n showed n e g l i g i b l e w e i g h t i n c r e a s e d u r i n g t h e r e a c t i o n i n d i c a t i n g t h a t almost a complete reduced s t a t e o f t h e c a t a l y s t m e t a l s was m a i n t a i n e d . A d s o r p t i o n c a p a c i t y o f hydrogen
A
20 mg p o r t i o n
of t h e c a t a l y s t sample i n t h e sample pan o f t h e thermo-
g r a v i m e t r i c a n a l y z e r was reduced in s i t u w i t h hydrogen a t 400°C f o r 20 min, and t h e n c o o l e d t o room temperature i n hydrogen f l o w . To t h i s sample 1.3% 02 d i l u t e d w i t h He was a l l o w e d t o f l o w w i t h a f l o w r a t e o f 50 ml/min. The w e i g h t i n c r e a s e i n t h e e a r l y stage was g r a d u a l l y decreased w i t h t i m e on stream and a t t a i n e d a c o n s t a n t l e v e l f o r each c a t a l y s t . The excess w e i g h t i n c r e a s e o t h e r t h a n o x i d a t i o n i s a t t r i b u t e d t h e adsorbed H20 which i s formed by t h e o x i d a t i o n o f absorbed hydrogen. The hydrogen amounts measured by t h i s method a r e l i s t e d i n T a b l e 3 f o r v a r i o u s c a t a l y s t s o f N i - C e e O y P t system. As can be seen i n t h i s d a t a hydrogen a d s o r p t i o n c a p a c i t y corresponds t o t h e c a t a l y t i c a c t i v i t y . TABLE 3 Uptakes o f oxygen and hydrogen f o r v a r i o u s c a t a l y s t s Catalyst Catalyst number constituent
9 11 13 12 14 5
Ni Pt Ni-Pt Ni-Ce203 CepO3-Pt Ni-Ce707-Pt
Oxygen u p t a k e o f Amount o f H20 formed Amount of absorbed t h e reduced by t h e room-tempera- hydrogen c a t a l y s t a t 100°C t u r e o x i d a t i o n o f adsorbed hydrogen (mg/g-catalyst) (mg/g-catalyst) (mg/g-catalyst) 13.30 0 0 4.77 0.53 0 8.75 1.93 0.21 0 0 11.15 10.75 1.20 0 11.90 25.45 2.83
CONCLUSION On t h e b a s i s o f e x p e r i m e n t a l r e s u l t s t h e s y n e r g i s t i c e f f e c t s o f t h e t h r e e component c a t a l y s t were i n t e r p r e t e d as f o l l o w s , u s i n g t h e example o f t h e N i Ce203-Pt c a t a l y s t . P t , which is c l o s e t o b u t s e p a r a t e f r o m Ni-Ce203,
a c t s as
t h e t r a n s p o r t a g e n t o f hydrogen a d s o r p t i o n , and t h e adsorbed hydrogen d i f f u s e s
by s p i l l o v e r t o Ni-Ce203 p a r t which a c t s as t h e hydrogen a c c e p t o r . Consequently,
190
t h e Ni m a i n t a i n s t h e reduced s t a t e and i t s c a t a l y t i c a c t i v i t y j o i n s t o t h e a c t i v i t y o f P t p a r t . The c a t a l y s t t e m p e r a t u r e i s r a i s e d by t h e r e a c t i o n h e a t and t h e pronounced c a t a l y t i c a c t i v i t y i s e x e r t e d .
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