- Email: [email protected]
Ruthenium Catalysts for Ammonia Synthesis Prepared by Different Methods
381
RUTHENIUM CATALYSTS FOR AMMONIA SYNTHESIS PREPARED BY DIFFERENT METHODS
A.
OZAKI, K.
URABE, K. SHIMAZAKI and S. SUMIYA
Research Laboratory of Resources U t i l i z a t i o n , Tokyo I n s t i t u t e of Technology, Nagatsuta, Midori-ku, Yokohama(Japan)
ABSTRACT c a t a l y s t s promoted by a l k a l i were prepared by impregnation of A1203 2 3 w i t h 1) RuCl -KN03(or CsN03) mixture, o r 2 ) K4Ru(CN16 followed by reduction with 3 hydrogen. The n i t r a t e anion was mostly coverted t o NH giving h i g h l y e f f i c i e n t 3 and w a t e r - r e s i s t a n t ammonia c a t a l y s t s . I n p a r t i c u l a r , t h e cesium oxide-promoted Ru/A1 0
c a t a l y s t was a s a c t i v e a s t h a t promoted by m e t a l l i c potassium.
I n t h e hydrogen
treatment of K4Ru(CN)6/A1203, t h r e e o r more CN l i g a n d s were converted t o CH4 and N H 3 , suggesting t h a t K ( 1 ) i n a d d i t i o n t o Ru(I1) i s reduced t o metal by hydrogen,
which i s supported by hydrogen evolution upon water treatment as w e l l a s d r a s t i c poisoning by water vapor.
INTRODUCTION I t has been shown t h a t t h e t r a n s i t i o n metals a r e remarkably promoted by a d d i t i o n
o f m e t a l l i c potassium f o r t h e ammonia s y n t h e s i s ( r e f . 1 and 2) as w e l l as f o r t h e i s o t o p i c e q u i l i b r a t i o n of n i t r o g e n ( r e f . 2 and 3) and t h a t ruthenium is t h e most a c t i v e element f o r t h i s c a t a l y s t system ( r e f . 1 and 3 ) .
The r o l e of potassium
has been shown t o be an e l e c t r o n donor, a s evidenced by remarkable i n c r e a s e i n t h e a c t i v i t y with decrease i n t h e i o n i z a t i o n p o t e n t i a l of a l k a l i metal a s the promoter, i . e . C s > K>Na ( r e f . 4 ) .
I n agreement with t h i s i d e a , Raney ruthenium
containing t h e e l e c t r o p o s i t i v e aluminum metal has been found t o be an e f f i c i e n t ammonia c a t a l y s t t h a t i s a c t i v e even a t 100°C a f t e r a d d i t i o n of potassium ( r e f . 5 ) . I t seems obvious, however, t h a t such c a t a l y s t s promoted by m e t a l l i c a l k a l i a r e
s u b j e c t t o i r r e v e r s i b l e poisoning by water, suggesting a p o s s i b l e drawback of t h e c a t a l y s t from a p r a c t i c a l p o i n t of view. r e s i s t a n t and e f f i c i e n t c a t a l y s t .
I t would be d e s i r a b l e t o g e t a water-
Thus a l t e r n a t i v e methods have been i n v e s t i g a t e d
f o r p r e p a r a t i o n of t h e ruthenium c a t a l y s t .
The p r e s e n t paper d e a l s with two methods
of potassium a d d i t i o n t o Ru/A1203, i n which potassium compounds were used a s t h e s t a r t i n g material.
382 CATALYST PREPARATION 1. Oxide-promoted
catalysts.
P o t a s h ( o r cesium oxide)-promoted ruthenium c a t a l y s t s were prepared by impregnating alumina with a mixture of RuCl3-KNO3(or CsN03)(l/lO,mol /mol ) . The alumina sample obtained from Tokai Konetsu Co. was i n a form of c y l i n d r i c a l pellet(3.5mm both i n diameter and i n length) with a c o a x i a l open hole and had a s p e c i f i c s u r f a c e a r e a 2 of 180 m /g. The alumina, i n t h e form of p e l l e t s or crushed g r a i n s of 10-20 mesh,
w a s immersed i n t h e mixed aqueous s o l u t i o n of 0.02 mol and evaporated t o dryness on a water bath.
The Ru/A1203 r a t i o w a s a d j u s t e d t o g i v e 2.0%(w/w)Ru.
The d r i e d
m a t e r i a l w a s t r e a t e d with c i r c u l a t i n g hydrogen a t i n c r e a s i n g temperatures, f i r s t a t 100°C u n t i l no hydrogen consumption w a s observed and f i n a l l y a t 45OOC f o r 40hr, during which t h e gaseous products of reduction were removed by a l i q u i d n i t r o g e n trap. Four c a t a l y s t s were used as follows: C a t a l y s t Number
Impregnation with
Support
N- 1
RUC13-KNO3
grains
N-2
RUC13-CSN03
grains
N- 3
RuC13-NH4N03
grains
X
RuCl3
pellet
2 . Cyanide-promoted c a t a l y s t s .
Two d i f f e r e n t samples,A and B , o f K4Ru(OU6 were prepared according t o t h e known methods.
The sample A was prepared by adding KCN t o K2Ru04 which w a s prepared
by a l k a l i f u s i o n of Ru metal and KNO3 ( r e f . 6 and 7 ) .
The sample B was prepared
by adding KCN t o RuC13 ( r e f . 8). B o t h samples were p u r i f i e d by r e c r y s t a l i z a t i o n from water t o g i v e a c o l o r l e s s t e t r a g o n a l p l a t y c r y s t a l .
a better yield
( @ 50% i
The former method gave
n Ru).
Both samples were examined by elemental a n a l y s i s f o r t h e i r i d e n t i t y as shown i n Table 1.
The sample B was dehydrated i n advance.
TABLE 1 Elemental a n a l y s i s of samples A and B Sample
H
C
N
K
A
found calc. as trihydrate
1.21 1.30
15.42 15.41
18.01 17.96
34.00 33.45
B
f0Wd c a l c . a s anhydride
0.05 0.00
17.41 17.43
19.73 20.31
37.82
-
There is a good agreement with t h e c a l c u l a t e d value f o r K4Ru(CN)6.
383 The sample A was f u r t h e r i d e n t i f i e d by X-ray d i f f r a c t i o n and I R absorption. Since K Ru(CN) -3H 0 and K4Fe(CNI6.3H 0 a r e isonorphous ( r e f . 9 ) , t h e i d e n t i t y 4 6 2 2 of t h e sample A was confirmed by a comparison of X-ray d i f f r a c t i o n p a t t e r n s with a known sample of K Fe(CNI6-3H20. The I R absorption spectrum of t h e sample A 4 was i n accord with t h e l i t e r a t u r e f o r K R U ( C N ) ~ - ~ H (~rOe f . 10). 4 The samples A and B were supported on t h e 10-20 mesh g r a i n s of alumina described above t o give a 2.0%(w/w) Ru/A1203, i.e.,
t h e alumina was impregnated
w i t h A o r B from i t s aqueous s o l u t i o n of 0.04 m o l / l by evaporation t o dryness. The supported samples were s u b j e c t e d t o decomposition i n c i r c u l a t i n g H2 o r 3 H /N 2
2
mixture a t i n c r e a s i n g temperatures up t o 450°C. Four c a t a l y s t samples were used as follows; Number
Reducing gas
K4Ru (CN)
c1
B
c2
B
c3
A
c4
A
3H - N 2 2 H2
H2 3H2-N2
PROCEDURES The hydrogen reduction and t h e a c t i v i t y measurements were c a r r i e d o u t i n a closed c i r c u l a t i n g system comprising a l i q u i d n i t r o g e n t r a p .
The ammonia s y n t h e s i s
gas. The numbers of s u r f a c e runs were c a r r i e d o u t a t 600torr using t h e 3H / N 2 2 ruthenium atoms were e s t i m a t e d by chemisorption of hydrogen which was determined by e x t r a p o l a t i o n o f l i n e a r p a r t of t h e isotherm a t OOC.
RESULTS AND DISCUSSIONS 1. Stoichiometry o f reduction by hydroqen
1-1 Hydrogen treatment of n i t r a t e - c o n t a i n i n q c a t a l y s t s .
A r a p i d consumption
o f hydrogen s t a r t e d a t around 100°C and was n e a r l y completed a t 200'C. consumption up t o 450'C
The t o t a l
f o r Cat.N-2 was 3.9 mol H /mol Ru which corresponded t o 94% 2
of t h e c a l c u l a t e d value f o r a r e a c t i o n ; RuC13
+
10CsN03
+
41.5H2+Ru
+
3CsC1
+
7CsOH
+
10NH3
+
23H20
I t i s obvious t h a t most of t h e n i t r a t e i s reduced t o NH3 and H 2 0 .
(1)
On t h e o t h e r
hand, t h e reduction of C a t . X (RuC1 / A 1 0 ) was much slower and t h e t o t a l consumption 3 2 3 up t o 45OOC w a s 1.38 m o l H /mol RU o r 92% of t h e c a l c u l a t e d value f o r t h e r e a c t i o n : 2 RuC13
+
1.5H2 + Ru
+
3HC1
The d e v i a t i o n from 100%would be due p a r t l y t o t h e l o s s during p r e p a r a t i o n .
(2) A t any
r a t e t h e presence of n i t r a t e c l e a r l y r e s u l t s i n t h e f a s t e r and l a r g e r consumption of hydrogen.
384 1 - 2 Hydrogen treatment of cyanide-containing c a t a l y s t s .
The r e a c t i o n of
unsupported K4Ru(CNl6 w i t h hydrogen was found t o be very slow even a t 500°C ( 2 mmol H
/mol Ru h r ) , while t h e hydrogen consumption was r e a d i l y observed a t 38OOC with t h e alumina-supported c a t a l y s t s a s i l l u s t r a t e d i n Fig. 1 f o r Cat. C-2.
500
-
L; 400
-
!-I
- 300 0
c,
a, !-I
1 rn
!-I g200
-
01 N
m
100
10
5
0
Time (hr) Fig. 1. Time course of hydrogen consumption a t 380'C f o r c a t a l y s t C-2.
The i n i t i a l slow r e a c t i o n (66 m o l H / m o l Ru h r ) is a c c e l e r a t e d a f t e r about 6 h r 2
by a f a c t o r o f about 25.
I t i s n o t i c e a b l e t h a t t h e r a t e of r e a c t i o n is independent
of hydrogen p r e s s u r e , suggesting t h a t a s o l i d phase process is r a t e - l i m i t i n g . The temperature was r a i s e d s t e p w i s e (400, 420 and 450°C) a f t e r t h e r a t e a t a temperature was slowed down. A f t e r t h e decomposition o f C a t . C-1 at 450°C t h e products c o l l e c t e d i n t h e t r a p were i d e n t i f i e d by massspectroscopy t o be methane and a much l a r g e r amount of ammonia, i n d i c a t i n g t h a t t h e ammonia s y n t h e s i s r e a c t i o n took p l a c e because t h e reducing gas was 3H2/N2.
Thus t h e decomposition w a s made i n H2 f o r C a t . C-2
and
t h e t o t a l hydrogen consumption up t o 450'C was determined t o be 10.1 mol/mol Ru, a value corresponding t o 3CN/Ru converted t o CH4 and NH3. decomposed i n H 2 ,
t h e amounts of CH4 and NH3 formed were determined i n a d d i t i o n
t o t h e hydrogen consumption as follows; CH4/Ru
:
4.8,
I n t h e case of Cat. C-3
NH3/Ru
:
4.6,
H2/Ru
:
16.5
2-
385 The hydrogen consumption corresponds t o 4.7 CN/Ru, i n a b e a u t i f u l agreement with t h e I n t h e case of C a t . C-4 decomposed i n 3H2/N2, both t h e values of CH4/Ru and NH /Ru. 3 CH4/Ru and CN/Ru values a r e found t o be 4.6. I t i s c l e a r from t h e above t h a t more than two CN l i g a n d s a r e hydrogenated i n t h e
decomposition, which implies t h a t some c a t i o n o t h e r than Ru(I1) i s reduced by hydrogen, a s represented f o r K ( 1 ) by
+
10.5H2*Ru
+
K
K4Ru(CN)6 +
14 H -Ru 2
+
2K
K4R~(CN)6
+
3KCN
+
+
3CH4
3NH3
(3)
or i2KCN
+
4CH4
+
4NH3.
(4)
I n f a c t t h e reduction o f KCN by H2 i s not impossible thermodynamically when t h e products a r e removed continuously.
The alumina support might be p a r t l y transformed
t o cyanide during t h e decomposition, r e s u l t i n g i n a reduction t o form m e t a l l i c aluminum, on which no information i s a v a i l a b l e .
N o information i s a l s o a v a i l a b l e
about t h e reason o f t h e d i f f e r e n c e i n e x t e n t of reduction observed f o r Cat. C-2 and C-3. 2 . Ammonia s y n t h e s i s a c t i v i t i e s
The r a t e s of ammonia s y n t h e s i s over t h e c a t a l y s t s are summarized i n Fig. 2 as Those c a t a l y s t s a r e commonly 2 % f w / w ) R u on t h e same alumina support,
Arrhenius p l o t s .
while d i f f e r e n c e s may be found i n t h e e x t e n t of d i s p e r s i o n a s w e l l as i n t h e n a t u r e of promoter.
The amounts of hydrogen chemisorption a t O°C a r e given i n Table 2
t o g e t h e r with t h e r a t e s of ammnia s y n t h e s i s p e r g c a t . a t 3OOOC and t h e apparent a c t i v a t i o n e n e r g i e s (Ea)
.
TABLE 2
C a t a l y t i c p r o p e r t i e s of c a t a l y s t s N-2
N-3
K20
Cs20
none
H2m1STP/g.cat
0.68
1.08
0.31
0.13
NH mlSTP/g.cat.hr
1.9
8:3
0.076*
0.039*
28
28
17
17
*
c-2
c-3
-
0.98
1.08
1.22
1.35
4.3
1.9
0.67
0.70
1.1
27
27
28
26
26
x K
N-1
3 Ea K c a l / m l
c-1
X none
Catalyst Promoter
c-4
- cyanide -
e s t i m a t i o n by e x t r a p o l a t i o n
Since l i t t l e d i f f e r e n c e i s observed i n Ea on those alkali-promoted c a t a l y s t s , t h e observed d i f f e r e n c e i n t h e c a t a l y t i c a c t i v i t y seems t o come from t h e e x t e n t of dispersion.
However t h e v a r i a t i o n i n hydrogen chemisorption i s t o o small t o
explain the difference.
The hydrogen chemisorptions on t h e cyanide s e r i e s of
c a t a l y s t s a r e g e n e r a l l y l a r g e r , whereas t h e c a t a l y t i c a c t i v i t i e s are lower those on Cat. N - 1 and N - 2 .
than
I f m e t a l l i c potassium i s formed during t h e decomposition
387 of Cat. C-2 o r C-3 a s suggested by t h e stoichiometry, hydrogen can be chemisorbed on K i n a d d i t i o n t o on Ru, giving r i s e to l a r g e r chemisorptions.
In f a c t , as
described l a t e r , t h e chemisorption value on Cat. C-2 decreased by about 30% a f t e r a t r e a t m e n t with water vapor.
The l a r g e r chemisorption on C a t . C-3 o r C-4 than on
Cat. C-2 can be due t o l a r g e r amount of potassium a s suggested by t h e stoichiometry. Thus t h e r e a l chemisorption on Ru would be l e s s than t h a t observed on t h e cyanide s e r i e s of c a t a l y s t s .
i s remarkable.
The high a c t i v i t y on Cat. N - 1 o r N-2
I n comparison with Cat. N-3
which has no promoter, Cat. N - 1 and N-2 a r e r e s p e c t i v e l y , 11 and 31 t i m e s more a c t i v e i n terms of t h e r a t e p e r chemisorption. When Cat. X was a c t i v a t e d by a d d i t i o n of m e t a l l i c potassium (K/Ru=2.9mol / m o l )
,
t h e e x t e n t o f promotion a t 3OOOC w a s 110
t i m e s a s shown i n Table 1. Since t h e potassium a d d i t i o n may give r i s e t o an increase i n d i s p e r s i o n of ruthenium on alumina as was t h e case with unsupported ruthenium ( r e f . 2 ) , t h e d i f f e r e n c e i n promoting e f f i c i e n c y between m e t a l l i c potassium and cesium oxide would be
smaller.
Thus cesium oxide is a promising promoter which i s
s t a b l e i n t h e presence of water vapor. K > C s 0 >K 0 , i s i n accord with 2 2
The e f f i c i e n c y sequence of promoters,
t h e o r d e r of e l e c t r o - p o s i t i v i t y ,
the e l e c t r o n donation t o ruthenium as has been suggested ( r e f . 4 ) .
i n conformity with The high a c t i v i t y
o f C a t . N-2 a r i s e s f i r s t l y from t h e high e l e c t r o p o s i t i v i t y of C s 0 and secondly from 2 t h e increased d i s p e r s i o n of ruthenium. Presumably t h e low temperature reduction which
i s r e a l i z e d i n t h e presence of n i t r a t e i s r e s p o n s i b l e f o r t h e higher d i s p e r s i o n . Although m e t a l l i c potassium i s e f f i c i e n t as t h e promoter and i s l i k e l y formed i n Cat. C-2 o r C-3, t h e s p e c i f i c a c t i v i t y p e r chemisorption f o r Cat. C-2 o r C-3 i s much l e s s than t h a t on Cat. N-2.
On t h e o t h e r hand it i s t o be noted t h a t t h e
decomposition of K R u ( W /A1203 made i n H2-N2 mixture r e s u l t s i n a higher a c t i v i t y 4 6 than t h a t i n H a , a s i s c l e a r form t h e comparison of Cat. C - 1 o r C-4 w i t h C-2 o r C-3. This r e s u l t seems t o be caused by a cooperative i n c o r p o r a t i o n of K and N2 i n t o Ru metal ( r e f . 111, which g i v e s r i s e t o a higher potassium content.
Hence t h e lower
a c t i v i t y of Cat. C-2 o r C-3 seems t o be caused by a l e s s extensive i n t e r a c t i o n o f K w i t h Ru o r a p o s s i b l e loss o f K during t h e decomposition of K Ru(CN) 4 6' 3. E f f e c t of water vapor treatment.
The c a t a l y s t N-1 was t r e a t e d with a c i r c u l a t i n g C O D (=3/1) mixture containing s a t u r a t e d water vapor a t 45OOC f o r 24
pr,
2 during which C n 4 and CO
were formed. 2 A f t e r evacuation a t 400°C f o r 2 h r , t h e ammonia s y n t h e s i s a c t i v i t y was found t o be 64% of t h e i n i t i a l value, while it was recovered t o 86% a f t e r hydrogen treatment a t 45OOC f o r 40 h r . treatments.
The amount o f hydrogen chemisorption changed l i t t l e i n t h e above
In c o n t r a s t , t h e Cat. X w i t h K was found t o s u f f e r a d r a s t i c poisoning
by water vapor. A f t e r a treatment with c i r c u l a t i n g water vapor (15 t o r r ) a t 45OoC
f o r 14 h r ( u n t i l t h e f i n i s h o f hydrogen e v o l u t i o n ) followed by evacuation a t 400'C f o r 2 h r , t h e ammonia s y n t h e s i s a c t i v i t y was found t o be 6% of t h e i n i t i a l value and recovered t o 15% a f t e r hydrogen treatment a t 45OoC f o r 40 h r , with the f i n a l
388 a c t i v i t y being comparable t o t h a t of Cat.N-1.
I n t e r e s t i n g l y t h e amount of hydrogen
chemisorption on Cat. X i n c r e a s e d by a f a c t o r of 2 . 8 i n t h e above treatments.
The
a d d i t i o n of potassium followed by t h e armnonia s y n t h e s i s runs presumably gave r i s e t o a c o r r o s i v e chemisorption of nitrogen t o form a compound ( r e f . ll), which was decomposed by water r e s u l t i n g i n t h e increased d i s p e r s i o n . n i t r o g e n uptake has been observed on Ru-K/Al
In fact, a large
c a t a l y s t s ( r e f . 1 2 ) , whereas l i t t l e
0
2 3
uptake on Cat. N-1. Since m e t a l l i c potassium i s l i k e l y formed i n t h e decomposition of cyanide s e r i e s of c a t a l y s t s , t h e e f f e c t of water vapor would be r e v e a l i n g .
Cat. C-2 w a s t r e a t e d
w i t h c i r c u l a t i n g water vapor a t 44OOC u n t i l t h e f i n i s h of p r e s s u r e i n c r e a s e .
The
evolved gas was confirmed t o be hydrogen (0.67 m l H /mol Ru), suggesting t h a t 2 1.3 mol K/mol Ru i s formed i n C a t . C-2, which i s near t o 1 moleK/moleRu as estimated from t h e hydrogen consumption during t h e decomposition.
The amount of
hydrogen chemisorption decreased t o 72% of t h e i n i t i a l v a l u e , i n d i c a t i n g t h a t t h e i n i t i a l value involved t h e chemisorption on K.
I t may be concluded from t h e above r e s u l t s t h a t m e t a l l i c potassium ( o r aluminum)
is formed by hydrogen reduction o f K R(CN) / A 1 0 4
promotion of Ru.
6
2 3
r e s u l t i n g i n an extensive
I t i s r e c a l l e d t h a t an i r o n c a t a l y s t
was used f o r a m o n i a s y n t h e s i s ( r e f . 1 3 ) .
derived from ferrocyanide
Although t h e formation of potassium
was n o t d e t e c t e d a t t h a t t i m e ( r e f . 1 4 ) , it might b e t h e reason of high a c t i v i t y .
REFERENCES
1 A. Ozaki, K. Aika and H . Hori, Bull. Chem. SOC. J a p . , 44, 3216 (1971) 2 K. Urabe, K. Aika and A. Ozaki, J. Cat., 42, 197 ( 1 9 7 6 r 3 K. Urabe, A. Ohya and A. Ozaki, t o be published 4 K. Aika, H. Hori and A. Ozaki, J . Cat., 21, 424 (1972) 5 K. Urabe, T. Yoshioka and A. Ozaki, J . Cat., i n p r e s s 6 F. Krauss, 2. Anorg. Allg. Chem., 59 (1927) 7 K. Masuno, S . Waku, Nippon Kagaku Zasshi 83, 161 (1962) 8 J . L . Howe, J. Am. Chem. SOC., I s 9 8 1 (1896) 9 V.A. Pospelov and G.S. Zhdanov, Zh. Fiz. a i m , SSSR, 21, 405 (1947) 1 0 I. Nakagawa and T. Shimanouchi, Spectrochim. A c t a , 2, 101 (1962) 11 A. Ohya, K. Urabe and A. Ozaki, Chem. L e t t . , 233 1 2 K. Urabe, K. S h i r a t o r i and A. Ozaki, J. Cat., submitted 13 A. Mittasch and E . Kuss, 2. Elektrochem., 34, 159 (1928) 14 A. Mittasch, E. Kuss and 0. E m e r t . 2 . Anorg. Allg. Chem., 193 (1928)
165,
1978,
170,
DISCUSSION J.G.
v a n OMMEN : I t i s n o t s t r a n g e t h a t C s z O a n d K m e t a l h a v e a
c o m p a r a b l e p r o m o t i n g e f f e c t o n Ru ?
Couldn't
i t be, t h a t y o u
(when C s 0 r e a c t s w i t h w a t e r i t 2 w i t h K 0 o r KOH. I n that c a s e , i n m y o p i n i o n it i s 2 e a s i e r t o understand t h e comparable promoter e f f e c t of CsOH and a c t u a l l y compare C s 2 0 o r C s O H forms CsOH)
KOH b e c a u s e
they are both s t r o n g bases.
YOU
a l s o showed t h a t p o i s o n i n g by w a t e r v a p o r w a s i r r e v e r s i b l e
b e c a u s e p o t a s s i u m i s c o n v e r t e d t o KOH.
But couldn't
it be possible
t h a t w a t e r v a p o r a l s o d e s t r o y e s t h e r u t h e n i u m s u r f a c e by r e c o n s t r u c t i n g i t , and so t h a t t h e l o w e r a c t i v i t y i s c a u s e d by t h i s effect ? In your paper you also mention an activity sequence of promotors, t h a t d i f f e r s from t h e sequence p r e s e n t e d in Fig. 2.
From this
f i g u r e , t h e s e q u e n c e is C s 2 0 > K > K 2 0 , w h i l e in t h e t e x t t h e s e q u e n c e
is K > C s 2 0 > K 2 0 .
Can you explain this ?
C a n y o u a l s o a g r e e t h a t if o n e c o m p a r e s t h e o v e r a l l a c t i v i t y of t h e C s 0 p r o m o t e d c a t a l y s t w i t h t h e K m e t a l promoted one, t h e 2 first o n e g i v e s t h e b e t t e r catalysts.
A. O Z A K I : K e x h i b i t s definitely a h i g h e r p r o m o t i n g e f f e c t t h a n K 2 0 a n d , after p o i s o n i n g w i t h H20, t h e p r o m o t i n g e f f e c t a p p r o a c h e s T h e s e q u e n c e K > C s 2 0 > K 0 is based o n t h e a c t i v i t y 2 p e r s u r f a c e a t o m , t h e latter being d e t e r m i n e d by H 2 chemisorption. t h a t o f K20.
A n activity i n c r e a s e d u e t o d e s i n t e g r a t i o n o f p a r t i c l e w a s f o u n d w i t h K-promoted Ru.
(J. Cat.
S,
430(1975)).
I f it t a k e s p l a c e ,
a n i n c r e a s e in a c t i v i t y i s o b s e r v e d in t h e second run.
P.E.H.
NIELSEN
: H a v e y o u o b s e r v e d a n y s u p p o r t effect i n g o i n g from
c a r b o n s u p p o r t e d Ru-K c a t a l y s t t o a l u m i n a s u p p o r t e d Ru-K c a t a l y s t ? A. O Z A K I
: T h e s e t w o catalysts, s h o u l d not b e c o m p a r e d i n t e r m s o f t h e
a c t i v i t y p e r s u r f a c e Ru.
But e v a l u a t i o n o f Ru d i s p e r s i o n o n c a r b o n
r e m a i n s d i f f i c u l t so t h a t n o c o m p a r i s o n h a s b e e n made.
K. J O H A N S E N : 1) W h i c h is t h e s p a c e v e l o c i t y used i n y o u r a c t i v i t y m e a s u r e m e n t s in T a b l e 2 ?
2) Have y o u any h i g h p r e s s u r e a c t i v i t y
measurements ?
A. O Z A K I : 1) 2 0 0 0 - 3 0 0 0 hr-'.
2) T h e r a t e o f N H 3 f o r m a t i o n on
Ru-K./Ac t e n d s t o a p p r o a c h a p l a t e a u v a l u e at h i g h pressures.
J.W.
H I G H T O W E R : F r o m T a b l e 2 and Fig. 2, I n o t i c e t h a t t h e l e a s t
a c t i v e u n p r o m o t e d c a t a l y s t s ( X and N-3) h a v e t h e l o w e s t a p p a r e n t a c t i v a t i o n energy.
B e c a u s e t h e r a t e s in t h e s e c a s e s a r e q u i t e low,
I d o not t h i n k t h a t d i f f u s i o n l i m i t a t i o n s c a n be r e s p o n s i b l e for t h e s e results.
T h e p r e - e x p o n e n t i a l f a c t o r m u s t be s o m e h o w g r e a t l y
390 altered by t h e promotor.
C a n you comment o n t h e s e r a t h e r s u r p r i s i n g
observations ?
A.
OZAKI
:
T h e p r o m o t o r effect of K on R u a p p a r e n t l y r e s u l t s in a
r e m a r k a b l e i n c r e a s e in t h e pre-exponential
factor.
T h i s must be
c a u s e d by a structural c h a n g e in Ru s u r f a c e , b u t we c a n n o t g i v e any f u r t h e r comment o n t h i s at present.
Z.G.
SZABO
: You have p u t e m p h a s i s o n t h e n i t r a t e effect, and not
w i t h o u t reason.
During t h e r e d u c t i o n , N H 3 i s first formed and
afterwards NHqN03. NZO.
U n d e r y o u r e x p e r i m e n t a l c o n d i t i o n s it p r o d u c e s
We a l s o u s e d t h i s d e c o m p o s i t i o n for m a k i n g h i g h l y active c a t a -
lysts.
It i s blowing u p t h e precipitate, r e s u l t i n g in very high
s u r f a c e development.
A s the activation energies are nearly the
same, it p o i n t s to a n increased p r e - e x p o n e n t i a l factor.
But t h i s
effect must operate at the most appropriate stage, during the preparation.
T h i s i s p e r h a p s t h e r e a s o n why p r e v i o u s l y added
i s n o t advantageous.
A.
OZAKI
:
T h a n k y o u for y o u r comment.
NH4N0
3
RUTHENIUM CATALYSTS FOR AMMONIA SYNTHESIS PREPARED BY DIFFERENT METHODS
A.
OZAKI, K.
URABE, K. SHIMAZAKI and S. SUMIYA
Research Laboratory of Resources U t i l i z a t i o n , Tokyo I n s t i t u t e of Technology, Nagatsuta, Midori-ku, Yokohama(Japan)
ABSTRACT c a t a l y s t s promoted by a l k a l i were prepared by impregnation of A1203 2 3 w i t h 1) RuCl -KN03(or CsN03) mixture, o r 2 ) K4Ru(CN16 followed by reduction with 3 hydrogen. The n i t r a t e anion was mostly coverted t o NH giving h i g h l y e f f i c i e n t 3 and w a t e r - r e s i s t a n t ammonia c a t a l y s t s . I n p a r t i c u l a r , t h e cesium oxide-promoted Ru/A1 0
c a t a l y s t was a s a c t i v e a s t h a t promoted by m e t a l l i c potassium.
I n t h e hydrogen
treatment of K4Ru(CN)6/A1203, t h r e e o r more CN l i g a n d s were converted t o CH4 and N H 3 , suggesting t h a t K ( 1 ) i n a d d i t i o n t o Ru(I1) i s reduced t o metal by hydrogen,
which i s supported by hydrogen evolution upon water treatment as w e l l a s d r a s t i c poisoning by water vapor.
INTRODUCTION I t has been shown t h a t t h e t r a n s i t i o n metals a r e remarkably promoted by a d d i t i o n
o f m e t a l l i c potassium f o r t h e ammonia s y n t h e s i s ( r e f . 1 and 2) as w e l l as f o r t h e i s o t o p i c e q u i l i b r a t i o n of n i t r o g e n ( r e f . 2 and 3) and t h a t ruthenium is t h e most a c t i v e element f o r t h i s c a t a l y s t system ( r e f . 1 and 3 ) .
The r o l e of potassium
has been shown t o be an e l e c t r o n donor, a s evidenced by remarkable i n c r e a s e i n t h e a c t i v i t y with decrease i n t h e i o n i z a t i o n p o t e n t i a l of a l k a l i metal a s the promoter, i . e . C s > K>Na ( r e f . 4 ) .
I n agreement with t h i s i d e a , Raney ruthenium
containing t h e e l e c t r o p o s i t i v e aluminum metal has been found t o be an e f f i c i e n t ammonia c a t a l y s t t h a t i s a c t i v e even a t 100°C a f t e r a d d i t i o n of potassium ( r e f . 5 ) . I t seems obvious, however, t h a t such c a t a l y s t s promoted by m e t a l l i c a l k a l i a r e
s u b j e c t t o i r r e v e r s i b l e poisoning by water, suggesting a p o s s i b l e drawback of t h e c a t a l y s t from a p r a c t i c a l p o i n t of view. r e s i s t a n t and e f f i c i e n t c a t a l y s t .
I t would be d e s i r a b l e t o g e t a water-
Thus a l t e r n a t i v e methods have been i n v e s t i g a t e d
f o r p r e p a r a t i o n of t h e ruthenium c a t a l y s t .
The p r e s e n t paper d e a l s with two methods
of potassium a d d i t i o n t o Ru/A1203, i n which potassium compounds were used a s t h e s t a r t i n g material.
382 CATALYST PREPARATION 1. Oxide-promoted
catalysts.
P o t a s h ( o r cesium oxide)-promoted ruthenium c a t a l y s t s were prepared by impregnating alumina with a mixture of RuCl3-KNO3(or CsN03)(l/lO,mol /mol ) . The alumina sample obtained from Tokai Konetsu Co. was i n a form of c y l i n d r i c a l pellet(3.5mm both i n diameter and i n length) with a c o a x i a l open hole and had a s p e c i f i c s u r f a c e a r e a 2 of 180 m /g. The alumina, i n t h e form of p e l l e t s or crushed g r a i n s of 10-20 mesh,
w a s immersed i n t h e mixed aqueous s o l u t i o n of 0.02 mol and evaporated t o dryness on a water bath.
The Ru/A1203 r a t i o w a s a d j u s t e d t o g i v e 2.0%(w/w)Ru.
The d r i e d
m a t e r i a l w a s t r e a t e d with c i r c u l a t i n g hydrogen a t i n c r e a s i n g temperatures, f i r s t a t 100°C u n t i l no hydrogen consumption w a s observed and f i n a l l y a t 45OOC f o r 40hr, during which t h e gaseous products of reduction were removed by a l i q u i d n i t r o g e n trap. Four c a t a l y s t s were used as follows: C a t a l y s t Number
Impregnation with
Support
N- 1
RUC13-KNO3
grains
N-2
RUC13-CSN03
grains
N- 3
RuC13-NH4N03
grains
X
RuCl3
pellet
2 . Cyanide-promoted c a t a l y s t s .
Two d i f f e r e n t samples,A and B , o f K4Ru(OU6 were prepared according t o t h e known methods.
The sample A was prepared by adding KCN t o K2Ru04 which w a s prepared
by a l k a l i f u s i o n of Ru metal and KNO3 ( r e f . 6 and 7 ) .
The sample B was prepared
by adding KCN t o RuC13 ( r e f . 8). B o t h samples were p u r i f i e d by r e c r y s t a l i z a t i o n from water t o g i v e a c o l o r l e s s t e t r a g o n a l p l a t y c r y s t a l .
a better yield
( @ 50% i
The former method gave
n Ru).
Both samples were examined by elemental a n a l y s i s f o r t h e i r i d e n t i t y as shown i n Table 1.
The sample B was dehydrated i n advance.
TABLE 1 Elemental a n a l y s i s of samples A and B Sample
H
C
N
K
A
found calc. as trihydrate
1.21 1.30
15.42 15.41
18.01 17.96
34.00 33.45
B
f0Wd c a l c . a s anhydride
0.05 0.00
17.41 17.43
19.73 20.31
37.82
-
There is a good agreement with t h e c a l c u l a t e d value f o r K4Ru(CN)6.
383 The sample A was f u r t h e r i d e n t i f i e d by X-ray d i f f r a c t i o n and I R absorption. Since K Ru(CN) -3H 0 and K4Fe(CNI6.3H 0 a r e isonorphous ( r e f . 9 ) , t h e i d e n t i t y 4 6 2 2 of t h e sample A was confirmed by a comparison of X-ray d i f f r a c t i o n p a t t e r n s with a known sample of K Fe(CNI6-3H20. The I R absorption spectrum of t h e sample A 4 was i n accord with t h e l i t e r a t u r e f o r K R U ( C N ) ~ - ~ H (~rOe f . 10). 4 The samples A and B were supported on t h e 10-20 mesh g r a i n s of alumina described above t o give a 2.0%(w/w) Ru/A1203, i.e.,
t h e alumina was impregnated
w i t h A o r B from i t s aqueous s o l u t i o n of 0.04 m o l / l by evaporation t o dryness. The supported samples were s u b j e c t e d t o decomposition i n c i r c u l a t i n g H2 o r 3 H /N 2
2
mixture a t i n c r e a s i n g temperatures up t o 450°C. Four c a t a l y s t samples were used as follows; Number
Reducing gas
K4Ru (CN)
c1
B
c2
B
c3
A
c4
A
3H - N 2 2 H2
H2 3H2-N2
PROCEDURES The hydrogen reduction and t h e a c t i v i t y measurements were c a r r i e d o u t i n a closed c i r c u l a t i n g system comprising a l i q u i d n i t r o g e n t r a p .
The ammonia s y n t h e s i s
gas. The numbers of s u r f a c e runs were c a r r i e d o u t a t 600torr using t h e 3H / N 2 2 ruthenium atoms were e s t i m a t e d by chemisorption of hydrogen which was determined by e x t r a p o l a t i o n o f l i n e a r p a r t of t h e isotherm a t OOC.
RESULTS AND DISCUSSIONS 1. Stoichiometry o f reduction by hydroqen
1-1 Hydrogen treatment of n i t r a t e - c o n t a i n i n q c a t a l y s t s .
A r a p i d consumption
o f hydrogen s t a r t e d a t around 100°C and was n e a r l y completed a t 200'C. consumption up t o 450'C
The t o t a l
f o r Cat.N-2 was 3.9 mol H /mol Ru which corresponded t o 94% 2
of t h e c a l c u l a t e d value f o r a r e a c t i o n ; RuC13
+
10CsN03
+
41.5H2+Ru
+
3CsC1
+
7CsOH
+
10NH3
+
23H20
I t i s obvious t h a t most of t h e n i t r a t e i s reduced t o NH3 and H 2 0 .
(1)
On t h e o t h e r
hand, t h e reduction of C a t . X (RuC1 / A 1 0 ) was much slower and t h e t o t a l consumption 3 2 3 up t o 45OOC w a s 1.38 m o l H /mol RU o r 92% of t h e c a l c u l a t e d value f o r t h e r e a c t i o n : 2 RuC13
+
1.5H2 + Ru
+
3HC1
The d e v i a t i o n from 100%would be due p a r t l y t o t h e l o s s during p r e p a r a t i o n .
(2) A t any
r a t e t h e presence of n i t r a t e c l e a r l y r e s u l t s i n t h e f a s t e r and l a r g e r consumption of hydrogen.
384 1 - 2 Hydrogen treatment of cyanide-containing c a t a l y s t s .
The r e a c t i o n of
unsupported K4Ru(CNl6 w i t h hydrogen was found t o be very slow even a t 500°C ( 2 mmol H
/mol Ru h r ) , while t h e hydrogen consumption was r e a d i l y observed a t 38OOC with t h e alumina-supported c a t a l y s t s a s i l l u s t r a t e d i n Fig. 1 f o r Cat. C-2.
500
-
L; 400
-
!-I
- 300 0
c,
a, !-I
1 rn
!-I g200
-
01 N
m
100
10
5
0
Time (hr) Fig. 1. Time course of hydrogen consumption a t 380'C f o r c a t a l y s t C-2.
The i n i t i a l slow r e a c t i o n (66 m o l H / m o l Ru h r ) is a c c e l e r a t e d a f t e r about 6 h r 2
by a f a c t o r o f about 25.
I t i s n o t i c e a b l e t h a t t h e r a t e of r e a c t i o n is independent
of hydrogen p r e s s u r e , suggesting t h a t a s o l i d phase process is r a t e - l i m i t i n g . The temperature was r a i s e d s t e p w i s e (400, 420 and 450°C) a f t e r t h e r a t e a t a temperature was slowed down. A f t e r t h e decomposition o f C a t . C-1 at 450°C t h e products c o l l e c t e d i n t h e t r a p were i d e n t i f i e d by massspectroscopy t o be methane and a much l a r g e r amount of ammonia, i n d i c a t i n g t h a t t h e ammonia s y n t h e s i s r e a c t i o n took p l a c e because t h e reducing gas was 3H2/N2.
Thus t h e decomposition w a s made i n H2 f o r C a t . C-2
and
t h e t o t a l hydrogen consumption up t o 450'C was determined t o be 10.1 mol/mol Ru, a value corresponding t o 3CN/Ru converted t o CH4 and NH3. decomposed i n H 2 ,
t h e amounts of CH4 and NH3 formed were determined i n a d d i t i o n
t o t h e hydrogen consumption as follows; CH4/Ru
:
4.8,
I n t h e case of Cat. C-3
NH3/Ru
:
4.6,
H2/Ru
:
16.5
2-
385 The hydrogen consumption corresponds t o 4.7 CN/Ru, i n a b e a u t i f u l agreement with t h e I n t h e case of C a t . C-4 decomposed i n 3H2/N2, both t h e values of CH4/Ru and NH /Ru. 3 CH4/Ru and CN/Ru values a r e found t o be 4.6. I t i s c l e a r from t h e above t h a t more than two CN l i g a n d s a r e hydrogenated i n t h e
decomposition, which implies t h a t some c a t i o n o t h e r than Ru(I1) i s reduced by hydrogen, a s represented f o r K ( 1 ) by
+
10.5H2*Ru
+
K
K4Ru(CN)6 +
14 H -Ru 2
+
2K
K4R~(CN)6
+
3KCN
+
+
3CH4
3NH3
(3)
or i2KCN
+
4CH4
+
4NH3.
(4)
I n f a c t t h e reduction o f KCN by H2 i s not impossible thermodynamically when t h e products a r e removed continuously.
The alumina support might be p a r t l y transformed
t o cyanide during t h e decomposition, r e s u l t i n g i n a reduction t o form m e t a l l i c aluminum, on which no information i s a v a i l a b l e .
N o information i s a l s o a v a i l a b l e
about t h e reason o f t h e d i f f e r e n c e i n e x t e n t of reduction observed f o r Cat. C-2 and C-3. 2 . Ammonia s y n t h e s i s a c t i v i t i e s
The r a t e s of ammonia s y n t h e s i s over t h e c a t a l y s t s are summarized i n Fig. 2 as Those c a t a l y s t s a r e commonly 2 % f w / w ) R u on t h e same alumina support,
Arrhenius p l o t s .
while d i f f e r e n c e s may be found i n t h e e x t e n t of d i s p e r s i o n a s w e l l as i n t h e n a t u r e of promoter.
The amounts of hydrogen chemisorption a t O°C a r e given i n Table 2
t o g e t h e r with t h e r a t e s of ammnia s y n t h e s i s p e r g c a t . a t 3OOOC and t h e apparent a c t i v a t i o n e n e r g i e s (Ea)
.
TABLE 2
C a t a l y t i c p r o p e r t i e s of c a t a l y s t s N-2
N-3
K20
Cs20
none
H2m1STP/g.cat
0.68
1.08
0.31
0.13
NH mlSTP/g.cat.hr
1.9
8:3
0.076*
0.039*
28
28
17
17
*
c-2
c-3
-
0.98
1.08
1.22
1.35
4.3
1.9
0.67
0.70
1.1
27
27
28
26
26
x K
N-1
3 Ea K c a l / m l
c-1
X none
Catalyst Promoter
c-4
- cyanide -
e s t i m a t i o n by e x t r a p o l a t i o n
Since l i t t l e d i f f e r e n c e i s observed i n Ea on those alkali-promoted c a t a l y s t s , t h e observed d i f f e r e n c e i n t h e c a t a l y t i c a c t i v i t y seems t o come from t h e e x t e n t of dispersion.
However t h e v a r i a t i o n i n hydrogen chemisorption i s t o o small t o
explain the difference.
The hydrogen chemisorptions on t h e cyanide s e r i e s of
c a t a l y s t s a r e g e n e r a l l y l a r g e r , whereas t h e c a t a l y t i c a c t i v i t i e s are lower those on Cat. N - 1 and N - 2 .
than
I f m e t a l l i c potassium i s formed during t h e decomposition
387 of Cat. C-2 o r C-3 a s suggested by t h e stoichiometry, hydrogen can be chemisorbed on K i n a d d i t i o n t o on Ru, giving r i s e to l a r g e r chemisorptions.
In f a c t , as
described l a t e r , t h e chemisorption value on Cat. C-2 decreased by about 30% a f t e r a t r e a t m e n t with water vapor.
The l a r g e r chemisorption on C a t . C-3 o r C-4 than on
Cat. C-2 can be due t o l a r g e r amount of potassium a s suggested by t h e stoichiometry. Thus t h e r e a l chemisorption on Ru would be l e s s than t h a t observed on t h e cyanide s e r i e s of c a t a l y s t s .
i s remarkable.
The high a c t i v i t y on Cat. N - 1 o r N-2
I n comparison with Cat. N-3
which has no promoter, Cat. N - 1 and N-2 a r e r e s p e c t i v e l y , 11 and 31 t i m e s more a c t i v e i n terms of t h e r a t e p e r chemisorption. When Cat. X was a c t i v a t e d by a d d i t i o n of m e t a l l i c potassium (K/Ru=2.9mol / m o l )
,
t h e e x t e n t o f promotion a t 3OOOC w a s 110
t i m e s a s shown i n Table 1. Since t h e potassium a d d i t i o n may give r i s e t o an increase i n d i s p e r s i o n of ruthenium on alumina as was t h e case with unsupported ruthenium ( r e f . 2 ) , t h e d i f f e r e n c e i n promoting e f f i c i e n c y between m e t a l l i c potassium and cesium oxide would be
smaller.
Thus cesium oxide is a promising promoter which i s
s t a b l e i n t h e presence of water vapor. K > C s 0 >K 0 , i s i n accord with 2 2
The e f f i c i e n c y sequence of promoters,
t h e o r d e r of e l e c t r o - p o s i t i v i t y ,
the e l e c t r o n donation t o ruthenium as has been suggested ( r e f . 4 ) .
i n conformity with The high a c t i v i t y
o f C a t . N-2 a r i s e s f i r s t l y from t h e high e l e c t r o p o s i t i v i t y of C s 0 and secondly from 2 t h e increased d i s p e r s i o n of ruthenium. Presumably t h e low temperature reduction which
i s r e a l i z e d i n t h e presence of n i t r a t e i s r e s p o n s i b l e f o r t h e higher d i s p e r s i o n . Although m e t a l l i c potassium i s e f f i c i e n t as t h e promoter and i s l i k e l y formed i n Cat. C-2 o r C-3, t h e s p e c i f i c a c t i v i t y p e r chemisorption f o r Cat. C-2 o r C-3 i s much l e s s than t h a t on Cat. N-2.
On t h e o t h e r hand it i s t o be noted t h a t t h e
decomposition of K R u ( W /A1203 made i n H2-N2 mixture r e s u l t s i n a higher a c t i v i t y 4 6 than t h a t i n H a , a s i s c l e a r form t h e comparison of Cat. C - 1 o r C-4 w i t h C-2 o r C-3. This r e s u l t seems t o be caused by a cooperative i n c o r p o r a t i o n of K and N2 i n t o Ru metal ( r e f . 111, which g i v e s r i s e t o a higher potassium content.
Hence t h e lower
a c t i v i t y of Cat. C-2 o r C-3 seems t o be caused by a l e s s extensive i n t e r a c t i o n o f K w i t h Ru o r a p o s s i b l e loss o f K during t h e decomposition of K Ru(CN) 4 6' 3. E f f e c t of water vapor treatment.
The c a t a l y s t N-1 was t r e a t e d with a c i r c u l a t i n g C O D (=3/1) mixture containing s a t u r a t e d water vapor a t 45OOC f o r 24
pr,
2 during which C n 4 and CO
were formed. 2 A f t e r evacuation a t 400°C f o r 2 h r , t h e ammonia s y n t h e s i s a c t i v i t y was found t o be 64% of t h e i n i t i a l value, while it was recovered t o 86% a f t e r hydrogen treatment a t 45OOC f o r 40 h r . treatments.
The amount o f hydrogen chemisorption changed l i t t l e i n t h e above
In c o n t r a s t , t h e Cat. X w i t h K was found t o s u f f e r a d r a s t i c poisoning
by water vapor. A f t e r a treatment with c i r c u l a t i n g water vapor (15 t o r r ) a t 45OoC
f o r 14 h r ( u n t i l t h e f i n i s h o f hydrogen e v o l u t i o n ) followed by evacuation a t 400'C f o r 2 h r , t h e ammonia s y n t h e s i s a c t i v i t y was found t o be 6% of t h e i n i t i a l value and recovered t o 15% a f t e r hydrogen treatment a t 45OoC f o r 40 h r , with the f i n a l
388 a c t i v i t y being comparable t o t h a t of Cat.N-1.
I n t e r e s t i n g l y t h e amount of hydrogen
chemisorption on Cat. X i n c r e a s e d by a f a c t o r of 2 . 8 i n t h e above treatments.
The
a d d i t i o n of potassium followed by t h e armnonia s y n t h e s i s runs presumably gave r i s e t o a c o r r o s i v e chemisorption of nitrogen t o form a compound ( r e f . ll), which was decomposed by water r e s u l t i n g i n t h e increased d i s p e r s i o n . n i t r o g e n uptake has been observed on Ru-K/Al
In fact, a large
c a t a l y s t s ( r e f . 1 2 ) , whereas l i t t l e
0
2 3
uptake on Cat. N-1. Since m e t a l l i c potassium i s l i k e l y formed i n t h e decomposition of cyanide s e r i e s of c a t a l y s t s , t h e e f f e c t of water vapor would be r e v e a l i n g .
Cat. C-2 w a s t r e a t e d
w i t h c i r c u l a t i n g water vapor a t 44OOC u n t i l t h e f i n i s h of p r e s s u r e i n c r e a s e .
The
evolved gas was confirmed t o be hydrogen (0.67 m l H /mol Ru), suggesting t h a t 2 1.3 mol K/mol Ru i s formed i n C a t . C-2, which i s near t o 1 moleK/moleRu as estimated from t h e hydrogen consumption during t h e decomposition.
The amount of
hydrogen chemisorption decreased t o 72% of t h e i n i t i a l v a l u e , i n d i c a t i n g t h a t t h e i n i t i a l value involved t h e chemisorption on K.
I t may be concluded from t h e above r e s u l t s t h a t m e t a l l i c potassium ( o r aluminum)
is formed by hydrogen reduction o f K R(CN) / A 1 0 4
promotion of Ru.
6
2 3
r e s u l t i n g i n an extensive
I t i s r e c a l l e d t h a t an i r o n c a t a l y s t
was used f o r a m o n i a s y n t h e s i s ( r e f . 1 3 ) .
derived from ferrocyanide
Although t h e formation of potassium
was n o t d e t e c t e d a t t h a t t i m e ( r e f . 1 4 ) , it might b e t h e reason of high a c t i v i t y .
REFERENCES
1 A. Ozaki, K. Aika and H . Hori, Bull. Chem. SOC. J a p . , 44, 3216 (1971) 2 K. Urabe, K. Aika and A. Ozaki, J. Cat., 42, 197 ( 1 9 7 6 r 3 K. Urabe, A. Ohya and A. Ozaki, t o be published 4 K. Aika, H. Hori and A. Ozaki, J . Cat., 21, 424 (1972) 5 K. Urabe, T. Yoshioka and A. Ozaki, J . Cat., i n p r e s s 6 F. Krauss, 2. Anorg. Allg. Chem., 59 (1927) 7 K. Masuno, S . Waku, Nippon Kagaku Zasshi 83, 161 (1962) 8 J . L . Howe, J. Am. Chem. SOC., I s 9 8 1 (1896) 9 V.A. Pospelov and G.S. Zhdanov, Zh. Fiz. a i m , SSSR, 21, 405 (1947) 1 0 I. Nakagawa and T. Shimanouchi, Spectrochim. A c t a , 2, 101 (1962) 11 A. Ohya, K. Urabe and A. Ozaki, Chem. L e t t . , 233 1 2 K. Urabe, K. S h i r a t o r i and A. Ozaki, J. Cat., submitted 13 A. Mittasch and E . Kuss, 2. Elektrochem., 34, 159 (1928) 14 A. Mittasch, E. Kuss and 0. E m e r t . 2 . Anorg. Allg. Chem., 193 (1928)
165,
1978,
170,
DISCUSSION J.G.
v a n OMMEN : I t i s n o t s t r a n g e t h a t C s z O a n d K m e t a l h a v e a
c o m p a r a b l e p r o m o t i n g e f f e c t o n Ru ?
Couldn't
i t be, t h a t y o u
(when C s 0 r e a c t s w i t h w a t e r i t 2 w i t h K 0 o r KOH. I n that c a s e , i n m y o p i n i o n it i s 2 e a s i e r t o understand t h e comparable promoter e f f e c t of CsOH and a c t u a l l y compare C s 2 0 o r C s O H forms CsOH)
KOH b e c a u s e
they are both s t r o n g bases.
YOU
a l s o showed t h a t p o i s o n i n g by w a t e r v a p o r w a s i r r e v e r s i b l e
b e c a u s e p o t a s s i u m i s c o n v e r t e d t o KOH.
But couldn't
it be possible
t h a t w a t e r v a p o r a l s o d e s t r o y e s t h e r u t h e n i u m s u r f a c e by r e c o n s t r u c t i n g i t , and so t h a t t h e l o w e r a c t i v i t y i s c a u s e d by t h i s effect ? In your paper you also mention an activity sequence of promotors, t h a t d i f f e r s from t h e sequence p r e s e n t e d in Fig. 2.
From this
f i g u r e , t h e s e q u e n c e is C s 2 0 > K > K 2 0 , w h i l e in t h e t e x t t h e s e q u e n c e
is K > C s 2 0 > K 2 0 .
Can you explain this ?
C a n y o u a l s o a g r e e t h a t if o n e c o m p a r e s t h e o v e r a l l a c t i v i t y of t h e C s 0 p r o m o t e d c a t a l y s t w i t h t h e K m e t a l promoted one, t h e 2 first o n e g i v e s t h e b e t t e r catalysts.
A. O Z A K I : K e x h i b i t s definitely a h i g h e r p r o m o t i n g e f f e c t t h a n K 2 0 a n d , after p o i s o n i n g w i t h H20, t h e p r o m o t i n g e f f e c t a p p r o a c h e s T h e s e q u e n c e K > C s 2 0 > K 0 is based o n t h e a c t i v i t y 2 p e r s u r f a c e a t o m , t h e latter being d e t e r m i n e d by H 2 chemisorption. t h a t o f K20.
A n activity i n c r e a s e d u e t o d e s i n t e g r a t i o n o f p a r t i c l e w a s f o u n d w i t h K-promoted Ru.
(J. Cat.
S,
430(1975)).
I f it t a k e s p l a c e ,
a n i n c r e a s e in a c t i v i t y i s o b s e r v e d in t h e second run.
P.E.H.
NIELSEN
: H a v e y o u o b s e r v e d a n y s u p p o r t effect i n g o i n g from
c a r b o n s u p p o r t e d Ru-K c a t a l y s t t o a l u m i n a s u p p o r t e d Ru-K c a t a l y s t ? A. O Z A K I
: T h e s e t w o catalysts, s h o u l d not b e c o m p a r e d i n t e r m s o f t h e
a c t i v i t y p e r s u r f a c e Ru.
But e v a l u a t i o n o f Ru d i s p e r s i o n o n c a r b o n
r e m a i n s d i f f i c u l t so t h a t n o c o m p a r i s o n h a s b e e n made.
K. J O H A N S E N : 1) W h i c h is t h e s p a c e v e l o c i t y used i n y o u r a c t i v i t y m e a s u r e m e n t s in T a b l e 2 ?
2) Have y o u any h i g h p r e s s u r e a c t i v i t y
measurements ?
A. O Z A K I : 1) 2 0 0 0 - 3 0 0 0 hr-'.
2) T h e r a t e o f N H 3 f o r m a t i o n on
Ru-K./Ac t e n d s t o a p p r o a c h a p l a t e a u v a l u e at h i g h pressures.
J.W.
H I G H T O W E R : F r o m T a b l e 2 and Fig. 2, I n o t i c e t h a t t h e l e a s t
a c t i v e u n p r o m o t e d c a t a l y s t s ( X and N-3) h a v e t h e l o w e s t a p p a r e n t a c t i v a t i o n energy.
B e c a u s e t h e r a t e s in t h e s e c a s e s a r e q u i t e low,
I d o not t h i n k t h a t d i f f u s i o n l i m i t a t i o n s c a n be r e s p o n s i b l e for t h e s e results.
T h e p r e - e x p o n e n t i a l f a c t o r m u s t be s o m e h o w g r e a t l y
390 altered by t h e promotor.
C a n you comment o n t h e s e r a t h e r s u r p r i s i n g
observations ?
A.
OZAKI
:
T h e p r o m o t o r effect of K on R u a p p a r e n t l y r e s u l t s in a
r e m a r k a b l e i n c r e a s e in t h e pre-exponential
factor.
T h i s must be
c a u s e d by a structural c h a n g e in Ru s u r f a c e , b u t we c a n n o t g i v e any f u r t h e r comment o n t h i s at present.
Z.G.
SZABO
: You have p u t e m p h a s i s o n t h e n i t r a t e effect, and not
w i t h o u t reason.
During t h e r e d u c t i o n , N H 3 i s first formed and
afterwards NHqN03. NZO.
U n d e r y o u r e x p e r i m e n t a l c o n d i t i o n s it p r o d u c e s
We a l s o u s e d t h i s d e c o m p o s i t i o n for m a k i n g h i g h l y active c a t a -
lysts.
It i s blowing u p t h e precipitate, r e s u l t i n g in very high
s u r f a c e development.
A s the activation energies are nearly the
same, it p o i n t s to a n increased p r e - e x p o n e n t i a l factor.
But t h i s
effect must operate at the most appropriate stage, during the preparation.
T h i s i s p e r h a p s t h e r e a s o n why p r e v i o u s l y added
i s n o t advantageous.
A.
OZAKI
:
T h a n k y o u for y o u r comment.
NH4N0
3