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Promoting effects of CO2 on dehydrogenation of propane over a SiO2-supported Cr2O3 catalyst
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
419
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Promoting effects of C O 2 supported Cr203 catalyst
on
dehydrogenation of propane over a SiO2-
I. Takahara a, W.-C. Chang b, N. Mimura a and M. Saito a aNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan bjKF (Japan-Korea Industrial Technology Onogawa, Tsukuba-shi, Ibaraki 305, Japan
Co-operation Foundation)
researcher,
16-3
The effects of carbon dioxide on the dehydrogenation of C3H8 to produce C3H6 were investigated over several Cr203 catalysts supported on A1203, active carbon and SIO2. Carbon dioxide exerted promoting effects only on SiO2-supported Cr203 catalyst. The promoting effects of carbon dioxide over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation.
I. I N T R O D U C T I O N The utilization of carbon dioxide has recently received much attention since the global warming mainly due to carbon dioxide was recognized as one of the most serious problems in the world. The catalytic hydrogenation of CO2 to produce methanol, hydrocarbons, etc., and the CO2 reforming of methane to syngas have been extensively studied. Furthermore, it has been reported that CO2 has several promoting effects on the conversion of hydrocarbons, for example, oxidative coupling of methane [1], aromatization of propane [2] and dehydrogenation of ethylbenzene [3, 4]. The authors have investigated the effect of CO2 on the dehydrogenation of propane over several supported Cr203 catalysts, and found that CO2 has promoting effects on a silica-supported Cr203 catalysts. 14 2. EXPERIMENTAL
12
B
C
~lO Several supported Cr203 catalysts were prepared by an impregnation method using an aqueous solution of chromium nitrate. The supports used were y- A1203, active carbon (AC) and SiO2. The catalysts prepared were calcined at 823 K in air for 2 h. The catalysts were characterized by X-ray diffraction (XRD). The XRD pattems of CrzO3/A1203 and CrzO3/AC showed the diffraction lines ascribed only to the phases of respective
=~ 8 o 6 -~ ~ 4 2 06
~ x
2
4 6 0 2 4 0 ' t/h t/h 2/h Figure 1. Catalytic activities of several supported Cr203 catalysts as a function of time on stream. supports. In the case of Cr203/SiO2, a Cr203 phase A=Cr203(5wt%)/Ai203, B=Cr203(5wt%)/AC' and an amorphous SiO2 phase were observed. The C-Cr203(Swt%)/SiO2' 823 K, W/F=2 g-cat.h/mol, dehydrogenation of C3H8 was conducted under Feed gas:C3H8/CO2=l/l(O), C3H8/Ar=-I/I(O)
420 100
atomospheric pressure of C3H8+CO2 (At) at 823 K by using a fixed bed flow reactor. TPR/D studies were carried out for elucidating the behavior of Cr203 in the catalyst during treatment with H2 and CO,. Pulse reaction technique was also employed for examining the initial activity of the catalyst.
A
88~
B
~
-''-
.
_.
~:~ 60 Q ~ .,,,~ ..,.4
3. R E S U L T S A N D D I S C U S S I O N
"~ 20 r~
3 . 1 . Effects of CO2 on d e h y d r o g e n a t i o n o f C3Ha
%
The main products of the conversion of C3H8 in the presence of Ar were C3H6 and H2, while those in the presence of CO, were C3H6, H2, and CO. Since the selectivities for C3H6 were more than 90%, the dehydrogenation of C3H8 to C3H6 should be the main reaction both in the presence of CO2 and in the absence of CO2. The yield of H2+CO was found higher than C3H6 yield on all catalysts used in the present study. There might be three possible routes for CO formation; the first one via the successive reactions (1) and (2), the second o n e via the reaction (3) and the third one via CO2 reforming of C3H8 (reaction 4) as shown below.
' 2 ' 4 6 L 8 ' 10 Yield of C3H6 / % Yield of C3H 6 / %
Figure 2. Selectivities of H2 and CO in the
reactionof C3H8+CO2as a function of C3H6 yield. A=Cr203(5wt%)/Al203, B=Cr203(5wt%)/Si02, Selectivity" H2(ll), CO(O),823 K, W/F=2g-cat-h/tool, Feed gas:C3H8/CO2=l/1
7 6 ~
/
~
~5 ~:~4
~
rS 3
~
f a'~,,,,o
/
o .,-,2
C3H8- C3H6 + H2 CO2 + H2 = CO + H20 C3H8 + CO2 = C3H6 + CO + H20 C3H8 + 3CO2 = 6CO + 4H2 Figure
1 shows
the activities
(1) (2) (3) (4)
of
>' 1 06
:~
3, t/h ~i
~
10
Figure 3. Change in the C3H6 yield with alternate several feeds of C3Hs/Ar and C3Hs/CO2/Ar over a Cr203/SiO 2.
supported Cr203 catalyst for the dehydrogenation C3H8/CO2/Ar=I/2/7(O) ' C3Hs/Ar=I/9(O) ' of C3H8 in the presence of CO2 as well as in the 823 K, W/F----0.62g-catoh/mol ~. absence of CO2. The activity of the Cr203]AI203 . 1.3.~ catalyst was much lower in the presence of CO2 l ~ ,~ than that in the absence of CO2. The activity of the 8 Cr203/AC was independent of the presence of CO2. ~ - t.2~ On the other hand, the activity of Cr203/SiO2 ~:1~6~.~' ~,~ L) catalyst in the presence of CO2 was surprisingly ~2/~\ "~ found to be 40% higher than that in the absence of .~ 4 t ~ "d CO2. ~ ~.~r~ Figure "~ C shows the O selectivities _ for HE and ~2t x~~__ ~ ~ as a function of C3H6 yield. In the case of 0/ i t ~ ~ t 1 o Cr203/AI203 catalyst, the, selectivity for H2 0 20 Cr20340/wt% 60 "= decreased with an increase in C3H6 yield, whereas Content of the selectivity for CO increased with an increase in Figure 4. Yield of C3H6 in the dehydrogenation of C3H8in the presence of CO2(l) and in the C3H6 yield, as shown in Figure 2A. On the other absenceof CO2(O) and their ratio(O) as hand, the selectivities of Cr203/SiO2 catalyst for a function of Cr203 contenton a Cr203/SiO2. CO and H2 did not change irrespective of C3H 6 C3Hs/CO2(Ar)=I/1,823K, W/F=2g-catoh/mol
421
yield, as shown in Figure 2B. These findings suggest that CO might be formed via successive reactions (1) and (2) over Cr203/A1203 catslyst, whereas both the reaction (1) and the reaction (3) could take place simultaneously over Cr203/SiO2 catalyst. In order to study the contribution of CO2 in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst, catalytic tests with alternate feeds of CaH8/Ar and C3H8/CO2/Ar were carried out. The results shown in Figure 3 clearly indicate that the presence of CO,,_ markedly improved the yield of C3H6. This catalytic performance is a proof that CO,, plays a promoting role in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst. Furthermore, the ratio of C3H6 yield in the reaction in the presence of CO2 to that in the absence of CO 2 decreased with increasing Cr203 content in the Cr203/SiO2 catalyst, as shown Figure 4. This suggests that the boundaries between Cr203 and SiO2 particles might have an important role in the promoting effect of CO2. The effect of CO2 addition on the deactivation of a Cr203/SiO2 catalyst was also examined (Figure 5). The reaction conditions for the both cases with and without CO2 were the same except the catalyst weight, which was 50% larger in the case of the reaction without CO2, in order to obtain the same initial yield of C3H6. The decrease in the yield of C3H6 was found much less in the presence of CO~ than in the absence of CO2. This finding suggests that the addition of CO2 could suppress the deactivation of the catalyst.
lo
8 :~ 6 e~
~ g7. O
2 I
I
I
2 4 6 0 t/h Figure 5. Effect of C O 2 o n the deactivation of Cr203(5wt%)/SiO2. Reaction conditions: 823K 9 : C3H8/CO2/Ar=1/2/7, W/F=0.62g-catoh/mol O: C3Hs/Ar=-1/9, W/F=0.93g-cat.h/mol
(A) in He -I
~B) in H2(10)/Ar
~ .~
3.2. TPR/D result Figure 6 shows TPR/D profiles during various treatments of Cr203/SiO2 catalyst. The Cr203 was (C) in He after CO2 treatmentat 823 K found to be reduced in a stream of H2. And smaller peaks were observed when the catalyst, which was reduced with H2 and then treated with CO2, was re(D) in H2(10)/Arafter (C) reduced with H2. This suggests that CO2 could 323 423 523 623 723 823 oxidize some part of the surface of Cr203 in the catalyst. Accordingly, these findings suggest that Temperature / K --823K= the surface of Cr203 on a Cr203/SiO2 catalyst could Figure 6. TPR/D profile for treatment be reduced during the dehydrogenation of C3H8 in A---*B----C----Dover a Cr203/SiO 2 catalyst. the absence of CO,_, whereas the surface of Cr203 Operation conditions 9H2(10)/Ar(90), 10 K/min might be maintained partially oxidized during the reaction in the presence of CO2. ,.._.___
-
- -
_.
_
422
3.3.
Pulse
reaction
A pulse reaction technique was employed tbr | 50 examining the initial activity of the catalyst. Figure 25 H2 treatment CO2 treatment / 7 shows the conversion of C3H8 and the yield of I hydrocarbon products as a function of the number 20 ~-._~:~ :i 40~. of C3H8 pulses on Cr203/SiO2 catalyst. The yield of C3H6 reached a maximum of about 21% at the - 30~ second pulse. After the second pulse, the yield of ~15 0 C3H6 slightly decreased with increasing pulse ~ "~ number. The yield of C3H6 decreased to 17% by ~,10 20-~ the treatment of the catalyst with H2 after filth pulse, o and remained constant from sixth pulse to ninth 5 10~ pulse. The treatment of the catalyst with CO2 after ninth pulse raised the yield of C3H6 to 19%, but the 0 yield of C3H6 gradually decreased with increasing 0 0 2 4 Pul6enumber810 12 14 pulses number from 10th pulses. These findings 7. Conversionof C3H8 and yield of confirm that partially oxidized Cr203/SiO2 catalyst Figure hydrocarbon products as function of could be more active for the dehydrogenation of the number of C3H8 pulse on Cr203/SiO2. C3H8 than a reduced catalyst. The results obtained Reaction conditions: 823K, He carrier from pulse reaction studies and from TPR/D studies Yield: CH4(i"I), C2H4(A), C2H6(O), C3H6(. ) might explain slower deactivation of the catalyst Conversion: C3H8(1) during the dehydrogenation in the presence of CO2. Furthemore, pulse reaction studies were performed for comparing between the yield of C3H6 during the reaction in stream of CO2 and in a stream of He. The C3H 6 yield at the first pulse and the second pulse in the presence of CO2 was found to be higher than those in the absence of CO2. This strongly suggests that CO2 could enhance the rate of the dehydrogenation of C3H8 over a Cr203/SiO2 catalyst. This promoting effects of CO2 might occur at the boundaries between Cr203 and SiO2 particles, as described in 3.1. 4. C O N C L U S I O N The effects of CO2 on the dehydrogenation of C3H8 to produce C3H6 exhibits only on SiO2-supported Cr203 catalyst. The promoting effects of CO, over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation. The partially oxidized Cr203 supported on SiO2 catalyst is active for the dehydrogenation of C3H8. In the presence of CO2, the surface of Cr203/SiO2 might be maintained partially oxidized during the reaction.
REFERENCES
1. T. Nishiyama and K. Aika, J. Catal., 122 (1990) 346. 2. S. Yamauchi, A. Satsuma, T. Hattori and Y. Murakami, Sekiyu Gakkaishi ( J. of Jpn. Petrol. Inst.), 37 (1994) 278. 3. S. Sato, M. Ohhara, T. Sodesawa and F. Nozaki, Appl. Catal., 37 (1988) 207. 4. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga and T. Suzuki, Appl. Catal. A:General, 121 (1995) 125.
419
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Promoting effects of C O 2 supported Cr203 catalyst
on
dehydrogenation of propane over a SiO2-
I. Takahara a, W.-C. Chang b, N. Mimura a and M. Saito a aNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan bjKF (Japan-Korea Industrial Technology Onogawa, Tsukuba-shi, Ibaraki 305, Japan
Co-operation Foundation)
researcher,
16-3
The effects of carbon dioxide on the dehydrogenation of C3H8 to produce C3H6 were investigated over several Cr203 catalysts supported on A1203, active carbon and SIO2. Carbon dioxide exerted promoting effects only on SiO2-supported Cr203 catalyst. The promoting effects of carbon dioxide over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation.
I. I N T R O D U C T I O N The utilization of carbon dioxide has recently received much attention since the global warming mainly due to carbon dioxide was recognized as one of the most serious problems in the world. The catalytic hydrogenation of CO2 to produce methanol, hydrocarbons, etc., and the CO2 reforming of methane to syngas have been extensively studied. Furthermore, it has been reported that CO2 has several promoting effects on the conversion of hydrocarbons, for example, oxidative coupling of methane [1], aromatization of propane [2] and dehydrogenation of ethylbenzene [3, 4]. The authors have investigated the effect of CO2 on the dehydrogenation of propane over several supported Cr203 catalysts, and found that CO2 has promoting effects on a silica-supported Cr203 catalysts. 14 2. EXPERIMENTAL
12
B
C
~lO Several supported Cr203 catalysts were prepared by an impregnation method using an aqueous solution of chromium nitrate. The supports used were y- A1203, active carbon (AC) and SiO2. The catalysts prepared were calcined at 823 K in air for 2 h. The catalysts were characterized by X-ray diffraction (XRD). The XRD pattems of CrzO3/A1203 and CrzO3/AC showed the diffraction lines ascribed only to the phases of respective
=~ 8 o 6 -~ ~ 4 2 06
~ x
2
4 6 0 2 4 0 ' t/h t/h 2/h Figure 1. Catalytic activities of several supported Cr203 catalysts as a function of time on stream. supports. In the case of Cr203/SiO2, a Cr203 phase A=Cr203(5wt%)/Ai203, B=Cr203(5wt%)/AC' and an amorphous SiO2 phase were observed. The C-Cr203(Swt%)/SiO2' 823 K, W/F=2 g-cat.h/mol, dehydrogenation of C3H8 was conducted under Feed gas:C3H8/CO2=l/l(O), C3H8/Ar=-I/I(O)
420 100
atomospheric pressure of C3H8+CO2 (At) at 823 K by using a fixed bed flow reactor. TPR/D studies were carried out for elucidating the behavior of Cr203 in the catalyst during treatment with H2 and CO,. Pulse reaction technique was also employed for examining the initial activity of the catalyst.
A
88~
B
~
-''-
.
_.
~:~ 60 Q ~ .,,,~ ..,.4
3. R E S U L T S A N D D I S C U S S I O N
"~ 20 r~
3 . 1 . Effects of CO2 on d e h y d r o g e n a t i o n o f C3Ha
%
The main products of the conversion of C3H8 in the presence of Ar were C3H6 and H2, while those in the presence of CO, were C3H6, H2, and CO. Since the selectivities for C3H6 were more than 90%, the dehydrogenation of C3H8 to C3H6 should be the main reaction both in the presence of CO2 and in the absence of CO2. The yield of H2+CO was found higher than C3H6 yield on all catalysts used in the present study. There might be three possible routes for CO formation; the first one via the successive reactions (1) and (2), the second o n e via the reaction (3) and the third one via CO2 reforming of C3H8 (reaction 4) as shown below.
' 2 ' 4 6 L 8 ' 10 Yield of C3H6 / % Yield of C3H 6 / %
Figure 2. Selectivities of H2 and CO in the
reactionof C3H8+CO2as a function of C3H6 yield. A=Cr203(5wt%)/Al203, B=Cr203(5wt%)/Si02, Selectivity" H2(ll), CO(O),823 K, W/F=2g-cat-h/tool, Feed gas:C3H8/CO2=l/1
7 6 ~
/
~
~5 ~:~4
~
rS 3
~
f a'~,,,,o
/
o .,-,2
C3H8- C3H6 + H2 CO2 + H2 = CO + H20 C3H8 + CO2 = C3H6 + CO + H20 C3H8 + 3CO2 = 6CO + 4H2 Figure
1 shows
the activities
(1) (2) (3) (4)
of
>' 1 06
:~
3, t/h ~i
~
10
Figure 3. Change in the C3H6 yield with alternate several feeds of C3Hs/Ar and C3Hs/CO2/Ar over a Cr203/SiO 2.
supported Cr203 catalyst for the dehydrogenation C3H8/CO2/Ar=I/2/7(O) ' C3Hs/Ar=I/9(O) ' of C3H8 in the presence of CO2 as well as in the 823 K, W/F----0.62g-catoh/mol ~. absence of CO2. The activity of the Cr203]AI203 . 1.3.~ catalyst was much lower in the presence of CO2 l ~ ,~ than that in the absence of CO2. The activity of the 8 Cr203/AC was independent of the presence of CO2. ~ - t.2~ On the other hand, the activity of Cr203/SiO2 ~:1~6~.~' ~,~ L) catalyst in the presence of CO2 was surprisingly ~2/~\ "~ found to be 40% higher than that in the absence of .~ 4 t ~ "d CO2. ~ ~.~r~ Figure "~ C shows the O selectivities _ for HE and ~2t x~~__ ~ ~ as a function of C3H6 yield. In the case of 0/ i t ~ ~ t 1 o Cr203/AI203 catalyst, the, selectivity for H2 0 20 Cr20340/wt% 60 "= decreased with an increase in C3H6 yield, whereas Content of the selectivity for CO increased with an increase in Figure 4. Yield of C3H6 in the dehydrogenation of C3H8in the presence of CO2(l) and in the C3H6 yield, as shown in Figure 2A. On the other absenceof CO2(O) and their ratio(O) as hand, the selectivities of Cr203/SiO2 catalyst for a function of Cr203 contenton a Cr203/SiO2. CO and H2 did not change irrespective of C3H 6 C3Hs/CO2(Ar)=I/1,823K, W/F=2g-catoh/mol
421
yield, as shown in Figure 2B. These findings suggest that CO might be formed via successive reactions (1) and (2) over Cr203/A1203 catslyst, whereas both the reaction (1) and the reaction (3) could take place simultaneously over Cr203/SiO2 catalyst. In order to study the contribution of CO2 in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst, catalytic tests with alternate feeds of CaH8/Ar and C3H8/CO2/Ar were carried out. The results shown in Figure 3 clearly indicate that the presence of CO,,_ markedly improved the yield of C3H6. This catalytic performance is a proof that CO,, plays a promoting role in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst. Furthermore, the ratio of C3H6 yield in the reaction in the presence of CO2 to that in the absence of CO 2 decreased with increasing Cr203 content in the Cr203/SiO2 catalyst, as shown Figure 4. This suggests that the boundaries between Cr203 and SiO2 particles might have an important role in the promoting effect of CO2. The effect of CO2 addition on the deactivation of a Cr203/SiO2 catalyst was also examined (Figure 5). The reaction conditions for the both cases with and without CO2 were the same except the catalyst weight, which was 50% larger in the case of the reaction without CO2, in order to obtain the same initial yield of C3H6. The decrease in the yield of C3H6 was found much less in the presence of CO~ than in the absence of CO2. This finding suggests that the addition of CO2 could suppress the deactivation of the catalyst.
lo
8 :~ 6 e~
~ g7. O
2 I
I
I
2 4 6 0 t/h Figure 5. Effect of C O 2 o n the deactivation of Cr203(5wt%)/SiO2. Reaction conditions: 823K 9 : C3H8/CO2/Ar=1/2/7, W/F=0.62g-catoh/mol O: C3Hs/Ar=-1/9, W/F=0.93g-cat.h/mol
(A) in He -I
~B) in H2(10)/Ar
~ .~
3.2. TPR/D result Figure 6 shows TPR/D profiles during various treatments of Cr203/SiO2 catalyst. The Cr203 was (C) in He after CO2 treatmentat 823 K found to be reduced in a stream of H2. And smaller peaks were observed when the catalyst, which was reduced with H2 and then treated with CO2, was re(D) in H2(10)/Arafter (C) reduced with H2. This suggests that CO2 could 323 423 523 623 723 823 oxidize some part of the surface of Cr203 in the catalyst. Accordingly, these findings suggest that Temperature / K --823K= the surface of Cr203 on a Cr203/SiO2 catalyst could Figure 6. TPR/D profile for treatment be reduced during the dehydrogenation of C3H8 in A---*B----C----Dover a Cr203/SiO 2 catalyst. the absence of CO,_, whereas the surface of Cr203 Operation conditions 9H2(10)/Ar(90), 10 K/min might be maintained partially oxidized during the reaction in the presence of CO2. ,.._.___
-
- -
_.
_
422
3.3.
Pulse
reaction
A pulse reaction technique was employed tbr | 50 examining the initial activity of the catalyst. Figure 25 H2 treatment CO2 treatment / 7 shows the conversion of C3H8 and the yield of I hydrocarbon products as a function of the number 20 ~-._~:~ :i 40~. of C3H8 pulses on Cr203/SiO2 catalyst. The yield of C3H6 reached a maximum of about 21% at the - 30~ second pulse. After the second pulse, the yield of ~15 0 C3H6 slightly decreased with increasing pulse ~ "~ number. The yield of C3H6 decreased to 17% by ~,10 20-~ the treatment of the catalyst with H2 after filth pulse, o and remained constant from sixth pulse to ninth 5 10~ pulse. The treatment of the catalyst with CO2 after ninth pulse raised the yield of C3H6 to 19%, but the 0 yield of C3H6 gradually decreased with increasing 0 0 2 4 Pul6enumber810 12 14 pulses number from 10th pulses. These findings 7. Conversionof C3H8 and yield of confirm that partially oxidized Cr203/SiO2 catalyst Figure hydrocarbon products as function of could be more active for the dehydrogenation of the number of C3H8 pulse on Cr203/SiO2. C3H8 than a reduced catalyst. The results obtained Reaction conditions: 823K, He carrier from pulse reaction studies and from TPR/D studies Yield: CH4(i"I), C2H4(A), C2H6(O), C3H6(. ) might explain slower deactivation of the catalyst Conversion: C3H8(1) during the dehydrogenation in the presence of CO2. Furthemore, pulse reaction studies were performed for comparing between the yield of C3H6 during the reaction in stream of CO2 and in a stream of He. The C3H 6 yield at the first pulse and the second pulse in the presence of CO2 was found to be higher than those in the absence of CO2. This strongly suggests that CO2 could enhance the rate of the dehydrogenation of C3H8 over a Cr203/SiO2 catalyst. This promoting effects of CO2 might occur at the boundaries between Cr203 and SiO2 particles, as described in 3.1. 4. C O N C L U S I O N The effects of CO2 on the dehydrogenation of C3H8 to produce C3H6 exhibits only on SiO2-supported Cr203 catalyst. The promoting effects of CO, over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation. The partially oxidized Cr203 supported on SiO2 catalyst is active for the dehydrogenation of C3H8. In the presence of CO2, the surface of Cr203/SiO2 might be maintained partially oxidized during the reaction.
REFERENCES
1. T. Nishiyama and K. Aika, J. Catal., 122 (1990) 346. 2. S. Yamauchi, A. Satsuma, T. Hattori and Y. Murakami, Sekiyu Gakkaishi ( J. of Jpn. Petrol. Inst.), 37 (1994) 278. 3. S. Sato, M. Ohhara, T. Sodesawa and F. Nozaki, Appl. Catal., 37 (1988) 207. 4. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga and T. Suzuki, Appl. Catal. A:General, 121 (1995) 125.