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Complete oxidation of active carbon at low temperatures by composite catalysts
Cwbon Vol. 20. No, 3. pp. 21347, Printed in Great Britain.
~223/82/030213~S~3.~/0 Q 1982 Pergamcn Press Ltd.
1982
COMPLETE OXIDATION OF ACTIVE CARBON AT LOW TEMPERATURES BY COMPOSITE CATALYSTS TOMOYUKIINUI, TOSHIROOTOWA,KADZUMASATSUTCHIHASHI and YOSHINOBUTAKEGAMI Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan (Receiued 8 Seprember 1981) .#stract--The reaction between oxygen and active carbon was studied at 500°C. Various composite catalysts having an iron-group metal as the main component were supported on active carbon. The synergistic effect of the oxidation activity was widely observed in the composite metal-metal oxide catalyst/carbon system. Several active carbons were studied in this catalyzed oxidation and the oxidation rate decreased with decreasing surface area of carbon materials. The rate-determining step of this reaction was considered to be the surface-diffusion rate of oxygen. Enhancement of oxygen transmission by active composite catalysts resulted in promotion of the reaction.
1. INTRODUCTION
In response to the demand for coal utilization, many studies have been conducted concerning the reaction between carbon and gaseous agents. It has been noticed that some metals or metal oxides in a coal work as catalysts. The oxidation rates of graphite were obtained for various single-component catalysts [ I]. The movement of various kinds of catalyst particles in the oxidation of graphite was directly observed, applying in situ electron microscopic technique[2,3]. The oxidation of highly graphitized carbon black was studied, and elemental rate constants based on active surface area were obtained[4]. In these studies, the mobility of catalyst particles [2], oxygen chemisorption [5], and other detailed kinetic observations [4] have been recorded; however, single component catalyst/graphite systems still formed the basis for research, and the possibilities of more active catalysts were not yet a common subject of investigation. In our study, we searched for more active catalyst systems for the oxidation of carbon at low temperatures. We have already shown that certain composite-catalyst system involving the iron-group metals, lanthanum oxide and platinum-group metals exerted high activities, both in direct hydrogenation of carbon[6] and in complete conversion of N0[7]. Since such reactions were accelerated by combining catalyst components, we assumed that the reaction proceeded with an accelerated transmission of gas onto carbon[ll]. In this paper, we apphed another composite catalyst to the oxidation of carbon in an attempt to develop a more practical, low cost catalyst system. Our ultimate aim is to obtain fundamental knowledge for application in other valuable reactions involving oxidative gases such as CO*, NO and HzO.
Catalysts. A catalyst supported on an active carbon was prepared by the impregnation method[6]. A-3 active carbon, 30-60 mesh in size, prepared by Shimadzu Seisakusho Co. Ltd. Japan, was used as the reaction material and, concurrently, as the catalyst support. It had a BETCAR Vol. 20, No. 3-D
surface area of 1230m2g-‘, a porosity of 0.46, a bulk density of 0.38 g cmm3,an average macro pore size of 3~ and an average micro pore size of 26 A. The ratio of macro pore to micro pore volume was 0.53. A watersoluble component was not detected. Volatile matter was negligible below 500°C in the stream of nitrogen containing 10% hydrogen. The active carbon was immersed in an impregnating aqueous solution of nitrate or chloride salt of catalyst metal, and was dried thoroughly by stirring over a boiling water bath, It was then exposed to a saturated vapor of 10% aqueous ammonia solution at 20°C for 2min, followed by heating in a nitrogen stream containing 10% hydrogen. The oxides were reduced into a metallic state, with the exception of lanthanum oxide. Several weight percent of an iron-group metal were supported as the main component of the catalyst. A transition metal oxide and a precious metal were combined with iron-group metal as promoters. The atomic ratios of the transition metal and the precious metal were set at ca. 0.2 and 0.04 of the substrate, respectively. A mixed solution consisting of nitrate salts of iron-group metal and a transition metal was used for the preparation of a two-component catalyst: however, in the case of the three-component catalyst, the precious metal was first dispersed on the support by the method described above, and the other two components were then added together. Reaction operation. An ordinary flow reaction apparatus was used at atmospheric pressure. A quartz tube with an inner diameter of 3Smm was chosen as the reactor. A 15mg(O.O5cm3) portion of the sample was packed in the reactor, which was heated by an external electric furnace. The catalyst-bed temperature was measured by inserting the top of a thermocouple shielded with stainless steel of 0.6 mm dia. into the exit side of the catalyst bed. The temperature was increased under nitrogen flow up to 500°C; 1.4% 02 diluted by NZ was then fed and effluent gas was analyzed by gas chromatography with columns of Porapak Q for CO;? and MS 5A for 02 and CO. The flow rate of feed gas was set at 50 cm3 n.t.p. mm’. Measurements of the oxidation state of the metal. The 213
214
T. INUI
oxidation states of the metal supported on the active carbon during the reaction were gravimetrically obtained by using Shimadzu’s TG-20 micro-thermogravimetric analyzer with a gas-flow control system. A 15 mg portion of the catalyst-supported active carbon was placed in an aluminum pan, having a 5 mm dia. and 2 mm depth. At 505”C, 1% oxygen was passed downward, through the quartz tube, at a flow rate of .50cm3min-‘. After a certain time, temperature was allowed to decrease until the oxidation reaction stopped; then the reaction gas replaced by 10%Hz/He and the temperature was elevated to 3OO*C,and the weight decrease due to hydrogen reduction was observed. The oxidation state of the catalyst metal was calculated from the weight difference between oxidative and reductive conditions. In this condition the amount of carbon-loss for methane formation was negligible.
3.1 Oxidationactivity of single component catalysts The oxidation activity of single component catalysts at 500°C is shown in Fig. 1. Only CO1 was found in the eflluent gas, except for the cases of CulC and NifC systems, Only a small amount of CO was yielded, such as 1.0 x low6and 0.8 x W6mol min-’ for CulC and Ni/C systems, respectively. In the light of the CO&O equilibrium composition of Boudouard reaction, it is likely that most of the CO was oxidized and was withdrawn before the equilibrium. The change of oxidation rate with carbon consumption was different for each catalyst, and also gave the kinetic data mentioned in 3.4. As can be clearly seen in Fig. I, the order of their activities was Cu>Co>Fe>Ni>Pt. Despite the difference in oxidation rate of each catalyst, the active carbon was consumed completely in every case when the operation proceeded for a sufficient time. Carbon bum-offs by Cu, Co, Fe and Ni at 500°C for 90 min were 100,59,39 and 12%, respectively.
9 ” t
1
2 u
a
a
60
30 TIME
90
(min)
Fig. 1. Comparison of carbon-oxidation activity for each metal catalyzed active carbon. SOOT 1.4%@/Nz 50 ml min-’ n&p. 0, 2.O%Cu/C: A, 4.7%Co/C; 0, 4.7% Fe/C; a, M%Ni/C; A, 3.0% Ptlc.
et at.
0
30
60 TIME
90
(min)
Fig. 2. The effect of Cu content in ~op~r~a~yzed rate of carbon oxidation. 0, 2.0%-; 0,5.0%-; A, I.&%-;A, O.SO%-; 0, 0.13%CUIC.
The effect of concentration of catalysts on the oxidation rate varied with Cu and Pt respectively. On the one hand, Cu showed a maximum activity at 2.O%Cu and then the activity decreased at 5.0% Cu, as shown in Fig. 2. However, on the other hand, no activity change was observed in Pt catalyst between 0.05 and 3.0% concentrations. 3.2 Syne~istic efect in the oxidation of Fe-Laz03Ptlactiue carbon system As the activity of copper catalyzed oxidation was found to be the highest of applied single metal/carbon systems, the copper-catalyzed oxidation for the copper~~phite system and copper-p~cle mobility has been investigated in detaiI[3]. However, we ascertained that Cu was ineffective for the reaction between active carbon and any other gaseous agent, such as carbon dioxide and water vapor, as was also shown using graphite as the carbon source by other workers[3]. Because of this, composite catalysts with iron-group metal as the main component were investigated in this reaction. The oxidation activity at 500°C is shown in Fig. 3, and is compared to each other in the early stage of the reaction. A notable enhancement was observed when small amounts of La203 and Pt were combined with the iron. Since the order of the CO, yield was not changed within 30min of reaction time, the carbon bum-off of each catalyst system was compared at 30min and became 66, 33, 17 and 3% for Fe-La203-Pt, La203, Fe and Pt, respectively. The activity of the three-component catalyst system was greater than the sum of the activities of each catalyst system. Such a synergistic effect was also observed when Co or Ni was used as the main component of the catalyst. Furthermore, although each main component by itself had a dtierent activity (as shown in Fig. l), the activities of these three-component catalysts were similarly high. This synergistic effect agrees with that observed in the previous reactions@].
Complete oxidation of active carbon at low temperatures Table 1. Oxidation activities of various active carbons
3t
cartmn rmenals
A3 DIAHOPE MERCK CARBON
0
215
0
30
60 TIME
90
(min)
Fig. 3. Synergistic effect of Fe-based three-component catalyst/active carbon system on oxidation rate. 0, 4.7% Fe2.6% La203-0.70% Pt/C; a, 4.7% Fe-2.6% La203/C; 2.8% La203/C; 0,4.7% Fe/C; Cl, 0.70% Pt/C.
A,
3.3 Oxidation activity of diflerent composite catalyst
systems In order to apply the composite catalyst system to practical use, it is essential to develop it economically. When expensive Pt was replaced with a small amount of Cu, it still exerted the same activity, as shown in Fig. 4. Furthermore, Mn20p was comparably active when it replaced La20j. The cost of the Fe-MnzOrCu/C system is one-thirtieth of that of the Fe-La*O,-Pt/C system. Other active carbons from various carbon materials were used for the catalyst system of Fe-La20rPt/C. Activities were compared with each other with respect to the carbon burn-off at 500°C in 30 min of reaction time. As shown in Table 1, higher activities were obtained for the carbon materials of larger BET surface area.
”
60
30 TIME
008*1 2186 BLACK*1
Surface area .2 g-l
Bulk dersjity g on-3
1230
0.38
1200
0.45
910
0.41
85
0.37
Pore “01LmE
on3 g-1
1.43
Partlcic
dmrterer
Carbon b”,“df-*
mm
0.4-0.8 O.J-0.6
1.03
-0.12 3x1~-j
64.6 53.2 47.6 27.5
3.4 Change of C02-formation rate with the consumption of active carbon In order to obtain kinetic data, the change in oxidation rate accompanying carbon consumption at 500°C was investigated. Prior to this study, we ascertained, upon adding 2% CO* to the reaction gas stream, that the CO2 produced has no effect on the oxidation rate of these catalyst systems. The oxidation rate was expressed as C02-formation per unit mass of residual carbon. With regard to the volume of the active carbon, it did not decrease until a carbon burn-off of 40%. As shown in Fig. 5, the rate of CO2 formation based on unit mass of residual catalyst tended to increase until a certain portion of carbon was consumed. This rate increase was larger and lasted longer with the more active catalysts. The maximum rates of CO2 formation in Fe-LazOrPt/C, Fe-La209/C and La2OJC were found at 75, 50 and 22% of carbon burn-off, respectively. Furthermore, changes in oxidation rate with reaction time was compared at a number of temperatures. Results for the Fe-Mn203-Cu catalyst system at 373, 403, 501 and 552°C are shown in Fig. 6. Linear relations between the rate of CO2 formation and the logarithm of reaction time were obtained at each temperature at early reaction times, where the diffusion of oxygen onto carbon was the rate-determining step. Assuming these linear relations to be kinetic data of carbon consumption, the initial rate of carbon consump-
90
(min) 50
”
Fig. 4. Alternative synergy in Fe-h&O&J catalyst/active carbon system. 0, 4.7% Fe-2.6% I&&0.70% Pt/C; a, 4.8% Fe2.6% LazO3-0.12%Cu/C; 0, 4.7% Fe-2.6% Mn203-0.12%Cu/C; q,o. 13%cu/c.
CARBON
BURN-OFF
100 (F)
Fig. 5. Change of C02-formation rate with the consumption of active carbon. Concentration of each catalyst is same as Fig. 3.
216
T. MI et of. \I I,
-
500
w P
,I' _
/ /' 8'
5
1
0
I
I
I
30
60
90
TIME
TIME
REDUCTION
OXIDATION
I I 120
-100
p
I 150°
(min)
Fig. 8. TG measurement of oxidation state of Cu/C in the reaction. 5.0% Culactive carbon (A3) 15.38mg, oxidation region 1.4%a/He 50 ml min-’ n.t.p., reduction region 10%Hz/He 60 ml min? n.t.p.
(min)
Fig. 6. Change of C&-formation rate at various temperatures. 4.7% Fe-2.6% M&h-0.12% Cu/C.
tion can be obtained by extrapolation of these lines. In this manner, the apparent activation energy was obtained as 17 kcal/mol from a reasonable linear relation as shown in Fig. 7. Laine et al. [9] emphasize the importance of the active surface area (ASA) for the gasification of graphite. They showed that the rate constant for carbon gasification (k,) based on ASA was essentially independent of carbon bum-off. However, in the case of our study, since the activity was markedly changed by promoters, the kinetics of the oxidation might be different from that of pure graphite. The activation energy obtained by Laine et al. for the oxidation of Graphon was 44? 2 kcal/mol. Baker and Chludzinski Jr. [3] obtained 21.7 + 2 kcal/mol for the copper catalyzed oxidation of graphite. The activation energy obtained for copper catalyzed oxidation was almost half as much as non-catalyzed oxidation. In the light of the surface diffusion, this value is relevant to the activation energy of surface diffusion of oxygen on polycrystalline copper; 17kcal/mol[lO].
3.5 The cause of rate enhancement in the composite catalyst Oxidation states of the catalyst metal during reaction were measured, as previously described. In the case of Fe-LaZOs-Pt/C, since La203 was not reduced under any conditions and the amount of Pt was small, the weight decrease was regarded as due to the reduction of the oxidized iron. It was confirmed by the dynamic adsorption measurement[ll]. The amount of oxygen adsorption on the active carbon was negligible at 500°C in l-3% 0*/N* stream. In this manner the oxidation state of iron in the Fe-LazOs-Pt/C was defined as Fe00.43 at 500°C. More than half of the bulk iron still remained in metallic form under these conditions. It is considered likely that, before the bulk iron is completely oxidized, oxygen is rapidly transferred onto carbon to form CO*. Results for copper are shown in Fig. 8. The oxidation state of Cu in 5.0% Cu/C was CuO, which coincided with the oxidation state of copper supported on graphite in previous findings[3]. This indicates that the oxidation rate of copper metal is faster than the reduction rate of copper oxide by active carbon at 500°C. On the basis of these observations, the rate enhancement of carbon-oxidation in the composite catalyst system, for example Fe-LazO3-Cu/C, can be explained as follows; oxygen in the gas phase is rapidly taken up by the Cu particles, which are separate from the Fe-La203 particles although in close contact with them. This adsorbed oxygen easily transfers successively to the carbon via the partially reduced iron oxide. In conclusion, the Cu particles played a role for taking up oxygen and the Fe-La203 particles for transferring
oxygen to the carbon. Since two different roles of each catalyst were combined in the composite catalyst, consequently, the oxidation reaction must be promoted with the synergistic effect.
1 .o
1.2
1.4 103/T
1.6
(K-l)
Fig. 7. Arrhenius plot of rate of Fe-Mn203-Cu catalyzed carbon oxidation.
1. D. W. McKee, Carbon 8, 131,623 (1970). 2. R. T. K. Baker and R. D. Sherwood, J. Catalysis 61, 378 (1980). 3. R. T. K. Baker and J. J. Chludzinski Jr., Carbon 19, 75 (1981).
Complete oxidation of active carbon at low temperatures 4. P. L. Walker Jr., Carbon 18,447 (1980). 5. A. Cl.W. Bradbury and F. Shafizadeh, Carbon 18,109 (1980). 6. T. Inui, K. Ueno, M. Funabiki, M. Suehiro, T. Sezume and Y. Takegami, .l. Chem. Sot. Faraday Trans. 1 75, 1495 (1979). 7. T. Inui, T. Otowa and Y. Takegami, J. Chem. Sot. Chem. Comm. 94 (1980).
217
8. T. Inui, T. Otowa and Y. Takegami, J. Caral. in press. 9. N. R. Laine, F. J. Vastola and P. L. Walker Jr., J. Phys. Chem. 67, 2030(1%3). 10. J. M. Blakely, Progress in Materials Science 10, 395 (1963). 11. T. Inui, M. Funabiki and Y. Takegami, J. Chem. Sot. Faraday Trans. 1 76, 2237(1980).
~223/82/030213~S~3.~/0 Q 1982 Pergamcn Press Ltd.
1982
COMPLETE OXIDATION OF ACTIVE CARBON AT LOW TEMPERATURES BY COMPOSITE CATALYSTS TOMOYUKIINUI, TOSHIROOTOWA,KADZUMASATSUTCHIHASHI and YOSHINOBUTAKEGAMI Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan (Receiued 8 Seprember 1981) .#stract--The reaction between oxygen and active carbon was studied at 500°C. Various composite catalysts having an iron-group metal as the main component were supported on active carbon. The synergistic effect of the oxidation activity was widely observed in the composite metal-metal oxide catalyst/carbon system. Several active carbons were studied in this catalyzed oxidation and the oxidation rate decreased with decreasing surface area of carbon materials. The rate-determining step of this reaction was considered to be the surface-diffusion rate of oxygen. Enhancement of oxygen transmission by active composite catalysts resulted in promotion of the reaction.
1. INTRODUCTION
In response to the demand for coal utilization, many studies have been conducted concerning the reaction between carbon and gaseous agents. It has been noticed that some metals or metal oxides in a coal work as catalysts. The oxidation rates of graphite were obtained for various single-component catalysts [ I]. The movement of various kinds of catalyst particles in the oxidation of graphite was directly observed, applying in situ electron microscopic technique[2,3]. The oxidation of highly graphitized carbon black was studied, and elemental rate constants based on active surface area were obtained[4]. In these studies, the mobility of catalyst particles [2], oxygen chemisorption [5], and other detailed kinetic observations [4] have been recorded; however, single component catalyst/graphite systems still formed the basis for research, and the possibilities of more active catalysts were not yet a common subject of investigation. In our study, we searched for more active catalyst systems for the oxidation of carbon at low temperatures. We have already shown that certain composite-catalyst system involving the iron-group metals, lanthanum oxide and platinum-group metals exerted high activities, both in direct hydrogenation of carbon[6] and in complete conversion of N0[7]. Since such reactions were accelerated by combining catalyst components, we assumed that the reaction proceeded with an accelerated transmission of gas onto carbon[ll]. In this paper, we apphed another composite catalyst to the oxidation of carbon in an attempt to develop a more practical, low cost catalyst system. Our ultimate aim is to obtain fundamental knowledge for application in other valuable reactions involving oxidative gases such as CO*, NO and HzO.
Catalysts. A catalyst supported on an active carbon was prepared by the impregnation method[6]. A-3 active carbon, 30-60 mesh in size, prepared by Shimadzu Seisakusho Co. Ltd. Japan, was used as the reaction material and, concurrently, as the catalyst support. It had a BETCAR Vol. 20, No. 3-D
surface area of 1230m2g-‘, a porosity of 0.46, a bulk density of 0.38 g cmm3,an average macro pore size of 3~ and an average micro pore size of 26 A. The ratio of macro pore to micro pore volume was 0.53. A watersoluble component was not detected. Volatile matter was negligible below 500°C in the stream of nitrogen containing 10% hydrogen. The active carbon was immersed in an impregnating aqueous solution of nitrate or chloride salt of catalyst metal, and was dried thoroughly by stirring over a boiling water bath, It was then exposed to a saturated vapor of 10% aqueous ammonia solution at 20°C for 2min, followed by heating in a nitrogen stream containing 10% hydrogen. The oxides were reduced into a metallic state, with the exception of lanthanum oxide. Several weight percent of an iron-group metal were supported as the main component of the catalyst. A transition metal oxide and a precious metal were combined with iron-group metal as promoters. The atomic ratios of the transition metal and the precious metal were set at ca. 0.2 and 0.04 of the substrate, respectively. A mixed solution consisting of nitrate salts of iron-group metal and a transition metal was used for the preparation of a two-component catalyst: however, in the case of the three-component catalyst, the precious metal was first dispersed on the support by the method described above, and the other two components were then added together. Reaction operation. An ordinary flow reaction apparatus was used at atmospheric pressure. A quartz tube with an inner diameter of 3Smm was chosen as the reactor. A 15mg(O.O5cm3) portion of the sample was packed in the reactor, which was heated by an external electric furnace. The catalyst-bed temperature was measured by inserting the top of a thermocouple shielded with stainless steel of 0.6 mm dia. into the exit side of the catalyst bed. The temperature was increased under nitrogen flow up to 500°C; 1.4% 02 diluted by NZ was then fed and effluent gas was analyzed by gas chromatography with columns of Porapak Q for CO;? and MS 5A for 02 and CO. The flow rate of feed gas was set at 50 cm3 n.t.p. mm’. Measurements of the oxidation state of the metal. The 213
214
T. INUI
oxidation states of the metal supported on the active carbon during the reaction were gravimetrically obtained by using Shimadzu’s TG-20 micro-thermogravimetric analyzer with a gas-flow control system. A 15 mg portion of the catalyst-supported active carbon was placed in an aluminum pan, having a 5 mm dia. and 2 mm depth. At 505”C, 1% oxygen was passed downward, through the quartz tube, at a flow rate of .50cm3min-‘. After a certain time, temperature was allowed to decrease until the oxidation reaction stopped; then the reaction gas replaced by 10%Hz/He and the temperature was elevated to 3OO*C,and the weight decrease due to hydrogen reduction was observed. The oxidation state of the catalyst metal was calculated from the weight difference between oxidative and reductive conditions. In this condition the amount of carbon-loss for methane formation was negligible.
3.1 Oxidationactivity of single component catalysts The oxidation activity of single component catalysts at 500°C is shown in Fig. 1. Only CO1 was found in the eflluent gas, except for the cases of CulC and NifC systems, Only a small amount of CO was yielded, such as 1.0 x low6and 0.8 x W6mol min-’ for CulC and Ni/C systems, respectively. In the light of the CO&O equilibrium composition of Boudouard reaction, it is likely that most of the CO was oxidized and was withdrawn before the equilibrium. The change of oxidation rate with carbon consumption was different for each catalyst, and also gave the kinetic data mentioned in 3.4. As can be clearly seen in Fig. I, the order of their activities was Cu>Co>Fe>Ni>Pt. Despite the difference in oxidation rate of each catalyst, the active carbon was consumed completely in every case when the operation proceeded for a sufficient time. Carbon bum-offs by Cu, Co, Fe and Ni at 500°C for 90 min were 100,59,39 and 12%, respectively.
9 ” t
1
2 u
a
a
60
30 TIME
90
(min)
Fig. 1. Comparison of carbon-oxidation activity for each metal catalyzed active carbon. SOOT 1.4%@/Nz 50 ml min-’ n&p. 0, 2.O%Cu/C: A, 4.7%Co/C; 0, 4.7% Fe/C; a, M%Ni/C; A, 3.0% Ptlc.
et at.
0
30
60 TIME
90
(min)
Fig. 2. The effect of Cu content in ~op~r~a~yzed rate of carbon oxidation. 0, 2.0%-; 0,5.0%-; A, I.&%-;A, O.SO%-; 0, 0.13%CUIC.
The effect of concentration of catalysts on the oxidation rate varied with Cu and Pt respectively. On the one hand, Cu showed a maximum activity at 2.O%Cu and then the activity decreased at 5.0% Cu, as shown in Fig. 2. However, on the other hand, no activity change was observed in Pt catalyst between 0.05 and 3.0% concentrations. 3.2 Syne~istic efect in the oxidation of Fe-Laz03Ptlactiue carbon system As the activity of copper catalyzed oxidation was found to be the highest of applied single metal/carbon systems, the copper-catalyzed oxidation for the copper~~phite system and copper-p~cle mobility has been investigated in detaiI[3]. However, we ascertained that Cu was ineffective for the reaction between active carbon and any other gaseous agent, such as carbon dioxide and water vapor, as was also shown using graphite as the carbon source by other workers[3]. Because of this, composite catalysts with iron-group metal as the main component were investigated in this reaction. The oxidation activity at 500°C is shown in Fig. 3, and is compared to each other in the early stage of the reaction. A notable enhancement was observed when small amounts of La203 and Pt were combined with the iron. Since the order of the CO, yield was not changed within 30min of reaction time, the carbon bum-off of each catalyst system was compared at 30min and became 66, 33, 17 and 3% for Fe-La203-Pt, La203, Fe and Pt, respectively. The activity of the three-component catalyst system was greater than the sum of the activities of each catalyst system. Such a synergistic effect was also observed when Co or Ni was used as the main component of the catalyst. Furthermore, although each main component by itself had a dtierent activity (as shown in Fig. l), the activities of these three-component catalysts were similarly high. This synergistic effect agrees with that observed in the previous reactions@].
Complete oxidation of active carbon at low temperatures Table 1. Oxidation activities of various active carbons
3t
cartmn rmenals
A3 DIAHOPE MERCK CARBON
0
215
0
30
60 TIME
90
(min)
Fig. 3. Synergistic effect of Fe-based three-component catalyst/active carbon system on oxidation rate. 0, 4.7% Fe2.6% La203-0.70% Pt/C; a, 4.7% Fe-2.6% La203/C; 2.8% La203/C; 0,4.7% Fe/C; Cl, 0.70% Pt/C.
A,
3.3 Oxidation activity of diflerent composite catalyst
systems In order to apply the composite catalyst system to practical use, it is essential to develop it economically. When expensive Pt was replaced with a small amount of Cu, it still exerted the same activity, as shown in Fig. 4. Furthermore, Mn20p was comparably active when it replaced La20j. The cost of the Fe-MnzOrCu/C system is one-thirtieth of that of the Fe-La*O,-Pt/C system. Other active carbons from various carbon materials were used for the catalyst system of Fe-La20rPt/C. Activities were compared with each other with respect to the carbon burn-off at 500°C in 30 min of reaction time. As shown in Table 1, higher activities were obtained for the carbon materials of larger BET surface area.
”
60
30 TIME
008*1 2186 BLACK*1
Surface area .2 g-l
Bulk dersjity g on-3
1230
0.38
1200
0.45
910
0.41
85
0.37
Pore “01LmE
on3 g-1
1.43
Partlcic
dmrterer
Carbon b”,“df-*
mm
0.4-0.8 O.J-0.6
1.03
-0.12 3x1~-j
64.6 53.2 47.6 27.5
3.4 Change of C02-formation rate with the consumption of active carbon In order to obtain kinetic data, the change in oxidation rate accompanying carbon consumption at 500°C was investigated. Prior to this study, we ascertained, upon adding 2% CO* to the reaction gas stream, that the CO2 produced has no effect on the oxidation rate of these catalyst systems. The oxidation rate was expressed as C02-formation per unit mass of residual carbon. With regard to the volume of the active carbon, it did not decrease until a carbon burn-off of 40%. As shown in Fig. 5, the rate of CO2 formation based on unit mass of residual catalyst tended to increase until a certain portion of carbon was consumed. This rate increase was larger and lasted longer with the more active catalysts. The maximum rates of CO2 formation in Fe-LazOrPt/C, Fe-La209/C and La2OJC were found at 75, 50 and 22% of carbon burn-off, respectively. Furthermore, changes in oxidation rate with reaction time was compared at a number of temperatures. Results for the Fe-Mn203-Cu catalyst system at 373, 403, 501 and 552°C are shown in Fig. 6. Linear relations between the rate of CO2 formation and the logarithm of reaction time were obtained at each temperature at early reaction times, where the diffusion of oxygen onto carbon was the rate-determining step. Assuming these linear relations to be kinetic data of carbon consumption, the initial rate of carbon consump-
90
(min) 50
”
Fig. 4. Alternative synergy in Fe-h&O&J catalyst/active carbon system. 0, 4.7% Fe-2.6% I&&0.70% Pt/C; a, 4.8% Fe2.6% LazO3-0.12%Cu/C; 0, 4.7% Fe-2.6% Mn203-0.12%Cu/C; q,o. 13%cu/c.
CARBON
BURN-OFF
100 (F)
Fig. 5. Change of C02-formation rate with the consumption of active carbon. Concentration of each catalyst is same as Fig. 3.
216
T. MI et of. \I I,
-
500
w P
,I' _
/ /' 8'
5
1
0
I
I
I
30
60
90
TIME
TIME
REDUCTION
OXIDATION
I I 120
-100
p
I 150°
(min)
Fig. 8. TG measurement of oxidation state of Cu/C in the reaction. 5.0% Culactive carbon (A3) 15.38mg, oxidation region 1.4%a/He 50 ml min-’ n.t.p., reduction region 10%Hz/He 60 ml min? n.t.p.
(min)
Fig. 6. Change of C&-formation rate at various temperatures. 4.7% Fe-2.6% M&h-0.12% Cu/C.
tion can be obtained by extrapolation of these lines. In this manner, the apparent activation energy was obtained as 17 kcal/mol from a reasonable linear relation as shown in Fig. 7. Laine et al. [9] emphasize the importance of the active surface area (ASA) for the gasification of graphite. They showed that the rate constant for carbon gasification (k,) based on ASA was essentially independent of carbon bum-off. However, in the case of our study, since the activity was markedly changed by promoters, the kinetics of the oxidation might be different from that of pure graphite. The activation energy obtained by Laine et al. for the oxidation of Graphon was 44? 2 kcal/mol. Baker and Chludzinski Jr. [3] obtained 21.7 + 2 kcal/mol for the copper catalyzed oxidation of graphite. The activation energy obtained for copper catalyzed oxidation was almost half as much as non-catalyzed oxidation. In the light of the surface diffusion, this value is relevant to the activation energy of surface diffusion of oxygen on polycrystalline copper; 17kcal/mol[lO].
3.5 The cause of rate enhancement in the composite catalyst Oxidation states of the catalyst metal during reaction were measured, as previously described. In the case of Fe-LaZOs-Pt/C, since La203 was not reduced under any conditions and the amount of Pt was small, the weight decrease was regarded as due to the reduction of the oxidized iron. It was confirmed by the dynamic adsorption measurement[ll]. The amount of oxygen adsorption on the active carbon was negligible at 500°C in l-3% 0*/N* stream. In this manner the oxidation state of iron in the Fe-LazOs-Pt/C was defined as Fe00.43 at 500°C. More than half of the bulk iron still remained in metallic form under these conditions. It is considered likely that, before the bulk iron is completely oxidized, oxygen is rapidly transferred onto carbon to form CO*. Results for copper are shown in Fig. 8. The oxidation state of Cu in 5.0% Cu/C was CuO, which coincided with the oxidation state of copper supported on graphite in previous findings[3]. This indicates that the oxidation rate of copper metal is faster than the reduction rate of copper oxide by active carbon at 500°C. On the basis of these observations, the rate enhancement of carbon-oxidation in the composite catalyst system, for example Fe-LazO3-Cu/C, can be explained as follows; oxygen in the gas phase is rapidly taken up by the Cu particles, which are separate from the Fe-La203 particles although in close contact with them. This adsorbed oxygen easily transfers successively to the carbon via the partially reduced iron oxide. In conclusion, the Cu particles played a role for taking up oxygen and the Fe-La203 particles for transferring
oxygen to the carbon. Since two different roles of each catalyst were combined in the composite catalyst, consequently, the oxidation reaction must be promoted with the synergistic effect.
1 .o
1.2
1.4 103/T
1.6
(K-l)
Fig. 7. Arrhenius plot of rate of Fe-Mn203-Cu catalyzed carbon oxidation.
1. D. W. McKee, Carbon 8, 131,623 (1970). 2. R. T. K. Baker and R. D. Sherwood, J. Catalysis 61, 378 (1980). 3. R. T. K. Baker and J. J. Chludzinski Jr., Carbon 19, 75 (1981).
Complete oxidation of active carbon at low temperatures 4. P. L. Walker Jr., Carbon 18,447 (1980). 5. A. Cl.W. Bradbury and F. Shafizadeh, Carbon 18,109 (1980). 6. T. Inui, K. Ueno, M. Funabiki, M. Suehiro, T. Sezume and Y. Takegami, .l. Chem. Sot. Faraday Trans. 1 75, 1495 (1979). 7. T. Inui, T. Otowa and Y. Takegami, J. Chem. Sot. Chem. Comm. 94 (1980).
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8. T. Inui, T. Otowa and Y. Takegami, J. Caral. in press. 9. N. R. Laine, F. J. Vastola and P. L. Walker Jr., J. Phys. Chem. 67, 2030(1%3). 10. J. M. Blakely, Progress in Materials Science 10, 395 (1963). 11. T. Inui, M. Funabiki and Y. Takegami, J. Chem. Sot. Faraday Trans. 1 76, 2237(1980).