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Hydrogen and methane oxidation performances of hybrid honeycomb catalyst for a tritium removal system
Fusion Engineering and Design 84 (2009) 1818–1822
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Hydrogen and methane oxidation performances of hybrid honeycomb catalyst for a tritium removal system M. Tanaka a,∗ , T. Uda a , Y. Shinozaki b , K. Munakata b a b
National Institute for Fusion Science, Oroshi-cho, Toki, Gifu 509-5292, Japan Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8251, Japan
a r t i c l e
i n f o
Article history: Available online 11 February 2009 Keywords: Honeycomb catalyst Hybrid catalyst Co-impregnation method Sequential impregnation method Tritium recovery system Methane oxidation Hydrogen oxidation
a b s t r a c t The oxidation properties of new type honeycomb catalysts using NA honeycomb support material were evaluated with hydrogen and methane gases in air to compare the simple catalysts with platinum or palladium to hybrid catalysts with both platinum and palladium. Hybrid catalysts impregnated platinum and palladium were made by a co-impregnation method (Pt + Pd) and a sequential impregnation method (Pt/Pd) and the total amount of noble metal was prepared to 2 g/L. The oxidation performance was conducted with specific flow of about 1.2 s−1 in the temperature range of less than 400 ◦ C. As the results, the NA honeycomb catalysts impregnated platinum gave the highest oxidation reaction rate for hydrogen gas among various type honeycomb catalysts. However, methane oxidation performances were inferior to other honeycomb catalysts. On the other hand, only hybrid NA catalyst manufactured by the co-impregnation method exhibited the high oxidation performance of both hydrogen and methane. It was suggested that the impregnation method of two noble metals was the key technique to take a synergic effect. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen isotopes of tritium and deuterium must be utilized as fuel in a nuclear fusion plant. Tritium is a beta radioactive isotope of hydrogen which has maximum energy of 18.6 keV, and decays into helium-3 with a half life of 12.3 years. From a viewpoint of radiation protection, tritium in the nuclear fusion plant must be safely confined in the tritium processing systems. However, if an accidental release of tritium to working area in the plant building occurred, it should be recovered by a tritium removal system. The widely used tritium removal technique is oxidation of tritiated hydrogen and hydrocarbon to water by packed type catalytic oxidation reactors, followed by adsorption process on a molecular sieve bed [1]. The pellet type noble metal catalysts made of alumina or zeolite, etc. are used in the packed type catalytic oxidation reactor. The oxidation properties of packed type catalysts with hydrogen and/or methane have been investigated under various experimental conditions [2–6]. Yoshida et al. investigated the oxidation performance with H2 , D2 and CH4 , and demonstrated a simulation test of tritium removal system [4]. Nishikawa et al. have reported that adsorbed water on the hydrophilic porous substrate had an influence on the tritium gas oxidation rate by the noble metal catalyst and have pro-
∗ Corresponding author. Tel.: +81 572 58 2087; fax: +81 572 58 2610. E-mail address: [email protected] (M. Tanaka). 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.01.020
posed a new arrangement for tritium recovery system [5,6]. On the other hand, if the tritium removal system is applied for safety issue of a tritium treatment in a large nuclear fusion facility, the processing gas volume and throughput will be large, and the packed type oxidation reactor is considered to have a large pressure drop. To reduce pressure drop in the packed bed oxidation reactor, we have proposed to apply for a honeycomb type catalyst, which exhibited a lower pressure drop than the packed bed catalyst reactor, made of a metal or a cordierite ceramic [7–9]. We examined the oxidizing characteristics of the honeycomb catalysts, which are generally used in the automotive industry, with hydrogen and methane in detail and discuss about design of oxidizing catalysts bed for the air cleanup system. A comparison of honeycomb and pellet type catalysts revealed that the honeycomb catalyst was expected to be evidently suitable for a high throughput of air. These honeycomb type catalysts impregnated with platinum or palladium had an adequate performance to oxide hydrogen and methane in air. It was found that a platinum catalyst exhibited higher oxidation reaction rate of hydrogen and a palladium catalyst exhibited higher that of methane. In this study, we applied a new type of ceramics honeycomb catalyst with platinum and/or palladium as noble metals, which have a larger specific surface area than previous honeycomb catalysts of metal and cordierite ceramics, and the oxidization properties with hydrogen and methane gases were evaluated. Hence, to make use of each catalyst characteristic, we attempted to prepare a hybrid honeycomb catalyst impregnated both platinum and palla-
M. Tanaka et al. / Fusion Engineering and Design 84 (2009) 1818–1822
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Fig. 1. Schematic diagram of the apparatus and the appearance of honeycomb catalyst.
dium, and discussed the oxidation reaction rate in comparison with previous results.
calculated by the following equation: Sv =
2. Experimental methods
The evaluations of oxidation performance were carried out by use of a flow-type fix bed catalyst reactor. The gas flow diagram of the experimental apparatus is shown in Fig. 1. To examine the oxidizing catalysts, the sample gas is blended with a mixture gas of 10% of hydrogen and methane gases and an air gas. The gas components were adjusted approximately 0.1% of hydrogen and methane in dry air. Then, the mixture gas is fed into the test piece of catalyst, which is set in an electric furnace. The temperature of the electric furnace was increased stepwise by a programmable temperature controller, and the temperature was kept at a constant value for 2.5 h at each step. The catalyst could be heated up to about 400 ◦ C. The temperature of test piece at downstream was measured by a thermocouple. The mixture gas flow rate (std) is measured with a file flow meter (STEC, SF-2). Process gases were sampled at the inlet and outlet of the catalyst bed and their concentrations were analyzed by a gas chromatograph (GTR Tech, G2800TF) with TCD (Thermo Conductivity Detector) and FID (Flame Ionization Detector) detectors. The conversion rate of oxidation, C (%) was defined by the following equation: Cin − Cout × 100 Cin
(1)
where Cin is the gas concentration at inlet of catalyst bed and Cout is the gas concentration at outlet of catalyst bed. From the experimental data of oxidizing efficiencies, decontamination factor K is shown as a function of temperature and the packing volume of the catalyst bed for the tritiated gas treatment system can be calculated using the following equation: K=
Cin = exp Cout
k × V
cat
Q
(3)
The specific gas flow rate: Sv was controlled to around 1.2 s−1 (4300 h−1 ) in all experiments.
2.1. Apparatus
C (%) =
Q Vcat
2.2. Honeycomb catalyst and the surface characteristics The appearance of the catalyst is also shown in Fig. 1. The test samples of honeycomb catalyst are prepared by Nagamine Manufacturing Co. Ltd. Specific dimensions of the honeycombs are 20 mm in diameter and 50 mm in length. The support materials of honeycomb were compound of CaO–Al2 O3 , SiO2 and TiO2 . The honeycomb cell density was 200 CPSI (cell per square inch). Platinum or palladium noble metals were impregnated by use of solution of them on the surface of honeycomb. The density of noble metal on the honeycomb was 2 g/L. In addition, two types of the hybrid catalyst were prepared by a co-impregnating method and a sequential impregnation method. One used the mixture solution of platinum and palladium: co-impregnating method. Other was further impregnated by use of a solution of platinum after it was impregnated by a solution of palladium: sequential impregnation method. The ratio of each catalyst was consisted of 1:1 and total amount of noble metals was 2 g/L. To activate the catalyst, before the catalyst performance test was carried out, the catalyst was baked at 350 ◦ C and hydrogen gas was fed to the catalyst at 500 cm3 /min for 3 h. Nitrogen gas was fed into the apparatus and displaced hydrogen gas sufficiently to prevent hydrogen explosions before and after the activation process. As for the surface characteristics of the NA honeycomb catalysts, specific surface and average pore size analyzed by BET multi-point nitrogen gas adsorbing method were about 67 m2 /g and 3.6 nm, respectively. 3. Experimental results and discussion
(2)
where K is the decontamination factor at oxidation (DF), k is the reaction rate constant (1/s), Vcat is the volume of catalyst (m3 ), and Q is the volume flow rate (m3 /s). Also the specific flow Sv (1/s) is
3.1. Oxidizing properties of honeycomb catalysts As basic research for the study of oxidizing properties of honeycomb catalyst, we evaluated the oxidation properties of simple
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Fig. 2. The relationships between temperature and oxidizing efficiency for platinum.
honeycomb catalyst with platinum or palladium. Fig. 2 shows the relationships between temperature and oxidizing efficiency for hydrogen and methane gases for platinum. The hydrogen gas was oxidized up to 44% conversion at 33 ◦ C and was completely oxidized above 100 ◦ C. For methane gas, the conversion rate was less than 40% at even 380 ◦ C, although methane gas began to be oxidized from 250 ◦ C. Fig. 3 shows the relationships between temperature and oxidizing efficiency for palladium. The hydrogen gas began to be oxidized around room temperature. However, temperature above 200 ◦ C was required to be completely oxidized. Whereas, methane gas began to be oxidized from 250 ◦ C, the conversion rate was more than 80% at 390 ◦ C. These results indicate that the platinum catalyst was particularly effective for hydrogen oxidation and the palladium catalyst showed good activity for methane gas. In other words, the former give a high rate of oxidation reaction of hydrogen with oxygen, the latter is superior in the rate of oxidation reaction of methane. The catalytic oxidation properties of each noble metal indicated similar tendencies in the results of our previous study due to other type honeycomb catalysts: metal and cordierite honeycomb catalysts [8]. 3.2. Performance of the hybrid catalysts We tried to examine the hybrid catalyst with platinum and palladium in anticipation of multiplier effects from individual noble metals. Fig. 4 shows the relationships between temperature and oxidizing efficiency for hybrid honeycomb by the sequential
Fig. 3. The relationships between temperature and oxidizing efficiency for palladium.
Fig. 4. The relationships between temperature and oxidizing efficiency for hybrid honeycomb by the sequential impregnation method [Pt/Pd].
impregnation method. The densities of noble metals were totally 2 g/L; platinum and palladium were 1 g/L and 1 g/L, respectively. The hydrogen gas was oxidized up to 51% conversion at even 27 ◦ C. The hydrogen oxidation performance of this type catalyst was more highly efficient than that of platinum catalyst impregnated 2 g/L, although amount of only platinum on the hybrid catalyst was 1 g/L. Meanwhile, methane gas began to be oxidized from 300 ◦ C. The conversion rate was less than 20% at even 380 ◦ C. In spite of the presence of palladium catalyst, the methane oxidation performance was not enhanced in comparison with the results of Fig. 2. The oxidation performances were similar to platinum catalyst characteristics. It would suggest that the only characteristics of second noble metal impregnated on the catalyst were appeared. Fig. 5 shows the relationships between temperature and oxidizing efficiency for hybrid honeycomb by the co-impregnation method. The hydrogen oxidation rate was up to 36% at 32 ◦ C. This oxidation performance seemed to be a little inferior to that of platinum catalyst impregnated 2 g/L as shown in Fig. 2. On the other hand, methane gas began to be oxidized from 200 ◦ C, and the conversion rate was up to 81% at 388 ◦ C. The methane oxidation performance might be almost equal to that of palladium impregnated 2 g/L as shown in Fig. 3, even though the amount of palladium was 1 g/L. It was indicated that the methane oxidation performance was enhanced due to hybrid catalyst prepared by co-impregnation method.
Fig. 5. The relationships between temperature and oxidizing efficiency for hybrid honeycomb by the co-impregnation method [Pt + Pd].
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Table 1 Comparison with the specifications of honeycom catalyst. Name
Cordierite
Metal
NA
Substrate material Catalyst Catalyst weight (g/L) Cell density (CPSI) Specific surface (m2 /g) Average pore size (nm) Specific flow (1/s) Manufacture
2MgO–2Al2 O3 –5SiO2 with alumina coat Pt, Pd 2 400 14 57 1.17–1.19 Tanaka Kikinzoku Kogyo K.K.
20Cr–5Al–Fe with wash coat Pt 2 400 4.8 – 1.20 Tanaka Kikinzoku Kogyo K.K.
CaO–Al2 O3 –SiO2 –TiO2 Pt, Pd, Pt + Pd 2 200 67 3.6 1.18–1.20 Nagamine Manufacturing Co. Ltd.
3.3. Comparison of oxidizing reaction rate coefficients with various catalysts In this section, we discuss the performances of oxidation reaction rate of NA honeycomb catalysts in comparison with other type honeycomb catalysts of cordierite and metal [8]. The specifications of these catalysts are summarized in Table 1. The specific surface area becomes lager in the order of metal, cordierite, NA honeycomb. Average pore sizes of NA are one order of magnitude less than that of other materials. Fig. 6 shows the Arrhenius plot of hydrogen oxidization reaction rate. NA honeycomb catalysts except for palladium catalyst of 2 g/L show the highest reaction rate among them. Comparing with each platinum catalyst of 2 g/L, the oxidation performances show a similar tendency in the order of the specific surface area; the size of the specific surface area is an important factor to oxide hydrogen efficiently. Furthermore, the oxidation performances of hybrid catalysts using NA honeycomb are almost the same performance as in the case of platinum catalyst of 2 g/L, although the amount of platinum is 1 g/L. It is suggested that oxidation performance is enhanced by use of hybrid catalyst with platinum and palladium. Fig. 7 shows the Arrhenius plot of methane oxidization reaction rate. As for the methane oxidation performances, NA honeycomb catalysts show lower reaction rate than other type catalysts, although they have the largest specific surface area among them. The one of the reasons is that the diffusion of methane gas to the catalyst metal might be a ratedetermining process in the methane oxidation properties of NA honeycomb catalysts, since the average pore size of NA honeycombs is one order of magnitude less than other type catalysts and an effective diffusion coefficient of methane in air is a few times less than that of hydrogen. Further systemic studies are required to clarify the problem. Among these NA honeycombs, hybrid catalyst manufactured by the sequential impregnation method shows lower performance of methane oxidation than platinum catalyst of 2 g/L in spite of the presence of palladium catalyst. On the other hand, hybrid catalyst manufactured by co-impregnation method shows
Fig. 6. The Arrhenius plot of hydrogen oxidization reaction rate.
higher methane oxidation performance than palladium catalyst of 2 g/L. In regard to the difference of oxidation characteristics among these hybrid type catalysts, these results suggested that hybrid catalyst by co-impregnation method would bring out a synergy effect. Tomishige and his research group have suggested that the trace second noble metal atoms introduced by the sequential impregnation method were preferentially located on the surface of first impregnated metal particles of the catalyst as the results of the analysis by TEM, EXAFS, FTIR, etc. [10–12]. On the other hand, the catalyst prepared by the coimpregnation method was contained with both noble metals in the mixture state. According to their results, the cross section of model structure of hybrid catalyst particles prepared by the co-impregnation method and the sequence impregnation method would be shown as Fig. 8. In the case of the hybrid catalytic metals prepared by the sequential impregnation method, since the noble metals of platinum and palladium were impregnated in a rate of 1:1, the noble metal atom introduced by the second impregnation may cover on the first impregnated catalyst as shown in Fig. 8(b). Therefore, only oxidation characteristic of platinum as second impregnated noble metal may be shown. Meanwhile, in the case of the hybrid catalyst prepared by the co-impregnation method, the catalytic particles on the support material were contained with both platinum and palladium catalytic metals. Therefore, both noble metals are segregated on the surface of the catalyst particles as shown in Fig. 8(a). Then, the oxidation characteristic of both platinum and palladium would be exhibited. It is suggested that the preparation method used in an impregnation was the key technique to take a synergic effect. 3.4. Application for tritium recovery system in a fusion test facility The National Institute for Fusion Science (NIFS) have planned to conduct plasma confinement experiments due to deuterium gas
Fig. 7. The Arrhenius plot of methane oxidization reaction rate.
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Fig. 8. The cross section of model structure of catalyst particles.
using the Large Helical Device (LHD), which is the largest helical coil type fusion test device with a superconducting magnet. A small amount of tritium is produced under the deuterium plasma experiments. After finishing plasma experiment period, workers will maintain and replace the equipments in the vacuum vessel. From a viewpoint of radiation protection, we would recovery tritium in the vacuum vessel by “Vacuum-Vessel Purge-Gas Treatment System” as tritium removal system, before workers enter there [13]. A target gas for the system is a very small amount of hydrogen isotope gas in air. Therefore, NA honeycombs with 2 g/L of platinum or hybrid type catalyst are useful for oxidation catalyst, because they indicated higher hydrogen oxidation performance as shown in Fig. 6. Assuming that the process gas is a flow rate of 300 N m3 /h, a temperature of around 80 ◦ C, and a target decontamination factor (DF) of 100 (namely, a conversion rate of 99%), these catalyst volume are calculated to be 0.08–0.10 m3 by use of reaction rate and Eq. (2). The volume of each catalyst is not large differences, but a cost of total noble metals is difference since the cost of platinum metal is several times as expensive as that of palladium metal: e.g., the average prices per ounce of platinum and palladium in 2007 were $1303 and $355, respectively [14]. Therefore, total cost of noble metal for hybrid catalyst is quite advantageous as the oxidation catalyst. Furthermore, if the co-impregnation method is used, it would reduce impregnated step of the catalytic processes than the sequential impregnation method. In conclusion, hybrid type NA honeycomb catalysts with platinum and palladium prepared by co-impregnation method would be suitable for an example of case in which applies for tritium recovery system in LHD from the viewpoints of both hydrogen oxidation performance and total cost. 4. Conclusions We examined new type honeycomb catalysts, “NA honeycomb catalyst”, with platinum or palladium for a tritium removal system. Hence, to make use of each catalyst characteristic, we attempted to prepare a hybrid type honeycomb catalyst with platinum and palladium prepared by the co-impregnation method and the sequential impregnation method. Then, the oxidation performances of hydrogen and methane in air were evaluated in the temperature range of less than 400 ◦ C and were compared with the previous results of metal and cordierite honeycomb catalyst. The following results are obtained: (1) On the NA honeycomb catalyst, the platinum catalyst had high activity for hydrogen gas oxidation, and the palladium catalyst was active for methane gas oxidation.
(2) It was found that the NA honeycomb catalysts gave the highest oxidation reaction rate for hydrogen gas among various type honeycomb catalysts. (3) The hybrid catalyst prepared by the co-impregnation method exhibited the oxidation characteristics of both noble metals of platinum and palladium. To reduce the total pressure drop of tritium removal system, the subject for a future study is to develop a honeycomb type adsorbent instead of adsorption process using a packed type molecular sieve bed. Acknowledgements The authors are grateful to Mr. T. Nagamine of Nagamine Manufacturing Co. Ltd. for preparing and analyzing the NA honeycomb catalyst. This study was supported by the NIFS budget (NIFS07UCSS004). References [1] Safe Handling of Tritium: Review of Data and Experience, Technical Reports Series No. 324, IAEA, Vienna, 1991, p. 34. [2] J.C. Bixel, C.J. Kershner, Catalytic oxidation and oxide adsorption for the removal of tritium from air, WASH-1332, 1974. [3] A.E. Sherwood, B.G. Monahan, R.A. McWilliams, F.S. Uribe, C.M. Griffith, Catalytic oxidation of tritium in air at ambient temperature, UCRL-52811, 1979. [4] H. Yoshida, T. Shimizu, K. Numata, K. Okuno, Y. Naruse, Simulation test of tritium removal system by using hydrogen and methane, J. Atom. Energy Soc. Jpn. 23 (1981) 923–929 (in Japanese). [5] M. Nishikawa, T. Takeishi, M. Enoeda, T. Higashijima, K. Munakata, I. Kumabe, Catalytic oxidation of tritium in wet gas, J. Nucl. Sci. Technol. 22 (1985) 922–933. [6] M. Nishikawa, K. Takahashi, K. Munakata, S. Fukada, K. Kotoh, T. Takeishi, A new arrangement for the air cleanup system to recover tritium, Fusion Technol. 31 (1997) 175–186. [7] T. Uda, T. Sugiyama, Y. Asakura, K. Munakata, M. Tanaka, Development of high performance catalyst for oxidation of tritiated hydrogen and methane gases, Fusion Sci. Technol. 48 (2005) 480–483. [8] T. Uda, M. Tanaka, K. Munakata, Characteristics of honeycomb catalysts to recover tritiated hydrogen and methane, Fusion Eng. Des. 83 (2008) 1715–1720. [9] T. Uda, M. Tanaka, Y. Shinozaki, K. Munakata, Characteristics of honeycomb catalysts for oxidization of tritiated gas cleanup system, in: Proceedings of the 2nd Japan-China Workshop on Blanket and Tritium Technology, Sendai, Japan, May 9–10, 2008, 2008. [10] B. Li, S. Kado, Y. Mukainakano, T. Miyazawa, T. Miyao, S. Naito, et al., Surface modification of Ni catalysts with trace Pt for oxidative steam reforming of methane, J. Catal. 245 (2007) 144–155. [11] Y. Mukainakano, K. Yoshida, K. Okumura, K. Kunimori, K. Tomishige, Catalytic performance and QXAFS analysis of Ni catalysts modified with Pd for oxidative steam reforming of methane, Catal. Today 132 (2008) 101–108. [12] K. Tomishige, Oxidative steam reforming of methane over Ni catalysis modified with noble metals, J. Jpn. Petrol. Inst. 50 (2007) 287–298. [13] Y. Asakura, Development of exhaust gas and effluent liquid treatment system for LHD, J. Plasma Fusion Res. 78 (2002) 1319–1324 (in Japanese). [14] Kitco Inc., http://www.kitco.com/charts/.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Hydrogen and methane oxidation performances of hybrid honeycomb catalyst for a tritium removal system M. Tanaka a,∗ , T. Uda a , Y. Shinozaki b , K. Munakata b a b
National Institute for Fusion Science, Oroshi-cho, Toki, Gifu 509-5292, Japan Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8251, Japan
a r t i c l e
i n f o
Article history: Available online 11 February 2009 Keywords: Honeycomb catalyst Hybrid catalyst Co-impregnation method Sequential impregnation method Tritium recovery system Methane oxidation Hydrogen oxidation
a b s t r a c t The oxidation properties of new type honeycomb catalysts using NA honeycomb support material were evaluated with hydrogen and methane gases in air to compare the simple catalysts with platinum or palladium to hybrid catalysts with both platinum and palladium. Hybrid catalysts impregnated platinum and palladium were made by a co-impregnation method (Pt + Pd) and a sequential impregnation method (Pt/Pd) and the total amount of noble metal was prepared to 2 g/L. The oxidation performance was conducted with specific flow of about 1.2 s−1 in the temperature range of less than 400 ◦ C. As the results, the NA honeycomb catalysts impregnated platinum gave the highest oxidation reaction rate for hydrogen gas among various type honeycomb catalysts. However, methane oxidation performances were inferior to other honeycomb catalysts. On the other hand, only hybrid NA catalyst manufactured by the co-impregnation method exhibited the high oxidation performance of both hydrogen and methane. It was suggested that the impregnation method of two noble metals was the key technique to take a synergic effect. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen isotopes of tritium and deuterium must be utilized as fuel in a nuclear fusion plant. Tritium is a beta radioactive isotope of hydrogen which has maximum energy of 18.6 keV, and decays into helium-3 with a half life of 12.3 years. From a viewpoint of radiation protection, tritium in the nuclear fusion plant must be safely confined in the tritium processing systems. However, if an accidental release of tritium to working area in the plant building occurred, it should be recovered by a tritium removal system. The widely used tritium removal technique is oxidation of tritiated hydrogen and hydrocarbon to water by packed type catalytic oxidation reactors, followed by adsorption process on a molecular sieve bed [1]. The pellet type noble metal catalysts made of alumina or zeolite, etc. are used in the packed type catalytic oxidation reactor. The oxidation properties of packed type catalysts with hydrogen and/or methane have been investigated under various experimental conditions [2–6]. Yoshida et al. investigated the oxidation performance with H2 , D2 and CH4 , and demonstrated a simulation test of tritium removal system [4]. Nishikawa et al. have reported that adsorbed water on the hydrophilic porous substrate had an influence on the tritium gas oxidation rate by the noble metal catalyst and have pro-
∗ Corresponding author. Tel.: +81 572 58 2087; fax: +81 572 58 2610. E-mail address: [email protected] (M. Tanaka). 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.01.020
posed a new arrangement for tritium recovery system [5,6]. On the other hand, if the tritium removal system is applied for safety issue of a tritium treatment in a large nuclear fusion facility, the processing gas volume and throughput will be large, and the packed type oxidation reactor is considered to have a large pressure drop. To reduce pressure drop in the packed bed oxidation reactor, we have proposed to apply for a honeycomb type catalyst, which exhibited a lower pressure drop than the packed bed catalyst reactor, made of a metal or a cordierite ceramic [7–9]. We examined the oxidizing characteristics of the honeycomb catalysts, which are generally used in the automotive industry, with hydrogen and methane in detail and discuss about design of oxidizing catalysts bed for the air cleanup system. A comparison of honeycomb and pellet type catalysts revealed that the honeycomb catalyst was expected to be evidently suitable for a high throughput of air. These honeycomb type catalysts impregnated with platinum or palladium had an adequate performance to oxide hydrogen and methane in air. It was found that a platinum catalyst exhibited higher oxidation reaction rate of hydrogen and a palladium catalyst exhibited higher that of methane. In this study, we applied a new type of ceramics honeycomb catalyst with platinum and/or palladium as noble metals, which have a larger specific surface area than previous honeycomb catalysts of metal and cordierite ceramics, and the oxidization properties with hydrogen and methane gases were evaluated. Hence, to make use of each catalyst characteristic, we attempted to prepare a hybrid honeycomb catalyst impregnated both platinum and palla-
M. Tanaka et al. / Fusion Engineering and Design 84 (2009) 1818–1822
1819
Fig. 1. Schematic diagram of the apparatus and the appearance of honeycomb catalyst.
dium, and discussed the oxidation reaction rate in comparison with previous results.
calculated by the following equation: Sv =
2. Experimental methods
The evaluations of oxidation performance were carried out by use of a flow-type fix bed catalyst reactor. The gas flow diagram of the experimental apparatus is shown in Fig. 1. To examine the oxidizing catalysts, the sample gas is blended with a mixture gas of 10% of hydrogen and methane gases and an air gas. The gas components were adjusted approximately 0.1% of hydrogen and methane in dry air. Then, the mixture gas is fed into the test piece of catalyst, which is set in an electric furnace. The temperature of the electric furnace was increased stepwise by a programmable temperature controller, and the temperature was kept at a constant value for 2.5 h at each step. The catalyst could be heated up to about 400 ◦ C. The temperature of test piece at downstream was measured by a thermocouple. The mixture gas flow rate (std) is measured with a file flow meter (STEC, SF-2). Process gases were sampled at the inlet and outlet of the catalyst bed and their concentrations were analyzed by a gas chromatograph (GTR Tech, G2800TF) with TCD (Thermo Conductivity Detector) and FID (Flame Ionization Detector) detectors. The conversion rate of oxidation, C (%) was defined by the following equation: Cin − Cout × 100 Cin
(1)
where Cin is the gas concentration at inlet of catalyst bed and Cout is the gas concentration at outlet of catalyst bed. From the experimental data of oxidizing efficiencies, decontamination factor K is shown as a function of temperature and the packing volume of the catalyst bed for the tritiated gas treatment system can be calculated using the following equation: K=
Cin = exp Cout
k × V
cat
Q
(3)
The specific gas flow rate: Sv was controlled to around 1.2 s−1 (4300 h−1 ) in all experiments.
2.1. Apparatus
C (%) =
Q Vcat
2.2. Honeycomb catalyst and the surface characteristics The appearance of the catalyst is also shown in Fig. 1. The test samples of honeycomb catalyst are prepared by Nagamine Manufacturing Co. Ltd. Specific dimensions of the honeycombs are 20 mm in diameter and 50 mm in length. The support materials of honeycomb were compound of CaO–Al2 O3 , SiO2 and TiO2 . The honeycomb cell density was 200 CPSI (cell per square inch). Platinum or palladium noble metals were impregnated by use of solution of them on the surface of honeycomb. The density of noble metal on the honeycomb was 2 g/L. In addition, two types of the hybrid catalyst were prepared by a co-impregnating method and a sequential impregnation method. One used the mixture solution of platinum and palladium: co-impregnating method. Other was further impregnated by use of a solution of platinum after it was impregnated by a solution of palladium: sequential impregnation method. The ratio of each catalyst was consisted of 1:1 and total amount of noble metals was 2 g/L. To activate the catalyst, before the catalyst performance test was carried out, the catalyst was baked at 350 ◦ C and hydrogen gas was fed to the catalyst at 500 cm3 /min for 3 h. Nitrogen gas was fed into the apparatus and displaced hydrogen gas sufficiently to prevent hydrogen explosions before and after the activation process. As for the surface characteristics of the NA honeycomb catalysts, specific surface and average pore size analyzed by BET multi-point nitrogen gas adsorbing method were about 67 m2 /g and 3.6 nm, respectively. 3. Experimental results and discussion
(2)
where K is the decontamination factor at oxidation (DF), k is the reaction rate constant (1/s), Vcat is the volume of catalyst (m3 ), and Q is the volume flow rate (m3 /s). Also the specific flow Sv (1/s) is
3.1. Oxidizing properties of honeycomb catalysts As basic research for the study of oxidizing properties of honeycomb catalyst, we evaluated the oxidation properties of simple
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Fig. 2. The relationships between temperature and oxidizing efficiency for platinum.
honeycomb catalyst with platinum or palladium. Fig. 2 shows the relationships between temperature and oxidizing efficiency for hydrogen and methane gases for platinum. The hydrogen gas was oxidized up to 44% conversion at 33 ◦ C and was completely oxidized above 100 ◦ C. For methane gas, the conversion rate was less than 40% at even 380 ◦ C, although methane gas began to be oxidized from 250 ◦ C. Fig. 3 shows the relationships between temperature and oxidizing efficiency for palladium. The hydrogen gas began to be oxidized around room temperature. However, temperature above 200 ◦ C was required to be completely oxidized. Whereas, methane gas began to be oxidized from 250 ◦ C, the conversion rate was more than 80% at 390 ◦ C. These results indicate that the platinum catalyst was particularly effective for hydrogen oxidation and the palladium catalyst showed good activity for methane gas. In other words, the former give a high rate of oxidation reaction of hydrogen with oxygen, the latter is superior in the rate of oxidation reaction of methane. The catalytic oxidation properties of each noble metal indicated similar tendencies in the results of our previous study due to other type honeycomb catalysts: metal and cordierite honeycomb catalysts [8]. 3.2. Performance of the hybrid catalysts We tried to examine the hybrid catalyst with platinum and palladium in anticipation of multiplier effects from individual noble metals. Fig. 4 shows the relationships between temperature and oxidizing efficiency for hybrid honeycomb by the sequential
Fig. 3. The relationships between temperature and oxidizing efficiency for palladium.
Fig. 4. The relationships between temperature and oxidizing efficiency for hybrid honeycomb by the sequential impregnation method [Pt/Pd].
impregnation method. The densities of noble metals were totally 2 g/L; platinum and palladium were 1 g/L and 1 g/L, respectively. The hydrogen gas was oxidized up to 51% conversion at even 27 ◦ C. The hydrogen oxidation performance of this type catalyst was more highly efficient than that of platinum catalyst impregnated 2 g/L, although amount of only platinum on the hybrid catalyst was 1 g/L. Meanwhile, methane gas began to be oxidized from 300 ◦ C. The conversion rate was less than 20% at even 380 ◦ C. In spite of the presence of palladium catalyst, the methane oxidation performance was not enhanced in comparison with the results of Fig. 2. The oxidation performances were similar to platinum catalyst characteristics. It would suggest that the only characteristics of second noble metal impregnated on the catalyst were appeared. Fig. 5 shows the relationships between temperature and oxidizing efficiency for hybrid honeycomb by the co-impregnation method. The hydrogen oxidation rate was up to 36% at 32 ◦ C. This oxidation performance seemed to be a little inferior to that of platinum catalyst impregnated 2 g/L as shown in Fig. 2. On the other hand, methane gas began to be oxidized from 200 ◦ C, and the conversion rate was up to 81% at 388 ◦ C. The methane oxidation performance might be almost equal to that of palladium impregnated 2 g/L as shown in Fig. 3, even though the amount of palladium was 1 g/L. It was indicated that the methane oxidation performance was enhanced due to hybrid catalyst prepared by co-impregnation method.
Fig. 5. The relationships between temperature and oxidizing efficiency for hybrid honeycomb by the co-impregnation method [Pt + Pd].
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Table 1 Comparison with the specifications of honeycom catalyst. Name
Cordierite
Metal
NA
Substrate material Catalyst Catalyst weight (g/L) Cell density (CPSI) Specific surface (m2 /g) Average pore size (nm) Specific flow (1/s) Manufacture
2MgO–2Al2 O3 –5SiO2 with alumina coat Pt, Pd 2 400 14 57 1.17–1.19 Tanaka Kikinzoku Kogyo K.K.
20Cr–5Al–Fe with wash coat Pt 2 400 4.8 – 1.20 Tanaka Kikinzoku Kogyo K.K.
CaO–Al2 O3 –SiO2 –TiO2 Pt, Pd, Pt + Pd 2 200 67 3.6 1.18–1.20 Nagamine Manufacturing Co. Ltd.
3.3. Comparison of oxidizing reaction rate coefficients with various catalysts In this section, we discuss the performances of oxidation reaction rate of NA honeycomb catalysts in comparison with other type honeycomb catalysts of cordierite and metal [8]. The specifications of these catalysts are summarized in Table 1. The specific surface area becomes lager in the order of metal, cordierite, NA honeycomb. Average pore sizes of NA are one order of magnitude less than that of other materials. Fig. 6 shows the Arrhenius plot of hydrogen oxidization reaction rate. NA honeycomb catalysts except for palladium catalyst of 2 g/L show the highest reaction rate among them. Comparing with each platinum catalyst of 2 g/L, the oxidation performances show a similar tendency in the order of the specific surface area; the size of the specific surface area is an important factor to oxide hydrogen efficiently. Furthermore, the oxidation performances of hybrid catalysts using NA honeycomb are almost the same performance as in the case of platinum catalyst of 2 g/L, although the amount of platinum is 1 g/L. It is suggested that oxidation performance is enhanced by use of hybrid catalyst with platinum and palladium. Fig. 7 shows the Arrhenius plot of methane oxidization reaction rate. As for the methane oxidation performances, NA honeycomb catalysts show lower reaction rate than other type catalysts, although they have the largest specific surface area among them. The one of the reasons is that the diffusion of methane gas to the catalyst metal might be a ratedetermining process in the methane oxidation properties of NA honeycomb catalysts, since the average pore size of NA honeycombs is one order of magnitude less than other type catalysts and an effective diffusion coefficient of methane in air is a few times less than that of hydrogen. Further systemic studies are required to clarify the problem. Among these NA honeycombs, hybrid catalyst manufactured by the sequential impregnation method shows lower performance of methane oxidation than platinum catalyst of 2 g/L in spite of the presence of palladium catalyst. On the other hand, hybrid catalyst manufactured by co-impregnation method shows
Fig. 6. The Arrhenius plot of hydrogen oxidization reaction rate.
higher methane oxidation performance than palladium catalyst of 2 g/L. In regard to the difference of oxidation characteristics among these hybrid type catalysts, these results suggested that hybrid catalyst by co-impregnation method would bring out a synergy effect. Tomishige and his research group have suggested that the trace second noble metal atoms introduced by the sequential impregnation method were preferentially located on the surface of first impregnated metal particles of the catalyst as the results of the analysis by TEM, EXAFS, FTIR, etc. [10–12]. On the other hand, the catalyst prepared by the coimpregnation method was contained with both noble metals in the mixture state. According to their results, the cross section of model structure of hybrid catalyst particles prepared by the co-impregnation method and the sequence impregnation method would be shown as Fig. 8. In the case of the hybrid catalytic metals prepared by the sequential impregnation method, since the noble metals of platinum and palladium were impregnated in a rate of 1:1, the noble metal atom introduced by the second impregnation may cover on the first impregnated catalyst as shown in Fig. 8(b). Therefore, only oxidation characteristic of platinum as second impregnated noble metal may be shown. Meanwhile, in the case of the hybrid catalyst prepared by the co-impregnation method, the catalytic particles on the support material were contained with both platinum and palladium catalytic metals. Therefore, both noble metals are segregated on the surface of the catalyst particles as shown in Fig. 8(a). Then, the oxidation characteristic of both platinum and palladium would be exhibited. It is suggested that the preparation method used in an impregnation was the key technique to take a synergic effect. 3.4. Application for tritium recovery system in a fusion test facility The National Institute for Fusion Science (NIFS) have planned to conduct plasma confinement experiments due to deuterium gas
Fig. 7. The Arrhenius plot of methane oxidization reaction rate.
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Fig. 8. The cross section of model structure of catalyst particles.
using the Large Helical Device (LHD), which is the largest helical coil type fusion test device with a superconducting magnet. A small amount of tritium is produced under the deuterium plasma experiments. After finishing plasma experiment period, workers will maintain and replace the equipments in the vacuum vessel. From a viewpoint of radiation protection, we would recovery tritium in the vacuum vessel by “Vacuum-Vessel Purge-Gas Treatment System” as tritium removal system, before workers enter there [13]. A target gas for the system is a very small amount of hydrogen isotope gas in air. Therefore, NA honeycombs with 2 g/L of platinum or hybrid type catalyst are useful for oxidation catalyst, because they indicated higher hydrogen oxidation performance as shown in Fig. 6. Assuming that the process gas is a flow rate of 300 N m3 /h, a temperature of around 80 ◦ C, and a target decontamination factor (DF) of 100 (namely, a conversion rate of 99%), these catalyst volume are calculated to be 0.08–0.10 m3 by use of reaction rate and Eq. (2). The volume of each catalyst is not large differences, but a cost of total noble metals is difference since the cost of platinum metal is several times as expensive as that of palladium metal: e.g., the average prices per ounce of platinum and palladium in 2007 were $1303 and $355, respectively [14]. Therefore, total cost of noble metal for hybrid catalyst is quite advantageous as the oxidation catalyst. Furthermore, if the co-impregnation method is used, it would reduce impregnated step of the catalytic processes than the sequential impregnation method. In conclusion, hybrid type NA honeycomb catalysts with platinum and palladium prepared by co-impregnation method would be suitable for an example of case in which applies for tritium recovery system in LHD from the viewpoints of both hydrogen oxidation performance and total cost. 4. Conclusions We examined new type honeycomb catalysts, “NA honeycomb catalyst”, with platinum or palladium for a tritium removal system. Hence, to make use of each catalyst characteristic, we attempted to prepare a hybrid type honeycomb catalyst with platinum and palladium prepared by the co-impregnation method and the sequential impregnation method. Then, the oxidation performances of hydrogen and methane in air were evaluated in the temperature range of less than 400 ◦ C and were compared with the previous results of metal and cordierite honeycomb catalyst. The following results are obtained: (1) On the NA honeycomb catalyst, the platinum catalyst had high activity for hydrogen gas oxidation, and the palladium catalyst was active for methane gas oxidation.
(2) It was found that the NA honeycomb catalysts gave the highest oxidation reaction rate for hydrogen gas among various type honeycomb catalysts. (3) The hybrid catalyst prepared by the co-impregnation method exhibited the oxidation characteristics of both noble metals of platinum and palladium. To reduce the total pressure drop of tritium removal system, the subject for a future study is to develop a honeycomb type adsorbent instead of adsorption process using a packed type molecular sieve bed. Acknowledgements The authors are grateful to Mr. T. Nagamine of Nagamine Manufacturing Co. Ltd. for preparing and analyzing the NA honeycomb catalyst. This study was supported by the NIFS budget (NIFS07UCSS004). References [1] Safe Handling of Tritium: Review of Data and Experience, Technical Reports Series No. 324, IAEA, Vienna, 1991, p. 34. [2] J.C. Bixel, C.J. Kershner, Catalytic oxidation and oxide adsorption for the removal of tritium from air, WASH-1332, 1974. [3] A.E. Sherwood, B.G. Monahan, R.A. McWilliams, F.S. Uribe, C.M. Griffith, Catalytic oxidation of tritium in air at ambient temperature, UCRL-52811, 1979. [4] H. Yoshida, T. Shimizu, K. Numata, K. Okuno, Y. Naruse, Simulation test of tritium removal system by using hydrogen and methane, J. Atom. Energy Soc. Jpn. 23 (1981) 923–929 (in Japanese). [5] M. Nishikawa, T. Takeishi, M. Enoeda, T. Higashijima, K. Munakata, I. Kumabe, Catalytic oxidation of tritium in wet gas, J. Nucl. Sci. Technol. 22 (1985) 922–933. [6] M. Nishikawa, K. Takahashi, K. Munakata, S. Fukada, K. Kotoh, T. Takeishi, A new arrangement for the air cleanup system to recover tritium, Fusion Technol. 31 (1997) 175–186. [7] T. Uda, T. Sugiyama, Y. Asakura, K. Munakata, M. Tanaka, Development of high performance catalyst for oxidation of tritiated hydrogen and methane gases, Fusion Sci. Technol. 48 (2005) 480–483. [8] T. Uda, M. Tanaka, K. Munakata, Characteristics of honeycomb catalysts to recover tritiated hydrogen and methane, Fusion Eng. Des. 83 (2008) 1715–1720. [9] T. Uda, M. Tanaka, Y. Shinozaki, K. Munakata, Characteristics of honeycomb catalysts for oxidization of tritiated gas cleanup system, in: Proceedings of the 2nd Japan-China Workshop on Blanket and Tritium Technology, Sendai, Japan, May 9–10, 2008, 2008. [10] B. Li, S. Kado, Y. Mukainakano, T. Miyazawa, T. Miyao, S. Naito, et al., Surface modification of Ni catalysts with trace Pt for oxidative steam reforming of methane, J. Catal. 245 (2007) 144–155. [11] Y. Mukainakano, K. Yoshida, K. Okumura, K. Kunimori, K. Tomishige, Catalytic performance and QXAFS analysis of Ni catalysts modified with Pd for oxidative steam reforming of methane, Catal. Today 132 (2008) 101–108. [12] K. Tomishige, Oxidative steam reforming of methane over Ni catalysis modified with noble metals, J. Jpn. Petrol. Inst. 50 (2007) 287–298. [13] Y. Asakura, Development of exhaust gas and effluent liquid treatment system for LHD, J. Plasma Fusion Res. 78 (2002) 1319–1324 (in Japanese). [14] Kitco Inc., http://www.kitco.com/charts/.