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Al2O3 catalyst
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Catalysis Communications 9 (2008) 1558–1562 www.elsevier.com/locate/catcom
Carbon dioxide reforming of methane over glow discharge plasma-reduced Ir/Al2O3 catalyst Yu Zhao a,b, Yun-xiang Pan a, Yongbing Xie a, Chang-jun Liu a,* a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering, Tianjin University, Tianjin 300072, China b School of Chemical Engineering, Tianjin University of Technology, Tianjin 300191, China Received 31 August 2007; received in revised form 4 December 2007; accepted 21 December 2007 Available online 3 January 2008
Abstract In this work, characterizations using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirm that argon glow discharge plasma can effectively reduce the Al2O3 supported Ir catalyst without the use of any conventional reducing chemicals. Such plasma-reduced Ir/Al2O3 catalyst shows a significantly enhanced dispersion. High conversions have been achieved on this plasma-reduced Ir/Al2O3 catalyst for CO2 reforming of methane. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Glow discharge; Plasma; Reduction; CO2 reforming; Methane; Syngas
1. Introduction CO2 reforming of methane is being considered as a very potential method for conversion of methane and carbon dioxide, the two major greenhouse gases, into valuable hydrocarbons or oxygenated hydrocarbons [1–5]. It can generate synthesis gas (CO + H2) with a 1/1 ratio of H2/ CO that is suitable for further syntheses. It also attracts much attention for fuel cell application [6,7]. CO2 reforming of methane is also a promising way for utilization of biogas and natural gas with a high concentration of CO2 [7]. In addition, interest in some important fundamental issues of catalysis like structure-sensitive surface, carbon nanotube and others makes CO2 reforming one of the frontiers of catalytic research. Various catalysts, including supported noble metal catalysts, have been investigated for CO2 reforming of methane [1,8,9]. Supported noble metal catalysts were less sensitive to carbon deposition. An industrial test of CO2 reforming of methane over supported Pd catalysts has been recently
*
Corresponding author. Tel.: +86 22 27406490; fax: +86 22 27890078. E-mail address: [email protected] (C.-j. Liu).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.12.024
conducted [10]. Moreover, several reported works focus on the supported Ir catalysts, which exhibit an excellent coke resistance property for CO2 reforming of methane. It was shown that the Ir dispersion has a significant influence on the catalytic activity of Ir/Al2O3 catalysts. [6,11]. Normally, the supported noble metal catalysts for CO2 reforming were reduced by flowing hydrogen at elevated temperatures [11–15]. We recently reported an alternative catalyst reduction method using room temperature nonhydrogen glow discharge plasma [9,16,17]. Glow discharge is a non-thermal plasma phenomenon, characterized by high electron temperature (10,000–100,000 K) and low gas temperature (as low as room temperature). The reducibility of metal ions by non-hydrogen plasmas can be evaluated by the value of standard electrode potential of metal ions [9]. Those metal ions with positive standard electrode potential can be easily reduced by non-hydrogen plasmas [9]. The energetic species (electrons, ions, and radicals) in plasmas can modify the metal particle size, morphology, and metal-support interactions of catalyst [4,18–20], leading to some specific catalytic properties [4,17]. In this work, we attempt to apply the glow discharge plasma reduction for the preparation of Ir/Al2O3 catalyst for CO2 reforming of methane. A significantly improved dispersion of Ir on
Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
the support has been achieved, leading to high conversions of methane and carbon dioxide. 2. Experimental 2.1. Catalyst preparation H2IrCl6 (Delan Fine Chemical Plant, Tianjin, China) and c-Al2O3 powder (SBET = 230 m2/g; Tianjin Taixing Reagent Plant of China) were commercially obtained. The c-Al2O3 powder was calcined at 500 °C for 4 h before use. To prepare the catalysts with 1 wt% loading of Ir, the alumina support was firstly impregnated with an aqueous solution containing a given amount of the metal precursor for 12 h. Then the obtained samples were reduced using argon glow discharge plasma. For the purpose of comparison, the hydrogen-reduced catalyst was also prepared with no use of glow discharge plasma reduction. The setup for glow discharge plasma catalyst reduction has been previously described [9,17]. The sample loaded on a quartz boat was placed in the glow discharge cell. After adjustment of the pressure in the range of 100– 200 Pa, the glow discharge plasma was generated by applying 900 V to the electrode using a high voltage amplifier (Trek 20/20B). The signal input for the high voltage amplifier is supplied by a function/arbitrary waveform generator (HP 33120A) with a 100 Hz square wave. Argon (>99.999% in purity) was used as the discharge plasma-forming gas. The discharge was initiated at room temperature without external heating or cooling. According to the temperature measurement using infrared imaging (Ircon 100PHT), the bulk temperature of the catalyst powder remained close to room temperature. No thermal effect of the glow discharge reduction could be seen. Because the glow discharge plasma reduction is a surface reaction with electrons, the powder only on the top can be effectively reacted. The plasma reduction is therefore normally repeated six times. The time of each plasma reduction operation is 10 min. The catalyst sample is stirred using a small glass rod between two reduction operations, in order to obtain a uniform reduction. In some cases, a drop of water is added during the stirring. In the end, the color of the catalyst powder had completely changed from light green to brown. This indicates the supported Ir catalyst can be effectively reduced by argon glow discharge at near room temperature. The samples after the glow discharge reduction were sieved to 40–60 mesh and ready for the catalytic reaction. 2.2. Catalyst characterization X-ray photoelectron spectroscopy (XPS) analyses were performed using a Perkin Elmer PHI-1600 spectrometer with Mg Ka radiation. The analyzer bandpass energy was 187.85 eV for survey spectra and 29.35 eV for high-resolution spectra. The pressure in the detection chamber was 1.6 10 6 Pa. The binding energy was calibrated using the Al2p peak (74.40 eV).
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X-ray powder diffraction (XRD) patterns were obtained by a Rigaku D/Max-2500V/PC diffractometer at a scanning speed of 8°/min. The diffractometer was equipped with a Ni-filtered Cu Ka radiation source. The X-ray source was operated at 40 kV and 200 mA. The phase identification was made by comparison with Joint Committee on Powder Diffraction Standards. CO chemisorption was carried out using a Quantachrome Autosorb-1-C system. Before CO chemisorption, the plasma-reduced catalyst was calcined at 600 °C for 3 h under protection of argon. The conventional catalyst was reduced at 600 °C for 3 h in flowing hydrogen. 2.3. Activity test Catalytic activity test was carried out in a quartz tube with an inner diameter of 4 mm at ambient pressure. A catalyst sample of 100 mg was packed in the quartz tube. The thermally prepared catalyst was pre-reduced in situ by flowing hydrogen at 50 cm3/min and 600 °C for 3 h. The plasma-reduced sample was first purged and annealed with flowing argon at 20 cm3/min and 600 °C for 3 h. After that, the temperature was raised to the reaction temperature (700 or 750 °C). Then the feed gases of CH4, CO2 and argon with a ratio of 1:1:2 were introduced into the micro reactor by mass flow controllers. The gases used were ultra high pure grade (>99.999%) and were directly used without further purification. After reaction, the effluents passed through an ice trap to remove water, and were then analyzed online by a gas chromatograph (Agilent 4890) equipped with a thermal conductivity detector (TCD) using a Porapak Q packed column. Argon was used as the carrier gas. 3. Results and discussion For clarity, the sample after argon plasma reduction is denoted as PR-Ir. The PR-Ir sample annealed at 600 °C for 3 h under the protection of argon is referred to as PRA-Ir. The sample reduced in flowing hydrogen at 600 °C for 3 h is denoted as HR-Ir. The deconvoluted Ir4f XPS spectra are shown in Fig. 1. One peak in Fig. 1a and b with binding energy of 61 eV corresponds to metallic Ir. The other three peaks with binding energies of 63 eV, 64 eV and 66 eV can be assigned to Al2p satellite peaks, partly overlapped with Ir4f peaks. The XPS results show that argon glow discharge plasma can reduce supported Ir ions into metallic Ir particles at near room temperature. CO chemisorption results in Table 1 confirm that the glow discharge plasma-reduced Ir catalyst shows a significantly enhanced dispersion. The average particle size of PRA-Ir catalyst is 1.18 nm, while it is 3.29 nm for the HR-Ir sample. Regarding the XRD analyses of PRA-Ir and HR-Ir with 1 wt% loading of Ir, no Ir peak could be identified. We had to use a higher metal loading in order to confirm the XRD
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Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
Fig. 1. The deconvoluted Ir4f XPS spectra of the catalysts: (a) PR-Ir; (b) HR-Ir.
peaks. Fig. 2 shows XRD patterns of the Ir catalysts with 5 wt% loading of Ir. The diffraction peaks at 2h = 37.1°, 45.4° and 66.2° belong to c-Al2O3 within the XRD patterns of the PR-Ir sample. After annealing in argon at 600 °C, a
weak and broad shoulder peak at 2h = 40.56° appears, which can be assigned to Ir(1 1 1), suggesting that argon glow discharge plasma-reduced Ir metal particles exist with a high dispersion on the support. Compared with c-Al2O3,
Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
respectively. This indicates that bigger Ir nanoparticles have been obtained in the HR-Ir sample. Both PRA-Ir and HR-Ir catalysts show high selectivities for hydrogen and CO. No significant difference can be observed between two catalysts. The values and the variation trend (with reaction temperature) of the selectivities are almost the same as those obtained over Rh catalysts [9]. On the other hand, the plasma-reduced and hydrogen reduced Rh catalysts did not show big differences in the conversions [9]. However, the conversions of methane and CO2 are very different with Ir catalysts. Normally,
Table 1 CO chemisorption results of Ir catalysts with 1 wt% loading of Ir Catalysts
Active metal surface area (m2/g)
Metal dispersion (%)
Average metal particle size (nm)
PRA-Ir HR-Ir
2.268 0.813
83.95 30.09
1.18 3.29
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five intense diffraction peaks (2h = 40.56°, 47.16°, 69.14°, 83.28° and 88.10°) are present in HR-Ir sample, which can be assigned to Ir(1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), 700
600
Intensity
500
HR-Ir
400
300
PRA-Ir
200 PR-Ir
100 A -Al 2 O3
0 10
20
30
40
50
60
70
80
90
100
2θ (degree) Fig. 2. XRD patterns of 5 wt%Ir/Al2O3 catalysts.
700 °c
60
Conversion (%)
50
CH4 (P)
40
CO2 (P) CH4 (C)
30
CO2 (C)
20
10
0 20
30
40
50
60
70
80
90
100 110 120 130 140 150 160
Time on stream (min) Fig. 3. Conversion of the Ir/Al2O3 catalysts for carbon dioxide reforming of methane at 700 °C (P: PRA-Ir; C: HR-Ir).
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Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
80
750 °c
70
Conversion (%)
60 50 40 30 CH4 (P)
20
CO2 (P)
10
CO2 (C)
CH4 (C)
0 20
30
40
50
60
70
80
90
100 110 120 130 140 150 160
Time on stream (min) Fig. 4. Conversion of the Ir/Al2O3 catalysts for carbon dioxide reforming of methane at 750 °C (P: PRA-Ir; C: HR-Ir).
the Ir/Al2O3 catalyst exhibits low conversions in the reported work [11]. High conversions have been achieved in this work with PRA-Ir catalyst. Figs. 3 and 4 present comparative results of conversions of PRA-Ir and HR-Ir catalysts at reaction temperatures of 700 and 750 °C. The increased conversions over the plasma-reduced Ir/Al2O3 catalyst benefit from the improved Ir dispersion on the alumina support. Because of the reverse water gas shift reaction [7], the conversion of CO2 is always higher than the methane conversion [3,7,9]. 4. Conclusion Argon glow discharge plasma was successfully used to reduce the c-Al2O3 supported Ir catalyst. The argon glow discharge plasma catalyst reduction at room temperature can be easily operated. The operation is fast, simple and efficient. Compared to the hydrogen-reduced catalyst, the average particle size of plasma-reduced Ir/Al2O3 catalyst was much smaller on the support. A significantly improved dispersion has been achieved with the argon glow discharge plasma reduction, leading to high conversions of methane and carbon dioxide for CO2 reforming of methane. Acknowledgments The supports from the 973 Project (under contract 2005CB221406) and National Natural Science Foundation of China (under contract 20490203) are greatly appreci-
ated. The donation of instrument and equipment from ABB Switzerland is also appreciated. References [1] Y.H. Hu, E. Ruckenstein, Adv. Catal. 48 (2004) 297. [2] E.J. Beckman, CO2 as a raw material in chemical production, Abstracts of CHEMRAWN-XVII and ICCDU-IX Conference, Kingston, Ontario, Canada, July 8–12, 2007, p. 18. [3] W.D. Zhang, B.S. Liu, Y.L. Tian, Catal. Commun. 8 (2007) 661. [4] X. Zhu, Y.-P. Zhang, C.-J. Liu, Catal. Lett. 118 (2007) 306. [5] H. Sun, H. Wang, J. Zhang, Appl. Catal. B 73 (2007) 158. [6] P.K. Cheekatamarla, C.M. Finnerty, J. Power Sources 160 (2006) 490. [7] M. Wisniewski, A. Bore´ave, P. Ge´lin, Catal. Commun. 6 (2005) 596. [8] S. Yokota, K. Okumura, M. Niwa, Appl. Catal. A 310 (2006) 122. [9] Z.-J. Wang, Y. Zhao, L. Cui, H. Du, P. Yao, C.-J. Liu, Green Chem. 9 (2007) 554. [10] F. Yagi, R. Kanai, S. Wakamatsu, R. Kajiyama, Y. Suehiro, M. Shimura, Catal. Today 104 (2005) 2. [11] M.F. Mark, W.F. Maier, J. Catal. 164 (1996) 122. [12] M. Yang, H. Papp, Catal. Today 115 (2006) 199. [13] M.M.V.M. Souza, M. Schmal, Appl. Catal. A 255 (2003) 83. [14] Z.Y. Hou, P. Chen, H.L. Fang, X.M. Zheng, T. Yashima, Int. J. Hydrogen Energy 31 (2006) 555. [15] X.E. Verykios, Appl. Catal. A 255 (2003) 101. [16] J.-J. Zou, Y.-P. Zhang, C.-J. Liu, Langmuir 22 (2006) 11388. [17] Y. Zhao, Y.-X. Pan, L. Cui, C.-J. Liu, Diam. Relat. Mater. 16 (2007) 229. [18] J.-J. Zou, C.-J. Liu, Y.-P. Zhang, Langmuir 22 (2006) 2334. [19] C. Shi, R. Hoisington, B.W.L. Jang, Ind. Eng. Chem. Res. 46 (2007) 4390. [20] C. Ratanatawanate, M. Macias, B.W.L. Jang, Ind. Eng. Chem. Res. 44 (2005) 9868.
Catalysis Communications 9 (2008) 1558–1562 www.elsevier.com/locate/catcom
Carbon dioxide reforming of methane over glow discharge plasma-reduced Ir/Al2O3 catalyst Yu Zhao a,b, Yun-xiang Pan a, Yongbing Xie a, Chang-jun Liu a,* a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering, Tianjin University, Tianjin 300072, China b School of Chemical Engineering, Tianjin University of Technology, Tianjin 300191, China Received 31 August 2007; received in revised form 4 December 2007; accepted 21 December 2007 Available online 3 January 2008
Abstract In this work, characterizations using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirm that argon glow discharge plasma can effectively reduce the Al2O3 supported Ir catalyst without the use of any conventional reducing chemicals. Such plasma-reduced Ir/Al2O3 catalyst shows a significantly enhanced dispersion. High conversions have been achieved on this plasma-reduced Ir/Al2O3 catalyst for CO2 reforming of methane. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Glow discharge; Plasma; Reduction; CO2 reforming; Methane; Syngas
1. Introduction CO2 reforming of methane is being considered as a very potential method for conversion of methane and carbon dioxide, the two major greenhouse gases, into valuable hydrocarbons or oxygenated hydrocarbons [1–5]. It can generate synthesis gas (CO + H2) with a 1/1 ratio of H2/ CO that is suitable for further syntheses. It also attracts much attention for fuel cell application [6,7]. CO2 reforming of methane is also a promising way for utilization of biogas and natural gas with a high concentration of CO2 [7]. In addition, interest in some important fundamental issues of catalysis like structure-sensitive surface, carbon nanotube and others makes CO2 reforming one of the frontiers of catalytic research. Various catalysts, including supported noble metal catalysts, have been investigated for CO2 reforming of methane [1,8,9]. Supported noble metal catalysts were less sensitive to carbon deposition. An industrial test of CO2 reforming of methane over supported Pd catalysts has been recently
*
Corresponding author. Tel.: +86 22 27406490; fax: +86 22 27890078. E-mail address: [email protected] (C.-j. Liu).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.12.024
conducted [10]. Moreover, several reported works focus on the supported Ir catalysts, which exhibit an excellent coke resistance property for CO2 reforming of methane. It was shown that the Ir dispersion has a significant influence on the catalytic activity of Ir/Al2O3 catalysts. [6,11]. Normally, the supported noble metal catalysts for CO2 reforming were reduced by flowing hydrogen at elevated temperatures [11–15]. We recently reported an alternative catalyst reduction method using room temperature nonhydrogen glow discharge plasma [9,16,17]. Glow discharge is a non-thermal plasma phenomenon, characterized by high electron temperature (10,000–100,000 K) and low gas temperature (as low as room temperature). The reducibility of metal ions by non-hydrogen plasmas can be evaluated by the value of standard electrode potential of metal ions [9]. Those metal ions with positive standard electrode potential can be easily reduced by non-hydrogen plasmas [9]. The energetic species (electrons, ions, and radicals) in plasmas can modify the metal particle size, morphology, and metal-support interactions of catalyst [4,18–20], leading to some specific catalytic properties [4,17]. In this work, we attempt to apply the glow discharge plasma reduction for the preparation of Ir/Al2O3 catalyst for CO2 reforming of methane. A significantly improved dispersion of Ir on
Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
the support has been achieved, leading to high conversions of methane and carbon dioxide. 2. Experimental 2.1. Catalyst preparation H2IrCl6 (Delan Fine Chemical Plant, Tianjin, China) and c-Al2O3 powder (SBET = 230 m2/g; Tianjin Taixing Reagent Plant of China) were commercially obtained. The c-Al2O3 powder was calcined at 500 °C for 4 h before use. To prepare the catalysts with 1 wt% loading of Ir, the alumina support was firstly impregnated with an aqueous solution containing a given amount of the metal precursor for 12 h. Then the obtained samples were reduced using argon glow discharge plasma. For the purpose of comparison, the hydrogen-reduced catalyst was also prepared with no use of glow discharge plasma reduction. The setup for glow discharge plasma catalyst reduction has been previously described [9,17]. The sample loaded on a quartz boat was placed in the glow discharge cell. After adjustment of the pressure in the range of 100– 200 Pa, the glow discharge plasma was generated by applying 900 V to the electrode using a high voltage amplifier (Trek 20/20B). The signal input for the high voltage amplifier is supplied by a function/arbitrary waveform generator (HP 33120A) with a 100 Hz square wave. Argon (>99.999% in purity) was used as the discharge plasma-forming gas. The discharge was initiated at room temperature without external heating or cooling. According to the temperature measurement using infrared imaging (Ircon 100PHT), the bulk temperature of the catalyst powder remained close to room temperature. No thermal effect of the glow discharge reduction could be seen. Because the glow discharge plasma reduction is a surface reaction with electrons, the powder only on the top can be effectively reacted. The plasma reduction is therefore normally repeated six times. The time of each plasma reduction operation is 10 min. The catalyst sample is stirred using a small glass rod between two reduction operations, in order to obtain a uniform reduction. In some cases, a drop of water is added during the stirring. In the end, the color of the catalyst powder had completely changed from light green to brown. This indicates the supported Ir catalyst can be effectively reduced by argon glow discharge at near room temperature. The samples after the glow discharge reduction were sieved to 40–60 mesh and ready for the catalytic reaction. 2.2. Catalyst characterization X-ray photoelectron spectroscopy (XPS) analyses were performed using a Perkin Elmer PHI-1600 spectrometer with Mg Ka radiation. The analyzer bandpass energy was 187.85 eV for survey spectra and 29.35 eV for high-resolution spectra. The pressure in the detection chamber was 1.6 10 6 Pa. The binding energy was calibrated using the Al2p peak (74.40 eV).
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X-ray powder diffraction (XRD) patterns were obtained by a Rigaku D/Max-2500V/PC diffractometer at a scanning speed of 8°/min. The diffractometer was equipped with a Ni-filtered Cu Ka radiation source. The X-ray source was operated at 40 kV and 200 mA. The phase identification was made by comparison with Joint Committee on Powder Diffraction Standards. CO chemisorption was carried out using a Quantachrome Autosorb-1-C system. Before CO chemisorption, the plasma-reduced catalyst was calcined at 600 °C for 3 h under protection of argon. The conventional catalyst was reduced at 600 °C for 3 h in flowing hydrogen. 2.3. Activity test Catalytic activity test was carried out in a quartz tube with an inner diameter of 4 mm at ambient pressure. A catalyst sample of 100 mg was packed in the quartz tube. The thermally prepared catalyst was pre-reduced in situ by flowing hydrogen at 50 cm3/min and 600 °C for 3 h. The plasma-reduced sample was first purged and annealed with flowing argon at 20 cm3/min and 600 °C for 3 h. After that, the temperature was raised to the reaction temperature (700 or 750 °C). Then the feed gases of CH4, CO2 and argon with a ratio of 1:1:2 were introduced into the micro reactor by mass flow controllers. The gases used were ultra high pure grade (>99.999%) and were directly used without further purification. After reaction, the effluents passed through an ice trap to remove water, and were then analyzed online by a gas chromatograph (Agilent 4890) equipped with a thermal conductivity detector (TCD) using a Porapak Q packed column. Argon was used as the carrier gas. 3. Results and discussion For clarity, the sample after argon plasma reduction is denoted as PR-Ir. The PR-Ir sample annealed at 600 °C for 3 h under the protection of argon is referred to as PRA-Ir. The sample reduced in flowing hydrogen at 600 °C for 3 h is denoted as HR-Ir. The deconvoluted Ir4f XPS spectra are shown in Fig. 1. One peak in Fig. 1a and b with binding energy of 61 eV corresponds to metallic Ir. The other three peaks with binding energies of 63 eV, 64 eV and 66 eV can be assigned to Al2p satellite peaks, partly overlapped with Ir4f peaks. The XPS results show that argon glow discharge plasma can reduce supported Ir ions into metallic Ir particles at near room temperature. CO chemisorption results in Table 1 confirm that the glow discharge plasma-reduced Ir catalyst shows a significantly enhanced dispersion. The average particle size of PRA-Ir catalyst is 1.18 nm, while it is 3.29 nm for the HR-Ir sample. Regarding the XRD analyses of PRA-Ir and HR-Ir with 1 wt% loading of Ir, no Ir peak could be identified. We had to use a higher metal loading in order to confirm the XRD
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Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
Fig. 1. The deconvoluted Ir4f XPS spectra of the catalysts: (a) PR-Ir; (b) HR-Ir.
peaks. Fig. 2 shows XRD patterns of the Ir catalysts with 5 wt% loading of Ir. The diffraction peaks at 2h = 37.1°, 45.4° and 66.2° belong to c-Al2O3 within the XRD patterns of the PR-Ir sample. After annealing in argon at 600 °C, a
weak and broad shoulder peak at 2h = 40.56° appears, which can be assigned to Ir(1 1 1), suggesting that argon glow discharge plasma-reduced Ir metal particles exist with a high dispersion on the support. Compared with c-Al2O3,
Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
respectively. This indicates that bigger Ir nanoparticles have been obtained in the HR-Ir sample. Both PRA-Ir and HR-Ir catalysts show high selectivities for hydrogen and CO. No significant difference can be observed between two catalysts. The values and the variation trend (with reaction temperature) of the selectivities are almost the same as those obtained over Rh catalysts [9]. On the other hand, the plasma-reduced and hydrogen reduced Rh catalysts did not show big differences in the conversions [9]. However, the conversions of methane and CO2 are very different with Ir catalysts. Normally,
Table 1 CO chemisorption results of Ir catalysts with 1 wt% loading of Ir Catalysts
Active metal surface area (m2/g)
Metal dispersion (%)
Average metal particle size (nm)
PRA-Ir HR-Ir
2.268 0.813
83.95 30.09
1.18 3.29
1561
five intense diffraction peaks (2h = 40.56°, 47.16°, 69.14°, 83.28° and 88.10°) are present in HR-Ir sample, which can be assigned to Ir(1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), 700
600
Intensity
500
HR-Ir
400
300
PRA-Ir
200 PR-Ir
100 A -Al 2 O3
0 10
20
30
40
50
60
70
80
90
100
2θ (degree) Fig. 2. XRD patterns of 5 wt%Ir/Al2O3 catalysts.
700 °c
60
Conversion (%)
50
CH4 (P)
40
CO2 (P) CH4 (C)
30
CO2 (C)
20
10
0 20
30
40
50
60
70
80
90
100 110 120 130 140 150 160
Time on stream (min) Fig. 3. Conversion of the Ir/Al2O3 catalysts for carbon dioxide reforming of methane at 700 °C (P: PRA-Ir; C: HR-Ir).
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Y. Zhao et al. / Catalysis Communications 9 (2008) 1558–1562
80
750 °c
70
Conversion (%)
60 50 40 30 CH4 (P)
20
CO2 (P)
10
CO2 (C)
CH4 (C)
0 20
30
40
50
60
70
80
90
100 110 120 130 140 150 160
Time on stream (min) Fig. 4. Conversion of the Ir/Al2O3 catalysts for carbon dioxide reforming of methane at 750 °C (P: PRA-Ir; C: HR-Ir).
the Ir/Al2O3 catalyst exhibits low conversions in the reported work [11]. High conversions have been achieved in this work with PRA-Ir catalyst. Figs. 3 and 4 present comparative results of conversions of PRA-Ir and HR-Ir catalysts at reaction temperatures of 700 and 750 °C. The increased conversions over the plasma-reduced Ir/Al2O3 catalyst benefit from the improved Ir dispersion on the alumina support. Because of the reverse water gas shift reaction [7], the conversion of CO2 is always higher than the methane conversion [3,7,9]. 4. Conclusion Argon glow discharge plasma was successfully used to reduce the c-Al2O3 supported Ir catalyst. The argon glow discharge plasma catalyst reduction at room temperature can be easily operated. The operation is fast, simple and efficient. Compared to the hydrogen-reduced catalyst, the average particle size of plasma-reduced Ir/Al2O3 catalyst was much smaller on the support. A significantly improved dispersion has been achieved with the argon glow discharge plasma reduction, leading to high conversions of methane and carbon dioxide for CO2 reforming of methane. Acknowledgments The supports from the 973 Project (under contract 2005CB221406) and National Natural Science Foundation of China (under contract 20490203) are greatly appreci-
ated. The donation of instrument and equipment from ABB Switzerland is also appreciated. References [1] Y.H. Hu, E. Ruckenstein, Adv. Catal. 48 (2004) 297. [2] E.J. Beckman, CO2 as a raw material in chemical production, Abstracts of CHEMRAWN-XVII and ICCDU-IX Conference, Kingston, Ontario, Canada, July 8–12, 2007, p. 18. [3] W.D. Zhang, B.S. Liu, Y.L. Tian, Catal. Commun. 8 (2007) 661. [4] X. Zhu, Y.-P. Zhang, C.-J. Liu, Catal. Lett. 118 (2007) 306. [5] H. Sun, H. Wang, J. Zhang, Appl. Catal. B 73 (2007) 158. [6] P.K. Cheekatamarla, C.M. Finnerty, J. Power Sources 160 (2006) 490. [7] M. Wisniewski, A. Bore´ave, P. Ge´lin, Catal. Commun. 6 (2005) 596. [8] S. Yokota, K. Okumura, M. Niwa, Appl. Catal. A 310 (2006) 122. [9] Z.-J. Wang, Y. Zhao, L. Cui, H. Du, P. Yao, C.-J. Liu, Green Chem. 9 (2007) 554. [10] F. Yagi, R. Kanai, S. Wakamatsu, R. Kajiyama, Y. Suehiro, M. Shimura, Catal. Today 104 (2005) 2. [11] M.F. Mark, W.F. Maier, J. Catal. 164 (1996) 122. [12] M. Yang, H. Papp, Catal. Today 115 (2006) 199. [13] M.M.V.M. Souza, M. Schmal, Appl. Catal. A 255 (2003) 83. [14] Z.Y. Hou, P. Chen, H.L. Fang, X.M. Zheng, T. Yashima, Int. J. Hydrogen Energy 31 (2006) 555. [15] X.E. Verykios, Appl. Catal. A 255 (2003) 101. [16] J.-J. Zou, Y.-P. Zhang, C.-J. Liu, Langmuir 22 (2006) 11388. [17] Y. Zhao, Y.-X. Pan, L. Cui, C.-J. Liu, Diam. Relat. Mater. 16 (2007) 229. [18] J.-J. Zou, C.-J. Liu, Y.-P. Zhang, Langmuir 22 (2006) 2334. [19] C. Shi, R. Hoisington, B.W.L. Jang, Ind. Eng. Chem. Res. 46 (2007) 4390. [20] C. Ratanatawanate, M. Macias, B.W.L. Jang, Ind. Eng. Chem. Res. 44 (2005) 9868.