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A novel approach for CO-free H2 production via catalytic decomposition of hydrazine
International Journal of Hydrogen Energy 30 (2005) 1081 – 1089 www.elsevier.com/locate/ijhydene
A novel approach for CO-free H2 production via catalytic decomposition of hydrazine Mingyuan Zhenga, b , Ruihua Chenga, b , Xiaowei Chena , Ning Lia , Lin Lia, b , Xiaodong Wanga , Tao Zhanga, b,∗ a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China b Graduate School of Chinese Academy of Sciences, China
Received 30 July 2004; received in revised form 26 August 2004 Available online 6 November 2004
Abstract Supported group VIII (8, 9, and 10) metal catalysts, such as Fe/SiO2 , Ru/SiO2 , Co/SiO2 , Rh/SiO2 , Ir/SiO2 , Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 have been prepared for use in the producing of CO-free H2 via catalytic decomposition of hydrazine. Evaluation of the catalysts was conducted in a fixed-bed continuous-flow microreactor. All of the catalysts could catalyze the decomposition of hydrazine to form H2 and N2 at temperatures higher than 300 ◦ C. Differing from the other catalysts, the Ni, Pd, and Pt catalysts were also able to produce hydrogen with very high selectivities under rather mild conditions. Ni/SiO2 was the best catalyst owing to its high activity and excellent H2 selectivity of higher than 90% at an initial temperature of ∼ 30 ◦ C. The selectivity was sensitive to the reaction temperature. Low temperatures (30–60 ◦ C) were favorable for producing H2 with high selectivity. The preparation method and the support have remarkable influence on the catalytic activity and selectivity of the nickel catalysts. The reasons for such dependences are discussed according to the TPR and H2 -TPD results of the nickel catalysts. 䉷 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: CO-free hydrogen; Hydrazine decomposition; Group VIII metals; Nickel catalyst; Autothermal
1. Introduction Proton exchange membrane (PEM) fuel cells are one of the new generation power generators. They attract considerable attentions due to their high efficiency and ultra-low emission of pollutants. At present, the hydrogen feeds for PEM fuel cells are provided by two approaches: One is by utilizing hydrogen storage techniques, including
∗ Corresponding author. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China. Tel.: +86 411 8437 9015; fax: +86 411 8469 1570. E-mail address: [email protected] (T. Zhang).
high-pressure tanks, metal hydrides [1–3], carbon nanotubules [4–6], etc. The other is by steam reforming and/or partial oxidation of hydrocarbons and alcohols [7,8]. Unfortunately, with the former approach, the storage capacities of hydrogen are low. As for the latter means, it is not easy to completely eliminate traces of CO (< 10 ppm), which poisons the fuel cell anode and leads to a drastic decrease in efficiency. Therefore, it is highly desirable to develop a novel and high-performance technology for the production of CO-free hydrogen. The use of ammonia as a hydrogen source appears to be attractive because it produces no COx , and the unconverted NH3 can be reduced to a less than 200 ppb level by means of adsorbents. However, a reaction temperature of higher than
0360-3199/$30.00 䉷 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.09.014
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M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
600 ◦ C is usually required for satisfactory activities of the catalysts used, and energy must be provided from outside to maintain the high reaction temperature [9,10]. N2 H4 is a substance which is hydrogen-rich and with high energy content. Its hydrogen percentage is as high as 12.5 wt%. Although hydrazine is a toxic material, its products after complete decomposition are N2 and H2 , which are nonpoisonous. The products are also CO-free since no carbon atoms are present in hydrazine. Furthermore, hydrazine is chemically a very active compound and can be decomposed over catalysts at room temperature [11–14]. The decomposition is an exothermic reaction and does not need any energy to be provided from outside. Hence, hydrazine is very attractive as a CO-free hydrogen source for PEM fuel cells, particularly when the fuel cells are operated under special conditions such as in unmanned space vehicles or satellites where the energy is very limited, while at the same time hydrazine is used as the propellant for the thrusters of the space vessels. The group VIII (8, 9, and 10) metals are versatile for many catalytic processes, especially for reactions involving hydrogen. Hence, the investigations on H2 production from catalytic decomposition of hydrazine will focus on these metal catalysts in this paper.
2. Experimental 2.1. Catalyst preparation Metal catalysts Fe/SiO2 (8.0 wt%), Ru/SiO2 (8.0 wt%), Co/SiO2 (8.0 wt%), Rh/SiO2 (8.0 wt%), Ir/SiO2 (8.0 wt%), Ni/SiO2 (12.0 wt%), Pd/SiO2 (12.0 wt%) and Pt/SiO2 (12.0 wt%) were prepared and tested in this work. The catalysts were prepared by repeated incipient wetness impregnations of the SiO2 support (SBET = 310 m2 /g) with aqueous solutions of Fe(NO3 )3 ·9H2 O, RuCl3 ·3H2 O, Co(NO3 )2 ·6H2 O, RhCl3 , H2 IrCl6 ·6H2 O, Ni(NO3 )2 · 6H2 O, PdCl2 , and H4 PtCl6 ·6H2 O, respectively. After drying at 120 ◦ C for 12 h, the samples were calcined in air at 450 ◦ C for 2 h, followed by reduction in pure H2 for 2 h. The reduction temperatures were 350 ◦ C for Ir/SiO2 , Pd/SiO2 , and Pt/SiO2 , 450 ◦ C for Ru/SiO2 and Rh/SiO2 , 500 ◦ C for Co/SiO2 and Ni/SiO2 , and 600 ◦ C for Fe/SiO2 . The influences of metal loadings and supports for the nickel catalysts were examined on the following catalysts: Ni/SiO2 (21.0 wt%), Ni/SiO2 (29.0 wt%), Ni/Al2 O3 (21.0 wt%, -Al2 O3 (SBET = 198 m2 /g)), and Ni/AC (21.0 wt%, active carbon (SBET = 950 m2 /g)). They were prepared by the same method mentioned above, with the exception of calcining the Ni/AC in an inert atmosphere. The reduction temperatures for Ni/Al2 O3 and Ni/AC were also 500 ◦ C. To examine the influence of the reduction temperature, the Ni/SiO2 (21.0 wt%) catalyst was reduced at 350, 500, and 700 ◦ C, respectively.
2.2. Catalyst characterization To understand the influence of different supports, temperature-programmed reduction (TPR), chemical H2 adsorption, and temperature-programmed desorption of H2 (H2 -TPD) for the nickel catalysts were performed on a Micromeritics AutoChem II 2920 Automated Catalyst Characterization System. For the TPR experiments, the samples were reduced at a rate of 10 ◦ C/min in H2 /Ar (10% v/v). A cold trap froze the produced water and the signals were recorded by a TCD (thermal conductivity detector). During the TPR experiment of NiO/Ac, the products were analyzed by a mass spectrograph (Omnistar). For the chemical H2 -adsorption and H2 -TPD experiments, the catalysts were first reduced at 500 ◦ C with pure H2 for 1 h, and then flushed with Ar at 510 ◦ C for 1 h. Next, they were dosed with H2 /Ar (10% v/v) to saturation of H2 adsorption at 40 ◦ C, and from this the metal dispersion and particle diameter could be calculated. Finally, the catalysts were flushed again with Ar at 40 ◦ C and then heated in flowing argon by ramping the temperature from 40 to 450 ◦ C at a rate of 10 ◦ C/min. 2.3. Catalyst evaluation 2.3.1. Hydrazine decomposition The hydrazine decomposition reaction was performed in a fixed-bed continuous-flow microreactor at atmospheric pressure. In a U-shaped quartz reactor, catalysts (0.1 cm3 , 20–40 meshes) were physically mixed with inactive quartz powders (0.2 cm3 , 20–40 meshes) in order to prevent local hightemperature spots caused by the exothermic reaction. The evaluations were performed in situ after the preparations of catalyst in the reactor. The reaction temperature was adjusted with a water bath when below 100 ◦ C, or an oven at higher temperatures. The feedstock (∼3 vol% N2 H4 in Ar) was introduced by a bubbling method and was maintained at a rate of 85 ml/min (GHSV = 51,000 h−1 ) by a mass flowmeter. The products N2 , H2 , and NH3 , as well as any remaining N2 H4 , were detected by an on-line gas chromatograph (Agilent 6890A with 13X and Chromossorb 103 columns) equipped with a TCD detector. Hydrazine decomposition can be described as following reactions (1) and (2). The produced ammonia decomposes further into hydrogen and nitrogen according to reaction (3) at high temperatures (> 300 ◦ C). The overall reaction can be expressed as Eq. (4). The H2 selectivity of the catalyst, X, is taken as the decomposition percentage according to reaction (1), and is calculated from the amounts of hydrogen and ammonia (see Eq. (5)). N2 H4 (g) = N2 (g) + 2H2 (g),
H = −95.4 kJ mol−1 , (1)
3N2 H4 (g) = 4NH3 (g) + N2 (g),
H = −157 kJ mol−1 , (2)
2NH3 (g) = N2 (g) + 3H2 (g),
H = 46.2 kJ mol−1 ,
3N2 H4 = 4(1 − X)NH3 + (1 + 2X)N2 + 6XH2 , X=
(4)
2[H2 ] × 100%. 2[H2 ] + 3[NH3 ]
(5)
Generally, the H2 selectivity of a catalyst is equal to the percent of decomposition of the intermediate ammonia at high temperatures. Hence, the H2 selectivity of catalyst, X, is also used to evaluate the extent of ammonia decomposition according to reaction (3). 2.3.2. NH3 decomposition NH3 decomposition was performed over Ni/SiO2 , Pd/SiO2 and Pt/SiO2 . The reaction conditions were the same as for hydrazine decomposition mentioned above, except that the feedstock was pure NH3 maintained at a feeding rate of 30 ml/min.
N2H4 conversion (%)
(3)
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M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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Fig. 2. N2 H4 conversions and H2 selectivities over different catalysts: (, ) Fe/SiO2 (8.0 wt%) and (◦, •) Ru/SiO2 (8.0 wt%). Hollow symbols: N2 H4 conversions. Solid symbols: H2 selectivities.
decompose readily due to kinetic limitations when there is no catalyst existing.
3. Results and discussion 3.2. Performances of Fe/SiO2 and Ru/SiO2 catalysts 3.1. Result of N2 H4 thermal decomposition
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Fig. 1 illustrates the result of hydrazine thermal decomposition without any catalysts. Hydrazine decomposed slightly at 170 ◦ C and rose to 100% conversion at 250 ◦ C. Within the experimental temperature range, only very small amount of hydrogen was detected. Even at as high as 600 ◦ C, the H2 selectivity was still less than 5%. This indicates that thermal decomposition of hydrazine dominantly follows reaction (2) to produce ammonia and nitrogen. Although the produced NH3 would decompose into H2 and N2 subsequently at higher temperatures according to the thermodynamic calculations, Fig. 1 shows that it is considerably stable within the experimental temperature range. This demonstrates that the produced ammonia would not
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Fig. 1. Results of hydrazine thermal decomposition: () N2 H4 conversion, (◦) H2 selectivity.
The results of reaction performance over Fe/SiO2 (8.0 wt%) and Ru/SiO2 (8.0 wt%) are shown in Fig. 2. It can be seen that Fe/SiO2 is not very efficient for hydrazine decomposition, and hydrazine conversion is only 6.4% at 30 ◦ C. On the contrary, Ru/SiO2 is much more active for hydrazine decomposition, giving a 100% conversion within the experimental temperature range. Below 300 ◦ C, both Fe/SiO2 and Ru/SiO2 do not produce apparent amount of H2 , indicating that N2 H4 decomposes mainly according to reaction (2) over these two catalysts. However, the reaction path does not vary with the activity of the catalyst, no matter whether it is the inactive iron or the very active ruthenium catalyst. Elemental iron and ruthenium belong to the same group of the periodic table, thus their similar chemical characteristics might lead to the same reaction route of hydrazine decomposition. As the reaction temperature was increased to higher than 300 ◦ C, the H2 selectivity gradually rose and finally leveled off at 100% at 450 ◦ C over the Ru/SiO2 , and at 600 ◦ C over the Fe/SiO2 . As has been mentioned in the experimental section, the produced ammonia decomposed into hydrogen and nitrogen at high temperatures, so the H2 selectivity reflects the extent of ammonia decomposition in this case. Ruthenium catalysts are known to be very efficient for ammonia synthesis/decomposition [15,16]. So it is no wonder that the Ru/SiO2 exhibited very high H2 selectivity at high temperatures. Also, iron catalysts are traditional for ammonia synthesis. As we know, iron catalysts for industrial applications are comprised of many important additives for improving their activities. Hence, the simple catalyst Fe/SiO2 might not be a very good catalyst for ammonia cracking, and would not readily give high H2 selectivity when compared with the Ru catalyst at high temperatures.
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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Fig. 3. N2 H4 conversions and H2 selectivities over different catalysts: (, ) Co/SiO2 (8.0 wt%), (◦, •) Rh/SiO2 (8.0 wt%) and (, ) Ir/SiO2 (8.0 wt%). Hollow symbols: N2 H4 conversions. Solid symbols: H2 selectivities.
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Fig. 4. N2 H4 conversions over different catalysts: () Ni/SiO2 (12.0 wt%), (◦) Pd/SiO2 (12.0 wt%) and () Pt/SiO2 (12.0 wt%).
3.3. Performances of Co/SiO2 , Rh/SiO2 , and Ir/SiO2 catalysts
100
3.4. Performances of Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 catalysts Fig. 4 presents the hydrazine conversions over Ni/SiO2 (12.0 wt%), Pd/SiO2 (12.0 wt%), and Pt/SiO2 (12.0 wt%)
H2 selectivity (%)
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Fig. 3 shows that the hydrazine conversions over Co/SiO2 (8.0 wt%), Rh/SiO2 (8.0 wt%), and Ir/SiO2 (8.0 wt%) are all 100% within the experimental temperature range. Apparently, catalysts of the metals of this group are so active that their activities cannot be discriminated under these evaluation conditions. The results of H2 selectivity are also shown in Fig. 3. These three catalysts clearly did not produce much H2 at low temperatures (< 300 ◦ C). Similar to Fe/SiO2 and Ru/SiO2 , the reaction paths over them were mainly reaction (2) for form ammonia and nitrogen. When the reaction temperature was increased to 300 ◦ C and higher, the produced ammonia disassociated further into hydrogen and nitrogen. The Rh/SiO2 was the most active among these three catalysts, showing a complete decomposition of ammonia at 450 ◦ C. The results of this series of catalysts as well as that of the Ru/SiO2 discussed above suggest that CO-free H2 can be produced if hydrazine decomposes at high enough temperatures over these catalysts. It is known that when hydrazine decomposes in thrusters as a propellant, the temperature of the catalyst bed usually rises to higher than 700 ◦ C [12,17,18]. Calculations have shown that the temperature of the decomposition products can be 604 ◦ C when liquid hydrazine decomposes into H2 and N2 under adiabatic conditions. Hence, by considering the results of the present work, we can conclude that producing CO-free H2 by catalytic decomposition of hydrazine under autothermal conditions is highly feasible.
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Fig. 5. H2 selectivities of different catalysts: () Ni/SiO2 (12.0 wt%), (◦) Pd/SiO2 (12.0 wt%) and () Pt/SiO2 (12.0 wt%).
catalysts. Generally, these three catalysts were less active than the other two series of catalysts just discussed above. They catalyzed the decomposition of hydrazine mildly or even weakly. The hydrazine conversions were 41.7%, 7.2%, and 9.6%, respectively for Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 at 30 ◦ C. The nickel catalyst was the most active among these catalysts. The H2 selectivities are shown in Fig. 5. Obviously, they are different from those discussed above. At low temperatures (< 200 ◦ C), the hydrazine decomposed over these three catalysts with very high H2 selectivities. Especially between 30 and 80 ◦ C, the selectivities reached to higher than 90%. The value is 95.4% for Ni/SiO2 at 40 ◦ C, 96.8% for Pd/SiO2 at 80 ◦ C, and 97.4% for Pt/SiO2 at 50 ◦ C, respectively. It is known that nickel, palladium and platinum catalysts possess outstanding ability in activating H species in many hydrogen-involved reactions. This might be very helpful to deprive hydrazine of its hydrogen atoms that would recombine further into H2 and escape from the surfaces of
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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Fig. 6. NH3 decomposition over different catalysts: () Ni/SiO2 (12.0 wt%), (◦) Pd/SiO2 (12.0 wt%) and () Pt/SiO2 (12.0 wt%).
the catalysts. An investigation has found that H–N cleavage was preferential to N–N as N2 H4 that adsorbed on Pt(1 1 1), and this is attributed to the H–Pt(1 1 1) bond being stronger than the N–Pt(1 1 1) bond [19]. Another interesting phenomenon is that the H2 selectivity is very sensitive to the reaction temperature. The profiles of H2 selectivity exhibited volcano shapes. Take Pd/SiO2 as an example. Its H2 selectivity increased with the reaction temperature in the range of 30–80 ◦ C. In this case, reaction (1) is preferred to reaction (2). After reaching the maximum, the H2 selectivity decreased quickly when the reaction temperature was increased further. This indicates that probably two reaction paths are proceeding and competing during hydrazine decomposition. In the range of 80–200 ◦ C, a higher temperature is disadvantageous to reaction (1). For the Ni/SiO2 and Pt/SiO2 , the H2 selectivities decreased to less than 5% at 200 ◦ C. As the reaction temperature was increased further from 200 to 650 ◦ C, Pt/SiO2 did not exhibit very high ability to decompose the produced ammonia, whereas Ni/SiO2 showed a better activity. The behavior of Pd/SiO2 seems to be unique among the three catalysts. Unlike Ni/SiO2 and Pt/SiO2 , its H2 selectivity remained at 31.3% even at 300 ◦ C. This should not be attributed to the further dissociation of ammonia produced over Pd/SiO2 . The results of NH3 decomposition (shown in Fig. 6) confirmed this supposition. It can be found from Fig. 6 that Pd/SiO2 and Pt/SiO2 were also inactive for the dissociation of ammonia, and below 450 ◦ C, ammonia did not decompose over them to any larger extent. Thus, the high H2 selectivity at 200–450 ◦ C in hydrazine decomposition over Pd/SiO2 should be attributed to the co-proceeding of reactions (1) and (2), but not to ammonia decomposition. From all the results discussed above, one can notice that all group VIII catalysts are rather active for catalytic decomposition of hydrazine. In general, their activities and H2 selectivities depend on the sub-group characteristics of group
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80
Temperature (oC) Fig. 7. N2 H4 conversions over different catalysts: () Ni/SiO2 (29.0 wt%), (◦) Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), () Ni/SiO2 (12.0 wt%).
VIII. A volcanic curve of the activity exists as the sub-group goes from the left to the right of group VIII. Cobalt, rhodium, and iridium catalysts are all effective for hydrazine decomposition, whereas nickel, palladium, and platinum catalysts are not so active as the other two sub-groups of metals. Meanwhile, they differ from the others in that their H2 selectivities are very high at low temperatures. There is a popular understanding in catalysis that only a suitable interaction between the catalyst and the reactants can yield a high catalytic activity, while too strong or too weak interaction would yield a low activity. As the heat of formation of the respective metal nitride decreases from the left to the right of the periodic table, the strength of the metal–N interaction on the surface of the catalyst decreases as well [20,21]. On the other hand, the metal–H interaction also decreases almost linearly as the d band orbital of the metal become more occupied [22]. Hence, the trends of catalytic activity and selectivity in hydrazine decomposition over these catalysts might be related to the regular variation of chemisorption energy of hydrogen and nitrogen on the metals. 3.5. Influences of loading, reduction temperature, and support on nickel catalysts Since Ni/SiO2 , Pd/SiO2 and Pt/SiO2 exhibited distinguished H2 selectivities at low temperatures, and by considering a balance between activity and cost of the catalyst, nickel catalysts were further investigated in this paper. The influence of nickel loading on Ni/SiO2 was investigated and the results are presented in Figs. 7 and 8. Apparently, as shown in Fig. 7, a catalyst with a higher nickel loading was more active for the reaction. As the nickel loading was increased from 12.0 to 21.0 and 29.0 wt%, the hydrazine conversion at 30 ◦ C also rose from 41.6% to 68.7%, and 76.3%, respectively. However, no apparent influence of
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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H2 selectivity
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Fig. 8. H2 selectivities of different catalysts: () Ni/SiO2 (29.0 wt%), (◦) Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), () Ni/SiO2 (12.0 wt%).
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Fig. 10. H2 selectivities of Ni/SiO2 (21.0 wt%) reduced at () 350 ◦ C, (◦) 500 ◦ C, and () 700 ◦ C, respectively.
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Fig. 9. Hydrazine conversions over Ni/SiO2 (21.0 wt%) reduced at () 350 ◦ C, (◦) 500 ◦ C, and () 700 ◦ C, respectively.
Fig. 11. N2 H4 conversions over different catalysts: () Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), (◦) Ni/Al2 O3 (21.0 wt%) and () Ni/AC (21.0 wt%).
the loading on the H2 selectivities at low or high temperatures was observed (shown in Fig. 8). These results demonstrate that the increase in nickel loading provides more active sites for the catalytic process, but this is merely a quantitative variation and does not produce any essential impact on the reaction path. The influence of reduction temperature on Ni/SiO2 (21.0 wt%) is shown in Figs. 9 and 10. There were apparent effects on the catalytic activities as well as on the H2 selectivities by different reduction temperatures 350, 500, and 700 ◦ C). Under these conditions, the catalyst reduced at 500 ◦ C was the most active at 30 ◦ C, whilst the highest reduction temperature led to the lowest activity. The turnover frequencies (TOF) of these catalysts were calculated. At 30 ◦ C, the TOF increased from 0.04 to 0.13 and 0.40 s−1 as the reduction temperature was increased from
350 to 500 and 700 ◦ C, respectively. So, a higher reduction temperature will lead to a higher TOF. The low conversion of hydrazine over Ni/SiO2 reduced at 700 ◦ C might be attributed to the fewer catalytic active sites due to nickel sintering at such a high temperature. In addition, with an increase in the reduction temperature, strong metal–support interaction (SMSI) would be enhanced and the properties of the nickel species would be affected. The phenomenon that the H2 selectivity within 40–80 ◦ C increased with the reduction temperature might also be related to the change in the extent of SMSI. Hence, the treatment or preparation method of the nickel catalysts also played an important role in the catalytic performance. Fig. 11 compares the activities of the nickel catalysts on different supports. N2 H4 conversions over Ni/AC (21.0 wt%) and Ni/Al2 O3 (21.0 wt%) were merely 13.7%
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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H2 selectivity (%)
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Fig. 12. H2 selectivities of different catalysts: () Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), (◦) Ni/Al2 O3 (21.0 wt%) and () Ni/AC (21.0 wt%).
0.45 0.40 0.35 TCD signal
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Fig. 13. TPR profiles of different catalysts: (1) NiO (from calcination of Ni(NO3 )2 · 6H2 O at 450 ◦ C) (2) Ni/SiO2 (21.0 wt%), (3) Ni/AC (21.0 wt%), and (4) Ni/Al2 O3 (21.0 wt%).
and 17.1%, respectively, at 30 ◦ C, and these values were ca. 50% lower than that of Ni/SiO2 (21.0 wt%). It is no doubt that the interaction between the nickel and the supports will result in different catalytic activities. Similarly, the supports also affect the selectivities. As shown in Fig. 12, the H2 selectivities of Ni/Al2 O3 and Ni/AC were 83.5% and 65.7%, respectively, at 30 ◦ C. These values were apparently lower than the 93.5% H2 selectivity of the Ni/SiO2 . This provides further evidence that SMSI plays an important role in the H2 selectivity. The TPR profiles of the catalysts (Fig. 13) reflect to some extent the interaction between nickel and the supports. They might be helpful for understanding the difference in catalytic activity as well as H2 selectivity among these catalysts at low
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temperatures. From Fig. 13, it can be seen that the reduction peak temperature of Ni/SiO2 is 340 ◦ C, and is basically the same as that of 355 ◦ C of unsupported NiO, indicating that the interaction between nickel and silica was quite weak. In contrast to this weak interaction with silica, nickel interacted with alumina more strongly. The reduction peak temperature of Ni/Al2 O3 was 540 ◦ C, which was 200 ◦ C higher than that of Ni/SiO2 . Thus, the SMSI of Ni/Al2 O3 will affect the chemical properties of nickel more significantly and accounts for its low activity or selectivity. As for Ni/AC, it was a bit different. There are three reduction peaks on the profile, i.e. peaks at 250, 395, and 500 ◦ C, respectively. The former two are attributed to nickel reduction, whereas the third peak should be attributed to methane production. This was validated by a mass spectrograph taken during the reduction. Accordingly, it can be inferred that there are at least two kinds of nickel species existing on the AC support. One kind of nickel can be reduced even easier than unsupported NiO (the reduction temperature is 355 ◦ C), and the other interacts with AC more strongly and needs a higher reduction temperature. It seems that a weak interaction between nickel and AC would be advantageous for yielding high activity and H2 selectivity. However, the result of H2 chemisorption shows that this is not the case. From the H2 chemisorption experiments it was found that the nickel dispersion over AC was the lowest among the three catalysts. It was merely 0.3% over AC, whereas the nickel dispersions over SiO2 and Al2 O3 were 2.7% and 4.4%, respectively. That is to say, the diameter of the nickel particles (343 nm) over AC was much larger than those of Ni/SiO2 (37 nm) and Ni/Al2 O3 (23 nm). No doubt, this nickel aggregation over AC should be attributed to the too weak interaction between nickel and AC support. In addition, the carbon loss during the reduction would also enhance the aggregation of the nickel metal. Hence, a too weak interaction between nickel and AC is disadvantageous and will lead to a low catalytic activity. The results of H2 -TPD experiments are displayed in Fig. 14. The profiles show that H2 desorbed at different temperatures on Ni/SiO2 , Ni/Al2 O3 , and Ni/AC. On Ni/SiO2 , H2 desorption commenced at the beginning of heating, whereas on Ni/Al2 O3 it began at 100 ◦ C. This indicates that H2 desorbed more facilely at the surface sites of Ni/SiO2 than at those of Ni/Al2 O3 . H2 desorption on Ni/AC also began at the start of applying the desorption temperature, however, its amount was considerably smaller than those of the other two due to low nickel dispersion. Therefore, the surface characteristics of Ni/SiO2 were the best among the three catalysts for the reaction of hydrogen-production, and favored high H2 selectivity. The results and discussion given above show that some important factors of the nickel catalysts must be considered in order to produce high activity and H2 selectivity, i.e. kind of support, reduction temperature, and nickel loading, as well as procedures capable of adjusting the interaction between the nickel and the support.
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M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
0.007 0.006 TCD signal
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Fig. 14. H2 -TPD profiles of the catalysts: (1) Ni/AC (21.0 wt%), (2) Ni/SiO2 (21.0 wt%), and (3) Ni/Al2 O3 (21.0 wt%).
100
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80 hydrazine conversion H2 selectivity
60
N2 with more than 90% selectivity at 30–40 ◦ C. The preparation method and the support of the nickel catalysts played important roles on the catalytic activity and H2 selectivity. These findings suggest that Ni/SiO2 might be a good candidate for catalytic decomposition of hydrazine at rather mild temperatures for producing CO-free H2 for PEM fuel cells. In addition, it was found that all catalysts of group VIII metals were efficient for hydrazine decomposition and were able to produce H2 at high temperatures (> 350 ◦ C). Especially, over Ru, Co, Rh, and Ir catalysts, hydrazine decomposed readily into H2 and N2 at high temperatures. Theoretically, production of H2 can also be realized by hydrazine decomposition over these catalysts under autothermal conditions.
Acknowledgements This work was supported by a grant from the National Natural Science Foundation of China (NSFC) for Outstanding Youth (No. 20325620) and a grant from NSFC for Youth (No. 20303017). The authors gratefully acknowledge helpful discussions with Professor Dongbai Liang of State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Chinese Sciences.
40 20 0
References 0
2
4
6
8
10 12 14 16 18 20 22 24 Time (h)
Fig. 15. N2 H4 conversion and H2 selectivity over Ni/SiO2 at 50 ◦ C (21.0 wt%, reduced at 500 ◦ C).
3.6. Life test of Ni/SiO2 The life test of Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C) was conducted at 50 ◦ C (GHSV = 25,500 h−1 ) and the result is shown Fig. 15. During the 24 h reaction, hydrazine conversion and H2 selectivity maintained steadily at 100% and more than 80%, respectively. These suggest that Ni/SiO2 possesses a high catalytic activity and a reasonable H2 selectivity for hydrazine decomposition.
4. Conclusions Hydrazine decomposition was investigated in a micro reactor for producing CO-free H2 for PEM fuel cells. It is found that Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 all catalyzed hydrazine decomposition with a high H2 selectivity at low temperatures. N2 H4 decomposed over Ni/SiO2 into H2 and
[1] Kong VCY, Foulkes FR, Kirk DW, Hinatsu JT. Development of hydrogen storage for fuel cell generators: i. Hydrogen generation using hydrolysis hydrides. Int J Hydrogen Energy 1999;24(7):665–75. [2] Esayed AY. Metal hydrides. Energy Sources 2001;23(3): 257–65. [3] Dantzer P. Properties of intermetallic compounds suitable for hydrogen storage applications. Mater Sci Eng A 2002;329–331:313–20. [4] Darkrim FL, Malbrunot P, Tartaglia GP. Review of hydrogen storage by adsorption in carbon nanotubes. Int J Hydrogen Energy 2002;27:193–202. [5] Bünger U, Zittel W. Hydrogen storage in carbon nanostructures—still a long road from science to commerce?. Appl Phys A 2001;72(2):147–51. [6] Wronski ZS. Materials for rechargeable batteries and clean hydrogen energy sources. Int Mater Rev 2001;46(1):1–49. [7] Liu DJ, Kaun TD, Liao HK, Ahmed S. Characterization of kilowatt-scale autothermal reformer for production of hydrogen from heavy hydrocarbons. Int J Hydrogen Energy 2004;29(10):1035–46. [8] Joensen F, Rostrup-Nielsen JR. Conversion of hydrocarbons and alcohols for fuel cells. J Power Sources 2002;105(2): 195–201. [9] Choudhary TV, Sivadinarayana C, Goodman DW. Catalytic ammonia decomposition: COx -free hydrogen production for fuel cell applications. Catal Lett 2001;72(3–4):197–201.
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089 [10] Yin SF, Zhang QH, Xu BQ, Zhu WX, Ng CF, Au CT. Investigation on the catalysis of COx -free hydrogen generation from ammonia. J Catal 2004;224:384–96. [11] Schmidt E. Hydrazine and its derivatives: preparation, properties, applications. 2nd Ed., NY, USA: Wiley; 2001. [12] Chen XW, Zhang T, Ying PL, Zheng MY, Wu WC, Xia LG, Li T, Wang XD, Li C. A novel catalyst for hydrazine decomposition: molybdenum carbide supported on gammaAl2 O3 . Chem Commun 2002; 288–9. [13] Chen XW, Zhang T, Xia LG, Li T, Zheng MY, Wu ZL, Wang XD, Wei ZB, Xin Q, Li C. Catalytic hydrazine decomposition over supported molybdenum nitride in a monopropellant thruster. Catal Lett 2002;79(1–4):21–5. [14] Chen XW, Zhang T, Zheng MY, Wu ZL, Wu WC, Li C. The reaction route and active site of catalytic decomposition of hydrazine over molybdenum nitride catalyst. J Catal 2004;224:473–8. [15] Liang CH, Li ZL, Qiu JS, Li C. Graphitic nanofilaments as novel support of Ru–Ba catalysts for ammonia synthesis. J Catal 2002;211(1):278–82.
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[16] Dahl S, Taylor PA, Tornqvist E, Chorkendorff I. The synthesis of ammonia over a ruthenium single crystal. J Catal 1998;178(2):679–86. [17] Neto TGS, Cobo AJG, Cruz GM. Textural properties evolution of Ir and Ru supported on alumina catalysts during hydrazine decomposition in satellite thruster. Appl Catal A 2003;250(2):331–40. [18] Rodrigues JAJ, Cruz GM, Bugli G, Boudart M, DjegaMariadassou G. Nitride and carbide of molybdenum and tungsten as substitutes of iridium for the catalysts used for space communication. Catal Lett 1997;45(1–2):1–3. [19] Alberas DJ, Kiss J, Liu ZM, White JM. Surface chemistry of hydrazine on Pt(1 1 1). Surf Sci 1992;278:51–61. [20] Grunze M. The interaction of hydrazine with an surface Fe(1 1 1). Surf Sci 1979;81:603–25. [21] Oyama ST. Introduction to the chemistry of transition metal carbides and nitrides. In: Oyama ST., editor. The chemistry of transition metal carbides and nitrides. Glasgow: Blackie Academic & Professional; 1996. p. 1–27. [22] Nordlander P, Holloway S, NZrskov JK. Hydrogen adsorption on metal surfaces. Surf Sci 1984;136:59–81.
A novel approach for CO-free H2 production via catalytic decomposition of hydrazine Mingyuan Zhenga, b , Ruihua Chenga, b , Xiaowei Chena , Ning Lia , Lin Lia, b , Xiaodong Wanga , Tao Zhanga, b,∗ a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China b Graduate School of Chinese Academy of Sciences, China
Received 30 July 2004; received in revised form 26 August 2004 Available online 6 November 2004
Abstract Supported group VIII (8, 9, and 10) metal catalysts, such as Fe/SiO2 , Ru/SiO2 , Co/SiO2 , Rh/SiO2 , Ir/SiO2 , Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 have been prepared for use in the producing of CO-free H2 via catalytic decomposition of hydrazine. Evaluation of the catalysts was conducted in a fixed-bed continuous-flow microreactor. All of the catalysts could catalyze the decomposition of hydrazine to form H2 and N2 at temperatures higher than 300 ◦ C. Differing from the other catalysts, the Ni, Pd, and Pt catalysts were also able to produce hydrogen with very high selectivities under rather mild conditions. Ni/SiO2 was the best catalyst owing to its high activity and excellent H2 selectivity of higher than 90% at an initial temperature of ∼ 30 ◦ C. The selectivity was sensitive to the reaction temperature. Low temperatures (30–60 ◦ C) were favorable for producing H2 with high selectivity. The preparation method and the support have remarkable influence on the catalytic activity and selectivity of the nickel catalysts. The reasons for such dependences are discussed according to the TPR and H2 -TPD results of the nickel catalysts. 䉷 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: CO-free hydrogen; Hydrazine decomposition; Group VIII metals; Nickel catalyst; Autothermal
1. Introduction Proton exchange membrane (PEM) fuel cells are one of the new generation power generators. They attract considerable attentions due to their high efficiency and ultra-low emission of pollutants. At present, the hydrogen feeds for PEM fuel cells are provided by two approaches: One is by utilizing hydrogen storage techniques, including
∗ Corresponding author. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China. Tel.: +86 411 8437 9015; fax: +86 411 8469 1570. E-mail address: [email protected] (T. Zhang).
high-pressure tanks, metal hydrides [1–3], carbon nanotubules [4–6], etc. The other is by steam reforming and/or partial oxidation of hydrocarbons and alcohols [7,8]. Unfortunately, with the former approach, the storage capacities of hydrogen are low. As for the latter means, it is not easy to completely eliminate traces of CO (< 10 ppm), which poisons the fuel cell anode and leads to a drastic decrease in efficiency. Therefore, it is highly desirable to develop a novel and high-performance technology for the production of CO-free hydrogen. The use of ammonia as a hydrogen source appears to be attractive because it produces no COx , and the unconverted NH3 can be reduced to a less than 200 ppb level by means of adsorbents. However, a reaction temperature of higher than
0360-3199/$30.00 䉷 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.09.014
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600 ◦ C is usually required for satisfactory activities of the catalysts used, and energy must be provided from outside to maintain the high reaction temperature [9,10]. N2 H4 is a substance which is hydrogen-rich and with high energy content. Its hydrogen percentage is as high as 12.5 wt%. Although hydrazine is a toxic material, its products after complete decomposition are N2 and H2 , which are nonpoisonous. The products are also CO-free since no carbon atoms are present in hydrazine. Furthermore, hydrazine is chemically a very active compound and can be decomposed over catalysts at room temperature [11–14]. The decomposition is an exothermic reaction and does not need any energy to be provided from outside. Hence, hydrazine is very attractive as a CO-free hydrogen source for PEM fuel cells, particularly when the fuel cells are operated under special conditions such as in unmanned space vehicles or satellites where the energy is very limited, while at the same time hydrazine is used as the propellant for the thrusters of the space vessels. The group VIII (8, 9, and 10) metals are versatile for many catalytic processes, especially for reactions involving hydrogen. Hence, the investigations on H2 production from catalytic decomposition of hydrazine will focus on these metal catalysts in this paper.
2. Experimental 2.1. Catalyst preparation Metal catalysts Fe/SiO2 (8.0 wt%), Ru/SiO2 (8.0 wt%), Co/SiO2 (8.0 wt%), Rh/SiO2 (8.0 wt%), Ir/SiO2 (8.0 wt%), Ni/SiO2 (12.0 wt%), Pd/SiO2 (12.0 wt%) and Pt/SiO2 (12.0 wt%) were prepared and tested in this work. The catalysts were prepared by repeated incipient wetness impregnations of the SiO2 support (SBET = 310 m2 /g) with aqueous solutions of Fe(NO3 )3 ·9H2 O, RuCl3 ·3H2 O, Co(NO3 )2 ·6H2 O, RhCl3 , H2 IrCl6 ·6H2 O, Ni(NO3 )2 · 6H2 O, PdCl2 , and H4 PtCl6 ·6H2 O, respectively. After drying at 120 ◦ C for 12 h, the samples were calcined in air at 450 ◦ C for 2 h, followed by reduction in pure H2 for 2 h. The reduction temperatures were 350 ◦ C for Ir/SiO2 , Pd/SiO2 , and Pt/SiO2 , 450 ◦ C for Ru/SiO2 and Rh/SiO2 , 500 ◦ C for Co/SiO2 and Ni/SiO2 , and 600 ◦ C for Fe/SiO2 . The influences of metal loadings and supports for the nickel catalysts were examined on the following catalysts: Ni/SiO2 (21.0 wt%), Ni/SiO2 (29.0 wt%), Ni/Al2 O3 (21.0 wt%, -Al2 O3 (SBET = 198 m2 /g)), and Ni/AC (21.0 wt%, active carbon (SBET = 950 m2 /g)). They were prepared by the same method mentioned above, with the exception of calcining the Ni/AC in an inert atmosphere. The reduction temperatures for Ni/Al2 O3 and Ni/AC were also 500 ◦ C. To examine the influence of the reduction temperature, the Ni/SiO2 (21.0 wt%) catalyst was reduced at 350, 500, and 700 ◦ C, respectively.
2.2. Catalyst characterization To understand the influence of different supports, temperature-programmed reduction (TPR), chemical H2 adsorption, and temperature-programmed desorption of H2 (H2 -TPD) for the nickel catalysts were performed on a Micromeritics AutoChem II 2920 Automated Catalyst Characterization System. For the TPR experiments, the samples were reduced at a rate of 10 ◦ C/min in H2 /Ar (10% v/v). A cold trap froze the produced water and the signals were recorded by a TCD (thermal conductivity detector). During the TPR experiment of NiO/Ac, the products were analyzed by a mass spectrograph (Omnistar). For the chemical H2 -adsorption and H2 -TPD experiments, the catalysts were first reduced at 500 ◦ C with pure H2 for 1 h, and then flushed with Ar at 510 ◦ C for 1 h. Next, they were dosed with H2 /Ar (10% v/v) to saturation of H2 adsorption at 40 ◦ C, and from this the metal dispersion and particle diameter could be calculated. Finally, the catalysts were flushed again with Ar at 40 ◦ C and then heated in flowing argon by ramping the temperature from 40 to 450 ◦ C at a rate of 10 ◦ C/min. 2.3. Catalyst evaluation 2.3.1. Hydrazine decomposition The hydrazine decomposition reaction was performed in a fixed-bed continuous-flow microreactor at atmospheric pressure. In a U-shaped quartz reactor, catalysts (0.1 cm3 , 20–40 meshes) were physically mixed with inactive quartz powders (0.2 cm3 , 20–40 meshes) in order to prevent local hightemperature spots caused by the exothermic reaction. The evaluations were performed in situ after the preparations of catalyst in the reactor. The reaction temperature was adjusted with a water bath when below 100 ◦ C, or an oven at higher temperatures. The feedstock (∼3 vol% N2 H4 in Ar) was introduced by a bubbling method and was maintained at a rate of 85 ml/min (GHSV = 51,000 h−1 ) by a mass flowmeter. The products N2 , H2 , and NH3 , as well as any remaining N2 H4 , were detected by an on-line gas chromatograph (Agilent 6890A with 13X and Chromossorb 103 columns) equipped with a TCD detector. Hydrazine decomposition can be described as following reactions (1) and (2). The produced ammonia decomposes further into hydrogen and nitrogen according to reaction (3) at high temperatures (> 300 ◦ C). The overall reaction can be expressed as Eq. (4). The H2 selectivity of the catalyst, X, is taken as the decomposition percentage according to reaction (1), and is calculated from the amounts of hydrogen and ammonia (see Eq. (5)). N2 H4 (g) = N2 (g) + 2H2 (g),
H = −95.4 kJ mol−1 , (1)
3N2 H4 (g) = 4NH3 (g) + N2 (g),
H = −157 kJ mol−1 , (2)
2NH3 (g) = N2 (g) + 3H2 (g),
H = 46.2 kJ mol−1 ,
3N2 H4 = 4(1 − X)NH3 + (1 + 2X)N2 + 6XH2 , X=
(4)
2[H2 ] × 100%. 2[H2 ] + 3[NH3 ]
(5)
Generally, the H2 selectivity of a catalyst is equal to the percent of decomposition of the intermediate ammonia at high temperatures. Hence, the H2 selectivity of catalyst, X, is also used to evaluate the extent of ammonia decomposition according to reaction (3). 2.3.2. NH3 decomposition NH3 decomposition was performed over Ni/SiO2 , Pd/SiO2 and Pt/SiO2 . The reaction conditions were the same as for hydrazine decomposition mentioned above, except that the feedstock was pure NH3 maintained at a feeding rate of 30 ml/min.
N2H4 conversion (%)
(3)
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Fig. 2. N2 H4 conversions and H2 selectivities over different catalysts: (, ) Fe/SiO2 (8.0 wt%) and (◦, •) Ru/SiO2 (8.0 wt%). Hollow symbols: N2 H4 conversions. Solid symbols: H2 selectivities.
decompose readily due to kinetic limitations when there is no catalyst existing.
3. Results and discussion 3.2. Performances of Fe/SiO2 and Ru/SiO2 catalysts 3.1. Result of N2 H4 thermal decomposition
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Fig. 1 illustrates the result of hydrazine thermal decomposition without any catalysts. Hydrazine decomposed slightly at 170 ◦ C and rose to 100% conversion at 250 ◦ C. Within the experimental temperature range, only very small amount of hydrogen was detected. Even at as high as 600 ◦ C, the H2 selectivity was still less than 5%. This indicates that thermal decomposition of hydrazine dominantly follows reaction (2) to produce ammonia and nitrogen. Although the produced NH3 would decompose into H2 and N2 subsequently at higher temperatures according to the thermodynamic calculations, Fig. 1 shows that it is considerably stable within the experimental temperature range. This demonstrates that the produced ammonia would not
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Fig. 1. Results of hydrazine thermal decomposition: () N2 H4 conversion, (◦) H2 selectivity.
The results of reaction performance over Fe/SiO2 (8.0 wt%) and Ru/SiO2 (8.0 wt%) are shown in Fig. 2. It can be seen that Fe/SiO2 is not very efficient for hydrazine decomposition, and hydrazine conversion is only 6.4% at 30 ◦ C. On the contrary, Ru/SiO2 is much more active for hydrazine decomposition, giving a 100% conversion within the experimental temperature range. Below 300 ◦ C, both Fe/SiO2 and Ru/SiO2 do not produce apparent amount of H2 , indicating that N2 H4 decomposes mainly according to reaction (2) over these two catalysts. However, the reaction path does not vary with the activity of the catalyst, no matter whether it is the inactive iron or the very active ruthenium catalyst. Elemental iron and ruthenium belong to the same group of the periodic table, thus their similar chemical characteristics might lead to the same reaction route of hydrazine decomposition. As the reaction temperature was increased to higher than 300 ◦ C, the H2 selectivity gradually rose and finally leveled off at 100% at 450 ◦ C over the Ru/SiO2 , and at 600 ◦ C over the Fe/SiO2 . As has been mentioned in the experimental section, the produced ammonia decomposed into hydrogen and nitrogen at high temperatures, so the H2 selectivity reflects the extent of ammonia decomposition in this case. Ruthenium catalysts are known to be very efficient for ammonia synthesis/decomposition [15,16]. So it is no wonder that the Ru/SiO2 exhibited very high H2 selectivity at high temperatures. Also, iron catalysts are traditional for ammonia synthesis. As we know, iron catalysts for industrial applications are comprised of many important additives for improving their activities. Hence, the simple catalyst Fe/SiO2 might not be a very good catalyst for ammonia cracking, and would not readily give high H2 selectivity when compared with the Ru catalyst at high temperatures.
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Fig. 3. N2 H4 conversions and H2 selectivities over different catalysts: (, ) Co/SiO2 (8.0 wt%), (◦, •) Rh/SiO2 (8.0 wt%) and (, ) Ir/SiO2 (8.0 wt%). Hollow symbols: N2 H4 conversions. Solid symbols: H2 selectivities.
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Fig. 4. N2 H4 conversions over different catalysts: () Ni/SiO2 (12.0 wt%), (◦) Pd/SiO2 (12.0 wt%) and () Pt/SiO2 (12.0 wt%).
3.3. Performances of Co/SiO2 , Rh/SiO2 , and Ir/SiO2 catalysts
100
3.4. Performances of Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 catalysts Fig. 4 presents the hydrazine conversions over Ni/SiO2 (12.0 wt%), Pd/SiO2 (12.0 wt%), and Pt/SiO2 (12.0 wt%)
H2 selectivity (%)
80
Fig. 3 shows that the hydrazine conversions over Co/SiO2 (8.0 wt%), Rh/SiO2 (8.0 wt%), and Ir/SiO2 (8.0 wt%) are all 100% within the experimental temperature range. Apparently, catalysts of the metals of this group are so active that their activities cannot be discriminated under these evaluation conditions. The results of H2 selectivity are also shown in Fig. 3. These three catalysts clearly did not produce much H2 at low temperatures (< 300 ◦ C). Similar to Fe/SiO2 and Ru/SiO2 , the reaction paths over them were mainly reaction (2) for form ammonia and nitrogen. When the reaction temperature was increased to 300 ◦ C and higher, the produced ammonia disassociated further into hydrogen and nitrogen. The Rh/SiO2 was the most active among these three catalysts, showing a complete decomposition of ammonia at 450 ◦ C. The results of this series of catalysts as well as that of the Ru/SiO2 discussed above suggest that CO-free H2 can be produced if hydrazine decomposes at high enough temperatures over these catalysts. It is known that when hydrazine decomposes in thrusters as a propellant, the temperature of the catalyst bed usually rises to higher than 700 ◦ C [12,17,18]. Calculations have shown that the temperature of the decomposition products can be 604 ◦ C when liquid hydrazine decomposes into H2 and N2 under adiabatic conditions. Hence, by considering the results of the present work, we can conclude that producing CO-free H2 by catalytic decomposition of hydrazine under autothermal conditions is highly feasible.
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Fig. 5. H2 selectivities of different catalysts: () Ni/SiO2 (12.0 wt%), (◦) Pd/SiO2 (12.0 wt%) and () Pt/SiO2 (12.0 wt%).
catalysts. Generally, these three catalysts were less active than the other two series of catalysts just discussed above. They catalyzed the decomposition of hydrazine mildly or even weakly. The hydrazine conversions were 41.7%, 7.2%, and 9.6%, respectively for Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 at 30 ◦ C. The nickel catalyst was the most active among these catalysts. The H2 selectivities are shown in Fig. 5. Obviously, they are different from those discussed above. At low temperatures (< 200 ◦ C), the hydrazine decomposed over these three catalysts with very high H2 selectivities. Especially between 30 and 80 ◦ C, the selectivities reached to higher than 90%. The value is 95.4% for Ni/SiO2 at 40 ◦ C, 96.8% for Pd/SiO2 at 80 ◦ C, and 97.4% for Pt/SiO2 at 50 ◦ C, respectively. It is known that nickel, palladium and platinum catalysts possess outstanding ability in activating H species in many hydrogen-involved reactions. This might be very helpful to deprive hydrazine of its hydrogen atoms that would recombine further into H2 and escape from the surfaces of
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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Fig. 6. NH3 decomposition over different catalysts: () Ni/SiO2 (12.0 wt%), (◦) Pd/SiO2 (12.0 wt%) and () Pt/SiO2 (12.0 wt%).
the catalysts. An investigation has found that H–N cleavage was preferential to N–N as N2 H4 that adsorbed on Pt(1 1 1), and this is attributed to the H–Pt(1 1 1) bond being stronger than the N–Pt(1 1 1) bond [19]. Another interesting phenomenon is that the H2 selectivity is very sensitive to the reaction temperature. The profiles of H2 selectivity exhibited volcano shapes. Take Pd/SiO2 as an example. Its H2 selectivity increased with the reaction temperature in the range of 30–80 ◦ C. In this case, reaction (1) is preferred to reaction (2). After reaching the maximum, the H2 selectivity decreased quickly when the reaction temperature was increased further. This indicates that probably two reaction paths are proceeding and competing during hydrazine decomposition. In the range of 80–200 ◦ C, a higher temperature is disadvantageous to reaction (1). For the Ni/SiO2 and Pt/SiO2 , the H2 selectivities decreased to less than 5% at 200 ◦ C. As the reaction temperature was increased further from 200 to 650 ◦ C, Pt/SiO2 did not exhibit very high ability to decompose the produced ammonia, whereas Ni/SiO2 showed a better activity. The behavior of Pd/SiO2 seems to be unique among the three catalysts. Unlike Ni/SiO2 and Pt/SiO2 , its H2 selectivity remained at 31.3% even at 300 ◦ C. This should not be attributed to the further dissociation of ammonia produced over Pd/SiO2 . The results of NH3 decomposition (shown in Fig. 6) confirmed this supposition. It can be found from Fig. 6 that Pd/SiO2 and Pt/SiO2 were also inactive for the dissociation of ammonia, and below 450 ◦ C, ammonia did not decompose over them to any larger extent. Thus, the high H2 selectivity at 200–450 ◦ C in hydrazine decomposition over Pd/SiO2 should be attributed to the co-proceeding of reactions (1) and (2), but not to ammonia decomposition. From all the results discussed above, one can notice that all group VIII catalysts are rather active for catalytic decomposition of hydrazine. In general, their activities and H2 selectivities depend on the sub-group characteristics of group
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Temperature (oC) Fig. 7. N2 H4 conversions over different catalysts: () Ni/SiO2 (29.0 wt%), (◦) Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), () Ni/SiO2 (12.0 wt%).
VIII. A volcanic curve of the activity exists as the sub-group goes from the left to the right of group VIII. Cobalt, rhodium, and iridium catalysts are all effective for hydrazine decomposition, whereas nickel, palladium, and platinum catalysts are not so active as the other two sub-groups of metals. Meanwhile, they differ from the others in that their H2 selectivities are very high at low temperatures. There is a popular understanding in catalysis that only a suitable interaction between the catalyst and the reactants can yield a high catalytic activity, while too strong or too weak interaction would yield a low activity. As the heat of formation of the respective metal nitride decreases from the left to the right of the periodic table, the strength of the metal–N interaction on the surface of the catalyst decreases as well [20,21]. On the other hand, the metal–H interaction also decreases almost linearly as the d band orbital of the metal become more occupied [22]. Hence, the trends of catalytic activity and selectivity in hydrazine decomposition over these catalysts might be related to the regular variation of chemisorption energy of hydrogen and nitrogen on the metals. 3.5. Influences of loading, reduction temperature, and support on nickel catalysts Since Ni/SiO2 , Pd/SiO2 and Pt/SiO2 exhibited distinguished H2 selectivities at low temperatures, and by considering a balance between activity and cost of the catalyst, nickel catalysts were further investigated in this paper. The influence of nickel loading on Ni/SiO2 was investigated and the results are presented in Figs. 7 and 8. Apparently, as shown in Fig. 7, a catalyst with a higher nickel loading was more active for the reaction. As the nickel loading was increased from 12.0 to 21.0 and 29.0 wt%, the hydrazine conversion at 30 ◦ C also rose from 41.6% to 68.7%, and 76.3%, respectively. However, no apparent influence of
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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Fig. 8. H2 selectivities of different catalysts: () Ni/SiO2 (29.0 wt%), (◦) Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), () Ni/SiO2 (12.0 wt%).
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Fig. 10. H2 selectivities of Ni/SiO2 (21.0 wt%) reduced at () 350 ◦ C, (◦) 500 ◦ C, and () 700 ◦ C, respectively.
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Fig. 9. Hydrazine conversions over Ni/SiO2 (21.0 wt%) reduced at () 350 ◦ C, (◦) 500 ◦ C, and () 700 ◦ C, respectively.
Fig. 11. N2 H4 conversions over different catalysts: () Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), (◦) Ni/Al2 O3 (21.0 wt%) and () Ni/AC (21.0 wt%).
the loading on the H2 selectivities at low or high temperatures was observed (shown in Fig. 8). These results demonstrate that the increase in nickel loading provides more active sites for the catalytic process, but this is merely a quantitative variation and does not produce any essential impact on the reaction path. The influence of reduction temperature on Ni/SiO2 (21.0 wt%) is shown in Figs. 9 and 10. There were apparent effects on the catalytic activities as well as on the H2 selectivities by different reduction temperatures 350, 500, and 700 ◦ C). Under these conditions, the catalyst reduced at 500 ◦ C was the most active at 30 ◦ C, whilst the highest reduction temperature led to the lowest activity. The turnover frequencies (TOF) of these catalysts were calculated. At 30 ◦ C, the TOF increased from 0.04 to 0.13 and 0.40 s−1 as the reduction temperature was increased from
350 to 500 and 700 ◦ C, respectively. So, a higher reduction temperature will lead to a higher TOF. The low conversion of hydrazine over Ni/SiO2 reduced at 700 ◦ C might be attributed to the fewer catalytic active sites due to nickel sintering at such a high temperature. In addition, with an increase in the reduction temperature, strong metal–support interaction (SMSI) would be enhanced and the properties of the nickel species would be affected. The phenomenon that the H2 selectivity within 40–80 ◦ C increased with the reduction temperature might also be related to the change in the extent of SMSI. Hence, the treatment or preparation method of the nickel catalysts also played an important role in the catalytic performance. Fig. 11 compares the activities of the nickel catalysts on different supports. N2 H4 conversions over Ni/AC (21.0 wt%) and Ni/Al2 O3 (21.0 wt%) were merely 13.7%
M. Zheng et al. / International Journal of Hydrogen Energy 30 (2005) 1081 – 1089
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200 300 400 Temperature (oC)
500
600
Fig. 12. H2 selectivities of different catalysts: () Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C), (◦) Ni/Al2 O3 (21.0 wt%) and () Ni/AC (21.0 wt%).
0.45 0.40 0.35 TCD signal
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(2)
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(3) (4)
0.00 0
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Fig. 13. TPR profiles of different catalysts: (1) NiO (from calcination of Ni(NO3 )2 · 6H2 O at 450 ◦ C) (2) Ni/SiO2 (21.0 wt%), (3) Ni/AC (21.0 wt%), and (4) Ni/Al2 O3 (21.0 wt%).
and 17.1%, respectively, at 30 ◦ C, and these values were ca. 50% lower than that of Ni/SiO2 (21.0 wt%). It is no doubt that the interaction between the nickel and the supports will result in different catalytic activities. Similarly, the supports also affect the selectivities. As shown in Fig. 12, the H2 selectivities of Ni/Al2 O3 and Ni/AC were 83.5% and 65.7%, respectively, at 30 ◦ C. These values were apparently lower than the 93.5% H2 selectivity of the Ni/SiO2 . This provides further evidence that SMSI plays an important role in the H2 selectivity. The TPR profiles of the catalysts (Fig. 13) reflect to some extent the interaction between nickel and the supports. They might be helpful for understanding the difference in catalytic activity as well as H2 selectivity among these catalysts at low
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temperatures. From Fig. 13, it can be seen that the reduction peak temperature of Ni/SiO2 is 340 ◦ C, and is basically the same as that of 355 ◦ C of unsupported NiO, indicating that the interaction between nickel and silica was quite weak. In contrast to this weak interaction with silica, nickel interacted with alumina more strongly. The reduction peak temperature of Ni/Al2 O3 was 540 ◦ C, which was 200 ◦ C higher than that of Ni/SiO2 . Thus, the SMSI of Ni/Al2 O3 will affect the chemical properties of nickel more significantly and accounts for its low activity or selectivity. As for Ni/AC, it was a bit different. There are three reduction peaks on the profile, i.e. peaks at 250, 395, and 500 ◦ C, respectively. The former two are attributed to nickel reduction, whereas the third peak should be attributed to methane production. This was validated by a mass spectrograph taken during the reduction. Accordingly, it can be inferred that there are at least two kinds of nickel species existing on the AC support. One kind of nickel can be reduced even easier than unsupported NiO (the reduction temperature is 355 ◦ C), and the other interacts with AC more strongly and needs a higher reduction temperature. It seems that a weak interaction between nickel and AC would be advantageous for yielding high activity and H2 selectivity. However, the result of H2 chemisorption shows that this is not the case. From the H2 chemisorption experiments it was found that the nickel dispersion over AC was the lowest among the three catalysts. It was merely 0.3% over AC, whereas the nickel dispersions over SiO2 and Al2 O3 were 2.7% and 4.4%, respectively. That is to say, the diameter of the nickel particles (343 nm) over AC was much larger than those of Ni/SiO2 (37 nm) and Ni/Al2 O3 (23 nm). No doubt, this nickel aggregation over AC should be attributed to the too weak interaction between nickel and AC support. In addition, the carbon loss during the reduction would also enhance the aggregation of the nickel metal. Hence, a too weak interaction between nickel and AC is disadvantageous and will lead to a low catalytic activity. The results of H2 -TPD experiments are displayed in Fig. 14. The profiles show that H2 desorbed at different temperatures on Ni/SiO2 , Ni/Al2 O3 , and Ni/AC. On Ni/SiO2 , H2 desorption commenced at the beginning of heating, whereas on Ni/Al2 O3 it began at 100 ◦ C. This indicates that H2 desorbed more facilely at the surface sites of Ni/SiO2 than at those of Ni/Al2 O3 . H2 desorption on Ni/AC also began at the start of applying the desorption temperature, however, its amount was considerably smaller than those of the other two due to low nickel dispersion. Therefore, the surface characteristics of Ni/SiO2 were the best among the three catalysts for the reaction of hydrogen-production, and favored high H2 selectivity. The results and discussion given above show that some important factors of the nickel catalysts must be considered in order to produce high activity and H2 selectivity, i.e. kind of support, reduction temperature, and nickel loading, as well as procedures capable of adjusting the interaction between the nickel and the support.
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0.007 0.006 TCD signal
0.005 0.004
(3)
0.003 0.002
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0.001
(1)
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Fig. 14. H2 -TPD profiles of the catalysts: (1) Ni/AC (21.0 wt%), (2) Ni/SiO2 (21.0 wt%), and (3) Ni/Al2 O3 (21.0 wt%).
100
Percent (%)
80 hydrazine conversion H2 selectivity
60
N2 with more than 90% selectivity at 30–40 ◦ C. The preparation method and the support of the nickel catalysts played important roles on the catalytic activity and H2 selectivity. These findings suggest that Ni/SiO2 might be a good candidate for catalytic decomposition of hydrazine at rather mild temperatures for producing CO-free H2 for PEM fuel cells. In addition, it was found that all catalysts of group VIII metals were efficient for hydrazine decomposition and were able to produce H2 at high temperatures (> 350 ◦ C). Especially, over Ru, Co, Rh, and Ir catalysts, hydrazine decomposed readily into H2 and N2 at high temperatures. Theoretically, production of H2 can also be realized by hydrazine decomposition over these catalysts under autothermal conditions.
Acknowledgements This work was supported by a grant from the National Natural Science Foundation of China (NSFC) for Outstanding Youth (No. 20325620) and a grant from NSFC for Youth (No. 20303017). The authors gratefully acknowledge helpful discussions with Professor Dongbai Liang of State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Chinese Sciences.
40 20 0
References 0
2
4
6
8
10 12 14 16 18 20 22 24 Time (h)
Fig. 15. N2 H4 conversion and H2 selectivity over Ni/SiO2 at 50 ◦ C (21.0 wt%, reduced at 500 ◦ C).
3.6. Life test of Ni/SiO2 The life test of Ni/SiO2 (21.0 wt%, reduced at 500 ◦ C) was conducted at 50 ◦ C (GHSV = 25,500 h−1 ) and the result is shown Fig. 15. During the 24 h reaction, hydrazine conversion and H2 selectivity maintained steadily at 100% and more than 80%, respectively. These suggest that Ni/SiO2 possesses a high catalytic activity and a reasonable H2 selectivity for hydrazine decomposition.
4. Conclusions Hydrazine decomposition was investigated in a micro reactor for producing CO-free H2 for PEM fuel cells. It is found that Ni/SiO2 , Pd/SiO2 , and Pt/SiO2 all catalyzed hydrazine decomposition with a high H2 selectivity at low temperatures. N2 H4 decomposed over Ni/SiO2 into H2 and
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