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Preparation of Ru–Cs catalyst and its application on hydrogen production by ammonia decomposition
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 3 2 3 3 e3 2 4 0
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Preparation of RueCs catalyst and its application on hydrogen production by ammonia decomposition De-Chin Huang a, Cheng-Hong Jiang a, Feng-Jiin Liu a, Yu-Chi Cheng a, Yen-Cho Chen b, Kan-Lin Hsueh a,b,c,* a
National United University (NUU), Department of Chemical Engineering, Miaoli, Taiwan National United University (NUU), Department of Energy and Resources, Miaoli, Taiwan c Industrial Technology Research Institute (ITRI), Energy and Environment Laboratory, Chutung, Taiwan b
article info
abstract
Article history:
Ammonia can be a hydrogen source for many applications including fuel cells. Using Ru or
Received 19 May 2012
CseRu as the catalyst, hydrogen is generated from ammonia by decomposition reaction.
Received in revised form
These catalysts are deposited on carbon powder by either chemical reduction or precipi-
27 October 2012
tation method in this study. Different carbon powder pre-treatment solutions and catalyst
Accepted 27 October 2012
deposition conditions are evaluated. Nitric acid pre-treatment followed by precipitation at
Available online 26 January 2013
pH of 6 produces the highest catalyst loading from solution with given concentration of catalyst precursor. Hydrogen generation rate is measured at different catalyst composi-
Keywords:
tions, ammonia inlet flow rates, decomposition temperatures, amount of catalyst packing,
Hydrogen generation
and ratio of Cs/Ru. The optimal condition for the ammonia decomposition reaction is
Ru catalyst
Cs/Ru weight ratio at 3, ammonia inlet flow at 6 ml min1, reaction temperature at 400 C.
Cs promoter
At this condition, the ammonia conversion rate reaches 90% and hydrogen generation rate
Ammonia decomposition
reaches 29.8 mmol/min-gcat. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is considered as a clean and environmental friendly fuel for the future. It can be used as the fuel for hydrogen feedvehicles or for fuel cell to produce the electricity. Water is the only product without any emission of carbon dioxide. Hydrogen can also be produced from many resources, such as reforming of fossil fuels [1e3], water electrolysis, biological processes, thermal decomposition, and biomass gasification. However, hydrogen generated from reforming, thermal decomposition, and biological processes produce CO2. To control and to reduce the emission of CO2, it requires an effective technology of separation and capture of CO2 [4,5].
Hydrogen storage still remains a technical barrier to be solved. Present hydrogen storage technologies are high-pressure tank, cryogenic storage, metal hydride, physic absorption, and chemical absorption. The hydrogen storage density is far less than 6%, a border line specified by DOE for transportation application. Although hydrogen has many advantages, it has few barriers to be overcome before it can be used in our daily life. The hydrogen distribution and transportation infrastructure is not ready and is costly. Low flash point and explosion danger are hydrogen safety issues to be considered. Ammonia was proposed as a promising energy carrier instead of hydrogen [6]. Zamfirescu and Dincer [7] compared various
* Corresponding author. National United University, Department of Energy and Resources, 1, Lien-Da Rd., Kung-Ching Li, Miaoli 36003, Taiwan. Tel.: þ886 958 647 100; fax: þ886 37 381237. E-mail address: [email protected] (K.-L. Hsueh). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.105
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properties of several fuels, such as gasoline, natural gas, liquid petroleum gas, methanol, hydrogen, and ammonia. At storage pressure of 10 bar, gravimetric energy densities of gasoline and ammonia are 46.7 and 22.5 MJ kg1, respectively. Their volumetric energy densities are 34.4 and 13.6, MJ L3, respectively. Ammonia has much higher energy density than compressed hydrogen and liquefied hydrogen. In addition, hydrogen generated from ammonia (NH3) is zero CO2 emission. Ammonia was proposed as alternate energy carrier instead of hydrogen [7,8]. Ammonia is commercially produced from hydrogen and atmospheric nitrogen by HabereBosch process. Producing ammonia from hydrogen and then decomposing ammonia to generate hydrogen is not cost effective and the overall energy efficiency is low. However, ammonia is attractive to portable application where easy storage and energy density are primary concern. For portable application, reactor miniaturization and operation at ambient temperature are critical issues. Ammonia decomposition is a reversible reaction as given by equation (1). Calculated from web site data [9], the equilibrium constant of equation (1) is higher at lower temperature. High reaction temperature favors ammonia decomposition. To completely decompose ammonia at fast reaction rate, low reaction temperature is a technical issue to be overcome. 92:5 kJ þ 2NH3 )/3H2 þ N2
contact angle meter (CA), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction analyzer (XRD).
2.
Experimental
The experiment was divided into three consecutive parts, the carbon powder pre-treatment, catalyst deposition, and ammonia decomposition reaction. Entire experimental procedure is schematic presented on Fig. 1. The carbon powder was used as the substrate. It was pre-treated with alcohols or acids. Various carbon powder pre-treatment conditions were evaluated first. Then Ru/C and CseRu/C catalysts were deposited on the pre-treated carbon by two different processes. Finally, the catalyst was placed in a tubular reactor and the hydrogen production rate was measured. The experimental procedure and conditions were described below.
2.1.
Pre-treatment of carbon power
Carbon powder (Cobot, XC-72R) is used as the substrate for catalyst. It has surface area around 250 m2 g1 and electrical conductivity 2.77 S cm1. Carbon powder was mixed with pre-
(1)
Two major steps involved in this reaction are the cleavage of the NeH bond of adsorbed NH3 and the recombinative desorption of nitrogen atoms. Yin et al. made a mini-review on ammonia decomposition reaction [9]. Many metals, alloys, and compounds of noble metal characters have been tested for ammonia decomposition. These include Fe, Ni, Ir, Pt, Rh, Pd, Ru, and their alloys and compounds. The Ru based catalyst is one of the promising catalysts for ammonia decomposition due to its high reactivity [4,5,11,12]. Yin et al. [10] found that 15% of Ru is the optimal Ru loading for high conversion efficiency of ammonia. The ammonia conversion efficiency is also affected by the catalyst substrates such as active carbon, graphite, carbon nano-tube, aluminum oxide, titanium oxide, magnesium oxides, etc [1e4,10]. Ru on carbon nano-tube has the best catalytic activity. Good substrate not only has good catalyst dispersion ability but also electronic conductive. Conductive substrate makes the electron transfer easy between catalyst and promoter. Promoter accelerates the recombination desorption of nitrogen atoms. Alkali and alkali earth metals such as K, Na, Li, Ba, Ca, Cs, and La were studied as the promoter [5,11,13]. Ru on carbon with Cs as the promoter, has high ammonia decomposition rate even at temperature below 400 C [11]. This is very attractive for proton exchange membrane fuel cell application. In this study, Ru catalyst was prepared by reduction and precipitation method. The effect of solution pH and reducing agent concentration on the Ru deposition was evaluated. Cesium promoter was deposited on the Ru catalyst. The decomposition rate of ammonia as well as the hydrogen generation rate was measured at different RueCs catalyst compositions, ammonia inlet flows, decomposition temperatures, and amount of catalyst packing. Catalyst was characterized by Fourier transform infrared spectroscopy (FTIR),
Pre-treatment of carbon powder (XC-72R)
Characterization of carbon powder (FTIR, contact angle)
Catalyst preparation -reduction method
Catalyst preparation - precipitation method
Ru Catalyst
Deposition of Cs promotor
Characterization of catalyst (EDS, SEM, XRD Mapping)
Hydrogen generation from ammonium decomposition Fig. 1 e Experimental procedure.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 3 2 3 3 e3 2 4 0
treatment solution at 80 C for 12 h under reflux condition. Different solutions were tested. They are water, methanol (Echo Chem. CO., 99.5%), ethanol (Echo Chem. Co., 99.9%), sulfuric acid (Shimakyu, 98%), hydrochloric acid (Shimakyu, 32%), hydrogen peroxide (Shimakyu, 35%), and nitric acid (Shimakyu, 69%). The pH value of the mixture was adjusted to 7.0 by ammonia after the mixture was cold. The carbon powder was centrifugation separated, D.I. water rinsed, and vacuum oven dried at 60 C. The powder was grinded by a mortar to fine powder and they are ready for catalyst deposition. The carbon powder was examined by FTIR and contact angle measurement. For FTIR measurement, a 0.1 mg carbon powder was mixed with 0.1 g KBr by mortar. This mixture was then pressed into a pellet. Measurement was carried out with an FTIR spectrometer (JASCO FT/IT-470). For contact angle measurement, carbon powder was adhesive on a carbon tape. Excessive powder was blow away by air. A water drop was rest on the carbon powder surface. After 3 min, the contact angle was measured by contact angle analysis system (APPR FTA-125).
2.2.
Preparation of RueCs catalyst
Deposition of Ru on carbon powder was prepared either by chemical reduction or by precipitation. In both methods, carbon powder (XC-72) was pre-treated with acid first. One gram of the pre-treated carbon powder was then mixed with de-ionized water. Two precursor solutions were prepared. One solution was 5 g of RuCl3 dissolved in 200 ml de-ionized water. The other one was 10 g Ru(NO)(NO3)3 dissolved in 200 ml de-ionized water. Proper amount of these precursor solutions were well mixed with carbon powder suspension. Then, the pH of this suspension solution was adjusted by NaOH solution to desired value. In chemical reduction, reducing agent, such as NaBH4, C6H12O6, CH3OH, or C2H5OH was then added into the carbon suspension solution. This mixture was reflux at 70 C for 6 h and Ru/C catalyst was produced. In the precipitation method, the mixture of carbon and precursor suspension was reflux at 70 C for 12 h without reducing agent. In both methods, the Ru/ C catalyst was centrifugation separated, rinsed with D.I. water, vacuum oven dried at 60 C. Several reduction parameters were investigated in this study. These parameters are: solution pH values (pH ¼ 4, 6, 8, and 10), type of reducing agent, and their concentration. Reducing agents were sodium borohydride (NaBH4, SigmaeAldrich, 99%), methanol (CH3OH, Echo, 99.9%), ethanol (C2H5OH, Echo, 99.5%), and glucose (C6H12O6, Merck, D(þ)glucose). Various amount of cesium promoter was deposited on the Ru catalyst. The weight ratio of RueCs is in the range of Cs/Ru ¼ 0, 1, 2, 3, 4, 5, and 6. Mapping of energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), X-ray diffraction meter (XRD) were used to characterize synthesized catalysts. Amount of Ru in the catalyst was measured by EDS attached within the field effect scanning electron microscopy (FESEM). The RueCs catalyst was prepared from Ru/C catalyst. The Ru/C catalyst powder was pressed into a thin porous sheet. Given amount of the CsNO3 solution was slowly added on the Ru/C sheet. This RueCs sheet was then dried at 100 C. The
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ammonia decomposition efficiency was tested with catalyst containing different Cs/Ru ratios (Cs/Ru ¼ 0, 1, 2, 3, 4, 5, 6).
2.3.
Hydrogen production by ammonia reformer test
Synthesized catalyst was placed into a tubular reactor. The tubular reactor was a stainless steel tube with 1/4 inch ID. Both end of the tube was filled with fiber glass. Catalyst was filled between both ends of fiber glass. The reactor was heated by a cylindrical heater. Ammonia was fed into the reactor through a mass flow controller (flow rate 3, 4.5, 6, and 7.5 ml min1). Various reaction conditions were tested, such as catalyst composition (percentage of Ru, Cs, and carbon), amount of catalyst, flow rate of ammonia, and reactor temperature. Effluent gas from the reactor was bubble through a trap containing 10 vol% sulfuric acid solution. Residue ammonia from the reactor was removed by this trap. The ammonia conversion efficiency and hydrogen production rate were calculated. The flow rate of effluent gas (VEffluent) was measured by a bubble flow meter. Effects of gas temperature and pressure were corrected during data analysis. Concentration of hydrogen in the effluent gas ðCH2 Þ was measured by a gas chromatography (China Gas Chromatography, GC 3000) with an Ultrafast column (Thermo Fisher, Trace UFM-M1). Measuring temperature of TCD, injection port, and oven were at 110 C. The TCD current was set at 50 mA. Operating condition was The hydrogen production rate ðVH2 Þ is the product of VEffluent and CH2 $ðVH2 ¼ VEffluent CH2 Þ The ammonia conversion efficiency is the ratio of consumed ammonia ðVNH3 ;cons Þ to the inlet ammonia ðVNH3 ;inlet Þ. The inlet flow rate of ammonia was read from the mass flow controller. The ammonia consumption rate of ammonia can be calculated from the hydrogen production rate ðVH2 Þ. According to the ammonia decomposition reaction (equation (1)), one mole hydrogen produced consumes 0.67 mol of ammonia.
3.
Results and discussion
3.1.
Effects of acid treatment on carbon properties
In order to increase functional groups on the carbon power surface, the carbon surface was treated with different solutions. Solutions have been tested are nitric acid, sulfuric acid, hydrochloric acid, hydrogen peroxide, ethanol, and methanol. The type of functional groups on the surface was analyzed by Fourier transform infrared spectroscopy (FTIR). Fig. 2 is the FTIR spectra of carbon powder after solution treatment. Legends from 1 to 8 marked on Fig. 2 were the FTIR spectra of carbon after treated by nitric acid, sulfuric acid, hydrochloric acid, hydrogen dioxide, ethanol, methanol, de-ionized water. For comparison, legend 8 was the FTIR spectrum of untreated carbon. Six functional group were detected on the acid treated carbon surface, They are carboxylic group (CeO stretching in 1210e1320 cm1, and C]O stretching in 1710e1780 cm1), alcohol (OH in-plane bending in 1330e1430 cm1 and OH stretching in 3200e3550 cm1), aromatic (C]C bending in 1500e1680 cm1), and alkyl (CeH stretching at 2850 and
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Table 2 e Amount of Ru deposited by reduction method and by precipitation method. Ru precursor Preparation conditions
RuCl3
Ru(NO)(NO3)3
Ru (wt.%)
Ru (wt.%)
5.53 4.84 3.92 4.84 3.13 4.94 4.93 8.61 5.12 4.04
4.49 4.12 2.2 3.92 1.75 1.7 5.12 8.28 4.6 5.35 Ru (wt.%) ¼ 4.32
Reduction
Fig. 2 e FTIR spectra of carbon powder after solution treatment. (1) by nitric acid, (2) by sulfuric acid, (3) by hydrochloric acid, (4) by hydrogen dioxide, (5) by ethanol, (6) by methanol, (7) by de-ionized water, (8) untreated carbon.
2950 cm1). Nitric acid is a strong oxidant. The carbon surface can be oxidized into various functional groups by nitric acid. Carbon surface treated with nitric acid has the most distinguish peaks on FTIR spectra. The C]O bond signals in the wave number 1710e1780 cm1 and a OH (in-plane) bond signals in wave number 1330e1430 cm1 were observed. The function group on the carbon powder is increased after pre-treatment. These functional groups, such as C]O and CeOH are hydrophilic. Carbon powder with hydrophilic surface helps the penetration of metal precursor solution into the pores on carbon surface. The level of hydrophilic surface is evaluated by measuring the contact angle of droplet on the carbon surface. Contact angle measurement results are listed on Table 1. The original carbon powder is hydrophobic surface. The contact angles of surface treated with water or alcohols are similar to the surface of original carbon powder. Water and alcohol treatment do not improve the hydrophobicity of carbon powder surface. Among acidic treatment, nitric acid is the most effective treatment. The contact angle of nitric acid treatment is 45 . It is followed by hydrogen peroxide, hydrochloric acid, and sulfuric acid. From the FTIR spectra and contact angle measurement, result shown that nitric acid is the most effective pre-treatment. In the following study, all the carbon powder was pre-treated with nitric acid.
Table 1 e The contact angle between water and carbon powder after a variety of solution treatment. Pre-treatment solution
Average. contact angle
Original carbon power Water Methanol Ethanol Sulfuric acid Hydrochloric acid Hydrogen peroxide Nitric acid
133 130 130 117 104 101 96 45
NaBH4 (acid) NaBH4 (pH ¼ 7) NaBH4 (pH ¼ 10) C6H12O6 (pH ¼ 10) CH3OH (pH ¼ 10) C2H5OH (pH ¼ 10) Precipitation pH ¼ 4 pH ¼ 6 pH ¼ 8 pH ¼ 10 Commercial 5 wt.% Ru/C(Aldrich)
3.2.
Deposition of Ru and Ru/Cs catalyst
The reactor performance of ammonia decomposition is strongly affected by the amount of Ru metal on the catalyst or the weight percentage of Ru in the catalyst (Ru%). In order to control the Ru% in the catalyst, it is important to find out the relationship between Ru% and deposition conditions. The Ru catalyst can be prepared by chemical reduction and by precipitation. The Ru% prepared from these two processes was evaluated. Table 2 is the list of Ru% prepared from two different precursor (RuCl3 and Ru(NO)(NO3)3) and two deposition processes (Chemical reduction and precipitation). The pH values for each deposition process are also listed on Table 2. They were prepared from the solution containing fixed amount of Ru precursor. Ten weight percent of Ru shall be obtained if all the Ru ions in the solution were deposited on the carbon powder. In the reduction process, different reducing agents were tested as listed on Table 2. For comparison, the solution pH value was adjusted to 10. In the precipitation, the solutions with pH value at 4, 6, 8, and 10 were tested. For a given amount of precursor, catalyst prepared by sodium borohydride (NaBH4) has the most Ru content as compared to other reducing agent. About 55% of the precursor ion was deposited on the carbon powder. In the precipitation method, solution with pH value at 6 produces the highest Ru content. About 86% of the precursor was deposited on the carbon powder. The type of precursor (RuCl3, Ru(NO)(NO3)3) plays a minor role on the Ru%. Catalyst prepared by precipitation method with solution pH at 6 has the most Ru deposited on the carbon powder.
Table 3 e Amount of Ru deposited by precipitation method at pH [ 6. RuCl3
Ru(NO)(NO3)3
Theoretical content
Ru (wt.%)
Ru (wt.%)
5 wt.% 10 wt.% 15 wt.% 20 wt.% 30 wt.%
2.35 8.61 10.66 11.22 18.8
3.38 8.28 11.31 13.69 24.92
Ru precursor
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Table 4 e The catalyst composition used in ammonia reforming test. No.
Ru precursor
Actual content of Ru (wt.%)
C01 C02 C03 C04 C05 C06 C07 C08
RuCl3 RuCl3 RuCl3 RuCl3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3
2.35 8.61 11.22 18.8 3.38 8.28 13.69 24.92
The effect of precursor concentration on the catalyst Ru % was also studied. Both precursors, (Ru(NO)(NO3)3 and RuCl3) were evaluated. Precursor concentration range was from 5% to 30% theoretical content. The solution pH value was controlled at 6 for these experiments. As shown on Table 3, both precursors produce similar result and the Ru % is linearly proportional to the precursor concentration. Less Ru was deposited from the RuCl3 solution than from the Ru(NO)(NO3)3 solution (Table 4).
3.3. Morphology and crystal structure of Ru and Ru/Cs catalyst Catalyst with different Cs/Ru ratio (from 0 to 6) was prepared. The CsNO3 solution was added into the Ru/C catalyst and it
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was then dried at 100 C. Both Ru/C and CsRu/C catalyst morphology was examined as given on Fig. 3a for Ru/C and Fig. 3b for CsRu/C. The size of Cs particle is in the order of 10 nm. From the element mapping (Fig. 3c for Ru and Fig. 3d for Cs), both Ru and Cs were uniformly dispersed on the carbon powder. The crystal structure of Ru/C and CsRu/C catalyst was examined by XRD. No obvious crystal signal was observed for the Ru/C catalyst prepared from either chemical reduction or precipitation methods as shown on Fig. 4. Sample 4 and 5 were the Ru/C catalyst prepared from RuCl3 and Ru(NO)(NO3)3, respectively. Sample 6 and 7 were the CsRu/C catalyst prepared from Ru/C catalyst 4 and 5, respectively. The XRD result of commercial Ru/C catalyst (5% Ru Aldrich) also has no crystal signal detected. One possible reason is that the Ru content is too low to have significant XRD signal or the Ru deposit was in amorphous form. However, the XRD of CsRu/C catalyst (Cs/Ru ¼ 4) observes clear Cs signals at 19.9 , 28.3 , 34.8 , 40.5 , 45.4 , and 50 . Possible phases at these angles are (110), (004), (103), (112), (105), and (311).
3.4. Effects of Ru contents on ammonia decomposition reaction Ammonia decomposition is endothermic reaction. High temperature favors decomposition. At 400 C, equilibrium conversion reaches 99.1% [10]. Calculated from web site data
Fig. 3 e Pictures of SEM picture and element mapping. (a) SEM of Ru/C, (b) SEM of CsRu/C, (c) element mapping of Ru, (d) element mapping of Cs.
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Fig. 4 e The XRD analysis results of RueC catalyst with four times cesium promoter (Cs/Ru [ 4 (wt. ratio)). Legend 4 and 5 were the Ru/C catalyst prepared from RuCl3 and Ru(NO)(NO3)3, respectively. Legend 6 and 7 were the CsRu/C catalyst prepared from Ru/C catalyst 4 and 5, respectively.
[9], the equilibrium constant at 300, 400, 500, and 600 K, are 2.30 102, 6.10 103, 6.90 104, 4.40 105. From these data, the equilibrium constant of ammonia decomposition (Keq) was calculated as given by equation (2). Keq ¼ 6 105 e½4505=T ¼ H2 3 ½N2 ½NH3 2
(2)
In our reaction temperature range (between 375 and 500 C), the Keq are 5.74 102 (at 375 C) and 1.77 103 (at 500 C). At equilibrium, the conversion efficiency of ammonia decomposition reaction is nearly 100%. The ammonia conversion efficiency and hydrogen production rate with different Ru catalyst loading were given on Fig. 5. These tests were carried out at ammonia inlet flow of 6 ml min1, 0.012 g Ru catalyst packing, reaction temperature at 400 C and at 450 C. Three types of catalyst were examined, catalyst prepared from RuCl3, Ru(NO)(NO3)3, and commercial grade Ru catalyst. Gas sample was taken after 2.5 reaction time. The ammonia GHSV was at 30,000 ml h1 g1 cat. In general, higher reaction temperature produces higher conversion efficiency ð3 NH3 Þ and hydrogen production rate ðVH2 Þ. Higher Ru loading also has higher 3 NH3 and VH2 . The improvement of 3 NH3 and VH2 becomes minor for Ru contain higher than 15%. This is due to high Ru catalyst loading results a short catalyst packing length (constant catalyst packing 0.012 g and ammonia inlet flow rate). At a given reaction temperature and Ru loading, catalyst prepared from Ru(NO)(NO3)3, has the highest 3 NH3 and VH2 . The catalyst prepared from RuCl3 may have residual Cl on the catalyst. It reacts with NH3 and produces NH4Cl precipitation on the catalyst surface. This reduces the 3 NH3 and VH2 for catalyst prepared from RuCl3.
3.5.
Fig. 5 e Effects of Ru content on (a) ammonia conversion rate and (b) hydrogen production rate at different reaction temperatures.
examined at reaction temperature of 375 C, 400 C, 425 C and 450 C. Result is plotted on Fig. 6. High ammonia flow rate enhance the mass transfer of ammonia to the catalyst surface and it results a high hydrogen production rate. This is especially true for high reaction temperature i.e., 450 C than for low reaction temperature, i.e., 375 C. At a given flow rate, the decomposition reaction of ammonia is shifted from kinetic
Effects of ammonia inlet flow
For a given amount of catalyst packing (0.024 g), the effect of ammonia flow rate on the hydrogen production rate is
Fig. 6 e Effects of ammonia inlet flow on hydrogen production rate at different reaction temperatures. Loading of catalyst was 0.024 g.
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3.5 Catalyst loading: 0.0024 g
Logarithm of H Formation Rate m mol/min-g
3 2.5 2 1.5
Inlet flow rate, ml/min 3.0
1
4.5 6.0
0.5 7.5
0 0.0013
0.0014
0.0015
0.0016
Fig. 8 e Effects of Cs/Ru ratio on hydrogen production rate at 0.012 g catalyst loading, 6 ml minL1 ammonia flow rate, and GHSV [ 30,000 ml hL1 gL1.
Temperature, 1/T (K )
Fig. 7 e Effects of reaction temperature on hydrogen production rate at different ammonia inlet flow rates. Loading of catalyst was 0.024 g.
controlled to mass transfer controlled as the reaction temperature increased.
3.6.
Effect of reaction temperature
The hydrogen production rate is also a function of reaction temperature as shown on Fig. 7. The hydrogen production rate increased as the reaction temperature (T ) increased. According to Arrhenius equation, the plot of logarithm rate against 1/ T shall yield a straight line as given on equation (3). The activation energy of ammonia decomposition reaction (Ea) can be calculated from the slope of this line. lnðrateÞ ¼ lnðko Þ
Ea 1 RT
(3)
The value of Ea is in the range between 54 and 72 kJ mol1 in the ammonia inlet flow rate of 3.0e7.5 ml min1 Table 5 lists the activation energy of various catalysts found in the literature. The activation energy may be substrate dependence. Catalyst Ru on oxide substrate has activation energy around 80e90 kJ mol1 (Table 5). Catalyst Ru on conductive substrate, such as carbon in our case, the activation energy is around 54e72 kJ mol1. Conductive substrate may provide an electronic path for the redox reaction taking place on the catalyst surface.
3.7. Effect of catalyst Ru/Cs ratio on hydrogen production rate At a given catalyst packing (0.012 g), ammonia inlet flow (6 ml min1), the hydrogen production rate of catalyst with different weight ratio of cesium promoter (Cs/Ru ¼ 0e6) is plotted on Fig. 8. A maximum production rate of hydrogen is at Cs/Ru ¼ 3 for reaction temperature at 375 C and 400 C. At his Cs/Ru ratio, the ammonia conversion efficiency reaches 90% and hydrogen production rate reaches 29.8 mmol/min-gcat. at 400 C. The Cs is acting a promoter, the hydrogen production rate is higher for catalyst containing higher Cs in the region Cs/Ru 3. The hydrogen production rate is lower for catalyst containing higher Cs in the region Cs/Ru 3. In this region, excessive amount of Cs converts the Ru surface and the hydrogen production rate is reduced.
3.8.
Short-term durability test of catalysts
A 24 h continue running of ammonia decomposition was carried out under reaction temperature at 400 C and ammonia flow rate at 6 ml min1. Three catalysts were tested. They are
Table 5 e The activation energy of various catalysts for ammonia decomposition reaction. Catalyst
Activation energy, kJ mole1
Reference
Ru/C Ru/Al2O3 Ru/Al2O3 Ru/SiO2 Ir Ir Mo Fe Fe4N
54e72 88 80 81.6 79.5 131 176 16 34
This work [14] [14] [14] [14] [14] [15] [16] [16]
Fig. 9 e Hydrogen production rate over a 24 h running time at 0.012 g catalyst loading, 6 ml minL1 ammonia flow rate, 400 C, and GHSV [ 30,000 ml hL1 gL1.
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Ru:Cs ratio of 1:0, 1:3, and 1:4 CseRu/C catalyst. Results were shown on Fig. 9. The Ru/C catalyst has the lowest performance among catalyst under testing. The average conversion efficiency of Ru/C over 24 h was about 37% and hydrogen production rate was about 12 m mol min1 g1 cat. The ammonia conversion rate of CseRu/C (1:3) was about 90% and the hydrogen production rate was 29.8 m mol min1 g1 cat. After 23 h reaction, the ammonia conversion rate dropped 3.1% and the hydrogen production rate was dropped 0.74 m mol min1 g1 cat. The ammonia conversion rate of CseRu/C (1:4) was about 76% and the hydrogen production rate was 25.3 m mol min1 g1 cat. After 23 h reaction, the ammonia conversion rate dropped 2.1% and the hydrogen production rate dropped 0.7 m mol min1 g1 cat. The ammonia conversion efficiency and hydrogen production rate was relative stable without significant degradation over 24 h of reaction time.
4.
Conclusion
The XC-72 carbon power treated with nitric acid has the most significant effects on its surface property among acids and alcohols tested in this study. Nitric acid changes the carbon powder surface from hydrophobic to hydrophilic. The NaBH4 in acid solution reduces the highest amount of Ru deposit in chemical reduction. However, precipitation method at pH ¼ 6 was higher then by chemical reduction method. The EFSEM mapping result shows that both Ru and Cs deposits were uniformly dispersed over the entire carbon sample. The ammonia conversion efficiency and hydrogen production rate are higher as the Ru wt% increased up to 15 wt.% of Ru. Excessive Ru on carbon powder may be aggregated and sintered which leads to low catalyst activity. Higher ammonia conversion efficiency and hydrogen production rate was obtained with slow ammonia inlet flow rate and high reaction temperature. Reaction temperature has significant effects on the ammonia decomposition reaction. As the catalyst packing quantity increased, the ammonia conversion efficiency is improved; however the hydrogen production rate (per unit catalyst) is decreased. Ammonia conversion efficiency of 90% and hydrogen production rate of 30 m mol min1 g1 cat was obtained with CseRu/C (1:3) catalyst. Catalyst tested over 24 h continuous operation, no significant performance degradation was observed.
Acknowledgment Part of this work was carried out under subcontract from ITRI (Industrial Technology Research Institute). The authors would like to thank the financial assistant from the Bureau of Energy (BOE), Ministry of Economic Affair (MOEA), Taiwan, R.O.C. and
the financial support from National Science Council (NSC), Taiwan, R.O.C.
references
[1] Yin S-F, Zhang Q-H, Xu B-Q, Zhu W-X, Ng C-F, Au C-T. Investigation on the catalysis of COx-free hydrogen generation from ammonia. Journal of Catalysis 2004;224(2): 384e96. [2] Yin S-F, Xu B-Q, Ng C-F, Au C-T. Nano Ru/CNTs: a highly active and stable catalyst for the generation of COx-free hydrogen in ammonia decomposition. Applied Catalysis B: Environmental 2004;48(4):237e41. [3] Yin S-F, Xu B-Q, Wang S-J, Ng C-F, Au C-T. Magnesia-carbon nanotubes (MgO-CNTs) nanocomposite: novel support of Ru catalyst for the generation of COx-free hydrogen from ammonia. Catalysis Letters 2004;96(3e4):113e6. [4] Yin S-F, Xu B-Q, Zhu W-X, Ng C-F, Zhou X-P, Au C-T. Carbon nanotubes-supported Ru catalyst for the generation of COx-free hydrogen from ammonia. Catalysis Today 2004;93-95:27e38. [5] Wang S-J, Yin S-F, Li L, Xu B-Q, Ng C-F, Au C-T. Investigation on modification of Ru/CNTs catalyst for the generation of COx-free hydrogen from ammonia. Applied Catalyst B: Environmental 2004;52(4):287e99. [6] Christensen CH, Johannessen T, Sorensen RZ, Norskov JK. Towards an ammonia-mediated hydrogen economy? Catalyst Today 2006;111:140e4. [7] Zamfirescu C, Dincer I. Using ammonia as a sustainable fuel. Journal of Power Sources 2008;185:459e65. [8] Li L, Hurley JA. Ammonia-based hydrogen source for fuel cell applications. International Journal of Hydrogen Energy 2007; 32:6e10. [9] Web site http://en.wikipedia.org/wiki/Haber_process. [10] Yin S-F, Xu B-Q, Zhou X-P, Au C-T. A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications. Applied Catalysis A: General 2004;277:1e9. [11] Sørensen RZ, Klerke A, Quaade U, Jensen S, Hansen O, Christensen CH. Promoted Ru on high-surface area graphite for efficient miniaturized production of hydrogen from ammonia. Catalysis Letters 2006;112(1e2):77e81. [12] Yin S-F, Xu B-Q, Wang S-J, Au C-T. Nanosized Ru on highsurface-area superbasic ZrO2eKOH for efficient generation of hydrogen via ammonia decomposition. Applied Catalysis A: General 2006;301:202e10. [13] Raro´g-Pilecka W, Szmigiel D, Kowalczyk Z, Jodzis S, Zielinski J. Ammonia decomposition over the carbon-based ruthenium catalyst promoted with barium or cesium. Journal of Catalysis 2003;218:465e9. [14] Choudhary TV, Sivadinarayana C, Goodman DW. Catalytic ammonia decomposition: COx-free hydrogen production for fuel cell applications. Catalysis Letters 2001;72(3e4):197e201. [15] Tsuchiya S, Ozaki A. Decomposition of ammonia and reactivity of nitrogen absorbed on molybdenum. Bulletin of the Chemical Society of Japan 1969;42:344e8. [16] Pelka R, Moszyriska I, Arabczyk MW. Catalytic ammonia decomposition over Fe/Fe4N. Catalysis Letters 2009;128(1): 72e6.
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Preparation of RueCs catalyst and its application on hydrogen production by ammonia decomposition De-Chin Huang a, Cheng-Hong Jiang a, Feng-Jiin Liu a, Yu-Chi Cheng a, Yen-Cho Chen b, Kan-Lin Hsueh a,b,c,* a
National United University (NUU), Department of Chemical Engineering, Miaoli, Taiwan National United University (NUU), Department of Energy and Resources, Miaoli, Taiwan c Industrial Technology Research Institute (ITRI), Energy and Environment Laboratory, Chutung, Taiwan b
article info
abstract
Article history:
Ammonia can be a hydrogen source for many applications including fuel cells. Using Ru or
Received 19 May 2012
CseRu as the catalyst, hydrogen is generated from ammonia by decomposition reaction.
Received in revised form
These catalysts are deposited on carbon powder by either chemical reduction or precipi-
27 October 2012
tation method in this study. Different carbon powder pre-treatment solutions and catalyst
Accepted 27 October 2012
deposition conditions are evaluated. Nitric acid pre-treatment followed by precipitation at
Available online 26 January 2013
pH of 6 produces the highest catalyst loading from solution with given concentration of catalyst precursor. Hydrogen generation rate is measured at different catalyst composi-
Keywords:
tions, ammonia inlet flow rates, decomposition temperatures, amount of catalyst packing,
Hydrogen generation
and ratio of Cs/Ru. The optimal condition for the ammonia decomposition reaction is
Ru catalyst
Cs/Ru weight ratio at 3, ammonia inlet flow at 6 ml min1, reaction temperature at 400 C.
Cs promoter
At this condition, the ammonia conversion rate reaches 90% and hydrogen generation rate
Ammonia decomposition
reaches 29.8 mmol/min-gcat. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is considered as a clean and environmental friendly fuel for the future. It can be used as the fuel for hydrogen feedvehicles or for fuel cell to produce the electricity. Water is the only product without any emission of carbon dioxide. Hydrogen can also be produced from many resources, such as reforming of fossil fuels [1e3], water electrolysis, biological processes, thermal decomposition, and biomass gasification. However, hydrogen generated from reforming, thermal decomposition, and biological processes produce CO2. To control and to reduce the emission of CO2, it requires an effective technology of separation and capture of CO2 [4,5].
Hydrogen storage still remains a technical barrier to be solved. Present hydrogen storage technologies are high-pressure tank, cryogenic storage, metal hydride, physic absorption, and chemical absorption. The hydrogen storage density is far less than 6%, a border line specified by DOE for transportation application. Although hydrogen has many advantages, it has few barriers to be overcome before it can be used in our daily life. The hydrogen distribution and transportation infrastructure is not ready and is costly. Low flash point and explosion danger are hydrogen safety issues to be considered. Ammonia was proposed as a promising energy carrier instead of hydrogen [6]. Zamfirescu and Dincer [7] compared various
* Corresponding author. National United University, Department of Energy and Resources, 1, Lien-Da Rd., Kung-Ching Li, Miaoli 36003, Taiwan. Tel.: þ886 958 647 100; fax: þ886 37 381237. E-mail address: [email protected] (K.-L. Hsueh). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.105
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properties of several fuels, such as gasoline, natural gas, liquid petroleum gas, methanol, hydrogen, and ammonia. At storage pressure of 10 bar, gravimetric energy densities of gasoline and ammonia are 46.7 and 22.5 MJ kg1, respectively. Their volumetric energy densities are 34.4 and 13.6, MJ L3, respectively. Ammonia has much higher energy density than compressed hydrogen and liquefied hydrogen. In addition, hydrogen generated from ammonia (NH3) is zero CO2 emission. Ammonia was proposed as alternate energy carrier instead of hydrogen [7,8]. Ammonia is commercially produced from hydrogen and atmospheric nitrogen by HabereBosch process. Producing ammonia from hydrogen and then decomposing ammonia to generate hydrogen is not cost effective and the overall energy efficiency is low. However, ammonia is attractive to portable application where easy storage and energy density are primary concern. For portable application, reactor miniaturization and operation at ambient temperature are critical issues. Ammonia decomposition is a reversible reaction as given by equation (1). Calculated from web site data [9], the equilibrium constant of equation (1) is higher at lower temperature. High reaction temperature favors ammonia decomposition. To completely decompose ammonia at fast reaction rate, low reaction temperature is a technical issue to be overcome. 92:5 kJ þ 2NH3 )/3H2 þ N2
contact angle meter (CA), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction analyzer (XRD).
2.
Experimental
The experiment was divided into three consecutive parts, the carbon powder pre-treatment, catalyst deposition, and ammonia decomposition reaction. Entire experimental procedure is schematic presented on Fig. 1. The carbon powder was used as the substrate. It was pre-treated with alcohols or acids. Various carbon powder pre-treatment conditions were evaluated first. Then Ru/C and CseRu/C catalysts were deposited on the pre-treated carbon by two different processes. Finally, the catalyst was placed in a tubular reactor and the hydrogen production rate was measured. The experimental procedure and conditions were described below.
2.1.
Pre-treatment of carbon power
Carbon powder (Cobot, XC-72R) is used as the substrate for catalyst. It has surface area around 250 m2 g1 and electrical conductivity 2.77 S cm1. Carbon powder was mixed with pre-
(1)
Two major steps involved in this reaction are the cleavage of the NeH bond of adsorbed NH3 and the recombinative desorption of nitrogen atoms. Yin et al. made a mini-review on ammonia decomposition reaction [9]. Many metals, alloys, and compounds of noble metal characters have been tested for ammonia decomposition. These include Fe, Ni, Ir, Pt, Rh, Pd, Ru, and their alloys and compounds. The Ru based catalyst is one of the promising catalysts for ammonia decomposition due to its high reactivity [4,5,11,12]. Yin et al. [10] found that 15% of Ru is the optimal Ru loading for high conversion efficiency of ammonia. The ammonia conversion efficiency is also affected by the catalyst substrates such as active carbon, graphite, carbon nano-tube, aluminum oxide, titanium oxide, magnesium oxides, etc [1e4,10]. Ru on carbon nano-tube has the best catalytic activity. Good substrate not only has good catalyst dispersion ability but also electronic conductive. Conductive substrate makes the electron transfer easy between catalyst and promoter. Promoter accelerates the recombination desorption of nitrogen atoms. Alkali and alkali earth metals such as K, Na, Li, Ba, Ca, Cs, and La were studied as the promoter [5,11,13]. Ru on carbon with Cs as the promoter, has high ammonia decomposition rate even at temperature below 400 C [11]. This is very attractive for proton exchange membrane fuel cell application. In this study, Ru catalyst was prepared by reduction and precipitation method. The effect of solution pH and reducing agent concentration on the Ru deposition was evaluated. Cesium promoter was deposited on the Ru catalyst. The decomposition rate of ammonia as well as the hydrogen generation rate was measured at different RueCs catalyst compositions, ammonia inlet flows, decomposition temperatures, and amount of catalyst packing. Catalyst was characterized by Fourier transform infrared spectroscopy (FTIR),
Pre-treatment of carbon powder (XC-72R)
Characterization of carbon powder (FTIR, contact angle)
Catalyst preparation -reduction method
Catalyst preparation - precipitation method
Ru Catalyst
Deposition of Cs promotor
Characterization of catalyst (EDS, SEM, XRD Mapping)
Hydrogen generation from ammonium decomposition Fig. 1 e Experimental procedure.
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treatment solution at 80 C for 12 h under reflux condition. Different solutions were tested. They are water, methanol (Echo Chem. CO., 99.5%), ethanol (Echo Chem. Co., 99.9%), sulfuric acid (Shimakyu, 98%), hydrochloric acid (Shimakyu, 32%), hydrogen peroxide (Shimakyu, 35%), and nitric acid (Shimakyu, 69%). The pH value of the mixture was adjusted to 7.0 by ammonia after the mixture was cold. The carbon powder was centrifugation separated, D.I. water rinsed, and vacuum oven dried at 60 C. The powder was grinded by a mortar to fine powder and they are ready for catalyst deposition. The carbon powder was examined by FTIR and contact angle measurement. For FTIR measurement, a 0.1 mg carbon powder was mixed with 0.1 g KBr by mortar. This mixture was then pressed into a pellet. Measurement was carried out with an FTIR spectrometer (JASCO FT/IT-470). For contact angle measurement, carbon powder was adhesive on a carbon tape. Excessive powder was blow away by air. A water drop was rest on the carbon powder surface. After 3 min, the contact angle was measured by contact angle analysis system (APPR FTA-125).
2.2.
Preparation of RueCs catalyst
Deposition of Ru on carbon powder was prepared either by chemical reduction or by precipitation. In both methods, carbon powder (XC-72) was pre-treated with acid first. One gram of the pre-treated carbon powder was then mixed with de-ionized water. Two precursor solutions were prepared. One solution was 5 g of RuCl3 dissolved in 200 ml de-ionized water. The other one was 10 g Ru(NO)(NO3)3 dissolved in 200 ml de-ionized water. Proper amount of these precursor solutions were well mixed with carbon powder suspension. Then, the pH of this suspension solution was adjusted by NaOH solution to desired value. In chemical reduction, reducing agent, such as NaBH4, C6H12O6, CH3OH, or C2H5OH was then added into the carbon suspension solution. This mixture was reflux at 70 C for 6 h and Ru/C catalyst was produced. In the precipitation method, the mixture of carbon and precursor suspension was reflux at 70 C for 12 h without reducing agent. In both methods, the Ru/ C catalyst was centrifugation separated, rinsed with D.I. water, vacuum oven dried at 60 C. Several reduction parameters were investigated in this study. These parameters are: solution pH values (pH ¼ 4, 6, 8, and 10), type of reducing agent, and their concentration. Reducing agents were sodium borohydride (NaBH4, SigmaeAldrich, 99%), methanol (CH3OH, Echo, 99.9%), ethanol (C2H5OH, Echo, 99.5%), and glucose (C6H12O6, Merck, D(þ)glucose). Various amount of cesium promoter was deposited on the Ru catalyst. The weight ratio of RueCs is in the range of Cs/Ru ¼ 0, 1, 2, 3, 4, 5, and 6. Mapping of energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), X-ray diffraction meter (XRD) were used to characterize synthesized catalysts. Amount of Ru in the catalyst was measured by EDS attached within the field effect scanning electron microscopy (FESEM). The RueCs catalyst was prepared from Ru/C catalyst. The Ru/C catalyst powder was pressed into a thin porous sheet. Given amount of the CsNO3 solution was slowly added on the Ru/C sheet. This RueCs sheet was then dried at 100 C. The
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ammonia decomposition efficiency was tested with catalyst containing different Cs/Ru ratios (Cs/Ru ¼ 0, 1, 2, 3, 4, 5, 6).
2.3.
Hydrogen production by ammonia reformer test
Synthesized catalyst was placed into a tubular reactor. The tubular reactor was a stainless steel tube with 1/4 inch ID. Both end of the tube was filled with fiber glass. Catalyst was filled between both ends of fiber glass. The reactor was heated by a cylindrical heater. Ammonia was fed into the reactor through a mass flow controller (flow rate 3, 4.5, 6, and 7.5 ml min1). Various reaction conditions were tested, such as catalyst composition (percentage of Ru, Cs, and carbon), amount of catalyst, flow rate of ammonia, and reactor temperature. Effluent gas from the reactor was bubble through a trap containing 10 vol% sulfuric acid solution. Residue ammonia from the reactor was removed by this trap. The ammonia conversion efficiency and hydrogen production rate were calculated. The flow rate of effluent gas (VEffluent) was measured by a bubble flow meter. Effects of gas temperature and pressure were corrected during data analysis. Concentration of hydrogen in the effluent gas ðCH2 Þ was measured by a gas chromatography (China Gas Chromatography, GC 3000) with an Ultrafast column (Thermo Fisher, Trace UFM-M1). Measuring temperature of TCD, injection port, and oven were at 110 C. The TCD current was set at 50 mA. Operating condition was The hydrogen production rate ðVH2 Þ is the product of VEffluent and CH2 $ðVH2 ¼ VEffluent CH2 Þ The ammonia conversion efficiency is the ratio of consumed ammonia ðVNH3 ;cons Þ to the inlet ammonia ðVNH3 ;inlet Þ. The inlet flow rate of ammonia was read from the mass flow controller. The ammonia consumption rate of ammonia can be calculated from the hydrogen production rate ðVH2 Þ. According to the ammonia decomposition reaction (equation (1)), one mole hydrogen produced consumes 0.67 mol of ammonia.
3.
Results and discussion
3.1.
Effects of acid treatment on carbon properties
In order to increase functional groups on the carbon power surface, the carbon surface was treated with different solutions. Solutions have been tested are nitric acid, sulfuric acid, hydrochloric acid, hydrogen peroxide, ethanol, and methanol. The type of functional groups on the surface was analyzed by Fourier transform infrared spectroscopy (FTIR). Fig. 2 is the FTIR spectra of carbon powder after solution treatment. Legends from 1 to 8 marked on Fig. 2 were the FTIR spectra of carbon after treated by nitric acid, sulfuric acid, hydrochloric acid, hydrogen dioxide, ethanol, methanol, de-ionized water. For comparison, legend 8 was the FTIR spectrum of untreated carbon. Six functional group were detected on the acid treated carbon surface, They are carboxylic group (CeO stretching in 1210e1320 cm1, and C]O stretching in 1710e1780 cm1), alcohol (OH in-plane bending in 1330e1430 cm1 and OH stretching in 3200e3550 cm1), aromatic (C]C bending in 1500e1680 cm1), and alkyl (CeH stretching at 2850 and
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Table 2 e Amount of Ru deposited by reduction method and by precipitation method. Ru precursor Preparation conditions
RuCl3
Ru(NO)(NO3)3
Ru (wt.%)
Ru (wt.%)
5.53 4.84 3.92 4.84 3.13 4.94 4.93 8.61 5.12 4.04
4.49 4.12 2.2 3.92 1.75 1.7 5.12 8.28 4.6 5.35 Ru (wt.%) ¼ 4.32
Reduction
Fig. 2 e FTIR spectra of carbon powder after solution treatment. (1) by nitric acid, (2) by sulfuric acid, (3) by hydrochloric acid, (4) by hydrogen dioxide, (5) by ethanol, (6) by methanol, (7) by de-ionized water, (8) untreated carbon.
2950 cm1). Nitric acid is a strong oxidant. The carbon surface can be oxidized into various functional groups by nitric acid. Carbon surface treated with nitric acid has the most distinguish peaks on FTIR spectra. The C]O bond signals in the wave number 1710e1780 cm1 and a OH (in-plane) bond signals in wave number 1330e1430 cm1 were observed. The function group on the carbon powder is increased after pre-treatment. These functional groups, such as C]O and CeOH are hydrophilic. Carbon powder with hydrophilic surface helps the penetration of metal precursor solution into the pores on carbon surface. The level of hydrophilic surface is evaluated by measuring the contact angle of droplet on the carbon surface. Contact angle measurement results are listed on Table 1. The original carbon powder is hydrophobic surface. The contact angles of surface treated with water or alcohols are similar to the surface of original carbon powder. Water and alcohol treatment do not improve the hydrophobicity of carbon powder surface. Among acidic treatment, nitric acid is the most effective treatment. The contact angle of nitric acid treatment is 45 . It is followed by hydrogen peroxide, hydrochloric acid, and sulfuric acid. From the FTIR spectra and contact angle measurement, result shown that nitric acid is the most effective pre-treatment. In the following study, all the carbon powder was pre-treated with nitric acid.
Table 1 e The contact angle between water and carbon powder after a variety of solution treatment. Pre-treatment solution
Average. contact angle
Original carbon power Water Methanol Ethanol Sulfuric acid Hydrochloric acid Hydrogen peroxide Nitric acid
133 130 130 117 104 101 96 45
NaBH4 (acid) NaBH4 (pH ¼ 7) NaBH4 (pH ¼ 10) C6H12O6 (pH ¼ 10) CH3OH (pH ¼ 10) C2H5OH (pH ¼ 10) Precipitation pH ¼ 4 pH ¼ 6 pH ¼ 8 pH ¼ 10 Commercial 5 wt.% Ru/C(Aldrich)
3.2.
Deposition of Ru and Ru/Cs catalyst
The reactor performance of ammonia decomposition is strongly affected by the amount of Ru metal on the catalyst or the weight percentage of Ru in the catalyst (Ru%). In order to control the Ru% in the catalyst, it is important to find out the relationship between Ru% and deposition conditions. The Ru catalyst can be prepared by chemical reduction and by precipitation. The Ru% prepared from these two processes was evaluated. Table 2 is the list of Ru% prepared from two different precursor (RuCl3 and Ru(NO)(NO3)3) and two deposition processes (Chemical reduction and precipitation). The pH values for each deposition process are also listed on Table 2. They were prepared from the solution containing fixed amount of Ru precursor. Ten weight percent of Ru shall be obtained if all the Ru ions in the solution were deposited on the carbon powder. In the reduction process, different reducing agents were tested as listed on Table 2. For comparison, the solution pH value was adjusted to 10. In the precipitation, the solutions with pH value at 4, 6, 8, and 10 were tested. For a given amount of precursor, catalyst prepared by sodium borohydride (NaBH4) has the most Ru content as compared to other reducing agent. About 55% of the precursor ion was deposited on the carbon powder. In the precipitation method, solution with pH value at 6 produces the highest Ru content. About 86% of the precursor was deposited on the carbon powder. The type of precursor (RuCl3, Ru(NO)(NO3)3) plays a minor role on the Ru%. Catalyst prepared by precipitation method with solution pH at 6 has the most Ru deposited on the carbon powder.
Table 3 e Amount of Ru deposited by precipitation method at pH [ 6. RuCl3
Ru(NO)(NO3)3
Theoretical content
Ru (wt.%)
Ru (wt.%)
5 wt.% 10 wt.% 15 wt.% 20 wt.% 30 wt.%
2.35 8.61 10.66 11.22 18.8
3.38 8.28 11.31 13.69 24.92
Ru precursor
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Table 4 e The catalyst composition used in ammonia reforming test. No.
Ru precursor
Actual content of Ru (wt.%)
C01 C02 C03 C04 C05 C06 C07 C08
RuCl3 RuCl3 RuCl3 RuCl3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3
2.35 8.61 11.22 18.8 3.38 8.28 13.69 24.92
The effect of precursor concentration on the catalyst Ru % was also studied. Both precursors, (Ru(NO)(NO3)3 and RuCl3) were evaluated. Precursor concentration range was from 5% to 30% theoretical content. The solution pH value was controlled at 6 for these experiments. As shown on Table 3, both precursors produce similar result and the Ru % is linearly proportional to the precursor concentration. Less Ru was deposited from the RuCl3 solution than from the Ru(NO)(NO3)3 solution (Table 4).
3.3. Morphology and crystal structure of Ru and Ru/Cs catalyst Catalyst with different Cs/Ru ratio (from 0 to 6) was prepared. The CsNO3 solution was added into the Ru/C catalyst and it
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was then dried at 100 C. Both Ru/C and CsRu/C catalyst morphology was examined as given on Fig. 3a for Ru/C and Fig. 3b for CsRu/C. The size of Cs particle is in the order of 10 nm. From the element mapping (Fig. 3c for Ru and Fig. 3d for Cs), both Ru and Cs were uniformly dispersed on the carbon powder. The crystal structure of Ru/C and CsRu/C catalyst was examined by XRD. No obvious crystal signal was observed for the Ru/C catalyst prepared from either chemical reduction or precipitation methods as shown on Fig. 4. Sample 4 and 5 were the Ru/C catalyst prepared from RuCl3 and Ru(NO)(NO3)3, respectively. Sample 6 and 7 were the CsRu/C catalyst prepared from Ru/C catalyst 4 and 5, respectively. The XRD result of commercial Ru/C catalyst (5% Ru Aldrich) also has no crystal signal detected. One possible reason is that the Ru content is too low to have significant XRD signal or the Ru deposit was in amorphous form. However, the XRD of CsRu/C catalyst (Cs/Ru ¼ 4) observes clear Cs signals at 19.9 , 28.3 , 34.8 , 40.5 , 45.4 , and 50 . Possible phases at these angles are (110), (004), (103), (112), (105), and (311).
3.4. Effects of Ru contents on ammonia decomposition reaction Ammonia decomposition is endothermic reaction. High temperature favors decomposition. At 400 C, equilibrium conversion reaches 99.1% [10]. Calculated from web site data
Fig. 3 e Pictures of SEM picture and element mapping. (a) SEM of Ru/C, (b) SEM of CsRu/C, (c) element mapping of Ru, (d) element mapping of Cs.
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Fig. 4 e The XRD analysis results of RueC catalyst with four times cesium promoter (Cs/Ru [ 4 (wt. ratio)). Legend 4 and 5 were the Ru/C catalyst prepared from RuCl3 and Ru(NO)(NO3)3, respectively. Legend 6 and 7 were the CsRu/C catalyst prepared from Ru/C catalyst 4 and 5, respectively.
[9], the equilibrium constant at 300, 400, 500, and 600 K, are 2.30 102, 6.10 103, 6.90 104, 4.40 105. From these data, the equilibrium constant of ammonia decomposition (Keq) was calculated as given by equation (2). Keq ¼ 6 105 e½4505=T ¼ H2 3 ½N2 ½NH3 2
(2)
In our reaction temperature range (between 375 and 500 C), the Keq are 5.74 102 (at 375 C) and 1.77 103 (at 500 C). At equilibrium, the conversion efficiency of ammonia decomposition reaction is nearly 100%. The ammonia conversion efficiency and hydrogen production rate with different Ru catalyst loading were given on Fig. 5. These tests were carried out at ammonia inlet flow of 6 ml min1, 0.012 g Ru catalyst packing, reaction temperature at 400 C and at 450 C. Three types of catalyst were examined, catalyst prepared from RuCl3, Ru(NO)(NO3)3, and commercial grade Ru catalyst. Gas sample was taken after 2.5 reaction time. The ammonia GHSV was at 30,000 ml h1 g1 cat. In general, higher reaction temperature produces higher conversion efficiency ð3 NH3 Þ and hydrogen production rate ðVH2 Þ. Higher Ru loading also has higher 3 NH3 and VH2 . The improvement of 3 NH3 and VH2 becomes minor for Ru contain higher than 15%. This is due to high Ru catalyst loading results a short catalyst packing length (constant catalyst packing 0.012 g and ammonia inlet flow rate). At a given reaction temperature and Ru loading, catalyst prepared from Ru(NO)(NO3)3, has the highest 3 NH3 and VH2 . The catalyst prepared from RuCl3 may have residual Cl on the catalyst. It reacts with NH3 and produces NH4Cl precipitation on the catalyst surface. This reduces the 3 NH3 and VH2 for catalyst prepared from RuCl3.
3.5.
Fig. 5 e Effects of Ru content on (a) ammonia conversion rate and (b) hydrogen production rate at different reaction temperatures.
examined at reaction temperature of 375 C, 400 C, 425 C and 450 C. Result is plotted on Fig. 6. High ammonia flow rate enhance the mass transfer of ammonia to the catalyst surface and it results a high hydrogen production rate. This is especially true for high reaction temperature i.e., 450 C than for low reaction temperature, i.e., 375 C. At a given flow rate, the decomposition reaction of ammonia is shifted from kinetic
Effects of ammonia inlet flow
For a given amount of catalyst packing (0.024 g), the effect of ammonia flow rate on the hydrogen production rate is
Fig. 6 e Effects of ammonia inlet flow on hydrogen production rate at different reaction temperatures. Loading of catalyst was 0.024 g.
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3.5 Catalyst loading: 0.0024 g
Logarithm of H Formation Rate m mol/min-g
3 2.5 2 1.5
Inlet flow rate, ml/min 3.0
1
4.5 6.0
0.5 7.5
0 0.0013
0.0014
0.0015
0.0016
Fig. 8 e Effects of Cs/Ru ratio on hydrogen production rate at 0.012 g catalyst loading, 6 ml minL1 ammonia flow rate, and GHSV [ 30,000 ml hL1 gL1.
Temperature, 1/T (K )
Fig. 7 e Effects of reaction temperature on hydrogen production rate at different ammonia inlet flow rates. Loading of catalyst was 0.024 g.
controlled to mass transfer controlled as the reaction temperature increased.
3.6.
Effect of reaction temperature
The hydrogen production rate is also a function of reaction temperature as shown on Fig. 7. The hydrogen production rate increased as the reaction temperature (T ) increased. According to Arrhenius equation, the plot of logarithm rate against 1/ T shall yield a straight line as given on equation (3). The activation energy of ammonia decomposition reaction (Ea) can be calculated from the slope of this line. lnðrateÞ ¼ lnðko Þ
Ea 1 RT
(3)
The value of Ea is in the range between 54 and 72 kJ mol1 in the ammonia inlet flow rate of 3.0e7.5 ml min1 Table 5 lists the activation energy of various catalysts found in the literature. The activation energy may be substrate dependence. Catalyst Ru on oxide substrate has activation energy around 80e90 kJ mol1 (Table 5). Catalyst Ru on conductive substrate, such as carbon in our case, the activation energy is around 54e72 kJ mol1. Conductive substrate may provide an electronic path for the redox reaction taking place on the catalyst surface.
3.7. Effect of catalyst Ru/Cs ratio on hydrogen production rate At a given catalyst packing (0.012 g), ammonia inlet flow (6 ml min1), the hydrogen production rate of catalyst with different weight ratio of cesium promoter (Cs/Ru ¼ 0e6) is plotted on Fig. 8. A maximum production rate of hydrogen is at Cs/Ru ¼ 3 for reaction temperature at 375 C and 400 C. At his Cs/Ru ratio, the ammonia conversion efficiency reaches 90% and hydrogen production rate reaches 29.8 mmol/min-gcat. at 400 C. The Cs is acting a promoter, the hydrogen production rate is higher for catalyst containing higher Cs in the region Cs/Ru 3. The hydrogen production rate is lower for catalyst containing higher Cs in the region Cs/Ru 3. In this region, excessive amount of Cs converts the Ru surface and the hydrogen production rate is reduced.
3.8.
Short-term durability test of catalysts
A 24 h continue running of ammonia decomposition was carried out under reaction temperature at 400 C and ammonia flow rate at 6 ml min1. Three catalysts were tested. They are
Table 5 e The activation energy of various catalysts for ammonia decomposition reaction. Catalyst
Activation energy, kJ mole1
Reference
Ru/C Ru/Al2O3 Ru/Al2O3 Ru/SiO2 Ir Ir Mo Fe Fe4N
54e72 88 80 81.6 79.5 131 176 16 34
This work [14] [14] [14] [14] [14] [15] [16] [16]
Fig. 9 e Hydrogen production rate over a 24 h running time at 0.012 g catalyst loading, 6 ml minL1 ammonia flow rate, 400 C, and GHSV [ 30,000 ml hL1 gL1.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 3 2 3 3 e3 2 4 0
Ru:Cs ratio of 1:0, 1:3, and 1:4 CseRu/C catalyst. Results were shown on Fig. 9. The Ru/C catalyst has the lowest performance among catalyst under testing. The average conversion efficiency of Ru/C over 24 h was about 37% and hydrogen production rate was about 12 m mol min1 g1 cat. The ammonia conversion rate of CseRu/C (1:3) was about 90% and the hydrogen production rate was 29.8 m mol min1 g1 cat. After 23 h reaction, the ammonia conversion rate dropped 3.1% and the hydrogen production rate was dropped 0.74 m mol min1 g1 cat. The ammonia conversion rate of CseRu/C (1:4) was about 76% and the hydrogen production rate was 25.3 m mol min1 g1 cat. After 23 h reaction, the ammonia conversion rate dropped 2.1% and the hydrogen production rate dropped 0.7 m mol min1 g1 cat. The ammonia conversion efficiency and hydrogen production rate was relative stable without significant degradation over 24 h of reaction time.
4.
Conclusion
The XC-72 carbon power treated with nitric acid has the most significant effects on its surface property among acids and alcohols tested in this study. Nitric acid changes the carbon powder surface from hydrophobic to hydrophilic. The NaBH4 in acid solution reduces the highest amount of Ru deposit in chemical reduction. However, precipitation method at pH ¼ 6 was higher then by chemical reduction method. The EFSEM mapping result shows that both Ru and Cs deposits were uniformly dispersed over the entire carbon sample. The ammonia conversion efficiency and hydrogen production rate are higher as the Ru wt% increased up to 15 wt.% of Ru. Excessive Ru on carbon powder may be aggregated and sintered which leads to low catalyst activity. Higher ammonia conversion efficiency and hydrogen production rate was obtained with slow ammonia inlet flow rate and high reaction temperature. Reaction temperature has significant effects on the ammonia decomposition reaction. As the catalyst packing quantity increased, the ammonia conversion efficiency is improved; however the hydrogen production rate (per unit catalyst) is decreased. Ammonia conversion efficiency of 90% and hydrogen production rate of 30 m mol min1 g1 cat was obtained with CseRu/C (1:3) catalyst. Catalyst tested over 24 h continuous operation, no significant performance degradation was observed.
Acknowledgment Part of this work was carried out under subcontract from ITRI (Industrial Technology Research Institute). The authors would like to thank the financial assistant from the Bureau of Energy (BOE), Ministry of Economic Affair (MOEA), Taiwan, R.O.C. and
the financial support from National Science Council (NSC), Taiwan, R.O.C.
references
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