Environ. Sci. Technol. 2007, 41, 3758-3762
Catalytic Ammonia Decomposition over Industrial-Waste-Supported Ru Catalysts P E I F A N G N G , † L I L I , †,§ S H A O B I N W A N G , ‡ Z H O N G H U A Z H U , * ,† G A O Q I N G L U , † A N D ZIFENG YAN§ ARC Functional Nanomaterials, The University of Queensland, St Lucia, Queensland 4072, Australia, Department of Chemical Engineering, Curtin University of Technology, Australia, and State Key Laboratory of Heavy Oil Processing, China University of Petroleum, China
Industrial solid wastes (fly ash and red mud) have been employed as supports for preparation of Ru-based catalysts. Physical and chemical treatments on red mud were conducted and these modified supports were also used for preparation of Ru-based catalysts. Those Ru catalysts were characterized by various techniques such as N2 adsorption, H2 adsorption, XRD, XPS, and temperatureprogrammed reduction (TPR), and were then tested for catalytic ammonia decomposition to hydrogen. It was found that red-mud-supported Ru catalyst exhibits higher ammonia conversion and hydrogen production than fly-ashsupported catalyst. Heat and chemical treatments of the red mud greatly improve the catalytic activity. Moreover, a combination of acid and heat treatments produces the highest catalytic conversion of ammonia.
1. Introduction Due to its environmentally benign properties, hydrogen has been proposed as a future “energy vector” (1). Key applications of hydrogen include carbon-free fuel and hydrogendriven fuel cells for automotives. Currently, the major routes of on-board H2 generation are steam reforming, autothermal reforming, and partial oxidation of hydrocarbons. However, COx is inevitably produced in these processes, which degrades cell electrodes even at lower COx concentrations. Due to the electrode poison by COx, COx-free hydrogen production is highly required. Catalytic ammonia decomposition is an attracting and promising route for H2 production in fuel cells because of no COx generation, high energy density, and hydrogen capacity (2-4). Ammonia decomposition on metal surfaces has been extensively studied for understanding of the industrially important ammonia synthesis process. However, only a few of these investigations provide information related to the interesting topic of H2 production (5). Recently, extensive efforts have been undertaken to explore ammonia decomposition for fuel cell application (6-16). Ru catalysts supported on carbon materials have been considered as the most active catalyst system for ammonia decomposition compared with other transition metals and oxide supports (17-20). However, the methanation reaction * Corresponding author e-mail: fax: 61 7 3365 4199. † The University of Queensland. ‡ Curtin University of Technology. § China University of Petroleum. 3758
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of the carbon supports in H2 atmosphere usually occurs at a relatively high temperature and H2 pressure, which has an adverse effect on the catalyst stability, especially in ammonia synthesis (21). Fly ash (FA) and red mud (RM) are two important solid wastes. Fly ash is usually generated from coal-firing power stations. Red mud is a byproduct of the Bayer process in aluminum industry and causes serious environmental problems due to its high alkalinity (pH >10) upon disposal. For every ton of alumina produced, between 1 and 2 tons (dry weight) of red mud residues are produced. It is composed primarily of particles of silicon, aluminum, iron, calcium, and titanium oxides and hydroxides (22). Due to its high calcium and sodium hydroxide content, red mud is relatively toxic and can pose a serious pollution hazard. In recent years, a great deal of research has been undertaken to utilize fly ash (23) and red mud (24-27) for wastewater treatment, such as removal of toxic heavy metals and dyes. However, few reports have been published on application of fly ash and red mud in catalyst supports (28-32). In this work, we explored new application of fly ash and red mud as catalyst support for Ru catalysts in catalytic ammonia decomposition reaction. Due to the strong basicity of red mud, heat and acid treatments were applied to the red mud sample for modification of the surface and bulk properties. These modified red mud samples were also employed as supports for Ru-based catalysts and tested in ammonia decomposition for hydrogen production.
2. Experimental Section 2.1 Supports and Treatment. A fly ash sample was obtained from an electric power plant in Western Australia. A red mud sample was obtained from a downstream slurry pond in Worsley Alumina, Australia. The physically and chemically treated RM samples were obtained by the following methods. One portion of the raw red mud sample was heat treated at 800 °C overnight under N2 gas (RM-800) and the two other portions of the samples were treated in 2 M HNO3 and HCl solutions, respectively, for 24 h at room temperature. After acid treatment, they were filtered, washed, and dried at 110 °C overnight (referred to as RM-HNO3 and RM-HCl, respectively). In addition, the two acid treated samples were further heat treated at 700 °C for 4 h under N2 gas, denoted as RMHNO3-700 and RM-HCl-700, respectively. 2.2 Preparation of Catalysts. All Ru-based catalysts were prepared by wetness incipient impregnation using RuCl3‚ xH2O as active component precursor with a nominal metal loading of 5 wt % on all samples. After impregnation, the samples were calcined in air flow at 500 °C for 2 h. All the catalysts were thus designated as 5%Ru/support. 2.3 Catalytic Testing. Catalytic ammonia decomposition reaction was performed in a vertical fixed-bed flow reactor in pure ammonia stream (purity >99.9%) under atmospheric pressure. Catalysts with loading of 0.1 g (60-80 mesh) were placed in the central section of the reactor. Prior to the reaction, the catalysts were first purged with Ar flow (purity >99.99%) and then heated to 500 °C in Ar flow at a heating rate of 10 °C/min. Subsequently, the catalysts were reduced in situ in pure H2 flow at 500 °C for 2 h. After that, the catalytic testing was carried out under pure NH3 (gas flow rate ) 100 mL/min) at 550 °C for 2 h. Product analysis was performed on-line using a gas chromatograph (Shimadzu) equipped with TCD with Ar as carrier gas. 2.4 Characterization Methods. The XRD analyses of the FA, RM, and treated RM supports, as well as Ru-based catalysts, were performed on a Rigaku Miniflex diffractometer 10.1021/es062326z CCC: $37.00
2007 American Chemical Society Published on Web 04/11/2007
TABLE 1. Physicochemical Properties of Fly Ash and Red Mud chemical composition SiO2 A12O3 Fe2O3 TiO2 Na2O CaO MgO K2O SO3 L.O.I. (%) pH SBET (m2/g) V (cm3/g)
fly ash (%) 55.0 29.3 8.8 0.3 1.6 1.0 0.4 0.1 5.2 3.7 4.7 0.014
red mud (%) 5 15 60 5 16
12 22.7 0.057
with Co KR radiation at a scanning rate of 2°/min in the 2θ range from 10° to 80°. Specific surface areas and pore volumes of samples were determined by nitrogen adsorption-desorption isotherms using a NOVA 1200 adsorption analyzer (Quantachrome, USA). Prior to measurement the samples were degassed at 200 °C for 8 h. Surface areas of the samples (SBET) were obtained from the BET equation, which was applied in relative pressure range from 0.05 to 0.25. The total pore volume was derived from the amount of vapor adsorbed at a relative pressure of 0.98, by assuming that the pores were then filled with liquid N2. Ru dispersion on catalysts was obtained using H2 chemisorption. Static volumetric H2 chemisorption measurements were carried out with an Autosorb 2010 adsorption analyzer (Quantachrome, USA). For the measurement, a catalyst sample was reduced in H2 at 500 °C and then evacuated at 400 °C. After cooling to 80 °C in He flow, a H2 chemisorption isotherm was measured between 3 and 35 kPa. The amounts of total and reversible hydrogen were determined by extrapolating the flat portion of the isotherm to zero pressure. A spherical model for the metallic particles and a H/Ru adsorption stoichiometry ) 1 were assumed in calculating the metal dispersion. The X-ray photoelectron spectroscopy (XPS) measurements were conducted using a PHI-560 ESCA system (PerkinElmer). All spectra were acquired at a basic pressure 2 × 10-7 Torr with Mg KR excitation at 15 kV and recorded in the ∆E ) constant mode, at pass energies of 50 and 100 eV. H2-TPR measurement of catalysts was investigated in a thermogravimetric analyzer (Shimadzu TGA-50). The samples were loaded into a platinum pan and heated under N2 atmosphere from room temperature to 110 °C and held at this temperature for 60 min. The temperature was further increased to 350 °C and held at this temperature for another 30 min. The temperature was then decreased to 30 °C and then heated to 500 °C under 10% H2/N2 atmosphere with a heating rate of 10 °C/min. NH3 adsorption and temperature-programmed desorption (TPD) measurements of the catalysts were performed in a simply set up reactor consisting of a cold trap filled with
desiccators and a glass tube to hold the sample. The Ru catalysts in the glass tube were first degassed at 400 °C for 2 h. After degassing, the sample was put in the ammonia flow for 2 h, and then the flow was switched to pure Ar for 1 h. The adsorbed sample was degassed for 1 h at 200 °C and again degassed for 1 h at 400 °C.
3. Results and Discussion 3.1 Characterization of Supports and Catalysts. The composition and some properties of the received fly ash and red mud samples are listed in Table 1. Most of the chemical compounds in fly ash are silica and alumina with some iron oxide and calcium oxide, while the red mud is mainly composed of iron oxide, alumina, and silica with sodium oxide. The solid fly ash shows acid property while red mud exhibits basic property. The textural characteristics of fly ash and red mud are different. The red mud has higher surface area and pore volume than those of fly ash. The chemical phases of fly ash and red mud were determined by XRD measurements. The XRD patterns of fly ash and red mud indicate that the major phases of fly ash are quartz and mullite, with minor hematite and magnetite, while four major phases, hematite, goethite, quartz, and calcite, are existent in raw red mud. Table 2 presents the textural properties of modified red mud samples and their supported Ru catalysts. Acid-treated RM supports have greater surface area and total pore volume if compared with raw RM. This shows that acid treatment is essential to improve the porosity of RM support. HNO3 treatment is more effective than HCl treatment since it provides higher surface area and comparable total pore volume. Thermal treatment, on the other hand, causes a significant decline in surface area and pore volume. The surface area of RM support is decreased 2-fold. Further heat treatment after acid treatment will generally decrease the surface area and pore volume. This is probably because of the sintering of oxides and thermal decomposition of surface functional groups. XRD patterns of various treated red mud supports are shown in Figure 1which shows that heat treatment and acid treatment will result in some changes. RM-HCl and RMHNO3 present similar XRD patterns, and RM-800, RM-HCl700, and RM-HNO3-700 exhibit similar profiles. After heat treatment, the peak intensities for quartz and goethite are significantly reduced and the intensities for hematite are remarkable enhanced, making it the dominant phase in RM800, RM-HCl-700, and RM-HNO3-700 samples. After acid treatment, the phase of calcite disappears and the intensities of quartz are increased, suggesting phase transformation also occurred on acid-treated RM. For FA-support and RM-supported catalysts, the surface area and pore volume are listed in Table 2. Compared with supports, impregnation of Ru on fly ash and red mud supports generally increases the surface area and pore volume except the HNO3 treated samples. Ru/RM exhibits the highest surface area and Ru/RM-HNO3-700 shows the lowest value. This is probably due to the high dispersion of Ru on the supports, acting as adsorbed sites for N2. Whereas for HNO3 treated
TABLE 2. Textural Parameters of Various Red Mud Samples and Their Supported Catalysts sample
SBET (m2/g)
Vtot (mL/g)
sample
SBET (m2/g)
Vtot (mL/g)
FA RM RM-800 RM-HCl RM-HNO3 RM-HCl-700 RM-HNO3-700
4.9 22.7 10.8 28.5 38.2 24.8 27.0
0.0137 0.0566 0.0038 0.0779 0.0658 0.0840 0.0209
5%Ru/FA 5%Ru/RM 5%Ru/RM-800 5%Ru/RM-HCl 5%Ru/RM-HNO3 5%Ru/RM-HCl-700 5%Ru/RM-HNO3-700
13.3 50.6 38.8 40.3 31.4 25.9 13.9
0.0225 0.116 0.115 0.150 0.129 0.0896 0.0103
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FIGURE 1. XRD patterns of RM and treated RM samples: a, RM; b, RM-800; c, RM-HCl; d, RM-HNO3; e, RM-HCl-700; f, RM-HNO3-700.
FIGURE 2. XRD patterns of Ru/RM and Ru/ RM-treated catalysts: a, Ru/RM; b, Ru/RM-800; c, Ru/RM-HCl; d, Ru/RM-HNO3; e, Ru/RM-HCl-700; f, Ru/RM-HNO3-700. RM samples, which have a large number of narrow pore channels, impregnation of Ru blocks the channels, resulting in decrease of surface area. The XRD patterns of various RM-supported Ru catalysts are illustrated in Figure 2. A strong Ru diffraction peak occurs on all samples. Due to further heat treatment via calcination of the catalysts, phase changes are also observed for all catalysts. Figure 3 shows the acidity of Ru-based catalysts based on ammonia adsorption. Both acid and heat treatments will increase the acidity. HCl treatment will produce the strongest acidity. Treatment of RM with acid HCl and HNO3 will remove Na2O or K2O compounds and increases acid sites. Heat treatment can also cause the decomposition of hydroxide on RM, resulting in more acid sites. The order of ammonia adsorption is Ru/RM-HCl > Ru/RM-HCl-700 > Ru/RM-800 > Ru/RM-HNO3-700 > Ru/RM-HNO3 > Ru/RM. In addition, ammonia TPD profiles also demonstrate that ammonia 3760
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desoprtion occurs at low temperature, suggesting that the acid sites are weak and not thermally stable for all Ru catalysts. H2-TPR profiles of Ru/FA and Ru/RM catalysts are presented in Figure 4. It is seen that reducibilities of the two catalysts are different. Ru/FA exhibits a weak reduction between 300 and 400 °C and a stronger reduction from 480 °C. For Ru/RM catalyst, reduction is occurring at 350-500 °C. These suggest that Ru/RM has a higher redox potential than Ru/FA. Further investigation of modified RM supported catalysts also indicated that modification of RM reduced the reducibility temperature of the catalysts. 3.2 Catalytic Activity of Ru/Fly-Ash and Ru/Red-Mud. The catalytic activities of Ru/FA and Ru/RM were first investigated and the results are presented in Table 3. Ru/RM shows higher catalytic conversion of ammonia and H2 production, and the ammonia conversion is 11% and 7% for Ru/RM and Ru/FA, respectively. Li et al. (33) investigated ammonia decomposition on Ru and Ni catalysts based on
FIGURE 3. Ammonia adsorption on various Ru/RM catalysts. candidate for ammonia decomposition though its activity is lower than promoted Ru/graphite. The difference of the catalytic behavior of Ru/RM and Ru/FA can be related with metal dispersion and ammonia adsorption. From Table 3, it is seen that Ru/RM has higher Ru dispersion and ammonia adsorption than Ru/FA. It has been widely reported that ammonia decomposition depends on metal dispersion. Higher dispersion will result in high ammonia decomposition. In addition, support will also affect the catalytic activity. A support of strong basicity is highly beneficial for high catalytic efficiency (34). Thus, it is concluded that RM is a better support for Ru catalyst. 3.3 Catalytic Activity of Modified Ru/Red-Mud Catalysts. Further investigations on catalytic activities of the modified RM-supported Ru catalysts were conducted and the results are listed in Table 4. One can see that the catalytic activities FIGURE 4. H2-TPR profiles of Ru/FA and Ru/RM catalysts.
TABLE 3. Comparison of Catalytic Activities of Ru/FA and Ru/RM H2 NH3 Ru ammonia conversion formation rate dispersion adsorbed (%) (mmol/min‚gcat) Sample (%) (g NH3/g catalyst) Ru/FA Ru/RM
6.9 11
4.2 7.36
4.2 19.7
0.9962 1.0807
various silica supports and found that 5%Ru/silica and 7% Ni/silica exhibited around 30% and 13% conversion at 550 °C. Yin et al. (14) also conducted an investigation of ammonia decomposition to H2 over Ru catalysts supported on various supports: MgO, Al2O3, ZrO2, TiO2, and carbon. The H2 production was found to be between 8.5 and 28 mmol/min‚ gcat at 500 °C. Some investigations have demonstrated that promoted Ru on porous graphitized carbon is the most active catalyst system for decomposition of ammonia. Kowalczyk et al. (19) investigated various Ru/C catalysts and found the most effective Ru-Cs/C exhibiting H2 production of 185 at 400 °C. Recently, Christensen group reported Ru-Cs/C activity at 188 mmol/min‚gcat for H2 production at 430 °C (20). Compared with these results, Ru/RM is a promising
TABLE 4. Catalytic Activities of Ammonia Decomposition on Ru Catalysts sample
NH3 conversion (%)
H2 formation rate (mmol/min‚gcat)
Ru/RM Ru/RM-800 Ru/RM-HCl Ru/RM-HNO3 Ru/RM-HCl-700 Ru/RM-HNO3-700
11 16.6 13.3 15.8 16.6 16.9
7.36 11.11 8.91 10.55 11.10 11.31
of the modified Ru/RM catalysts are higher than those of raw Ru/RM. The catalysts based on heat-treated RM will produce better catalytic activity and the catalysts on RM supports after acid and heat treatment present even higher activity than those with either heat or acid treatment. The highest ammonia conversion of 17% can be achieved over Ru/RMHNO3-700. The activity order of Ru-supported catalysts is Ru/RM-HNO3-700 > Ru/RM-HCl-700 ) Ru/RM-800 > Ru/ RM-HNO3 > Ru/RM-HCl > Ru/RM. The higher catalytic activity of the modified red-mudsupported Ru catalysts can be attributed to the changes of surface and bulk properties caused by physical and chemical treatments. It have been found from TPR experiments that VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the modified red mud catalysts present higher reducibility than raw red-mud-supported Ru catalyst, which will result in more dispersive Ru metals on catalyst. As more active Ru is on the catalyst, higher catalytic activity will be produced. For all Ru/RM catalysts, Figure 3 also demonstrates that the modified red mud catalysts have higher acidity, which favors the ammonia adsorption on the surface of catalysts and thus promotes its decomposition. In this investigation, it has been shown that acid and heat treatments will increase the catalytic activity of Ru/RM in ammonia decomposition. However, compared with the most active catalyst Ru/C, the modified Ru/RM catalysts exhibit lower activity. Many investigations have found that addition of a promoter to a catalyst system will significantly increase the activity. For Ru/C, Cs and Ba have been reported to induce stronger promotion effect in ammonia decomposition (19, 20). Au et al. (35) systematically investigated the effects of promoter cations such as rare earth, alkali, and alkaline earth metals on Ru/carbon-nanotube and found that K would be the best promoter. Thus, it is possible to modify the Ru/RM system with various promoters to further enhance the catalytic activity and the research is being carried out. FA- and RM-supported Ru catalysts were tested for catalytic ammonia decomposition. Modification of RM with heat and acid treatments was conducted and these were used as supports for Ru catalysts. Ru/RM exhibits higher catalytic activity and hydrogen production due to the higher metal dispersion and ammonia adsorption. Modification of RM improves the surface area and ammonia adsorption as well as Ru reduction, resulting in higher catalytic activity. Modified RM-supported Ru catalysts demonstrate a catalytic activity comparable to that of other catalysts systems.
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Received for review September 28, 2006. Revised manuscript received March 2, 2007. Accepted March 8, 2007. ES062326Z
