CA2676333A1 - Peat derived carbon-based metal catalysts - Google Patents

Abstract

The present invention provides catalysts, and methods of making the catalysts using peat as a raw material for highly stable activated carbon-based catalysts based on the transition metals and noble metals. Particular embodiments include two new carbon-based Ni/Fe catalysts useful for catalytic decomposition of ammonia into N2 and H2. These catalysts are prepared using a porous activated carbon (AC) support derived from peat by H3PO4 activation. The newly developed catalysts proved to be highly active for ammonia decomposition. The conversion of 2000 ppm NH3 diluted in helium over the Fe catalyst reached as high as 90% at 750°C and at a space velocity of 45000 h-1, compared with only about 15% with the activated carbon alone without metal loading. The new catalyst of Fe/AC was also much more active than a previously reported Fe catalyst supported on a commercial activated carbon. In addition, the new Fe/Ni catalysts showed superior performance with respect to their resistance to catalyst deactivation. Both catalysts remained active as the reaction time increased up to 10 hours without showing signs of deactivation. Fresh and spent catalysts were characterized by XRD, XPS and TPD.
Possible catalytic mechanisms are discussed, and a cycle mechanism, involving both metal phosphides and metal nitrides, is proposed for the NH3 decomposition reactions over the new Fe/Ni catalysts.

Description

PEAT DERIVED CARBON-BASED METAL CATALYSTS
FIELD OF THE INVENTION

The present invention relates to use of peat as a raw material to make highly stable activated carbon-based catalysts based on the transition metals and noble metals.

BACKGROUND OF THE INVENTION

Currently, with respect to catalysts for ammonia decomposition, the most common catalysts tested include metals or alloys of Ni [6, 8, 10,11 ], Fe [10,12], Pt [10], Ru [6,10,13,14], Pd and Rh [10], Ni-Pt [15] and Ni-Ru [3]. Most of the catalyst metals are supported on solid acids such as A12O3 and SiO2. Goodman and coworkers [16] found that the catalytic activity of NH3 decomposition decreased in the order Ru > Ir > Ni when pure NH3 was employed as the reactant. Yin et al.
[10]

conducted a systematic investigation on the effects of carbon nanotube (CNT) supported metals (Ru, Rh, Pt, Pd, Ni, Fe) on the reaction, and found that, under the same reaction conditions, the NH3 conversion over Ru is much higher than over the other metals, and the reaction rates in terms of turn-over frequencies (TOF) decreased in the order of Ru > Rh - Ni > Pt = Pd > Fe. It should however be noted that noble metals of Ru, Ir, and Rh are very expensive compared with Fe and Ni-based catalysts.

As far as the cost is concerned, Ni and Fe could be an attractive alternative.
It has been widely accepted that the roles of the metals in ammonia decomposition involve formation and decomposition of active metal-nitrogen compounds, i.e., metal nitrides, while these active nitrides can be deactivated by the presence of a small amount of 02 or H2O [17,18]. Arabczyk and Zamlynny [12] examined ammonia decomposition over a fused iron catalyst with the grain sizes from 1 to 1.5 mm, containing nanoparticles (about 30 nm) of elementary Fe after reduction and 3.3% AI2O3, 3.2% CaO and 0.8% K2O. The kinetics of the ammonia decomposition on the iron catalysts were studied using a differential reactor with internal mixing in the temperature range from 325 to 500 C under the atmospheric pressure. The ammonia decomposition reaction was found to be first-order with respect to the partial pressure of ammonia whose rate expression was shown as follows:

r = ko exp(Ea/R/T)pNH3 (1) where Ea is the activation energy of ammonia decomposition over the Fe-catalysts.
The value of Ea was found to be 96 kJ/mol for the Fe-catalyst with potassium as a promoter, and 87 kJ/mol for the catalyst without potassium. Chellappa et al.
[15]
investigated ammonia decomposition using Ni-Pt/A1203 catalyst, and the reaction also proved to be first order with an activation energy of about 200 kJ/mol.

A good support for catalysts can not only enhance the dispersion and surface area of the active components, but greatly affect the activities of the catalysts. For instance, the AI2O3-supported Ru or Ir catalysts showed lower activities for ammonia decomposition than those supported on SiO2, and the activity of Ni/HZSM-5 was much lower than that of Ni/Si02 [16]. Yin et al. [5, 10]
demonstrated that the catalytic performance of Ru catalyst was strongly dependent on support materials: under similar reaction conditions, NH3 conversion decreased in the order of Ru/CNTs > Ru/MgO > Ru/Ti02 = Ru/A1203 = Ru/Zr02 > Ru/AC. The excellent

2 catalytic performance of Ru/CNTs was believed to be related to the high dispersion of Ru on the CNTs. Moreover, Yin et al. [5, 10] proposed that the conductivity of the support might also be an important factor for catalytic activity.

A conductive support is beneficial for the transfer of electrons from promoter and/or support to Ru, which would facilitate desorption of surface N atoms to form N2. It was further demonstrated by Yin et al. [5, 10] that a support of high acidity is unsuitable for NH3 decomposition. Accordingly, CNTs (of high conductivity due to the graphitization of carbon atoms) combined with a basic support (MgO) may lead to enhanced activities for supported Ru catalysts. This has been evidenced by another study by Yin et al. [19], where the Ru/MgO-CNTs catalyst with an equal weight of MgO and CNTs exhibited catalytic activity higher than those of Ru/MgO
and Ru/CNTs.

NH3 (1000-5000 ppm), together with other inorganic/organic impurities such as H2S, HCI and alkali metals, as well as tars and particulates, is present in the fuel gas produced from coal/biomass gasification [21]. These impurities would cause detrimental impacts on the process, e.g., the impurities of H2S, HCI, NH3 are corrosive for the downstream piping, and tar can clog pipes, filters, fuel lines etc. In order to efficiently utilize biomass by gasification, it is necessary to remove, convert or deconstruct these impurities in the gas product from gasification.
Moreover, in some applications, e.g., PEM fuel cells requiring pure H2, complete elimination of tar and ammonia may be necessary [21]. In addition, gas turbine power generating plants rely on syngas from gasification from either coal or biomass, and ammonia present in the fuel stream can form nitrogen oxides under combustion conditions,

3 which are significant contributors to air pollution [22]. Ammonia in the fuel gas is conventionally eliminated using wet scrubbers. Wet scrubbing is an effective gas conditioning method that can remove significant amounts of ammonia and tars from the product gas, but it generates waste water streams and requires that the gas be cooled, and if the final application requires that the gas remain at high temperatures then there is a cost of reheating the gas. In some cases tars in the form of aerosols are difficult to remove even at temperatures below the boiling points, and may remain in the vapour phase, and NH3 concentrations may not be low enough to comply with environmental regulations [23].

Ammonia in the hot syngas can be removed by catalytic decomposition at high temperature into N2 and H2 as per reaction (1) described previously, commonly called hot gas cleanup. By converting the tar and NH3 at a high temperature, the temperature of the final gas product is not reduced as it would be in wet scrubbing, and the gas does not need to be cooled and reheated for final uses, hence making it more advantageous than the wet scrubbing technologies with respect to thermal efficiency. A variety of catalysts have been tested for ammonia decomposition and cracking of tars. Catalysts such as dolomite (calcium-magnesium carbonates), olivine ((Mg, Fe)2SiO4), iron-based and Ni-based catalysts have been commonly used as secondary catalysts to reduce the tar and NH3 content of the gasification product gas [21, 24, 25-29, 30-32]. Other widely used catalysts for ammonia decomposition are A12O3-supported catalysts of Ni and Ru [30-34]. A major problem for these A12O3-supported Ni or Ru catalysts can be the deactivation by fouling of the catalyst and by H2S [35, 36].

4 Compared with the above-mentioned expensive Ru catalysts, less expensive and readily available carbon-based catalysts are of interest. Xu et al. [37]
reported the use of pyrolysis chars of low rank coals as catalysts for ammonia decomposition, being more active than a commercial activated carbon. Ohtsuka et al. [29] investigated decomposition of NH3 with Fe catalysts supported on brown coal chars. The catalyst was prepared by pyrolyzing a brown coal with Fe ion added. In the decomposition of 2000 ppm NH3 diluted with helium at 750 C and at a space velocity of 45,000 h"1, the coal-char supported 6 wt% Fe catalyst led to about 95% ammonia conversion, compared with less than 30% conversion for the 8 wt% Fe catalyst loaded on a commercial activated carbon.

Activated carbon (AC) is a highly porous, microcrystalline and amorphous form of carbon. AC can be produced using a variety of carbonaceous precursors including wood, coconut shells, peat, coal etc, by two categories of carbon activation methods: physical or chemical methods. Physical activation involves thermal treatment of the carbonaceous precursor at high temperatures followed by activation with oxidizing gases such as CO2 and steam at temperatures ranging >800 C. In chemical activation of the carbonaceous materials, they are first impregnated with a chemical agent such as H2SO4, H3PO4, ZnCl2, KOH or NaOH
before being carbonized at relatively low temperatures <800 C [38,39].
Different methods used for activation can result in different surface characteristics such as the BET surface area, porosity, number of active sites, and functional groups [39].
ACs have been widely used as adsorbents or catalytic supports because of their

5 high surface areas, ranging from 250-3000 m2/g, pore distribution and several oxygenated surface functional groups which provide a means of adsorption [40,41].
Although AC and AC supported catalysts were found to be much less active for ammonia decomposition than those supported on other materials such as carbon nanotubes (CNTs), MgO, TiO2, A1203, SiO2, coal char, etc.
[29,37,42,43], it would be desirable to develop effective AC supported catalysts for NH3 decomposition by properly designing its chemical and textural structures and properties. This is mainly because activated carbon materials have high surface areas and relatively low costs. Chemical and textural structures and properties of the activated carbon materials could be altered by the activation process (activation reagent and conditions) and the precursor materials (coal, biomass or peat).

The publication to Xu, C. entitled "Catalytic decomposition of ammonia gas with metal cations present naturally in low rank coals" in Fuel 84 (2005) 1957-1967, discloses a hot gas cleanup method that uses low rank coal such as char to decompose a low concentration of ammonia. The char naturally contains Fe and Ca. The study showed that the Fe plays an important role in ammonia decomposition.

The publication Y. Ohtsuka, C. Xu, D. Kong, N. Tsubouchi. Fuel, 83 (2004), 685, discloses catalysts prepared by actively loading the char with the metals Fe and Ca.

In the publication entitled "Study on ammonia decomposition catalyzed by active carbon-supported Ni or Fe" by Yang in Hunan University Xuebao 33(5)

6 (2006) 100-104, active carbon (AC) supported by Fe or Ni with K, Mo or Co as the promoter were prepared as catalysts for ammonia decomposition.

The paper entitled "Commercial Fe- or Co-containing carbon nanotubes as catalysts for NH3 decomposition" by Su et al., Chem. Commun. (2007) 1916-1918 discloses commercial carbon nanotubes (CNTs) containing residual Co or Fe nanoparticles used for NH3 decomposition.

In another industrial application, specifically fuel cells, due to the rapid development of proton exchange membrane fuel cell (PEMFC) and the PEMFC
vehicles, on-site generation of hydrogen has attracted significant research efforts [1-8]. The hydrogen generated from carbonaceous feedstock (e.g. methanol, methane) has the disadvantage of formation of CO and CO2 that degrade cell electrodes even at extremely low concentrations [2,4]. The use of ammonia as a hydrogen source through the following decomposition reaction has been demonstrated to be promising and economical [9] as its decomposition generates H2 and inert N2 gas without formation of CO/CO2.

2NH3- N2 + 3H2, OH = 92.4 kJ/mol (2) Thus it would be very useful to provide activated carbon based catalysts that are economical and based on readily available carbon sources.

7 SUMMARY OF THE INVENTION

An object of this patent is to prepare novel less expensive carbon-based catalysts (Fe, Ni) derived from peat useful for hydrogen generation from ammonia and for hot syngas cleanup.

In an embodiment of the present invention there is provided a method of synthesizing carbon-based catalysts, comprising the steps of:

a) preparing activated carbon by mixing particles of dried peat with an activation agent to form a first slurry, thermally treating the slurry at a temperature in a range from about 200 C to about 800 C and cooling to room temperature; and b) mixing the activated carbon with a liquid solution containing salts of: one or more transition metals, one or more noble metals, or any combination of transition metal and noble metal, to form a second slurry with the liquid solution impregnating the activated carbon to give metal-loaded activated carbon, removing the liquid and drying the second slurry, followed by calcination in an inert gas to produce particles of the carbon-based catalysts.

Another embodiment of the present invention provides a method of catalytic decomposition of ammonia into N2 and H2 using the carbon-based catalyst of claim 23, comprising the steps of;

heating a catalyst bed containing the carbon-based catalyst to a temperature in a range from about 500 C to about 1000 C while flowing an inert gas over the catalyst in the catalyst bed and, once the catalyst bed reaches a desired operating temperature in said temperature range, replacing the flow of inert gas with a flow of ammonia gas diluted with an inert gas in a preselected ratio, or pure ammonia, the

8 flow of diluted ammonia gas or pure ammonia being at a pre-selected space velocity.

More particularly, two new carbon-based Ni/Fe catalysts are provided for catalytic decomposition of ammonia into N2 and H2. These catalysts can be prepared using a porous activated carbon (AC) support derived from peat by activation. The newly developed catalysts are highly active for ammonia decomposition. The conversion of 2000 ppm NH3 diluted in helium over the Fe catalyst reached as high as 90% at 750 C and at the space velocity of 45000 h"', compared with only about 15% with the activated carbon alone without metal loading. The new catalyst of Fe/AC was also much more active than the Fe catalyst supported on a commercial activated carbon reported previously. In addition, the new Fe/Ni catalysts showed superior performance with respect to their resistance to catalyst deactivation. Both catalysts remained active as the reaction time increased up to 10 hours without showing a sign of deactivation. Fresh and spent catalysts were characterized by XRD, XPS and TPD. Possible catalytic mechanisms are discussed, which will be understood by those skilled in the art to be non-limiting and not to restrict the present invention in any way, and a cycle mechanism, involving both metal phosphides and metal nitrides, are proposed for the NH3 decomposition reactions over the new Fe/Ni catalysts.

A further understanding of the functional and advantageous aspects of the invention can be realized by reference to the following detailed description and drawings.

9 BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention are described with reference to the attached figures, wherein:

Fig. 1 shows a process flow diagram of production of activated carbon from peat;

Fig. 2 shows NH3 conversions vs. time on stream at 750 C with catalysts of Fe/AC, Ni/AC as well as AC (space velocity of 45000 h-1);

Fig. 3 shows plots of BJH Desorption dV/dlog(D) pore volume distribution of AC, Fe/AC and Ni/AC catalysts, wherein results from duplicate tests for each catalyst are presented;

Fig. 4 shows measurements of the total amount of CO2, CH4 and CO
evolved during decomposition of 2000 ppm NH3/He over AC for 6 h, Ni/AC for 10 h and Fe/AC for 10 h;

Fig. 5 shows XRD profiles of the fresh Ni/AC catalyst (a), the Ni/AC catalyst after 2 h H2 reduction at 500 C (b), and the Ni/AC catalyst after the NH3 decomposition experiment at 750 C for 4 h (c) and for 10 h (d);

Fig. 6 shows XRD profiles of the fresh Fe/AC catalyst (a), the Fe/AC catalyst after 2 h H2 reduction at 500 C (b), and the Fe/AC catalyst after the NH3 decomposition experiment at 750 C for 4 h (c) and for 10 h (d);

Fig. 7 shows the x-ray photoelectron spectroscopy (XPS) spectra of Ni 2p for the Ni/AC catalyst before and after H2 reduction, and after the NH3 decomposition for 4 and 10 hours;

Fig. 8 shows the XPS spectra of Fe 2p for the Fe/AC catalyst before and after H2 reduction, and after the NH3 decomposition for 4 and 10 hours;

Fig. 9 shows changes in atomic Fe/P and Ni/P ratios determined by XPS
during the NH3 decomposition;

Fig. 10 shows the XPS spectra of N 1 s for the Fe/AC catalyst before and after H2 reduction, and after NH3 decomposition for 4 and 10 hours;

Fig. 11 shows the effects of the Fe2P/AC and Ni2P/AC catalysts on NH3 decomposition (experimental conditions: 750 C, 2000 ppm NH3/He and space velocity of 45000 h-1);

Fig. 12 shows the evolution of N2 during TPD of the fresh Fe/AC catalyst after H2 reduction (a) and after NH3 treatment at 500 C for 4 h (b); and Fig. 13 presents plots showing dependencies of standard Gibbs free energies (AG ) on temperature for reactions (1), (2), (3), (4), (5) and (6).

DETAILED DESCRIPTION OF THE INVENTION

The systems described herein are directed, in general, to embodiments of carbon-based catalysts based on the transition metals and noble metals in which the carbon support is porous activated carbon (AC) support derived from peat.
Although embodiments of the present invention are disclosed herein, the disclosed embodiments are merely exemplary and it should be understood that the invention relates to many alternative forms, including different shapes and sizes.

Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for enabling someone skilled in the art to employ the present invention in a variety of manners. For purposes of instruction and not limitation, the illustrated embodiments are all carbon-based catalysts based on the transition metals and noble metals in which the carbon support is porous activated carbon (AC) support derived from peat, with exemplary embodiments being Ni/Fe catalysts useful for catalytic decomposition of ammonia into N2 and H2.

As used herein, the term "about", when used in conjunction with ranges of dimensions of particles, compositions of mixtures, ppm, space, velocity or other physical properties or characteristics, is meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where, on average, most of the dimensions are satisfied but where statistically dimensions may exist outside this range. It is not the intention to exclude embodiments such as these from the present invention.

As used herein the phrase "activated carbon" means the porous carbon-based materials derived from coal, peat and biomass by a variety of carbonization and activation processes.

As used herein the phrase "porous" means materials with micro-pores (<2 nm pore diameter) and meso-pores (2-50 nm diameter) identified using characterization techniques such as gas adsorption and microscopy (SEM, TEM, etc.).

The present invention provides novel and less expensive carbon-based catalysts based on the transition metals (e.g., Fe, Ni, etc.) and noble metals (e.g., Ru, Rh, Pd, Ag, Os, Ir, Pt, Au). The carbon support for these catalysts is derived from activated peat, rather than traditional carbon supports.

As used herein the term "peat" refers to carbon based material which is an accumulation of partially decayed vegetation matter from wetlands. Over the past 50 years, the use of peat fuel in industry and for large-scale power generation has been common in Europe, in particular in Finland, Ireland, Russia, Belarus and Sweden. Peat can also be used as a raw material for the production bio-oil and bio-char [44]. Peatlands cover an estimated 400 million hectares (about 3%) of the Earth's land surface and Canada contains some 40% of the world's peatlands -about 170 million hectares [45]. Therefore, peat can be an immense resource for the production of both fuel and carbon materials.

Broadly speaking, the present invention provides a method of synthesizing carbon-based catalysts derived from peat by preparing activated carbon by mixing particles of dried peat with an activation agent to form a slurry and allowing the slurry to sit for a period time. The slurry is then thermally treated at a temperature in a range from about 200 C to about 800 C and thereafter it is cooled to room temperature.

Following preparation of the dried peat-based activated carbon, the catalyst(s) are then incorporated into the porous carbon by mixing the activated carbon with a liquid solution containing salts of: one or more transition metals, one or more noble metals, or any combination of transition metal and noble metal, to form another slurry with the liquid solution impregnating the activated carbon, removing the liquid and drying the slurry followed by calcination in an inert gas to produce particles of the carbon-based catalysts.

The activation agent is preferably H3PO4, but other activation agents may be used to activate the peat, including but not limited to ZnC12 and KOH.

The ground particles of sieved peat-based activated carbon preferably have sizes in a range from about 300 to about 850 pm, while larger particles or pellets could also be used depending on the scale and size of the system.

The transition metals and noble metals may be any one of Fe, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, and any combination thereof.

In general the transition metals may be any one or combination of 40 chemical elements 21 to 30, 39 to 48, 71 to 80, and 103 to 112 of the Periodic Table of Elements. Preferably the transition metals are selected from the group Cr, Mn, Co, Cu, Fe, Ni, and combinations thereof.

In an embodiment of the catalyst the transition metal is any one or combination of Fe and Ni.

The particles of sieved peat-based activated carbon may be micro- or meso-porous with a specific surface area >50 m2/g.

The solution containing the salts of the transition and/or noble metals is an aqueous solution of water and an organic solvent. The organic solvent may conveniently be alcohol, acetone, acetic acid, or a combination thereof.

A preferred solution is an alcohol and water solution and preferably methanol or ethanol and water.

The washed thermally treated sample may be dried at a temperature in a range from about 40 C to about 150 C in a flow of gas before being crushed.
The gas may be any one or combination of air, nitrogen, argon, helium, and CO2.

The washed thermally treated sample is preferably dried at a temperature of about 105 C in air before being crushed.

The present invention will now be illustrated regarding catalysts based on Fe and Ni prepared for decomposing ammonia gas to hydrogen gas; however it will be understood that this is just to illustrate the present invention and it is not limited in any way to Fe and Ni.

EXAMPLE
Fe, Ni derived from peat for hydrogen generation from ammonia and for hot syngas cleanup.

Synthesis of these novel carbon-based Ni/Fe catalysts and testing/characterization of these catalysts for ammonia decomposition are disclosed. The manufacture of the catalysts involved two processes: production of activated carbon (AC) as the catalyst support from peat, and addition of metal ions of Fe or Ni to the AC to an amount of about 13 wt% to produce the AC-supported Fe or Ni catalysts, denoted as Fe/AC and Ni/AC for convenience. Tests of the catalysts for NH3 decomposition were carried out with a flow-type, vertical quartz reactor placed in an electric furnace as detailed later.

Peat The peat sample used was obtained from Eastern Canada. Prior to use, the peat was dried for 24 hrs at 105 C, and then ground with a Wiley mill and screened to particles smaller than 40 mesh (-0.4 mm). Elemental analysis of the raw peat was carried out using a CEC (SCP) 240-XA Elemental Analyzer. The proximate analysis results were determined by Thermogravimetric Analyzer (TGA) and the compositions of inorganic matter in the peat sample were determined using ICP-AES (Inductively Coupled Plasma - Atomic Emission Spectrometer). The analysis results of the raw material of peat are given in Table 1.

Table 1 Proximate and ultimate analyses of the peat sample and concentrations of major inorganic elements in the peat.
Proximate analysis, Ultimate analysis, wt% (d.b.(1) wt% (d.a.f.(2)) VM FC Ash C H N S O
65.6 29.0 5.4 54.7 5. 2.1 0. 32.1 Major inorganic elements, wt% (d.b.) Na K Mg Ca P Fe S Al Si <0.1 <0.1 0.1 0.8 0.1 0.6 0.1 0.3 0.1 1 On a dry basis; 2On a dry-and-ash-free basis; 3By difference; 4Determined by ICP-AES

Production of Activated Carbon The activated carbon was prepared by a chemical activation method using H3PO4, and the overall synthesis process is shown in Figure 1. The AC samples were prepared by mixing 40 grams of the dried raw peat powder with 100 ml of 60wt% H3PO4. The mixture was stirred thoroughly to form uniform slurry, and allowed to soak overnight at room temperature. The sample was then thermally treated in a muffle furnace pre-set at 200 C for 15 min in air, followed by treatment at 450 C for 45 min in air. After thermal treatment, the sample was cooled to room temperature, and then washed with distilled water repeatedly until a neutral pH was obtained. To assist the washing process, the sample was first sonicated in distilled water for 30 minutes, and filtered using a Whatman No. 5 filter paper. This washing process was repeated at least six times to obtain a neutral pH. The washed sample was dried overnight at 105 C in air before being crushed and sieved into particles sized 300-850 pm.

Production of Activated Carbon Supported Fe/Ni Catalysts The Fe and Ni loaded catalysts were prepared using the peat-derived activated carbon by a wet impregnation method using Fe(NO3)3-9H2O or Ni(NO3)2.6H2O as the metal sources. To prepare 13 wt% Fe/AC or 13 wt% Ni/AC,

10 grams of the activated carbon support (300-850 pm) was mixed with 9.38 g of Fe(NO3)3.9H2O or 6.41 g of Ni(NO3)2.6H2O in 200 ml 50 wt% methanol/H20 solution. The mixture was sonicated for 40 min to form uniform slurry, and the CH3OH and H2O were then evaporated under reduced pressure at 40 C and 85 C, respectively. The samples were further dried in air at 105 C for 4 hr and then calcinated in a flow of 200m1/min of N2 heated at a heating rate of 5 C/min from room temperature up to 500 C for 4 hrs in a tubular reactor.

Catalytic Tests of the Catalysts on Ammonia Decomposition NH3 decomposition experiments were carried out with a flow-type, vertical quartz reactor placed in an electric furnace. The catalyst bed within the reactor measured approximately 8 mm in height, and was held in place with fine grade quartz wool. Prior to NH3 decomposition, the samples were heated to 500 C at a heating rate of 15 C/min in a helium flow of 180 ml/min, and then subjected to reduction using 200 ml/min of H2 for 2 h. After H2 reduction, the reactor was heated to 750 C with a heating rate of 15 C/min in a He flow of 180 ml/min. As the temperature reached 750 C, the helium flow was replaced with 2000 ppm NH3 diluted with high purity He under the space velocity of 45000 W. A high speed micro GC and a photo acoustic multi-gas monitor were used to determine N2 formed and the un-reacted NH3, respectively. The apparatus and procedures have been described in more detail elsewhere [29].

Characterization of Catalysts The prepared AC sample and the AC-supported Ni/Fe catalysts were analyzed by N2 isothermal adsorption (77K) for surface area and textual structures.
N2 isothermal adsorption (77 K) was conducted with a bench-top high speed gas sorption analyzer "NOVA 1200e/TO", manufactured by Quantachrome Instruments, USA. X-ray diffraction (XRD) with Cu Ka and Fe Ka radiation was used to characterize the crystalline structures of the catalysts before and after the ammonia decomposition tests. The XRD measurements were carried out on a Shimadzu XRD-6000 x-ray diffractometer with Cu and Fe Ka radiation using a current of mA and a voltage of 40 kV. The average crystalline size of the particles was calculated using the Debye-Scherrer equation as follows:

0.89, Lc=D- (3) (B - Bs, )cos 0 where the B is the width of the half peak of species, BS; is the width of the half peak of the silicon standard, X is the wavelength, Lc is the particle diameter, and 0 is the diffraction angle. X-ray photoelectron spectroscopy (XPS) was employed to characterize the chemical composition on the surfaces of the catalysts before and after ammonia decomposition experiments. The XPS experiments were performed on a ULVAC PHI 5600 spectrometer with an Al anode for Ka, X-ray source operating at 200W. Charging effects were corrected by adjusting the binding energy of Cl., peak of carbon contamination to 284.6 eV.

Results and Discussion Performance of the Catalysts in NH3 Decomposition The activities of the new Fe/AC, Ni/AC catalysts towards NH3 decomposition were investigated at 750 C for various lengths of time ranging from 4 to 10 hours.
The ammonia decomposition efficiencies or ammonia conversions to N2 and H2 of these two new catalysts are compared with the peat-derived activated carbon without metal loading (AC) in Figure 2. Conversion of NH3 into N2 and H2 over the Ni catalyst attained approximately 75%, and the Fe catalyst was found to have the greatest activity for ammonia decomposition reaching as high as 90%, while the conversion with AC alone was only about 15%. Our new catalyst of Fe/AC is thus much more active than the Fe catalyst supported on a commercial AC as reported in previous work by Ohtsuka et al. [29], where the commercial AC-supported Fe catalyst led to ammonia conversion of only 30% under the same reaction conditions (750 C, 2000 ppm NH3/He, and SV of 45000 h-1). Compared with other patented carbon-based Fe catalysts [46], our new Fe/AC and Ni/AC catalysts showed superior performance with respect to their stable activity. As shown in Figure 2, both catalysts remained stably active as the reaction time increased up to 10 hours without showing a sign of deactivation, while the activity of the previously patented carbon based Fe catalysts declined significantly after about 3 hours on the stream [29, 46]. The ammonia decomposition catalyzed by the Ni/AC or Fe/AC mainly yielded N2 and H2 as the dominant products, although it was also observed that very small amount of HCN was formed as by-product (at a selectivity generally <5%) from NH3.

Surface Area and Textual Properties The fresh as-prepared AC, Fe/AC and Ni/AC catalysts and the spent catalyst after ammonia decomposition experiments were analyzed using N2 isothermal adsorption (77K) for their surface areas and textual structures, and the results are summarized in Table 2.

Table 2 Surface areas and textual properties of the as-synthesized activated carbon (AC) and the AC-supported Fe and Ni catalysts, and the spent catalysts after the ammonia decomposition experiments.

Catalyst Multi-point BET Total pore volume BJH desorption (m2/g) (< 163 nm) (cm3/g) average pore diameter (nm) AC-fresh 710 0.58 3.7 AC-4h 678 0.54 3.7 AC-6h 655 0.51 3.7 Fe/AC-fresh 205 0.14 3.7 Fe/AC-4h 209 0.16 3.7 Fe/AC-1 Oh 236 0.18 3.7 Ni/AC-fresh 393 0.25 3.7 Ni/AC-4h 327 0.24 3.7 Ni/AC-10h 346 0.31 3.7 The as-synthesized AC has a BET surface area of 710 m2/g, much greater than either the fresh Fe/AC (205 m2/g) or the fresh Ni/AC (393 m2/g). It can however be expected that the addition of metal ions by impregnation caused reduction in surface area due to the deposition of the metal ions (Fe or Ni) in the pores, as evidenced by the significantly reduced total pore volumes. The obtained AC
product has a total pore (< 163 nm) volume of 0.58 cm3/g, mainly mesopores as shown in Figure 3. The total pore volumes dropped to 0.14 cm3/g for Fe/AC and 0.25 cm3/g for Ni/AC due to the metal addition. Interestingly, however, the average pore diameters of the three materials (fresh AC, Fe/AC and Ni/AC) remained the same as 3.7 nm (mesopores), as clearly shown in both Table 2 and Figure 3.

After NH3 decomposition for 4-6 hours, the BET surface areas and the total pore volumes for the AC decreased slightly. For Ni/AC catalyst, its BET
surface area also decreased slightly from 393 m2/g to 327 m2/g after 4 hours on stream for NH3 decomposition, while it climbed to 346 m2/g after 10 hours on stream for decomposition, accompanied by an increase in total pore volume, as shown in Table 2. Interestingly, the BET surface area and total pore volume of the Fe/AC
catalyst both increased consistently during the ammonia decomposition. For instance, its BET surface area increased from 205 m2/g (fresh) to 209 m2/g (4h) and 236 m2/g (10h). These increases in surface area and total pore volume for the Ni/AC and Fe/AC catalysts during the ammonia decomposition might be explained by the release of the carbon element from the support material at an elevated temperature, generating more micro/meso-pores in the support. The release of elemental carbon from the support materials was evidenced by the measurements of the CO2, CH4 and CO evolved during the ammonia decomposition, as illustrated in Figure 4. The in-situ climbing of the surface area and pore volume might at least in part account for the steady increase in the catalytic activities versus time on stream for both catalysts as shown in Figure 2.
Crystalline Structures Figure 5 shows the XRD (Cu-Ka) profiles for the Ni/AC catalyst before and after H2 reduction at 500 C, and after NH3 decomposition at 750 C for 4 hours and 10 hours. In the fresh catalyst (Figure 5a), diffraction lines of metallic Ni were observed, existing as nanoparticles with an average crystalline size L, = 14 nm, calculated using the Debye-Scherrer Equation. After H2 reduction at 500 C
(Figure 5b), the formation of Ni12P5 and Ni3P were observed, accompanied by decreased intensities of the metallic Ni signals (Lc = 23 nm). After the subsequent NH3 decomposition at 750 C for 4 hours (Figure 5c), the diffraction lines of Ni12P5 increased in intensities, compared with those after H2 reduction at 500 C.
Interestingly, the signals of Ni3P were replaced by relatively strong signals of Ni2P
(Lc = 30 nm). When the time on stream (of NH3) increased further to 10 hours, the signals of Ni12P5 weakened but the signals of Ni2P (Lc = 45 nm) became the dominate signals in the sample.

Figure 6 shows the XRD (Fe-K(x) profiles for the Fe/AC catalyst before and after H2 reduction at 500 C, and after NH3 decomposition at 750 C for 4 hours and 10 hours. No diffraction lines of any Fe species could be detected with the fresh catalyst. A very weak XRD peak, probably due to a-Fe, was detectable at 20 (Fe Ka) of about 57 degrees after 2 h H2 reduction at 500 C. These observations strongly suggest that Fe particles before NH3 decomposition are finely dispersed on the AC support. When the Fe catalyst was subjected to the 10 h decomposition at 750 C, as seen in Figure 6d, the distinct XPD signals of Fe2P appeared, and they were present as highly dispersed nanoparticles with an average crystalline size Lc _ 26 nm calculated by the Debye-Scherrer Equation (Eq. 3).

The detection of nickel/iron phosphides (Ni12P5, Ni3P, and Ni2P, Fe2P) in the AC-supported catalysts after the hydrogen reduction and/or after the ammonia decomposition reveals the reaction of the Ni/Fe metal with P species remaining in the AC at elevated temperatures. The presence of P in the AC support was due to the use of H3PO4 as the chemical activation agent in the AC production process.
The presence of P in both the AC-supported catalysts was also evidenced by XPS
analyses, which will be discussed below. Interestingly, no metal nitride such as NiXN

or FeRN was detectable by XRD in the spent catalysts after ammonia decomposition. This is discussed further below.

Surface Chemical States Chemical states and compositions over the catalyst surfaces for fresh and spent catalysts of Ni/AC and Fe/AC (after the ammonia decomposition experiments at 750 C for 4h and 10h) were analyzed by XPS. Figure 7 illustrates the XPS

spectra of Ni 2p for the fresh and spent catalysts of Ni/AC. Ni exists primarily in the forms of Ni3+ or Ni2+ (Ni 2p3/2 peak at around 856.8 eV) resulting from Ni203 and NiO formed by air oxidation of sample prior to or during the XPS measurements.
Another Ni 2p peak observed at around 853.8 0.1 eV in the spent catalysts after the ammonia decomposition experiments for 4h and 10h may be ascribable to the phosphides (Ni12P5, Ni3P, and Ni2P) and metallic Ni. The detection of phosphides coincides with the observation by XRD (Figure 5).

The XPS spectra of Fe 2p for the fresh and spent catalysts of Fe/AC are shown in Figure 8. According to the Fe 2p spectra for all samples (fresh and spent), Fe exists mainly in the forms of Fe'+ (x = 2 - 3) cations (Fe 2p3/2 peak at around 711.8 eV) resulting from Fe2O3, Fe304 and FeO formed by air oxidation of sample prior to or during the XPS measurements. In addition, a new Fe 2p peak at around 707 eV (although weak in intensity) might be identified in the spent catalysts after 4h and 10h on the stream of 2000 ppm NH3/He at 750 C. This new peak may be ascribable to the phosphides (Fe2P) and metallic a-Fe, as evidenced by the XRD
observation discussed previously (Figure 6).

Quantitative analysis of atomic ratios was accomplished by determining the elemental peak areas, using Shirley background subtraction with the sensitivity factors supplied from the instrument maker. Table 3 shows the atomic ratios of several elements (0, P, N, Ni and Fe) in relation to carbon on the surfaces of the fresh and spent catalysts of Ni/AC and Fe/AC determined by XPS. Atomic ratios of P/C with the fresh Fe and Ni catalysts were 0.02 and 0.07, respectively, indicating significant retention of the P from H3PO4, used for the chemical activation of the raw peat. The P 2p XPS spectra were observed at about 134 eV, regardless of the kind of sample, and may be identified as mainly Ni- or Fe-phosphate species such as PO43- (133.75 eV) and metaphosphates (134.3 0.3eV), and there was a weak peak at approximately 130.5 eV which may correspond to NiXP or FeXP (similar to what is shown in Figures 7 and 8 from the XPS spectra of Ni 2p and Fe 2p).
Atomic ratios of Fe/P and Ni/P are plotted against time on stream in Figure 9, where the data at time zero mean those for the fresh catalysts. The value Ni/P is about 1.5, lower than its bulk Ni/P ratio (2.0-3.0) as estimated by XRD (Figure 5), suggesting non-uniformity in the composition of the sample. The value Fe/P is slightly greater than 2.0, which is consistent with the bulk Fe/P ratios estimated by XRD

measurement demonstrating the presence of Fe2P (Figure 6).

As also shown in Table 3, the surface O/C ratios for both catalysts decreased during the ammonia decomposition experiments, in particular after 10 hours on stream. This may be explained by the release of elemental oxygen from the AC support or combined oxygen from residual P2O5 during the experiment under elevated temperature (750 C), as evidenced by formation of CO and CO2 in the effluent gases (Figure 4). However, the surface N/C ratios for both catalysts significantly increased during the ammonia decomposition experiments, as compared with the respective fresh catalyst. The increase in N/C ratios during the ammonia decomposition over the two catalysts implies the chemsorption of NH3 to the catalysts or the formation of nitrogen-containing compounds (e.g., metallic nitrides).

Figure 10 illustrates the N 1 s XPS spectra for the fresh and spent catalysts of Fe/AC. In the fresh catalyst, the widened peak at around 400 eV may be attributed to nitrogen present in an organic matrix, but this peak intensity was reduced after H2 reduction, possibly due to the release of nitrogen from the samples during reduction. After NH3 decomposition at 750 C over the catalyst the intensity increases, and it is further increased with time on stream. After 4 and 10 hour NH3 decomposition, the XPS spectra revealed NH3 (399.1 0.1 eV) and cyanides (399 1.5) on the surface of the catalysts due to the adsorption of the reactant of NH3 and a by-product (HCN) from the NH3 decomposition reactions. It is note-worthy that there were shoulder peaks detectable at the binding energy of 397.4 0.1 eV, which might be attributed to Fe nitrides (FeRN). FeRN were detected in some previous studies on ammonia decomposition using limonite Fe catalysts and brown coal char supported Fe catalysts, and they were considered as the active intermediates that were involved in the catalytic mechanism for NH3 decomposition [27,29].

Catalytic Mechanisms As discussed above, high activities of new catalysts of Fe/AC and Ni/AC for the decomposition of 2000 ppm NH3/He at 750 C are shown in Figure 2, where ammonia decomposition efficiencies (conversion of ammonia into N2 and H2) reached 90% with the Fe catalyst and 75% for the Ni catalyst, compared to only 15% with AC alone. Both catalysts showed superior resistance to deactivation versus other patented carbon-based Fe catalysts [46]. As shown in Figure 2, both catalysts remained active with slightly climbing activities as the reaction time increased up to 10 hours, showing no sign of deactivation, whereas the activity of the previously patented carbon based Fe catalysts declined after about 3 hours on the stream [29, 46]. Possible mechanisms to explain the high activities of the new catalysts are thus discussed as follows.

First, interestingly in the spent catalysts even after ammonia decomposition for 10 hours, no metal nitride such as NiXN or FeXN and no metal carbides were detectable by XRD or XPS (Figure 5-8). In contrast, in some previous work with limonite Fe catalysts and brown coal char supported Fe catalysts, FeXN species were detected after NH3 decomposition, and these metal nitrides were considered as the active intermediates that were involved in the catalytic mechanism for decomposition [27,29]. Furthermore, it was demonstrated in the previous work with brown coal char supported Fe catalysts that Fe carbide was formed during the ammonia decomposition process at 750 C [29]. It was also reported that the formation of iron carbide led to deactivation of the Fe catalyst [29]. For our two new catalysts (Ni/AC and Fe/AC), Fe carbides were not detected in the spent catalysts even after ammonia decomposition for 10 hours as shown in Figures 5 and 6. As such, the ammonia decomposition over the new catalysts may proceed with a different mechanism.

The presence of P in both the AC-supported catalysts was evidenced by XPS
analyses (Figures 7-9 and Table 3), and nickel/iron phosphides (Ni12P5, Ni3P, and Ni2P, Fe2P) were observed by both XRD and XPS in the new catalysts after the hydrogen reduction or after the ammonia decomposition. This suggests that these metal phosphides (Ni12P5, Ni3P, and Ni2P, Fe2P) might play a role in the ammonia decomposition reactions. To investigate whether these metal phosphides were responsible for the conversion of NH3 to N2 and H2 or not, commercial compounds of Fe2P and Ni2P were mixed physically with the AC alone to make 25 wt %
Fe2P/AC and 25 wt % Ni2P/AC respectively, and their catalytic effects were examined at 750 C. The results are shown in Figure 11. Although these bulk compounds were much less active than the two new AC-supported Fe and Ni catalysts, the activities of Ni2P and Fe2P were still evident, in particular for Ni2P.

Without being bound by any theory, it is possible the fine dispersion of these phosphides formed in-situ in the two new catalysts (Ni/AC and Fe/AC) during the ammonia decomposition process may be responsible for the high activities of the two new catalysts.

Table 3 Atomic ratios of the fresh and spent catalysts of Ni/AC and Fe/AC.
Sample Atomic Ratios O/C P/C N/C Ni/C Fe/C
Ni/AC, Fresh 0.44 0.07 0.03 0.10 -Ni/AC, 4 h 0.29 0.05 0.06 0.07 -Ni/AC, 10 h 0.20 0.03 0.05 0.04 -Fe/AC, Fresh 0.26 0.02 0.02 - 0.05 Fe/AC, 4 h 0.26 0.02 0.05 - 0.05 Fe/AC, 10 h 0.19 0.02 0.04 - 0.04 The Fe/AC catalyst was subjected to TPD measurements, and the results are shown in Figure 12. For curve (b) in Figure 12, the catalyst was first reduced by H2 at 500 C for 2 hours, and it was then treated with 2000 ppm/He NH3 for 4 h at 500 C before being quenched to room temperature and subjected to TPD
measurement. The TPD measurements were carried out by heating the catalyst sample in high-purity helium at 800 C or 900 C and held at this temperature for 30 minutes, when the desorbed N2 was detected by a GC-TCD. As shown in Figure 12, for both samples evolution of N2 proceeded significantly at temperatures >_600 C. At approximately 730 C a significant amount of N2 was detected from the ammonia-treated sample, and as the temperature was increased to 800 C there was further N2 detected at an increased rate. The fresh Fe/AC catalyst was also subjected to TPD measurements after H2 reduction to distinguish between the nitrogen present in the fresh Fe catalyst and the Fe-nitride complexes formed during NH3 decomposition. As shown from the XPS results (Table 3), the fresh catalyst of Fe/AC or Ni/AC contained a significant amount of inherent nitrogen (atomic ratio N/C of 0.02 or 0.03). Although the inherent nitrogen was detectable significantly at temperatures >_600 C, the N2 desorption rate peaked at 870 C for the catalyst without ammonia treatment. This might imply that the peaks formed in the Fe catalyst after NH3 treatment at temperatures greater than 800 C were more likely a result of the inherent nitrogen present in the fresh catalyst, while the N2 desorbed at the lower temperatures (600-800 C) might be due to the chemisorbed NH3 on the catalyst's surface or the decomposition of Fe-nitrides that could be formed during the NH3 treatment.

By analogy with the cycle mechanism involving Fe metal and nitrides proposed in the Fe-catalyzed NH3 decomposition studies [27, 29, 37], the following reaction schemes, involving intermediates of iron phosphide (Fe2P) and iron nitride (Fe4N), are proposed:

2Fe + P -) Fe2P (1) 6Fe + 3P + 2NH3(g)-3Fe2P+ N2(g) + 3H2(9) (2) Fe2P + 0.5NH3 (g) + 0.75H2 - 0.5Fe4N + PH3 (g) (3) PH3 (g) + 2Fe - Fe2P + 1.5H2 (g) (4) Fe4N 4 4Fe + 0.5N2 (g) (5) 4Fe + NH3 (g) 4 Fe4N + 1.5H2 (g) (6) The dependencies of standard Gibbs free energies (AGO) with temperature for these reactions are given in Figure 13. As seen in Figure 13, all of these reactions listed above are thermodynamically favourable at 750 C. Because the presence of Fe2P was identified and suggested by the XRD (Figure 6) and XPS
(Figure 8), this species could be formed via reactions (1) and (2) between a-Fe and the P in the AC support at elevated temperatures. The Fe2P might react with and active H2 or H atom on the catalyst surface to form Fe4N and PH3 (reaction 3).
The Fe2P could be regenerated by the reaction of PH3 and a-Fe (reaction 4).
The iron nitride species is very unstable and/or in a highly dispersed state in the catalyst (implied by the analyses of XRD and XPS), so that it would readily decompose to a-Fe and N2 at temperatures of >_ 400 C according to equation (5) [27, 29, 37].

Reaction of a-Fe and NH3 would regenerate Fe4N and produce H2 (reaction 6). It is thus possible that NH3 decomposition over the new AC-supported Fe/Ni catalyst may proceed through a cycle mechanism involving metal phosphides and nitrides.

It will be noted that while helium is used as a diluting gas in the examples described above, other inert gases other than helium could likely be used as diluting gases to save costs, e.g., nitrogen. In addition, it will be appreciated by those skilled in the art that it is not necessary to activate the catalysts with H2 before they are used.

Conclusion New Ni/Fe catalysts were prepared using an activated carbon (AC) support derived from peat by H3PO4 activation. The newly developed catalysts proved to be highly active for ammonia decomposition. The conversion of 2000 ppm NH3 diluted in helium over the Fe catalyst reached as high as 90% at 750 C and space velocity of 45000 h-1, compared with only about 15% with the AC alone. The new catalyst of Fe/AC was also much more active than the Fe catalyst supported on a commercial AC reported previously, and the two Fe/Ni catalysts showed superior performance with respect to their resistance to catalyst deactivation. Both catalysts remained active as the reaction time increased up to 10 hours without showing a sign of deactivation.

The fresh Ni/AC catalyst and Fe/AC catalyst were porous materials with an average pore diameter of 3.7 nm. In the fresh catalyst, very fine nanoparticles of metallic Ni or Fe were present, proved by XRD. In the spent catalysts, no metal nitride such as NiXN or FeRN, and no metal carbides were detectable by XRD, probably due to their unstable and highly dispersed states. The presence of nickel/iron phosphides (Ni12P5, Ni3P, and Ni2P, Fe2P) was observed by both XRD
and XPS in the spent catalysts after the hydrogen reduction and/or after the ammonia decomposition. The commercial available bulk compounds of Ni2P, Fe2P
were demonstrated to be active for ammonia decomposition, although they were much less active than the two new AC-supported Fe and Ni catalysts. Possibly, the fine dispersion of these phosphides formed in-situ in the two new catalysts (Ni/AC
and Fe/AC) during the ammonia decomposition process may be responsible for the high activities of these catalysts. TPD measurements of the Fe/AC catalyst suggested that the N2 desorbed at the lower temperatures (600-800 C) might be due to the decomposition of Fe-nitrides. A cycle mechanism, involving both metal phosphides and metal nitrides, was thus proposed for the NH3 decomposition reactions over the new Fe/Ni catalysts.

It will be appreciated that the present invention as applied to decomposition of ammonia to hydrogen is not limited to the above-described examples. For example, the inert gas may be any one or combination of helium, nitrogen, and argon.
The desired operating temperature may be in the range from about 600 to about 900 C, while 750 C is a preferred temperature. The space velocity may be in a range from about 5000 to about 50,000 W.

The inert gas flow may be replaced with about 2000 ppm NH3 diluted with high purity inert gas which could be any one or combination of helium, nitrogen, and argon.

When transition metal is Fe only, it may be present in a range from about 2 wt% Fe/activated carbon to about 20 wt% Fe/activated carbon, and preferably at a concentration of about 10 to about 20 wt% Fe/activated carbon.

When the transition metal is Ni only, it may be present in a range from about 2 wt% Ni/activated carbon to about 20 wt% Ni/activated carbon, and preferably it may be present at a concentration from about 10 to about 20 wt% Ni/activated carbon.

It is not required to activate the catalyst with hydrogen but if one chooses to do so, it can be exposed to a flow of hydrogen at a preselected temperature in a range from about 400 to about 600 C.

Thus, the present invention very advantageously provides for the first time a method of making activated carbon catalysts which makes use of peat as a raw material to make highly stable activated carbon catalysts suitable for ammonia decomposition.

Compared to the publication of Xu, C. entitled "Catalytic decomposition of ammonia gas with metal cations present naturally in low rank coals" in Fuel 84 (2005) 1957-1967, and their earlier work with Fe-/Ni-loaded coal chars (Y.
Ohtsuka, C. Xu, D. Kong, N. Tsubouchi. Fuel, 83 (2004), 685), the present invention is very advantageous because the new peat-derived catalysts disclosed herein are more active and showed better stability (no significant decrease in decomposition efficiency after 10 hours time on stream).

It is difficult to compare activities between the present invention and the activated carbon-supported Ni and Ni catalysts reported in the publication entitled "Study on ammonia decomposition catalyzed by active carbon-supported Ni or Fe"

by Yang in Hunan University Xuebao 33(5) (2006) 100-104, where pure ammonia was used and the decomposition tests operated at different temperatures.
Nevertheless, according to the results from previous work (Y. Ohtsuka, C. Xu, D.
Kong, N. Tsubouchi. Fuel, 83 (2004), 685), where the commercial AC-supported Fe catalyst led to ammonia conversion of only 30% under the same reaction conditions (750 C, 2000 ppm NH3/He, and SV of 45000 h-1), the present invention is also advantageous with respect to better and more stable activity.

As compared to the paper entitled "Commercial Fe- or Co-containing carbon nanotubes as catalysts for NH3 decomposition" by Su et al., Chem. Commun.

(2007) 1916-1918, the present invention is more advantageous because the production cost of carbon nanotubes is much higher than that of the peat-derived activated carbon of the present invention.

The catalysts according to the present invention may be used in many applications, including but not limited to the cleanup of hot gas from biomass gasification processes. Currently, syngas must be cooled and run through water scrubbers to remove ammonia. This process results in loss of energy due to the cooling step and production of waste water that must be treated prior to disposal. The method disclosed herein makes possible cleaning of syngas at a higher temperature, thus reducing or eliminating the cooling step. The present method also eliminates the production of waste water.

The catalysts may also be used for the cleanup of exhaust gas in semiconductor production, and as catalysts for production of hydrogen for fuel cells from stored ammonia. Ammonia may be stored in tanks (instead of having to store gaseous hydrogen); then when H2 is needed, the ammonia is broken down by the catalyst to form N2 and H2. Such H2 is then used in the fuel cell and N2 is expelled into the atmosphere. Although the catalysts from this invention were tested and demonstrated high activity mainly in a diluted stream of ammonia (1000-5000 ppm), they are expected to be effective when a pure ammonia stream is used for hydrogen generation for fuel cells.

As used herein, the coordinating conjunction "and/or" is meant to be a selection between a logical disjunction and a logical conjunction of the adjacent words, phrases, or clauses. Specifically, the phrase "X and/or Y" is meant to be interpreted as "one or both of X and Y" where X and Y are any word, phrase, or clause.

As used herein, the terms "comprises" and "comprising" are to be construed as being inclusive and open-ended, and not exclusive. Specifically, when used in this specification including claims, the terms "comprises" and "comprising"
and variations thereof mean that the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.

The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiments illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.

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Claims (46)

THEREFORE WHAT IS CLAIMED IS:

1. A method of synthesizing carbon-based catalysts, comprising the steps of:
a) preparing activated carbon by mixing particles of dried peat with an activation agent to form a first slurry, thermally treating the slurry at a temperature in a range from about 200°C to about 800°C and cooling to room temperature; and b) mixing the activated carbon with a liquid solution containing salts of: one or more transition metals, one or more noble metals, or any combination of transition metal and noble metal, to form a second slurry with the liquid solution impregnating the activated carbon to give metal-loaded activated carbon, removing the liquid and drying the second slurry, followed by calcination in an inert gas to produce particles of the carbon-based catalysts.

2. The method according to claim 1 including, prior to mixing the peat with the activation agent, drying the peat and crushing and sieving the peat to obtain peat particles with sizes in a desired size range.

3. The method according to claim 1 or 2 including, after cooling to room temperature in step a), washing with a second solution until a substantially neutral pH is obtained.

4. The method according to claim 3 wherein the thermally treated sample is washed with water until the substantially neutral pH is obtained.

5. The method according to claim 1, 2, 3 or 4 wherein said activation agent is any one or combination of H3PO4, ZnCl2 and KOH.

6. The method according to claim 1 wherein said activation agent is H3PO4.

7. The method according to any one of claims 1 to 6 wherein said particles of carbon-based catalysts are micro- or meso-porous with a specific surface area >50 m2/g.

8. The method according to any one of claims 1 to 7 wherein said particles of carbon-based catalysts are crushed and sieved to give particles having sizes in a range from about 300 to about 850 µm.

9. The method according to any one of claims 1 to 7 wherein said particles of carbon-based catalysts are crushed and sieved to give particles having a size greater than about 850 µm.

10. The method according to any one of claims 1 to 9 wherein said noble metals are selected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, and any combination thereof.

11. The method according to any one of claims 1 to 9 wherein said transition metals are selected from the group consisting of Fe, Ni, and combinations thereof.

12. The method according to any one of claims 1 to 9 wherein said transition metals are selected from the group consisting of Cr, Mn, Co, Cu, Fe, Ni, and combinations thereof.

13. The method according to any one of claims 1 to 12 wherein said transition metals are selected from any one or combination of 40 chemical elements 21 to 30, 39 to 48, 71 to 80, and 103 to 112 of the Periodic Table of Elements.

14. The method according to any one of claims 1 to 13 wherein said liquid solution is an aqueous solution of water and an organic solvent.

15. The method according to claim 14 wherein said organic solvent is any one of an alcohol, acetone, acetic acid, and any combination thereof.

16. The method according to claim 15 wherein said alcohol is methanol or ethanol, or a combination thereof.

17. The method according to any one of claims 1 to 16 including, after cooling to room temperature, drying at a temperature from about 40°C to about 150°C in vacuum or a gas atmosphere before being crushed.

18. The method according to claim 17 wherein said gas is any one or combination of air, nitrogen, argon, helium, and CO2.

19. The method according to claim 3 wherein said washed thermally treated sample is dried at a temperature of about 105°C in air before being crushed.

20. The method according to claim 1 wherein step a) includes, after cooling to room temperature, crushing the activated carbon and sieving into particles of peat-based activated carbon having sizes in a first desired range of sizes, and wherein step b) includes, after calcination, crushing the particles of carbon-based catalysts and sieving into particles of a second desired range of sizes, wherein the first and second desired ranges of sizes may or may not be the same.

21. A carbon-based catalyst produced by any one of claims 1 to 20.

22. The method according to any one of claims 1 to 9 wherein said transition metal is Fe, Ni, or a combination of Fe and Ni.

23. A carbon-based catalyst produced by the method of claim 22.

24. A method of catalytic decomposition of ammonia into N2 and H2 using the carbon-based catalyst of claim 23, comprising the steps of;

heating a catalyst bed containing the carbon-based catalyst to a temperature in a range from about 500°C to about 1000°C while flowing an inert gas over the catalyst in the catalyst bed and, once the catalyst bed reaches a desired operating temperature in said temperature range, replacing the flow of inert gas with a flow of ammonia gas diluted with an inert gas in a preselected ratio, or pure ammonia, the flow of diluted ammonia gas or pure ammonia being at a pre-selected space velocity.

25. The method according to claim 24 wherein the inert gas is any one or combination of helium, nitrogen, and argon.

26. The method according to claim 24 or 25 wherein the temperature is in a range from about 600 to about 900°C.

27. The method according to claim 24 or 25 wherein the temperature is about 750°C.

28. The method according to any one of claims 24 to 27 wherein the pre-selected space velocity is in a range from about 5000 to about 50,000 h-1.

29. The method according to any one of claims 24 to 27 wherein the inert gas flow is replaced with 2000 ppm NH3 diluted with high purity inert gas under a space velocity of about 45000 h-1.

30. The method according to claim 24 wherein, prior to NH3 decomposition, the catalyst bed is heated to about 500°C at a heating rate of about 15°C/min in an inert gas flow of about 180 ml/min, and then subjected to reduction using H2 at a flow rate of about 200 ml/min for about 2 h, and wherein after H2 reduction, the catalyst bed is heated to about 750°C with a heating rate of about 15°C/min in an inert gas flow of about 180 ml/min and, when the temperature reaches 750°C, the inert gas flow is replaced with about 1000 to about 5000 ppm NH3 diluted with high purity inert gas under a space velocity in a range from about 5000 to about 50,000 h-1.

31. The method according to claim 30 wherein the inert gas flow is replaced with about 2000 ppm NH3 diluted with high purity inert gas under a space velocity of about 45,000 h-1.

32. The method according to claim 31 wherein the inert gas is any one or combination of helium, nitrogen, and argon.

33. The method according to any one of claims 24 to 32 wherein said transition metal is Fe present in a range from about 2 wt% Fe/activated carbon to about wt% Fe/activated carbon.

34. The method according to any one of claims 24 to 32 wherein said transition metal is Fe present at a concentration of about 10 to about 20 wt%
Fe/activated carbon.

35. The method according to any one of claims 24 to 32 wherein said transition metal is Ni present in a range from about 2 wt% Ni/activated carbon to about wt% Ni/activated carbon.

36. The method according to any one of claims 24 to 32 wherein said transition metal is Ni present at a concentration from about 10 to about 20 wt%
Ni/activated carbon.

37. The method according to any one of claims 24 to 36 including a step of activating the carbon-based catalyst by exposing it to a flow of hydrogen at a preselected temperature in a range from about 400 to about 600°C.

38. A peat-based catalyst, comprising:

porous peat-based activated carbon particles impregnated with a pre-selected amount of: one or more transition metals, one or more noble metals, or any combination of transition metal and noble metal.

39. The catalyst according to claim 38 wherein said metal is Fe present in a range from about 2 wt% Fe/activated carbon to about 20 wt% Fe/activated carbon.

40. The catalyst according to claim 38 wherein said metal is Fe present at a concentration of about 10 to about 20 wt% Fe/activated carbon.

41. The catalyst according to claim 38 wherein said metal is Ni present in a range from about 2 wt% Ni/activated carbon to about 20 wt% Ni/activated carbon.

42. The catalyst according to claim 38 wherein said metal is Ni present at a concentration from about 10 to about 20 wt% Ni/activated carbon.

43. The catalyst according to any one of claims 38 to 42 wherein said peat-based activated carbon particles are activated by any one or combination of H3PO4, ZnCl2 and KOH.

44. The catalyst according to any one of claims 38 to 43 wherein said porous peat-based activated carbon particles are micro- or meso-porous with a specific surface area > 50 m2/g.

45. The catalyst according to any one of claims 38 to 43 wherein said porous peat-based activated carbon particles have sizes in a range from about 300 to about 850 µm.

46. The catalyst according to any one of claims 38 to 43 wherein said porous peat-based activated carbon particles have a size greater than about 850 µm.