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Preparation of Nano-NiO Particles and Evaluation of Their Catalytic Activity in Pyrolyzing Biomass Components† Jianfen Li,‡,§ Rong Yan,*,§ Bo Xiao,‡ David Tee Liang,§ and Dong Ho Lee§ School of EnVironmental Science & Engineering, Huazhong UniVersity of Science and Technology, Wuhan, 430074, People’s Republic of China, and Institute of EnVironmental Science and Engineering, Nanyang Technological UniVersity, InnoVation Center, Block 2, Unit 237, 18 Nanyang DriVe, Singapore 637723 ReceiVed May 27, 2007. ReVised Manuscript ReceiVed July 26, 2007
This study focuses on the preparation of nano-NiO particles and their applications as catalysts in biomass pyrolysis. First, nano-NiO particles were prepared via precursors, which were obtained by homogeneous precipitation involving an aqueous solution of nickel nitrate hexahydrate and urea. Different approaches such as TGA, FTIR, XRD, BET, and TEM were used to characterize the precursors and nano-NiO particles. The formula of the precursor was identified as NiCO3 · 2Ni(OH)2 · nH2O, and it could completely translate into NiO nanoparticles below 360 °C under an air atmosphere. The prepared nano-NiO particles were found to be spherical in shape and well dispersed with weak agglomeration. They had generally a high purity and fine crystal phase of cubic syngony with a mean size of ∼7.5 nm and specific surface area of 188.0 m2/g. Furthermore, the catalytic activity of nano-NiO particles in pyrolyzing three biomass components (cellulose, xylan, and lignin) was preliminarily investigated using a thermogravimetric analyzer, and the results were compared with those of micro-NiO particles under the same conditions. With the presence of a catalyst, the weight loss of three biomass components mainly occurred at relatively lower temperature, with the final yield of residues and the activation energy of biomass pyrolysis being reduced markedly. Nano-NiO particles demonstrated a more effective catalytic effect in biomass pyrolysis over micro-NiO particles. The pathway of biomass pyrolysis in the presence of catalyst was briefly discussed.
Biomass gasification/pyrolysis is one of the promising technologies converting biomass to bioenergy. One of the major issues in biomass gasification/pyrolysis is dealing with the tar formed during the process.1,2 Catalytic cracking, which could operate at relatively lower temperatures and generate high tar removal efficiency, is recognized as the most efficient method to diminish the tar content in the gasification gas mixture.3 Numerous materials have been tested as potential catalysts for tar removal, including Ni-based catalysts, calcined dolomites and magnesites, zeolites, iron catalysts, and so forth. Among them, dolomite and Ni-based catalysts are believed to be the most popular and effective.4 Although it is cheap, calcined dolomite gets easily eroded and is now used mostly as a guard bed additive in secondary treatment of double bed. The development of Ni-based catalysts and its commercial application is promising, due to its high
effectiveness in tar removal and production of high yields of syn-gas.5 Still, there are certain technical barriers of applying Ni-based catalysts in real industrial application, which are mainly associated with their deactivation due to carbon deposition on the catalyst and poisoning with the presence of H2S. Further studies are hence needed to develop novel nickel-based catalysis with improved performance. In recent years, nanomaterials have attracted extensive interest for their unique properties in various fields (such as catalytic, electronic, and magnetic properties) in comparison with their bulk counterparts.6,7 In particular, nickel oxide (NiO) has demonstrated excellence when being used as a catalyst.8–10 Due to the different effects in terms of volume, quantum size, surface, and macroscopic quantum tunnel, nanometer-sized NiO (nano-NiO) particles are expected to possess many improved properties over those of bulk
† Presented at the International Conference on Bioenergy Outlook 2007, Singapore, April 26–27, 2007. * Corresponding author. Phone: (65)67943244. Fax: (65)67921291. E-mail:
[email protected]. ‡ Huazhong University of Science and Technology. § Nanyang Technological University. (1) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Pretreated olivine as tar removal catalyst for biomass gasifiers: investigation using naphthalene as model biomass tar. Fuel Process. Technol. 2005, 86 (6), 707–730. (2) Devi, L.; Ptasinski, K. J.; Janssen, F. J. G. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003, 24 (2), 125–140. (3) Sutton, D.; Kelleher, B.; Ross, J. R. H. Review of literature on catalysts for biomass gasification. Fuel Process. Technol. 2001, 73 (3), 155– 173. (4) Furusawa, T.; Tsutsumi, A. Comparison of Co/MgO and Ni/MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification. Appl. Catal., A 2005, 278 (2), 207– 212.
(5) Czernik, S.; French, R.; Feik, C.; Chornet, E. Hydrogen by Catalytic Steam Reforming of Liquid Byproducts from Biomass Thermoconversion Processes. Ind. Eng. Chem. Res. 2002, 41 (17), 4209–4215. (6) Li, Q.; Wang, L.-S.; Hu, B.-Y.; Yang, C.; Zhou, L.; Zhang, L. Preparation and characterization of NiO nanoparticles through calcination of malate gel. Mater. Lett. 2007, 61 (8–9), 1615–1618. (7) Bhargava, R. N. Doped nanocrystalline materials)Physics and applications. J. Lumin. 1996, 70 (1–6), 85–94. (8) Wang, Y.; Zhu, J.; Yang, X.; Lu, L.; Wang, X. Preparation of NiO nanoparticles and their catalytic activity in the thermal decomposition of ammonium perchlorate. Thermochim. Acta 2005, 437 (1–2), 106–109. (9) Ichiyanagi, Y.; Wakabayashi, N.; Yamazaki, J.; Yamada, S.; Kimishima, Y.; Komatsu, E.; Tajima, H. Magnetic properties of NiO nanoparticles. Physica B 2003, 329–333 (Part 2), 862–863. (10) Biju, V.; Abdul Khadar, M. Analysis of AC electrical properties of nanocrystalline nickel oxide. Mater. Sci. Eng., A 2001, 304–306, 814– 817.
1. Introduction
10.1021/ef700283j CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2007
Preparation of Nano-NiO Particles and micrometer-sized NiO (micro-NiO) particles.11,12 There are several methods available on the preparation of NiO nanoparticles,13,14 in which the precipitation method exhibits such advantages as a simple procedure and technology, high yield, easily controlled particle size, and so forth. Although nano-NiO particles as catalysts have been widely used,8–10 to our knowledge, their application in biomass pyrolysis/ gasification has not been studied. Besides, the specific surface area of a nano-NiO catalyst would be much bigger than that of a conventional nickel-based catalyst or micro-NiO catalyst, and in particular, nano-NiO particles can be loaded on the surface of certain carriers for savings cost. On the basis of all of the understanding mentioned, it is thus highly expected that nano-NiO catalyst would have a better catalytic activity in enhancing the performance of biomass gasification/pyrolysis. This present work focuses on developing a novel nano-NiO catalyst and evaluating its catalytic effects in biomass pyrolysis. With the success of this study, the loaded nano-NiO catalyst on the surface of certain carriers is to be developed and applied in biomass pyrolysis/gasification using a bench-scale dual-bed reactor (including a fluidized bed gasifier and a fixed bed packed with the Ni-based catalyst for hot gas cleaning), which is believed to be more advantageous in practical application. This study includes two main aspects: (1) To prepare and characterize the nano-NiO catalysts. Catalysts are synthesized by a homogeneous precipitation method involving an aqueous solution of nickel nitrate hexahydrate and urea. (2) To investigate the catalytic activity of the as-synthesized nano-NiO catalyst on biomass pyrolysis. The results are compared with those of micro-NiO under the same conditions. The three biomass components (cellulose, hemicellulose, and lignin) were used as the model biomass in pyrolysis, which was performed in the presence and absence of nano-NiO or micro-NiO catalysts, in the thermogravimetric analyzer. The results obtained would promote the further development of a novel catalyst for efficient biomass pyrolysis.
2. Experiment and Methods 2.1. Materials. All of the chemicals such as nickel nitrate hexahydrate (Ni(NO3)2 · 6H2O) and urea (CO(NH2)2) used in the preparation of nano-NiO particles were analytical grade, and they were used without further purification. Deionized water was used throughout the study. In testing the catalytic activity, the microNiO particles (325 mesh, approximate 44 µm) used for a comparison were purchased from Johnson Matthey Company (Alfa Aesar) and the nano-NiO particles were prepared by ourselves. Three biomass components, including cellulose, hemicellulose (xylan), and lignin, were purchased from Sigma-Aldrich Chemie GmbH. Here, xylan has been used as a representative of hemicellulose because the commercial hemicellulose is difficult to obtain and xylan has been widely used (instead of hemicellulose) in pyrolysis processes.15–17 The cellulose, xylan, and lignin are in the form of fibrous powder, yellow powder, and brown alkali powder, respectively. The average (11) Carnes, C. L.; Klabunde, K. J. The catalytic methanol synthesis over nanoparticle metal oxide catalysts. J. Mol. Catal. A: Chem. 2003, 194 (1–2), 227–236. (12) Biju, V.; Abdul Khadar, M. Fourier transform infrared spectroscopy study of nanostructured nickel oxide. Spectrochim. Acta, Part A 2003, 59 (1), 121–134. (13) Deki, S.; Iizuka, S.; Mizuhata, M.; Kajinami, A. Fabrication of nanostructured materials from aqueous solution by liquid phase deposition. J. Electroanal. Chem. 2005, 584 (1), 38–43. (14) Dierstein, A.; Natter, H.; Meyer, F.; Stephan, H. O.; Kropf, C.; Hempelmann, R. Electrochemical deposition under oxidizing conditions (EDOC): a new synthesis for nanocrystalline metal oxides. Scr. Mater. 2001, 44 (8–9), 2209–2212. (15) Varhegyi, G.; Antal, M. J.; Jakab, E.; Szabo, P. Kinetic modeling of biomass pyrolysis. J. Anal. Appl. Pyrolysis 1997, 42 (1), 73–87. (16) Orfao, J. J. M.; Antunes, F. J. A.; Figueiredo, J. L. Pyrolysis kinetics of lignocellulosic materials--three independent reactions model. Fuel 1999, 78 (3), 349–358. (17) Rao, T. R.; Sharma, A. Pyrolysis rates of biomass materials. Energy 1998, 23 (11), 973–978.
Energy & Fuels, Vol. 22, No. 1, 2008 17 particle size of xylan is ∼100 µm, and those of cellulose and lignin are each ∼50 µm. 2.2. Preparation and Characterization of Nano-NiO Particles. 2.2.1. Preparation of Nano-NiO Particles. Nickel oxide nanoparticles were prepared through the following process. First, the precursor of the nano-NiO was synthesized using a homogeneous precipitation method. According to the molar ratio of nickel nitrate hexahydrate to urea at 1:4, a stoichiometric amount of Ni(NO3)2 · 6H2O (0.08 mol) and CO(NH2)2 (0.32 mol) was accurately weighed and dissolved into 80 mL of deionized water, respectively. The two solutions were mixed in a beaker and stirred with a magnetic stirrer at room temperature until a homogeneous solution obtained. Thereafter, the mixture was transferred into a round bottom flask, sealed, and maintained heating at 115 °C for 1.5 h in an oil bath. In this process, a kind of light green sediment (i.e., the precursor) was formed. After the reaction was completed, the precipitated powders were filtered and washed with deionized water to neutral and colorless. This was to remove the possibly adsorbed ions and chemicals to reduce the potential of agglomeration. After being dried in an oven at 90 °C for 6 h, the precursors were calcined in a muffle furnace at 400 °C for 1 h to obtain the products in dark color (i.e., NiO nanoparticles). The calcined products were then collected for further analyses. 2.2.2. Characterization of Nano-NiO Particles and Their Precursor. Infrared (IR) spectra of the precursor were obtained on a Fourier transform infrared (FTIR) spectrometer (BioRad Excalibur Series, model FTS 3000) equipped with a deuterated triglycine sulfate (DTGS) detector using the KBr pellet technique to determine the chemical structure of the precursor. Samples of IR analysis were prepared by mixing the precursor powders with KBr (1:300 by weight ratio) in a steel die. The thermal decomposition behavior of the precursor was studied in a thermogravimetric analyzer (TGA 2050, TA, U.S.), and operated in a nitrogen atmosphere at a flow rate of 80 mL/min. The temperature procedure of thermogravimetric analysis (TGA) tests was from the ambient temperature to 700 °C at a heating rate of 10 °C/min and held at 700 °C for 3 min to make sure that decomposition was completed. The crystalline structures of both the catalyst product and its precursor were identified with a powder X-ray diffractometer (BDX3200, operated at 36 kV, 20 mA), employing Cu KR radiation (λ ) 0.15418 nm). The sample was scanned from 10 to 85° (2θ) with a scanning rate of 4°/min. The average crystalline size of the nano-NiO product was estimated according to the Scherrer equation and further confirmed by its transmission electron microscopy (TEM) results. TEM modeled FEI TECNAIG2 working at 100 kV was used to understand the grain size and morphology of the nanoNiO product. An accelerated surface area porosimetry (ASAP 2010) instrument, which used liquid nitrogen at 77 K, was applied to measure the BET surface area of NiO nanoparticles for estimating its catalytic activity. 2.3. Catalytic Activity of Nano-NiO Particles in Pyrolysis of Biomass Components. In order to investigate the catalytic activity of the as-synthesized nano-NiO on pyrolysis of biomass, three biomass components (cellulose, hemicellulose, and lignin) were used as a substrate. Both the purchased micro-NiO and the prepared nano-NiO particles were used as catalysts in biomass pyrolysis. The pyrolysis experiments were performed in the thermogravimetric analyzer (model TGA 2050). Approximately 15 mg of each pure biomass component either alone or mixed with 0.5 mg of catalyst (accounts for 3%) was placed in the sample pan, with the sample size of every experiment maintained the same. To ensure a good dispersion of catalyst in the substrate, the substrate and the catalyst particles at a controlled amount of each were mixed via physical mixing with iterative stirring until a homogenous color was attained. Meanwhile, the scanning electron microscopy (SEM) photographs (not shown here) of the mixed samples were obtained, confirming a good mixing of particles. Then, the sample was heated under conditions of linear temperature increase (10 °C/min) from the ambient temperature to 900 °C and finally held at 900 °C for 3 min to make sure that pyrolysis was completed. Purified nitrogen
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Figure 1. FTIR spectra of the as-synthesized precursor.
at a constant flow rate of 40 mL/min was used as the purge gas to provide an inert atmosphere for pyrolysis and to remove any gaseous and condensable products that evolved. Under the selected conditions, the limitation of heat transfer inside the particles is negligible, based on the results generated from pretesting of the same sample at different heating rates. In addition, it is recognized that the Ni element serves as the active component in the Ni-base catalysts; thus, catalysts comprising NiO are usually activated through reduction to Ni before being used. On the other hand, an earlier work18 indicated that the commercial Ni-base catalyst (NiO as the main component) without reduction could be used in biomass catalytic pyrolysis and gasification, as the catalyst could be reduced and activated by the resulting gas products (including H2 and CO) in the initial pyrolysis period. With the focus of this study on the preparation and characterization of nano-NiO catalyst, the performance of the developed catalyst was preliminarily evaluated using TGA, in comparison with the microNiO catalyst, where the prereduction of catalysts was ignored for the purpose of simplification.
3. Results and Discussion 3.1. Analysis and Characterization of the Precursor. 3.1.1. FTIR Analysis of the Precursor. The FTIR spectra of the precursor powder dried at 105 °C for 6 h are illustrated in Figure 1 shown in transmittance percentage. Several absorption peaks were observed. The broad absorption band centered at 3445 cm-1 was attributable to the O–H bond stretching vibrations, and the band near 1620 cm-1 was assigned to H–O–H bending vibrations. This provided the evidence for the water of hydration in the structure, and it also implied the presence of hydroxyl in the structure. The wide absorption band around 1384 cm-1 in Figure 1 indicated the existence of CO32ions, and the three bands appearing around 1114, 833, and 620 cm-1 were correlated with the stretching and bending vibrations of the intercalated C–O species in the precursor. This further confirmed the presence of CO32- ions.19 Thus, the IR spectrum of the as-synthesized precursor was characteristic of hydrated basic carbonates. Furthermore, the broad absorption bands appearing around 2205 cm-1 in Figure 1 indicated the existence of CO2 which derived from the powder samples prepared in air.20 The strong absorption band around 410 cm-1 was assigned to Ni–O (18) Zhang, R.; Brown, R. C.; Suby, A.; Cummer, K. Catalytic destruction of tar in biomass derived producer gas. Energy ConVers. Manage. 2004, 45 (7–8), 995–1014. (19) Xin, X.; Lu, Z.; Zhou, B.; Huang, X.; Zhu, R.; Sha, X.; Zhang, Y.; Su, W. Effect of synthesis conditions on the performance of weakly agglomerated nanocrystalline NiO. J. Alloys Compd. 2007, 427 (1–2), 251– 255. (20) Liu, X.-M.; Zhang, X.-G.; Fu, S.-Y. Preparation of urchinlike NiO nanostructures and their electrochemical capacitive behaviors. Mater. Res. Bull. 2006, 41 (3), 620–627.
Figure 2. XRD pattern of the synthesized precursor.
Figure 3. TG–DTG curves of the precursor.
stretching vibration, indicating the Ni–O structure presented in the as-synthesized precursor. Therefore, the as-synthesized precursor could possibly be the nickel salt of hydrated basic carbonates. 3.1.2. X-ray Diffraction (XRD) Analysis of the Precursor. The crystalline structures of the precursor were characterized using an X-ray diffractometer, and the result is shown in Figure 2. The diffraction data were in good agreement with the standard spectrum (JCPDS, No. 35-0501),20 which indicated that the precursor probably corresponded to Ni2(CO3)(OH)2, namely, NiCO3 · Ni(OH)2 with poor crystallization. The structure of the precursor for Ni2(CO3)(OH)2 resulting from XRD analysis was consistent with the conclusion of IR spectrum analysis on the precursor. Combining the results of XRD and FTIR analysis on the precursor indicated that the precursor was nickel basic carbonate hydrated, and its molecular formula could be NiCO3 · Ni(OH)2 · nH2O. 3.1.3. ThermograVimetric Analysis of the Precursor. Figure 3 shows the thermal decomposition result of the precursor from the ambient temperature to 700 °C under a nitrogen atmosphere, with both the thermogravimetry curves (TG, in units of wt %) and differential thermogravimetry curves (DTG, in units of %/°C) given. The TG curve indicated that the weight loss of precursors occurred from 50 to 360 °C, suggesting that the precursors decomposed completely to become nickel oxide below 360 °C. Three distinct intervals of weight loss were observed in the TG curves, accompanied by three peaks of weight loss rate in the DTG curves. The first peak of weight loss rate located between 50 and 153 °C might be attributed to
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Figure 5. TEM micrograph of NiO nanoparticles. Figure 4. XRD pattern of the products.
the thermal dehydration of the precursors and the evaporation of the physically absorbed impurities such as urea, and the corresponding weight loss was 5.3%. The second peak of weight loss rate located between 233 and 291 °C may be related to the decomposition of nickel hydroxide in the precursors accompanying a weight loss of 9.3%, which is close to the theoretically stoichiometric weight loss for 8.5% calculated by dehydration of nickel basic carbonate (NiCO3 · Ni(OH)2). The biggest peak of weight loss shown between 291 and 360 °C in the DTG curve may correlate with the decomposition of nickel carbonate in the precursors, with correspondingly a weight loss of 21.4%, which was also very close to the theoretically stoichiometric value of 20.8% calculated by the percent of subtracting CO2 from nickel basic carbonate. The TGA/DTG results confirmed again the earlier conclusion of the precursor formula: NiCO3 · Ni(OH)2 · nH2O. The following progress of thermal decomposition reaction on the precursor was thus suggested: 50 - 153 °C ⁄ -nH2O
NiCO3 · Ni(OH)2 · nH2O 98 233 - 291 °C ⁄ -H2O
NiCO3 · Ni(OH)2 98 291 - 360 °C ⁄ -CO2
NiCO3 · NiO 98 2NiO 3.2. Characteristics of the Nano-NiO Particle. 3.2.1. XRD Analysis of the Nano-NiO Particle. Figure 4 shows XRD patterns of the products obtained after the precursors were calcined at 400 °C for 1 h. All of these diffraction peaks in Figure 4, not only the peak positions appearing at 2θ ) 37.28, 43.28, 62.88, 75.28, and 79.48 but also their lattice parameters, were quite consistent with those of the standard JCPDS Card No. 04-0835 for the standard spectrum of the pure and cubic NiO.20,21 It was seen that these characteristic diffraction peaks in the pattern had a marked broadening effect. The results indicated that the products were nano-NiO crystal of cubic structure; they have a high purity and small particle size with a fine crystal phase. According to the Scherrer formula D ) 0.89λ/β cos θ, where D represents the average particle size, β stands for the full width at half-height of the peaks, λ is the X-ray wavelength, and θ is the diffraction angle of the peak, (21) Li, G.-J.; Huang, X.-X.; Shi, Y.; Guo, J.-K. Preparation and characteristics of nanocrystalline NiO by organic solvent method. Mater. Lett. 2001, 51 (4), 325–330.
the mean particle size of the as-synthesized products was thus calculated at about 7.5 nm. 3.2.2. TEM Analysis of the Nano-NiO Particle. Figure 5 shows a TEM micrograph of the NiO nanoparticles. TEM analysis of the products provided information on the size and morphology of NiO nanoparticles and their status of agglomeration. It can be seen from Figure 5 that the NiO nanoparticles had spherical shapes and were well dispersed with weak agglomeration. Meanwhile, the as-calcined NiO particles were found to have a small and narrow size distribution in a range from 7 to 9 nm. This is in good agreement with the result calculated by the Scherrer formula and the data of powder X-ray diffraction. According to the TEM image, it could be concluded that this preparation method of the precursor obtained by homogeneous precipitation had successfully overcome the problem of agglomeration and the calcination condition (400 °C for 1 h) was appropriate to obtain the NiO nanoparticles of less crystalline size. 3.2.3. BET Analysis of the Nano-NiO Particle. The specific surface area of the nano-NiO products calculated by the BET method after being measured by an ASAP 2010 instrument was 188.0 m2/g, while that of the micro-NiO particles purchased was 28.6 m2/g. In particular, the external surface area and micropore area of the nano-NiO particles were 179.2 and 8.8 m2/g, respectively, and that of the micro-NiO particles was correspondingly 24.5 and 4.1 m2/g. It showed that both nanoNiO and micro-NiO particles were almost nonporous, testified by their extremely low micropore areas. The much higher external surface area of nano-NiO particles was most likely attributed to their smaller size of particles. As we know, the external surface of particles was used as a contact surface in catalytic pyrolysis of biomass components. The larger the external surface area of a catalyst, the higher the chance of gas contacting particles there is, for both cases of CO and H2 contacting NiO for reduction, and volatile organics contacting Ni for catalytic pyrolysis. Overall, the much higher external surface areas of the nano-NiO products indicated their high possibility of application as an efficient catalytic material. 3.3. Evaluation on the Catalytic Activity of Nano-NiO Particles. 3.3.1. TG Analysis of the Nano-NiO Effect on Pyrolysis of Biomass Components. The TG analysis of either nano-NiO or micro-NiO particles alone was first conducted under the same conditions as those for biomass samples. The results showed that a weight loss of 2.8% for nano-NiO and 2.3% for micro-NiO occurred near 100 °C due to the release of moisture, and after that, there was no obvious weight loss with further increase in temperature to 900 °C (data were not shown
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istics of the three components (cellulose, xylan, and lignin) in the absence of catalyst were similar to those reported in the literature.24–26 The cellulose and xylan decomposed in a single, well-defined, rather narrow peak, as observed in earlier investigations.15,27,28 Most of the weight loss for cellulose happened from 330 to 390 °C. The pyrolysis of pure xylan (the typical hemicellulose) focused at 240–320 °C. In comparison to the sharp decomposition of cellulose and xylan, the lignin decomposed over a broad temperature range with a very low weight loss rate (
