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J. Phys. Chem. C 2007, 111, 9504-9508
Preparation and Formation Mechanism of Hollow Microspherical Tungsten Carbide with Mesoporosity Chun’an Ma,*,†,‡ Nigel Brandon,‡ and Guohua Li† State Key Laboratory Breeding Base for Green Chemistry Synthesis Technology, Research Center of Nano Science and Technology, Zhejiang UniVersity of Technology, Hangzhou 310032, China, and Energy Futures Laboratory, Department of Earth Science and Engineering, Faculty of Engineering, Imperial College London, London, SW7 2AZ, United Kingdom ReceiVed: March 26, 2007; In Final Form: April 26, 2007
Hollow microspherical tungsten carbide with mesoporosity was prepared by spray-drying sphere miniaturization/ gas-solid reaction, using ammonium metatungstate (AMT) as a precursor, carbon monoxide as a reducing gas, and carbon dioxide as a carrier gas. The samples were characterized by XRD, SEM, and EDS. XRD and EDS results show that the samples are phase pure WC, with a ratio of W/C ≈ 1. SEM results show that the hollow microsphere forms during spray-drying sphere miniaturization, and mesoporosity forms by gas-solid reaction during subsequent heat treatment. To investigate the phase transition during the gas-solid reaction, the samples were monitored by in situ XRD in an Anton Paar XRX900 container. The in situ XRD results show that when the reaction temperature is raised to 1023 K slowly and continuously, the phase transition of the sample follows the pattern AMT f WO3 f WO2 f W2C f WC, but when the reaction temperature is raised to 673 K slowly and then raised to 1023 K quickly, the phase transition of the sample follows AMT f WO3 f WO2 f WC. These results indicate that the phase transition of the sample during the heat treatment step is connected to the temperature and the time of the reaction, as well as the rate of the temperature rise.
1. Introduction Since the catalytic capability of tungsten carbide (WC) has been shown to be analogous to that platinum,1-3 extensive attention has been paid to its preparation and application in recent years.4-12 However, the catalytic activity of WC is much less than that of platinum metal.2 Therefore, methods to improve the catalytic activity of WC, so that its activity approaches that of platinum, are a key area of interest. Nikolov et al. proposed that the starting materials, and the carburization atmosphere, do not influence the activity of tungsten carbide.14,15 The catalytic activity of tungsten carbide was defined only as a function of the specific surface area.15 The differences in catalytic activities of tungsten carbides result from differences in surface properties, such as particle morphology and the chemical composition of the surface layer, which could be significantly different from the bulk.15,16 Particle morphology was shown to have a strong influence on electrocatalytic properties of tungsten carbide catalysts.15 Thus, adjusting the morphology of the particles of tungsten carbide is an effective way to control its catalytic activity. Tungsten carbide can be divided into two categories according to its stoichiometric ratio, namely stoichiometric and nonstoichiometric. The former includes WC, W2C, and WC2, and the latter includes WC1-x (x ) 0.18-0.42). Among these, only WC and W2C display catalytic activity to some extent, and WC is the better. This is because WC exhibits a porous lattice structure, in which tungsten metal atoms are constructed into a hexagonal close-packed framework and carbon occupies the pores and is * Corresponding author. E-mail:
[email protected], science@ zjut.edu.cn, or
[email protected]. † Zhejiang University of Technology. ‡ Imperial College London.
dispersed between interlamellar tungsten atoms. Hence an investigation of the phase transition in the preparation process of tungsten carbide could help to improve its catalytic activity. Ross et al.17 proposed that the main chemical reaction steps of WC formation using WO3 precursors involved WO3 reduction to W by CO coupled with the production of C from CO, and the subsequent diffusion of C into the W lattice to form WC. Lemaiter et al. proposed that the main chemical reactions in the preparation of tungsten carbide powders were1
2W + 2CO ) W2C + CO2
(1)
W2C + 2CO ) 2WC + CO2
(2)
2CO ) Cat + CO2
(3)
W2C + Cat f 2WC
(4)
nCat f Cn
(5)
W2C + (1/n)Cn f 2WC
(6)
During the reduction and carbonization of WO3, there were three kinds of reactions: one was reduction of the surface of oxide particle, the second was diffusion in metal particles, and the last was the reaction between gas and solid on the solid particle surface. These three reactions competed with each other; though the process of chemical reaction competition was complex, the main steps were WO3 f W20O58 f WO2 f W2C f WC. Xiao et al.18 proposed that when WO3 was reduced and carbonized in an atmosphere of CH4/H2 and C2H6/H2, the phase transition of the sample followed WO3 f WO2 f (WOxCy) f W2C f WC. Also, two mechanisms existed for
10.1021/jp072378q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007
Formation of Hollow Porous Microspherical WC the reduction-carburization of powder WO3 by CH4/H2 mixture for the synthesis of WC catalysts.19 These contrasting results indicate that there are different views about the phase transition in tungsten carbide preparation through different approaches. It is therefore interesting and necessary to investigate the phase transition further. To improve the catalytic activity of WC, Ma et al.20,21 reported the preparation of hollow microspherical tungsten carbide with mesoporosity, and its electrocatalytic properties. However, questions remain: namely, how do hollow microspheres form, what is the phase transition of tungsten carbide during the reduction and carbonization step, and how is this influenced by process parameters? This paper addresses these questions, using a variety of characterization techniques, including scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), and in situ XRD. The results indicate that a hollow microspherical structure with mesoporosity forms gradually during the preparation process, and that the phase transition differs from that previously reported in the literature. 2. Experimental Section An appropriate amount of ammonium metatungstate (AMT) was dissolved in deionized water to form an aqueous solution with a weight percentage of 15-20%. This solution was passed into a spray desiccator while stirring with a magnetic force stirrer at room temperature. During the drying process, the velocity of hot air flow was 35 m3/h, the velocity of aqueous solution was 15 mL/min, the temperature of drying air at the threshold was 438 K, and the temperature of the mixture of gas and powder at the exit was 378 K. In this way, the powder precursor was prepared. The powder precursor was then put into a ceramic boat, and was sent into a tube furnace that had been purified with a mixture of carbon monoxide and carbon dioxide for 0.5 h. The temperature of the furnace was raised to 673 K and held for 1 h, and then raised to 1023 K for 10-12 h, while a mixture of carbon monoxide and carbon dioxide at a ratio of 10:1 (CO: CO2) was passed through the furnace. After that, the ceramic boat was pushed into an ice-water bath to cool and the sample was passivated. The morphology, crystal phase, and chemical components of the sample were characterized by SEM, XRD, and EDS, respectively. SEM was carried out on a Hitachi S-4700 equipped with energy dispersive X-ray spectroscopy (EDS, Cambridge). X-ray diffraction was performed with a Thermo ARL SCINTAG X’TRA at room temperature, using a Cu KR1 radiation source (k ) 1.541 nm) under a voltage of 45 kV and a current of 40 mA. The XRD patterns were recorded with a step size of 0.04° from 15° to 90° at a speed of 2.4°/min. In situ XRD was carried out by putting the sample in an Anton Paar XRX900 container. The phase transition of the sample was monitored under the same conditions used for sample preparation, with a step size of 0.04° from 15° to 70° at a speed of 2.4°/min. 3. Results and Discussion 3.1. XRD. XRD patterns of the product are shown in Figure 1. There are three intensive diffraction peaks with 2θ values of 31.48°, 35.76°, and 48.40°, which can be assigned to the planes of WC (001), (100), and (101), respectively, according to JCPDS 25-1047. In addition, there are several other diffraction peaks with 2θ values of 64.04°, 65.32°, 73.20°, 75.60°, 76.92°, and 84.16°, which can be assigned to the planes of WC (110), (002),
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9505
Figure 1. XRD patterns of tungsten carbide.
(111), (200), (102), and (201), respectively, according to JCPDS 25-1047. This indicates that the crystal phase of the sample is tungsten carbide (WC) with a hexagonal structure. Comparison of the 2θ values of the sample with that of JCPDS 25-1047 reveals some difference between them. This difference can be attributed to the adsorption of oxygen during passivation in the ice-water bath, verified by EDS results in the following. 3.2. EDS. The sample was analyzed by EDS, and the result is shown in Figure 2. The chemical components of the sample are W, C, and O according to the EDS spectra shown in Figure 2, where O is probably adsorbed on the surface of the particles during passivation. Passivation is a necessary step in tungsten carbide preparation. If the sample as prepared is not passivated in an ice-water bath immediately, then it will suffer autogenous combustion in air. W, C, and O percentages, atom percentages, and the atom ratios were estimated according to the results of EDS analysis, and the results are shown in Table 1. The weight percentages of W, C, and O are 93.50%, 5.51%, and 0.98%, respectively, as Table 1 shows. The atom percentages of W, C, and O are 49.42%, 44.61, and 5.97%, respectively, and the ratio of W to C is 1.108, near 1. This indicates that there is C deficiency, which is a well-known phenomenon in tungsten carbide preparation by temperature programmed reduction methods. The ratio of W to (C + O) is 0.977. This implies that the oxygen is present as both lattice oxygen and adsorbed oxygen; the former is maintaining the charge balance resulting from the carbon deficiency and the latter is adsorbing on the surface of the sample during passivation. This is consistent with results of ref 22. 4. Formation Mechanism 4.1. Hollow Microspherical Structure with Mesoporosity. The SEM images of the sample at different stages during the preparation process are shown in Figure 3. The morphology of the sample processed by spray-drying sphere miniaturization is microspherical, with a diameter of 1-15 µm, as shown in Figure 3a. The outer surface of the microsphere is essentially smooth, though some defects can be seen on the surface of the microsphere, and a few are fractured, showing the thickness of the wall of the microsphere to be about 500-1000 nm, as shown in Figure 3b. This indicates that the microspheres are hollow. Based on the above results, it is evident that the hollow microspherical structure of the sample forms during spray-drying sphere miniaturization of AMT.
9506 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Ma et al.
Figure 2. EDS patterns of the sample.
Figure 4. In situ XRD patterns of the sample during reductioncarbonization at different temperatures.
Figure 3. SEM images of the sample at different stages: (a, b) sample of spray-drying sphere miniaturization; (c-f) final product.
TABLE 1: Estimated Atom Ratios According to EDS Results element
atom %
wt %
C O W
44.61 5.97 49.42
5.51 0.98 93.50
total
100.00
100.00
atom ratio W/C ) 1.108 W/(C + O) ) 0.977
The morphology of the sample after reduction and carbonization remains microspherical, with diameters of 1-10 µm, as shown in Figure 3c. The outer surfaces of the microspheres of the product have become rough, as shown in Figure 3d. The walls of the microspheres of the product are constituted of a lot of puncheons and cavities as shown in Figure 3e. The lengths of the puncheons are 100-800 nm and their widths are 80200 nm; these puncheons are surrounded by irregular intercon-
nected cavities with widths of 80-200 nm, as shown in Figure 3e. Some microspheres are fractured, indicating the wall thickness of a microsphere to be about 500-800 nm, as shown in Figure 3f. These results imply that the structure is now one of mesoporous hollow spheres, with the mesoporosity arising during the reduction and carbonization step. 4.2. Crystal Phase. The in situ XRD results of the sample during reduction and carbonization in CO/CO2 atmospheres are shown in Figure 4. Curve a displays the XRD results of AMT processed by spray-drying sphere miniaturization. It is clear that the as-processed phases of AMT are complex, and that WO3 phase is one of these phases, as curve a in Figure 4 shows. After the sample was reduced and carbonized in CO/CO2 atmosphere at 573 and 623 K, it is an essentially noncrystalline phase, as curves 573K and 623K in Figure 4 show. After processing at 673 K, the sample is largely composed of WO3. When the temperature is raised to 773 K, the intensity of the WO3 peak is the highest. When the temperature is raised to 823 K, some WO3 is reduced to WO2, as curve 823K in Figure 4 shows, and the diffraction intensity of WO3 becomes weaker, while that of WO2 becomes stronger, with increasing temperature. When the temperature is raised to 923 and 973 K, WO3 mostly transforms into WO2. If the reduction and carbonization temperature is held at 973 K for 2 h, the sample is composed
Formation of Hollow Porous Microspherical WC
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9507
WO3 + H2O ) WO2(OH)2v
Figure 5. In situ XRD patterns of the sample of different reductioncarbonization times at 1023 K.
of WC, W2C, and WO2, as curve 973K-2h shows in Figure 4. When the reduction and carbonization temperature is increased to 1023 K, the diffraction intensity of WC becomes stronger, while that of W2C becomes weaker. Keeping the temperature at 1023 K results in the diffraction intensity of WC becoming stronger and that of W2C becoming weaker. After 3.5 h, the sample is essentially WC, as curve 1023K-3.5h in Figure 4 shows. On the basis of the above results, one can see clearly that when the reduction and carbonization temperature is between 973 and 1023 K the sample is a mixture of WC and W2C. To get phase pure WC, the reaction temperature of the sample was raised to 673 K gradually and then it was raised to 1023 K rapidly. The crystal phase transition was monitored by in situ XRD as a function of reaction time, and the results are shown in Figure 5. When the reaction temperature is 673 K, the sample is largely composed of WO3. When the temperature is increased rapidly to 1023 K, the sample is composed of WO3 and WO2, as curve 1023K in Figure 5 shows. The diffraction intensity of WO2 becomes stronger, while that of WO3 becomes weaker, as the reduction and carbonization time increases. When the time is 2.6 h, the diffraction intensity of WO2 is the strongest, that of WO3 is hard to see, and the diffraction peak of WC is weak, as curve 2.6h in Figure 5 shows. As the reaction time increases, the diffraction intensity of WO2 becomes weaker, while that of WC becomes stronger. When the reaction time is 7 h, the diffraction peak of WO2 disappears and only that of WC can be seen. After 10 h, the intensity and the component of the diffraction peaks of WC are almost the same as that of phase pure crystalline WC. 5. Discussion The chemical components of the sample would be expected to be (NH4)+, (H2W12O40)6-, and H2O (crystal water), i.e., the same as ATM, because the process of spray-drying sphere miniaturization is only a physical one. The in situ XRD results show that, from room temperature to 773 K, the dominant phase of the sample is WO3, as shown in Figure 4. During this period, (NH4)+ will decompose into NH3 and escape from the sample. This will produce some porosity in the sample. H2O (crystal water) will also evaporate from the sample, and when H2O meets WO3, the following reaction will take place:23
This reaction will take some WO3 away from the sample and leave cavities in it. These reactions are responsible for the mesoporosity introduced into the sample during the heat treatment step. The phase transition of the sample during the reduction and carbonization in CO/CO2 atmosphere when the temperature is raised gradually can be divided into the following steps: below 773 K, it is governed by the decomposition and dehydration of AMT, and the dominant phase of the sample is of WO3; from 773 to 873 K, the sample is composed of WO3 and WO2; from 873 to 973 K, the dominant phase of the sample is WO2; from 973 to 1023 K, two phases, W2C and WC, coexist in the sample; when the reaction temperature is increased to 1023 K, the sample is composed of WC only, as shown in Figure 4. Based on the above results, the phase transition of the sample during the reduction and carbonization in CO/CO2 atmosphere when the reaction temperature is raised gradually can be summarized as
AMT f WO3 f WO2 f W2C f WC When the reaction temperature of the sample is raised to 673 K, the dominant phase of the sample is of WO3, as shown in Figure 5. When the reaction temperature of the sample is raised to 1023 K, the sample is composed of WO3 and WO2, as shown in Figure 5. When the reaction temperature is kept at 1023 K, the phase transition of the sample depends on the reaction time. When the reaction time is less than 5.5 h, the sample is composed of WO3 and WO2; when the reaction time is longer than 7 h, the sample is composed of WC only, as shown in Figure 5. Based on the above results, when the reaction temperature is raised to 673 K gradually and then raised to 1023 K rapidly, the phase transition of the sample can be summarized as
AMT f WO3 f WO2 f WC On the basis of the above, it can be concluded that the phase transition depends not only on the temperature and time of reaction, but also on the rate of change of temperature. Ross et al.17 suggested that, when WO3 was reduced by CO, metal W formed first, followed by WC. In our in situ XRD experiments, no metal W diffraction peaks were observed. This is probably because if the W metal is well dispersed or forms noncrystal phases, or the size of W metal particles is smaller than 4 nm, the diffraction peaks of W metal cannot be seen in XRD patterns.24,25 For the same reason, for the reductioncarbonization temperature at 1023 K, the rate of dissolution and diffusion of carbon into W metal is quick,17 and W2C phase cannot be detected by in situ XRD. 6. Conclusions On the base of the experimental results and discussions, the following conclusions can be drawn: (1) Hollow microspheres of tungsten carbide with mesoporosity were prepared by the approach of spray-drying sphere miniaturization/gas-solid reaction. (2) A hollow microspherical structure formed during spraydrying sphere miniaturization. A mesoporous structure formed during the reduction and carbonization step, arising from the reaction of ammonium metatungstate during the preparation process. (3) The phase transition of the sample depends on the temperature and time of reaction, and the rate of the reaction
9508 J. Phys. Chem. C, Vol. 111, No. 26, 2007 temperature rise. When the reaction temperature was raised gradually from room temperature to 1023 K, the phase transition of the sample reduced and carbonized in CO/CO2 atmosphere followed AMT f WO3 f WO2 f W2C f WC. When the reaction temperature was raised to 673 K gradually and then raised to 1023 K rapidly, the phase transition of the sample followed AMT f WO3 f WO2 f WC. This differs from results previously reported in the literature. (4) WO2 transformed into WC directly, with no intermediate phases, resulting in phase pure WC with high catalytic activity at lower temperature. Acknowledgment. The authors thank the National Nature Science of Foundation of China (Nos. 20476097 and 20276069), the Natural Science Foundation of Zhejiang Province (No. Y406094), and the Scientific Starting Foundation of Zhejiang University of Technology for financial support. References and Notes (1) Lemaiter, J.; Vidick, B.; Delmon, B. J. Catal. 1986, 99, 415. (2) Levy, R. B.; Boudart, M. Science 1973, 181, 547. (3) Bohm, H. Nature 1970, 227, 484. (4) Hudson, M. J.; Peckett, J. W.; Harris, P. J. F. Ind. Eng. Chem. Res. 2005, 44, 5575. (5) York, A. P. E.; Claridge, J. B.; Brungs, A. J.; Tsang, S. C.; Green, M. L. H. Chem. Commun. 1997, 9. (6) Hwu, H. H.; Chen, J. G. J. Phys. Chem. B 2003, 107, 2029. (7) Hwu, H. H.; Chen, J. G. J. Phys. Chem. B 2001, 105, 10037. (8) Hwu, H. H.; Polizzotti, B. D.; Chen, J. G. J. Phys. Chem. B 2001, 105, 10045.
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