Preparation of Ultrafine β-MoO3 from Industrial Grade MoO3 Powder

Aug 15, 2016 - ABSTRACT: This study reports a novel strategy to obtain ultrafine metastable monoclinic molybdenum trioxide powder by the method of ...
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Preparation of Ultrafine β‑MoO3 from Industrial Grade MoO3 Powder by the Method of Sublimation Lu Wang,† Guo-Hua Zhang,*,† Yuan-Jun Sun,‡ Xin-Wen Zhou,‡ and Kuo-Chih Chou† †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China Jinduicheng Molybdenum Co. Ltd., Xi’an 710077, China

‡

ABSTRACT: This study reports a novel strategy to obtain ultrafine metastable monoclinic molybdenum trioxide (β-MoO3) powder by the method of sublimation in the temperatures range of 1123−1373 K using the industrial grade MoO3 as raw material. The vapor-phase MoO3 could rapidly condensate and deposit in the bag filter after being quenched. It is found that the particle size has no obvious regularity with the sublimation temperature and the as-prepared βMoO3 appears to be perfectly spherical. Impurities such as SiO2 in raw MoO3 samples have a significant retarding influence on the sublimation rate. Both the mechanisms of sublimation of industrial grade MoO3 as well as the nucleation and growth process of ultrafine β-MoO3 have been shed light. This work may have a guide role in the large-scale production of ultrafine spherical β-MoO3 catalyst.

1. INTRODUCTION Molybdenum trioxide (MoO3 ) has received increasing attentions due to its important applications in recent years. Nowadays, it is widely used as catalysts,1,2 display devices,3,4 optical materials5 etc., owing to its excellent physical and electronic properties. What is more, molybdenum trioxide particle with a small size is of substantial interest and the demand is becoming larger and larger. MoO3 has different structures that can be mainly divided into four polymorphs: (1) orthorhombic phase,6−8 α-MoO3; (2) monoclinic phase,9,10 β-MoO3; (3) metastable high pressure phase,11−13 MoO3-II; and (4) hexagonal phase,14−17 h-MoO3. In all the MoO3 polymorphs, MoO6 octahedron is the basic building block used to construct MoO3 in various phases and its arrangement makes differences in the structures. The arrangement of the MoO6 octahedron (formation of any phase) depends on the external conditions, e.g., temperature, pressure and impurities. However, the two most commonly studied polymorphs forms are α-MoO3 and β-MoO3, respectively. The corresponding crystal structures are displayed in Figures 1 and 2, respectively. From Figure 1, it can be seen that α-MoO3 has a unique layer structure in which each layer is built up of MoO6 octahedron at two levels, connected along y-axis by common edges and corners, so as to form zigzag rows and along z-axis by common corners only. In addition, three kinds of structurally different lattice oxygens are existed, i.e., terminal oxygen (O1), asymmetric bridging oxygen (O2,) and symmetric bridging oxygen (O3). While as to β-MoO3, as shown in Figure 2, the structures are quite different from the case of α-MoO3. β-MoO3 is similar to WO3 and is related to the three-dimensional ReO3 structure, which is formed only by the shared corners MoO6 octahedron. Moreover, it has been found18 that β-MoO3 has a © 2016 American Chemical Society

Figure 1. Crystal structure of α-MoO3.

higher catalytic performance than α-MoO3 for the oxidation of methanol to formaldehyde at temperatures below 600 K. Therefore, the synthesis of β-MoO 3 has been widely investigated by many researchers. Ramirez and Cruz19 obtained β-MoO3 with a high purity by vacuum drying method. Mariotti et al.20 has successfully synthesized porous high surface area thin films of nanosheetshaped β-MoO3 using patch antenna-based atmospheric microplasma processing. Parise et al.21 produced the powdered Received: June 14, 2016 Revised: August 7, 2016 Published: August 15, 2016 19821

DOI: 10.1021/acs.jpcc.6b05982 J. Phys. Chem. C 2016, 120, 19821−19829

Article

The Journal of Physical Chemistry C

Figure 3. XRD pattern of the raw materials. Figure 2. Crystal structure of β-MoO3.

β-MoO3 by the gentle heat treatment of freeze-dried molybdic acid at 623 K for 1 h. Mizushima et al.10 synthesized the βMoO3 powder through the evaporation of molybdic acid solution prepared by a cation exchange of an aqueous solution of Na2MoO4·2H2O. In the present paper, a new method has been proposed to prepare β-MoO3 by the combination of sublimation at the high reaction temperatures and condensation by quenching using industrial grade MoO3 powders as experimental raw materials. Moreover, the sublimation mechanism of raw materials and the possible growth process of as-prepared β-MoO3 were discussed.

2. MATERIALS AND EXPERIMENTAL PROCEDURES 2.1. Materials. Industrial grade molybdenum trioxide powders from Jinduicheng Molybdenum Co. Ltd., which were generated by the oxidation roasting of molybdenite concentrates in air, were used for preparing ultrafine β-MoO3. The corresponding chemical compositions are given in Table 1.

Figure 4. SEM image of the raw MoO3 materials (obtained by SEM microscopy).

Table 1. Chemical Compositions of Industrial Grade Molybdenum Trioxide Obtained by X-ray Fluorescence Analysis (XRF) compositions mass % compositions mass %

MoO3 92.47 MgO 0.13

SiO2 3.24 PbO 0.1

Fe2O3 2.03 CuO 0.1

Al2O3 0.7 TiO2 0.06

CaO 0.4 MnO 0.04

K2O 0.35 Cl 0.4

Figure 3 shows the XRD patterns of the raw MoO3; it demonstrates that the main peaks are well-defined to be MoO3. In addition, from the XRD patterns, it can be indexed that raw MoO3 belongs to the orthorhombic system (PDF card No. 5508), α-MoO3. Figure 4 shows the SEM image of raw MoO3 materials, from which it can be seen that the particles have a unique layered structure. 2.2. Experimental Apparatus and Procedures. Figure 5 depicts the schematic diagram of the experimental apparatus. It mainly includes four parts, i.e., the inner quartz tube (1), the external quartz tube (2), stainless steel tube (6) and stainless steel barrel (7). A high-temperature resistance bag filter (8) was fixed between the steel tube and steel barrel in order to collect MoO3. Furthermore, a vacuum pump was used to make the

Figure 5. Schematic diagram of the experimental apparatus for the preparation process: (1) Inner quartz tube; (2) external quartz tube; (3) Si−C electrical resistance furnace; (4) samples; (5) NiCr−NiSi thermocouple; (6) stainless steel tube; (7) stainless steel barrel; (8) high-temperature resistance bag filter; (9) vacuum pump.

negative pressure and increase the flow velocity of MoO3 vapor which can enhance the cooling rate. When the device was assembled, a sample of about 5 g was weighted and filled into the alumina crucible with the dimension of (50 mm × 20 mm 19822

DOI: 10.1021/acs.jpcc.6b05982 J. Phys. Chem. C 2016, 120, 19821−19829

Article

The Journal of Physical Chemistry C × 20 mm) before putting them into the inner quartz tube. After the desired temperature was arrived and kept for 30 min to reach a steady state, the inner quartz tube was put into the Si− C electrical furnace carefully to start the vaporization reaction. After reacting for a certain time, the inner quartz tube was taken out to cool the residual sample down to the room temperature. The weight loss after the sublimation process was weighted. In addition, the powders obtained in the collection bag filter (8) were also weighted in order to calculate the yield. In the present study, the sublimation experiments in the temperatures ranges of 1123−1373 K were conducted. After collecting the sublimate MoO3 at different temperatures, the products were characterized by different technologies. X-ray diffraction technology (XRD) (model TTRIII, Rigaku Corportation, Japan) was carried out for the phase analysis. The morphologies of collected MoO3 were observed by using scanning electron microscopy (SEM; model S250MK3, CAMBRIDGE, American) and field emission scanning electron microscopy (FE-SEM; ZEISS SUPRA 55, Oberkochen, Germany) techniques.

when the sublimation temperature is 1373 K. However, from Figure 7b, it can be seen that a little of platelet-shaped MoO3 particles (may be α-MoO3) also exist, which may have fallen off from the quart tube wall. 3.3. Sublimation and Condensation. In order to evaluate the feasibility of the method adopted here to the large-scale production of ultrafine β-MoO3 powders, the sublimation losses and yields of MoO3 powders obtained at the higher temperatures (1273, 1323, and 1373 K) were calculated. The sublimation loss (W) of industrial grade MoO3 powders was calculated by the following eq 1 m − mt W= 0 × 100% m0 (1) where m0 and mt are the weights of initial sample and that reacted for a period of time t, respectively. Herein, m0 is fixed to be 5 g and the reaction time t is equal to 60 min. Then the yields of sublimate MoO3 powders can be identified by the ratio of mass of collected MoO3 powders in the collection bag filter mc to the sublimation loss m0 − mt as shown in eq 2 mc η= × 100% m 0 − mt (2)

3. RESULTS 3.1. X-ray Diffraction Analyses. The XRD patterns of collected MoO3 deposited in the high-temperature resistance bag filter (8) at different temperatures are depicted in Figure 6.

The corresponding values are listed in Table 2. From Table 2, it can be known that as expected the sublimation rate increases with the increasing temperature. More detailed analyses about the sublimation kinetics of industrial grade MoO3 will be discussed in the following sections. Moreover, the yields of the as-synthesized MoO3 are nearly above 60% at the three reaction temperatures, which indicate that most of the sublimate MoO3 can be collected in the bag filter. Of course, the absolute yield could be changed by changing the quenching and collecting conditions Figure 8 shows the XRD patterns of the residual sample after sublimating at 1273 K for 60 min. It can be seen that the main residue is MoO3, which indicates that raw material MoO3 has not been sublimated completely, which accords with the sublimation loss (67.04%) listed in Table 2. When compared with the XRD patterns of raw materials MoO3 shown in Figure 3, parts of impurities such as CaMoO4, Al2(MoO4)3, and SiO2 are also detected in the residue, which indicates that the impurities are difficult to evaporate and have been enriched after the sublimation reaction. Furthermore, another important parameter of the assynthesized MoO3 is the contents of the impurities elements. Due to that the elements of Si, Al, and Ca are very difficult to volatilize at the current temperatures (shown in Figure 8), but the elements of Cu, Pb, and K may be easy to evaporate for the low melting points of their oxides. Therefore, contents of Cu, Pb, and K in the as-synthesized MoO3 are measured and the corresponding values are listed in Table 3. As can be seen, increasing temperatures will increase the volatilization of impurities elements especially Pb and K. Therefore, in order to reduce the contents of impurities elements, the sublimation temperature should be controlled.

Figure 6. XRD patterns of collected MoO3 in the bag filter at different temperatures.

From Figure 6, it can be known that all of the products are composed of monoclinic molybdenum trioxide (β-MoO3, PDF card No. 47-1081, space group P21/c). The intensity of the strongest peak ((011) plane) of β-MoO3 increases with the increasing temperature, which indicates that the crystallinity also increases gradually. One interesting aspect of the test is the difference in the color of raw MoO3 (α-MoO3) (light-yellow) and the as-collected MoO3 (light-green), which also signaled the structural dissimilarity between the two kinds of materials. 3.2. FE-SEM for Microscopy Study. Figure 7 shows the typical images of as-synthesized MoO3 powders at different sublimation temperatures. From Figure 7, it can be known the crystallinity of MoO3 powders are increasing with the increasing temperatures, which are well agreement with the XRD patterns shown in Figure 6. In addition, the particles are almost spherical with the average sizes less than 1 μm especially

4. DISCUSSION 4.1. Mechanism of Sublimation. 4.1.1. Kinetics Analyses. In order to investigate the mechanism and kinetics of sublimation of raw materials MoO3 in details, another series of experiments were carried out. In each experiment run, a 19823

DOI: 10.1021/acs.jpcc.6b05982 J. Phys. Chem. C 2016, 120, 19821−19829

Article

The Journal of Physical Chemistry C

Figure 7. FESEM micrographs of as-synthesized MoO3 powders at different sublimation temperatures: (a) 1123 K; (b) 1173 K; (c) 1223 K; (d) 1273 K; (e) 1323 K; (f) 1373 K.

Table 2. Sublimation Losses and Yields of Raw MoO3 Powders Obtained at the Sublimation Temperatures of 1273, 1323, and 1373 K T (K) W (%) η (%)

1273 67.04 58.43

1323 79.50 65.04

1373 87.18 63.78

sample of about 300 mg in weight was used and filled into a small alumina crucible with 7 mm in diameter and 7 mm in height. The experimental procedures were the same as the previous description. The curves of mass loss versus reaction time are shown in Figure 9, which shows that the rate of sublimation is very rapid at the initial time and then gradually goes down until the end of reaction. Moreover, the maximal mass loss is almost equal to the theoretical value of 92.47% according to the content of MoO3 shown in Table 1. Because all the three experimental temperatures are higher than the melting point (1068 K)22 of MoO3, the raw materials will be melted. However, MoO3 can also have a very high volatilization rate even in solid state as the temperature is higher than 973 K.23,24 Therefore, the reaction mechanism may involve the volatilization of both the solid and liquid MoO3. In order to

Figure 8. XRD patterns of the residual sample after sublimating for 60 min at 1273 K.

further clarify the mechanism, the mass loss curves of high purity MoO3 powders versus reaction time were also obtained 19824

DOI: 10.1021/acs.jpcc.6b05982 J. Phys. Chem. C 2016, 120, 19821−19829

Article

The Journal of Physical Chemistry C

4.1.2. Cross Section Structure. In order to gain inside into the sublimation mechanism, both partial and complete sublimate samples were analyzed by SEM and EDS technologies. Figure 11 shows the cross section structure of the raw sample after different sublimation time at 1273 K. It can be seen that after 5 and 10 min, as shown in Figure 11a and c, the bottoms of the samples are relatively homogeneous and the surfaces are accumulated with impurities (Figure 11b can be further demonstrated the phenomena), which will hinder the diffusion of MoO3 from the bottom to the surface of the crucible. While reacting for 120 min, at which time no MoO3 could be evaporated, as shown in Figure 11d, the residual samples were mainly composed of impurities (the dark parts are the impurities of SiO2). Figure 12 shows the SEM images of sample after sublimation for 5 min at 1273 K, revealing three distinct regions which appear as light-gray, deep-gray and dark. In order to identify the element distribution, the EDS area scanning maps were used and the results are presented in Figure 13. It can be seen that light-gray, deep-gray, and dark are mainly comprised of Mo, Al, and Si, respectively. Furthermore, Ca was also detected in some regions. K, Cu, and Fe were uniformly distributed. To identify the three distinct phases, EDS technology was used and the EDS results of points 2 and 3 in Figure 12b are shown in Table 4, which indicates that points 2 and 3 are Al2 (MoO4)3 and SiO2, respectively. However, it is worth noting that Al in the Al2(MoO4)3 phase may result from both the impurity and the alumina crucible. Many investigators have investigated the spread behavior of MoO3 into Al2O3/SiO2 supports.25−28 Figure 14 shows the XRD patterns of the products after roasting the mixtures of Al2O3 and high purity MoO3 at 1273 K for 10 min, which further confirmed that Al2(MoO4)3 can be easily generated even after a short reaction time. 4.1.3. Mechanism of Sublimation. In the past few decades, many papers29−31 have shown that MoO3 sublimes to form gaseous polymers of the type (MoO3)n (n = 2, 3, 4, 5); that is to say, the vapors over the raw crystal are composed of various polymer molecules, and mainly include trimer, (MoO3)3(g). For simplicity, the following analyses are only based on the evaporation of MoO3 liquid. The mechanism of sublimation can be roughly divided into the following four steps: (I) MoO3 molecules diffuse from the bottom to the surface of crucible through the layer concentrated by impurities; (II) MoO3 molecules rearrange into polymorphs, mainly (MoO3)3(g); (III) Polymorphs desorption from the surface; (IV) Polymorphs diffuse through the gas boundary layer to the ambient environment. The sublimation process which undergoes marked chemical rearrangements during vaporization is, in general, controlled by the rate of a chemical reaction that takes place at a particular surface site. Besides, the surface structure also plays an important role in determining the rate of vaporization.32 Based on the above analysis and the current experimental conditions, gas molecules inside the sample transfer to the surface is the most likely rate controlling step because of the existence of impurities, which makes the concentration of MoO3 gradually decrease. At first, the sublimation rate of the surface is large, which leads to an enrichment of impurities on the surface (Figure 11a and b) which retards the gas molecule diffusion into the surface seriously. When the reaction continues, concentrations of impurities continuously increase,

Table 3. Contents of Volatile Impurities Elements (Cu, Pb, and K) in the As-Synthesized MoO3 Powders at the Sublimation Temperatures of 1273, 1323, and 1373 K with the Case of Raw MoO3 Powders Added as References T (K) Cu (%) Pb (%) K (%)

1273