Preliminary Synthesis and Characterization of Mesoporous Nanocrystalline Zirconia

Transcription

1 Journal of Natural Gas Chemistry 12(2003) Preliminary Synthesis and Characterization of Mesoporous Nanocrystalline Zirconia Xinmei Liu 1,2, Gaoqing Lu 1, Zifeng Yan 1,2, 1 State Key Laboratory for Heavy Oil Processing, Key Laboratory of Catalysis, CNPC, University of Petroleum, Dongying , China; 2 The Nanomaterials Centre and Department of Chemical Engineering, University of Queensland, Brisbane 4072, Australia [Manuscript received June 24, 2003; revised August 11, 2003] Abstract: A novel method to prepare mesoporous nano-zirconia was developed. The synthesis was carried out in the presence of PEO surfactants via a solid-state reaction. The materials exhibit a strong diffraction peak at low 2θ angle and their nitrogen adsorption/desorption isotherms are typical of type IV with H1 hysteresis loops. The pore structure imaged by TEM can be described as wormhole domains. The tetragonal zirconia nanocrystals are uniform in size (around 1.5 nm) and their mesopores focus on around 4.6 nm. The zirconia nanocrystal growth is tentatively postulated to be the result of an aggregation mechanism. This study also reveals that the PEO surfactants can interact with the Zr-O-Zr framework to reinforce the thermal stability of zirconia. The ratio of NaOH to ZrOCl 2, crystallization and calcination temperature play an important role in the synthesis of mesoporous nano-zirconia. Key words: solid-state reaction, zirconia, mesopore, nanosize, synthesis, characterization 1. Introduction Since the discovery of the M41S materials by the Mobil researchers, mesoporous materials have attracted tremendous interests due to their shape selectivity, extremely high specific surface areas and other desirable properties in catalysis, sorption and microelectronics. Apart from the silicate and aluminosilicate mesoporous materials, other non-silicabased mesostructured materials such as titanium, zirconium, lead and tungsten oxides have also been synthesized recently. As fluorite-type oxides, zirconium oxide has facecentered-cubic crystalline structure in which each tetravalent metal ion is surrounded by eight equivalent nearest O 2 ions forming the vertices of a cube. This structure renders zirconia high oxygen vacancy concentration and mobility properties [1]. Such zirconia also possesses unique bifunctional characteristics of weak acidic and basic properties [2,3] as well as high thermal stability under reducing or oxidizing atmospheres. Thus zirconia has been extensively investigated as a robust catalyst, catalyst promoter or support. In addition, zirconia is widely used in thermal barrier coating, electrode and oxygen sensors, etc. Mesoporous nanocrystalline zirconia is of particular interest because of its potential applications in oxygen sensors, solid oxide electrolyte for fuel cells, and catalysis. Large mesoporosity is desirable to facilitate transport of the larger reactant molecules to active sites, and is important in determining the length of the triple-phase boundary where charge transfer occurs for an electronically conducting electrode and is expedient to the percolation of electrons throughout the electrode microstructure [4 7]. Fine particle zirconia bears the better wear resistance [8] and lower diffusion resistance, which are favorable for catalyst applications. In addition, nanocrystalline zirconia having higher adsorptive capacity thus appears promising for adsorption applications. Corresponding author. zfyancat@hdpu.edu.cn.

2 162 Xinmei Liu et al./ Journal of Natural Gas Chemistry Vol. 12 No So far, most zirconium oxides are generally synthesized using the sol-gel or precipitation processes with a surfactant as the template. In this paper, we developed a novel method combining solid-state reaction and in situ crystallizing with polyethylene oxide surfactant to prepare the nanocystalline zirconia with mesoporous structure. The synthesized zirconia samples possess high surface area and thermal stability without doping of any stabilizer. 2. Experimental 2.1. Preparation of nanocrystalline zirconia The nanocrystalline zirconia was prepared via a solid-state reaction using zirconyl chloride (ZrOCl 2 8H 2 O) as the precursors. Several procedures were investigated to study the influence of Zr/NaOH ratios, calcination and crystallization temperature, and the role of surfactant. First, ZrOCl 2 8H 2 O and NaOH were milled into fine powder and mixed at ambient temperature. The mixture were then transferred to an autoclave and kept at a desired temperature for certain period of time. Subsequently, the mixture was washed with deionized water until it was free of Cl ions, and then washed with ethanol twice to remove water contained in the solid. Finally, the samples were dried at 383 K overnight. The dried samples were calcined at temperatures of K for 20 h Characterization The synthesized samples were characterized by nitrogen adsorption analyzer, X-ray diffraction (XRD), ultraviolet and visable spectrophotometer (UV visible), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The nitrogen adsorption was performed on Autosorb-1C (Quantachrome, USA) at 77.3 K. The mesopore size distribution was calculated from the desorption branch of the isotherm by the Barrett, Joyner and Halenda (BJH) method. XRD was performed on a Philips PW 1840 powder diffractometer, using Ni-filtered Cu K α radiation with a tube voltage of 40 kv, a tube current of 20 ma and scanned from o (2θ). The TEM images were taken by Philips CM200 Microscope in bright field transmission model. Thermal analysis was carried out using thermogravimetric analyzer (Shimadzu TGA-50). Measurements were performed in air flow with a heating rate of 2 K/min. 3. Results and discussion 3.1. XRD patterns It is interesting to note a rather broad low-angle peak occurred in the XRD patterns of as-synthesized zirconia shown in Figure 1. Figure 1. XRD patterns of zirconia with different calcination temperatures Insert: High-angle peaks

3 Journal of Natural Gas Chemistry Vol. 12 No The other four peaks appeared at high 2θ degrees, and showed that the synthesized samples are single-phase nanocrystalline zirconia with tetragonal structure. The long-range order exhibited by the low angle peaks means that such as-synthesized zirconia has a mesoporous framework. The broad shape of the XRD peaks at low angle indicated less ordered mesoporous structure and the particles might be ultrafine. The particle size estimated by the Sherrer equation is about 1.5 nm as confirmed later by TEM images and UV spectra. The diffuse reflectance UV spectra shown in Figure 2 demonstrated that such assynthesized mesoporous zirconia is nanosized particles because the specific absorption edge situated at lower wavelengths [9]. with the increase in pore size from nitrogen adsorption data. The increase in nanocrystal size with calcination temperature was shown in Table 1. The average crystal size increased from 1.52 nm of as-synthesized sample to 5.94 nm of the sample calcined at 500. The UV spectra in Figure 2 also showed that the nanocrystal size changes with the calcination temperature, i.e. UV absorption peak slightly shifts to higher wave number with increasing temperature. Table 1 also showed that the thermal stability of the zirconia prepared with surfactant is better than the samples prepared without surfactant. The aggregation of the mesoporous nanocrystalline zirconia particles treated by PEO is reduced. This can probably be attributed to that PEO surfactant can interact with the framework or surface Zr-O-Zr and reduce the surface tension of the nanocrystal particles, which effectively decrease the interaction among the particles thus inhibiting particle growth. Table 1. Sample Effect of calcination temperature on the nanocrystal size of zirconia Crystal size with surfactant (nm) Crystal size without surfactant (nm) As-synthesized Calcined at Calcined at Figure 2. Profile of the UV spectrum of zirconia (1) As-made, (2) 350, (3) 500 Figure 1 also illustrated that the XRD peaks tended to be sharper and stronger with the increase in calcination temperature. This indicated that the agglomeration and surface reconstruction of as-synthesized nanocrystalline zirconia occurred in the process of calcination. Such agglomeration and surface reconstruction might result in the growth of mesoporous nanocrystalline zirconia particle sizes. The growth of these particles can be attributed to condensation of the abounding surface hydroxyl groups, which causes the nucleation of new oxide nanocrystals and the growth of the existing one at higher temperatures [10]. From Figure 1 we can also see that the peak at low angle shifts to smaller degrees with the increase in calcinations temperature, which is consistent It is noteworthy that the crystallization temperature plays a significant role in the crystal phase transformation of zirconia. Figure 3 illustrated that the sample crystallized at ambient temperature exhibits broader diffraction peaks with rather weak intensity, which showed that the sample is amorphous and the nanocrystal particle size is ultrafine. The weak XRD peak indicates that mesoporous zirconia has tetragonal skeleton but contains many defects and/or lattice vacancies. The large number of lattice vacancies and local lattice disorder resulted in weak diffraction intensity [11]. Upon heating at elevated temperatures, the zirconyl clusters can agglomerate and generate many small nuclei [12]. The higher the temperature, the faster growth of the nuclei. This results in larger particle size and more ordered nanocrystalline structure at higher temperatures, therefore the diffraction patterns of the tetragonal phase steadily sharpen and strengthen with the increase in crystallization temperature. However, the monoclinic phase can be formed at temperatures up to 200 although the signal is not apparent. When the temperature is raised to 250

4 164 Xinmei Liu et al./ Journal of Natural Gas Chemistry Vol. 12 No , the pure monoclinic phase can be seen. Thus crystallizing temperature can control the crystalline phase of zirconia. At elevated temperatures, thermal lattice contraction might occur and the aggregate particle size can grow, resulting larger mesoporosity. The mesopore distribution and surface area as a function of calcination temperature is shown in Figure 5 and Table 2. Figure 4. Nitrogen adsorption/desorption isotherms of zirconia with different calcination temperatures (1) As-Made, (2) 350, (3) 500 Figure 3. XRD patterns of zirconia prepared at different crystallization temperature (1) Room temperature, (2) 80, (3) 110, (4) 150, (5) 200, (6) Nitrogen adsorption isotherm The nitrogen adsorption isotherms showed that the zirconia prepared with solid-state reaction posses larger surface area and mesoporous structure. The isotherms are of type IV with H1 hysteresis loops as shown in Figure 4. It was thought that zirconia prepared with this novel method is comprised of the aggregates (loose assemblages) of plate-like nanocrystals of zirconia thus forming slit-like pores. It also exhibited that the calcination temperature plays an important role in the pore structure formation. Figure 5. Mesopore distribution of zirconia obtained at different calcination temperatures

5 Journal of Natural Gas Chemistry Vol. 12 No Table 2. Sample Effect of calcination temperature on the surface area of zirconia Surface area with surfactant (m 2 /g) Surface area without surfactant (m 2 /g) As-synthesized Calcined at Calcined at Calcined at Table 2 showed that the surface area of the calcined sample treated with PEO is larger than that of the samples without PEO. This is another evidence of higher thermal stability due to the presence of surfactant PEO, which can be explained as follows: The lattice water of ZrOCl 2 8H 2 O or hygroscopic water of the hydroxides can induce micelle formation of PEO during powdering and mixing. The oxide groups of PEO surfactant would form the outer surface of the micelles, being in contact with the surface of hydrous zirconia and act as chemical bonds which would result in the closer packed structure of zirconia crystallites. Such surface interaction and lattice reconstruction render the synthesized mesoporous zirconia higher thermal stability. The surfactant micelles can also decrease the surface tension and thus prevent the collapse of Zr-O-Zr network. NaOH/Zr ratio is another key factor in influencing the properties of mesoporous nanocrystalline zirconia. Increasing this ratio from 2 4 resulted in a large increase in the adsorption capacity of zirconia synthesized without surfactant. However, the adsorption capacity will be slightly decreased when the NaOH/Zr ratio is above 4.0. Consequently, the specific surface area changed from to m 2 /g, and then decreased to m 2 /g when the ratio is 5.0. Of interest is that the inception point of the hysteresis loop shifts to the lower pressure region with the increase in NaOH/Zr ratio. This indicated that the mesopore diameter of synthesized zirconia is reduced at higher NaOH/Zr ratio. This could be due to the role of NaOH in the crystallization of zirconia. As shown in Table 3, pore sizes of zirconia prepared with solidstate reaction can be tuned by choosing the ratio of NaOH to ZrOCl 2. Table 3. Effect of NaOH/ZrOCl 2 ratio on the pore structure of zirconia prepared without surfactant Mole ratio of Surface area Pore volume Average pore NaOH to ZrOCl 2 (m 2 /g) (cm 3 /g) diameter (nm) It is interesting to note that we have not found appreciable difference in surface area among the samples synthesized with different ratios of zirconyl chloride to surfactant, which contradicts previous reports that the surface area is strongly influenced by the ratio of Zr/surfactant [13] TEM images TEM images of nanocrystalline zirconia samples depicted in Figure 6 supported the data of nanocrystal size acquired by XRD and nitrogen adsorption/desorption results. Figure 6. TEM images of as-prepared zirconia

6 166 Xinmei Liu et al./ Journal of Natural Gas Chemistry Vol. 12 No It is clear that zirconia samples obtained using this novel solid-state method have uniform mesopores, and they are nanocrystalline aggregate particles. The mesopore structure is best described as the wormhole type. Such a pore structure has been considered to be very favorable in catalysis and adsorption owing to its greater accessibility to surface sites for gases [4]. The lattice images exhibit the necking between nanocrystallites while a void region representing the mesopores winding extensively throughout the structure Thermal analyses Two weight loss stages were observed in the TGA profile of nanocrystalline zirconia as illustrated in Figure 7. The first one is located before 150 corresponding to the loss of water adsorbed in the sample. The weight loss presented between 350 and 500 is due to the removal of terminal hydroxyl groups bonded on the surface of zirconia. Such large weight loss between 350 and 500 means that large number of hydroxyl groups exist on the surface of as-synthesized zirconia. When the samples were annealed at temperatures above 500, no further weight loss was observed. This means that the thermal stability of the zirconia prepared with solid-state reaction is high. Figure 7. TGA curves of as-prepared ZrO 2 4. Conclusions In this work, mesoporous nanocrystalline zirconia has been synthesized by a novel solid-state reaction method. The following conclusions are drawn: 1. Zirconia sample prepared without PEO is sensitive to the calcination temperature. The PEO molecules can interact with the Zr-O-Zr framework and reduce the surface tension and thus improve the thermal stability of zirconia. 2. The mesopore size can be tailored by changing the ratio of NaOH to ZrOCl The different crystal phases can be formed at different crystallizing temperatures. 4. The growth of zirconia nanoparticles is mainly via an aggregation mechanism Acknowledgements We thank Dr. J. Riches for his kind help in TEM analysis, and Mr. L. Bekessy and Ms A.J.E. Yago for kind assistance in XRD experiments. References [1] Zhou R X, Yu T M, Jiang X Y et al. Appl Surf Sci, 1999, 148: 263 [2] Su C L, Li J R, He D H et al. Appl Catal A, 2000, 202: 81 [3] Wong M S, Antonelli D M, Ying J Y. Nanostruct Mater, 1997, 9: 165 [4] Mamak M, Coombs N, Ozin G. J Am Chem Soc, 2000, 122: 8932 [5] Verweij H. Adv Mater, 1998, 10: 1483 [6] Ziehfreund A, Simon U, Maier W F. Adv Mater, 1996, 8: 424 [7] van Berkel F P F, van Heuveln F H, Huijsmans J P P. Solid State Ionics, 1994, 72: 240 [8] He Y J, Winnubst L, Burggraaf A J et al. J Am Ceram Soc, 1996, 79: 3090 [9] Brus L E. J Chem Phys, 1984, 80: 4403 [10] Wang J A, Valenzuela M A, Salmones J et al. Catal Today, 2001, 68: 21 [11] Siu G G, Stokes M J, Liu Y L. Phy Rev B, 1999, 59: 3173 [12] Hu M Z-C, Harris M T, Byers C H. J Colloid Interface Sci, 1998, 198: 87 [13] Parvulescu V I, Bonnemann H, Parvulescu V et al. Appl Catal A, 2001, 214: 273