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Ceramic montmorillonite nanocomposites by electrochemical synthesis
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Applied Clay Science 42 (2008) 310 – 317 www.elsevier.com/locate/clay
Ceramic montmorillonite nanocomposites by electrochemical synthesis Adele Qi Wang a , Nandika D'Souza b , Teresa Diane Golden a,⁎ a
University of North Texas, Department of Chemistry, Denton, Texas 76203, United States b University of North Texas, Department of Materials Science, United States
Received 5 November 2007; received in revised form 8 February 2008; accepted 14 February 2008 Available online 23 February 2008
Abstract Nanocomposite powders of montmorillonite and cerium oxide are synthesized using an electrochemical technique. X-ray diffraction of the nanocomposites shows a random orientation of the powders with particle sizes as small as 4–6 nm. The montmorillonite percentage in the nanocomposite is also shown to affect sintering rates and particle size growth as shown by X-ray diffraction. Fourier transmission infrared measurements confirm the presence of individual platelets of the montmorillonite within the nanocomposite matrix. Nanocomposites containing 1% of the montmorillonite in the matrix increase crystallization, while higher percentages of montmorillonite inhibit particle growth during sintering. © 2008 Elsevier B.V. All rights reserved. Keywords: Montmorillonite; Nanocomposites; Ceramics
1. Introduction Catalytic applications such as the combustion of volatile organic compounds are an important research area (Zwinkels et al., 1993; Noordally et al., 1993; Larsson et al., 1997; Papaefthimiou et al., 1998; Gandia et al., 2002). For many combustion processes, high efficiency, high surface area and lower operating temperatures are the driving forces for the production of new materials. As a material, cerium oxide has been produced for catalyst applications in a variety of fields. There are several issues that affect the application of ceria in catalysis. The most severe is aging of the catalyst system over operating time. This problem is related to the increase of the cerium oxide crystallite size within the catalyst during heating. In this study we explore the electrochemical synthesis of ceria and the influence of montmorillonite from the phyllosilicate family during heating on particle size and growth phenomena. Clay minerals mixed with metal oxides are of current interest because of their high thermal stability, surface area and catalytic properties. Ceramics such as Fe3O4, SiO2, Al2O3, Fe2O3, and other mixed oxides have been intercalated into the interlayer of montmorillonite, with the montmorillonite as the structure ⁎ Corresponding author. Tel.: +1 940 565 2888. E-mail address: [email protected] (T.D. Golden). 0169-1317/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.02.004
matrix and another ceramic material as the additive (Pinnavaia, 1983; Rightor et al., 1991; Han et al., 1997; Mishra and Parida, 1998; Mamedov et al., 2000). Several methods have been used to produce these composites, such as spin coating, spraying, ion-exchange, sol–gel processing, and layer-by-layer assembly. Some of these composites have also been studied for optical, electrical and magnetic properties. In this paper, we explore a solution method that uses electrosynthesis to produce composites of montmorillonite and cerium oxide, each of which exhibit catalytic properties. Recently, we developed a solution technique using anodic electrochemical synthesis to produce nanocrystalline cerium oxide powders and films with particle sizes as small as 6 nm (Wang and Golden, 2003; Golden and Wang, 2003; Wang et al., 2003). We have previously developed a solution technique for crystalline ceria and iron substituted montmorillonite (Wang et al., 2006). In this paper, we explore the sintering effects on nanocomposites with no metal catalyst impurities. In this research, this solution technique is used to synthesize nanocomposite powders of cerium oxide/ montmorillonite. The ceramic oxide is used as the main structure matrix and the exfoliated montmorillonite as the additive to form the nanocomposite. X-ray diffraction (XRD), fourier transmission infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) are used to study the structure and properties of the resulting nanocomposites.
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2. Experimental
Table 1 Comparison between experimental and PDF data for CeO2
Sodium montmorillonite (untreated) (provided by Nanocor) was added to D.I. water and stirred or ultrasonicated overnight. The resulting dispersion was slowly decanted into a mixture of cerium nitrate salt and sodium acetate. Concentration of the montmorillonite varied between 0.5 to 50% (w/w) to the cerium nitrate concentration. The pH of the cerium salt solution was adjusted to ~ 10 with NaOH. The resulting solution mixture was used as the electrolyte in the electrochemical experiments. The anodic electrochemical synthesis of the nanocomposite powder was done using an EG&G Princeton Applied Research (PAR) Model 273A potentiostat/ galvanostat. Constant temperature was maintained with a Fisher Scientific Model 1016D circulator. A three-electrode configuration was employed for the synthesis with stainless steel disk, chromel wire and the saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively. A galvanostatic method (J = 5.6 mA/cm2) was used to produce the nanocomposite powders. The formed precipitate was filtered and rinsed with water and ethanol to remove possible impurities. The precipitate was dried in air at room temperature and ground before being subjected to any characterization. A Siemens D500 diffractometer with Cu Kα radiation (λ = 0.15405 nm) was used to investigate the X-ray diffraction (XRD) of the pure montmorillonite and ceramic montmorillonite composites. All scans ranged from a 2θ of 2o to 100o, with the primary reflections of the montmorillonite appearing below 40o. To diagnostically identify the reflections, a pure montmorillonite sample was analyzed using a step size of 0.01o 2θ and a dwell time of 3 s per step. For the composites, the step size and dwell time were 0.01o 2θ and 1 s, respectively. FTIR was used to study the structure of the composites by mixing 1–2 mg of the composite powder sample with ~ 100 mg of KBr and then pressed to form a disc with a diameter of 1.0 cm. The discs were heated at 150 °C overnight to eliminate the absorbed moisture in the sample. Infrared spectra were obtained using a Perkin-Elmer 1760X FTIR spectrophotometer. For each sample, 40 scans were collected in the range of 4000 to 400 cm− 1 with a resolution of 4 cm− 1. The powders were processed by pressing uniaxially under a pressure of ~ 50 MPa in a die (diameter of the die − 1.0 cm). The pressed pellets were sintered at a constant heating rate of 1 °C/min. The final sintering temperatures ranged from 700 to 1100 °C.
hkl
3. Results and discussions The diffraction patterns of the pure montmorillonite (A) and electrogenerated nanocrystalline cerium oxide powder (B) are shown in Fig. 1. The reflections for the electrosynthesized
Fig. 1. XRD patterns for montmorillonite (A) and electrogenerated nanocrystalline CeO2 powder (B). The corresponding phase assignment for CeO2 is listed in Table 1.
111 200 220 311 222 400 331 420 422 511
Experimental data
PDF#34-0394
2θ
I/I0
2θ
I( f )
28.48 33.07 47.42 56.29 59.04 69.45 76.64 78.93 88.36 95.32
100 30 56 43 10 9 18 13 15 13
28.554 33.081 47.478 56.334 59.085 69.400 76.698 79.067 88.410 95.394
100 30 52 42 8 8 14 8 14 11
Experimental data from Fig. 1.
nanocrystalline CeO2 powder are compared with the random JCPDS powder pattern of CeO2 (PDF# 34-0394) in Table 1, showing a random XRD pattern with a fluorite structure. For the montmorillonite, it is well recognized that the (00l) basal reflections are important for structure interpretation. The distance between a certain plane in the layer and the corresponding plane in the next layer is called the basal or d spacing, which can be determined by X-ray diffraction (XRD). An absence of the reflection for these materials is indicative of layer delamination or exfoliation. The montmorillonite gives a strong (001) reflection at 6.85o 2θ, with a basal spacing of 12.9 Å. The (002) reflection is weakly present at 14.05o 2θ with a calculated d-spacing of 6.30 Å. Other higher-order reflections are weak, which reflects the disorder present in the natural clay mineral. The (060) reflection of the montmorillonite can provide supplementary information to distinguish dioctahedral and trioctahedral types according to the d-spacing and 2θ position (Pinnavaia, 1983). The (060) reflection of the untreated montmorillonite is at 61.87o 2θ with a d-spacing of 1.49 Å, which corresponds to a typical dioctahedral clay mineral, such as montmorillonite (2θ(060) =62.22 ~ 61.67o and d(060) = 1.492 ~ 1.504 Å). In addition, there are three twodimensional bands, (02;11) at 19.79o 2θ, (20;13) at 34.93o 2θ and (04;22) at 53.93o 2θ present in the montmorillonite XRD pattern (Fig. 1A). Typically the 20;13 band is recognized as the best parameter to evaluate turbostratic disorder since it is more asymmetric than the other two common bands (Moore and Reynolds, 1997). Since montmorillonites are naturally occurring clay minerals, there are minor amounts of impurities present, such as the sharp reflection at 26.63o 2θ coinciding with quartz silica (Moore and Reynolds, 1997; Chipera and Bish, 2001). Selected XRD patterns of the composites (CeO2/montmorillonite) are shown in Fig. 2 and indicate the electrosynthesized powders are randomly oriented. The electrosynthesized CeO2/ montmorillonite composites retain the fcc fluorite structure characteristic of CeO2 for increasing contents of montmorillonite in the composites (1, 2, 3, 5, 10, 20, and 30%). The Scherrer equation was used for the (111) reflection of CeO2 in the composites to estimate the particle size of the composites at varying montmorillonite contents. The particle sizes ranged from 4–6 nm, indicating the electrosynthesized CeO2/montmorillonite powders are nanocomposites.
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Fig. 2. XRD patterns for electrogenerated CeO2/montmorillonite nanocomposite powders at montmorillonite concentrations of 1, 3, 20 and 30%.
With the formation of the nanocomposite, the (001) reflection for the montmorillonite content (1–20%) is not present in the XRD patterns. At 30%, a small peak at 6.95o 2θ is detected for the (001) reflection. For all percentages of montmorillonite content in the nanocomposites, a very small reflection around 63o 2θ is seen in the diffraction patterns. This reflection corresponds to an α-quartz reflection and could result from the dramatic agitation or ultrasonication during the preparation of the montmorillonite dispersion and the electrochemical synthesis process. The ultrasonication and agitation
not only delaminate the layered structure in the z direction, but also break down to some degree, order in the x/y direction of the montmorillonite. The loss of layered structure indicates that the montmorillonite is exfoliated and dispersed as individual platelets throughout the ceria. Low temperature DSC of the nanocomposites show only one endo-peak for the nanocrystalline cerium oxide, which verifies no interlayer water present in the nanocomposites and hints at the exfoliation of the montmorillonite in these nanocomposites.
Fig. 3. FTIR spectra of montmorillonite (a) and electrogenerated nanocrystalline CeO2 powder (b) in the OH stretching (3850–3350 cm− 1) (A) and Si–O stretching (1300–400 cm− 1) (B) region. Insert shows magnified OH stretching band region.
A.Q. Wang et al. / Applied Clay Science 42 (2008) 310–317
Infrared spectra can give information on the occupancy of octahedral and tetrahedral sites and the stacking and arrangement of the sheets in clay minerals. For infrared, the absorbance bands due to structural OH and Si–O groups have significant roles in identification and analysis of clay minerals. Infrared spectra of the montmorillonite were collected in the 4000– 400 cm− 1 region. The spectra of sodium montmorillonite (a) and nanocrystalline CeO2 powder (b) are shown in Fig. 3, for the OH stretching regions (A) and Si–O stretching regions (B). To diminish the absorbed water, the samples were heated at 150 °C overnight (Madejová and Komadel, 2001; Madejová, 2003). Montmorillonite belongs to a dioctahedral smectite, in which the position and shape of the OH stretching band is mainly determined by the octahedral atoms coordinated with the hydroxyl groups (Farmer and Russell, 1964; Farmer, 2000; Madejová, 2003). The radius, valence charge, and hydration energies of the exchangeable cations also play critical roles (Yan et al., 1996; Xie et al., 2001). The broad absorption in the region of 3400–3500 cm− 1, which is centered at 3458 cm− 1, is usually assigned to OH stretching of water moisture in the montmorillonite sample. This band is observed to diminish when a KBrsample pellet was heated overnight at 150 °C. Two bands can be resolved from each other in the range of 3620–3660 cm− 1 and occur at 3634 and 3626 cm− 1. These bands are assigned to structural OH stretching vibrations of the clay mineral. The peak close to 3620 cm− 1 is indicative of high concentrations of Al in the octahedral (Farmer and Russell, 1964; Madejová, 2003). The OH bending vibrations can be found in the low frequency region of infrared spectra and the positions of such reflections indicate the occupancy of octahedral sheet as well.
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Dioctahedral clay minerals have been observed to have this vibration in the range of 950–800 cm− 1, different from the lower wavenumber vibrations at 700–600 cm− 1 for trioctahedral clay minerals (Madejová and Komadel, 2001; Madejová, 2003). In montmorillonite, the 914 cm− 1 and 847 cm− 1 absorption bands represent inner hydroxyl bending in Al–Al– OH and in Al–Mg–OH structure, respectively, reflecting partial magnesium substitution for aluminum in octahedral sheets (Madejová and Komadel, 2001; Madejová, 2003). The absence of the 885 cm− 1 band indicates no iron substitution for aluminum in the octahedral sheet. The broad band between 1130 and 1000 cm− 1, centered at 1045 cm− 1 in Fig. 3B is the Si–O characteristic band for montmorillonite (Madejová and Komadel, 2001; Madejová, 2003). Since quartz impurity also contributes to this broad band, FTIR as well as XRD was run to confirm its presence. The band at ~ 799 cm− 1 also comes from quartz impurity in this montmorillonite. Bands at 522 cm− 1 and 468 cm− 1 in montmorillonite are assigned to Si–O–Al (octahedral Al) and Si–O–Si bending vibration, respectively. According to the literature, the presence of the 624 cm− 1 band is due to Si–O and Al–O coupled out-of-plane vibrations (Madejová and Komadel, 2001). FTIR for the nanocrystalline cerium oxide powder (b) is also shown in Fig. 3. Usually simple metal oxides are infrared inactive, however CeO2 has two oxygen connected to cerium atoms, resulting in infrared active vibrations in the range of 1100–825 cm− 1. It is recognized that the 550 cm− 1 band is a characteristic peak in phonon modes spectral range for cubic crystalline forms of rare earth oxides (Li et al., 1989; Li et al., 1993; Binet et al., 1999). Generally, stoichiometric cerium
Fig. 4. FTIR spectra of CeO2/montmorillonite nanocomposites of 1 (a), 3 (b), and 30% (c) in the OH stretching (3850–3350 cm− 1) (A) and Si–O stretching (1300–400 cm− 1) (B) region.
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dioxide is optically transparent in the wavenumber range of 4000 to 600 cm− 1. The 1300 to 600 cm− 1 region spectrum of cerium oxide powder shows: two bands at 1052 and 1021 cm− 1; one small band at 931 cm− 1; two sharp bands at 856 and 837 cm− 1; one small peak at 726 cm− 1; and two bands at ~ 654 and 621 cm− 1. The 726 cm− 1 band can be attributed to Ce–O vibration, the 1021 cm− 1 is usually assigned as the first overtone mode of the fundamental vibration at ~ 550 cm− 1. A band at ~ 2103 cm− 1 is also found in the FTIR spectrum, which can be regarded as a possible overtone for 1021 cm− 1. The peaks at 856 and 837 cm− 1 can be ascribed to adsorbed superoxide species, − (O2ads ) which is the characteristic frequency of O–O vibration for a bond order of 1.05. Some other researchers also assign these two peaks to carbonate due to the adsorption of CO2 in air. Since this electrochemical synthesis used acetate as a ligand, the
1021 and 1052 cm− 1 bands may also be due to residual acetate (Socrates, 1998). After electrochemical synthesis, FTIR is also run for the resulting composites. Fig. 4 shows the infrared spectra for the CeO2/montmorillonite nanocomposites at 1 (a), 3 (b), and 30% (c) montmorillonite content. The structural OH band of the montmorillonite (~ 3620–3660 cm− 1) can be detected in the CeO2/montmorillonite nanocomposites except for the 1% sample containing a low content of montmorillonite in the nanocomposite. The OH stretching vibration shifts negatively with decreasing montmorillonite and increasing CeO2 content in the nanocomposites to 3636, 3621, and 3620 cm− 1 for the 30, 3, and 1%, respectively. In the formation of CeO2/montmorillonite nanocomposites, the montmorillonite was suspended by ultrasonication or agitation before mixing with the cerium salt solution, it is
Fig. 5. A and B. XRD patterns for CeO2/montmorillonite nanocomposites before and after sintering at 700, 800, 900, 1000, 1100 °C.
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315
Fig. 5 (continued ).
believed that the montmorillonite loses the long range structural order and delaminates into platelets. With this exfoliation, more apical oxygen atoms are exposed in the surface of the octahedral or tetrahedral sheets, creating more chances for hydrogen bonding between inner hydroxyl group in the octahedral sheet and these tetrahedral apical oxygen atoms. Isomorphous substitution of Al3+ by Mg2+ in octahedral structure brings negative charge on the apical oxygens of the tetrahedra of the clay mineral. This oxygen interacts with the hydroxyl group by OH….Oap hydrogen bond formation. This hydrogen bond strength plays a significant role in the OH vibration frequency, which can be seen in the cation intercalation experiments. Cation (Li+, Cu2+, Cd2+) migration and intercalation into layer structures modify the environment and orientation of hydroxyl group and compensate for the positive charge deficit (Madejová et al., 1999). The absence of the negative
Oap weakens hydrogen bond interaction and OH stretching shifts to a higher wavenumber with increasing concentration of inserted cations. Therefore, the downward shift in the position of the OH stretching band in CeO2/montmorillonite composites is an evidence of hydrogen bonding for the hydroxyl containing structure. In addition, since acetate was used in the electrogeneration of the nanocomposites, oxygens in acetate may participate in hydrogen Table 2 Comparison of particle size (nm) for varying percentage of montmorillonite in nanocomposite Sample
Before sintering
700 °C
800 °C
900 °C
0% 1% 20%
5 5 5
16 23 8
54 114 14
54 – 46
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bonding. To eliminate this acetate effect, the complexation of cerium (III) ions with acetate was arranged prior to the addition of the montmorillonite suspension. Dilution enhances delamination of montmorillonite (Sivakumar et al., 1995), therefore, the lower concentration of clay mineral, leads to stronger hydrogen bonding and a lower OH stretching frequency. Si–O stretching and bending vibration is another set of significant IR frequencies for the clay mineral. The position and shape of these bands also fingerprint the different arrangement and stacking of montmorillonite. The Si–O stretching of the CeO2/montmorillonite composites, for the region of 1300 to 400 cm− 1, is shown in Fig. 4B. Positions of the Si–O–Al and Si–O–Si bendings at 522 and 468 cm− 1 do not change significantly with the formation of the nanocomposites and increasing clay mineral content in the nanocomposites. With exfoliation and electrochemical processing, long-range order of the montmorillonite is destroyed in the nanocomposites. Small tetrahedral sheet platelets possess less tension and exhibit more regularity in structure compared with ordered montmorillonite, resulting in a relative sharper absorption band centered around ~ 1035 cm− 1 in the Si–O infrared spectra. The observed downward shift of Si–O in the nanocomposites relative to the pristine montmorillonite contrasts with the previously reported Si–O absorption upward migration phenomenon associated with cation insertion into octahedral and tetrahedral sites (Madejová et al., 1999). With decreasing concentration of clay mineral in the nanocomposites, the band center position does not vary, but the shape of Si–O band changes with a more pronounced shoulder for the 30% nanocomposite, which is not present in lower concentrations of clay mineral composites. For high concentrations of montmorillonite in the nanocomposite, the band is mainly due to the Si–O of montmorillonite and the shoulder band from quartz impurity. At low concentrations of montmorillonite (1%), the Si–O stretching signal attenuates but there is a shoulder peak around 1023 cm− 1, indicating a minor amount of acetate impurity in the nanocomposite. Fig. 5A and B are the XRD patterns of the unsintered and sintered nanocomposite pellets at different sintering temperatures. A noticeable difference in the XRD’s of the nanocomposites is the increasing particle size with increasing sintering temperature. However, the percentage of montmorillonite in the nanocomposite definitely affects the rate of particle growth with sintering temperature. Table 2 lists the particle sizes as calculated from the Scherrer equation using the (111) reflection in the XRD pattern. The particle sizes for 1000 °C and 1100 °C sintering are not listed since the crystallite size has reached the microcrystalline range and cannot be measured by the XRD procedure. As shown by the XRD pattern, sintering of the nanocomposites containing 1% montmorillonite results in faster crystallization compared to nanocrystalline cerium oxide or nanocomposites with greater than 1% montmorillonite concentrations. A higher percentage of montmorillonite in the nanocomposite hinders particle growth during sintering, as seen by the broader peaks in the XRD patterns with increasing temperature. This inhibition of the growth rate in the nanocomposites by the addition of a montmorillonite is important for the nanocomposite field. The
mechanism for the incorporation of the montmorillonite in cerium oxide crystallization by an electrochemical method to form nanocomposites requires further study. 4. Conclusions CeO2 montmorillonite nanocomposites have been successfully produced using anodic electrochemical synthesis. FTIR measurement confirmed the presence of montmorillonite in the composites whereas X-ray diffraction and sintering experiments suggest the inclusion of exfoliated montmorillonite platelets into the cerium oxide particles affect rates of particle growth. Acknowledgment This work was financially supported by the Robert A. Welch Foundation (Grant No. B-1454) and a UNT Faculty Research Grant. References Binet, C., Daturi, M., Lavalley, J.C., 1999. IR study of polycrystalline ceria properties in oxidized and reduced states. Catalysis Today 50, 207–225. Chipera, S.J., Bish, D.L., 2001. Baseline studies of the clay minerals society source clays: powder x-ray diffraction analysis. Clays and Clay Mineral 49, 398–409. Farmer, V.C., Russell, J.D., 1964. The infrared spectra of layered silicates. Spectrochima Acta 20, 1149–1178. Farmer, V.C., 2000. Transverse and longitudinal crystal modes associated with OH stretching vibrations in single crystals of kaolinite and dickite. Spectrochima Acta Part A 56, 927–930. Gandia, L.M., Vicente, M.A., Gil, A., 2002. Complete oxidation of acetone over manganese oxide catalysts supported on alumina- and zirconia-pillared clays. Applied Catalysis B:Environmental 38, 295–307. Golden, T.D., Wang, A.Q., 2003. Anodic electrodeposition of cerium oxide thin films II. Mechanism studies. Journal of the Electrochemical Society 150 (9), C621–C624. Han, Y.-S., Matsunoto, H., Yamanaka, S., 1997. Preparation of new silica sol-based pillared clays with high surface area and high thermal stability. Chemisty of Materials 9, 2013–2018. Larsson, P.O., Berggren, H., Anderson, A., Augustsson, O., 1997. Supported metal oxides for catalytic combustion of CO and VOCs emissions: preparation of titania overlayers on a macroporous support. Catalysis Today 35, 137–144. Li, C., Domen, K., Maruya, K., Onishi, T., 1989. Dioxygen adsorption on welloutgassed and partially reduced cerium oxide studied by FT-IR. Journal of the American Chemical Society 111, 7683–7687. Li, C., Domen, K., Maruya, K., Onishi, T., 1993. An in situ FT-IR study of CO hydrogenation over cerium oxide. Journal of Catalysis 141, 540–547. Madejová, J., 2003. FTIR techniques in clay mineral studies. Vibrational Spectroscopy 31, 1–10. Madejová, J., Komadel, P., 2001. Baseline studies of the clay mineral society source clays: infrared methods. Clays and Clay Mineral 49, 410–432. Madejová, J., Arvaiová, B., Komadel, P., 1999. FTIR spectroscopic characterization of thermally treated Cu2+, Cd2+, and Li+ montmorillonites. Spectrochimica Acta Part A 55, 2467–2476. Mamedov, A., Ostrander, J., Aliev, F., Kotov, N.A., 2000. Stratified assemblies of magnetite nanoparticles and montmorillonite prepared by the layer-bylayer assembly. Langmuir 16, 3941–3949. Mishra, T., Parida, K., 1998. Transition metal oxide pillared clay: 5. Synthesis, characterization and catalytic activity of iron–chromium mixed oxide pillared montmorillonite. Applied Catalysis A; General 174, 91–98. Moore, D.M., Reynolds Jr., R.C., 1997. X-ray diffraction and the identification and analysis of clay minerals. Oxford university press, New York.
A.Q. Wang et al. / Applied Clay Science 42 (2008) 310–317 Noordally, E., Richmond, J.R., Tahir, S.F., 1993. Destruction of volatile organic compounds by catalytic oxidation. Catalysis Today 17, 359–366. Papaefthimiou, P., Ioannides, T., Verykios, X.E., 1998. Performance of doped Pt/TiO2 (W6+) catalysts for combustion of volatile organic compounds (VOCs). Applied Catalysis B 15, 75–92. Pinnavaia, T., 1983. Intercalated clay catalysts. Science 220, 365–371. Rightor, E.G., Tzou, M.S., Pinnavaia, T.J., 1991. Iron oxide pillared clay with large gallery height: synthesis and properties as a Fischer–Tropsch catalyst. Journal of Catalysis 130, 29–40. Sivakumar, A., Damodaran, A.D., Warrier, K.G.K., 1995. Delamination through sonication for hydroxy metal oxide sol intercalation of montmorillonite. Ceramics International 21, 85–88. Socrates, G., 1998. Infrared Characteristic group frequencies, 2nd ed. John Wiley & Sons, New York. Wang, A.Q., Golden, T.D., 2003. Anodic electrodeposition of cerium oxide thin films I. Formation of crystalline thin films. Journal of the Electrochemical Society 150 (9), C616–C620.
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Wang, A.Q., Punchaipetch, P., Wallace, R.M., Golden, T.D., 2003. XPS study of electrodeposited nanostructures CeO2 films. Journal of Vacuum Science Technology 21, 1169–1175. Wang, A.Q., D'Souza, N.A., Golden, T.D., 2006. Electrosynthesis of nanocrystalline cerium oxide/layered silicate powders. Journal of Materials Chemistry 16, 481–488. Xie, W., Gao, Z., Liu, K., Pan, W., Vaia, R., Hunter, D., Singh, A., 2001. Thermal characterization of organically modified montmorillonite. Thermochimica Acta 367–368, 339–350. Yan, L., Roth, C.B., Low, P., 1996. Effects of monovalent, exchangeable cations and electrolytes on the infrared vibrations of smectite layers and interlayer water. Journal of Colloid and Interface Science 184, 663–670. Zwinkels, M.F.M., Jaras, S.G., Menon, P.G., Griffin, T.A., 1993. Catalytic materials for high-temperature combustion. Catalysis Reviews Science and Engineering 35, 319–358.
Applied Clay Science 42 (2008) 310 – 317 www.elsevier.com/locate/clay
Ceramic montmorillonite nanocomposites by electrochemical synthesis Adele Qi Wang a , Nandika D'Souza b , Teresa Diane Golden a,⁎ a
University of North Texas, Department of Chemistry, Denton, Texas 76203, United States b University of North Texas, Department of Materials Science, United States
Received 5 November 2007; received in revised form 8 February 2008; accepted 14 February 2008 Available online 23 February 2008
Abstract Nanocomposite powders of montmorillonite and cerium oxide are synthesized using an electrochemical technique. X-ray diffraction of the nanocomposites shows a random orientation of the powders with particle sizes as small as 4–6 nm. The montmorillonite percentage in the nanocomposite is also shown to affect sintering rates and particle size growth as shown by X-ray diffraction. Fourier transmission infrared measurements confirm the presence of individual platelets of the montmorillonite within the nanocomposite matrix. Nanocomposites containing 1% of the montmorillonite in the matrix increase crystallization, while higher percentages of montmorillonite inhibit particle growth during sintering. © 2008 Elsevier B.V. All rights reserved. Keywords: Montmorillonite; Nanocomposites; Ceramics
1. Introduction Catalytic applications such as the combustion of volatile organic compounds are an important research area (Zwinkels et al., 1993; Noordally et al., 1993; Larsson et al., 1997; Papaefthimiou et al., 1998; Gandia et al., 2002). For many combustion processes, high efficiency, high surface area and lower operating temperatures are the driving forces for the production of new materials. As a material, cerium oxide has been produced for catalyst applications in a variety of fields. There are several issues that affect the application of ceria in catalysis. The most severe is aging of the catalyst system over operating time. This problem is related to the increase of the cerium oxide crystallite size within the catalyst during heating. In this study we explore the electrochemical synthesis of ceria and the influence of montmorillonite from the phyllosilicate family during heating on particle size and growth phenomena. Clay minerals mixed with metal oxides are of current interest because of their high thermal stability, surface area and catalytic properties. Ceramics such as Fe3O4, SiO2, Al2O3, Fe2O3, and other mixed oxides have been intercalated into the interlayer of montmorillonite, with the montmorillonite as the structure ⁎ Corresponding author. Tel.: +1 940 565 2888. E-mail address: [email protected] (T.D. Golden). 0169-1317/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.02.004
matrix and another ceramic material as the additive (Pinnavaia, 1983; Rightor et al., 1991; Han et al., 1997; Mishra and Parida, 1998; Mamedov et al., 2000). Several methods have been used to produce these composites, such as spin coating, spraying, ion-exchange, sol–gel processing, and layer-by-layer assembly. Some of these composites have also been studied for optical, electrical and magnetic properties. In this paper, we explore a solution method that uses electrosynthesis to produce composites of montmorillonite and cerium oxide, each of which exhibit catalytic properties. Recently, we developed a solution technique using anodic electrochemical synthesis to produce nanocrystalline cerium oxide powders and films with particle sizes as small as 6 nm (Wang and Golden, 2003; Golden and Wang, 2003; Wang et al., 2003). We have previously developed a solution technique for crystalline ceria and iron substituted montmorillonite (Wang et al., 2006). In this paper, we explore the sintering effects on nanocomposites with no metal catalyst impurities. In this research, this solution technique is used to synthesize nanocomposite powders of cerium oxide/ montmorillonite. The ceramic oxide is used as the main structure matrix and the exfoliated montmorillonite as the additive to form the nanocomposite. X-ray diffraction (XRD), fourier transmission infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) are used to study the structure and properties of the resulting nanocomposites.
A.Q. Wang et al. / Applied Clay Science 42 (2008) 310–317
311
2. Experimental
Table 1 Comparison between experimental and PDF data for CeO2
Sodium montmorillonite (untreated) (provided by Nanocor) was added to D.I. water and stirred or ultrasonicated overnight. The resulting dispersion was slowly decanted into a mixture of cerium nitrate salt and sodium acetate. Concentration of the montmorillonite varied between 0.5 to 50% (w/w) to the cerium nitrate concentration. The pH of the cerium salt solution was adjusted to ~ 10 with NaOH. The resulting solution mixture was used as the electrolyte in the electrochemical experiments. The anodic electrochemical synthesis of the nanocomposite powder was done using an EG&G Princeton Applied Research (PAR) Model 273A potentiostat/ galvanostat. Constant temperature was maintained with a Fisher Scientific Model 1016D circulator. A three-electrode configuration was employed for the synthesis with stainless steel disk, chromel wire and the saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively. A galvanostatic method (J = 5.6 mA/cm2) was used to produce the nanocomposite powders. The formed precipitate was filtered and rinsed with water and ethanol to remove possible impurities. The precipitate was dried in air at room temperature and ground before being subjected to any characterization. A Siemens D500 diffractometer with Cu Kα radiation (λ = 0.15405 nm) was used to investigate the X-ray diffraction (XRD) of the pure montmorillonite and ceramic montmorillonite composites. All scans ranged from a 2θ of 2o to 100o, with the primary reflections of the montmorillonite appearing below 40o. To diagnostically identify the reflections, a pure montmorillonite sample was analyzed using a step size of 0.01o 2θ and a dwell time of 3 s per step. For the composites, the step size and dwell time were 0.01o 2θ and 1 s, respectively. FTIR was used to study the structure of the composites by mixing 1–2 mg of the composite powder sample with ~ 100 mg of KBr and then pressed to form a disc with a diameter of 1.0 cm. The discs were heated at 150 °C overnight to eliminate the absorbed moisture in the sample. Infrared spectra were obtained using a Perkin-Elmer 1760X FTIR spectrophotometer. For each sample, 40 scans were collected in the range of 4000 to 400 cm− 1 with a resolution of 4 cm− 1. The powders were processed by pressing uniaxially under a pressure of ~ 50 MPa in a die (diameter of the die − 1.0 cm). The pressed pellets were sintered at a constant heating rate of 1 °C/min. The final sintering temperatures ranged from 700 to 1100 °C.
hkl
3. Results and discussions The diffraction patterns of the pure montmorillonite (A) and electrogenerated nanocrystalline cerium oxide powder (B) are shown in Fig. 1. The reflections for the electrosynthesized
Fig. 1. XRD patterns for montmorillonite (A) and electrogenerated nanocrystalline CeO2 powder (B). The corresponding phase assignment for CeO2 is listed in Table 1.
111 200 220 311 222 400 331 420 422 511
Experimental data
PDF#34-0394
2θ
I/I0
2θ
I( f )
28.48 33.07 47.42 56.29 59.04 69.45 76.64 78.93 88.36 95.32
100 30 56 43 10 9 18 13 15 13
28.554 33.081 47.478 56.334 59.085 69.400 76.698 79.067 88.410 95.394
100 30 52 42 8 8 14 8 14 11
Experimental data from Fig. 1.
nanocrystalline CeO2 powder are compared with the random JCPDS powder pattern of CeO2 (PDF# 34-0394) in Table 1, showing a random XRD pattern with a fluorite structure. For the montmorillonite, it is well recognized that the (00l) basal reflections are important for structure interpretation. The distance between a certain plane in the layer and the corresponding plane in the next layer is called the basal or d spacing, which can be determined by X-ray diffraction (XRD). An absence of the reflection for these materials is indicative of layer delamination or exfoliation. The montmorillonite gives a strong (001) reflection at 6.85o 2θ, with a basal spacing of 12.9 Å. The (002) reflection is weakly present at 14.05o 2θ with a calculated d-spacing of 6.30 Å. Other higher-order reflections are weak, which reflects the disorder present in the natural clay mineral. The (060) reflection of the montmorillonite can provide supplementary information to distinguish dioctahedral and trioctahedral types according to the d-spacing and 2θ position (Pinnavaia, 1983). The (060) reflection of the untreated montmorillonite is at 61.87o 2θ with a d-spacing of 1.49 Å, which corresponds to a typical dioctahedral clay mineral, such as montmorillonite (2θ(060) =62.22 ~ 61.67o and d(060) = 1.492 ~ 1.504 Å). In addition, there are three twodimensional bands, (02;11) at 19.79o 2θ, (20;13) at 34.93o 2θ and (04;22) at 53.93o 2θ present in the montmorillonite XRD pattern (Fig. 1A). Typically the 20;13 band is recognized as the best parameter to evaluate turbostratic disorder since it is more asymmetric than the other two common bands (Moore and Reynolds, 1997). Since montmorillonites are naturally occurring clay minerals, there are minor amounts of impurities present, such as the sharp reflection at 26.63o 2θ coinciding with quartz silica (Moore and Reynolds, 1997; Chipera and Bish, 2001). Selected XRD patterns of the composites (CeO2/montmorillonite) are shown in Fig. 2 and indicate the electrosynthesized powders are randomly oriented. The electrosynthesized CeO2/ montmorillonite composites retain the fcc fluorite structure characteristic of CeO2 for increasing contents of montmorillonite in the composites (1, 2, 3, 5, 10, 20, and 30%). The Scherrer equation was used for the (111) reflection of CeO2 in the composites to estimate the particle size of the composites at varying montmorillonite contents. The particle sizes ranged from 4–6 nm, indicating the electrosynthesized CeO2/montmorillonite powders are nanocomposites.
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Fig. 2. XRD patterns for electrogenerated CeO2/montmorillonite nanocomposite powders at montmorillonite concentrations of 1, 3, 20 and 30%.
With the formation of the nanocomposite, the (001) reflection for the montmorillonite content (1–20%) is not present in the XRD patterns. At 30%, a small peak at 6.95o 2θ is detected for the (001) reflection. For all percentages of montmorillonite content in the nanocomposites, a very small reflection around 63o 2θ is seen in the diffraction patterns. This reflection corresponds to an α-quartz reflection and could result from the dramatic agitation or ultrasonication during the preparation of the montmorillonite dispersion and the electrochemical synthesis process. The ultrasonication and agitation
not only delaminate the layered structure in the z direction, but also break down to some degree, order in the x/y direction of the montmorillonite. The loss of layered structure indicates that the montmorillonite is exfoliated and dispersed as individual platelets throughout the ceria. Low temperature DSC of the nanocomposites show only one endo-peak for the nanocrystalline cerium oxide, which verifies no interlayer water present in the nanocomposites and hints at the exfoliation of the montmorillonite in these nanocomposites.
Fig. 3. FTIR spectra of montmorillonite (a) and electrogenerated nanocrystalline CeO2 powder (b) in the OH stretching (3850–3350 cm− 1) (A) and Si–O stretching (1300–400 cm− 1) (B) region. Insert shows magnified OH stretching band region.
A.Q. Wang et al. / Applied Clay Science 42 (2008) 310–317
Infrared spectra can give information on the occupancy of octahedral and tetrahedral sites and the stacking and arrangement of the sheets in clay minerals. For infrared, the absorbance bands due to structural OH and Si–O groups have significant roles in identification and analysis of clay minerals. Infrared spectra of the montmorillonite were collected in the 4000– 400 cm− 1 region. The spectra of sodium montmorillonite (a) and nanocrystalline CeO2 powder (b) are shown in Fig. 3, for the OH stretching regions (A) and Si–O stretching regions (B). To diminish the absorbed water, the samples were heated at 150 °C overnight (Madejová and Komadel, 2001; Madejová, 2003). Montmorillonite belongs to a dioctahedral smectite, in which the position and shape of the OH stretching band is mainly determined by the octahedral atoms coordinated with the hydroxyl groups (Farmer and Russell, 1964; Farmer, 2000; Madejová, 2003). The radius, valence charge, and hydration energies of the exchangeable cations also play critical roles (Yan et al., 1996; Xie et al., 2001). The broad absorption in the region of 3400–3500 cm− 1, which is centered at 3458 cm− 1, is usually assigned to OH stretching of water moisture in the montmorillonite sample. This band is observed to diminish when a KBrsample pellet was heated overnight at 150 °C. Two bands can be resolved from each other in the range of 3620–3660 cm− 1 and occur at 3634 and 3626 cm− 1. These bands are assigned to structural OH stretching vibrations of the clay mineral. The peak close to 3620 cm− 1 is indicative of high concentrations of Al in the octahedral (Farmer and Russell, 1964; Madejová, 2003). The OH bending vibrations can be found in the low frequency region of infrared spectra and the positions of such reflections indicate the occupancy of octahedral sheet as well.
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Dioctahedral clay minerals have been observed to have this vibration in the range of 950–800 cm− 1, different from the lower wavenumber vibrations at 700–600 cm− 1 for trioctahedral clay minerals (Madejová and Komadel, 2001; Madejová, 2003). In montmorillonite, the 914 cm− 1 and 847 cm− 1 absorption bands represent inner hydroxyl bending in Al–Al– OH and in Al–Mg–OH structure, respectively, reflecting partial magnesium substitution for aluminum in octahedral sheets (Madejová and Komadel, 2001; Madejová, 2003). The absence of the 885 cm− 1 band indicates no iron substitution for aluminum in the octahedral sheet. The broad band between 1130 and 1000 cm− 1, centered at 1045 cm− 1 in Fig. 3B is the Si–O characteristic band for montmorillonite (Madejová and Komadel, 2001; Madejová, 2003). Since quartz impurity also contributes to this broad band, FTIR as well as XRD was run to confirm its presence. The band at ~ 799 cm− 1 also comes from quartz impurity in this montmorillonite. Bands at 522 cm− 1 and 468 cm− 1 in montmorillonite are assigned to Si–O–Al (octahedral Al) and Si–O–Si bending vibration, respectively. According to the literature, the presence of the 624 cm− 1 band is due to Si–O and Al–O coupled out-of-plane vibrations (Madejová and Komadel, 2001). FTIR for the nanocrystalline cerium oxide powder (b) is also shown in Fig. 3. Usually simple metal oxides are infrared inactive, however CeO2 has two oxygen connected to cerium atoms, resulting in infrared active vibrations in the range of 1100–825 cm− 1. It is recognized that the 550 cm− 1 band is a characteristic peak in phonon modes spectral range for cubic crystalline forms of rare earth oxides (Li et al., 1989; Li et al., 1993; Binet et al., 1999). Generally, stoichiometric cerium
Fig. 4. FTIR spectra of CeO2/montmorillonite nanocomposites of 1 (a), 3 (b), and 30% (c) in the OH stretching (3850–3350 cm− 1) (A) and Si–O stretching (1300–400 cm− 1) (B) region.
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dioxide is optically transparent in the wavenumber range of 4000 to 600 cm− 1. The 1300 to 600 cm− 1 region spectrum of cerium oxide powder shows: two bands at 1052 and 1021 cm− 1; one small band at 931 cm− 1; two sharp bands at 856 and 837 cm− 1; one small peak at 726 cm− 1; and two bands at ~ 654 and 621 cm− 1. The 726 cm− 1 band can be attributed to Ce–O vibration, the 1021 cm− 1 is usually assigned as the first overtone mode of the fundamental vibration at ~ 550 cm− 1. A band at ~ 2103 cm− 1 is also found in the FTIR spectrum, which can be regarded as a possible overtone for 1021 cm− 1. The peaks at 856 and 837 cm− 1 can be ascribed to adsorbed superoxide species, − (O2ads ) which is the characteristic frequency of O–O vibration for a bond order of 1.05. Some other researchers also assign these two peaks to carbonate due to the adsorption of CO2 in air. Since this electrochemical synthesis used acetate as a ligand, the
1021 and 1052 cm− 1 bands may also be due to residual acetate (Socrates, 1998). After electrochemical synthesis, FTIR is also run for the resulting composites. Fig. 4 shows the infrared spectra for the CeO2/montmorillonite nanocomposites at 1 (a), 3 (b), and 30% (c) montmorillonite content. The structural OH band of the montmorillonite (~ 3620–3660 cm− 1) can be detected in the CeO2/montmorillonite nanocomposites except for the 1% sample containing a low content of montmorillonite in the nanocomposite. The OH stretching vibration shifts negatively with decreasing montmorillonite and increasing CeO2 content in the nanocomposites to 3636, 3621, and 3620 cm− 1 for the 30, 3, and 1%, respectively. In the formation of CeO2/montmorillonite nanocomposites, the montmorillonite was suspended by ultrasonication or agitation before mixing with the cerium salt solution, it is
Fig. 5. A and B. XRD patterns for CeO2/montmorillonite nanocomposites before and after sintering at 700, 800, 900, 1000, 1100 °C.
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Fig. 5 (continued ).
believed that the montmorillonite loses the long range structural order and delaminates into platelets. With this exfoliation, more apical oxygen atoms are exposed in the surface of the octahedral or tetrahedral sheets, creating more chances for hydrogen bonding between inner hydroxyl group in the octahedral sheet and these tetrahedral apical oxygen atoms. Isomorphous substitution of Al3+ by Mg2+ in octahedral structure brings negative charge on the apical oxygens of the tetrahedra of the clay mineral. This oxygen interacts with the hydroxyl group by OH….Oap hydrogen bond formation. This hydrogen bond strength plays a significant role in the OH vibration frequency, which can be seen in the cation intercalation experiments. Cation (Li+, Cu2+, Cd2+) migration and intercalation into layer structures modify the environment and orientation of hydroxyl group and compensate for the positive charge deficit (Madejová et al., 1999). The absence of the negative
Oap weakens hydrogen bond interaction and OH stretching shifts to a higher wavenumber with increasing concentration of inserted cations. Therefore, the downward shift in the position of the OH stretching band in CeO2/montmorillonite composites is an evidence of hydrogen bonding for the hydroxyl containing structure. In addition, since acetate was used in the electrogeneration of the nanocomposites, oxygens in acetate may participate in hydrogen Table 2 Comparison of particle size (nm) for varying percentage of montmorillonite in nanocomposite Sample
Before sintering
700 °C
800 °C
900 °C
0% 1% 20%
5 5 5
16 23 8
54 114 14
54 – 46
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bonding. To eliminate this acetate effect, the complexation of cerium (III) ions with acetate was arranged prior to the addition of the montmorillonite suspension. Dilution enhances delamination of montmorillonite (Sivakumar et al., 1995), therefore, the lower concentration of clay mineral, leads to stronger hydrogen bonding and a lower OH stretching frequency. Si–O stretching and bending vibration is another set of significant IR frequencies for the clay mineral. The position and shape of these bands also fingerprint the different arrangement and stacking of montmorillonite. The Si–O stretching of the CeO2/montmorillonite composites, for the region of 1300 to 400 cm− 1, is shown in Fig. 4B. Positions of the Si–O–Al and Si–O–Si bendings at 522 and 468 cm− 1 do not change significantly with the formation of the nanocomposites and increasing clay mineral content in the nanocomposites. With exfoliation and electrochemical processing, long-range order of the montmorillonite is destroyed in the nanocomposites. Small tetrahedral sheet platelets possess less tension and exhibit more regularity in structure compared with ordered montmorillonite, resulting in a relative sharper absorption band centered around ~ 1035 cm− 1 in the Si–O infrared spectra. The observed downward shift of Si–O in the nanocomposites relative to the pristine montmorillonite contrasts with the previously reported Si–O absorption upward migration phenomenon associated with cation insertion into octahedral and tetrahedral sites (Madejová et al., 1999). With decreasing concentration of clay mineral in the nanocomposites, the band center position does not vary, but the shape of Si–O band changes with a more pronounced shoulder for the 30% nanocomposite, which is not present in lower concentrations of clay mineral composites. For high concentrations of montmorillonite in the nanocomposite, the band is mainly due to the Si–O of montmorillonite and the shoulder band from quartz impurity. At low concentrations of montmorillonite (1%), the Si–O stretching signal attenuates but there is a shoulder peak around 1023 cm− 1, indicating a minor amount of acetate impurity in the nanocomposite. Fig. 5A and B are the XRD patterns of the unsintered and sintered nanocomposite pellets at different sintering temperatures. A noticeable difference in the XRD’s of the nanocomposites is the increasing particle size with increasing sintering temperature. However, the percentage of montmorillonite in the nanocomposite definitely affects the rate of particle growth with sintering temperature. Table 2 lists the particle sizes as calculated from the Scherrer equation using the (111) reflection in the XRD pattern. The particle sizes for 1000 °C and 1100 °C sintering are not listed since the crystallite size has reached the microcrystalline range and cannot be measured by the XRD procedure. As shown by the XRD pattern, sintering of the nanocomposites containing 1% montmorillonite results in faster crystallization compared to nanocrystalline cerium oxide or nanocomposites with greater than 1% montmorillonite concentrations. A higher percentage of montmorillonite in the nanocomposite hinders particle growth during sintering, as seen by the broader peaks in the XRD patterns with increasing temperature. This inhibition of the growth rate in the nanocomposites by the addition of a montmorillonite is important for the nanocomposite field. The
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