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Preparation and characterization of a new layered double hydroxide, Co–Zr–Si
Journal of Colloid and Interface Science 297 (2006) 182–189 www.elsevier.com/locate/jcis
Preparation and characterization of a new layered double hydroxide, Co–Zr–Si Osama Saber Egyptian Petroleum Research Institute (EPRI), El-Zohour District, Nasr City, PO Box 11727, Cairo, Egypt Received 18 July 2005; accepted 14 October 2005 Available online 15 December 2005
Abstract The layered double hydroxides (LDHs) are nano-ordered layered compounds and well known for their ability to intercalate anionic compounds. Most LDH is prepared conventionally only with divalent and trivalent cations. In this study, Co–Zr–Si LDH, consisting of divalent, tetravalent, and tetravalent cations, was prepared and reacted with monocarboxylic acids at room temperature. The Co–Zr–Si LDH and intercalated compounds have been characterized by energy-dispersive X-ray spectrometry, X-ray powder diffraction, IR spectra, thermal analysis, and scanning electron microscopy (SEM). The insertion of cyanate and carbonate anions into LDH was confirmed by IR spectra. XRD patterns of the prepared Co–Zr– Si LDH showed that the interlayer spacing of the LDH is 0.78 nm. The spacing is similar to that of usual LDH in which chloride, carbonate, or bromide anion is the guest. SEM images showed that Co–Zr–Si LDH can exist as plate-like or fibrous structures. 2005 Elsevier Inc. All rights reserved. Keywords: Preparation; Intercalation; Layered double hydroxide; Characterizations
1. Introduction Nanostructured inorganic, organic, or hybrid organic–inorganic nanocomposites will contribute to the development of science and technology. Soft-chemistry-based processes clearly offer innovative strategies for obtain tailored nano-structured materials. An increasing interest exists in layered double-metal hydroxides, which are or may be used as catalysts [1–6], photocatalysts, catalyst supports [7,8], adsorbents [9,10], anion exchangers [11,12], medicines [13,14], and bonding materials. Also, LDH materials have been extensively studied in terms of thermal evolution [15–18], textural properties [18–20], and formation of nanosized metal particles [21,22] and for environmental purpose [23–31]. Layered double hydroxides (LDHs) are anionic clays comprising positively charged layers with anions and water molecules intercalated in the interlayer region [1]. The best-known compound in this class of materials is the double hydroxide of Mg with Al, known as hydrotalcite [1,9–12]. The general E-mail address: [email protected]. 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.10.010
formula for these LDHs is (Mg1−x Alx (OH)2 )x+ ·(A− )x ·nH2 O, where “A” represents the interlamellar anion that restore the electroneutrality of the compound. We call these the II–III LDHs. In the case of using Li with Al, the layers acquire the composition (Al2/3 Li1/3 (OH)2 )1/3+ and become positively charged. For charge neutrality, carbonates are intercalated into the interlayer region, resulting in a compound of the nominal composition (LiAl2 (OH)6 )(CO3 )1/2 ·nH2 O [32,33]. We call these I–III LDH. Although several examples of II–III LDHs have been reported, Li–Al LDH is the only known example of I–III LDH. Attempts by the authors to prepare Fe, Cr, and Ga analogues failed to yield LDHs with Li. We recently reported that the preparation of II–IV LDH, Zn– Ti, Zn–Sn, and Co–Ti LDHs, consisting of di- and tetravalent cations, is possible [34–36]. The present work examines the possibility of the preparation of the other kind of LDHs consisting of di-, tetra-, and tetravalent cations. This new LDH structure contains Co2+ , Zr4+ , and Si4+ cations in host layers and cyanate and carbonate anions as the guests. The effect of the aging procedure on the formation of LDH structure was clarified. Also, the intercalation reactions of this LDH with organic acids were tested.
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Horiba FT-720. Scanning electron microscopy (SEM) was performed with a JEOL: JSM-6330F (15 kV/12 mA).
2. Experimental 2.1. Materials
3. Results and discussion The Co–Zr–Si LDH was prepared by co-precipitation of cobalt, zirconium, and silicon salts from homogeneous solution [34–36]. A solution of cobalt nitrate, zirconyl chloride, and silicon chloride (0.039 mol) was mixed with urea solution under vigorous stirring and heated for a long time. The percentage of titanium and silicon is 24 mol%. After filtration and washing several times in distilled water, the products were dried under vacuum at room temperature. Sodium salts of monocarboxylic acids were obtained from WAKO and T.C.I. (Tokyo). 2.2. Intercalation Typically, appropriate amounts of sodium salts of organic acids (0.002 mol) dissolve in 10 ml of deionized–distilled water (concentration about 0.2 M) with ultrasonic treatment. The LDH (0.24 g) were mixed with the solution of organic acid under Ar and stirred at room temperature for 72 h. After filtration and washing, the samples were dried under vacuum at room temperature. 2.3. Characterization The SEM/EDS measurements were carried out with a JEOL JED-2140. Powder X-ray diffraction (XRD) spectra were registered between the 2θ limits 1.8◦ and 50◦ on a Rigaku RINT 2200 using CuKα (filtered) radiation (λ = 0.154 nm) at 40 kV and 20 mA. Thermal analyses (TG, DTG, and DTA) of powdered samples up to 800 ◦ C were carried out at a heating rate of 10 ◦ C/min in flow of nitrogen using a Seiko SSC 5200 apparatus. FT-IR spectra (KBr disc method) were recorded on a
3.1. Chemical analysis of Co–Zr–Si LDH Electron spectroscopy for chemical analysis (ESCA) supports the presence of cobalt in the solid as the divalent cation. The electron binding energy of Co(2p3/2 ) for Co–Ti–Si LDH is 786.4 eV and this value is closer to the binding energy of Co in CoO (783.3) than to the binding energy of Co in Co2 O3 (781.2). Scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM/EDS) analysis provides local information of the concentrations of different elements in the outermost layers of the platelet of LDH. Cobalt, zirconium, silicon, carbon, oxygen, and nitrogen are clearly identified in one spot on the platelet of Co–Zr–Si LDH, as shown in Fig. 1. Fig. 2 presents the results of the EDS analysis, carried out on two other randomly selected spots of LDH. The percentages of different elements of the two spots (Fig. 2) are slightly different from those of the first spot (Fig. 1), which indicates that the different elements are homogeneously distributed in the LDH at the micrometer scale. The scattered molecules of Co, Zr, Si, C, N, and O are shown in the point-mapping image in Fig. 3, which shows them (the white points are molecules) distributed throughout the specific area. Additionally, the white points found in the Co chart are located in positions similar to those of the white points found in the Zr and Si charts, although more Co molecules are found in the chart than Si and Zr charts. This agrees with the schematic representation of Co–Zr–Si LDH which is mentioned later. Also, if we overlapped C and N charts, major white points
Fig. 1. EDS spectra of one spot on platelets of Co–Zr–Si LDH.
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Fig. 2. EDS spectra of two other spots on different platelets of Co–Zr–Si LDH.
would overlie each other. These results prove that carbon and nitrogen are chemically bonded together, which suggests the presence of cyanate anions. This agrees with IR spectra. 3.2. X-ray powder diffraction The X-ray powder diffraction (XRD) for Co–Zr–Si LDH series is given in Fig. 4. Two types of samples were obtained for Co–Zr–Si LDH with different aging times. There are no XRD results reported in the literature for Co–Zr–Si LDH. However, the XRD results of Co–Zr–Si LDH are similar to the XRD results, which were previously published for Co–Al LDH [1,37–39] in the case of the guest is carbonate and chloride. The measured XRD pattern of Co–Zr–Si LDH fits well to a layered structure. The X-ray powder diffraction of Co–Zr–Si LDH (Fig. 4b and c) show the basal peaks of planes hkl (003), (006), and
(009). The good agreement between the values corresponding to successive diffractions by basal planes, i.e., d(003) = 2d(006) = 3d(009) for Co–Zr–Si LDH, reveals highly packed stacks of brucite-like layers ordered along axis c. Dimension c is calculated as three times the spacing for planes (003), i.e., 2.37 nm. The c dimension is very close to that reported for natural and synthetic hydrotalcite, 2.31 nm [37–39]. The XRD pattern of Co–Zr–Si LDH has a main peak at 0.78 nm, which corresponds to interlayer spacing of the LDH as shown in Fig. 4b and c. This value is related to the thickness of the brucite-like layers (0.48 nm for hydrotalcite), as well as the size of the anion (and, in some cases, its orientation) and the number of water molecules existing in the interlayer. Two weak reflection at d = 0.41 and 0.30 nm are observed. We assign those reflections as due to impurities of cobalt hydroxide. The peaks of the layered structure disappeared by the calcination at 500 ◦ C, and the appearance of new peaks at high 2θ
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Fig. 3. Mapping image of different elements of Co–Zr–Si LDH using EDS/SEM analysis.
values, as shown in Fig. 4d indicates the formation of metal oxides. By the reaction of Co–Zr–Si LDH with n-capric acid at room temperature, new peaks are observed at low 2θ , indicating interlayer spacing 3.0 nm, as shown in Fig. 4e. This suggests that the Co–Zr–Si LDH can perform intercalation reactions with organic acids. 3.3. FT-IR spectroscopy The FT-IR technique has been used to identify the nature and symmetry of interlayer anions. The insertion of cyanate anions (NCO–) into Co–Zr–Si LDH is demonstrated by the appearance of the ν1 vibration in the region 2230–2105 cm−1 , the ν2 vibration in the region 600–650 cm−1 , and the ν3 vibration in the
region 1190–1220 cm−1 of the infrared spectrum of the materials, as shown in Fig. 5b [39]. The absorption peak at 2204 cm−1 is assigned to the symmetrical stretching vibration mode of CNO. The band at 1384 cm−1 , together with its companion band at 1495 cm−1 , should be due to mode ν3 of interlayer carbonate species [39,40]. This band is recorded at 1450 cm−1 for free carbonate species, but it splits and shifts upon symmetry lowering. So, for aragonite, it is known that it splits into two bands at 1504 and 1492 cm−1 . In the present case, the splitting is probably due to the restricted symmetry in the interlayer space, in addition to the different electrostatic interactions because of the distortions originated by the three different-sized cations in the layers [39,40]. An important phenomenon for the presence of cyanate anions in the interlayer space is the presence of very sharp bands
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Fig. 6. Thermal analysis of Co–Zr–Si LDH in nitrogen gas: (a) TG, (b) DTG, and (c) DTA. Fig. 4. X-ray diffraction patterns of Co–Zr–Si LDH with aging time (a) 9 h, (b) 14 h, (c) 24 h, and (d) LDH after calcinations at 500 ◦ C and (e) LDH after intercalation with n-capric acid (CH3 (CH2 )8 COOH).
Fig. 5. IR spectra of Co–Zr–Si LDH (a) at aging time 14 h, (b) at aging time 24 h, and (c) after intercalation with n-capric acid.
recorded around 2856 cm−1 , as shown in Fig. 5. It has been ascribed to the OH stretching mode of interlayer water molecules hydrogen-bonded to interlayer anions [40–44]. This means that
cyanate anions may be connected together through hydrogen bonds, with water molecules in the interlayer spacing suggesting a self-assembling phenomenon for cyanate anions among the layers. The bending mode band of water molecules is observed clearly close to 1637 cm−1 , which suggests the existence of a large number of water molecules between the layers [45]. Also, thermal analysis supported this suggestion. The weak band observed at 1045 cm−1 can be ascribed to the ν1 mode of carbonate. Although this band is IR-inactive in the free carbonate, it becomes activated owing to lowering of symmetry of carbonate anions in the interlayer [46]. These results indicate that the prepared sample of Co–Zr–Si LDH has a structure similar to the usual LDH structure, and it confirms the presence of carbonate anions and cyanate anions, in addition to water molecules inside the interlayer space. Also, the existence of organic compounds inside Co–Zr–Si LDH after intercalation reactions is confirmed by the results of IR spectroscopy, as shown in Fig. 5c. In the IR spectrum of the reaction products of Co–Zr–Si LDH with straight chain aliphatic acids such as n-capric acid, the carbon–hydrogen stretch absorption near 2852 cm−1 and the carbon–hydrogen bending band at 1465 cm−1 are observed as shown in Fig. 5c. Also, new peaks appear near 1590 and 1405 cm−1 . The absorption at 1590 cm−1 is assigned to the symmetric stretching vibration of carboxylate, and the absorption at 1405 cm−1 is assigned to the asymmetric stretching vibration of carboxylate. On the other hand, the peaks of cyanate anions and carbonate anions disappeared after intercalation reactions. 3.4. Thermal analysis The thermal analyses (DTA, TG, and DTG) of Co–Zr–Si LDH are shown in Figs. 6 and 7. Because of the presence of the divalent cation cobalt and cyanate anion, and taking into account preliminary results that indicated their oxidation (and re-
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Fig. 7. Thermal analysis of Co–Zr–Si LDH in air: (a) TG, (b) DTG, and (c) DTA.
duction) during calcination in air at high temperature, the thermal analyses were recorded both in air and in nitrogen, in order to identify thermal processes related to oxidation/reduction processes. Thermal characteristics of Co–Zr–Si LDH in nitrogen gas at aging time 24 h were determined by TG, DTG, and DTA, as shown in Fig. 6. Major losses of weight occur mainly in four steps. The TG diagram showed that the first weight loss up to 100 ◦ C is 15.7% and the second weight loss up to 211 ◦ C is 21.4%. The third weight loss occurs from 211 to 272 ◦ C, while the fourth weight loss happens from 272 to 331 ◦ C. The first and second weight losses are due to the desorption of surface and intercalated water of LDH. The third weight loss corresponds to the decomposition of the cyanate anion and the fourth weight loss is due to the decomposition of carbonate anions and the dehydroxylation process. The main losses are confirmed from DTG and DTA diagrams. The DTA diagram shows three endothermic peaks, as shown in Fig. 6c. The peak around 62 ◦ C corresponds to the desorption of surface water and the peak at 250 ◦ C corresponds to the decomposition of cyanate. The third endothermic peak is due to the decomposition of carbonate anions and the dehydroxylation of layers. These processes (desorption of surface water and intercalated water, decomposition of cyanate, decomposition of carbonate, and dehydroxylation of layers) are clearly distinguished when the thermal analyses are performed in air instead of in nitrogen. The TG analysis in air, Fig. 7a, showed five weight losses, indicating five thermal processes: desorption of surface water, desorption of intercalated water, oxidation of cyanate anion, decomposition of carbonate anions, and dehydroxylation process. The DTG of Co–Zr–Si LDH in air, Fig. 7b, showed five peaks, confirming TG results. Also, the DTA of Co–Zr–Si
Fig. 8. SEM images of Co–Zr–Si LDH at different locations and magnifications: (a) ×1000, (b) ×5000, and (c) ×10,000.
LDH in air showed one broad endothermic peak and one broad exothermic peak. The endothermic peak is due to desorption of water and the exothermic peak is due to the oxidation of cyanate anion and Co2+ cation and the decomposition of carbonate. This agrees with the ESCA analysis that confirmed the presence of cobalt as divalent cation. 3.5. Scanning electron microscopy (SEM) SEM images of Co–Zr–Si LDH at aging time 24 h are shown in Fig. 8. SEM images show that the Co–Zr–Si LDH has platelike morphology, which was typical for the LDH morphology, as shown in Fig. 8a. Some platelets of Co–Zr–Si LDH started to convert to fibrous morphology, as shown in Fig. 8b. Other platelets completely converted to fibrous morphology, as shown in Fig. 8c. By intercalation reactions with n-capric acid, SEM
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Fig. 10. Schematic representation of Co–Zr–Si LDH for platelike morphology.
The first anion, mainly intercalated, is cyanate, which is produced from the decomposition of urea, as shown in the equation
Fig. 9. SEM images of Co–Zr–Si LDH after reaction with n-capric acid at different magnifications: (a) ×350 and (b) ×20,000.
images of Co–Zr–Si LDH showed that the fibrous morphology changed again to platelike morphology, as shown in Fig. 9. From these results, it can be concluded that the Co–Zr– Si LDH have the ability to exist with fibrous and platelike morphology depending on the kind of intercalated anion. This means that we can control the morphology of LDH by intercalation reactions, which has been recently published. 4. Discussion The results presented in this work show that the formation of layered double hydroxides is not limited to reactions between di- and trivalent cations, although these are the only reactions reported by many researchers [1–18]. The powder XRD patterns are not sufficiently high-quality to allow us to carry out structure determination. However, by interlayer spacing and the size of the guest ions, orientation of guest ions was considered. From known layer thickness 0.48 nm, the interlayer spacing available for the anion was calculated as 0.30 nm. By comparison with the size of the cyanate anion, 0.34 nm, it was considered that the cyanate anion makes an angle of 65.7◦ with the Co–Zr–Si layer and connects with Zr or Si cations through two different sides (above and below the layer) in order to neutralize the positive charge, as shown in Fig. 10. The experimental results show that two anions are intercalated into the interlayer spacing of Co–Zr–Si LDH, indicating one interlayer spacing, 0.78 nm.
− Urea → NH+ 4 CNO .
(1)
Earlier investigators [46–48] confirmed this equation and considered that ammonium cyanate is an intermediate in the decomposition of urea. Recently, Coebel and Elsener [49,50] mentioned that the thermal decomposition of urea can yield a variety of products apart from ammonia and isocyanic acid. The second anion is carbonate, produced from the complete decomposition of urea, as shown in the equation + − + NH+ 4 CNO + 2H2 O + 2H → NH4 + H2 CO3 .
(2)
The earlier investigators [46–48] indicated that the decomposition of urea is complete only under acidic conditions, as shown in Eq. (2). According to the synthetic conditions of our experiments, the decomposition of urea is not complete. This leads to the concentration of cyanate being higher than the concentration of carbonate in the interlayer spacing of Co–Zr–Si LDH. The schematic representation of Co–Zr–Si LDH, Fig. 10, may be valid only for the platelike structure, while in the case of the fibrous structure, we suggest that the above representation may be valid after some modification. According to the mechanism of our laboratory for fibrous structures formed from the reaction of zinc hydroxide with organic acids [28], we can apply the same idea to this fibrous structure. The presence of carbonate (−2) in the interlayer spacing pushes two cyanate anions (−1) to attack the layer (+2) from one side. This leads to stearic repulsion between bulky cyanide groups, which causes curling of the layers, giving a fibrous structure as shown in Fig. 11.
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Fig. 11. Schematic representation of Co–Zr–Si LDH for fibrous morphology.
5. Conclusions In this study, Co–Zr–Si layered double hydroxides consisting of divalent and two tetravalent cations have been prepared for the first time. The Co–Zr–Si LDH has interlayer spacing 0.78 nm. SEM images showed that the Co–Zr–Si LDH can exist as a platelike or a fibrous structure. Anion-exchange reactions were successful in replacing inorganic anions with organic anions in Co–Zr–Si LDH. Acknowledgments The author thanks Professor H. Tagaya and Yamagata University for their cooperation. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Preparation and characterization of a new layered double hydroxide, Co–Zr–Si Osama Saber Egyptian Petroleum Research Institute (EPRI), El-Zohour District, Nasr City, PO Box 11727, Cairo, Egypt Received 18 July 2005; accepted 14 October 2005 Available online 15 December 2005
Abstract The layered double hydroxides (LDHs) are nano-ordered layered compounds and well known for their ability to intercalate anionic compounds. Most LDH is prepared conventionally only with divalent and trivalent cations. In this study, Co–Zr–Si LDH, consisting of divalent, tetravalent, and tetravalent cations, was prepared and reacted with monocarboxylic acids at room temperature. The Co–Zr–Si LDH and intercalated compounds have been characterized by energy-dispersive X-ray spectrometry, X-ray powder diffraction, IR spectra, thermal analysis, and scanning electron microscopy (SEM). The insertion of cyanate and carbonate anions into LDH was confirmed by IR spectra. XRD patterns of the prepared Co–Zr– Si LDH showed that the interlayer spacing of the LDH is 0.78 nm. The spacing is similar to that of usual LDH in which chloride, carbonate, or bromide anion is the guest. SEM images showed that Co–Zr–Si LDH can exist as plate-like or fibrous structures. 2005 Elsevier Inc. All rights reserved. Keywords: Preparation; Intercalation; Layered double hydroxide; Characterizations
1. Introduction Nanostructured inorganic, organic, or hybrid organic–inorganic nanocomposites will contribute to the development of science and technology. Soft-chemistry-based processes clearly offer innovative strategies for obtain tailored nano-structured materials. An increasing interest exists in layered double-metal hydroxides, which are or may be used as catalysts [1–6], photocatalysts, catalyst supports [7,8], adsorbents [9,10], anion exchangers [11,12], medicines [13,14], and bonding materials. Also, LDH materials have been extensively studied in terms of thermal evolution [15–18], textural properties [18–20], and formation of nanosized metal particles [21,22] and for environmental purpose [23–31]. Layered double hydroxides (LDHs) are anionic clays comprising positively charged layers with anions and water molecules intercalated in the interlayer region [1]. The best-known compound in this class of materials is the double hydroxide of Mg with Al, known as hydrotalcite [1,9–12]. The general E-mail address: [email protected]. 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.10.010
formula for these LDHs is (Mg1−x Alx (OH)2 )x+ ·(A− )x ·nH2 O, where “A” represents the interlamellar anion that restore the electroneutrality of the compound. We call these the II–III LDHs. In the case of using Li with Al, the layers acquire the composition (Al2/3 Li1/3 (OH)2 )1/3+ and become positively charged. For charge neutrality, carbonates are intercalated into the interlayer region, resulting in a compound of the nominal composition (LiAl2 (OH)6 )(CO3 )1/2 ·nH2 O [32,33]. We call these I–III LDH. Although several examples of II–III LDHs have been reported, Li–Al LDH is the only known example of I–III LDH. Attempts by the authors to prepare Fe, Cr, and Ga analogues failed to yield LDHs with Li. We recently reported that the preparation of II–IV LDH, Zn– Ti, Zn–Sn, and Co–Ti LDHs, consisting of di- and tetravalent cations, is possible [34–36]. The present work examines the possibility of the preparation of the other kind of LDHs consisting of di-, tetra-, and tetravalent cations. This new LDH structure contains Co2+ , Zr4+ , and Si4+ cations in host layers and cyanate and carbonate anions as the guests. The effect of the aging procedure on the formation of LDH structure was clarified. Also, the intercalation reactions of this LDH with organic acids were tested.
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Horiba FT-720. Scanning electron microscopy (SEM) was performed with a JEOL: JSM-6330F (15 kV/12 mA).
2. Experimental 2.1. Materials
3. Results and discussion The Co–Zr–Si LDH was prepared by co-precipitation of cobalt, zirconium, and silicon salts from homogeneous solution [34–36]. A solution of cobalt nitrate, zirconyl chloride, and silicon chloride (0.039 mol) was mixed with urea solution under vigorous stirring and heated for a long time. The percentage of titanium and silicon is 24 mol%. After filtration and washing several times in distilled water, the products were dried under vacuum at room temperature. Sodium salts of monocarboxylic acids were obtained from WAKO and T.C.I. (Tokyo). 2.2. Intercalation Typically, appropriate amounts of sodium salts of organic acids (0.002 mol) dissolve in 10 ml of deionized–distilled water (concentration about 0.2 M) with ultrasonic treatment. The LDH (0.24 g) were mixed with the solution of organic acid under Ar and stirred at room temperature for 72 h. After filtration and washing, the samples were dried under vacuum at room temperature. 2.3. Characterization The SEM/EDS measurements were carried out with a JEOL JED-2140. Powder X-ray diffraction (XRD) spectra were registered between the 2θ limits 1.8◦ and 50◦ on a Rigaku RINT 2200 using CuKα (filtered) radiation (λ = 0.154 nm) at 40 kV and 20 mA. Thermal analyses (TG, DTG, and DTA) of powdered samples up to 800 ◦ C were carried out at a heating rate of 10 ◦ C/min in flow of nitrogen using a Seiko SSC 5200 apparatus. FT-IR spectra (KBr disc method) were recorded on a
3.1. Chemical analysis of Co–Zr–Si LDH Electron spectroscopy for chemical analysis (ESCA) supports the presence of cobalt in the solid as the divalent cation. The electron binding energy of Co(2p3/2 ) for Co–Ti–Si LDH is 786.4 eV and this value is closer to the binding energy of Co in CoO (783.3) than to the binding energy of Co in Co2 O3 (781.2). Scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM/EDS) analysis provides local information of the concentrations of different elements in the outermost layers of the platelet of LDH. Cobalt, zirconium, silicon, carbon, oxygen, and nitrogen are clearly identified in one spot on the platelet of Co–Zr–Si LDH, as shown in Fig. 1. Fig. 2 presents the results of the EDS analysis, carried out on two other randomly selected spots of LDH. The percentages of different elements of the two spots (Fig. 2) are slightly different from those of the first spot (Fig. 1), which indicates that the different elements are homogeneously distributed in the LDH at the micrometer scale. The scattered molecules of Co, Zr, Si, C, N, and O are shown in the point-mapping image in Fig. 3, which shows them (the white points are molecules) distributed throughout the specific area. Additionally, the white points found in the Co chart are located in positions similar to those of the white points found in the Zr and Si charts, although more Co molecules are found in the chart than Si and Zr charts. This agrees with the schematic representation of Co–Zr–Si LDH which is mentioned later. Also, if we overlapped C and N charts, major white points
Fig. 1. EDS spectra of one spot on platelets of Co–Zr–Si LDH.
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Fig. 2. EDS spectra of two other spots on different platelets of Co–Zr–Si LDH.
would overlie each other. These results prove that carbon and nitrogen are chemically bonded together, which suggests the presence of cyanate anions. This agrees with IR spectra. 3.2. X-ray powder diffraction The X-ray powder diffraction (XRD) for Co–Zr–Si LDH series is given in Fig. 4. Two types of samples were obtained for Co–Zr–Si LDH with different aging times. There are no XRD results reported in the literature for Co–Zr–Si LDH. However, the XRD results of Co–Zr–Si LDH are similar to the XRD results, which were previously published for Co–Al LDH [1,37–39] in the case of the guest is carbonate and chloride. The measured XRD pattern of Co–Zr–Si LDH fits well to a layered structure. The X-ray powder diffraction of Co–Zr–Si LDH (Fig. 4b and c) show the basal peaks of planes hkl (003), (006), and
(009). The good agreement between the values corresponding to successive diffractions by basal planes, i.e., d(003) = 2d(006) = 3d(009) for Co–Zr–Si LDH, reveals highly packed stacks of brucite-like layers ordered along axis c. Dimension c is calculated as three times the spacing for planes (003), i.e., 2.37 nm. The c dimension is very close to that reported for natural and synthetic hydrotalcite, 2.31 nm [37–39]. The XRD pattern of Co–Zr–Si LDH has a main peak at 0.78 nm, which corresponds to interlayer spacing of the LDH as shown in Fig. 4b and c. This value is related to the thickness of the brucite-like layers (0.48 nm for hydrotalcite), as well as the size of the anion (and, in some cases, its orientation) and the number of water molecules existing in the interlayer. Two weak reflection at d = 0.41 and 0.30 nm are observed. We assign those reflections as due to impurities of cobalt hydroxide. The peaks of the layered structure disappeared by the calcination at 500 ◦ C, and the appearance of new peaks at high 2θ
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Fig. 3. Mapping image of different elements of Co–Zr–Si LDH using EDS/SEM analysis.
values, as shown in Fig. 4d indicates the formation of metal oxides. By the reaction of Co–Zr–Si LDH with n-capric acid at room temperature, new peaks are observed at low 2θ , indicating interlayer spacing 3.0 nm, as shown in Fig. 4e. This suggests that the Co–Zr–Si LDH can perform intercalation reactions with organic acids. 3.3. FT-IR spectroscopy The FT-IR technique has been used to identify the nature and symmetry of interlayer anions. The insertion of cyanate anions (NCO–) into Co–Zr–Si LDH is demonstrated by the appearance of the ν1 vibration in the region 2230–2105 cm−1 , the ν2 vibration in the region 600–650 cm−1 , and the ν3 vibration in the
region 1190–1220 cm−1 of the infrared spectrum of the materials, as shown in Fig. 5b [39]. The absorption peak at 2204 cm−1 is assigned to the symmetrical stretching vibration mode of CNO. The band at 1384 cm−1 , together with its companion band at 1495 cm−1 , should be due to mode ν3 of interlayer carbonate species [39,40]. This band is recorded at 1450 cm−1 for free carbonate species, but it splits and shifts upon symmetry lowering. So, for aragonite, it is known that it splits into two bands at 1504 and 1492 cm−1 . In the present case, the splitting is probably due to the restricted symmetry in the interlayer space, in addition to the different electrostatic interactions because of the distortions originated by the three different-sized cations in the layers [39,40]. An important phenomenon for the presence of cyanate anions in the interlayer space is the presence of very sharp bands
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Fig. 6. Thermal analysis of Co–Zr–Si LDH in nitrogen gas: (a) TG, (b) DTG, and (c) DTA. Fig. 4. X-ray diffraction patterns of Co–Zr–Si LDH with aging time (a) 9 h, (b) 14 h, (c) 24 h, and (d) LDH after calcinations at 500 ◦ C and (e) LDH after intercalation with n-capric acid (CH3 (CH2 )8 COOH).
Fig. 5. IR spectra of Co–Zr–Si LDH (a) at aging time 14 h, (b) at aging time 24 h, and (c) after intercalation with n-capric acid.
recorded around 2856 cm−1 , as shown in Fig. 5. It has been ascribed to the OH stretching mode of interlayer water molecules hydrogen-bonded to interlayer anions [40–44]. This means that
cyanate anions may be connected together through hydrogen bonds, with water molecules in the interlayer spacing suggesting a self-assembling phenomenon for cyanate anions among the layers. The bending mode band of water molecules is observed clearly close to 1637 cm−1 , which suggests the existence of a large number of water molecules between the layers [45]. Also, thermal analysis supported this suggestion. The weak band observed at 1045 cm−1 can be ascribed to the ν1 mode of carbonate. Although this band is IR-inactive in the free carbonate, it becomes activated owing to lowering of symmetry of carbonate anions in the interlayer [46]. These results indicate that the prepared sample of Co–Zr–Si LDH has a structure similar to the usual LDH structure, and it confirms the presence of carbonate anions and cyanate anions, in addition to water molecules inside the interlayer space. Also, the existence of organic compounds inside Co–Zr–Si LDH after intercalation reactions is confirmed by the results of IR spectroscopy, as shown in Fig. 5c. In the IR spectrum of the reaction products of Co–Zr–Si LDH with straight chain aliphatic acids such as n-capric acid, the carbon–hydrogen stretch absorption near 2852 cm−1 and the carbon–hydrogen bending band at 1465 cm−1 are observed as shown in Fig. 5c. Also, new peaks appear near 1590 and 1405 cm−1 . The absorption at 1590 cm−1 is assigned to the symmetric stretching vibration of carboxylate, and the absorption at 1405 cm−1 is assigned to the asymmetric stretching vibration of carboxylate. On the other hand, the peaks of cyanate anions and carbonate anions disappeared after intercalation reactions. 3.4. Thermal analysis The thermal analyses (DTA, TG, and DTG) of Co–Zr–Si LDH are shown in Figs. 6 and 7. Because of the presence of the divalent cation cobalt and cyanate anion, and taking into account preliminary results that indicated their oxidation (and re-
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Fig. 7. Thermal analysis of Co–Zr–Si LDH in air: (a) TG, (b) DTG, and (c) DTA.
duction) during calcination in air at high temperature, the thermal analyses were recorded both in air and in nitrogen, in order to identify thermal processes related to oxidation/reduction processes. Thermal characteristics of Co–Zr–Si LDH in nitrogen gas at aging time 24 h were determined by TG, DTG, and DTA, as shown in Fig. 6. Major losses of weight occur mainly in four steps. The TG diagram showed that the first weight loss up to 100 ◦ C is 15.7% and the second weight loss up to 211 ◦ C is 21.4%. The third weight loss occurs from 211 to 272 ◦ C, while the fourth weight loss happens from 272 to 331 ◦ C. The first and second weight losses are due to the desorption of surface and intercalated water of LDH. The third weight loss corresponds to the decomposition of the cyanate anion and the fourth weight loss is due to the decomposition of carbonate anions and the dehydroxylation process. The main losses are confirmed from DTG and DTA diagrams. The DTA diagram shows three endothermic peaks, as shown in Fig. 6c. The peak around 62 ◦ C corresponds to the desorption of surface water and the peak at 250 ◦ C corresponds to the decomposition of cyanate. The third endothermic peak is due to the decomposition of carbonate anions and the dehydroxylation of layers. These processes (desorption of surface water and intercalated water, decomposition of cyanate, decomposition of carbonate, and dehydroxylation of layers) are clearly distinguished when the thermal analyses are performed in air instead of in nitrogen. The TG analysis in air, Fig. 7a, showed five weight losses, indicating five thermal processes: desorption of surface water, desorption of intercalated water, oxidation of cyanate anion, decomposition of carbonate anions, and dehydroxylation process. The DTG of Co–Zr–Si LDH in air, Fig. 7b, showed five peaks, confirming TG results. Also, the DTA of Co–Zr–Si
Fig. 8. SEM images of Co–Zr–Si LDH at different locations and magnifications: (a) ×1000, (b) ×5000, and (c) ×10,000.
LDH in air showed one broad endothermic peak and one broad exothermic peak. The endothermic peak is due to desorption of water and the exothermic peak is due to the oxidation of cyanate anion and Co2+ cation and the decomposition of carbonate. This agrees with the ESCA analysis that confirmed the presence of cobalt as divalent cation. 3.5. Scanning electron microscopy (SEM) SEM images of Co–Zr–Si LDH at aging time 24 h are shown in Fig. 8. SEM images show that the Co–Zr–Si LDH has platelike morphology, which was typical for the LDH morphology, as shown in Fig. 8a. Some platelets of Co–Zr–Si LDH started to convert to fibrous morphology, as shown in Fig. 8b. Other platelets completely converted to fibrous morphology, as shown in Fig. 8c. By intercalation reactions with n-capric acid, SEM
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Fig. 10. Schematic representation of Co–Zr–Si LDH for platelike morphology.
The first anion, mainly intercalated, is cyanate, which is produced from the decomposition of urea, as shown in the equation
Fig. 9. SEM images of Co–Zr–Si LDH after reaction with n-capric acid at different magnifications: (a) ×350 and (b) ×20,000.
images of Co–Zr–Si LDH showed that the fibrous morphology changed again to platelike morphology, as shown in Fig. 9. From these results, it can be concluded that the Co–Zr– Si LDH have the ability to exist with fibrous and platelike morphology depending on the kind of intercalated anion. This means that we can control the morphology of LDH by intercalation reactions, which has been recently published. 4. Discussion The results presented in this work show that the formation of layered double hydroxides is not limited to reactions between di- and trivalent cations, although these are the only reactions reported by many researchers [1–18]. The powder XRD patterns are not sufficiently high-quality to allow us to carry out structure determination. However, by interlayer spacing and the size of the guest ions, orientation of guest ions was considered. From known layer thickness 0.48 nm, the interlayer spacing available for the anion was calculated as 0.30 nm. By comparison with the size of the cyanate anion, 0.34 nm, it was considered that the cyanate anion makes an angle of 65.7◦ with the Co–Zr–Si layer and connects with Zr or Si cations through two different sides (above and below the layer) in order to neutralize the positive charge, as shown in Fig. 10. The experimental results show that two anions are intercalated into the interlayer spacing of Co–Zr–Si LDH, indicating one interlayer spacing, 0.78 nm.
− Urea → NH+ 4 CNO .
(1)
Earlier investigators [46–48] confirmed this equation and considered that ammonium cyanate is an intermediate in the decomposition of urea. Recently, Coebel and Elsener [49,50] mentioned that the thermal decomposition of urea can yield a variety of products apart from ammonia and isocyanic acid. The second anion is carbonate, produced from the complete decomposition of urea, as shown in the equation + − + NH+ 4 CNO + 2H2 O + 2H → NH4 + H2 CO3 .
(2)
The earlier investigators [46–48] indicated that the decomposition of urea is complete only under acidic conditions, as shown in Eq. (2). According to the synthetic conditions of our experiments, the decomposition of urea is not complete. This leads to the concentration of cyanate being higher than the concentration of carbonate in the interlayer spacing of Co–Zr–Si LDH. The schematic representation of Co–Zr–Si LDH, Fig. 10, may be valid only for the platelike structure, while in the case of the fibrous structure, we suggest that the above representation may be valid after some modification. According to the mechanism of our laboratory for fibrous structures formed from the reaction of zinc hydroxide with organic acids [28], we can apply the same idea to this fibrous structure. The presence of carbonate (−2) in the interlayer spacing pushes two cyanate anions (−1) to attack the layer (+2) from one side. This leads to stearic repulsion between bulky cyanide groups, which causes curling of the layers, giving a fibrous structure as shown in Fig. 11.
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Fig. 11. Schematic representation of Co–Zr–Si LDH for fibrous morphology.
5. Conclusions In this study, Co–Zr–Si layered double hydroxides consisting of divalent and two tetravalent cations have been prepared for the first time. The Co–Zr–Si LDH has interlayer spacing 0.78 nm. SEM images showed that the Co–Zr–Si LDH can exist as a platelike or a fibrous structure. Anion-exchange reactions were successful in replacing inorganic anions with organic anions in Co–Zr–Si LDH. Acknowledgments The author thanks Professor H. Tagaya and Yamagata University for their cooperation. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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