Abstract
Layered metal hydroxynitrates and mixed metal hydroxynitrates (copper/cobalt hydroxynitrates and zinc/cobalt hydroxynitrates at different mole ratios) were synthesized by hydrolysis of urea and metal nitrates at 140°C. Layered metal hydroxyl nitrates derive their structure from brucite mineral and generally crystallize in hexagonal and monoclinic phases. Isothermal decomposition studies of Cu2(OH)3(NO3), Co2(OH)3(NO3), Cu1.5Co0.5(OH)3(NO3), Cu1.34Co0.66(OH)3(NO3), Zn5(OH)8(NO3)2(H2O)2, Zn3.75Co1.25(OH)8(NO3)2(H2O)2, and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 samples were carried out at different intervals of temperature and the structural transformations during the process were monitored using powder X-ray diffractograms. Biphasic mixture of metal hydroxynitrate/metal oxide is observed in case of cobalt/zinc based layered hydroxynitrates, while copper hydroxynitrate or copper/cobalt metal hydroxynitrate decomposes in a single step. The decomposition temperatures of layered metal hydroxynitrates and mixed layered metal hydroxides depend on the method of preparation, their composition and the nature of metal ion, and their coordination.
1. Introduction
Layered metal hydroxysalts (LHS) are used in dye adsorption, catalysts in organic reactions such as catalyst supports, fire retardants, polymer composites, electrodes, magnetism, electrochromic devices, and photocatalysts [1–5]. The structure of layered metal hydroxysalt is comprised of a hydroxyl deficient positively charged compound and the composition [M(OH)1−x]x+ where M denotes metal ion; to 1 [6]. Anions (An−) will get intercalated to compensate its charge neutrality resulting in the formula [M(OH)2−x(An−)x/n] [7, 8]. The value of “” dictates the composition of the compound; that is, when , we get layered metal hydroxide-[M(OH)2] and, at “” = 2, metal nitrate MA2 to obtain two end members. The structure of metal hydroxide is comprised of hexagonally close packed OH− ions in which divalent metal ions occupy octahedral sites resulting in the formation of charge neutral layers. These charge neutral layers stack on top of each other having an interlamellar spacing of 4.6 Å. Intermediate values of “” will dictate the composition of the layered metal hydroxysalt. “” = 0.5 results in the formation of M(OH)(An−), and M2(OH)3X/M3(OH)4(NO3)2/M5(OH)8(NO3)2 will be obtained at “” = 0.67 where the above compounds crystallize in hexagonal or pseudo-hexagonal symmetry [9–13].
Several methods have been reported on the preparation of layered hydroxysalts [9–14]. Precipitation method involves addition of an alkali solution to metal nitrate solution to attain pH-6.8–7.0. Aging of metal oxide in aqueous solution of metal salt has also been adopted for the precipitation of layered hydroxysalts [3]. Hydrolysis of metal nitrates using urea/hexamine precipitates metal hydroxysalts [15, 16]. Thermodynamics and kinetic factors control the stoichiometry and composition product formation. The kinetics of the reaction can be controlled by adjusting the temperature, concentration of the reagents, pH, hydrolysis rate, and metal salt to hydrolyzing agent ratio.
Zinc, nickel, cobalt, and copper based layered hydroxysalts have been extensively studied. Cobalt and nickel based layered metal hydroxynitrates (CoHN/NiHN) crystallize in the hexagonal system and have been used as magnetic materials. While zinc hydroxynitrate is used as an anticorrosive agent and copper hydroxynitrate is used in vehicle air bags [17, 18]. Copper based hydroxynitrate (CuHN) with composition Cu2(OH)3(NO3) crystallizes in orthorhombic and monoclinic crystal systems [19, 20]. The crystal structure of monoclinic copper hydroxynitrate [Cu2(OH)3(NO3) or Cu(OH)1.5(NO3)0.5] is comprised of copper hydroxide layers in which some of the hydroxyl ions () are replaced by the nitrate ions and are directly coordinated to the sheets. Copper occupies two different distorted octahedral sites within the layer and the structure of copper hydroxynitrate-Cu2(OH)3NO3 is shown in Figure 1.
Divalent metal ions or cations occupy octahedral or tetrahedral sites in the sheets of layered hydroxynitrate. One of the classic examples is zinc hydroxynitrate (ZnHN) with the composition Zn5(OH)8(NO3)2(H2O)2 which is comprised of hexagonal close packing of hydroxide ions in which three-fourths of the octahedral sites are occupied by divalent Zn2+ ions and one-fourth of the octahedral sites are vacant in the layer. Two tetrahedral sites, one above and one below the layer, will be available if one of the octahedral sites is vacant in the sheet of zinc hydroxynitrate. Zn2+ ion occupies tetrahedral coordination in the sheets of zinc hydroxynitrate which are coordinated by 3 OH− ions on the layer and one of the OH− group rom the water molecule present in the interlayer region to form tetrahedron. Nitrate ions are present in the interlamellar region and do not have any bonding or coordination with the ions present in the lattice. The formula of zinc hydroxynitrate is Zn3(octa)Zn2(tetra)(OH)8(NO3)2(H2O)2 [21]. Figure 2 shows the crystal structure of zinc hydroxynitrate.
Extensive studies have been carried out on the decomposition of magnesium hydroxide, nickel hydroxide, and calcium hydroxide to obtain their respective metal oxides. There are also reports on the utilization of thermal energy storage density of magnesium hydroxide and calcium hydroxide during hydration-rehydration process [22, 23]. We had reported on the structural transformations during thermal decomposition of nickel hydroxide to nickel oxide, depending on the conditions of synthesis and crystallinity [24, 25]. Metal oxides have been extensively investigated in the last two decades in view of their application as magnetic materials, electrochromic devices, gas sensors, high-temperature solar selective absorbers, and catalysts [1, 26–29]. The information about the decomposition studies of layered metal hydroxysalt is important to prepare high surface area solid for adsorption, catalytic supports, and catalysts. Decomposition of layered hydroxysalt also results in formation of metal oxide. We have reported the thermal decomposition studies of cobalt and nickel based layered hydroxynitrates and their mechanism of structural transformation to metal oxides. On continuing our investigations on layered hydroxysalts, the present work is concerned with the synthesis and characterization of layered metal hydroxynitrates, layered mixed metal hydroxynitrates, and their thermal evolution studies. To the best of our knowledge, there are no reports on what is mentioned above and in this paper, we have reported on the thermal decomposition of metal hydroxynitrates (metal: Cu, Co, Zn, Cu/Co, and Zn/Co) and their phase evolution using powder X-ray diffraction analyses.
This work describes the preparation and characterization of zinc/copper/cobalt hydroxynitrate samples. The term zinc/copper/cobalt used in this paper covers not only mixed metal salts, but also a single metal ion present in the lattice of the compound. Thermal decomposition studies of CoHN, CuHN, ZnHN, Cu1−x-CoxHN (, 0.33, 0.5), and Zn1−x-CoxHN (, 0.33, 0.5) and their structural transformation have been investigated.
1.1. Experimental Section
1.1.1. Reagents
Analytical grade Co(NO3)26H2O, Cu(NO3)26H2O, and Zn(NO3)26H2O and urea were procured from Merck, India, and SD Fine Chemicals, India, and were used as received.
Stoichiometric quantities of the divalent transition metal nitrates, that is, M(NO3)2·6H2O (where M = Cu/Co/Zn), were mixed with 2 g of urea and 2 mL of distilled water in 100 mL beaker. The mixture was stirred well, and beaker was covered with watch glass and placed in hot air oven maintained at 140°C for 2 h. The above mixture was periodically stirred and after 2 h, it was cooled to room temperature (RT). The solid formed was washed with distilled water and dried at room temperature till constant weight is obtained. In separate experiments, mixed metal hydroxynitrates were prepared by following the procedure reported above by taking mixed metal nitrates [Zn(NO3)2·6H2O and Co(NO3)2·6H2O; Cu(NO3)2·6H2O and Co(NO3)2·6H2O] in the mole ratios of , 0.33, 0.5. Solid solution series of layered mixed metal hydroxynitrate (, 0.33, 0.5) have been prepared and the details are given in Table 1.
1.1.2. Thermal Decomposition Studies
Thermal conversion of layered metal hydroxynitrates and layered mixed metal hydroxynitrate samples to their respective oxides has been carried out on the following samples: Cu2(OH)3(NO3) (CuHN), Co2(OH)3(NO3) (CoHN), Cu1.5Co0.5(OH)3(NO3) [Cu1.5Co0.5(HN)], Cu1.34Co0.66(OH)3(NO3) [Cu1.34Co0.66(HN)], Zn5(OH)8(NO3)2(H2O)2(ZnHN), Zn3.75Co1.25(OH)8(NO3)2(H2O)2[Zn3.75Co1.25(HN)], and Zn3.35Co1.65(OH)8(NO3)2(H2O)2.
In a typical procedure, approximately 100 mg of as-prepared layered metal hydroxynitrate/layered mixed metal hydroxynitrates was placed in an alumina boat and isothermally heated to different temperatures (100, 150, 200, 250, 300, 350, 400, and 500°C) for 2 h and cooled to room temperature. The details of the starting compounds are given in Table 1. The decomposed products were collected for structural characterization using powder X-ray diffraction.
2. Characterization
All the samples were characterized by powder X-ray diffraction (pXRD) using Bruker D8 advanced diffractometer (CuKα source Å; 30 mA and 40 kV) in a 2θ range of 5° to 65°. Data was collected at a scan rate of 4°θ min−1 with 2θ steps of 0.05°. X-ray diffractogram of the following compounds: Cu2(OH)3(NO3), Cu3(OH)4(NO3)2(2H2O)2, Cu4(OH)6(NO3)2, CoO (cubic), CoO (monoclinic), Co2(OH)3(NO3), CoO (cubic), Co3O4 (cubic), CoCu2O3, Zn(OH)(NO3)(H2O), Zn3(OH)4(NO3)2, Zn5(OH)8(NO3)2(H2O)2, ZnO (hexagonal), and Zn0.85Co0.15O was obtained at different experimental conditions and their decomposed products were indexed using the powder diffraction data of from ICSD database.
3. Results and Discussion
3.1. Formation Mechanism of the Precursors
Precipitation of layered metal hydroxynitrates by mixing metal nitrates with alkali at appropriate pH range from 7 to 9 does not provide control over the crystal growth [30]. Hence urea, (NH2)2CO, is a nontoxic, cheap, stable, and water soluble hydrolyzing agent that was used to control the precipitation of layered metal hydroxysalts and the products obtained will be homogeneous in nature. Decomposition of urea decomposes at 70°C leading to the following reaction: In acidic or neutral conditions, HNCO will be converted to carbon dioxide and ammonia/ammonium cation: Consumption of H+ ions results in an increase in the solution pH. The precipitation of layered metal hydroxynitrate involves reaction of hydroxyl ions with metal nitrate solution.
Figure 3 shows the powder X-ray diffraction patterns of copper hydroxynitrate, Cu2(OH)3(NO3), Cu1.5Co0.5(OH)3(NO3), and Cu1.34Co0.66(OH)3(NO3) samples obtained by urea hydrolyses.
In the pXRD patterns of the CuHN, successive reflections, up to at least the third order, indicate the lamellar structure and could be indexed to monoclinic crystal system space group-P21 using the JCPDS-ICDD 75-1779 file. The cell parameters of copper hydroxynitrate are as follows: Å, Å, Å, , and Å3 with an interlayer distance of 6.9 Å. The powder X-ray diffraction patterns of Cu2(OH)3(NO3), Cu3(OH)4(NO3)2(H2O)2, and Cu4(OH)6(NO3)2 samples were simulated using the single crystal data obtained from ICDD database. The simulated powder X-ray diffraction pattern of Cu2(OH)3(NO3) matches the observed X-ray diffraction pattern of copper hydroxynitrate (see Figure 4).
Thermal decomposition of layered hydroxynitrate follows dehydration, denitration, and disruption of layered framework under atmospheric conditions. The elimination of hydroxyl and nitrate groups from the precursors has great influence on the crystallinity of metal oxides, and new intermediate phase might also form during the disruption of the original structure. The copper hydroxynitrate undergoes decomposition in one step producing CuO according to the equation: The theoretical weight loss for the reaction Cu2(OH)3(NO3) → 2CuO is 33.17%.
Powder XRD was very important in the identification of the phase formation and in Figure 5 the data of the copper hydroxynitrate heated to different temperatures is shown. The XRD results show that up to 200°C the samples do not show any structural changes. Thermal decomposition of copper hydroxynitrate occurs at single step in which dehydroxylation and decomposition of the nitrate in the interlayer space take place simultaneously leading to the destruction of the layered structure (250°C) (see Figure 5). The total weight loss on isothermal heating of the copper hydroxyl nitrate sample is 31%. Copper oxide can exist in monoclinic or cubic crystal system and in Table 2 the XRD data is shown. The decomposed product could be indexed to monoclinic phase of copper oxide. Figure 6 shows the crystal structure of copper oxide (monoclinic) formed on decomposition of copper hydroxynitrate.
3.2. Cu1.5Co0.5(OH)3(NO3) and Cu1.34Co0.66(OH)3(NO3) Series
In the structure of layered hydroxynitrate, divalent transition metal ions can be isomorphically substituted by the other divalent cations aross the whole range of composition “” which results in the formation of continuous solid solution series. Goldschmidt’s rule states that a continuous solid solution cannot be obtained if the difference in ionic radii is greater than 15% of the smallest cation. The ionic radii of Co2+ are 0.745 Å for high spin and 0.65 Å for low spin complexes. The ionic radius of Cu2+ is 0.73 Å when the coordination number is 6. Co2+ substitution in Cu2+ sites of layered metal hydroxynitrate is permitted as the difference in their ionic radii is <10% and hence the values are within the limits set by Goldschmidt, and solid solution series Cu/Co hydroxynitrates with the composition of Cu1.5Co0.5(OH)3(NO3) and Cu1.34Co0.66(OH)3(NO3) were prepared by using urea hydrolysis route and their powder X-ray diffraction patterns are shown in Figure 3. We observe structural similarities among Cu2(OH)3(NO3), Cu1.5Co0.5(OH)3(NO3), and Cu1.34Co0.66(OH)3(NO3) as the peak positions in their diffraction patterns appear at similar positions in 2θ. However, the Co2+ and Cu2+ have d7 and d9 electronic configurations, respectively. Cu2+ ion is prone to Jahn Teller distortion and results in the irregular octahedral coordination arrangement around it. On substitution of Co2+ into the lattice, we observed a decrease in its irregularity in the structure of cobalt substituted copper hydroxynitrate. The pXRD patterns of Cu1.5Co0.5(OH)3(NO3) and Cu1.34Co0.66(OH)3(NO3) heated to different temperatures are shown in Figures 7 and 8, respectively. The mixed metal hydroxynitrates [Cu1.5Co0.5(OH)3(NO3) and Cu1.34Co0.66(OH)3(NO3)] undergo dehydration and denitration at lower temperatures compared to pure zinc/copper/cobalt based hydroxynitrates. It was found that on substitution of cobalt into the structure of basic copper hydroxynitrate in mixed metal hydroxynitrates, the decomposition temperature is reduced by ~50°C. The dramatic decrease in the decomposition temperature was due to decarboxylation reaction on substitution of cobalt for copper in the cobalt substitute copper hydroxynitrates.
In Figure 9, the structural transformations of Cu1.5Co0.5(OH)3(NO3)/Cu1.34Co0.66(OH)3(NO3)/Cu1.0Co1.0(OH)3(NO3) to copper-cobalt oxide, respectively, are shown. The decomposed product, Co0.333Cu0.666O, could crystallize in cubic or monoclinic system. The powder X-ray diffraction patterns of Co0.25Cu0.75O/Co0.333Cu0.666O/Co0.5Cu0.5O shown in Figures 9 and 10 could be indexed (at temperatures >250°C) to monoclinic system.
Table 3 shows the decomposition temperatures of Cu2(OH)3(NO3), Cu1.5Co0.5(OH)3(NO3), and Cu1.34Co0.66(OH)3(NO3), respectively.
3.3. Zinc Hydroxynitrate
Figure 10 shows the powder X-ray diffraction pattern of zinc hydroxynitrate obtained by urea hydrolysis. Zinc hydroxynitrate can crystallize in different crystal structures and depends on the composition, that is, Zn(OH)(NO3)(H2O), Zn3(OH)4(NO3)2, and Zn5(OH)8(NO3)2(H2O)2. Figure 11 shows the simulated powder X-ray diffraction patterns of Zn(OH)(NO3)(H2O), Zn3(OH)4(NO3)2, and Zn5(OH)8(NO3)2(H2O)2, respectively. The experimentally observed pattern of zinc hydroxynitrate matches the simulated powder X-ray diffraction pattern of Zn5(OH)8(NO3)2(H2O)2. On isothermal heating of Zn5(OH)8(NO3)2(H2O)2 the total weight loss observed was 50%, while theoretically the total weight loss expected was 40% indicating higher water content. Figure 12 shows the powder X-ray diffraction patterns of the Zn5(OH)8(NO3)2(H2O)2 samples obtained on isothermal heating at different intervals of temperature.
The powder X-ray diffraction pattern of zinc hydroxynitrate heated to 100°C shows additional peaks indicating evolution of second phase of zinc hydroxynitrate with composition Zn3(OH)4(NO3)2. The formation of Zn3(OH)4(NO3)2 could arise due to dehydration of a Zn5(OH)5(NO3)2(H2O)2. At 175°C, peaks due to Zn5(OH)5(NO3)2(H2O)2 and Zn3(OH)4(NO3)2 disappear and the zinc oxide peaks are prominently observed. Tables 4 and 5 show the XRD data of zinc oxide in hexagonal and cubic systems, respectively. The experimental data of the decomposed product obtained from Zn5(OH)5(NO3)2(H2O)2 can be indexed to hexagonal phase of ZnO. The decomposition of layered metal hydroxynitrate results in the absorption of heat and the process is endothermic in nature. The decomposition reaction of this type yields metal oxide and water vapour and nitrates as by-products. Figure 13 shows the crystal structure of zinc oxide formed on decomposition of zinc hydroxynitrate-Zn5(OH)5(NO3)2(H2O)2.
3.4. Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2
Solid solution series of Co/Zn hydroxynitrates were prepared by urea hydrolysis route. The ionic radii of Co2+ are 0.745 Å for high spin and 0.65 Å for low spin complexes, respectively. Zn2+ with ionic radius of 0.74 Å indicates its coordination number to be 6. Co2+ substitution in Zn2+ sites is permitted as the difference in the ionic radii is <10% and hence the values are within the limits set by Goldschmidt. The powder X-ray diffraction patterns of Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 are shown in Figure 10. We observe structural similarities among Zn5(OH)5(NO3)2(H2O)2, Cu1 Zn3.75Co1.25(OH)8(NO3)2(H2O)2, and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 as the peak positions appear at similar positions. Co2+ and Zn2+ have d7 and d10 electronic configurations, respectively, and both Co2+ and Zn2+ do not induce any distortion of octahedral geometry. The pXRD patterns of Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 heated to different temperatures are shown in Figures 14 and 15, respectively. The mixed metal hydroxynitrates, Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2, undergo dehydration and denitration at 150 to 200°C temperatures compared to pure zinc/copper/cobalt based hydroxynitrates. The decomposition temperatures of Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 are shown in Table 3. The pXRD patterns of the decomposed products of Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 heated to 250°C could be indexed to hexagonal phase of zinc oxide. Figures 14 and 15 show the powder X-ray diffraction patterns of Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 on decomposition, respectively, to form (Zn1−xCox)O. Figure 16 shows the structural transformation of Zn3.75Co1.25(OH)8(NO3)2(H2O)2 and Zn3.35Co1.65(OH)8(NO3)2(H2O)2 on decomposition, respectively, to form (Zn1−xCox)O.
Tables 6 and 7 shows the lattice parameters, expected, of different phases of layered metal hydroxynitrate and mixed metal hydroxynitrates and their respective decomposed products. Table 8 shows the percentage weight loss of the layered metal hydroxynitrate and mixed metal hydroxynitrates, respectively.
4. Conclusion
Simple preparative method has been adopted for the synthesis of layered hydroxynitrates/mixed metal hydroxynitrates by urea hydrolyses. Thermal decomposition studies of mixed metal hydroxynitrates (zinc/cobalt hydroxynitrates and copper/cobalt hydroxynitrates) were investigated. The mechanism of thermal decomposition of the layered metal/mixed metal hydroxynitrates was deduced and the effect of substituting other metal cations into the structure of layered hydroxynitrate was determined. The single metal hydroxynitrate will have higher decomposition temperature than cobalt substituted copper hydroxynitrates. It was found that the mixed metal hydroxynitrate decomposed at lower temperatures than the single metal hydroxynitrate studied. The work will be used as a basis for further studies of mixed metal hydroxynitrate for the preparation of novel catalyst precursors. The mechanism of decomposition provides insight into the nature of phases formed at different temperatures which can significantly affect the catalytic activity. These metal/mixed metal hydroxynitrates with high surface area may exhibit unusual and unexpected properties for device based applications.
Conflict of Interests
The authors declare that they have no competing financial interests.
Acknowledgments
Ramesh Thimmasandra Narayan gratefully acknowledges the Council of Scientific and Industrial Research (CSIR) (project ref: 01/(2741)/13/EMR-II dated 18-04-2013), New Delhi, for the financial support. Ramesh Thimmasandra Narayan wishes to thank Professor P. Vishnu Kamath lab of Bangalore University for providing laboratory facilities. Authors gratefully thank Mr. Shivanna, Supreeth Nagendran, and Mr. Kiran of Bangalore University for their help in collection of powder-ray diffraction data. The author would like to thank Tumkur University for providing facilities.