J Therm Anal Calorim (2014) 118:1537–1541 DOI 10.1007/s10973-014-4067-x
Thermolysis mechanism of samarium nitrate hexahydrate P. Melnikov • I. V. Arkhangelsky • V. A. Nascimento A. F. Silva • L. Z. Zanoni Consolo
•
Received: 28 March 2014 / Accepted: 3 August 2014 / Published online: 28 August 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract Thermal decomposition of samarium nitrate hexahydrate Sm(NO3)36H2O has been investigated by thermogravimetry, differential scanning calorimetry, infrared spectroscopy, and X-ray diffractometry. This is a complex process that involves slow dehydration and fast concomitant internal hydrolysis. It is markedly different from the processes described for other members of the lanthanide series. At the first stage, pyrolysis is accompanied by removal of water and nitric acid to form samarium pentahydrate Sm(NO3)35H2O and intermediate oxonitrates containing O–Sm–OH groups. No traces of SmONO3 were detected. It is assumed that the existence of intermediate structures with six atoms of samarium best fits the experimental results. At higher temperatures, these products undergo further degradation, lose nitrogen dioxide, water, and oxygen, and finally, after having lost lattice water, are transformed into a cubic form of samarium oxide. Keywords Samarium nitrate hexahydrate Rare earths Lanthanides Thermal decomposition Oxynitrates
Introduction Previous investigation on the thermal decomposition of rare earth nitrates, including samarium nitrate, was P. Melnikov (&) V. A. Nascimento A. F. Silva L. Z. Zanoni Consolo School of Medicine/UFMS, Campo Grande, MS Caixa Postal 549, Brazil e-mail:
[email protected] I. V. Arkhangelsky School of Chemistry, Moscow State University, Vorobievy Gory, 119517 Moscow, Russia
carried out long time ago and concerned mainly with the compositions of the intermediate products. As a result, an oxynitrate SmONO3 has been reported as the only stoichiometric intermediate existing in the temperature range 450–490 °C [1]. The proposed mechanism does not involve the formation of hydrates or anhydrous Sm(NO3)3, supposing that further heating would lead directly to samarium oxide. More recent investigations have been extended to thermal decomposition of MeIII(NO3)3xH2O, where MeIII = Ga, Y, Gd, Cr, Al, Fe [2–9], establishing that all attempts to prepare the anhydrous compounds under hydrothermal conditions were unsuccessful. Despite expected similarities with coordination chemistry of the aforementioned nitrates, the thermal decomposition of samarium nitrate appears to be a more complex process because of the lower basicity of the element and distinct mechanism of inner hydrolysis. Since X-ray data are unavailable for samarium nitrate, we can use the results of the study of the parent compound Dy(NO3)36H2O. In this structure, each of Dy(III) ions is coordinated by ten oxygen atoms of three nitrate groups and four water molecules forming a distorted icosahedron [10]. In a continuation of previous studies, the present work was undertaken to revert to the thermolysis of samarium nitrate hexahydrate in order to obtain comprehensive data on the mechanisms involved, and elaborate a realistic scheme of its thermal transformations. It intends to deal with the scarcity of experimental structural information on samarium oxynitrates, as an option to fill the gap.
Materials and methods The starting reagent used was samarium nitrate hexahydrate Sm(NO3)36H2O, of analytical grade purity (99.9 %),
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80
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Fig. 1 DSC curve of Sm(NO3)36H2O 40
Results and discussion The DSC analysis of Sm(NO3)36H2O is presented in Fig. 1. No solid–solid phase transition was found, at least over -40 °C. From approximately 50 °C on, small endothermic effects become evident, reflecting the decomposition processes. Unfortunately, owing to extremely complex
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Fig. 2 TG curve of Sm(NO3)36H2O
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purchased from Sigma-Aldrich. Direct heating of the commercial reagent resulted in mass loss of 59.94 % confirming the water number slightly lower than six (calc. 60.76 %). Thermal gravimetric analysis (TG) and differential scanning calorimetry (DSC) were used to study thermal behavior, employing a 50H Shimadzu Instrumentation, and a Netsch STA Jupiter 449C Instrumentation. Test specimens, in the first case, were heated in a flux of nitrogen (temp. range -40 to 500 °C), and in the second case in a flux of argon (temp. range 25–500 °C), always at a heating rate 5 °C min-1. Mass losses during heating were analyzed and compared to previously calculated values. Melting point, gas liberation, and crystallization processes were additionally monitored by visual observation, allowing the visualization of NO2 vapors, and other peculiarities. The evolution of volatiles was measured using a Netsch STA Jupiter 449C apparatus coupled with a FTIR spectrometer. Infrared spectroscopy of evolved gases was performed using a Tensor 27 Bruker spectrometer. The spectra were detected in a range 400–4,000 cm-1. Temperature of the transport gas line was 240 °C. The spectra were taken for 12 s at a frequency accuracy of 1 cm-1. The identification of the IR spectra was done on the basis of NIST Chemistry WebBook [11]. The samples were sealed in glass ampoules in a hot condition in order to avoid the impact of water vapors from the air. X-ray powder patterns of solid samples (CuKa radiation) were registered with a Siemens Kristalloflex diffractometer with a graphite diffracted beam monochromator and Ni filter. Data were collected in the range of 2h = 108–708 with a scan step 2h = 0.028 and scan rate of 18min-1. Phase identification was performed using ICDD PDF-2 database.
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Fig. 3 Dynamics of nitric acid removal in relation to DSC curve
TG pattern, they could not be used for a rigorous interpretation of results. The representative TG curve of Sm(NO3)36H2O is shown in Fig. 2. The first mass loss is due to water evaporation after fusion of the original hexahydrate at 76–78 °C. This temperature is also confirmed by direct visual observation, when the melt loses gas bubbles and turns into a clear yellow liquid. Our working hypothesis was that samarium nitrate is hydrolyzed by the crystallization water, and as a result nitric acid should have been produced. Indeed, this acid, or rather the azeotrope 68 % HNO3–32 % H2O, is detected by the IR sensor of the volatile products from the very beginning of the thermal treatment (Fig. 3). At this step, we do not have the ability to judge about precise compositions, since the mixture is in the liquid state, and the removal of HNO3 is essentially a continuous process. It would be reasonable to assume that the pyrolysis curve shows the presence of various chemical products, not a single stoichiometric compound of definite composition. At temperatures above 200 °C melt becomes viscous, and then solidifies at *250 °C. The X-ray powder
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Thermolysis mechanism of samarium
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Fig. 4 X-ray identification of samarium pentahydrate a diffractogram of Yb(NO3)35H2O; b diffractogram of the sample obtained at 250 °C
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Fig. 5 IR spectrum of the sample obtained at 250 °C
diffractogram of this product, with a small deviation in angles corresponds closely with the diffractograms published for pentahydrates of europium, ytterbium, and other rare earth nitrates of the composition Ln(NO3)35H2O. For purposes of confirming isostructurality, Fig. 4 shows the diffractogram of Yb(NO3)35H2O (ISDD file 89–6391) and the pattern obtained in this work. As for IR spectrum of the same sample (Fig. 5), this is typical for a nitrate complex(es) with water of crystallization [12]. In this case, the presence of a strong sharp band at 1,000 cm-1 unambiguously indicates the symmetry C2v. Meanwhile, D3h symmetry can be ruled out as the aforementioned band does not need to be active in the IR spectrum. So we can assume that the sample heated at 250 °C contains Sm(NO3)35H2O as a crystalline phase. The experimental mass loss of 27.5 % is greater than the amount of water corresponding to one mole of H2O, indicating that in part the nitrate ions were also removed. So the system does not seem to be in equilibrium, and the rest of the material is supposed to remain in the amorphous
Fig. 6 Dynamics of nitric dioxide removal in relation to DSC curve with NO2 spectrum used as reference
state. Unfortunately, amorphous oxynitrate(s) cannot be unambiguously identified by means of IR spectra due to the overlapping bands of nitrate ion and water. However, we must bear in mind that the early stages of thermal decomposition comprise at least two processes that occur simultaneously: dehydration and hydrolysis of samarium nitrate hexahydrate by its own water of crystallization. It is important that the former process is substantially independent on temperature, while the latter is characterized by fast kinetics, leading to recombination of ligands in closer coordination shell. That is why a number of rare earth lower hydrates could have been isolated at room temperature after prolonged desiccation over concentrated H2SO4. Between 320 and 430 °C, the compound loses 23.6 % of mass. At this step, the composition of the gaseous phase changes. The observation of the characteristic brown color of nitrogen dioxide and the known instability of HNO3 at high temperatures suggest that the acid in its molecular form is not actually present. Thermal decomposition of nitric acid represents a reversible endothermic reaction and almost quantitatively proceeds according to the reaction [13]: HNO3 ¼ NO2 þ 0:5H2 O þ 0:25O2 þ 7:8 kcal mol1 : ð1Þ Thereupon, at 430 °C, IR sensor detects nitrogen dioxide by its characteristic absorbance bands 1,597 and 1,630 cm-1 (Fig. 6). Another product of HNO3 thermal disproportionation is oxygen, but this element in principle cannot be registered in the spectrum due to the lack of changing dipole moment. The DCS curve (Fig. 1) supplements the mass loss curve showing the splitting of the main endothermic effect into minima at 359 and 378 °C. This separation, unnoticeable in the TG curve, may reflect the two-step removal of HNO3 in this temperature range. The next loss of mass (2.02 %) takes place between 430 and 484 °C. As seen in Fig. 6, this also corresponds to the removal of nitric acid in the form of NO2, H2O, and O2, but
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Fig. 7 IR spectrum of the sample obtained at 547 °C
in a smaller quantity. It has to be said that this loss is always clearly expressed in the curves of TG and DSC, with no overlapping with the previous one, so this acid is likely to have a completely different structural origin. In all probability, the oxynitrates producing these gaseous products must be similar to samarium hydroxo complexes of the composition [Sm6O(OH)8(H2O)12(NO3)6](NO3)26H2O. They were early obtained in the form of single crystals by heating of a clear solution of samarium oxide in an excess of concentrated nitric acid, until decomposition started. The melt was treated with ethyl alcohol and water, after which fine precipitate was collected [14]. Later, for the same purpose, pentahydrates of earth nitrates were shown to be suitable as starting reactants. It appears that in both cases the compounds contain [Ln6O(OH)8(NO3)6]2? cations, still present at 300 °C [15]. Obviously, because of the fluctuations in stoichiometric coefficients [16], these crystalline complexes are not necessarily the same species that are present in the melt during thermolysis. Nevertheless, an arrangement of six samarium atoms is apparently maintained. It is well known that the system of hydrogen bonds is extremely sensitive to external conditions. Therefore, it should be born in mind that state of water and hydroxyl groups cannot be accurately ascertained. After the denitrification was completed at 485 °C, the solid phase slowly loses water in two stages: between 485 and 560 °C and 560–700 °C. The former corresponds to the elimination of hydroxo groups to form oxo and oxohydroxo bridges (Ln–O–Ln and O–Sm–OH). Indeed, the IR spectrum of the sample heated at 547 °C (Fig. 7) shows the absorption at 1,500 and 1,450 cm-1 which can be attributed to the presence of O–Sm–OH groups. A strong band observed at 3,500 cm-1 is further confirmation.
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Fig. 8 X-ray diffractograms of the samples obtained at different temperatures: a 250 °C, b 547 °C, c 600 °C, d 700 °C Table 1 Mass losses at different stages of Sm(NO3)36H2O thermal decomposition and composition of the volatile products in relation to the initial six monomers* Stage
Mass loss/%
Volatile products of thermal decomposition/ mol
Exp.
Calc.
H2O
HNO3
NO2
O
I
27.51
27.76
20.5
6
–
–
II
51.17
57.36
5
{11}
10
5
III
55.89
56.08
1
{2}
2
1
IV
57.90
58.09
3
–
–
–
V
59.91
60.1
3
–
–
–
* Calculated taking into account slight dehydration of the initial sample
It is remarkable that the diffraction pattern of the same sample (Fig. 8) clearly exhibits a set of interplanar distances and intensities belonging samarium oxide (ICSD file 15–815) with no traces of SmONO3 mentioned in earlier works. As the solid product already contains the cubic form of Sm2O3, we can assume that during this phase the reaction is partially completed, although some remains of amorphous hydroxo complexes are still in the process of decomposition. The last mass loss corresponds to the removal of lattice water. At this stage, the IR spectrum is of little help, as the librational modes couple not only among themselves, but also with internal modes of non-coordinated HOH bendings [12]. At 600 °C, dehydration is already completed. According to the X-Ray diffraction pattern of the samples heated at 600 and 700 °C (Fig. 8) they contain only samarium oxide. Two small additional reflections with 2h = 27.438 and 31.768 seem to belong the monoclinic form of Sm2O3. It can be seen (Table 1) that the experimental mass losses actually correspond to the values calculated for each stage.
Thermolysis mechanism of samarium
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It is obvious that the loss of mass which takes place during the first two stages, a total of 51.17 %, cannot possibly result from the disintegration of the single mol of Sm(NO3)36H2O, if only because its formula unit contains no more than one atom of metal, whereas at least two are required to form Sm2O3. Consequently, we took into consideration the condensation processes, typical of the chemistry of cations with the charge 3? and not solely limited to nitrates [17]. This approach is consistent with the existing published data on the basic nitrates identified before [14, 15]. As practically all of them contain six lanthanide atoms, it was reasonable to consider the condensation of at least six monomer units Sm(NO3)36H2O. Naturally, in order to obtain Sm2O3, we might start considering the condensation of two or four starting units, but in this case, at the end of the process, we would have to resort to fractional values of stoichiometric coefficients for the last two losses of water, which would not have physical sense. So, the whole process of thermal decomposition can be described as: 6 SmðNO3 Þ3 6H2 O ¼ 3Sm2 O3 þ 6HNO3 þ 12NO2 þ 3O2 þ 33H2 O:
ð2Þ
The present research should provide a base for further investigation of the chemistry of rare earth nitrates.
Conclusions The thermal decomposition of samarium nitrate hexahydrate Sm(NO3)36H2O is a complex process that involves slow dehydration and fast concomitant hydrolysis. It is markedly different from the processes described for other members of the lanthanide series. At the first stage, pyrolysis is accompanied by removal of water and nitric acid to form samarium pentahydrate Sm(NO3)35H2O, and intermediate oxonitrates containing O–Sm–OH groups. No traces of SmONO3 were detected. It is assumed that the existence of intermediate structures with six atoms of samarium best fits the experimental results. At higher temperatures, these products undergo further degradation, lose nitrogen dioxide, water, and oxygen, and finally, after having lost lattice water, are transformed into the cubic form of samarium oxide. Acknowledgements The authors are indebted to CNPq and FUNDECT (Brazilian agencies) for financial support.
References 1. Wendlandt WW. The thermolysis of the rare earth and other metal nitrates. Anal Chim Acta. 1956;1515:435–9. 2. Melnikov P, Nascimento VA, Zanoni Consolo LZ. Thermal decomposition of gallium nitrate hydrate and modeling of thermolysis products. J Therm Anal Calorim. 2012;107:1117–21. 3. Melnikov P, Nascimento VA, Consolo LZZ, Silva AF. Mechanism of thermal decomposition of yttrium nitrate hexahydrate Y(NO3)36H2O and modeling of intermediate oxynitrates. J Therm Anal Calorim. 2013;111:115–9. 4. Melnikov P, Nascimento VA, Zanoni Consolo LZ. Computerized modeling of intermediate compounds formed during thermal decomposition of gadolinium nitrate hydrate. Russ. J Phys Chem. 2012;86:1659–63. 5. Melnikov P, Nascimento VA, Arkhangelsky IV, Zanoni Consolo LZ. Thermal decomposition mechanism of aluminum nitrate octahydrate and characterization of intermediate products by the technique of computerized modeling. J Therm Anal Calorim. 2013;111:543–8. 6. Melnikov P, Nascimento VA, Arkhangelsky IV, Zanoni Consolo LZ, de Oliveira LCS. Thermolysis mechanism of chromium nitrate nonahydrate and computerized modeling of intermediate products. J Therm Anal Calorim. 2013;114:1021–7. 7. Wieczorek-Ciurowa K, Kozak AJ. The thermal decomposition of Fe(NO3)39H2O. J Therm Anal Calorim. 1999;58:647–51. 8. Elmasry MAA, Gaber A, Khater EMH. Thermal decomposition of Ni(II) and Fe(III) nitrates and their mixture. J Therm Anal Calorim. 1998;52:489–95. 9. Melnikov P, Nascimento VA, Arkhangelsky IV, Zanoni Consolo LZ, de Oliveira LCS. Thermal decomposition mechanism of iron (III) nitrate and characterization of intermediate products by the technique of computerized modeling. J Therm Anal Calorim. 2014;115:145–51. 10. Shengli G, Huairang M, Zupei Y. Crystal structure of dysprosium(III) nitrate hexahydrate. J Northwest Univ (Natural Science Edition). 1990;20:53–8. 11. NIST chemistry WebBook, NIST standard reference database number 69. www.http//webbook.nist/chemistry. Accessed 21 May 2013. 12. Nakamoto k. Infrared and Raman spectra of inorganic and coordination compounds. Part A: theory and applications in inorganic chemistry. 6th ed. New Jersey; 2009. 13. Manelis GB, Nazin GM, Rubtsov YuT, Strunin VA. Thermal decomposition and combustion of explosives and propellants. Boca Raton: CRC Press; 2003. 14. Giester G, Unfried P, Zak Z. Synthesis and crystal structure of some new rare earth basic nitrates II: [Ln6O(OH)8(H2O)12 (NO3)6](NO3)2xH2O, Ln = Sm, Dy, Er; x(Sm) = 6, x(Dy) = 5, x(Er) = 4. J Alloy Compd. 1997;257:175–81. 15. Mahe´ N, Guillou O, Daiguebonne C, Ge´rault Y, Caneschi A, Sangregorio C, Chane-Ching JY, Car PE, Roisnel T. Polynuclear lanthanide hydroxo complexes: new chemical precursors for coordination polymers. Inorg Chem. 2005;44:7743–50 (and references therein). 16. Haschke JM. Preparation, phase equilibria, crystal chemistry and some properties of lanthanide hydroxide nitrates. Inorg Chem. 1974;13:1812–8. 17. Logvinenko VA, Belyaev AV, Vorobieva SN. Dehydration process of rhodium sulfate crystalline hydrate. J Therm Anal Calorim. 2013;114:1177–81.
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