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Influence of lanthanum carbonate additions on thermal stability of eutectic lithium–sodium carbonate near its melting point
Thermochimica Acta 551 (2013) 33–39
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Influence of lanthanum carbonate additions on thermal stability of eutectic lithium–sodium carbonate near its melting point Stefano Frangini a,∗ , Silvera Scaccia b a b
Hydrogen and Fuel Cell Laboratory (UTRINN-IFC), Via Anguillarese 301, I-00123 Rome, Italy Sustainable Combustion Laboratory (UTTEI-COMSO), C.R. ENEA, Casaccia, Via Anguillarese 301, I-00123 Rome, Italy
a r t i c l e
i n f o
Article history: Received 11 July 2012 Accepted 9 October 2012 Available online 3 November 2012 Keywords: DSC EGA, Li/Na eutectic carbonate Lanthanum carbonate Lanthanum oxycarbonate
a b s t r a c t Thermal behavior and stability of eutectic 52/48 Li/Na carbonate mixture containing various concentrations of lanthanum carbonate in the range 0.5–2.0 mol% was investigated by a combination of thermal analysis (DSC) and spectroscopic (EGA-FTIR) techniques under CO2 and N2 gas flows. Thermal decomposition of lanthanum carbonate with CO2 gas release was observed in the 470–480 ◦ C range close to the melting point of eutectic alkali carbonate giving raise to formation of lanthanum oxycarbonate species. It was also found that the lanthanum carbonate phase promptly reformed, when the eutectic carbonate melt samples were cooled down to 440–460 ◦ C under CO2 atmosphere. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Alkali molten carbonates are an important class of oxoanion salts that are increasingly recognized in importance for various technological applications due to optimal combination of functional and eco-friendly features as compared to other molten salt systems. Presently, molten carbonate salts hold best promise in operations at intermediate temperatures for use as a fill electrolyte medium in fuel cells and in innovative solar-driven electrolysis processes without CO2 emission [1] or alternatively as versatile reaction flux solvent in homogeneous catalysis, metal surface treatment and energy-related processes that could otherwise produce high carbon footprint or noxious by-products, including coal
earth (RE) cation additions are the most effective in promoting the formation of highly basic carbonates [5–8]. Lanthanum is the most suitable RE additive due to its low cost, high basicity and solubility properties in molten carbonates compared to the other rare earth cations [9]. In a typical fuel cell molten carbonate environment lanthanum exists prevalently as lanthanum dioxycarbonate La2 O2 CO3 , which is a stable reaction intermediate in the process of thermal decomposition of lanthanum carbonate to La2 O3 . Thermal stability of lanthanum carbonate has been studied in details only at solid phase (see, for example [10]). The process spans over a wide range of temperature through two major intermediate steps that can be schematically summarized as follows [11,12]:
(1) combustion, spent nuclear fuel reprocessing, destruction of hardto-burn organic-containing wastes [2–4]. Recent studies have shown that notable changes in the acid–base properties of alkali molten carbonates can be obtained through minor additions of non-alkali metals. In particular, rare
∗ Corresponding author. Tel.: +39 063 0483138. E-mail address: [email protected] (S. Frangini). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.016
The loss of CO2 from stoichiometric lanthanum carbonate falls below 500 ◦ C, being most frequently reported at around 470 ◦ C and giving raise to formation of lanthanum dioxycarbonate La2 O2 CO3 and possibly of some monooxycarbonate La2 O(CO3 )2 [11,13]. Lanthanum dioxycarbonate may exist in three different polymorphic forms, namely tetragonal (Type I) and monoclinic (Type IA) both having a cubic LaOCl-related structure, and hexagonal (Type II) with a La2 O3 sesquioxide structure [14]. Thermal decomposition studies of dioxycarbonate phases indicate that the hexagonal polymorph
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S. Frangini, S. Scaccia / Thermochimica Acta 551 (2013) 33–39
Table 1 DSC experimental design for melting and freezing points determination of alkali carbonate samples under different atmospheres.
structure forms at temperatures higher than the cubic phases forms, at around 552 ◦ C [14]. The final decomposition temperature at which La2 O3 is formed from the hexagonal solid La2 O2 CO3 is slightly above 700 ◦ C, the exact temperature much depending on exact experimental conditions and gas composition [10]. In agreement with these results, there are indications in recent literature that confirm that the hexagonal oxycarbonate phase is already stabilized at 600 ◦ C in lithium-containing alkali molten carbonate eutectic salts [15]. However, the exact decomposition temperature of La2 (CO3 )3 in the mixed alkali phase has not yet reported, although of its important implications to explain the acid–base alkali carbonate chemistry changes. In particular, thermal stability near the alkali carbonate melting point is identified as a most interesting region to explore because of its proximity to the reported first decomposition step of pure lanthanum carbonate solids and secondly because, in virtue of lanthanum cation valency, charge asymmetry effects could be expected to potentially affect melting/freezing points of eutectic carbonate mixtures. As useful example, the present paper is therefore aimed to investigate the effect of lanthanum carbonate additions on the thermal stability of an eutectic 52/48 Li/Na carbonate mixture with emphasis to the region close to the carbonate melting point, where maximum possibility exists of lanthanum carbonate decomposition phenomena. Commercial La2 (CO3 )3 powders usually contain relatively high levels of crystallization water, which is difficult to remove [16]. Therefore, anhydrous La2 O3 was used in this work as a suitable La2 (CO3 )3 precursor to inhibit fast hydrolysis of carbonate salt mixtures during their preparation.
2. Experimental 2.1. Preparation and characterization of molten salts Alkali metal carbonate mixtures with and without La2 O3 additions were prepared as described in our previous works [7]. In brief, commercial powders of analytical reagent grade of Li2 CO3 (Carlo Erba), Na2 CO3 (Carlo Erba), La2 O3 (Rudi-Point) were initially dried at 300 ◦ C for 24 h in air. The baseline alkali eutectic composition was prepared by exactly weighing 52 mol% Li2 CO3 + 48 mol% Na2 CO3 . Lanthanum-containing samples were obtained by adding 0, 0.5, 1.0, and 2.0 mol of La2 O3 to the baseline alkali eutectic composition. Each powder mixture was then transferred into a 50-cm3 polyethylene bottle together with 4 mm diameter glass-balls using a ball-to powder volume ratio of about 1:1. The bottle was then rolled in a jar mill for 24 h using a rotating speed of 60 rpm for optimal mixing effect. Check for any wrong elemental composition of
the jar milled powders (Li, Na, and La) was performed after sample dissolution in dilute nitric acid by flame atomic absorption spectrometry (Varian 220 FS). The effect of exposure to moist air was also checked due to the known tendency of lanthanum oxide to easily react with water present as humidity. A Varian 640 Fourier transform infrared (FTIR) spectrometer was used for the analysis of solid alkali carbonate mixtures by the KBr technique. FTIR spectra were taken at 4 cm−1 resolution and 32 scans in the spectral region 4000–400 cm−1 . 2.2. DSC measurements A TGA/DSC 1 STARe System analyzer (Mettler-Toledo, Switzerland) was used. Calibration was performed by determining the melting point of indium. Inert N2 was used as purge gas at 30 mL min−1 for balance protection. N2 and CO2 were used as testing gases at 80 mL min−1 flow rate. A medium-sized alumina crucible of 30 L was used as sample container. Typical mass sample was 5 mg. Various multiple-step thermal programs were designed to evaluate the thermal behavior on crystallized melt samples. The necessary preliminary steps to prepare the crystallized melt samples from the initial powder samples are summarized in Table 1 as segments I–IV. In particular, the powder samples were heated from ambient temperature up to 400 ◦ C at 20 ◦ C min−1 under CO2 to prevent carbonate decomposition. Subsequently, in proximity of the melting point, heating rate was slowed down at 10 ◦ C min−1 up to 550 ◦ C to avoid sudden boiling of the molten salt. After isotherm time of 15 min, the temperature was decreased at 20 ◦ C min−1 down to 200 ◦ C to ensure complete transformation of the initial La2 O3 precursor into La2 (CO3 )3 [14]. From DSC scans (segments V–X in Fig. 1) the temperatures of onset (Tonset ), end (Tend ), and peak maximum (Tpeak ) as well the peak area (endothermic/exothermic heat) were calculate automatically by computer using the STARe software version 9.2. To asses reproducibility, the DSC measurements were repeated twice on each sample. 2.3. Gas evolution analysis (EGA) Real-time analysis of gas evolution phenomena on crystallized melt samples was performed by coupling the Mettler-Toledo TGA/DSC 1 Stare System with a FTIR spectrometer (Varian 640) via a stainless steel transfer line (1 m length and 3.175 mm internal diameter) with temperature control set to 200 ◦ C. A 100 mm path length infrared gas cell (cell volume 38.5 mL, Pike Technologies Inc.) was used. KBr windows and MTC (Cryogenic Mercury
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Fig. 1. Typical FTIR spectra of La2 O3 -containing eutectic 52/48 Li/Na carbonate mixtures of as-prepared powders.
Cadmium Telluride) detector cooled with liquid nitrogen were used. FTIR spectra were recorded at 4 cm−1 resolution and 1.5 sensitivity. The interferograms were collected with a temporal resolution of about 1.1 s and post-processed by the Resolutions ProTM software packages to obtain the gas absorption spectra. The DSC thermal program used for EGA measurements is summarized in Table 2. 3. Results Typical FTIR spectra of the as-prepared powders of eutectic 52/48 Li/Na carbonate containing La2 O3 additive in the concentration range of 0.5–2.0 mol% are given in Fig. 1. The FTIR spectra of all the samples show a O H stretching vibration mode at 3610 cm−1 [13], whose peak intensity increases with increasing the lanthanum oxide concentration. This finding clearly reflects the well-known lanthanum oxide tendency to absorb some moisture from surrounding atmosphere. A noticeable peak at 1088 cm−1 and others minor intense bands at 740 and 700 cm−1 may also be noted. These peaks can be ascribed to the symmetric stretching modes of CO3 2− groups belonging to rare earth carbonate and oxycarbonate series [11,14]. It is plausible that the some formation of rare earth carbonate species may have taken place by anion exchange effects during the prolonged mixing of lanthanum oxide with the alkali carbonates. Fig. 2 shows the most significant temperature portion of the DSC heating/cooling curves of a crystallized melt sample of the
baseline eutectic Li/Na molten carbonate under both CO2 and N2 gas flows. During heating the curves display a sharp endothermic peak at Tpeak = 499 ◦ C due to fusion of the carbonate salt, whereas in cooling runs the corresponding freezing points are indicated by the exothermic peak at Tpeak = 492 ◦ C. Under these dynamic experimental conditions, it is evident that the surrounding gas atmosphere does not exert any significant influence on the molten salt thermal behavior. Fig. 3 shows the effect of La2 O3 additions on the DSC heating/cooling curves of crystallized melt samples of eutectic Li/Na carbonate, under the CO2 atmosphere. The heating pattern (Fig. 3(a)) shows a weak extra endothermic signal in the 470–480 ◦ C range, namely in pre-melting region, that could be assigned to the thermal decomposition of lanthanum carbonate. With the increase of the additive concentration the Tonset of the pre-melting peak shifts to higher temperature with the result that the absorbed heat flow tends to overlap to the fusion heat of the carbonate melt. Moreover, the incorporation of lanthanum into the alkali carbonate composition provokes a temperature depression of the melting Tpeak with a maximum of a 10 ◦ C for the 2.0 mol% additive concentration (Table 3). During the cooling down (Fig. 3(b)) the additional peak is positioned below the carbonate freezing point suggesting that the exchange of CO2 with the surrounding atmosphere should be a reversible process. In general, the 1.0 and 2.0 mol% La2 O3 containing carbonate samples exhibit very similar cooling curves, being markedly different from that of 0.5 mol% La2 O3 addition. In the latter case the additional peak moves to lower temperatures of
Table 2 DSC experimental design for EGA analysis of 1 mol% La2 O3 -containing eutectic 52/48 Li/Na carbonate sample.
Thermal pre-treatment
Segment
Mode
Rate (◦ C min−1 )
I–IV XI XII XIII
Isotherm Heating Cooling
0 20 20
Temperature range (◦ C) 200 200–550 550–200
Hold time (min)
Gas type
25
N2 N2 N2
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Fig. 2. Heating and cooling curves of crystallized melt samples of eutectic 52/48 Li/Na under CO2 (—) and N2 (- - -) atmospheres.
about 20 ◦ C. The freezing point of molten carbonate salt similarly shows a depression effect yet as a linear function of lanthanum concentration (Table 3). This linearity is typical of a colligative property behavior, thus indicating that lanthanum oxycarbonate is soluble in the alkali melt for all the concentrations used in this work. Moreover, any effect due to structural asymmetry introduced with the lanthanum three valency cation could be reasonably excluded from this linear behavior. Due to a sufficiently wide temperature spacing, it has been possible to accurately estimate shape and heat flow of the exothermic peaks during cooling down. The sharp freezing peaks of the lanthanum-containing carbonate samples indicate that lanthanum does not appreciably affect the eutectic behavior of the baseline alkali mixed carbonate. The freezing enthalpy of molten carbonate is slightly higher in presence of La2 O3 additive and does not appreciably change with its concentration. Conversely, the area under the extra peak exhibits a distinct linear increase with the increasing La2 O3 concentration. All these thermal processes have been found to be reversible during repeated heating/cooling cycles. Fig. 4 shows the DSC curves of a crystallized melt sample of 1.0 mol% La2 O3 -containing eutectic carbonate after three consecutive heating/cooling cycles, the first one under CO2 and the subsequent two ones under N2 . The pre-melting peak shows a reversible profile only in the first run under N2 , namely after the initial CO2 heating/cooling cycle (curve 1 in Fig. 4(a) and curve 4 in Fig. 4(b)). Conversely, the pre-melting peak almost disappears
Table 3 Thermoanalytical data of eutectic 52/48 Li/Na carbonate melt with and without La2 O3 additions. Sample
LN LN + 0.5L LN + 1.0L LN + 2.0L
Melting point
Freezing point
Tonset
Tend
Tpeak
Tonset
Tend
Tpeak
496 485 n.a.a n.a.a
509 504 502 499
499 492 492 489
494 493 483 480
483 476 473 468
492 489 482 478
LN = baseline alkali carbonate mixture; L = La2 O3 . a n.a. = not appreciable.
in the second N2 heating run that had been preceded by a N2 cooling run. However, the pre-melting thermal event (not showed here) can be promptly restored if a subsequent heating run is performed under CO2 flow. Incidentally, it has to be noted that the decrease of freezing enthalpy observed in curve 6 is merely due to carbonate sample vaporization after several scans in inert atmosphere. All these results show that complete reversibility of the pre-melting peak is possible only with presence of CO2 in the atmosphere. In order to confirm beyond any doubt that thermal decomposition of lanthanum carbonate is the effective cause of the pre-melting thermal events, EGA analysis has been performed during selected DSC measurements of the 1.0 mol% La2 O3 -containing Li/Na carbonate crystallized melt sample. The FTIR analysis of the evolved gases in coincidence with the pre-melting peak at 470 ◦ C in N2 flow shows the characteristic band at 2360 cm−1 of CO2 (Fig. 5). Consequently, the loss of CO2 can be safely associated to La2 (CO3 )3 decomposition to give oxycarbonate species such as La2 O(CO3 )2 or, which is much more probably, La2 O2 CO3 . The decomposition reaction of La2 (CO3 )3 shows high reversibility indicating that the exothermic peak during the cooling down must be due to a fast reaction of CO2 with the lanthanum oxycarbonate to regenerate La2 (CO3 )3 .
4. Discussion As shown in the previous section, the decomposition temperature starting at 470 ◦ C correlates well with that reported in literature for pure lanthanum carbonate. This findings could be strongly indicative for a Type I, IA phase oxycarbonate formation even in the mixed alkali carbonate. For the same reason, it is also highly probable that the phase transition from Type I, IA to Type II oxycarbonate should take place at around 552 ◦ C in the liquid phase as in the pure solid. As mentioned in Section 1, the decomposition reaction of lanthanum carbonate with CO2 evolution has direct consequences on the acid–base balance of the eutectic molten carbonate since molten carbonate salts can be treated as oxide-transfer
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Fig. 3. Heating (a) and cooling (b) curves of crystallized melt samples of eutectic 52/48 Li/Na with La2 O3 additions (0.5–2.0 mol%) under CO2 atmosphere.
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systems, where the CO2 gas behaves as the acidic component during thermal decomposition reaction of carbonate ions according to: CO3 2− = CO2 + O2−
Fig. 4. Heating (a) and cooling (b) curves of crystallized melt 1.0 mol% La2 O3 -containing Li/Na carbonate sample in subsequent cycles under different atmospheres (1, segment V; 2, segment VII; 3, segment IX; 4, segment VI; 5, segment VIII; 6, segment X).
(2)
Adopting this scheme, it is straightforward to assume that the degree of basicity in molten carbonates is mostly determined by its oxide ion content in the melt. Consequently, the evolution of the acidic CO2 gas accompanying the lanthanum oxycarbonate formation leads to an increased oxide basicity in the melt. The effect of an increased melt basicity has been already proposed in literature to explain most intriguing properties of these electrolyte systems. However, basicity effects have been mostly observed at temperatures higher than 550 ◦ C [5–8], where the Type II oxycarbonate phase is the likely prevalent species. Based on reported literature [14,17], it is possible to speculate that this Type II phase may be in reality a non-stoichiometric phase, La2 O3 ·yCO2 , with y being less than unity and variable with the temperature. This non-stoichiometric oxycarbonate corresponds to a variable mixture of stoichiometric La2 O2 CO3 Type II and La2 O3 suggesting that decomposition of oxycarbonate to La2 O3 may proceed via this non-stoichiometric oxycarbonate phase. Consequently, an increased basicity effect can be expected in alkali molten carbonates containing the Type II oxycarbonate phase for its likely lower CO2 content than the cubic oxycarbonate phases. In summary, the authors have clearly showed with this work that the increasing oxide-like character of lanthanum-containing eutectic lithium–sodium carbonate mixtures is effectively due to CO2 gas evolution accompanying lanthanum oxycarbonate formation that specifically takes place in the pre-melting region of the eutectic carbonate mixture. It is speculated that the reversible CO2 exchange properties of these carbonate systems may offer new potentials, not only for electrolytic applications, but also for broader applications in chemical engineering, for instance, as liquid membranes for gas separation or for CO2 chemical looping capture schemes.
Fig. 5. FTIR spectrum of evolved gas from crystallized melt sample of 1.0 mol% La2 O3 -containing 52/48 Li/Na carbonate at 470 ◦ C under N2 atmosphere.
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5. Conclusions Thermal behavior of eutectic Li/Na 52/48 carbonate mixture containing lanthanum carbonate additive was studied in CO2 and N2 atmospheres up to 700 ◦ C by DSC. Thermal decomposition of the added lanthanum carbonate took place in the pre-melting region of carbonate mixture giving rise to lanthanum oxycarbonate formation and concomitant CO2 gas evolution. The influence of lanthanum additions was noted also on the melting/freezing behavior of the baseline eutectic Li/Na carbonate melt. References [1] S. Licht, H. Wu, C. Hettige, B. Wang, J. Asercion, J. Lau, J. Stuart, STEP cement: solar thermal electrochemical production of CaO without CO2 emission, Chem. Commun. 48 (2012) 6019–6021. [2] I. Naruse, S. Raharjo, S. Iwasaki, T. Takuwa, R. Yoshiie, Gasification and desulfurization characteristics of carbonaceous materials in molten alkali carbonates, Trans. Jpn. Soc. Mech. Eng. B 74 (2008) 2636–2641. [3] T.R. Griffiths, V.A. Volkovich, S.M. Yakimov, I. May, C.A. Sharrad, J.M. Charnock, Reprocessing spent nuclear fuel using molten carbonates and subsequent precipitation of rare earth fission products using phosphate, J. Alloys Compd. 418 (2006) 116–121. [4] Z. Yao, J. Li, X. Zhao, Molten salt oxidation: a versatile and promising technology for the destruction of organic-containing wastes, Chemosphere 84 (2011) 1167–1174. [5] S. Scaccia, S. Frangini, Effect of various electrolyte compositions on the NiO degradation in molten carbonates, J. Fuel Cell Sci. Technol. 3 (2006) 208–212.
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[6] S. Frangini, Corrosion of metallic stack components in molten carbonates: critical issues and recent findings, J. Power Sources 182 (2008) 462–468. [7] S. Scaccia, S. Frangini, S. Dellepiane, Enhanced O2 solubility by RE2 O3 (RE = La, Gd) additions in molten carbonate electrolytes for MCFC, J. Mol. Liq. 138 (2008) 107–112. [8] K. Ota, Y. Matsuda, K. Matsuzawa, S. Mitsushima, N. Kamiya, Effect of rare earth oxides for improvement of MCFC, J. Power Sources 160 (2006) 811–815. [9] K. Matsuzawa, Y. Akinaga, S. Mitsushima, K. Ota, Solubilities of NiO and LaNiO3 in Li/Na eutectic carbonate with rare-earth oxide, J. Power Sources 196 (2011) 5007–5011. [10] A.N. Shirsat, M. Ali, K.N.G. Kaimal, S.R. Bharadwaj, D. Das, Thermochemistry of La2 O2 CO3 decomposition, Thermochim. Acta 399 (2003) 167. [11] J.A. Goldsmith, S.D. Ross, Factors affecting the infra-red spectra of some planar anions with D3h symmetry-III. The spectra of rare-earth carbonates and their thermal decomposition products, Spectrochim. Acta A 23 (1967) 1909–1915. [12] S. Bernal, F.J. Botana, J.M. Rodriguez-Izquierdo, Thermal evolution of a sample of La2 O3 exposed to the atmosphere, Thermochim. Acta 66 (1993) 139–145. [13] L. Paama, I. Pitkänen, H. Halttunen, P. Perämäki, Infrared evolved gas analysis during thermal investigation of lanthanum, europium and samarium carbonates, Thermochim. Acta 403 (2003) 197–206. [14] R.P. Turcotte, J.O. Sawyer, L. Eyring, On the rare earth dioxymonocarbonates and their decomposition, Inorg. Chem. 8 (1969) 238–246. [15] T. Masui, K. Koyabu, S. Tamura, N. Imanaka, Synthesis of a new green-emitting phosphor based on lanthanum oxycarbonate (La2 O2 CO3 -II), J. Mater. Sci. 40 (2005) 4121–4123. [16] M.S. Wickleder, Inorganic lanthanide compounds with complex anions, Chem. Rev. 102 (2002) 2011–2087. [17] A.M. Maitra, Determination of solid state basicity of rare earth oxides by thermal analysis of their carbonates, J. Therm. Anal. 36 (1990) 657–675.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Influence of lanthanum carbonate additions on thermal stability of eutectic lithium–sodium carbonate near its melting point Stefano Frangini a,∗ , Silvera Scaccia b a b
Hydrogen and Fuel Cell Laboratory (UTRINN-IFC), Via Anguillarese 301, I-00123 Rome, Italy Sustainable Combustion Laboratory (UTTEI-COMSO), C.R. ENEA, Casaccia, Via Anguillarese 301, I-00123 Rome, Italy
a r t i c l e
i n f o
Article history: Received 11 July 2012 Accepted 9 October 2012 Available online 3 November 2012 Keywords: DSC EGA, Li/Na eutectic carbonate Lanthanum carbonate Lanthanum oxycarbonate
a b s t r a c t Thermal behavior and stability of eutectic 52/48 Li/Na carbonate mixture containing various concentrations of lanthanum carbonate in the range 0.5–2.0 mol% was investigated by a combination of thermal analysis (DSC) and spectroscopic (EGA-FTIR) techniques under CO2 and N2 gas flows. Thermal decomposition of lanthanum carbonate with CO2 gas release was observed in the 470–480 ◦ C range close to the melting point of eutectic alkali carbonate giving raise to formation of lanthanum oxycarbonate species. It was also found that the lanthanum carbonate phase promptly reformed, when the eutectic carbonate melt samples were cooled down to 440–460 ◦ C under CO2 atmosphere. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Alkali molten carbonates are an important class of oxoanion salts that are increasingly recognized in importance for various technological applications due to optimal combination of functional and eco-friendly features as compared to other molten salt systems. Presently, molten carbonate salts hold best promise in operations at intermediate temperatures for use as a fill electrolyte medium in fuel cells and in innovative solar-driven electrolysis processes without CO2 emission [1] or alternatively as versatile reaction flux solvent in homogeneous catalysis, metal surface treatment and energy-related processes that could otherwise produce high carbon footprint or noxious by-products, including coal
earth (RE) cation additions are the most effective in promoting the formation of highly basic carbonates [5–8]. Lanthanum is the most suitable RE additive due to its low cost, high basicity and solubility properties in molten carbonates compared to the other rare earth cations [9]. In a typical fuel cell molten carbonate environment lanthanum exists prevalently as lanthanum dioxycarbonate La2 O2 CO3 , which is a stable reaction intermediate in the process of thermal decomposition of lanthanum carbonate to La2 O3 . Thermal stability of lanthanum carbonate has been studied in details only at solid phase (see, for example [10]). The process spans over a wide range of temperature through two major intermediate steps that can be schematically summarized as follows [11,12]:
(1) combustion, spent nuclear fuel reprocessing, destruction of hardto-burn organic-containing wastes [2–4]. Recent studies have shown that notable changes in the acid–base properties of alkali molten carbonates can be obtained through minor additions of non-alkali metals. In particular, rare
∗ Corresponding author. Tel.: +39 063 0483138. E-mail address: [email protected] (S. Frangini). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.016
The loss of CO2 from stoichiometric lanthanum carbonate falls below 500 ◦ C, being most frequently reported at around 470 ◦ C and giving raise to formation of lanthanum dioxycarbonate La2 O2 CO3 and possibly of some monooxycarbonate La2 O(CO3 )2 [11,13]. Lanthanum dioxycarbonate may exist in three different polymorphic forms, namely tetragonal (Type I) and monoclinic (Type IA) both having a cubic LaOCl-related structure, and hexagonal (Type II) with a La2 O3 sesquioxide structure [14]. Thermal decomposition studies of dioxycarbonate phases indicate that the hexagonal polymorph
34
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Table 1 DSC experimental design for melting and freezing points determination of alkali carbonate samples under different atmospheres.
structure forms at temperatures higher than the cubic phases forms, at around 552 ◦ C [14]. The final decomposition temperature at which La2 O3 is formed from the hexagonal solid La2 O2 CO3 is slightly above 700 ◦ C, the exact temperature much depending on exact experimental conditions and gas composition [10]. In agreement with these results, there are indications in recent literature that confirm that the hexagonal oxycarbonate phase is already stabilized at 600 ◦ C in lithium-containing alkali molten carbonate eutectic salts [15]. However, the exact decomposition temperature of La2 (CO3 )3 in the mixed alkali phase has not yet reported, although of its important implications to explain the acid–base alkali carbonate chemistry changes. In particular, thermal stability near the alkali carbonate melting point is identified as a most interesting region to explore because of its proximity to the reported first decomposition step of pure lanthanum carbonate solids and secondly because, in virtue of lanthanum cation valency, charge asymmetry effects could be expected to potentially affect melting/freezing points of eutectic carbonate mixtures. As useful example, the present paper is therefore aimed to investigate the effect of lanthanum carbonate additions on the thermal stability of an eutectic 52/48 Li/Na carbonate mixture with emphasis to the region close to the carbonate melting point, where maximum possibility exists of lanthanum carbonate decomposition phenomena. Commercial La2 (CO3 )3 powders usually contain relatively high levels of crystallization water, which is difficult to remove [16]. Therefore, anhydrous La2 O3 was used in this work as a suitable La2 (CO3 )3 precursor to inhibit fast hydrolysis of carbonate salt mixtures during their preparation.
2. Experimental 2.1. Preparation and characterization of molten salts Alkali metal carbonate mixtures with and without La2 O3 additions were prepared as described in our previous works [7]. In brief, commercial powders of analytical reagent grade of Li2 CO3 (Carlo Erba), Na2 CO3 (Carlo Erba), La2 O3 (Rudi-Point) were initially dried at 300 ◦ C for 24 h in air. The baseline alkali eutectic composition was prepared by exactly weighing 52 mol% Li2 CO3 + 48 mol% Na2 CO3 . Lanthanum-containing samples were obtained by adding 0, 0.5, 1.0, and 2.0 mol of La2 O3 to the baseline alkali eutectic composition. Each powder mixture was then transferred into a 50-cm3 polyethylene bottle together with 4 mm diameter glass-balls using a ball-to powder volume ratio of about 1:1. The bottle was then rolled in a jar mill for 24 h using a rotating speed of 60 rpm for optimal mixing effect. Check for any wrong elemental composition of
the jar milled powders (Li, Na, and La) was performed after sample dissolution in dilute nitric acid by flame atomic absorption spectrometry (Varian 220 FS). The effect of exposure to moist air was also checked due to the known tendency of lanthanum oxide to easily react with water present as humidity. A Varian 640 Fourier transform infrared (FTIR) spectrometer was used for the analysis of solid alkali carbonate mixtures by the KBr technique. FTIR spectra were taken at 4 cm−1 resolution and 32 scans in the spectral region 4000–400 cm−1 . 2.2. DSC measurements A TGA/DSC 1 STARe System analyzer (Mettler-Toledo, Switzerland) was used. Calibration was performed by determining the melting point of indium. Inert N2 was used as purge gas at 30 mL min−1 for balance protection. N2 and CO2 were used as testing gases at 80 mL min−1 flow rate. A medium-sized alumina crucible of 30 L was used as sample container. Typical mass sample was 5 mg. Various multiple-step thermal programs were designed to evaluate the thermal behavior on crystallized melt samples. The necessary preliminary steps to prepare the crystallized melt samples from the initial powder samples are summarized in Table 1 as segments I–IV. In particular, the powder samples were heated from ambient temperature up to 400 ◦ C at 20 ◦ C min−1 under CO2 to prevent carbonate decomposition. Subsequently, in proximity of the melting point, heating rate was slowed down at 10 ◦ C min−1 up to 550 ◦ C to avoid sudden boiling of the molten salt. After isotherm time of 15 min, the temperature was decreased at 20 ◦ C min−1 down to 200 ◦ C to ensure complete transformation of the initial La2 O3 precursor into La2 (CO3 )3 [14]. From DSC scans (segments V–X in Fig. 1) the temperatures of onset (Tonset ), end (Tend ), and peak maximum (Tpeak ) as well the peak area (endothermic/exothermic heat) were calculate automatically by computer using the STARe software version 9.2. To asses reproducibility, the DSC measurements were repeated twice on each sample. 2.3. Gas evolution analysis (EGA) Real-time analysis of gas evolution phenomena on crystallized melt samples was performed by coupling the Mettler-Toledo TGA/DSC 1 Stare System with a FTIR spectrometer (Varian 640) via a stainless steel transfer line (1 m length and 3.175 mm internal diameter) with temperature control set to 200 ◦ C. A 100 mm path length infrared gas cell (cell volume 38.5 mL, Pike Technologies Inc.) was used. KBr windows and MTC (Cryogenic Mercury
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Fig. 1. Typical FTIR spectra of La2 O3 -containing eutectic 52/48 Li/Na carbonate mixtures of as-prepared powders.
Cadmium Telluride) detector cooled with liquid nitrogen were used. FTIR spectra were recorded at 4 cm−1 resolution and 1.5 sensitivity. The interferograms were collected with a temporal resolution of about 1.1 s and post-processed by the Resolutions ProTM software packages to obtain the gas absorption spectra. The DSC thermal program used for EGA measurements is summarized in Table 2. 3. Results Typical FTIR spectra of the as-prepared powders of eutectic 52/48 Li/Na carbonate containing La2 O3 additive in the concentration range of 0.5–2.0 mol% are given in Fig. 1. The FTIR spectra of all the samples show a O H stretching vibration mode at 3610 cm−1 [13], whose peak intensity increases with increasing the lanthanum oxide concentration. This finding clearly reflects the well-known lanthanum oxide tendency to absorb some moisture from surrounding atmosphere. A noticeable peak at 1088 cm−1 and others minor intense bands at 740 and 700 cm−1 may also be noted. These peaks can be ascribed to the symmetric stretching modes of CO3 2− groups belonging to rare earth carbonate and oxycarbonate series [11,14]. It is plausible that the some formation of rare earth carbonate species may have taken place by anion exchange effects during the prolonged mixing of lanthanum oxide with the alkali carbonates. Fig. 2 shows the most significant temperature portion of the DSC heating/cooling curves of a crystallized melt sample of the
baseline eutectic Li/Na molten carbonate under both CO2 and N2 gas flows. During heating the curves display a sharp endothermic peak at Tpeak = 499 ◦ C due to fusion of the carbonate salt, whereas in cooling runs the corresponding freezing points are indicated by the exothermic peak at Tpeak = 492 ◦ C. Under these dynamic experimental conditions, it is evident that the surrounding gas atmosphere does not exert any significant influence on the molten salt thermal behavior. Fig. 3 shows the effect of La2 O3 additions on the DSC heating/cooling curves of crystallized melt samples of eutectic Li/Na carbonate, under the CO2 atmosphere. The heating pattern (Fig. 3(a)) shows a weak extra endothermic signal in the 470–480 ◦ C range, namely in pre-melting region, that could be assigned to the thermal decomposition of lanthanum carbonate. With the increase of the additive concentration the Tonset of the pre-melting peak shifts to higher temperature with the result that the absorbed heat flow tends to overlap to the fusion heat of the carbonate melt. Moreover, the incorporation of lanthanum into the alkali carbonate composition provokes a temperature depression of the melting Tpeak with a maximum of a 10 ◦ C for the 2.0 mol% additive concentration (Table 3). During the cooling down (Fig. 3(b)) the additional peak is positioned below the carbonate freezing point suggesting that the exchange of CO2 with the surrounding atmosphere should be a reversible process. In general, the 1.0 and 2.0 mol% La2 O3 containing carbonate samples exhibit very similar cooling curves, being markedly different from that of 0.5 mol% La2 O3 addition. In the latter case the additional peak moves to lower temperatures of
Table 2 DSC experimental design for EGA analysis of 1 mol% La2 O3 -containing eutectic 52/48 Li/Na carbonate sample.
Thermal pre-treatment
Segment
Mode
Rate (◦ C min−1 )
I–IV XI XII XIII
Isotherm Heating Cooling
0 20 20
Temperature range (◦ C) 200 200–550 550–200
Hold time (min)
Gas type
25
N2 N2 N2
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Fig. 2. Heating and cooling curves of crystallized melt samples of eutectic 52/48 Li/Na under CO2 (—) and N2 (- - -) atmospheres.
about 20 ◦ C. The freezing point of molten carbonate salt similarly shows a depression effect yet as a linear function of lanthanum concentration (Table 3). This linearity is typical of a colligative property behavior, thus indicating that lanthanum oxycarbonate is soluble in the alkali melt for all the concentrations used in this work. Moreover, any effect due to structural asymmetry introduced with the lanthanum three valency cation could be reasonably excluded from this linear behavior. Due to a sufficiently wide temperature spacing, it has been possible to accurately estimate shape and heat flow of the exothermic peaks during cooling down. The sharp freezing peaks of the lanthanum-containing carbonate samples indicate that lanthanum does not appreciably affect the eutectic behavior of the baseline alkali mixed carbonate. The freezing enthalpy of molten carbonate is slightly higher in presence of La2 O3 additive and does not appreciably change with its concentration. Conversely, the area under the extra peak exhibits a distinct linear increase with the increasing La2 O3 concentration. All these thermal processes have been found to be reversible during repeated heating/cooling cycles. Fig. 4 shows the DSC curves of a crystallized melt sample of 1.0 mol% La2 O3 -containing eutectic carbonate after three consecutive heating/cooling cycles, the first one under CO2 and the subsequent two ones under N2 . The pre-melting peak shows a reversible profile only in the first run under N2 , namely after the initial CO2 heating/cooling cycle (curve 1 in Fig. 4(a) and curve 4 in Fig. 4(b)). Conversely, the pre-melting peak almost disappears
Table 3 Thermoanalytical data of eutectic 52/48 Li/Na carbonate melt with and without La2 O3 additions. Sample
LN LN + 0.5L LN + 1.0L LN + 2.0L
Melting point
Freezing point
Tonset
Tend
Tpeak
Tonset
Tend
Tpeak
496 485 n.a.a n.a.a
509 504 502 499
499 492 492 489
494 493 483 480
483 476 473 468
492 489 482 478
LN = baseline alkali carbonate mixture; L = La2 O3 . a n.a. = not appreciable.
in the second N2 heating run that had been preceded by a N2 cooling run. However, the pre-melting thermal event (not showed here) can be promptly restored if a subsequent heating run is performed under CO2 flow. Incidentally, it has to be noted that the decrease of freezing enthalpy observed in curve 6 is merely due to carbonate sample vaporization after several scans in inert atmosphere. All these results show that complete reversibility of the pre-melting peak is possible only with presence of CO2 in the atmosphere. In order to confirm beyond any doubt that thermal decomposition of lanthanum carbonate is the effective cause of the pre-melting thermal events, EGA analysis has been performed during selected DSC measurements of the 1.0 mol% La2 O3 -containing Li/Na carbonate crystallized melt sample. The FTIR analysis of the evolved gases in coincidence with the pre-melting peak at 470 ◦ C in N2 flow shows the characteristic band at 2360 cm−1 of CO2 (Fig. 5). Consequently, the loss of CO2 can be safely associated to La2 (CO3 )3 decomposition to give oxycarbonate species such as La2 O(CO3 )2 or, which is much more probably, La2 O2 CO3 . The decomposition reaction of La2 (CO3 )3 shows high reversibility indicating that the exothermic peak during the cooling down must be due to a fast reaction of CO2 with the lanthanum oxycarbonate to regenerate La2 (CO3 )3 .
4. Discussion As shown in the previous section, the decomposition temperature starting at 470 ◦ C correlates well with that reported in literature for pure lanthanum carbonate. This findings could be strongly indicative for a Type I, IA phase oxycarbonate formation even in the mixed alkali carbonate. For the same reason, it is also highly probable that the phase transition from Type I, IA to Type II oxycarbonate should take place at around 552 ◦ C in the liquid phase as in the pure solid. As mentioned in Section 1, the decomposition reaction of lanthanum carbonate with CO2 evolution has direct consequences on the acid–base balance of the eutectic molten carbonate since molten carbonate salts can be treated as oxide-transfer
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Fig. 3. Heating (a) and cooling (b) curves of crystallized melt samples of eutectic 52/48 Li/Na with La2 O3 additions (0.5–2.0 mol%) under CO2 atmosphere.
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systems, where the CO2 gas behaves as the acidic component during thermal decomposition reaction of carbonate ions according to: CO3 2− = CO2 + O2−
Fig. 4. Heating (a) and cooling (b) curves of crystallized melt 1.0 mol% La2 O3 -containing Li/Na carbonate sample in subsequent cycles under different atmospheres (1, segment V; 2, segment VII; 3, segment IX; 4, segment VI; 5, segment VIII; 6, segment X).
(2)
Adopting this scheme, it is straightforward to assume that the degree of basicity in molten carbonates is mostly determined by its oxide ion content in the melt. Consequently, the evolution of the acidic CO2 gas accompanying the lanthanum oxycarbonate formation leads to an increased oxide basicity in the melt. The effect of an increased melt basicity has been already proposed in literature to explain most intriguing properties of these electrolyte systems. However, basicity effects have been mostly observed at temperatures higher than 550 ◦ C [5–8], where the Type II oxycarbonate phase is the likely prevalent species. Based on reported literature [14,17], it is possible to speculate that this Type II phase may be in reality a non-stoichiometric phase, La2 O3 ·yCO2 , with y being less than unity and variable with the temperature. This non-stoichiometric oxycarbonate corresponds to a variable mixture of stoichiometric La2 O2 CO3 Type II and La2 O3 suggesting that decomposition of oxycarbonate to La2 O3 may proceed via this non-stoichiometric oxycarbonate phase. Consequently, an increased basicity effect can be expected in alkali molten carbonates containing the Type II oxycarbonate phase for its likely lower CO2 content than the cubic oxycarbonate phases. In summary, the authors have clearly showed with this work that the increasing oxide-like character of lanthanum-containing eutectic lithium–sodium carbonate mixtures is effectively due to CO2 gas evolution accompanying lanthanum oxycarbonate formation that specifically takes place in the pre-melting region of the eutectic carbonate mixture. It is speculated that the reversible CO2 exchange properties of these carbonate systems may offer new potentials, not only for electrolytic applications, but also for broader applications in chemical engineering, for instance, as liquid membranes for gas separation or for CO2 chemical looping capture schemes.
Fig. 5. FTIR spectrum of evolved gas from crystallized melt sample of 1.0 mol% La2 O3 -containing 52/48 Li/Na carbonate at 470 ◦ C under N2 atmosphere.
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5. Conclusions Thermal behavior of eutectic Li/Na 52/48 carbonate mixture containing lanthanum carbonate additive was studied in CO2 and N2 atmospheres up to 700 ◦ C by DSC. Thermal decomposition of the added lanthanum carbonate took place in the pre-melting region of carbonate mixture giving rise to lanthanum oxycarbonate formation and concomitant CO2 gas evolution. The influence of lanthanum additions was noted also on the melting/freezing behavior of the baseline eutectic Li/Na carbonate melt. References [1] S. Licht, H. Wu, C. Hettige, B. Wang, J. Asercion, J. Lau, J. Stuart, STEP cement: solar thermal electrochemical production of CaO without CO2 emission, Chem. Commun. 48 (2012) 6019–6021. [2] I. Naruse, S. Raharjo, S. Iwasaki, T. Takuwa, R. Yoshiie, Gasification and desulfurization characteristics of carbonaceous materials in molten alkali carbonates, Trans. Jpn. Soc. Mech. Eng. B 74 (2008) 2636–2641. [3] T.R. Griffiths, V.A. Volkovich, S.M. Yakimov, I. May, C.A. Sharrad, J.M. Charnock, Reprocessing spent nuclear fuel using molten carbonates and subsequent precipitation of rare earth fission products using phosphate, J. Alloys Compd. 418 (2006) 116–121. [4] Z. Yao, J. Li, X. Zhao, Molten salt oxidation: a versatile and promising technology for the destruction of organic-containing wastes, Chemosphere 84 (2011) 1167–1174. [5] S. Scaccia, S. Frangini, Effect of various electrolyte compositions on the NiO degradation in molten carbonates, J. Fuel Cell Sci. Technol. 3 (2006) 208–212.
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