Molten Alkali Metal Nitrates Research Articles

Uncovering the CO2 Capture Mechanism of NaNO3-Promoted MgO by 18O Isotope Labeling.

MgO-based CO2 sorbents promoted with molten alkali metal nitrates (e.g., NaNO3) have emerged as promising materials for CO2 capture and storage technologies due to their low cost and high theoretical CO2 uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO2 capture reaction) remains debated and poorly understood. Here, we utilize 18O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO3-promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the 18O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high 18O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the 18O label in the solid (MgCO3), molten (NaNO3), and gas (CO2) phases during the CO2 capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO2 and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO3. On the other hand, no oxygen exchange was observed between NaNO3 and CO2 or NaNO3 and MgO. Combining the results of the 18O labeling experiments, with insights gained from atomistic calculations, we propose a carbonation mechanism that, in the first stage, proceeds through a fast, surface-limited carbonation of MgO. These surface carbonates are subsequently dissolved as [Mg2+···CO3 2-] ionic pairs in the molten NaNO3 promoter. Upon reaching the solubility limit, MgCO3 crystallizes at the MgO/NaNO3 interface.

Alkali Metal Nitrate-Promoted High-Capacity MgO Adsorbents for Regenerable CO2 Capture at Moderate Temperatures

Regenerable high capacity CO2 sorbents are desirable for the establishment of widespread carbon capture and storage (CCS) systems to reduce global CO2 emissions. We report on the marked effects of molten alkali metal nitrates on CO2 uptake by MgO particles and their impact on the development of highly regenerable CO2 adsorbents with high capacity (>10.2 mmol g–1) at moderate temperatures (∼300 °C) under ambient pressure. The molten alkali metal nitrates are shown to prevent the formation of a rigid, CO2-impermeable, unidentate carbonate layer on the surfaces of MgO particles and promote the rapid generation of carbonate ions to allow the high rate of CO2 uptake.

Synthesis of pure phase BiFeO 3 powders in molten alkali metal nitrates

Pure phase BiFeO 3 powders were successfully synthesized in molten alkali metal nitrates (KNO 3–NaNO 3) at 500 °C. The as-prepared BiFeO 3 had a rhombohedral structure which was studied using X-ray diffraction. The plate-like morphologies were investigated through scanning electron microscopy and transmission electron microscopy. The average length and width of BiFeO 3 plates were 400 and 200 nm, respectively. Furthermore, the mechanism of formation of BiFeO 3 was also discussed through X-ray diffraction, thermogravimetry, differential thermal analysis and mass spectrometry.

Solubility of carbon dioxide in molten alkali-metal nitrates

The solubility of CO2 in the series of molten alkali-metal nitrates is volumetrically measured on improved equipment, and the thermodynamic characteristics of dissolution are calculated.

Lanthanum-based perovskites obtained in molten nitrates or nitrites

Abstract Lanthanum-based perovskites, LaMO3 (where M = Mn, Fe, Co, Ni), have been synthesised from lanthanum precursors and corresponding 3d transition metal salts in molten alkali metal nitrates or nitrites. The reactivity of eight precursors in three molten salts of different basicity was analysed. Single phase orthorhombic LaMnO3.11, and orthorhombic LaFeO3 have been obtained in molten NaNO2 at 723 K, for 2 h. LaCoO3 is also formed in molten nitrite, but an increase of basicity, by adding Na2O2, was necessary to obtain LaNiO3. The reaction systems were investigated by thermal analysis. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and specific surface area measurements (BET method) were used to characterise the synthesised nanopowders.

Molten salt syntheses of alkali metal titanates

Submicrometer dispersions of rod-like alkali metal titanates were prepared by the flux method, from the reaction of TiOSO4 or TiO2 precursors in molten alkali metal nitrates, doped with carbonates or hydroxides. Mono-, di-, tetra-, and hexatitanates are formed as a function of the precursor nature and the melt composition. As a rule, in these syntheses poorly crystalline or amorphous solids are obtained, showing the structure of polytitanates on the nanoscopic level. Lamellar potassium hexatitanate can be exchanged by action of a diluted acid, leading to protonic form, free from the alkali metal but retaining initial morphology. Reactivity of TiOSO4 and TiO2 in molten alkali metal nitrates and their mixtures with the corresponding carbonates was studied by mass spectrometry of the gases evolved during heating of the reaction mixtures. For both pure and doped nitrates it changes in the expected row Li>Na>K, following the melts oxobasicity sequence.

Investigation of the reactivity of AlCl 3 and CoCl 2 toward molten alkali-metal nitrates in order to synthesize CoAl 2O 4

Cobalt aluminate CoAl 2O 4 powder, constituted of nano-sized crystallites, is prepared, involving the reactivity of AlCl 3 and CoCl 2 with molten alkali-metal nitrates. The reaction at 450 °C for 2 h leads to a mixture of spinel oxide Co 3O 4 and amorphous γ-Al 2O 3. It is transformed into the spinel oxide CoAl 2O 4 by heating at 1000 °C. The powders are mainly characterized by XRD, FTIR, ICP, electron microscopy and diffraction, X-EDS and diffuse reflection. Their properties are compared to those of powders obtained by solid state reactions of a mechanical mixture of chlorides or oxides submitted to the same thermal treatment.

Study of the redox behaviour of high surface area CeO 2–SnO 2 solid solutions

Molten alkali metal nitrites or nitrates and co-precipitation methods were used to prepare CeO 2, SnO 2 and Ce 0.5Sn 0.5O 2 solid solutions with high surface areas. In the case of the single oxides prepared by the molten salt method, the use of KNO 3 leads to better crystallised solids than with KNO 2, but on the opposite with higher particle sizes and lower BET surface areas. The same effect was observed for the preparation of the mixed oxides with different resulting structures. With KNO 3, the mixed oxide appears as a mechanical mixture of the single oxides, whereas with KNO 2, a mixture of amorphous SnO 2 and of solid solution with a specific surface area (SSA) of 100 m 2 g −1 has been synthesised, but without evidence of Ce 2Sn 2O 8 pyrochlore formation. The solid prepared by co-precipitation is also well dispersed. Its amorphous structure by X-ray diffraction (XRD) analysis does not allow to conclude to the formation of a solid solution. TPR experiments with hydrogen have shown a beginning of reduction at lower temperature in presence of the solid solution compared to the mechanical mixture of the single oxides. An improved reducibility was also observed with the oxide prepared by co-precipitation, which is in favour of a solid solution formation also in this case. Therefore, the resulting hydrogen consumption obtained when reducing the solid solutions at 450 °C is very high and the oxygen storage capacities (OSCs) reach about 1 mmol O 2 g −1, value higher than those obtained with ceria-zirconia solid solutions. The OSC values were confirmed by oxygen chemisorption on the reduced solid between 25 and 450 °C. The full recovery of the initial solid solution was shown by XRD analysis and a second redox cycle has demonstrated the reversibility of the redox properties at this temperature.

Study of the reaction of tungsten carbide in molten alkali metal nitrates. Syntheses of divalent (s and d blocks) metal tungstates

WC is tested as precursor to synthesize metal tungstates by reaction in molten alkali metal nitrates. This constitutes a complex redox system with two reducing agents, W and C, and an oxidizer having several oxidation states. The mass loss due to the evolution of gases reveals the reaction steps. The infrared analyses of the gas phase show what kind of reaction develops according to the temperature. WC produces a water-soluble alkali metal tungstate. The reaction of a mixture of WC and a divalent metal chloride (Mg, Ca, Ba, Ni, Cu, Zn) leads to water-insoluble metal tungstates. As the reactivity of the cations increases in the order Zn, Ni, Cu, the reaction of WC is modified by their presence. The physico-chemical characterizations of the products show that some of them are contaminated either by WC or by metal oxide. Some others are rather pure products. These differences, in relationship with the other analyses, allow to propose first reaction pathways of the tungsten carbide in molten salts.

Catalysis assisted characterizations of nanosized TiO 2–Al 2O 3 mixtures obtained in molten alkali metal nitrates : Effect of the metal precursor

Mixtures of TiO 2 and Al 2O 3 (0, 5, 10 and 20 mol% Al 2O 3) were prepared by reaction of metal salts with molten alkali metal nitrates. The powders are constituted from nanosized anatase crystallites and amorphous alumina. Physico-chemical characterizations reveal the powders to be homogeneous. Ni–Mo sulphide catalysts were elaborated using these powders as supports and tested in the reaction of hydrogenation of tetralin. The decrease of the intrinsic catalytic activity per Mo atom, related to the alumina content, is higher than expected for a regular mixing of oxides, suggesting that alumina coats the anatase particles. On the contrary, it was found that changing the aluminium precursor salt leads to a metastable solid solution which increases significantly the intrinsic activity.

Preparation of Mixed Phosphates in Molten Alkali Metal Nitrates

Mixed phosphates of alkali metals and main or transition group metals were obtained from the reactions of different starting salts of metals (nitrates, chlorides) and ammonium hydrogen monophosphate at 673−823 K in the alkali metal nitrate fluxes. The products obtained include submicrometer dispersions of known commercially important compounds, among which are KTiOPO4 (KTP), a number of ABPO4 phases (where A stands for alkali metal; B, bivalent transition or main group metal), as well as various substituted phases, derived from the NASICON NaZr2(PO4)3 and lamellar zirconium phosphate structure. Sometimes novel mixed phosphates were formed, such as those of sodium and nickel or cobalt, for which stoichiometric composition and/or elementary cell parameters have been established. The dependence of composition of solid products on the nature of reacting species and reaction conditions was discussed. Possible extensions of method by means of chemical modifications with admixtures of nitrite and fluoride specie...

A novel preparation of calcia fully stabilised zirconia from molten alkali-metal nitrate

A novel preparation of cubic calcia stabilised zirconia (CaO–ZrO2) which employs the coprecipitation of the constituents from zirconium oxychloride (ZrO1.22Cl1.56) and calcium nitrate [Ca(NO3)2] with eutectic lithium nitrate-potassium nitrate (LiNO3–KNO3) at around 600 °C is described. The optimisation and advantages of the preparation are discussed.

Zirconia formation by reaction of zirconium sulfate in molten alkali-metal nitrates or nitrites

Thermogravimetry up to 500 °C reveals several multi-stage reactions between Zr(SO4)2 and molten alkali-metal nitrates which form poorly crystallised mixtures of tetragonal and monoclinic zirconia. The use of NaNO3 gives incomplete transformations. The addition of NaCl as well as the use of LiNO3-KN03, leads the reaction to completion; the increase of basicity by the addition of Na2CO3 also has the same effect, if it is assumed that Na2ZrO3 is formed together with ZrO2. Pure and finely divided tetragonal zirconia is produced by a single-stage reaction at a temperature lower than 300 °C, when the basicity of the medium is increased by replacing LiN03–KNO3 by NaNO2–KNO3.

The effect of NaCl addition on the corrosion of mild steel in NaNO 3 melt

Molten alkali metal nitrates are considered to be a promising heat transfer and storage system and the addition of NaCl in these melts increases their specific heats which increases their energy storage capacity. The effect of the addition of 7.5 wt% NaCl in NaNO 3 melt on the hot corrosion of mild steel has been investigated in the temperature range from 350° to 550°C by applying various techniques. Gravimetric test results up to 10 h of immersion showed higher oxidation rates in the presence of NaCl. Scale and melt analysis results indicated the diffusion of more iron from the scale into the NaNO 3NaCl melt. This suggests that the addition of NaCl increases the basicity of the melt. The higher corrosion rate, a lower polarization resistance and a decrease of activation energy were found in the presence of NaCl by electrochemical studies. Metallography study revealed the existence of localized attack at 350°C and the scale became non-adherent at 450°C in the NaNO 3NaCl melt.

Infrared Emission Spectra of a Thin Film of Molten Alkali-metal Nitrates.

Infrared Emission Spectra of a Thin Film of Molten Alkali-metal Nitrates.

The Structure of Ag4Br(3+) in Molten Alkali-Metal Nitrate.

The Structure of Ag4Br(3+) in Molten Alkali-Metal Nitrate.

Fluorodilithium(1+) and hydroxodilithium(1+) in molten alkali-metal nitrate

Fluorodilithium(1+) and hydroxodilithium(1+) in molten alkali-metal nitrate

Cationic lead(II) hydroxide complexes in molten alkali-metal nitrate

The complexation between lead(II) and hydroxide has been studied in molten (K,Na)NO3 at 280 °C by various techniques. An inert platinized platinum wire bathed with a gaseous mixture of oxygen and water served as a hydroxide electrode in potentiometric determinations of the hydroxide activity in the melts. Only PbOH+ and Pb2OH3+ can be detected at the low hydroxide concentrations used. Raman spectra and 1H NMR experiments gave strong indications that hydroxide retains the proton also when coordinated to lead(II). Oxide ions added to the melts readily transform into hydroxide even at very low water activities. Studies of the solubility of Pb(NO3)2 in melts with various concentrations of dissolved PbO show the formation of [PbOH+]q-species at fairly high hydroxide concentrations. Liquid X-ray scattering data yield q= 2 at these hydroxide concentrations, i.e. Pb2(OH)2+2 is the predominant complex ion. The Pb—O and Pb—Pb distances are 2.7 and 3.4 A, respectively. The short Pb—Pb distance is the same as that observed in the halide species Pb2X3+, and it is suggested that the complex is stabilized by a Pb—Pb interaction.

Thermodynamic stability and structure of Tl2Br+ in molten alkali-metal nitrate

The complexation between thallium(l) and bromide has been studied in molten 57:43 (K,Li)NO3 between 150 and 250°C, and in 50 : 50 (K,Na)NO3 between 250 and 300°C in systems TlBr–TlNO3–AlkNO3(Alk = K,Li or K,Na). Solubility data and e.m.f. measurements with Ag/AgBr electrodes give consistent evidence for TlBr and Tl2Br+ complexes. ΔH°mn and ΔS°mn have been evaluated for the stepwise associations in (K,Li)NO3. The results are: ΔH°11=–7.4 kJ mol–1, ΔS°11= 14.2 J K–1 mol–1 for Tl++ Br–→ TlBr and ΔH°21=–31.1 kJ mol–1, ΔS°21=–47 J K–1 mol–1 for Tl++ TlBr → Tl2Br+. A qualitatively similar pattern is observed for the reactions in (K,Na)NO3. Liquid X-ray scattering measurements on TlNO3–(K,Na)NO3 and TlBr–TlNO3–(K,Na)NO3 melts at 280°C reveal that Tll is solvated by four nitrate ions, which coordinate to Tl in an asymmetrical bidentate manner, the Tl–O distances being 2.66 and 3.23 A. The Tl–Br–Tl angle in Tl2Br+ is 92° and the Tl–Tl separation in the complex, 4.54 A, does not imply any direct Tl–Tl interaction.

Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 3.—The structure of Pb2X3+ and the solvated PbII ion, determined by liquid X-ray scattering and raman spectroscopy

Liquid X-ray scattering and Raman spectroscopy measurements have been made at 280 °C on binary and ternary mixtures in the system PbX2–Pb(NO3)2–(K,Na)NO3, where X = F, Cl, or Br. The furnaces and experimental techniques are described. The nitrate coordination of Pb(II) is shown to be Pb(NO3)2–4 in halide-free melts of Pb(NO3)2–(K,Na)NO3. The nitrate ions are asymmetrically bound via two oxygen atoms at distances of 2.4 and 3.1 A and the lead(II) atoms is positioned slightly out of the nitrate plane. In melts containing halide ions X– complexes of formal composition Pb2X3+ are formed, with Pb—X—Pb angles of 84 and 81° for Pb2Cl3+ and Pb2Br3+, respectively. In Pb2F3+ only the Pb—Pb distance can be experimentally determined. The Pb—F—Pb angle ranges between 96 and 139°. A remarkable characteristic of these complexes is the occurrence of very shot Pb—Pb distance of 3.4–3.8 A, suggesting a significant metal–metal interaction.