Third dissociation constant of phosphoric acid in H2O and D2O from 75 to 300 °C at p = 20.4 MPa using Raman spectroscopy and a titanium-sapphire flow cell

Characterization of Orthophosphate and Orthovanadate in Aqueous Solution Using Polarized Raman Spectroscopy

Polarized Raman spectroscopy was used to analyze aqueous solutions of sodium orthophosphate and orthovanadate over a wide concentration range (0.00891-0.702 mol/L) at 23 °C. The isotropic scattering profiles were obtained by measuring polarized Raman scattering spectra. Furthermore, R-normalized spectra were calculated and presented. The tetrahedral ions, VO43-(aq) and PO43-(aq), demand four Raman active bands which have been subsequently characterized and assigned. For the PO43-(aq) ion, the deformation modes ν2(e) and ν4(f2) appear at 415 and 557 cm-1, and these modes are depolarized. In the P-O stretching region, the strongest Raman band appears at 936.5 cm-1, which is totally polarized with a depolarization ratio (ρ-value) of 0.002. The broad and depolarized mode at 1010 cm-1 constitutes the antisymmetric stretching band ν3(f2). The Raman spectrum of VO43- shows two depolarized deformation modes ν2(e) and ν4(f2) at 327 and 345.6 cm-1, which are severely overlapped. These bands are very weak. The strongest band in the Raman spectrum of VO43-(aq) is the symmetric stretching mode ν1(a1) at 820.2 cm-1 which is totally polarized with a ρ-value at 0.004. The depolarized antisymmetric stretching mode ν3(f2) appeared at 785 cm-1 as a broad and weak band. Both anions are strongly hydrated and showed extensive hydrolysis in an aqueous solution. Orthovanadate is a much stronger base than orthophosphate in aqueous solution. Therefore, a large amount of NaOH was used to suppress the hydrolysis of VO43-(aq) sufficiently, so, it was possible to characterize the VO43- modes. Quantitative Raman spectroscopy was applied to follow the hydrolysis of PO43- over a wide concentration range from 0.00891 to 0.592 mol/L. The hydrolysis data allowed the calculation of the pKa3 value for H3PO4 to be 12.330 ± 0.02 (25 °C). The hydrolysis of the VO43- ion is ∼21 times larger than that of the PO43-. The pKa3 value for H3VO4 is estimated to be 13.65 ± 0.1 (25 °C).

Nanostructured amorphous Ni-Co-Fe phosphide as a versatile electrocatalyst towards seawater splitting and aqueous zinc-air batteries

Electrocatalysis provides a desirable approach for moving toward a sustainable energy future. Herein, a rapid and facile potential pulse method was implemented for a one-pot electrosynthesis of the amorphous Ni-Co-Fe-P (NCFP) electrocatalyst. The 2 mg cm-2 loaded electrode displayed excellent trifunctional electrocatalytic activities toward the hydrogen evolution reaction (η HER j=10 = 102 mV), oxygen evolution reaction (η OER j=10 = 250 mV), and oxygen reduction reaction (E ORR 1/2 = 0.73 V) in alkaline solutions. Interestingly, even a lower overpotential of η HER j=10 = 86 mV was obtained at a super-high mass loading of 18.7 mg cm-2, demonstrating its feasibility for industrial-level applications. The NCFP electrocatalyst also offered superior catalytic activity in alkaline seawater electrolysis at industrially required current rates (500 mA cm-2). When implemented as an air cathode catalyst of an aqueous and quasi-solid state zinc-air battery, both devices delivered excellent performance. This study provides insights into a transformative technology towards a sustainable energy future.

In situ Raman investigation of Dy complexation in Cl-bearing aqueous solutions at 20-300 °C

Raman spectroscopy provides a versatile tool for in situ characterization of aqueous rare earth elements (REE) speciation at the molecular level. Complexation of REE with ligands such as Cl- and OH- is of particular interest for understanding the mobility of REE in NaCl-bearing hydrothermal fluids responsible for enriching REE to economic levels in nature. Raman spectroscopic studies of REE speciation in Cl-bearing aqueous fluids are primarily conducted at ambient temperature, whereas natural systems indicate temperatures of >100-600 °C. In this study, the speciation of Dy in acidic chloride-bearing hydrothermal solutions was investigated using confocal Raman spectroscopy with a new capillary Raman heating stage at 20-300 °C. Background solutions (pure water, NaCl-solutions) and solutions with 0.14-1.8 mol kg-1 dissolved DyCl3 were sealed in quartz capillary cells. Comparison of the spectra for Dy chloride solutions with those for background solutions and the spectra for reference Dy-bearing solids was used to identify Raman bands specific to Dy-O and Dy-Cl bonds. The Raman band for the Dy-O stretching mode of hydrated Dy3+ aqua ions was measured at 365-384 cm-1 and a Raman band for the Dy-Cl stretching modes of Dy chloride complexes was measured near 240 cm-1. The Dy-O band decreases systematically with temperature, whereas the Dy-Cl band systematically increases, indicating a systematic increase in the stability of Dy chloride complexes with temperature. This study provides the framework for expanding the use of in situ Raman spectroscopy to investigate the speciation of REE in aqueous solutions to hydrothermal conditions.

Dittmarite-type magnesium phosphates for highly efficient capture of Cs. JOURNAL OF HAZARDOUS MATERIALS 2023;453:131385. [PMID: 37043858 DOI: 10.1016/j.jhazmat.2023.131385] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The presence of cesium ions (Cs⁺) in radioactive wastewater has attracted considerable attention owing to their extreme toxic effects. Thus, there is an urgent need to develop adsorbents for Cs⁺ with high adsorption capacities (q). While phosphate-based adsorbents have advantages for their disposal, previous adsorbents have shown limited q because of their limited capacity for ion exchange, despite showing high theoretical q values. In this study, two dittmarite-type magnesium phosphates, KMgPO₄·H₂O (KMP) and NH₄MgPO₄·H₂O (NMP), were synthesized because of their ability to contain readily exchangeable cations in their interlayers. KMP and NMP demonstrated remarkable adsorption capacities for Cs⁺ (q_e^KMP = 630 mg g^-1 and q_e^NMP = 711 mg g^-1), which were the highest among all reported adsorbents and are ∼84 % of their theoretical values. Their distribution coefficients in waters with high divalent ion concentrations were low, which limits their use for the adsorption of Cs⁺ from such environments. After adsorption, KMP and NMP were structurally transformed into struvite-type CsMgPO₄·6H₂O (CsMP), which has two different stacking structures, either cubic or hexagonal, depending on the pH of the solution. The high q values of KMP and NMP enable them to reduce the volume of radioactive waste for disposal.

Second ionization constant of sulfuric acid in H2O and D2O from 150 to 300 °C at p = 11.5 MPa using flow AC conductivity

A custom-built flow-through AC conductivity instrument was used to measure the deuterium isotope effect on the ionization quotient of bisulfate from 150 to 300 °C, at p = 11.5 MPa. Standardized solutions of KCl, HCl, KOH, KHSO₄, K₂SO₄, and H₂SO₄ were prepared in light and heavy waters and their conductivities were measured and fitted with the Quint-Viallard conductivity model to obtain single ion conductivities at infinite dilution for K⁺, Cl^-, H⁺, OH^-, HSO₄^-, and SO₄^2-. These are the first conductivities of DSO₄^- and SO₄^2- measured in heavy water at any temperature, and the first ionization constants for bisulfate reported in heavy water above 225 °C. The deuterium isotope effect on the chemical equilibrium constant, ΔpK_2a = pK_2a,D - pK_2a,H, was found to increase with temperature, in contrast to the behaviour seen for other simple oxyacids.

Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

Electrochemical synthesis possesses substantial promise to utilize renewable energy sources to power the conversion of abundant feedstocks to value-added commodity chemicals and fuels. Of the potential system architectures for these processes, only systems employing 3-D structured porous electrodes have the capacity to achieve the high rates of conversion necessary for industrial scale. However, the phenomena and environments in these systems are not well understood and are challenging to probe experimentally. Fortunately, continuum modeling is well-suited to rationalize the observed behavior in electrochemical synthesis, as well as to ultimately provide recommendations for guiding the design of next-generation devices and components. In this review, we begin by presenting an historical review of modeling of porous electrode systems, with the aim of showing how past knowledge of macroscale modeling can contribute to the rising challenge of electrochemical synthesis. We then present a detailed overview of the governing physics and assumptions required to simulate porous electrode systems for electrochemical synthesis. Leveraging the developed understanding of porous-electrode theory, we survey and discuss the present literature reports on simulating multiscale phenomena in porous electrodes in order to demonstrate their relevance to understanding and improving the performance of devices for electrochemical synthesis. Lastly, we provide our perspectives regarding future directions in the development of models that can most accurately describe and predict the performance of such devices and discuss the best potential applications of future models.