The hydration structure of aqueous carbonic acid from X-ray absorption spectroscopy

Agglomeration Drives the Reversed Fractionation of Aqueous Carbonate and Bicarbonate at the Air-Water Interface

In the course of our investigations of the adsorption of ions to the air-water interface, we previously reported the surprising result that doubly charged carbonate anions exhibit a stronger surface affinity than singly charged bicarbonate anions. In contrast to monovalent, weakly hydrated anions, which generally show enhanced concentrations in the interfacial region, multivalent (and strongly hydrated) anions are expected to show a much weaker surface propensity. In the present work, we use resonantly enhanced deep-UV second-harmonic generation spectroscopy to measure the Gibbs free energy of adsorption of both carbonate (CO32-) and bicarbonate (HCO3-) anions to the air-water interface. Contrasting the predictions of classical electrostatic theory and in support of our previous findings from X-ray photoelectron spectroscopy, we find that carbonate anions do indeed exhibit much stronger surface affinity than do the bicarbonate anions. Extensive computer simulations reveal that strong ion pairing of CO32- with the Na+ countercation in the interfacial region results in the formation of near-neutral agglomerate clusters, consistent with a theory of interfacial ion adsorption based on hydration free energy and capillary waves. Simulated X-ray photoelectron spectra predict a 1 eV shift in the carbonate spectra compared to that of bicarbonate, further confirming our experiments. These findings not only advance our fundamental understanding of ion adsorption chemistry but also impact important practical processes such as ocean acidification, sea-spray aerosol chemistry, and mammalian respiration physiology.

Coupled Biohydrogen Production and Bio-Nanocatalysis for Dual Energy from Cellulose: Towards Cellulosic Waste Up-Conversion into Biofuels

Hydrogen, an emergent alternative energy vector to fossil fuels, can be produced sustainably by fermentation of cellulose following hydrolysis. Fermentation feedstock was produced hydrolytically using hot compressed water. The addition of CO2 enhanced hydrolysis by ~26% between 240 and 260 °C with comparable hydrolysis products as obtained under N2 but at a 10 °C lower temperature. Co-production of inhibitory 5-hydromethyl furfural was mitigated via activated carbon sorption, facilitating fermentative biohydrogen production from the hydrolysate by Escherichia coli. Post-fermentation E. coli cells were recycled to biomanufacture supported Pd/Ru nanocatalyst to up-convert liquid-extracted 5-HMF to 2,5-dimethyl furan, a precursor of ‘drop in’ liquid fuel, in a one-pot reaction. This side stream up-valorisation mitigates against the high ‘parasitic’ energy demand of cellulose bioenergy, potentially increasing process viability via the coupled generation of two biofuels. This is discussed with respect to example data obtained via a hydrogen biotechnology with catalytic side stream up-conversion from cellulose feedstock.

Electron Flow Characterization of Charge Transfer for Carbonic Acid to Strong Base Proton Transfer in Aqueous Solution

Protonation of the strong base methylamine CH₃NH₂ by carbonic acid H₂CO₃ in aqueous solution, HOCOOH···NH₂CH₃ → HOCOO^-···⁺HNH₂CH₃, has been previously studied ( J. Phys. Chem. B 2016, 109, 2271-2280; J. Phys. Chem. B 2016, 109, 2281-2290) via Car-Parinnello molecular dynamics. This proton transfer (PT) reaction within a hydrogen (H)-bonded complex was found to be barrierless and very rapid, with key reaction coordinates comprising the proton coordinate, the H-bond separation R_ON, and a solvent coordinate, reflecting the water solvent rearrangement involved in the neutral to ion pair conversion. In the present work, the reaction's charge flow aspects are analyzed in detail, especially a description via Mulliken charge transfer for PT (MCTPT). A natural bond orbital analysis and some extensions of them are employed for the complex's electronic structure during the reaction trajectories. Results demonstrate that consistent with the MCTPT picture, the charge transfer (CT) occurs from a methylamine base nonbonding orbital to a carbonic acid antibonding orbital. A complementary MCTPT reaction product perspective of CT from the antibonding orbital of the HN⁺ moiety to the nonbonding orbital of the oxygen in the H-bond complex is also presented. σ_OH and σ_HN⁺ bond order expressions show this CT to occur within the H-bond OHN triad, an aspect key for simultaneous bond-breaking and -forming in the PT reaction.

Reactive force fields for aqueous and interfacial magnesium carbonate formation

We develop Mg/C/O/H ReaxFF parameter sets for two environments: an aqueous force field for magnesium ions in solution and an interfacial force field for minerals and mineral-water interfaces. Since magnesium is highly ionic, we choose to fix the magnesium charge and model its interaction with C/O/H through Coulomb, Lennard-Jones, and Buckingham potentials. We parameterize the forcefields against several crystal structures, including brucite, magnesite, magnesia, magnesium hydride, and magnesium carbide, as well as Mg²⁺ water binding energies for the aqueous forcefield. Then, we test the forcefield for other magnesium-containing crystals, solvent separated and contact ion-pairs and single-molecule/multilayer water adsorption energies on mineral surfaces. We also apply the forcefield to the forsterite-water and brucite-water interface that contains a bicarbonate ion. We observe that a long-range proton transfer mechanism deprotonates the bicarbonate ion to carbonate at the interface. Free energy calculations show that carbonate can attach to the magnesium surface with an energy barrier of about 0.22 eV, consistent with the free energy required for aqueous Mg-CO₃ ion pairing. Also, the diffusion constant of the hydroxide ions in the water layers formed on the forsterite surface are shown to be anisotropic and heterogeneous. These findings can help explain the experimentally observed fast nucleation and growth of magnesite at low temperature at the mineral-water-CO₂ interface in water-poor conditions.

Laminar flow in microfluidics investigated by spatially-resolved soft X-ray absorption and infrared spectroscopy

The application of soft X-ray absorption spectroscopy (XAS) to liquid cells based on microfluidics for chemical state analysis of light elements is much more difficult than hard X-ray absorption since soft X-rays cannot deeply penetrate a microfluidic cell. In this study, we have newly developed a microfluidic cell for spatially resolved XAS, where a 100 nm thick Si₃N₄ membrane is used for the measurement window to transmit soft X-rays for keeping the microfluidic flow at a width and depth of 50 µm. The π^* peak of pyridine near the N K-edge XAS shows characteristic energy shifts near the liquid-liquid interface in a laminar flow of pyridine and water. The distributions of the molar fractions of pyridine and water near the liquid-liquid interface have been determined from the energy shifts of the π^* peak probed at different geometric positions, where pyridine is mixed in the water part of the laminar flow and vice versa. The spatial distribution of both species has also been studied by infrared microscopy, using the same microfluidic setup. The present work clearly shows that these spectroscopic techniques are easily applicable to chemical and biological reactions prepared by microfluidics.

Molecular Hydration of Carbonic Acid: Ab Initio Quantum Chemical and Density Functional Theory Investigation

Molecular hydration of carbonic acid (H₂CO₃) is investigated in terms of bonding patterns in H₂CO₃···(H₂O) _n [ n = 1-4] hydrogen-bonded clusters within ab initio quantum chemical and density functional theory (DFT) frameworks. Successive addition of water molecules to H₂CO₃···H₂O entails elongation of O-H (hydroxyl) bond as well as contraction of specific intermolecular hydrogen bonds signifying hydration of carbonic acid; these structural features get markedly enhanced under the continuum solvation framework. A comparison between the structurally similar clusters H₂CO₃···(H₂O) _n and HCOOH···(H₂O) _n [ n = 1-3] brings out the structural stability of the former. The present investigation in conjunction with the binding energy behavior of approaching water molecule(s) should serve as a precursor for pathways exploring aqueous dissociation of H₂CO₃ for larger clusters, as well as development of force-field potentials for acid dissociation process.

Characterization of a trans-trans Carbonic Acid-Fluoride Complex by Infrared Action Spectroscopy in Helium Nanodroplets

The high Lewis basicity and small ionic radius of fluoride promote the formation of strong ionic hydrogen bonds in the complexation of fluoride with protic molecules. Herein, we report that carbonic acid, a thermodynamically disfavored species that is challenging to investigate experimentally, forms a complex with fluoride in the gas phase. Intriguingly, this complex is highly stable and is observed in abundance upon nanoelectrospray ionization of an aqueous sodium fluoride solution in the presence of gas-phase carbon dioxide. We characterize the structure and properties of the carbonic acid–fluoride complex, F^–(H₂CO₃), and its deuterated isotopologue, F^–(D₂CO₃), by helium nanodroplet infrared action spectroscopy in the photon energy range of 390–2800 cm^–1. The complex adopts a C_2v symmetry structure with the carbonic acid in a planar trans–trans conformation and both OH groups forming ionic hydrogen bonds with the fluoride. Substantial vibrational anharmonic effects are observed in the infrared spectra, most notably a strong blue shift of the symmetric hydrogen stretching fundamental relative to predictions from the harmonic approximation or vibrational second-order perturbation theory. Ab initio thermostated ring-polymer molecular dynamics simulations indicate that this blue shift originates from strong coupling between the hydrogen stretching and bending vibrations, resulting in an effective weakening of the OH···F^– ionic hydrogen bonds.

Influence of Local Environment on Inner Shell Excitation Spectra, Studied by Electron and X-ray Spectroscopy and Spectromicroscopy

Inner shell excitation spectroscopy is a local probe of the unoccupied electronic structure in the immediate vicinity of the core excited atom. As such, one might expect the inner shell spectrum of a given unit (a molecular fragment or a repeat unit of a solid) to be largely independent of where that unit is located. This is often an implicit assumption in spectral analysis and analytical applications. However, there are situations where inner shell excitation spectra exhibit significant sensitivity to their local environment. Here I categorize the ways in which inner shell spectra are affected by their local environment, and give examples from a career dedicated to developing a better understanding of inner shell excitation spectroscopy, its experimental techniques, and applications.

Probing electrode/electrolyte interfaces in situ by X-ray spectroscopies: old methods, new tricks

Electrode/electrolyte interfaces play a vital role in various electrochemical systems, but in situ characterization of such buried interfaces remains a major challenge. Several efforts to develop techniques or to modify existing techniques to study such interfaces are showing great promise to overcome this challenge. Successful examples include electrochemical scanning tunneling microscopy (EC-STM), surface-sensitive vibrational spectroscopies, environmental transmission electron microscopy (E-TEM), and surface X-ray scattering. Other techniques such as X-ray core-level spectroscopies are element-specific and chemical-state-specific, and are being widely applied in materials science research. Herein we showcase four types of newly developed strategies to probe electrode/electrolyte interfaces in situ with X-ray core-level spectroscopies. These include the standing wave approach, the meniscus approach, and two liquid cell approaches based on X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy. These examples demonstrate that with proper modifications, many ultra-high-vacuum based techniques can be adapted to study buried electrode/electrolyte interfaces and provide interface-sensitive, element- and chemical-state-specific information, such as solute distribution, hydrogen-bonding network, and molecular reorientation. At present, each method has its own specific limitations, but all of them enable in situ and operando characterization of electrode/electrolyte interfaces that can provide important insights into a variety of electrochemical systems.

Properties of aqueous nitrate and nitrite from x-ray absorption spectroscopy

Nitrate and nitrite ions are of considerable interest, both for their widespread use in commercial and research contexts and because of their central role in the global nitrogen cycle. The chemistry of atmospheric aerosols, wherein nitrate is abundant, has been found to depend on the interfacial behavior of ionic species. The interfacial behavior of ions is determined largely by their hydration properties; consequently, the study of the hydration and interfacial behavior of nitrate and nitrite comprises a significant field of study. In this work, we describe the study of aqueous solutions of sodium nitrate and nitrite via X-ray absorption spectroscopy (XAS), interpreted in light of first-principles density functional theory electronic structure calculations. Experimental and calculated spectra of the nitrogen K-edge XA spectra of bulk solutions exhibit a large 3.7 eV shift between the XA spectra of nitrate and nitrite resulting from greater stabilization of the nitrogen 1s energy level in nitrate. A similar shift is not observed in the oxygen K-edge XA spectra of NO3 (-) and NO2 (-). The hydration properties of nitrate and nitrite are found to be similar, with both anions exhibiting a similar propensity towards ion pairing.