Observation of Carbonic Acid Formation from Interaction between Carbon Dioxide and Ice by Using In Situ Modulation Excitation IR Spectroscopy

Carbonic acid, H₂ CO₃ , is of fundamental importance in nature both in living and non-living systems. Providing direct spectroscopic evidence for carbonic acid formation is however a challenge. Here we provide clear evidence by in situ attenuated total reflection IR spectroscopy combined with modulation excitation spectroscopy and phase-sensitive detection that CO₂ adsorption on ice surfaces is accompanied by carbonic acid formation. We demonstrate that carbonic acid can be formed from CO₂ on ice in the absence of high-energy irradiation and without protonation by strong acids. The formation of carbonic acid is favored at low temperature, whereas at high temperature it rapidly dissociates to form bicarbonate (HCO₃ ^- ) and carbonate (CO₃ ^2- ). The direct formation of carbonic acid from adsorption of CO₂ on ice could play a role in the upper troposphere in cirrus clouds, where all the necessary ingredients to form carbonic acid, that is, low temperature, CO₂ gas, and ice, are present.

Heterogeneous Chemistry of CaCO3 Aerosols with HNO3 and HCl

Calcite (CaCO₃) aerosols often serve as an idealized proxy for calcium-rich mineral dust. Their use has also previously been proposed for stratospheric solar radiation management (SSRM). Little is known about the heterogeneous chemistry of calcite aerosols with trace gases HNO₃ and HCl and therefore their potential impact on stratospheric ozone (O₃). Here we report the results of an experimental study of the uptake of HNO₃ and HCl onto submicron CaCO₃ particles in two different flow reactors. Products and reaction kinetics were observed by impacting aerosolized CaCO₃ onto ZnSe windows, exposing them to the reagent gases at a wide range of concentrations, at 296 K and under dry conditions, and analyzing the particles before and after trace gas exposure using Fourier transform infrared spectroscopy (FTIR). A Ca(OH)(HCO₃) termination layer was detected in the form of a HCO₃^- peak in the FTIR spectra, indicating a hydrated surface even under dry conditions. The results demonstrate the reaction of HNO₃ with Ca(OH)(HCO₃) to produce Ca(NO₃)₂, water, and CO₂. HCl reacted with Ca(OH)(HCO₃) to produce CaCl₂ and also water and CO₂. The depletion of the Ca(OH)(HCO₃)/Ca(CO₃) signal due to reaction with HNO₃ or HCl followed pseudo-first-order kinetics. From the FTIR analysis, the reactive uptake coefficient for HNO₃ was determined to be in the range of 0.013 ≤ γ_HNO3 ≤ 0.14, and that for HCl was 0.0011 ≤ γ_HCl ≤ 0.012 within the reported uncertainty. The reaction of HCl with airborne CaCO₃ aerosols was also studied in an aerosol flow tube coupled with a quadrupole chemical ionization mass spectrometer (CIMS) under similar conditions to the FTIR study, and γ_HCl was determined to be 0.013 ± 0.001. Following previous modeling studies, these results suggest that the reactions of HCl and HNO₃ with calcite in the stratosphere could ameliorate the potential for stratospheric solar radiation management to lead to stratospheric ozone depletion.

Alpha-Carbonic Acid Revisited: Carbonic Acid Monomethyl Ester as a Solid and its Conformational Isomerism in the Gas Phase

In this work, earlier studies reporting α‐H₂CO₃ are revised. The cryo‐technique pioneered by Hage, Hallbrucker, and Mayer (HHM) is adapted to supposedly prepare carbonic acid from KHCO₃. In methanolic solution, methylation of the salt is found, which upon acidification transforms to the monomethyl ester of carbonic acid (CAME, HO‐CO‐OCH₃). Infrared spectroscopy data both of the solid at 210 K and of the evaporated molecules trapped and isolated in argon matrix at 10 K are presented. The interpretation of the observed bands on the basis of carbonic acid [as suggested originally by HHM in their publications from 1993–1997 and taken over by Winkel et al., J. Am. Chem. Soc. 2007 and Bernard et al., Angew. Chem. Int. Ed. 2011] is inferior compared with the interpretation on the basis of CAME. The assignment relies on isotope substitution experiments, including deuteration of the OH‐ and CH₃‐ groups as well as ¹²C and ¹³C isotope exchange and on variation of the solvents in both preparation steps. The interpretation of the single molecule spectroscopy experiments is aided by a comprehensive calculation of high‐level ab initio frequencies for gas‐phase molecules and clusters in the harmonic approximation. This analysis provides evidence for the existence of not only single CAME molecules but also CAME dimers and water complexes in the argon matrix. Furthermore, different conformational CAME isomers are identified, where conformational isomerism is triggered in experiments through UV irradiation. In contrast to earlier studies, this analysis allows explanation of almost every single band of the complex spectra in the range between 4000 and 600 cm⁻¹.

Carbonic acid monoethyl ester as a pure solid and its conformational isomerism in the gas-phase

The monoesters of carbonic acid are deemed to be unstable and decompose to alcohol and carbon dioxide. In spite of this, we here report the isolation of the elusive carbonic acid monoethyl ester (CAEE) as a pure solid from ethanolic solutions of potassium bicarbonate. The hemiester is surprisingly stable in acidic solution and does not experience hydrolysis to carbonic acid. Furthermore, it is also stable in the gas phase, which we demonstrate by subliming the hemiester without decomposition. This could not be achieved in the past for any hemiester of carbonic acid. In the gas phase the hemiester experiences conformational isomerism at 210 K. Interestingly, the thermodynamically favored conformation is only reached for the torsional movement of the terminal ethyl group, but not the terminal hydrogen atom on the millisecond time scale. Accordingly, IR spectra of the gas phase trapped in an argon matrix are best explained on the basis of a 5 : 1 mixture of monomeric conformers. Our findings necessitate reevaluation of claims of the formation of a carbonic acid polymorph in methanolic solution, which is the subject of a forthcoming publication.

Stable solid and aqueous H2CO3 from CO2 and H2O at high pressure and high temperature

Carbonic acid (H₂CO₃) forms in small amounts when CO₂ dissolves in H₂O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H₂CO₃ plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO₂/H₂O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H₂CO₃ inside celestial bodies. We present a novel method to prepare solid H₂CO₃ by heating CO₂/H₂O mixtures at high pressure with a CO₂ laser. Furthermore, we found that, contrary to present understanding, neutral H₂CO₃ is a significant component in aqueous CO₂ solutions above 2.4 GPa and 110 °C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C–O–H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H₂CO₃, a role in subduction related phenomena is inferred.

Real-time observation of carbonic acid formation in aqueous solution

Despite the widespread importance of aqueous bicarbonate chemistry, its conjugate acid, carbonic acid, has remained uncharacterized in solution. Here we report the generation of deuterated carbonic acid in deuterium oxide solution by ultrafast protonation of bicarbonate and its persistence for nanoseconds. We follow the reaction dynamics upon photoexcitation of a photoacid by monitoring infrared-active marker modes with femtosecond time resolution. By fitting a kinetic model to the experimental data, we directly obtain the on-contact proton-transfer rate to bicarbonate, previously inaccessible with the use of indirect methods. A Marcus free-energy correlation supports an associated pKa (Ka is the acid dissociation constant) of 3.45 +/- 0.15, which is substantially lower than the value of 6.35 that is commonly assumed on the basis of the overall carbon dioxide-to-bicarbonate equilibrium. This result should spur further exploration of acid-base reactivity in carbon dioxide-rich aqueous environments such as those anticipated under sequestration schemes.

H2CO3(s): a new candidate for CO2 capture and sequestration

To reduce the magnitude of anthropogenic global warming it is necessary to remove CO2(g) from the effluent streams of coal-fired power plants and to sequester the CO2 either as a liquid or by reaction with other compounds. A major difficulty in achieving this goal arises from the very weak acidity of CO2(g), causing it to react only incompletely with weak bases, although this weak interaction does provide a means for "stripping" the CO2 from the acid-base complex at high temperatures. Reaction with strong bases like Na0H yields more stable complexes, but massive amounts of chemical reactants would need to be purchased and chemical products like NaHCO3 then stored. However, when gas-phase CO2 reacts with the weak base water (or when bicarbonate reacts with strong acid) the unstable product monomeric "H2CO3" can be formed. The free energy required is about 16 kcal/mol in the gas phase and about 10 kcal/mol in aqueous solution. This energy can be supplied by particle or photon excitation and is only a small fraction ofthe energy released when a mole of CH4 is converted to a mole of CO2. Although this monomeric compound is highly unstable, its oligomers are considerably more stable, due to internal H-bonding, with free energies for the larger oligomers in the gas phase which are about 4 kcal/(mol of H2CO3) lower, only about 6 kcal/mol H2CO3 higher than the gas-phase combination of CO2 and H2O at room temperature. Also, at lower temperature the entropic penalty for the oligomer is less and oligomeric H2CO3 becomes stable around the sublimation temperature of dry ice. This indicates that it may be possible to capture gas-phase CO2 directly, using only cheap and abundant H2O as a reactant, and to store the resulting (H2CO3)n as a oligomeric solid at only moderately cold temperatures. These conclusions are based on quantum computations that accurately reproduce the structures, spectra, and stabilities of H2CO3 oligomers. Methods for producing and characterizing the H2CO3 oligomers are discussed. However, some aspects of the proposed scheme are quite speculative and will require additional investigation. Several important questions need to be answered before the feasibility of this procedure on a planetary scale can be assessed, particularly those involving the vapor pressure curve, heat of sublimation, density, and compressibility of (H2CO3)n.

Liquid-like relaxation in hyperquenched water at ≤140 K

Micrometre-sized water droplets were hyperquenched on a solid substrate held at selected temperatures between 150 and 77 K. These samples were characterized by differential scanning calorimetry (DSC) and X-ray diffraction. 140 K is the upper temperature limit to obtain mainly amorphous samples on deposition within 16-37 min. DSC scans of glassy water prepared at 140 K exhibit on heating an endothermic step assignable to glass --> liquid transition, with an onset temperature (T(g)) of 136 +/- 2 K on heating at 30 K min(-1). For T(g) of approximately 136 K, water relaxes during deposition at 140 K for 16 min, moving towards metastable equilibrium. The apparent increase in heat capacity (deltaC(p)) depends, for a given rate of heating, on the rate of prior cooling, and a so-called overshoot develops. 140 K deposits cooled at a rate of 5, 2 or 0.2 K min(-1) show on subsequent reheating at a rate of 30 K min(-1) deltaC(p) values of 0.7, 1.1 and 1.7 J K(-1) mol(-1). This is consistent with liquid-like relaxation at 140 K, and it indicates that different limiting structures are obtained. When these 140 K deposits are in addition annealed at 130 K for 90 min, after slow-cooling at 5, 2 or 0.2 K min(-1), their deltaC(p) values on subsequent reheating are similar to those of hyperquenched glassy water (HGW) deposits made at 77 K and annealed at 130 K. Thus, the previous deltaC(p) value of 1.6 J K(-1) mol(-1) obtained with glassy water samples annealed at 130 K (A. Hallbrucker, E. Mayer and G. P. Johari, Philos. Mag. B, 1989, 60, 179) must be an upper-bound limit because it contains a contribution from an overshoot. The T(g) value of 140 K deposits, which had relaxed during deposition towards metastable equilibrium, is within experimental error the same as that of 140 K deposits annealed in addition at 130 K. This contradicts Yue and Angell's (Y. Yue and C. Angell, Nature, 2004, 427, 717) claim for assigning the endothermic step to a sub-T(g) peak or a "shadow" T(g). Our new data further support the proposed fragile-to-strong transition on cooling liquid water from ambient temperature into the deeply supercooled and glassy state. We also describe in detail experimental aspects to obtain HGW specimens, show the ultrastructure of the deposits using electron microscopy, and discuss the mechanism of our hyperquenching method.

Carbonic acid: an important intermediate in the surface chemistry of calcium carbonate

Calcium carbonate is an important and ubiquitous component of biological and geochemical systems. In this study, the surface chemistry of calcium carbonate with several trace atmospheric gases including HNO3, SO2, HCOOH, and CH3COOH is investigated with infrared spectroscopy. Adsorbed carbonic acid, H2CO3, is found to be an intermediate in these reactions. In the absence of adsorbed water, carbonic acid is stable on the surface at room temperature. However, upon water adsorption, carbonic acid dissociates as indicated by the evolution of gaseous CO2 and the disappearance of infrared absorption bands associated with adsorbed carbonic acid. Thus, it is postulated that under ambient conditions, carbonic acid may be an important albeit short-lived intermediate in the surface chemistry of calcium carbonate.