Molecular-scale mechanisms of CO2 mineralization in nanoscale interfacial water films

Nanoscale Understanding on CO2 Diffusion and Adsorption in Clay Matrix Nanopores: Implications for Carbon Geosequestration

Carbon capture and storage (CCS) in subsurface reservoirs represents a highly promising and viable strategy for mitigating global carbon emissions. In the context of CCS implementation, it is particularly crucial to understand the complex molecular diffusive and adsorptive behaviors of anthropogenic carbon dioxide (CO2) in the subsurface at the nanoscale. Yet, conventional molecular models typically represent only single-slit pores and overlook the complexity of interconnected nanopores. In this work, finite kaolinite lamellar assemblages with abundant nanopores (r < 2 nm) were used. Molecular dynamics simulations were performed to quantify the spatial distribution correlations, adsorption preference, diffusivity, and residence time of the CO2 molecules in kaolinite nanopores. The movement of the CO2 molecules primarily occurs in the central and proximity regions of the siloxane surfaces, progressing from larger to smaller nanopores. CO2 prefers smaller nanopores over larger ones. The diffusion coefficients increase, while residence times decrease, with the pore size increasing, differing from typical slit-pore models due to the pore shape and interconnectivity. The perspectives in this study, which would be challenging in conventional slit-pore models, will facilitate our comprehension of the CO2 molecular behaviors in the complex subsurface clay sediments for developing quantitative estimation techniques throughout the CCS project durations.

Insights into the structure and dynamics of K+ ions at the muscovite-water interface from machine learning potential simulations

The surfaces of many minerals are covered by naturally occurring cations that become partially hydrated and can be replaced by hydronium or other cations when the surface is exposed to water or an aqueous solution. These ion exchange processes are relevant to various chemical and transport phenomena, yet elucidating their microscopic details is challenging for both experiments and simulations. In this work, we make a first step in this direction by investigating the behavior of the native K+ ions at the interface between neat water and the muscovite mica (001) surface with ab-initio-based machine learning molecular dynamics and enhanced sampling simulations. Our results show that the desorption of the surface K+ ions in pure ion-free water has a significant free energy barrier irrespective of their local surface arrangement. In contrast, facile K+ diffusion between mica's ditrigonal cavities characterized by different Al/Si orderings is observed. This behavior suggests that the K+ ions may favor a dynamic disordered surface arrangement rather than complete desorption when exposed to deionized water.

Influence of Cations on Direct CO2 Capture and Mineral Film Formation: The Role of KCl and MgCl2 at the Air/Electrolyte/Iron Interface

Uncovering the mechanisms associated with CO2 capture through mineralization is vital for addressing rising CO2 levels. Iron in planetary soils, the mineral cycle, and atmospheric dust react with CO2 through complex surface chemistry. Here, the effect of cations on the growth of carbonate films on iron surfaces was investigated. In situ polarized modulated infrared reflection absorption spectroscopy was used to measure CO2 adsorption and oxidation of iron in MgCl2(aq) and KCl(aq), compared to FeCl2(aq) at the air/electrolyte/iron interface. The cation was found to influence the film composition and growth rates, as corroborated by infrared and photoelectron spectroscopy. In MgCl2(aq), a mixture of hydromagnesite, magnesite, and a Mg hydroxy carbonate film was grown on iron, while in KCl(aq), a potassium-rich bicarbonate film was grown. The cations were found to affect the rates of hydroxylation and carbonation, confirming a specific cation effect on carbonate film growth. In the submerged region, a heterogeneous mixture of lepidocrocite and iron hydroxy carbonate was produced, suggesting that Fe2+ dominates the surface products. Surface roughness measurements from in situ atomic force microscopy indicate iron initially corrodes faster in MgCl2(aq) than KCl(aq), due to the Cl- ions that initiate pitting and corrosion. In this region, cations were not found to affect the morphologies. This study shows surface corrosion is necessary to provide nucleation sites for film growth and that the cations influence the carbonate film, relevant for CO2 capture and planetary processes.

A ReaxFF Potential for Modeling Organic Matter Degradation with Oxybromine Oxidants

Oxidation of organic matter with oxybromine oxidants is ushering in a new era of enhanced hydrocarbon recovery. While these potent reagents are being tested in laboratory and field experiments, there is a pressing demand to delineate the molecular processes governing oxidation reactions at geological depth. Here, we parameterize a ReaxFF potential to model the oxidative decompositions of aliphatic and aromatic hydrocarbons in the presence of water-NaBr solutions that contain oxybromine (BrOn)- oxidizers. Our parameterization results in a reliable empirical bond-order potential that accurately calculates bond energies, exhibiting an RMSE of ∼1.18 eV, corresponding to 1.36 % average error. Reproducing bond dissociation and binding energies from Density Functional Theory (DFT), our parameterization proves transferable to aqueous environments. This H/C/O/Na/Br ReaxFF potential accurately reproduces the oxidation pathways of small hydrocarbons with oxybromine oxidizers. This force field captures proton and oxygen transfer, C-C bond tautomerization, and cleavage, leading to ring-opening and chain fragmentation. Molecular dynamic simulations demonstrate the oxidative degradation of aromatic and aliphatic kerogen-like moieties in bulk solutions. We envision that such reactive force fields will be useful to understand better the oxidation reactions of organic matter formed in geological reservoirs for enhanced shale gas recovery and improved carbon dioxide treatments.

Parts-Per-Million Carbonate Mineral Quantification with Thermogravimetric Analysis-Mass Spectrometry

Mitigating the deleterious effects of climate change requires the development and implementation of carbon capture and storage technologies. To expand the monitoring, verification, and reporting (MRV) capabilities of geologic carbon mineralization projects, we developed a thermogravimetric analysis-mass spectrometry (TGA-MS) methodology to enable quantification of <100 ppm calcite (CaCO3) in complex samples. We extended TGA-MS calcite calibration curves to enable a higher measurement resolution and lower limits of quantification for evolved CO2 from a calcite-corundum mixture. We demonstrated <100 ppm carbonate mineral quantification with TGA-MS for the first time, an outcome applicable across earth, environmental, and materials science fields. We applied this carbonate quantification method to a suite of Columbia River Basalt Group (CRBG) well cuttings recovered in 2009 from Pacific Northwest National Laboratory's Wallula #1 Well. Our execution of this new combined calcite and calcite-corundum calibration curve TGA-MS method on our CRBG sample suite indicated average carbonate contents of 0.050 wt % in flow interiors (caprocks) and 0.400 wt % in interflow zones (reservoirs) in the upper 1250 m of the Wallula #1 Well. By advancing our knowledge of continental flood basalt-hosted carbonates in the mafic subsurface and reaching new TGA-MS quantification limits for carbonate minerals, we expand MRV capabilities and support the commercial-scale deployment of carbon mineralization projects in the Pacific Northwest United States and beyond.

Reaction Layer Formation on MgO in the Presence of Humidity

Mineralization by MgO is an attractive potential strategy for direct air capture (DAC) of CO2 due to its tendency to form carbonate phases upon exposure to water and CO2. Hydration of MgO during this process is typically assumed to not be rate limiting, even at ambient temperatures. However, surface passivation by hydrated phases likely reduces the CO2 capture capacity. Here, we examine the initial hydration reactions that occur on MgO(100) surfaces to determine whether they could potentially impact CO2 uptake. We first used atomic force microscopy (AFM) to explore changes in reaction layers in water (pH = 6 and 12) and MgO-saturated solution (pH = 11) and found the reaction layers on MgO are heterogeneous and nonuniform. To determine how relative humidity (R.H.) affects reactivity, we reacted samples at room temperature in nominally dry N2 (∼11-12% R.H.) for up to 12 h, in humid (>95% R.H.) N2 for 5, 10, and 15 min, and in air at 33 and 75% R.H. for 8 days. X-ray reflectivity and electron microscopy analysis of the samples reveal that hydrated phases form rapidly upon exposure to humid air, but the growth of the hydrated reaction layer slows after its initial formation. Reaction layer thickness is strongly correlated with R.H., with denser reaction layers forming in 75% R.H. compared with 33% R.H. or nominally dry N2. The reaction layers are likely amorphous or poorly crystalline based on grazing incidence X-ray diffraction measurements. After exposure to 75% R.H. in air for 8 days, the reaction layer increases in density as compared to the sample reacted in humid N2 for 5-15 min. This may represent an initial step toward the crystallization of the reaction layer. Overall, high R.H. favors the formation of a hydrated, disordered layer on MgO. Based on our results, DAC in a location with a higher R.H. will be favorable, but growth may slow significantly from initial rates even on short timescales, presumably due to surface passivation.

Acid-Base Chemistry of a Model IrO2 Catalytic Interface

Iridium oxide (IrO2) is one of the most efficient catalytic materials for the oxygen evolution reaction (OER), yet the atomic scale structure of its aqueous interface is largely unknown. Herein, the hydration structure, proton transfer mechanisms, and acid-base properties of the rutile IrO2(110)-water interface are investigated using ab initio based deep neural-network potentials and enhanced sampling simulations. The proton affinities of the different surface sites are characterized by calculating their acid dissociation constants, which yield a point of zero charge in agreement with experiments. A large fraction (≈80%) of adsorbed water dissociation is observed, together with a short lifetime (≈0.5 ns) of the resulting terminal hydroxy groups, due to rapid proton exchanges between adsorbed H2O and adjacent OH species. This rapid surface proton transfer supports the suggestion that the rate-determining step in the OER may not involve proton transfer across the double layer into solution, as indicated by recent experiments.