Induced Migration of CO2 from Hydrate Cages to Amorphous Solid Water under Ultrahigh Vacuum and Cryogenic Conditions

Understanding the Kinetics of CO2 Hydrate Formation in Dry Water for Carbon Capture and Storage: X-ray Diffraction and In Situ Raman Studies

Hydrate-based carbon capture and storage (HBCS) is a sustainable and promising approach to combating global warming by utilizing water, which is a ubiquitous resource. Here, we report a comprehensive study of CO2 hydrate formation in dry water (DW), a water-in-air dispersion confined in silica particles, for improving the kinetics of hydrate growth. Utilizing a combination of a home-built high-pressure reactor, in situ Raman spectroscopy, and powder X-ray diffraction (PXRD), we elucidate the crystal structure, growth dynamics, and morphology of CO2 hydrates formed in DW, with and without the kinetic hydrate promoter, l-tryptophan. Our analysis reveals that CO2 forms structure I (sI) hydrate in DW, with hydrate growth occurring both on and beneath the silica shell. This results in a substantial increase in CO2 uptake─approximately 2.8 times higher than that observed in pure water (∼134 v/v compared to ∼48 v/v). Moreover, incorporation of l-tryptophan in DW formation markedly accelerates the DW-CO2 hydrate formation process, reducing both the induction time and the time required to achieve 90% gas uptake at 274.65 K. These findings offer crucial insights into the formation of CO2 hydrate in DW, highlighting its potential to improve the efficiency and scalability of HBCS technologies.

Laboratory Studies of the Clathrate Hydrate Formation in the Carbon Dioxide-Water Mixtures at Interstellar Conditions

This study investigates the formation of carbon dioxide clathrate hydrates under conditions simulating interstellar environments, a process of significant astrophysical and industrial relevance. Clathrate hydrates, where gas molecules are trapped within water ice cages, play an essential role in both carbon sequestration strategies and understanding of the behavior of ices in space. We employed a combination of Fourier Transform Infrared (FTIR) spectroscopy, mass spectrometry, temperature-programmed desorption (TPD), and Density Functional Theory (DFT) calculations to explore thin films of H2O:CO2 ice mixtures with varying CO2 concentrations (5-75%) prepared by vapor deposition at temperatures ranging between 11 and 180 K. The study revealed the influence of CO2 concentration and deposition temperature on the formation mechanism of diverse structures, including clathrate hydrates, polyaggregates, and segregated CO2 groups. Spectral features associated with CO2 encapsulation in clathrate hydrates were observed at 2335, 2349, and 3698 cm-1 in the 5 and 15% mixtures after deposition at 11 K and after warming at temperatures above 100 K. The observed increase in CO2 sublimation temperature to 145-155 K and co-condensation of CO2 molecules at 172 K with water molecules at a pressure of 0.5 μTorr can be attributed to the encapsulation of CO2 molecules within the robust hydrogen-bonded framework of the clathrate cages under specific conditions. These findings enhance our understanding of the intricate processes involved in clathrate and hydrate formation in CO2 and H2O mixtures, shedding light on their physical properties and the dependence of their specific characteristics on the formation method.

Growth of Clathrate Hydrates in Nanoscale Ice Films Observed Using Electron Diffraction and Infrared Spectroscopy

Clathrate hydrates (CHs) are believed to exist in cold regions of space, such as comets and icy moons. While spectroscopic studies have explored their formation under similar laboratory conditions, direct structural characterization using diffraction techniques has remained elusive. We present the first electron diffraction study of tetrahydrofuran (THF) and 1,3-dioxolane (DIOX) CHs in the form of nanometer-thin ice films under an ultrahigh vacuum at cryogenic temperatures. Using reflection high-energy electron diffraction, we show that THF CH grows readily on various substrates during thermal annealing of an amorphous ice mixture of THF and water, and the formation is independent of the nature of the substrate. The growth of DIOX CHs on a Au(111) substrate is similar. A comparison of electron diffraction patterns with calculated X-ray diffraction patterns indicates that THF and DIOX form structure II CH (51264) with a lattice constant of ∼17.2 Å (cubic Fd3̅m). Both CHs were also grown on Ru(0001) and were examined by reflection absorption infrared spectroscopy. A direct comparison of diffraction data to infrared spectra as a function of the temperature further demonstrates the strength of multiple probes in examining complex systems possessing diverse molecular interactions.

Partitioning photochemically formed CO2 into clathrate hydrate under interstellar conditions

Clathrate hydrates (CHs), host-guest compounds of water forming hydrogen-bonded cages around guest molecules, are now known to exist under interstellar conditions. Experimental evidence demonstrated that prolonged thermal treatment of a solid mixture of water and CO2/CH4 produces CHs at 10-30 K under simulated interstellar conditions. However, in the current study, we show that CO2 produced photochemically by vacuum ultraviolet irradiation of H2O-CO mixtures at 10 K and ∼10-10 mbar, gets partitioned into its CH phase and a matrix phase embedded in amorphous ice. The process occurring under simulated interstellar conditions was studied at different temperatures and H2O-CO compositions. The formation of CO2 CH and other photoproducts was confirmed using reflection absorption infrared spectroscopy. The UV-induced photodesorption event of CO2 may provide the mobility required for the formation of CHs, while photoproducts like methanol can stabilize such CH structures. Our study suggests that new species originating during such energetic processing in ice matrices may form CH, potentially altering the chemical composition of astrophysical environments.

Mechanical properties of amorphous CO2 hydrates: insights from molecular simulations

Understanding physicochemical properties of amorphous gas hydrate systems is of great significance to reveal structural stabilities of polycrystalline gas hydrate systems. Furthermore, amorphous gas hydrates can occur ordinarily in the nucleation events of gas hydrate systems. Herein, the mechanical properties of amorphous carbon dioxide hydrates are examined by means of all-atom classical molecular dynamic simulations. Our molecular simulation results reveal that mechanical strengths of amorphous carbon dioxide hydrates are evidently governed by temperatures, confining pressures, and ratios of water to carbon dioxide molecules. Notably, under compressive loads, amorphous carbon dioxide hydrates firstly exhibit monotonic strain hardening, followed by an interesting distinct phenomenon characterized by a steady flow stress at further large deformation strains. Furthermore, structural evolutions of amorphous carbon dioxide hydrates are analyzed on the basis of the N-Hbond DOP order parameter. These important findings can not only contribute to our understanding of the structural stabilities of amorphous gas hydrate systems, but also help to develop fundamental understandings about grain boundaries of gas hydrate systems.

Formation and Transformation of Clathrate Hydrates under Interstellar Conditions

ConspectusContinuing efforts by many research groups have led to the discovery of ∼240 species in the interstellar medium (ISM). Observatory- and laboratory-based astrochemical experiments have led to the discovery of these species, including several complex organic molecules (COMs). Interstellar molecular clouds, consisting of water-rich icy grains, have been recognized as the primordial sources of COMs even at extremely low temperatures (∼10 K). Therefore, it is paramount to understand the chemical processes of this region, which may contribute to the chemical evolution and formation of new planetary systems and the origin of life.This Account discusses our effort to discover clathrate hydrates (CHs) of several molecules and their structural varieties, transformations, and kinetics in a simulated interstellar environment. CHs are nonstochiometric crystalline host-guest complexes in which water molecules form cages of different sizes to entrap guest molecules. CHs are abundant on earth and require moderate temperatures and high pressures for their formation. Our focus has been to form CHs at extremely low pressure and temperature as in the ISM, although their existence under such conditions has been a long-standing question since water and guest molecules (CH4, CO2, CO, etc.) exist in space. In multiple studies conducted at ∼10-10 mbar, we showed that CH4, CO2, and C2H6 hydrates could be formed at 30, 10, and 60 K, respectively. Well-defined IR spectroscopic features supported by quantum chemical simulations and temperature-programmed desorption mass spectrometric analyses confirmed the existence of the 512 (for CH4 and CO2) and 51262 (for C2H6) CH cages. Mild thermal activation for long periods under ultrahigh vacuum (UHV) allowed efficient molecular diffusion, which is crucial for forming CHs. We also explored the formation of THF hydrate (a promoter/stabilizer for binary CHs), and a spontaneous method was found for its formation under UHV. In a subsequent study, we observed a binary THF-CO2 hydrate and its thermal processing at 130 K leading to the transportation of CO2 from the hydrate cages to the matrix of amorphous water. The findings imply that such systems possess a dynamic setting that facilitates the movement of molecules, potentially accounting for the chemical changes observed in the ISM. Furthermore, an intriguing fundamental phenomenon is the consequences of these CHs and their dynamics. We showed that preformed acetone and formaldehyde hydrates dissociate to form cubic (Ic) and hexagonal (Ih) ices at 130-135 K, respectively. These unique processes could be the mechanistic routes for the formation of various ices in astrophysical environments.Other than adding a new entry, namely, CHs, to the list of species found in ISM, its existence opens new directions to astrochemistry, observational astronomy, and astrobiology. Our work provides a molecular-level understanding of the formation pathways of CHs and their transformation to crystalline ices, which sheds light on the chemical evolution of simple molecules to COMs in ISM. Furthermore, CHs can be potential candidates for studies involving radiation, ionization, and electron impact to initiate chemical transformations between the host and guest species and may be critical in understanding the origin of life.