61st Annual Report on Research 2016

Thermomechanical Reactions of Volatiles in Deep Earth Environments: Carbon Reduction by Hydrogen at High Pressures Choong-Shik Yoo (PI), Washington State University, Pullman, WA99164 Summary: This project is to investigate the formation and evolution of abiotic hydrocarbons at high pressure-temperature conditions deep in the Earth's interior. In this reporting period, we have investigated thermomechanical reduction of carbon (graphite) by hydrogen in diamond anvil cell, using in-situ Raman spectroscopy and synchrotron x-ray diffraction. The results show clear evidences for the pressure-induced hydrogen intercalation into graphite at 5 GPa and a series of phase transitions to 2D hetero-layered structures and 3D hydrocarbons. These results provide important constraints for high-pressure or low-temperature storage of solid hydrogen in graphite as well as other interesting phenomena related to low-dimensional hydrogen-rich compounds. Research Objectives: Hydrocarbons are fundamental to materials synthesis, technology applications, and energy resources. They are the main building blocks of petroleum and natural gas products. While most of the hydrocarbons in petroleum result from the decomposition of living organisms in the Earth's sediments (a biogenic origin), some particularly simple forms of methane and small hydrocarbons are also known to form through purely 'inorganic' chemical reactions among Earth's minerals and 'volatiles' such as water and carbon dioxide at various depth of the mantle (an abiogenic origin). However, the mechanisms of these thermomechanical reactions and evolutions of simple hydrocarbons to more complex hydrocarbons are not well understood. Similarly, the significance of these reactions to natural energy resources such as 'rock' oils, shale gases, and carbon budgets are not fully understood. To remedy this situation, we must first understand the detailed chemical mechanism to form abiotic hydrocarbons and the thermomechanical and kinetic properties to control the evolution of these hydrocarbons in the Earth's interior at high pressure and temperature with various minerals and volatiles. Progress in this reporting period: We have investigated the most fundamental process to form hydrocarbons; that is, carbon reduction by hydrogen. In this study, we have considered low-dimensional carbons such as graphite, graphene, and nanotubes, which have high interfacial and surface energies and provide enhanced opportunities to develop new materials and control the structure and properties. Carbon atoms on the surface, for example, can serve as an atomistic template for growing metastable structures modulated by the specific carbon lattice. At the same time, they can react with other chemicals to produce new hybridized nanoparticles of carbon in various metastable structures. The large open structure of low-dimensional carbons, on the other hand, is subject to chemical diffusion in the presence of small molecules and ions, which can produce new intercalated, encapsulated, and doped hydrocarbon structures at high pressures and temperatures. The Raman spectrum consists of single vibron at 4.6 GPa - same with that of pure hydrogen vibron ?0 at 4204.1 cm-1 and two additional peaks ?_0^1 and ?_0^2 at the higher frequency tail of hydrogen vibron ?0, when measured in the graphite-rich area (Fig. 1). These side bands are, however, absent in the H2 rich area, which signifies their origin from intercalated hydrogen in the graphite. Such intercalation is absent in fluid H2 below 5 GPa. We attribute the former to a large intermolecular distance of hydrogen fluids (~3.64 ) with respect to the interlayer distance (~3.13  at 5 GPa). Note that two hydrogen bands from intercalated hydrogen (at 4252.4 cm-1 and 4269.7 cm-1 at 9.8 GPa) are significantly shifted toward the blue (or higher frequencies) from that of bulk hydrogen (at 4228.1 cm-1). The strong blue shifts of these bands indicate the repulsive interaction between hydrogen and graphite, which increases with increasing pressure as shown in Fig. 2. It is also interesting to note that the frequency difference has a linear dependence on pressure. The repulsive interaction between the ?* electrons of graphite and the ? electrons of hydrogen would confine the intercalated hydrogen molecules at the center of hexagonal carbons rings of the graphitic layers and elevate the internal pressure acting on intercalated H2 molecules. This in turns results in intercalated hydrogen molecules to arrange a hexagonal structure following the interstitial lattice points of hexagonal graphitic layers, which will split the hydrogen vibron no into two ?_0^1 and ?_0^2, as observed. The x-ray data also support the hydrogen intercalation at ~5 GPa, abruptly expanding the c-axis of graphite as shown in Fig. 3. The intercalation occurs over a large pressure range to ~50 GPa, where all sp2-hybridized in graphite convert into sp3-hybridized carbons. The pressure-induced x-ray diffraction changes (inset) in fact indicate that hydrogen intercalated graphite undergoes a series of phase transitions eventually to diamond-like structures. However, the calculated density is substantially lower than that of pure diamond, presumably due to the incorporation of hydrogen molecules in carbons to form hetero 2D layered carbon-hydrogen materials and/or 3D hydrocarbons. We plan to perform neutron diffraction studies to elucidate the structural details. Significance of the Result: The present result provides some constraints for high-pressure or low-temperature storage of solid hydrogen in graphite, as well as the synthesis of novel 2D hetero-layered structure that can convert to 3D hydrocarbons at elevated temperatures. Furthermore, because of hydrogen's unparalleled electron/ion mobility and vibration frequency, it is likely that doping hydrogen in low-dimensional carbons can give rise to superionicity and superconductivity as observed in alkali metal intercalated graphite and Ca-intercalated graphene bilayers. Figure 1 (a) High-frequency Raman spectra of graphite-H2 mixture (Inset) to 57 GPa. (b) Pressure-induced shifts of the surrounding pure H2 vibron (?0) and H2 vibrons (?01 and ?02) in graphite. The inset plots the pressure dependence of the vibron frequency difference. Figure 2 (a) The pressure dependent lattice changes of graphite, showing an abrupt increase of the c-axis upon the intercalation of solid hydrogen (Inset). (b) X-ray diffraction patterns of graphite and H2 mixtures, showing phase transitions at ~15 and 45 GPa to carbon-hydrogen structures. Table of Content Image: Micro-photographs of hydrogen intercalated graphite in DAC at (a) 9.8, (b) 24, (c) 36, and (d) 57 GPa, showing a gradual transition of sp2 graphite (opaque) to sp3 diamond-like (transparent) structure.