In-situ abiogenic methane synthesis from diamond and graphite under geologically relevant conditions

High pressure study of sodium trihydride

The reactivity between NaH and H2 has been investigated through a series of high-temperature experiments up to pressures of 78 GPa in diamond anvil cells combined with first principles calculations. Powder X-ray diffraction measurements show that heating NaH in an excess of H2 to temperatures around 2000 K above 27 GPa yields sodium trihydride (NaH3), which adopts an orthorhombic structure (space group Cmcm). Raman spectroscopy measurements indicate that NaH3 hosts quasi-molecular hydrogen (H 2 δ - ) within a NaH lattice, with the H 2 δ - stretching mode downshifted compared to pure H2 (Δν ∼-120 cm-1 at 50 GPa). NaH3 is stable under room temperature compression to at least 78 GPa, and exhibits remarkable P-T stability, decomposing at pressures below 18 GPa. Contrary to previous experimental and theoretical studies, heating NaH (or NaH3) in excess H2 between 27 and 75 GPa does not promote further hydrogenation to form sodium polyhydrides other than NaH3.

Hydrogenation of different carbon substrates into light hydrocarbons by ball milling

The conversion of carbon-based solids, like non-recyclable plastics, biomass, and coal, into small molecules appears attractive from different points of view. However, the strong carbon-carbon bonds in these substances pose a severe obstacle, and thus-if such reactions are possible at all-high temperatures are required1-5. The Bergius process for coal conversion to hydrocarbons requires temperatures above 450 °C6, pyrolysis of different polymers to pyrolysis oil is also typically carried out at similar temperatures7,8. We have now discovered that efficient hydrogenation of different solid substrates with the carbon-based backbone to light hydrocarbons can be achieved at room temperature by ball milling. This mechanocatalytic method is surprisingly effective for a broad range of different carbon substrates, including even diamond. The reaction is found to proceed via a radical mechanism, as demonstrated by reactions in the presence of radical scavengers. This finding also adds to the currently limited knowledge in understanding mechanisms of reactions induced by ball milling. The results, guided by the insight into the mechanism, could induce more extended exploration to broaden the application scope and help to address the problem of plastic waste by a mechanocatalytic approach.

Massive abiotic methane production in eclogite during cold subduction

Methane (CH₄) is a critical but overlooked component in the study of the deep carbon cycle. Abiotic CH₄ produced by serpentinization of ultramafic rocks has received extensive attention, but its formation and flux in mafic rocks during subduction remain poorly understood. Here, we report massive CH₄-rich fluid inclusions in well-zoned garnet from eclogites in Western Tianshan, China. Petrological characteristics and carbon-hydrogen isotopic compositions confirm the abiotic origin of this CH₄. Reconstructed P-T-fO₂-fluid trajectories and Deep Earth Water modeling imply that massive abiotic CH₄ was generated during cold subduction at depths of 50-120 km, whereas CO₂ was produced during exhumation. The massive production of abiotic CH₄ in eclogites may result from multiple mechanisms during prograde high pressure-ultrahigh pressure metamorphism. Our flux calculation proposes that abiotic CH₄ that has been formed in HP-UHP eclogites in cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH₄ on Earth.

Formation and Stability of Dense Methane-Hydrogen Compounds

Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH_{4}-H_{2} system. We find that pressures as low as 4.8 GPa can stabilize CH_{4}(H_{2})_{2} and (CH_{4})_{2}H_{2}, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H_{2} units within the structure. On further compression, a unique structural composition, (CH_{4})_{3}(H_{2})_{25}, emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H_{2}. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GPa.