HR: 0830h
AN: OS31C-0216    [PDF]
TI: Hydrothermal Reduction of Carbon Dioxide at $250\deg$C and 500 bar
AU: * Fu, Q
EM: fuxx0033@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr.SE, Minneapolis, MN 55455-0219 United States
AU: Seyfried, W E
EM: wes@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr.SE, Minneapolis, MN 55455-0219 United States
AB: Fischer-Tropsch type catalysis reactions are believed to play important roles in abiotic synthesis of organic compounds in mid-ocean ridge vent fluids. However, little is known about the mechanisms of these processes, especially the reaction intermediates in the formation of abiotic hydrocarbons from reduction of CO$_{2}$. To better understand abiotic synthesis of hydrocarbons, an experiment involving CO$_{2}$ reduction was carried out in a flexible-Au/Ti-cell hydrothermal solution apparatus at $250\deg$C, 500 bar. To limit sources of contamination and simplify the pathway of this reduction process, this experiment was conducted by injecting CO$_{2}$ and then H$_{2}$ into the reaction cell, instead of using organic acids as surrogate sources. Magnetite was used as a catalyst for organic synthesis. To exclude organic contaminants on the catalyst and their potential effects on this reduction process, however, the magnetite was reacted with hydrogen peroxide prior to the experiment. Subsequently, H$_{2}$ was injected into the reaction cell to restore the magnetite structure. Dissolved CO$_{2}$ (aq) (35 mmol/kg) was then injected into the reaction cell. This was followed by systematic addition of dissolved H$_{2}$. With the addition of H$_{2}$ into the system, trace amounts of dissolved CO and CH$_{4}$ were produced. 7 $\mu$mol/kg formate (HCOO$^{-}$) and 8 $\mu$mol/kg acetate (CH$_{3}$COO$^{-}$) were also detected by ion chromatography. More formate was produced with adding more H$_{2}$. When injecting H$_{2}$ to a final concentration of 11 mmol/kg, the concentration of formate increased to 0.16 mmol/kg, CO and CH$_{4}$ remain constant, while CO$_{2}$ decreased to 4 mmol/kg, and no acetate was observed. Surface compositional analysis of magnetite was conducted using X-ray Photoelectron Spectroscopy (XPS). The concentration of carbon increased following reaction. The prominent C$_{1s}$ peak from the reacted magnetite occurred at 284.5 eV, which corresponds to graphite. The subsidiary peak was consistent with that expected for trace carbon on metal surfaces, which also can be seen in the unreacted magnetite. Results suggest that at $250\deg$C, 500 bar, graphite and formate exist as two primary reduced carbon phases in the system, while the amount of CH$_{4}$ and CO were much less than predicted from theoretical data. We speculate that the coating of graphite on the magnetite surface and concomitant loss of magnetite catalytic activity prevent the formation of methane. A similar experiment with temperature stepping scheme (from 250 to $400\deg$C) is in progress. It will allow us to assess the potential contribution of reduced carbon phase formation to the abiotic synthesis of organic compounds under seafloor hydrothermal conditions.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3035 Midocean ridge processes
DE: 4850 Organic marine chemistry
DE: 4851 Oxidation/reduction reactions
DE: 8135 Hydrothermal systems (8424)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting