The role of Fischer‐Tropsch catalysis in solar nebula chemistry

Abstract

Abstract-Fischer-Tropsch catalysis, the ironhickel catalyzed conversion of CO and H2 to hydrocarbons, would have been the only thermally-driven pathway available in the solar nebula to convert CO into other forms of carbon. A major issue in meteoritics is to determine the origin of meteoritic organics: are they mainly formed from CO in the solar nebula via a process such as Fischer-Tropsch, or are they derived from interstellar organics? In order to determine the role that Fischer-Tropsch catalysis may have played in the organic chemical evolution of the solar nebula, we have developed a kinetic model for this process. Our model results agree well with experimental data from several existing laboratory studies. In contrast, empirical rate equations, which have been derived from experimental rate data for a limited temperature (7") and pressure (P) range, are inconsistent with experimental rate data for higher T and lower P.We have applied our model to pressure and temperature profiles for the solar nebula, during the epoch in which meteorite parent bodies condensed and agglomerated. We find that, under nebular conditions, the conversion rate of CO to CH4 does not simply increase with temperature as the empirically-derived equations suggest. Instead, our model results show that this process would have been most efficient in a fairly narrow region that coincides with the present position of the asteroid belt. Our results support the hypothesis that Fischer-Tropsch catalysis may have played a role in solar nebula chemistry by converting CO into less processed in the nebula and in parent bodies.

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…4,[8][9] This process is believed to be the most efficient way to convert CO into other forms of alkanes under high temperature regimes in astrophysical environments. 3 Previous studies by Nuth et al have found that when the surface of the grain becomes poisoned the surface acts as a better catalyst then the bare grain itself. Under normal circumstances decreasing the surface area of a catalyst will decrease the reaction rate but what they have found is by building up macromolecular grain coating the surface area of the catalyst essentially increases rather than being rendered unreactive.…”

“…17 Physisorption processes have already been treated at low temperatures, where atoms and molecules don't have sufficient energy to cross into a chemisorption well, so we'll be looking at high temperature grain reactions where chemisorption is believed to dominate. 1,3,19 We'll be looking at gas phase temperatures of up to 800 K, and grain temperatures equal to gas temperatures. This is expected to have implications on the way the chemistry is treated in each step as well as increased binding energies with respect to the physisorption values.…”

“…This is expected to have implications on the way the chemistry is treated in each step as well as increased binding energies with respect to the physisorption values. 3 We're expecting a change in fractional abundances that, when augmented with both observational data and our knowledge of astrophysical environments, will offer insight into the importance of these new processes. There are three general types of surface reactions: Langmuir-Hinshelwood (LH) mechanisms, Eley-Rideal (ER) mechanism and hot atom mechanisms ( Figure 1.2 above).…”

“…Fischer-Tropsch catalysis is the family of surface reactions in which carbon monoxide (CO) and molecular hydrogen (H 2 ) are converted to alkanes and alkenes using a catalyst. 3,23 This catalyst is typically a transition metal such as iron or nickel but here the dust grain will act as the catalyst. 4,[8][9] This process is believed to be the most efficient way to convert CO into other forms of alkanes under high temperature regimes in astrophysical environments.…”

“…[3][4] Although the effect of gas-surface reactions has been studied in industry and academia, to our knowledge it hasn't been explored in any great detail in astrophysical environments . 4 We will attempt to theoretically model high temperature-gas grain chemistry using a rate equation approach where chemisorption sites will be considered.…”