Formation of H2 on an olivine surface: a computational study

Details Formation of H2 on an olivine surface: a computational study Formation of H2 on an olivine surface: a computational study

Mar 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009mnras.393.1403g&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 393, Issue 4, pp. 1403-1407.

Astronomy and Astrophysics

Astronomy

5

Astrochemistry, Molecular Processes, Ism: Molecules

Scientific paper

The formation of H2 on a pristine olivine surface [forsterite (010)] is investigated computationally. Calculations show that the forsterite surface catalyzes H2 formation by providing chemisorption sites for H atoms. The chemisorption route allows for stepwise release of the reaction exothermicity and stronger coupling to the surface, which increases the efficiency of energy dissipation. This suggests that H2 formed on a pristine olivine surface should be much less rovibrationally excited than H2 formed on a graphite surface. Gas-phase H atoms impinging on the surface will first physisorb relatively strongly (Ephys = 1240 K). The H atom can then migrate via desorption and re-adsorption, with a barrier equal to the adsorption energy. The barrier for a physisorbed H atom to become chemisorbed is equal to the physisorption energy, therefore there is almost no gas-phase barrier to chemisorption. An impinging gas-phase H atom can easily chemisorb (Echem = 12200 K), creating a defect where a silicate O atom is protonated and a single electron resides on the surface above the adjacent magnesium ion. This defect directs any subsequent impinging H atoms to chemisorb strongly (39 800 K) on the surface electron site. The two adjacent chemisorbed atoms can subsequently recombine to form H2 via a barrier (5610 K) that is lower than the chemisorption energy of the second H atom. Alternatively, the adsorbed surface species can react with another incoming H atom to yield H2 and regenerate the surface electron site. This double chemisorption `relay mechanism' catalyzes H2 formation on the olivine surface and is expected to attenuate the rovibrational excitation of H2 thus formed.

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