Formation of H2 on an olivine surface: a computational study

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= 12 200 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.