Formation of H 2 on an olivine surface: a computational study

The formation of H 2 on a pristine olivine surface [forsterite (010)] is investigated computationally. Calculations show that the forsterite surface catalyzes H 2 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 H 2 formed on a pristine olivine surface should be much less rovibrationally excited than H 2 formed on a graphite surface. Gas‐phase H atoms impinging on the surface will first physisorb relatively strongly ( E phys = 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 ( E chem = 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 H 2 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 H 2 and regenerate the surface electron site. This double chemisorption ‘relay mechanism’ catalyzes H 2 formation on the olivine surface and is expected to attenuate the rovibrational excitation of H 2 thus formed.