Quantum study of hydrogen–oxygen–graphite interactions

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

“…The fact that molecular hydrogen binds to a graphite surface in a physisorption site, atomic hydrogen binds in a chemisorption site and molecular hydrogen dissociatively absorbs onto the surface is well established in literature and we therefore believe these processes can be trusted. 39,53,[57][58]62 It's worth noting we used a value of 14,000 K for the binding energy of atomic hydrogen in accordance with the work of Cazaux et al 51 , this value is likely quite high, Petucci et al 63 found the entrance barrier for atomic hydrogen to chemisorb on graphite to be .2 eV and this value seems to be well established by multiple groups and should be adjusted accordingly. 39,53,[57][58]62 Considering these new chemisorption processes in this model only decreased the rate of formation roughly an order of magnitude, which is still much higher then what has been found using the Monte Carlo approach by Iqbal et al 25 It's important to note Iqbal et al used a different hydrogen density, varied the grain size in their model and considered that species bind in a physisorbed precursor state before diffusing to a chemisorbed site.…”

“…One is the binding energy of atomic oxygen (BO), which according to Jelea et al binds to the surface of a grain with a binding energy of 29,046 K. 53 Having this high of a binding energy causes some oxygen to be accreted onto the grain (see Figures A1-A6 in appendix for fractional abundances of species on the grain surface). As a result atomic oxygen will be depleted from the gas phase until it has enough energy to thermally desorb, which seem to take place at 500 K. The second contributing factor to the change in atomic oxygen in the gas phase has to do with gas-grain formation routes for OH and H 2 O.…”

“…As a result atomic oxygen will be depleted from the gas phase until it has enough energy to thermally desorb, which seem to take place at 500 K. The second contributing factor to the change in atomic oxygen in the gas phase has to do with gas-grain formation routes for OH and H 2 O. According to Jelea et al H 2 O weakly binds to the grain surface in a physisorption site with a binding energy of 5700 K and OH binds to the surface in a chemisorption site with a slightly higher binding energy of 6074 K. 53 In both cases there isn't an appreciable amount of either species on the surface to make the grain-surface reaction efficient enough to compete with the gas phase reaction pathways from 500-800 K. However, OH forms efficiently from reactive desorption from 200-400 K since both atomic hydrogen and oxygen bind to the grain surface in a chemisorption site. Atomic oxygen being bound to the grain surface in a chemisorption site is well established, as is molecular oxygen being bound in a physisorption site.…”

“…Molecular hydrogen is weakly bound to the grain surface with a binding energy of 440 K. The activation energy for reaction 3.2 is taken from work by Jelea et al for the chemisorption case, for physisorption the reaction is also assumed to be barrierless. 53 Molecular hydrogen has been known to be over produced in the gas phase using the rate equation approach. 25, 49-51 Iqbal et al explored the formation of molecular hydrogen using the monte carlo approach, where they considered both chemisorption and physisorption sites.…”

“…According to Jelea et al OH binds to the surface in a chemisorption site with a slightly higher binding energy of 6074 K. 53 In both cases there isn't an appreciable amount of either species on the surface to make the grain-surface reaction efficient enough to compete with the gas phase reaction pathways at these temperatures. From 200-400 K the change in abundance of OH is due to the following reactive desorption reaction showing up as a secondary formation route:…”