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Understanding H2 Formation on Hydroxylated Pyroxene Nanoclusters: Ab Initio Study of the Reaction Energetics and Kinetics
journal contribution
posted on 2019-10-18, 13:37 authored by Boutheïna Kerkeni, Marie-Christine Bacchus-Montabonel, Xiao Shan, Stefan T. BromleyThe
rate constants of H2 formation on five models of
silicate nanoclusters with varying degrees of hydroxylation, (Mg4Si4O12)(H2O)N, were computed over a wide temperature range [180–2000
K]. We tested nine combinations of density functional methods and
basis sets for their suitability for calculating reaction energies
and barrier heights, and we computed the minimum energy H + H →
H2 reaction paths on each nanocluster. Subsequently, we
computed the rate constants employing three semiclassical approaches
that take into account tunneling and nonclassical reflection effects
by means of the zero curvature tunneling (ZCT), the small curvature
tunneling (SCT), and the one-dimensional semiclassical transition
state theory (SCTST) methods, which all provided comparable results.
Our investigations show that the H2 formation process following
the Langmuir–Hinshelwood (LH) mechanism is more efficient on
the hydroxylated (N = 1–4) nanoclusters than
on the bare (N = 0) one due to relatively higher
reaction barrier height on the latter. H2 formation is
found to have the smallest barrier and the most exothermic reaction
for the moderately hydroxylated (Mg4Si4O12)(H2O)2 nanocluster for all nine considered
methods. Overall, we conclude that all the considered nanoclusters
are very efficient catalyzing grains for H2 formation in
the physical conditions of the interstellar medium (ISM) with pyroxene
nanosilicates having moderate to high hydroxylation being more efficient
than bare nanograins.