The dissociation of water on 2D monolayer molybdenum disulfide (MoS 2 ) edges was studied with density functional theory. The catalytically active sites for H 2 O, H, and OH adsorption on MoS 2 edges with 0% (Mo-edge), 50% (S50-edge), and 100% (S100-edge) sulfur coverage were determined, and the Mo-edge was found to be the most favorable for adsorption of all species. The water dissociation reaction was then simulated on all edges using the climbing image nudged elastic band (CI-NEB) technique. The reaction was found to be endothermic on the S100-edge and exothermic for the S50-and Mo-edges, with the Mo-edge having the lowest activation energy barrier. Water dissociation was then explored on the Mo-edge using metadynamics biased ab initio molecular dynamics (AIMD) methods to explore the reaction mechanism at finite temperature. These simulations revealed that water dissociation can proceed by two mechanisms: the first by splitting into adsorbed OH and H species produced a particularly small activation free energy barrier of 0.06 eV (5.89 kJ/mol), and the second by formation of desorbed H 2 and adsorbed O atom had a higher activation barrier of 0.36 eV (34.74 kJ/mol) which was nevertheless relatively small. These activation barrier results, along with reaction rate calculations, suggest that water dissociation will occur spontaneously at room temperature on the Mo-edge.
The
recent discovery of frustrated Lewis pairs (FLPs) capable of
heterolytically splitting hydrogen gas at the surface of hydroxylated
indium oxide (In2O3–x
(OH)
y
) nanoparticles has led to interesting
implications for heterogeneous catalytic reduction of CO2. Although the role of surface FLPs in the reverse water-gas shift
(RWGS) reaction (CO2 + H2 → CO + H2O) has been experimentally and theoretically demonstrated,
the interplay between surface FLPs and temperature and their consequences
for the reaction mechanism have yet to be understood. Here we use
well-tempered metadynamics-biased ab initio molecular dynamics to
obtain the free energy landscape of the multistep RWGS reaction at
finite temperatures. The reaction is simulated at 20 and 180 °C,
and the minimum energy reaction pathways and energy barriers corresponding
to H2 dissociation and CO2 reduction are obtained.
The reduction of CO2 at the surface FLP catalytically active
site, where H2 is heterolytically dissociated and bound,
is found to be the rate-limiting step and is mostly unaffected by
increased temperature conditions; however, at 180 °C the energetic
barriers associated with the splitting of H2 and the subsequent
adsorption of CO2 are reduced by 0.15 and 0.19 eV, respectively.
It is suggested that increased thermal conditions may enhance reactivity
by enabling the surface FLP to become further spatially separated.
Product H2O is found to favor dissociative adsorption over
direct desorption from the surface of In2O3–x
(OH)
y
and may therefore
impede sustained catalytic activity by blocking surface sites.
In this work, the tribological behavior of ultrathin-MoS2 was investigated to understand the independent roles of water and oxidation. Water adsorption was identified as the primary interfacial mechanism for both SiO2/pristine-MoS2 and SiO2/graphene interfaces, however, tribological behavior of pristine-MoS2 was observed to be more sensitive to presence of water due to stronger MoS2-water interaction.Comparison of pristine-MoS2 and oxidized-MoS2 revealed that the oxidation of MoS2 significantly increased its friction and sensitivity to water by play a more detrimental role. The specific effect of oxygen on friction via chemical interactions was studied in isolation through density functional theory (DFT) simulations of a tip sliding on MoS2 basal planes and over edges before and after oxidation. The maximum change in energy, or energy barrier correlating with friction, as the tip moved across the surface, increased after oxidation by up to 66% for the basal plane and by 25% at the edge. Charge density analysis suggests that the more localized and non-uniform interfacial charge distribution on oxygen rich surfaces, as compared to pristine surfaces, leads to higher resistance to sliding. This confirms that oxygen presence alone increases friction and when coupled with the presence of water, both effects are additive in increasing friction.
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