90 research outputs found
Unified non-equilibrium simulation methodology for flow through nanoporous carbon membrane
The emergence of new nanoporous materials, based e.g. on 2D materials, offers
new avenues for water filtration and energy. There is accordingly a need to
investigate the molecular mechanisms at the root of the advanced performances
of these systems in terms of nanofluidic and ionic transport. In this work, we
introduce a novel unified methodology for Non-Equilibrium classical Molecular
Dynamic simulations (NEMD), allowing to apply likewise pressure, chemical
potential and voltage drops across nanoporous membranes and quantifying the
resulting observables characterizing confined liquid transport under such
external stimuli. We apply the NEMD methodology to study a new type of
synthetic Carbon NanoMembranes (CNM), which have recently shown outstanding
performances for desalination, keeping high water permeability while
maintaining full salt rejection. The high water permeance of CNM, as measured
experimentally, is shown to originate in prominent entrance effects associated
with negligible friction inside the nanopore. Beyond, our methodology allows to
fully calculate the symmetric transport matrix and the cross-phenomena such as
electro-osmosis, diffusio-osmosis, streaming currents, etc. In particular, we
predict a large diffusio-osmotic current across the CNM pore under
concentration gradient, despite the absence of surface charges. This suggests
that CNMs are outstanding candidates as alternative, scalable membranes for
osmotic energy harvesting.Comment: 13 pages, 16 figures, submitted to J. Chem Phy
Coupled interactions at the ionic graphene/water interface
We compute ionic free energy adsorption profiles at aqueous graphene
interface by developing a self-consistent approach. To do so, we design a
microscopic model for water and put the liquid on an equal footing with the
graphene described by its electronic band structure. By evaluating
progressively the electronic/dipolar coupled electrostatic interactions, we
show that the coupling level including mutual graphene/water screening permits
to recover remarkably the precision of extensive quantum simulations. We
further derive the potential of mean force evolution of several alkali cations
to gain insight on the ionic size effects
Enhanced interfacial water dissociation on a hydrated iron porphyrin single-atom catalyst in graphene
Single Atom Catalysis (SAC) is an expanding field of heterogeneous catalysis in which single metallic atoms embedded in different materials catalyze a chemical reaction, but these new catalytic materials still lack fundamental understanding when used in electrochemical environments. Recent characterizations of non-noble metals like Fe deposited on N-doped graphitic materials have evidenced two types of Fe-N4 fourfold coordination, either of pyridine type or of porphyrin type. Here, we study these defects embedded in a graphene sheet and immersed in an explicit aqueous medium at the quantum level. While the Fe-pyridine SAC model is clear cut and widely studied, it is not the case for the Fe-porphyrin SAC that remains ill-defined, because of the necessary embedding of odd-membered rings in graphene. We first propose an atomistic model for the Fe-porphyrin SAC. Using spin-polarized ab initio molecular dynamics, we show that both Fe SACs spontaneously adsorb two interfacial water molecules from the solvent on opposite sides. Interestingly, we unveil a different catalytic reactivity of the two hydrated SAC motives: while the Fe-porphyrin defect eventually dissociates an adsorbed water molecule under a moderate external electric field, the Fe-pyridine defect does not convey water dissociation
Pyridine Adsorption on Single-Layer Iron Phthalocyanine on Au(111)
The adsorption of pyridine on monolayers of well-ordered, flat-lying iron phthalocyanine molecules on Au(111) is investigated by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and density functional theory. It is found that pyridine both coordinates to the iron site of iron phthalocyanine and binds weakly to other sites. The iron coordination causes significant changes in the electronic structure of the iron phthalocyanine compound, with the implication of a change of the spin properties of the iron atoms due to the strong ligand field created by the pyridine axial ligand. Both low coverages and multilayer coverages of pyridine are considered. At low doses, the pyridine molecules are ordered, whereas in multilayers, no preferred orientation is observed. The orientation of the FePc molecules with respect to the Au(111) surface is not affected by the adsorption of pyridine
Ammonia adsorption on iron phthalocyanine on Au(111): Influence on adsorbate-substrate coupling and molecular spin.
The adsorption of ammonia on Au(111)-supported monolayers of iron phthalocyanine has been investigated by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory calculations. The ammonia-induced changes of the x-ray photoemission lines show that a dative bond is formed between ammonia and the iron center of the phthalocyanine molecules, and that the local spin on the iron atom is quenched. This is confirmed by density functional theory, which also shows that the bond between the iron center of the metalorganic complex and the Au(111) substrate is weakened upon adsorption of ammonia. The experimental results further show that additional adsorption sites exist for ammonia on the iron phthalocyanine monolayer
Is graphene on Ru(0001) a nanomesh?
The electronic structure of a single layer graphene on Ru(0001) is compared
with that of a single layer hexagonal boron nitride nanomesh on Ru(0001). Both
are corrugated sp2 networks and display a pi-band gap at the K point of their 1
x 1 Brillouin zone. Graphene has a distinct Fermi surface which indicates that
0.1 electrons are transferred per 1 x 1 unit cell. Photoemission from adsorbed
xenon identifies two distinct Xe 5p1/2 lines, separated by 240 meV, which
reveals a corrugated electrostatic potential energy surface. These two Xe
species are related to the topography of the system and have different
desorption energies.Comment: 5 pages, 4 figures, 1 tabl
Direct quantitative identification of the "surface trans-effect"
The strong parallels between coordination chemistry and adsorption on metal surfaces, with molecules and ligands forming local bonds to individual atoms within a metal surface, have been established over many years of study. The recently proposed "surface trans-effect" (STE) appears to be a further manifestation of this analogous behaviour, but so far the true nature of the modified molecule-metal surface bonding has been unclear. The STE could play an important role in determining the reactivities of surface-supported metal-organic complexes, influencing the design of systems for future applications. However, the current understanding of this effect is incomplete and lacks reliable structural parameters with which to benchmark theoretical calculations. Using X-ray standing waves, we demonstrate that ligation of ammonia and water to iron phthalocyanine (FePc) on Ag(111) increases the adsorption height of the central Fe atom; dispersion corrected density functional theory calculations accurately model this structural effect. The calculated charge redistribution in the FePc/H2O electronic structure induced by adsorption shows an accumulation of charge along the σ-bonding direction between the surface, the Fe atom and the water molecule, similar to the redistribution caused by ammonia. This apparent σ-donor nature of the observed STE on Ag(111) is shown to involve bonding to the delocalised metal surface electrons rather than local bonding to one or more surface atoms, thus indicating that this is a true surface trans-effect
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