Deprotonation of formic acid in collisions with a liquid water surface studied by molecular dynamics and metadynamics simulations

Deprotonation of organic acids at aqueous surfaces has important implications in atmospheric chemistry and other disciplines, yet it is not well-characterized or understood. This article explores the interactions of formic acid (FA), including ionization, in collisions at the air-water interface. Ab initio molecular dynamics simulations with dispersion-corrected density functional theory were used. The 8-50 picosecond duration trajectories all resulted in the adsorption of FA within the interfacial region, with no scattering, absorption into the bulk or desorption into the vapor. Despite the known weak acidity of FA, spontaneous deprotonation of the acid was observed at the interface on a broad picosecond timescale, ranging from a few picoseconds typical for stronger acids to tens of picoseconds. Deprotonation occurred in 4% of the trajectories, and was followed by Grotthuss proton transfer through adjacent water molecules. Both sequential and ultrafast concerted proton transfer were observed. The formation of contact ion pairs and solvent-separated ion pairs, and finally the reformation of neutral FA, both trans and cis conformers, occurred in different stages of the dynamics. To better understand the deprotonation mechanisms at the interface compared with the process in bulk water, we used well-tempered metadynamics to obtain deprotonation free energy profiles. While in bulk water FA deprotonation has a free energy barrier of 14.8 kJ mol(-1), in fair agreement with the earlier work, the barrier at the interface is only 7.5 kJ mol(-1). Thus, at the air-water interface, FA may dissociate more rapidly than in the bulk. This finding can be rationalized with reference to the dissimilar aqueous solvation and hydrogen-bonding environments in the interface compared to those in bulk liquid water.Peer reviewe

Ab initio molecular dynamics studies of formic acid dimer colliding with liquid water

Ab initio molecular dynamics simulations of formic acid (FA) dimer colliding with liquid water at 300 K have been performed using density functional theory. The two energetically lowest FA dimer isomers were collided with a water slab at thermal and high kinetic energies up to 68k(B)T. Our simulations agree with recent experimental observations of nearly a complete uptake of gas-phase FA dimer: the calculated average kinetic energy of the dimers immediately after collision is 5 +/- 4% of the incoming kinetic energy, which compares well with the experimental value of 10%. Simulations support the experimental observation of no delayed desorption of FA dimers following initial adsorption. Our analysis shows that the FA dimer forms hydrogen bonds with surface water molecules, where the hydrogen bond order depends on the dimer structure, such that the most stable isomer possesses fewer FA-water hydrogen bonds than the higher energy isomer. Nevertheless, even the most stable isomer can attach to the surface through one hydrogen bond despite its reduced hydrophilicity. Our simulations further show that the probability of FA dimer dissociation is increased by high collision energies, the dimer undergoes isomerization from the higher energy to the lowest energy isomer, and concerted double-proton transfer occurs between the FA monomers. Interestingly, proton transfer appears to be driven by the release of energy arising from such isomerization, which stimulates those internal vibrational degrees of freedom that overcome the barrier of a proton transfer.Peer reviewe

Oxygen Evolution Reaction on Nitrogen-Doped Defective Carbon Nanotubes and Graphene

The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3-1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone-Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone-Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties. © 2018 American Chemical Society.Peer reviewe

Oxygen Evolution Reaction Kinetic Barriers on Nitrogen-Doped Carbon Nanotubes

We investigate kinetic barriers for the oxygen evolution reaction (OER) on singly and doubly nitrogen-doped single-walled carbon nanotubes (NCNTs) using the climbing image nudged elastic band method with solvent effects represented by a 45-water-molecule droplet. The studied sites were chosen based on a previous study of the same systems utilizing a thermodynamic model which ignored both solvent effects and kinetic barriers. According to that model, the two studied sites, one on a singly nitrogen-doped CNT and the other on a doubly doped CNT, were approximately equally suitable for OER. For the four-step OER process, however, our reaction barrier calculations showed a clear difference in the rate-determining *OOH formation step between the two systems, with barrier heights differing by more than 0.4 eV. Thus, the simple thermodynamic model may alone be insufficient for identifying optimal OER sites. Of the remaining three reaction steps, the two H2O forming ones were found to be barrierless in all cases. We also performed solvent-free barrier calculations on NCNTs and undoped CNTs. Substantial differences were observed in the energies of the intermediates when the solvent was present. In general, the observed low activation energy barriers for these reactions corroborate both experimental and theoretical findings of the utility of NCNTs for OER catalysis.Peer reviewe

First and second deprotonation of H₂SO₄ on wet hydroxylated (0001) α-quartz.

We present an ab initio molecular dynamics study of deprotonation of sulfuric acid on wet quartz, a topic of atmospheric interest. The process is preferred, with 65% of our trajectories at 250 K showing deprotonation. The time distribution of the deprotonation events shows an exponential behavior and predicts an average deprotonation time of a few picoseconds. The process is exoergic, with most of the temperature increase being due to formation of hydrogen bonds prior to deprotonation. In agreement with existing studies of H2SO4 in water clusters, in liquid water, and at the air-water interface, the main determinant of deprotonation is the degree of solvation of H2SO4 by neighboring water molecules. However, we find that if both hydrogens of H2SO4 are simultaneously donated to water oxygens, deprotonation is disfavored. Predicted spectroscopic signatures showing the presence of solvated hydronium and bisulfate are presented. Increasing the temperature up to 330 K accelerates the process but does not change the main features of the deprotonation mechanisms or the spectroscopic signatures. The second deprotonation of H2SO4, studied only at 250 K, occurs provided there is sufficient solvation of the bisulfate by additional water molecules. In comparison to HCl deprotonation on the identical surface examined in our previous work, the first deprotonation of H2SO4 occurs more readily and releases more energy

First and second deprotonation of H₂SO₄ on wet hydroxylated (0001) α-quartz

International audienceWe present an ab initio molecular dynamics study of deprotonation of sulfuric acid on wet quartz, a topic of atmospheric interest. The process is preferred, with 65% of our trajectories at 250 K showing deprotonation. The time distribution of the deprotonation events shows an exponential behavior and predicts an average deprotonation time of a few picoseconds. The process is exoergic, with most of the temperature increase being due to formation of hydrogen bonds prior to deprotonation. In agreement with existing studies of H2SO4 in water clusters, in liquid water, and at the air–water interface, the main determinant of deprotonation is the degree of solvation of H2SO4 by neighboring water molecules. However, we find that if both hydrogens of H2SO4 are simultaneously donated to water oxygens, deprotonation is disfavored. Predicted spectroscopic signatures showing the presence of solvated hydronium and bisulfate are presented. Increasing the temperature up to 330 K accelerates the process but does not change the main features of the deprotonation mechanisms or the spectroscopic signatures. The second deprotonation of H2SO4, studied only at 250 K, occurs provided there is sufficient solvation of the bisulfate by additional water molecules. In comparison to HCl deprotonation on the identical surface examined in our previous work, the first deprotonation of H2SO4 occurs more readily and releases more energy