Diabatic model for electrochemical hydrogen evolution based on constrained DFT configuration interaction

The accuracy of density functional theory (DFT) based kinetic models for electrocatalysis is diminished by spurious electron delocalization effects, which manifest as uncertainties in the predicted values of reaction and activation energies. In this work, we present a constrained DFT (CDFT) approach to alleviate overdelocalization effects in the Volmer-Heyrovsky mechanism of the hydrogen evolution reaction (HER). This method is applied a posteriori to configurations sampled along a reaction path to correct their relative stabilities. Concretely, the first step of this approach involves describing the reaction in terms of a set of diabatic states that are constructed by imposing suitable density constraints on the system. Refined reaction energy profiles are then recovered by performing a configuration interaction (CDFT-CI) calculation within the basis spanned by the diabatic states. After a careful validation of the proposed method, we examined HER catalysis on open-ended carbon nanotubes and discovered that CDFT-CI increased activation energies and decreased reaction energies relative to DFT predictions. We believe that a similar approach could also be adopted to treat overdelocalization effects in other electrocatalytic proton-coupled electron transfer reactions, e.g., in the oxygen reduction reaction.

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.

Atomic Layer Deposition of Zinc Oxide: Study on the Water Pulse Reactions from First-Principles

Atomic layer deposition (ALD) of zinc oxide thin films has been under intense research in the past few years. The most common precursors used in this process are diethyl zinc (DEZ) and water. The surface chemistry related to the growth of a zinc oxide thin film via atomic layer deposition is not entirely clear, and the ideal model of the process has been contradicted by experimental data, e.g., the incomplete elimination of the ethyl ligands from the surface and the non-negative mass change during the water pulse. In this work we investigate the surface reactions of water during the atomic layer deposition of zinc oxide. The adsorption and ligand-exchange reactions of water are studied on ethyl-saturated surface structures to grasp the relevant surface chemistry contributing to the deposition process. The complex ethyl-saturated surface structures are adopted from a previous publication on the DEZ/H2O-process, and different configurations are sampled using ab initio molecular dynamics in order to find a suitable minimum structure. Water molecules are found to adsorb exothermically onto the ethyl-covered surface at all the ethyl concentrations considered. We do not observe an adsorption barrier for water at 0 K; however, the adsorption energy for any additional water molecules decreases rapidly at high ethyl concentrations. Ligand-exchange reactions are studied at various surface ethyl coverages. The water pulse ligand-exchange reactions have overall larger activation energies than surface reactions for diethyl zinc pulse. For some of the configurations considered, the reaction barriers may be inaccessible at the process conditions, suggesting that some ligands may be inert toward ligand-exchange with water. The activation energies for the surface reactions show only a weak dependence on the surface ethyl concentration. The sensitivity of the adsorption of water at high ethyl coverages suggests that at high ligand-coverages the kinetics may be somewhat hindered due to steric effects. Calculations on the ethyl-covered surfaces are compared to a simple model containing a single monoethyl zinc group. The calculated activation energy for this model is in line with calculations done on the complex model, but the adsorption of water is poorly described. The weak adsorption bond onto a single monoethyl zinc is probably due to a cooperative effect between the surface zinc atoms. A cooperative effect between water molecules is also observed; however, the effect on the activation energies is not as significant as has been reported for other ALD processes.

Hydrogen adsorption trends on Al-doped Ni2P surfaces for optimal catalyst design

Nanoparticles of nickel phosphide are promising materials to replace the currently used rare Pt-group metals at cathode-side electrodes in devices for electrochemical hydrogen production.

Computational exploration of Fe55@C240-catalyzed Fischer–Tropsch synthesis

DFT calculations showed possible hydrocarbon chain growth on Fe55@C240 preferentially via a CO insertion mechanism.

Efficient Constrained Density Functional Theory Implementation for Simulation of Condensed Phase Electron Transfer Reactions

Constrained density functional theory (CDFT) is a versatile tool for probing the kinetics of electron transfer (ET) reactions. In this work, we present a well-scaling parallel CDFT implementation relying on a mixed basis set of Gaussian functions and plane waves, which has been specifically tailored to investigate condensed phase ET reactions using an explicit, quantum chemical representation of the solvent. The accuracy of our implementation is validated against previous theoretical results for predicting electronic couplings and charge transfer energies. Subsequently, we demonstrate the efficiency of our method by studying the intramolecular ET reaction of an organic mixed-valence compound in water using a CDFT based molecular dynamics simulation.

Hydrogen adsorption on MoS2-surfaces: a DFT study on preferential sites and the effect of sulfur and hydrogen coverage

H-Adsorption on MoS₂-surfaces is studied as a function of structural parameters and an assessment of the intricate structure–property relations is conducted.

Isomerism of Trimeric Aluminum Complexes in Aqueous Environments: Exploration via DFT-Based Metadynamics Simulation

The chemistry of aluminum or oxo-aluminum in water is still relatively unknown, although it is the basis for many chemical and industrial processes, including flocculation in water treatment plants. Trimeric species have a predominant role in the formation of the Keggin cations, which are the basic building blocks of aluminum-based chemicals. Despite this, details of the structural evolution of these small solvated clusters and how this is related to the processes leading to the formation of larger aggregates are still an open issue. To address these questions, here, we have applied the metadynamics (MTD) simulation technique [ Barducci , A. ; Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2010 , 1 , 826 - 843 ] with density functional theory-based molecular dynamics to disclose the dynamics and structural conversions of trimeric aluminum complexes in an aqueous environment. The existence of a variety of competing metastable conformations, for example, book-like, cyclic boat, and linear shape conformations, is revealed in the MTD simulation. Furthermore, equilibrium simulations of the various intermediate states encountered along the MTD trajectory are used to assess their (meta)stability, determine the rearrangement of the OH ligands, and discuss the role of the solvating water.

DFT simulations and microkinetic modelling of 1-pentyne hydrogenation on Cu20 model catalysts

Adsorption and dissociation of H2 and hydrogenation of 1-pentyne on neutral and anionic Cu20 clusters have been investigated using the density functional theory and microkinetic modelling. Molecular adsorption of H2 is found to occur strictly at atop sites. The H2 dimer is activated upon adsorption, and the dissociation occurs with moderate energy barriers. The dissociated H atoms reside preferentially on 3-fold face and 2-fold edge sites. Based on these results, the reaction paths leading to the partial and total hydrogenation of 1-pentyne have been studied step-by-step. The results suggest that copper clusters can display selective activity on the hydrogenation of alkyne and alkene molecules. The hydrogenated products are more stable than the corresponding initial reactants following an energetic staircase with the number of added H atoms. Stable semi-hydrogenated intermediates are formed before the partial (1-pentene) and total (pentane) hydrogenation stages of 1-pentyne. The microkinetic model analysis shows that C5H10 is the dominant product. Increasing the reactants (C5H8/H2) ratio enhances the formation of products (C5H10 and C5H12).

Dissociative adsorption of O2 on negatively charged nitrogen-doped single-walled carbon nanotubes: first-principles calculations

Spin unrestricted DFT calculations have been used to study the molecular and dissociative adsorption of O₂ on achiral substitutional nitrogen-doped single-walled carbon nanotubes with and without additional charges.

Charge distribution and Fermi level in bimetallic nanoparticles

Fermi level equilibration driven charge redistribution and electric dipole formation was quantified using a simple nanocapacitor model for bimetallic nanoparticles.

Theoretical Insight into the Hydrogen Evolution Activity of Open-Ended Carbon Nanotubes

Carbon nanotubes (CNTs), while inactive by themselves, are often used as a platform in the search of new catalysts for the hydrogen evolution reaction (HER) by introducing metal nanoparticles or other dopants. Here, we examine the HER activity of pristine open-ended CNTs considering both the effects of chirality and hydrogen coverage using electronic structure calculations. The results indicate that the formation of different 5-ring structures at the end of the CNT introduces surface sites that are highly active toward HER, whereas the activity of traditional 6-ring sites is not greatly altered by tube termination. At fixed hydrogen coverage, the enhanced activity of these sites was attributed to valence orbitals residing close to the highest occupied molecular level facilitating electron transfer to protons.

First principles study of the atomic layer deposition of alumina by TMA-H2O-process

Atomic layer deposition (ALD) is a coating technology used to produce highly uniform thin films. Aluminiumoxide, Al2O3, is mainly deposited using trimethylaluminium (TMA) and water as precursors and is the most studied ALD-process to date. However, only few theoretical studies have been reported in the literature. The surface reaction mechanisms and energetics previously reported focus on a gibbsite-like surface model but a more realistic description of the surface can be achieved when the hydroxylation of the surface is taken into account using dissociatively adsorbed water molecules. The adsorbed water changes the structure of the surface and reaction energetics change considerably when compared to previously studied surface model. Here we have studied the TMA-H2O process using density functional theory on a hydroxylated alumina surface and reproduced the previous results for comparison. Mechanisms and energetics during both the TMA and the subsequent water pulse are presented. TMA is found to adsorb exothermically onto the surface. The reaction barriers for the ligand-exchange reactions between the TMA and the surface hydroxyl groups were found to be much lower compared to previously presented results. TMA dissociation on the surface is predicted to saturate at monomethylaluminium. Barriers for proton diffusion between surface sites are observed to be low. TMA adsorption was also found to be cooperative with the formation of methyl bridges between the adsorbants. The water pulse was studied using single water molecules reacting with the DMA and MMA surface species. Barriers for these reactions were found to reasonable in the process conditions. However, stabilizing interactions amongst water molecules were found to lower the reaction barriers and the dynamical nature of water is predicted to be of importance. It is expected that these calculations can only set an upper limit for the barriers during the water pulse.

Impact of Ga-V Codoping on Interfacial Electron Transfer in Dye-Sensitized TiO2

The improvement of charge transfer between an organic molecule and a semiconductor is an important and challenging goal in the fields of photovoltaics and photocatalysis. In this work, we present a time-dependent density functional theory investigation of the impact of Ga-V codoping of TiO2 on the excited-state electron injection from perylene-3-carboxylic acid. The doping is shown to raise the charge-transfer efficiency for the highest possible surface dye uptake by ∼16%. The strength of the effect depends on the dopant-pair-dye separation, dopant concentration, and distribution of Ga, V atoms in TiO2. The doping of the superficial level turns out to be more favorable than those in the bulk. The changes in electron injection dynamics are attributed to the modification of accepting semiconductor levels and hybridization profile between molecular and semiconductor states.

CO oxidation catalyzed by neutral and anionic Cu20 clusters: relationship between charge and activity

Reactions of CO and O2 on neutral and anionic Cu20 clusters have been investigated by spin-polarized density functional theory. Three reaction mechanisms of CO oxidation are explored: reactions with atomic oxygen (dissociated O2) as well as reactions with molecular oxygen, including Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The adsorption energies, reaction pathways, and reaction barriers for CO oxidation are calculated systematically. The anionic Cu20(-) cluster can adsorb CO and O2 more strongly than the neutral counterpart due to the superatomic shell closing of 20 valence electrons which leaves one electron above the band gap. The activation of O2 molecule upon adsorption is crucial to determine the rate of CO oxidation. The CO oxidation proceeds efficiently on both Cu20 and Cu20(-) clusters, when O2 is pre-adsorbed dissociatively. The ER mechanism has a lower reaction barrier than the LH mechanism on the neutral Cu20. In general, CO oxidation occurs more readily on the anionic Cu20(-) (effective reaction barriers 0.1-0.3 eV) than on the neutral Cu20 cluster (0.3-0.5 eV). Moreover, Cu20(-) exhibits enhanced binding for CO2. From the analysis of the reverse direction of CO oxidation, it is observed that the transition of CO2 to CO + O can occur on the Cu20(-) cluster, which demonstrates that Cu clusters may serve as good catalyst for CO2 chemistry.

Ab initio Kinetic Monte Carlo simulations of dissolution at the NaCl-water interface

We have used ab initio molecular dynamics (AIMD) simulations to study the interaction of water with the NaCl surface. As expected, we find that water forms several ordered hydration layers, with the first hydration layer having water molecules aligned so that oxygen atoms are on average situated above Na sites. In an attempt to understand the dissolution of NaCl in water, we have then combined AIMD with constrained barrier searches, to calculate the dissolution energetics of Na(+) and Cl(-) ions from terraces, steps, corners and kinks of the (100) surface. We find that the barrier heights show a systematic reduction from the most stable flat terrace sites, through steps to the smallest barriers for corner and kink sites. Generally, the barriers for removal of Na(+) ions are slightly lower than for Cl(-) ions. Finally, we use our calculated barriers in a Kinetic Monte Carlo as a first order model of the dissolution process.

The molecular and magnetic structure of carbon-enclosed and partially covered Fe55 particles

The structure and magnetic moment distribution are studied for an iron nanoparticle with varying degree of carbon adatom coverage. The limiting models of the study are the clean icosahedral Fe55 particle and the iron particle completely enclosed in carbon cages. Between the two extrema, partially covered particles are considered. The iron cluster with partial coverage of carbon adatoms represents a model of active catalysts in the chemical vapor deposition synthesis of carbon nanotubes. The investigated structures are the bare Fe55 cluster, Fe55N4C(x) (x = 27, 37, 47, 54, 65), and Fe55 encapsulated inside C180 and C240. The two latter are extreme examples of an iron particle completely enclosed in a carbon network. Fe55@C180 and Fe55@C240 present novel structures resembling the endohedral metallofullerenes. Two structural isomers of the Fe55@C180 are considered. Enclosing the Fe55 cluster inside C180 and C240 fullerenes gives rise to changes in the Fe-Fe bond lengths. This alters the magnetic structure of the iron cluster considerably. The interaction between the fullerenes and the enclosed iron cluster is reflected in a charge transfer of 8-13 electrons in the considered endohedral complexes. The localization of atomic charges on the C180 and C240 cages suggests site-selective reactivity of the endohedral complexes. The total magnetic moments of the Fe55N4C(x) nanoparticles vary with the degree of adatom coverage. The magnetic moments of individual Fe atoms depend strongly on the element of the nearest-neighbor atoms and on the coordination number and carry therefore information about the local chemistry.

Dissolution of NaCl nanocrystals: an ab initio molecular dynamics study

NaCl nanocrystal dissolution was investigated in atomistic detail revealing a difference in the solvation of two different ionic species.