Showing papers on "Elementary reaction published in 2020"

One-pot, room-temperature conversion of dinitrogen to ammonium chloride at a main-group element.

Abstract: The industrial reduction of dinitrogen (N2) to ammonia is an energy-intensive process that consumes a considerable proportion of the global energy supply. As a consequence, species that can bind N2 and cleave its strong N-N bond under mild conditions have been sought for decades. Until recently, the only species known to support N2 fixation and functionalization were based on a handful of metals of the s and d blocks of the periodic table. Here we present one-pot binding, cleavage and reduction of N2 to ammonium by a main-group species. The reaction-a complex multiple reduction-protonation sequence-proceeds at room temperature in a single synthetic step through the use of solid-phase reductant and acid reagents. A simple acid quench of the mixture then provides ammonium, the protonated form of ammonia present in fertilizer. The elementary reaction steps in the process are elucidated, including the crucial N-N bond cleavage process, and all of the intermediates of the reaction are isolated.

ReacNetGenerator: an automatic reaction network generator for reactive molecular dynamics simulations

Abstract: Reactive molecular dynamics (MD) simulation makes it possible to study the reaction mechanism of complex reaction systems at the atomic level. However, the analysis of MD trajectories which contain thousands of species and reaction pathways has become a major obstacle to the application of reactive MD simulation in large-scale systems. Here, we report the development and application of the Reaction Network Generator (ReacNetGenerator) method. It can automatically extract the reaction network from the reaction trajectory without any predefined reaction coordinates and elementary reaction steps. Molecular species can be automatically identified from the cartesian coordinates of atoms and the hidden Markov model is used to filter the trajectory noises which makes the analysis process easier and more accurate. The ReacNetGenerator has been successfully used to analyze the reactive MD trajectories of the combustion of methane and 4-component surrogate fuel for rocket propellant 3 (RP-3), and it has great advantages in terms of efficiency and accuracy compared to traditional manual analysis.

Design criteria for the competing chlorine and oxygen evolution reactions: avoid the OCl adsorbate to enhance chlorine selectivity.

Abstract: The formation of gaseous chlorine within chlor-alkali electrolysis is accompanied by a selectivity problem, as the evolution of gaseous oxygen constitutes a detrimental side reaction in the same potential range. As such, the development of electrode materials with high selectivity toward the chlorine evolution reaction is of particular importance to the chemical industry. Insight into the elementary reaction steps is ultimately required to comprehend chlorine selectivity on a molecular level. Commonly, linear scaling relationships are analyzed by the construction of a volcano plot, using the binding energy of oxygen, ΔEO, as a descriptor in the analysis. The present article reinvestigates the selectivity problem of the competing chlorine and oxygen evolution reactions by applying a different strategy compared to previous literature studies. On the one hand, a unifying material-screening framework that, besides binding energies, also includes the applied overpotential, kinetics, and the electrochemical-step symmetry index is used to comprehend trends in this selectivity issue for transition-metal oxide-based electrodes. On the other hand, the free-energy difference between the adsorbed oxygen and adsorbed hydroxide, ΔG2, rather than ΔEO is used as a descriptor in the analysis. It is demonstrated that the formation of the OCl adsorbate within the chlorine evolution reaction inherently limits chlorine selectivity, whereas, in the optimum case, the formation of the Cl intermediate can result in significantly higher chlorine selectivity. This finding is used to derive the design criteria for highly selective chlorine evolution electrocatalysts, which can be used in the future to search for potential electrode compositions by material-screening techniques.

The vital role of step-edge sites for both CO activation and chain growth on cobalt Fischer-Tropsch catalysts revealed through first-principles based microkinetic modeling including lateral interactions

Abstract: Microkinetic modeling is a bottom-up approach that pinpoints activity and selectivity controlling elementary reaction steps. We have applied this method to the Fischer-Tropsch (FT) reaction by computing all relevant elementary reaction steps at Co(11"2" 1) step-edge and Co(0001) terrace sites. Our model includes important aspects such as the impact of coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. We found that CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers, while the overall barriers via CO insertion are much higher. A comparison with the CO dissociation barrier indicates why cobalt is such a good FT catalyst: CO bond scission and chain growth compete, while termination to olefins has a slightly higher barrier. The simulations predict kinetic parameters that correspond well with experimental kinetic data. They show that the Co(11"2" 1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model leads to a substantially higher CH4 selectivity at the expense of the C2+-hydrocarbons selectivity. The chain-growth probability decreases with increasing temperature and H2/CO ratio, which is caused by faster hydrogenation of the hydrocarbon chain. The elementary reaction steps for O removal and CO dissociation significantly control the overall CO consumption rate. Chain growth occurs almost exclusively at step-edge sites, while additional CH4 stems from CH and CH3 migration from step-edge to terrace sites. Replacing CO by CO2 as the reactant shifts the product distribution nearly completely to CH4. We show that the much higher H/CO coverage ratio during CO2 hydrogenation causes this high CH4 selectivity. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint low-reactive terrace sites near step-edge sites as the origin of unwanted CH4.

High-Throughput Approach Exploitation: Two-Dimensional Double-Metal Sulfide (M2S2) of Efficient Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

Abstract: A transition-metal sulfide (M2S2) nanolayer as a catalyst for the oxygen reduction reaction (ORR) has been investigated by the density functional theory (DFT) method to explore the underlying mechanisms of the elementary reaction steps for the ORR process. Both the O2 dissociation and O2 hydrogenation paths are probably possible in the ORR on the M2S2 surface. All of the possible intermediate reaction steps of the ORR are exothermic for O2 hydrogenation. This indicates that the four-electron reaction path (4e– ORR) process is the most favorable path, and it is preferred over the two-electron path (2e– ORR) process. The changes in the reaction free energy diagrams were determined, and these diagrams showed that oxygen hydrogenation (OOH) is the rate-determining step. Meanwhile, different working potentials for our studied catalysts were also considered, and we observed that the double-transition-metal sulfide catalysts are energetically favorable (exothermic) catalysts via a 4e transfer mechanism of the ORR processes. According to the formation energies of the ORR intermediates (*O, *OH, *OOH) and the scaling relations between them on different slabs, the volcano plot for the overpotential of the catalyst is also an important index of the catalytic activities, and we found that a smaller overpotential is appropriate to determine better catalytic activities for the ORR process.

High-Throughput Calculation Investigations on the Electrocatalytic Activity of Codoped Single Metal–Nitrogen Embedded in Graphene for ORR Mechanism

Abstract: Two types of single metal atoms embedded in graphene were investigated as a potential electrocatalyst for oxygen reduction reaction (ORR) for the application in a fuel cell. ORR was considered in the four elementary reaction steps of oxygen hydrogenation, perhydroxyl production, atomic oxygen hydrogenation, and final water form. All calculations of catalytic activity were performed with the Vienna Ab Initio Simulation Package (VASP) on an M@Gra (M = Mn, Fe, Co, and Ir)–embedded structure, indicating that high-efficiency catalytic activity in the oxidation reaction takes place on the top of metal atom sites. Our calculations revealed that ORR is profiled via four-electron transfer pathway. Activity of these catalysts is closely related to the same scaling linear relations between the adsorption energies of the ORR intermediates on different catalytic surfaces; this can improve their catalytic activity for O2 reduction through a high-efficiency 4e reaction path. Mn- and Ir-doped of cell A graphene exhibited excellent ORR catalytic performance in case of their small overpotential (less than 0.23 V) and low-energy barrier (less than 0.64 eV) of the Ir-doped graphene rate-determining step. Mn@Gra and Fe@Gra of cell B monolayers showed poor ORR catalytic performance due to the strong interaction between various ORR-involved species. Based on the free energy change and activation energy of each intermediate reaction in ORR, Fe@Gra and Ir@Gra are promising catalysts for ORR processes in fuel cells. This provides useful guidance for different types of catalysts in applications to fuel cells.

Investigation of hydrogen oxidation in supercritical H2O/CO2 mixtures using ReaxFF molecular dynamics simulation

Abstract: Hydrogen oxidation kinetics in supercritical H2O/CO2 mixtures is a fundamental topic in supercritical water oxidation (SCWO) technology. A series of reactive molecular dynamics (ReaxFF-MD) simulations were performed to investigate hydrogen oxidation process in supercritical mixtures. The results showed that HO2 and H2O2 radicals played key roles in the reaction kinetics. High concentration H2O suppressed the production of OH radical and increased steric hindrance for effective collisions, exerting negative influence on hydrogen oxidation. The presence of CO2 advanced the oxidation rate of hydrogen mainly through the elementary reaction CO2 + H → CO + OH. It was found that high O2 concentration promoted the oxidation of hydrogen and also affected the reaction induction time. The hydrogen reaction mechanisms under supercritical conditions were illustrated, showing different characteristics from those at atmospheric condition. The results will facilitate the development of continuum-scale kinetic models for accurate prediction of hydrogen oxidation behavior in supercritical systems.

Critical Hydrogen Coverage Effect on the Hydrogenation of Ethylene Catalyzed by δ-MoC(001): An Ab Initio Thermodynamic and Kinetic Study

Abstract: The molecular mechanism of ethylene (C2H4) hydrogenation on a δ-MoC(001) surface has been studied by periodic density functional theory methods. Activation energy barriers and elementary reaction r...

DFT insights into electrocatalytic CO2 reduction to methanol on α-Fe2O3(0001) surfaces

Abstract: Electrocatalytic reduction of CO2 to manufacture fuels and other useful chemicals is one of the appealing methods to reuse CO2. Herein, electrocatalytic CO2 reduction on a model α-Fe2O3(0001) surface catalyst has been investigated by means of density functional theory. This systematic study, involving 20 reaction intermediates and 63 distinct elementary reaction steps, has allowed the identification of a novel mechanism for the decomposition of the key intermediate *COOH. Methanol is the preferred product, with an overpotential of 0.8 V, over carbon monoxide (CO), formic acid (HCOOH), and formaldehyde (CH2O). Formaldehyde formed on the surface will be converted into methanol. This work demonstrates the need for a complete investigation of possible pathways to find the most favourable one, beyond chemical intuition. Moreover, it suggests that hematite could be an interesting material for CO2 reduction.

DFT analysis elementary reaction steps of catalytic activity for ORR on metal-, nitrogen- co-doped graphite embedded structure

Abstract: Metal-nitrogen coordinated graphite coordination structures are becoming more and more attractive for its novel catalytic activity in oxygen reduction reaction (ORR) at the fuel cells. In this work, single copper atom on graphitic carbon nitride acting as electrocatalyst for ORR have been investigated by using the density functional theory method. Our study results that the Cu site is the active center for all the possible elementary steps of the ORR. Further studies the elementary reaction steps are used to explore the underlying mechanisms to gain insights into ORR. Both the O2 dissociation and O2 hydrogenation paths are probably to ORR on the CuN4-Gra surface. All the possible elementary reaction steps for the ORR are exothermic with small reaction barriers (less than 1.98 eV) for O2 hydrogenation. Meanwhile, with large reaction barrier (3.16 eV) for O2 dissociation to go through the rate-limiting steps. The Gibbs free energy for each elementary step of ORR is used to clarify which path determine the ORR/OER on the CuN4 co-doped graphene. Scaling relation and surface phase diagram are obtained by calculated Gibbs free energy of intermediates at surface active sites with various adsorption species. The different working potentials are also considered for the studied catalysts, as the overpotential of ORR is also an important indexes of the catalytic activities of the catalyst, we calculated the overpotential for each active site on the structures and determined the minimum overpotential for ORR.

Thermodynamics of non-elementary chemical reaction networks

Abstract: We develop a thermodynamic framework for closed and open chemical networks applicable to non-elementary reactions that do not need to obey mass action kinetics. It only requires the knowledge of the kinetics and of the standard chemical potentials, and makes use of the topological properties of the network (conservation laws and cycles). Our approach is proven to be exact if the network results from a bigger network of elementary reactions where the fast-evolving species have been coarse grained. Our work should be particularly relevant for energetic considerations in biosystems where the characterization of the elementary dynamics is seldomly achieved.

A chemical dynamics study on the gas-phase formation of triplet and singlet C5H2 carbenes.

Abstract: Since the postulation of carbenes by Buchner (1903) and Staudinger (1912) as electron-deficient transient species carrying a divalent carbon atom, carbenes have emerged as key reactive intermediates in organic synthesis and in molecular mass growth processes leading eventually to carbonaceous nanostructures in the interstellar medium and in combustion systems. Contemplating the short lifetimes of these transient molecules and their tendency for dimerization, free carbenes represent one of the foremost obscured classes of organic reactive intermediates. Here, we afford an exceptional glance into the fundamentally unknown gas-phase chemistry of preparing two prototype carbenes with distinct multiplicities-triplet pentadiynylidene (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C5H2) carbene-via the elementary reaction of the simplest organic radical-methylidyne (CH)-with diacetylene (HCCCCH) under single-collision conditions. Our combination of crossed molecular beam data with electronic structure calculations and quasi-classical trajectory simulations reveals fundamental reaction mechanisms and facilitates an intimate understanding of bond-breaking processes and isomerization processes of highly reactive hydrocarbon intermediates. The agreement between experimental chemical dynamics studies under single-collision conditions and the outcome of trajectory simulations discloses that molecular beam studies merged with dynamics simulations have advanced to such a level that polyatomic reactions with relevance to extreme astrochemical and combustion chemistry conditions can be elucidated at the molecular level and expanded to higher-order homolog carbenes such as butadiynylcyclopropenylidene and triplet heptatriynylidene, thus offering a versatile strategy to explore the exotic chemistry of novel higher-order carbenes in the gas phase.