Showing papers on "Elementary reaction published in 2019"

Elucidating the Mechanism of Electrochemical N2 Reduction at the Ru(0001) Electrode

Abstract: Density functional theory calculations are used to elucidate the mechanism of the nitrogen reduction reaction (NRR) in an electrochemical double layer on the Ru(0001) electrode, where all possible proton–electron transfer steps and N–N scission steps during NRR are considered. The model includes a negatively charged electrode and an explicit solvation bilayer of water including hydronium ion(s). We find that all elementary steps in which a proton is transferred to an adsorbate have small additional barriers on the order of 0.0–0.25 eV at an applied potential of −0.6 V versus the reversible hydrogen electrode. The first proton–electron transfer step where an end-on N2 admolecule is reduced to NNH has a negligible additional barrier, whereas the thermochemical barrier is 0.8 eV at −0.6 V. This elementary reaction step is found to be rate-limiting and potential-limiting for NRR on Ru(0001). We predict that NRR will follow an associative distal pathway where after the third proton–electron transfer, the N–N b...

Factors controlling surface oxygen exchange in oxides.

Abstract: Reducing the working temperature of solid oxide fuel cells is critical to their increased commercialization but is inhibited by the slow oxygen exchange kinetics at the cathode, which limits the overall rate of the oxygen reduction reaction. We use ab initio methods to develop a quantitative elementary reaction model of oxygen exchange in a representative cathode material, La0.5Sr0.5CoO3−δ, and predict that under operating conditions the rate-limiting step for oxygen incorporation from O2 gas on the stable, (001)-SrO surface is lateral (surface) diffusion of O-adatoms and oxygen surface vacancies. We predict that a high vacancy concentration on the metastable CoO2 termination enables a vacancy-assisted O2 dissociation that is 102–103 times faster than the rate limiting step on the Sr-rich (La,Sr)O termination. This result implies that dramatically enhanced oxygen exchange performance could potentially be obtained by suppressing the (La,Sr)O termination and stabilizing highly active CoO2 termination. The performance of solid oxide fuel cells relies on oxygen exchange kinetics, which can limit oxygen reduction at the cathode. Here the authors use ab initio methods to model oxygen exchange in a representative cathode material to better understand the active molecular mechanism toward optimized design.

Structure- and Temperature-Dependence of Pt-Catalyzed Ammonia Oxidation Rates and Selectivities

Abstract: Ammonia oxidation is operated at different temperatures over Pt catalysts of different structures to recover different products. In this work, we elucidate the dependency of ammonia oxidation rates and selectivities on both Pt structure and temperature. We perform density functional theory (DFT) computations to compare the reaction and activation energies of elementary reactions on Pt(211) and Pt(111). We develop a microkinetic model parametrized with the DFT results. We show that barriers to product formation are lower on stepped Pt than on terrace, leading to a much higher step rate at low temperature to selectively oxidize ammonia to nitrogen. At high temperature, however, both step and terrace perform comparably in rate to selectively produce nitric oxide. While N2 is always the thermodynamic product, relative N and O coverages interact to make NO the kinetic product at high temperature. The predicted rate and selectivity are consistent with experiments. We further show rate-controlling steps on the t...

Systematic Enumeration of Elementary Reaction Steps in Surface Catalysis

Abstract: The direct synthesis of complex chemicals from simple precursors (such as syngas) is one of the main objectives of current research in heterogeneous catalysis. To rationally design catalytic materials for this purpose, it is essential to identify the critical elementary reaction steps that ultimately determine a catalyst's activity and selectivity with respect to a desired product. Unfortunately, the number of potentially relevant elementary steps is in the thousands, even for relatively simple target species like ethanol. The challenge of identifying the critical steps is thus akin to finding the proverbial needle in a haystack. Recently, a model-reduction scheme has been proposed, which tackles this problem by prescreening the barriers of all potential reactions with computationally inexpensive approximations. Although this route appears highly promising, it raises the question of how the starting point of the model-reduction process can be determined. In this contribution, we present a systematic method for enumerating all intermediates and elementary reactions relevant to a chemical process of interest. Using this approach, we construct reaction networks for C,H,O-containing systems consisting of up to four non-hydrogen atoms (more than 1 million reactions). Importantly, the scheme goes beyond simple bond-breaking reactions and allows considering rearrangement and transfer reactions as well. The presented reaction networks thus cover the chemistry of syngas-based processes (and beyond) to an unprecedented scale.

Facile Synthesis of Unsolvated Alkali Metal Octahydrotriborate Salts MB 3 H 8 (M=K, Rb, and Cs), Mechanisms of Formation, and the Crystal Structure of KB 3 H 8

Abstract: A facile synthesis of heavy alkali metal octahydrotriborates (MB3 H8 ; M=K, Rb, and Cs) has been developed. It is simply based on reactions of the pure alkali metals with THF⋅BH3 , does not require the use of electron carriers or the addition of other reaction media such as mercury, silica gel, or inert salts as for previous procedures, and delivers the desired products at room temperature in very high yields. However, no reactions were observed when pure Li or Na was used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. The low sublimation energies of K, Rb, and Cs were found to be key for initiation of the reactions. The syntheses can be carried out at room temperature because all of the elementary reaction steps have low energy barriers, whereas reactions of LiBH4 /NaBH4 with THF⋅BH3 have to be carried out under reflux. The high stability and solubility of KB3 H8 were examined, and a crystal structure thereof was obtained for the first time.

Mechanism Deduction from Noisy Chemical Reaction Networks.

Abstract: We introduce KiNetX, a fully automated meta-algorithm for the kinetic analysis of complex chemical reaction networks derived from semiaccurate but efficient electronic structure calculations. It is designed to (i) accelerate the automated exploration of such networks and (ii) cope with model-inherent errors in electronic structure calculations on elementary reaction steps. We developed and implemented KiNetX to possess three features. First, KiNetX evaluates the kinetic relevance of every species in a (yet incomplete) reaction network to confine the search for new elementary reaction steps only to those species that are considered possibly relevant. Second, KiNetX identifies and eliminates all kinetically irrelevant species and elementary reactions to reduce a complex network graph to a comprehensible mechanism. Third, KiNetX estimates the sensitivity of species concentrations toward changes in individual rate constants (derived from relative free energies), which allows us to systematically select the mo...

Automatic Proposal of Multistep Reaction Mechanisms using a Graph-Driven Search.

Abstract: Proposing and testing mechanistic hypotheses stands as one of the key applications of contemporary computational chemistry. In the majority of computational mechanistic analyses, the individual elementary steps leading from reactants to products are proposed by the user, based on learned chemical knowledge, intuition, or comparison to an existing well-characterized mechanism for a closely related chemical reaction. However, the prerequisite of prior chemical knowledge is a barrier to automated (or "black box") mechanistic generation and assessment, and it may simultaneously preclude mechanistic proposals that lie outside the "standard" chemical reaction set. In this Article, we propose a simple random-walk algorithm that searches for the set of elementary chemical reactions that transform defined reactant structures into target products. Our approach operates exclusively in the space of molecular connectivity matrices, seeking the set of chemically sensible bonding changes that link connectivity matrices for input reactant and product structures. We subsequently illustrate how atomic coordinates for each elementary reaction can be generated under the action of a graph-restraining potential, prior to further analysis by quantum chemical calculations. Our approach is successfully demonstrated for carbon monoxide oxidation, the water-gas shift reaction, and n-hexane aromatization, all catalyzed by Pt nanoparticles.

The kinetics of elementary thermal reactions in heterogeneous catalysis.

Abstract: The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as microkinetic models of atmospheric chemistry provided the predictive power that led to the Montreal Protocol reversing loss of stratospheric ozone, pursuing a microkinetic approach to heterogeneous catalysis has tremendous potential for societal impact. However, the development of this approach for catalysis faces great challenges. Methods for measuring rate constants are quite limited, and the present predictive theoretical methods remain largely unvalidated. Here, we present a short Perspective on recent experimental advances in the measurement of rates of elementary reactions at surfaces that rely on a stroboscopic pump–probe concept for neutral matter. We present the principles behind successful measurement methods and discuss a recent implementation of those principles. The topic is discussed within the context of a specific but highly typical surface reaction, CO oxidation on Pt, which, despite more than 40 years of study, was only clarified after experiments with velocity-resolved kinetics became possible. This deceptively simple reaction illustrates fundamental lessons concerning the coverage dependence of activation energies, the nature of reaction mechanisms involving multiple reaction sites, the validity of transition-state theory to describe reaction rates at surfaces and the dramatic changes in reaction mechanism that are possible when studying reactions at low temperatures. In this Perspective, recent advances in measuring rates of elementary reactions at model catalyst surfaces are presented. A recent ion-imaging-based technique — velocity-resolved kinetics — is discussed in the context of a typical surface reaction, CO oxidation on Pt.

Elementary kinetics of nitrogen electroreduction on Fe surfaces

Abstract: Electrochemical ammonia synthesis could provide a sustainable and efficient alternative to the energy intensive Haber-Bosch process. Development of an active and selective N2 electroreduction catalyst requires mechanism determination to aid in connecting the catalyst composition and structure to performance. Density functional theory (DFT) calculations are used to examine the elementary step energetics of associative N2 reduction mechanisms on two low index Fe surfaces. Interfacial water molecules in the Heyrovsky-like mechanism help lower some of the elementary activation barriers. Electrode potential dependent barriers show that cathodic potentials below -1.5 V-RHE (reversible hydrogen electrode) are necessary to give a significant rate of N2 electroreduction. DFT barriers suggest a larger overpotential than expected based on elementary reaction free energies. Linear Bronsted-Evans-Polanyi relationships do not hold across N-H formation steps on these surfaces, further confirming that explicit barriers should be considered in DFT studies of the nitrogen reduction reaction.

The mechanism and activity of oxygen reduction reaction on single atom doped graphene: a DFT method

Abstract: Heteroatom doped graphene as a single-atom catalyst for oxygen reduction reaction (ORR) has received extensive attention in recent years. In this paper, the ORR activity of defective graphene anchoring single heteroatom (IIIA, IVA, VA, VIA and VIIA) was systematically investigated using a dispersion-corrected density functional theory method. For all of the 34 catalysts, 14 of which were further analyzed, and the Gibbs free energy of each elementary reaction was calculated. According to the scaling relationship between ΔGOOH* and ΔGOH*, we further analyzed the rate-determining step of the remaining 20 catalysts. The results show that when the ORR reaction proceeds in the path O2 → OOH → O → OH → H2O, the reaction energy barriers are lower than 0.8 eV for Te-SV, Sb-DV, Pb-SV, Pb-DV, As-SV, As-DV, B-SV, Sn-SV and N-SV. Our result provides a theoretical basis for further exploration of carbon-based single-atom catalysts for ORR.

Comprehensive evolution mechanism of SOx formation during pyrite oxidation

Abstract: Pyrite (FeS2) oxidation during coal combustion is one of the main sources of harmful SO2 emission from coal-fired power plants. Density functional theory (DFT) study was performed to uncover the evolution mechanism of SOx formation during pyrite oxidation. The results show that chemisorption mechanism is responsible for O2, SO2 and SO3 adsorption on FeS2 surface. The presence of formed oxidation layer (Fe2O3) weakens the interaction between O2 molecule and FeS2 surface. The adsorbed O2 molecule easily dissociates into active surface O atom for SOx formation. The dissociation reaction of O2 is activated by 77.38 kJ/mol, and exothermic by 138.46 kJ/mol. Compared to the further oxidation of SO2 into SO3, SO2 prefers to desorb from FeS2 surface. The dominant reaction pathway of SO2 formation from the oxidation of the outermost FeS2 surface layer is a three-step process: surface lattice S oxidation, SO2 desorption and replenishment of S vacancy by activated surface O atom. The elementary reaction of surface lattice S oxidation has an activation energy barrier of 197.96 kJ/mol, and is identified as the rate-limiting step. SO2 formation from the further oxidation of bulk FeS2 layer is controlled by a four-step process: bulk lattice S migration, lattice S oxidation, SO2 desorption and surface O atom deposition. Migration of lattice S from bulk position to the outermost surface shows a high activation energy barrier of 175.83 kJ/mol. The deposition process of surface O atom is a relatively easy step, and is activated by 21.05 kJ/mol.

Acceptorless Alcohol Dehydrogenation Catalysed by Pd/C

Abstract: Although the selective oxidation of alcohols to carbonyl compounds is a critical reaction, it is often plagued by several challenges related to sustainability. Here, the continuous, acceptorless dehydrogenation of alcohols to carbonyl compounds over heterogeneous catalysts was demonstrated, in the absence of oxidants, bases or acceptor molecules. In addition to improving selectivity and atom efficiency, the absence of an acceptor resulted in the co‐production of molecular H2, a clean energy source, and permitted dehydrogenation to proceed at >98 % selectivity at turnover frequency values amongst the highest in the literature. Moreover, excellent durability was observed during continuous operation over 48 h, reaching space‐time yields of 0.683 g(product) mL−1 h−1, better than the state of the art by over two orders of magnitude. Alongside these breakthroughs, the basic kinetic parameters of the reaction were also determined, allowing some of the elementary reaction steps to be identified.

Neural Network Based in Silico Simulation of Combustion Reactions.

Abstract: Understanding and prediction of the chemical reactions are fundamental demanding in the study of many complex chemical systems. Reactive molecular dynamics (MD) simulation has been widely used for this purpose as it can offer atomic details and can help us better interpret chemical reaction mechanisms. In this study, two reference datasets were constructed and corresponding neural network (NN) potentials were trained based on them. For given large-scale reaction systems, the NN potentials can predict the potential energy and atomic forces of DFT precision, while it is orders of magnitude faster than the conventional DFT calculation. With these two models, reactive MD simulations were performed to explore the combustion mechanisms of hydrogen and methane. Benefit from the high efficiency of the NN model, nanosecond MD trajectories for large-scale systems containing hundreds of atoms were produced and detailed combustion mechanism was obtained. Through further development, the algorithms in this study can be used to explore and discovery reaction mechanisms of many complex reaction systems, such as combustion, synthesis, and heterogeneous catalysis without any predefined reaction coordinates and elementary reaction steps.

Gas phase synthesis of [4]-helicene.

Abstract: A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4) yields [4]-helicene (C18H12) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C18H13). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

Molecular Dynamics Simulations of an Initial ChemicalReaction Mechanism of Shocked CL-20 Crystals Containing Nanovoids

Abstract: To understand the initial chemical reaction mechanism of the heterogeneous explosive hexanitrohexaazaisowurtzitane (CL-20), it is necessary to study the shock initiation mechanism of this nanovoid-containing crystal. In this paper, supercells of CL-20 with different void sizes were constructed. The chemical reactions induced by different impact velocities were calculated using molecular dynamics based on the ReaxFF-lg reactive force field. The effects of impact velocities and void sizes on the chemical reactions of the CL-20 crystal were discussed. The initial reaction of CL-20 molecules around the voids was analyzed, and the evolution of the formation and breakage of chemical bonds as well as the elementary reactions were also obtained. It is found that under an impact, the CL-20 molecules around the voids first undergo polymerization of the N–O bonds and then breakage of the C–N, N–N, and C–H bonds occurs. Increased void size and impact velocity lead to higher temperature “hot spots” and more intense ch...

Solid-to-liquid phase transitions of sub-nanometer clusters enhance chemical transformation.

Abstract: Understanding the nature of active sites is crucial in heterogeneous catalysis, and dynamic changes of catalyst structures during reaction turnover have brought into focus the dynamic nature of active sites. However, much less is known on how the structural dynamics couples with elementary reactions. Here we report an anomalous decrease in reaction free energies and barriers on dynamical sub-nanometer Au clusters. We calculate temperature dependence of free energies using ab initio molecular dynamics, and find significant entropic effects due to solid-to-liquid phase transitions of the Au clusters induced by adsorption of different states along the reaction coordinate. This finding demonstrates that catalyst dynamics can play an important role in catalyst activity. Understanding the dynamic evolution of the catalysts’ structure under reaction conditions is crucial in heterogeneous catalysis. Here the authors use ab initio molecular dynamics simulations to show an anomalous decrease in reaction free energies and barriers on dynamical sub-nanometer Au clusters supported on MgO(001).

Extracting the mechanisms and kinetic models of complex reactions from atomistic simulation data.

Abstract: Determining reaction mechanisms and kinetic models, which can be used for chemical reaction engineering and design, from atomistic simulation is highly challenging. In this study, we develop a novel methodology to solve this problem. Our approach has three components: (1) a procedure for precisely identifying chemical species and elementary reactions and statistically calculating the reaction rate constants; (2) a reduction method to simplify the complex reaction network into a skeletal network which can be used directly for kinetic modeling; and (3) a deterministic method for validating the derived full and skeletal kinetic models. The methodology is demonstrated by analyzing simulation data of hydrogen combustion. The full reaction network comprises 69 species and 256 reactions, which is reduced into a skeletal network of 9 species and 30 reactions. The kinetic models of both the full and skeletal networks represent the simulation data well. In addition, the essential elementary reactions and their rate constants agree favorably with those obtained experimentally. © 2019 Wiley Periodicals, Inc.

Elucidating the Chemical Dynamics of the Elementary Reactions of the 1-Propynyl Radical (CH3CC; X2A1) with Methylacetylene (H3CCCH; X1A1) and Allene (H2CCCH2; X1A1)

Abstract: The reactions of the 1-propynyl radical (CH3CC; X2A1) with two C3H4 isomers, methylacetylene (H3CCCH; X1A1) and allene (H2CCCH2; X1A1), along with their (partially) deuterated counterparts were explored at collision energies of 37 kJ mol-1, exploiting crossed molecular beams to unravel the chemical reaction dynamics to synthesize distinct C6H6 isomers under single collision conditions. The forward convolution fitting of the laboratory data along with ab initio and statistical calculations revealed that both reactions have no entrance barrier, proceed via indirect (complex-forming) reaction dynamics involving C6H7 intermediates with life times longer than their rotation period(s), and are initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density of the unsaturated hydrocarbon at the terminal carbon atoms of methylacetylene (C1) and allene (C1/C3). In the methylacetylene system, the initial collision complexes undergo unimolecular decomposition via tight exit transition states by atomic hydrogen loss, forming dimethyldiacetylene (CH3CCCCCH3) and 1-propynylallene (H3CCCHCCCH2) in overall exoergic reactions (123 and 98 kJ mol-1) with a branching ratio of 9.4 ± 0.1; the methyl group of the 1-propynyl reactant acts solely as a spectator. On the other hand, in the allene system, our experimental data exhibit the formation of the fulvene (c-C5H4CH2) isomer via a six-step reaction sequence with two higher energy isomers-hexa-1,2-dien-4-yne (H2CCCHCCCH3) and hexa-1,4-diyne (HCCCH2CCCH3)-also predicted to be formed based on our statistical calculations. The pathway to fulvene advocates that, in the allene-1-propynyl system, the methyl group of the 1-propynyl reactant is actively engaged in the reaction mechanism to form fulvene. Because both reactions are barrierless and exoergic and all transition states are located below the energy of the separated reactants, the hydrogen-deficient C6H6 isomers identified in our investigation are predicted to be synthesized in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and their moons such as Titan along with cold molecular clouds such as Taurus Molecular Cloud-1.

Theoretical study of the effect of hydrogen radicals on the formation of HCN from pyrrole pyrolysis

Abstract: As a typical fossil fuel, coal is a major contributor to nitrogen oxide (NOx) pollution The detailed mechanism of NOx generation from coal pyrolysis need to be clarified Within this research, we used density functional theory (DFT) to investigate the formation mechanism of HCN as a NOx precursor during pyrolysis of pyrrole in the presence of hydrogen (H) radicals Firstly, three different reaction positions for hydrogen radical attacking were compared It was identified that hydrogen radical initially reacts with pyrrole at the location adjacent to N through a single elementary reaction step with an activation energy of 7712 kJ/mol Additionally, to examine the role of hydrogen radical in the pyrrole pyrolysis to form HCN, 12 subsequent reaction pathways were theoretically investigated It was found that one of the pathway (Pathway a-4) involving hydrogen transfer followed by carbon-carbon cleavage processes is the route with the lowest energy barrier of all of the mechanisms reported, thus it plays an important role in the formation of HCN from the pyrrolic components of coal These results further indicated that the hydrogen radicals significantly reduce the energy barrier of the pyrrole pyrolysis