Showing papers on "Elementary reaction published in 2014"

Reaction Pathway for Oxygen Reduction on FeN4 Embedded Graphene.

Abstract: The detailed reaction pathways for oxygen reduction on FeN4 embedded graphene have been investigated using density functional theory transition-state calculations. Our first-principles calculation results show that all of the possible ORR elementary reactions could take place within a small region around the embedded FeN4 complex. It is predicted that the kinetically most favorable reaction pathway for ORR on the FeN4 embedded graphene would be a four-electron OOH dissociation pathway, in which the rate-determining step is found to be the OOH dissociation reaction with an activation energy of 0.56 eV. Consequently, our theoretical study suggests that nonprecious FeN4 embedded graphene could possess catalytic activity for ORR comparable to that of precious Pt catalysts.

The optimally performing Fischer-Tropsch catalyst

Abstract: Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.

Mechanistic aspects of the water-gas shift reaction on isolated and clustered Au atoms on CeO2(110) : a density functional theory study

Abstract: Density functional theory was employed to study the water–gas shift (WGS) reaction for two structural models—namely, a single Au atom and a Au nanorod—supported on the (110) surface of ceria. The carboxyl mechanism involving a COOH intermediate is strongly preferred over the redox mechanism, which would require O–H bond cleavage of ceria-bound hydroxyl groups. Two candidate rate-controlling elementary reaction steps were identified in the carboxyl mechanism: oxygen vacancy formation and COOH formation from CO and OH adsorbed to Au and the ceria support, respectively. A reaction energy analysis shows that both steps are more favorable on clustered Au atoms than on a single Au atom. CO adsorption on a single Au atom is hindered because of its negative charge. Comparison to literature data shows that the WGS reaction is preferred for a gold cluster on the CeO2(110) surface over the CeO2(111) one because of the lower binding energy of OH on the former surface. These results are discussed in the light of a lar...

The pyrolysis of 2-methylfuran: a quantum chemical, statistical rate theory and kinetic modelling study

Abstract: Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice–Ramsperger–Kassel–Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1–C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

Density functional theory analysis of methanation reaction of CO2 on Ru nanoparticle supported on TiO2 (1 0 1)

Abstract: The methanation reaction of CO 2 on a Ru nanoparticle supported on TiO 2 catalyst has been investigated by density functional theory (DFT) using the generalized gradient approximation with periodic boundary conditions. Two plausible reaction paths were found for the transformation of CO 2 to CH 4 on TiO 2 -supported Ru nanoparticles. The origin of the high activity of the catalyst is discussed based on the overall reaction energy diagram obtained from DFT calculations. The CO 2 is readily and stably adsorbed on Ru cluster at moderate temperature as compared with that on bulk Ru surface. It is due to the difference of the Ru structure between the Ru nanoparticle and the bulk Ru surface. The elementary reactions of the hydrogenation of adsorbed CO and of the production of CH 4 are possible to become the rate-determining steps over the methanation reaction, because these two reactions have a higher potential energy barrier than that of other elementary reactions in the overall reaction path. These potential energy barriers for the hydrogenation of CO and the production of CH 4 on TiO 2 -supported Ru nanoparticles were lower than those on bulk Ru surface, which explains the high activity of the Ru nanoparticle-loaded TiO 2 catalyst. The lowering of these potential energy barriers can be caused by weak charge transfer between Ru atoms and adsorbed species on the TiO 2 -supported Ru nanoparticles. As the results, the catalytic activity of the Ru nanoparticles supported on TiO 2 catalyst is characterized by the structure of Ru nanoparticles and by the weak charge transfer between Ru atoms and adsorbed species.

Perspective: Bimolecular chemical reaction dynamics in liquids

Abstract: Bimolecular reactions in the gas phase exhibit rich and varied dynamical behaviour, but whether a profound knowledge of the mechanisms of isolated reactive collisions can usefully inform our understanding of reactions in liquid solutions remains an open question. The fluctuating environment in a liquid may significantly alter the motions of the reacting particles and the flow of energy into the reaction products after a transition state has been crossed. Recent experimental and computational studies of exothermic reactions of CN radicals with organic molecules indicate that many features of the gas-phase dynamics are retained in solution. However, observed differences may also provide information on the ways in which a solvent modifies fundamental chemical mechanisms. This perspective examines progress in the use of time-resolved infra-red spectroscopy to study reaction dynamics in liquids, discusses how existing theories can guide the interpretation of experimental data, and suggests future challenges for this field of research.

Timing of inorganic phosphate release modulates the catalytic activity of ATP-driven rotary motor protein.

Abstract: F1-ATPase is a rotary motor protein driven by ATP hydrolysis. The rotary motion of F1-ATPase is tightly coupled to catalysis, in which the catalytic sites strictly obey the reaction sequences at the resolution of elementary reaction steps. This fine coordination of the reaction scheme is thought to be important to achieve extremely high chemomechanical coupling efficiency and reversibility, which is the prominent feature of F1-ATPase among molecular motor proteins. In this study, we intentionally change the reaction scheme by using single-molecule manipulation, and we examine the resulting effect on the rotary motion of F1-ATPase. When the sequence of the products released, that is, ADP and inorganic phosphate, is switched, we find that F1 frequently stops rotating for a long time, which corresponds to inactivation of catalysis. This inactive state presents MgADP inhibition, and thus, we find that an improper reaction sequence of F1-ATPase catalysis induces MgADP inhibition.

Mercury Oxidation via Chlorine, Bromine, and Iodine under Atmospheric Conditions: Thermochemistry and Kinetics

Abstract: Emissions of gaseous mercury from combustion sources are the major source of Hg in the atmosphere and in environmental waters and soils. Reactions of Hgo(g) with halogens are of interest because they relate to mercury and ozone depletion events in the Antarctic and Arctic early spring ozone hole events, and the formation of Hg-halides (HgX2) is a method for removal of mercury from power generation systems. Thermochemistry and kinetics from published theoretical and experimental studies in the literature and from computational chemistry are utilized to compile a mechanism of the reactions considered as contributors to the formation of HgX2 (X = Cl, Br, I) to understand the reaction paths and mechanisms under atmospheric conditions. Elementary reaction mechanisms are assembled and evaluated using thermochemistry for all species and microscopic reversibility for all reactions. Temperature and pressure dependence is determined with quantum Rice Ramsperger Kassel (RRK) analysis for k(E) and master equation ana...

Development of linear free energy relationships for aqueous phase radical-involved chemical reactions.

Abstract: Aqueous phase advanced oxidation processes (AOPs) produce hydroxyl radicals (HO•) which can completely oxidize electron rich organic compounds. The proper design and operation of AOPs require that we predict the formation and fate of the byproducts and their associated toxicity. Accordingly, there is a need to develop a first-principles kinetic model that can predict the dominant reaction pathways that potentially produce toxic byproducts. We have published some of our efforts on predicting the elementary reaction pathways and the HO• rate constants. Here we develop linear free energy relationships (LFERs) that predict the rate constants for aqueous phase radical reactions. The LFERs relate experimentally obtained kinetic rate constants to quantum mechanically calculated aqueous phase free energies of activation. The LFERs have been applied to 101 reactions, including (1) HO• addition to 15 aromatic compounds; (2) addition of molecular oxygen to 65 carbon-centered aliphatic and cyclohexadienyl radicals; (3) disproportionation of 10 peroxyl radicals, and (4) unimolecular decay of nine peroxyl radicals. The LFERs correlations predict the rate constants within a factor of 2 from the experimental values for HO• reactions and molecular oxygen addition, and a factor of 5 for peroxyl radical reactions. The LFERs and the elementary reaction pathways will enable us to predict the formation and initial fate of the byproducts in AOPs. Furthermore, our methodology can be applied to other environmental processes in which aqueous phase radical-involved reactions occur.

Car–Parrinello Molecular Dynamics + Metadynamics Study of High-Temperature Methanol Oxidation Reactions Using Generic Collective Variables

Abstract: We used Car–Parrinello molecular dynamics (CPMD) and metadynamics in conjunction with the recently introduced social permutation invariant collective coordinates to study the mechanism of high-temperature methanol oxidation. Using a set of biased MD trajectories, we collected specific elementary reactions that arise during the simulations and assembled their connectivity in a small reaction network. A subset of the reaction network generated with metadynamics is compared to a consensus reaction network generated from many literature sources, and the many similarities indicate that this approach may be a useful way to enumerate bimolecular radical reactions in complex systems. We also demonstrated some intrinsic similarities to atomic contact maps used in our metadynamics approach and the reaction matrix/operators that are found in common mechanism generation algorithms. Extending the capabilities of these new generic collective variables for the study of complex reaction networks can help overcome limitat...

Single-molecule chemical reaction reveals molecular reaction kinetics and dynamics

Abstract: Understanding the microscopic elementary process of chemical reactions, especially in condensed phase, is highly desirable for improvement of efficiencies in industrial chemical processes. Here we show an approach to gaining new insights into elementary reactions in condensed phase by combining quantum chemical calculations with a single-molecule analysis. Elementary chemical reactions in liquid-phase, revealed from quantum chemical calculations, are studied by tracking the fluorescence of single dye molecules undergoing a reversible redox process. Statistical analyses of single-molecule trajectories reveal molecular reaction kinetics and dynamics of elementary reactions. The reactivity dynamic fluctuations of single molecules are evidenced and probably arise from either or both of the low-frequency approach of the molecule to the internal surface of the SiO2 nanosphere or the molecule diffusion-induced memory effect. This new approach could be applied to other chemical reactions in liquid phase to gain more insight into their molecular reaction kinetics and the dynamics of elementary steps.

Catalytic oxidation of CO with N2O on isolated copper cluster anions

Abstract: A catalytic redox reaction involving N2O and CO on size-selected copper cluster anions, Cun−, was investigated in the gas phase using a guided ion-beam tandem mass spectrometer. When Cun− is exposed to a mixture of N2O and CO, CunO− is produced via the decomposition of N2O. Increase of the CO partial pressure results in the reproduction of Cun− and decrease of CunO− through the oxidation of CO. The present results demonstrate that a full catalytic cycle for the reaction, N2O + CO → N2 + CO2, takes place on copper cluster anions. Furthermore, in the investigations of the elementary reactions of Cun− + N2O and CunO− + CO, we found that the catalytic oxidation of CO with N2O on Cun− proceeds most efficiently at n = 7 in the size range of n = 5–16.

The power-law reaction rate coefficient for an elementary bimolecular reaction

Abstract: The power-law TST reaction rate coefficient for an elementary bimolecular reaction is studied when the reaction takes place in a nonequilibrium system with power-law distributions. We derive a generalized TST rate coefficient, which not only depends on a power-law parameter but also on the reaction coordinate frequency of transition state. The numerical analyses show a very strong dependence of the TST rate coefficient on the power-law parameter, and clearly indicate that a tiny deviation from unity in the parameter (thus from a Boltzmann–Gibbs distribution) would result in significant changes in the rate coefficient. We take an elementary reaction, F + H 2 → FH + H , as an application example to calculate the reaction rate coefficient, and yield the rate values being exactly in agreement with the measurement values in all the experimental studies in the temperature range 190–765 K.

Spectroscopy and dynamics of the HOCO radical: insights into the OH + CO → H + CO2 reaction.

Abstract: After more than forty years of scrutiny, crucial new details regarding the elementary reaction OH + CO → H + CO2 are still emerging from experimental and theoretical studies of the HOCO radical intermediate. In this perspective, previous studies of this elementary reaction and the structure and energetics of the HOCO radical will be briefly reviewed. Particular attention will be paid to the experimental techniques used in our laboratory to prepare excited HOCO radicals by both photodetachment and dissociative photodetachment of HOCO−. These experiments directly probe the dynamics occurring on the ground and excited states of the HOCO radical, and are sensitive to both direct and tunneling-induced dissociation. Photoelectron–photofragment coincidence experiments on HOCO− in particular have been used to study tunneling from the HOCO well to form H + CO2 products. In addition, new experimental insights into the OH + CO entrance channel for the reaction will be presented. These studies have provided a number of constraints on the potential energy surface for this system from an energetic and dynamical perspective, and have helped spur a renewed effort to characterize the global potential energy surface and reaction dynamics of this fundamental chemical reaction. Outstanding questions and new directions for future work on the HOCO radical will be discussed.

Constant-Charge Reaction Theory for Potential-Dependent Reaction Kinetics at the Solid–Liquid Interface

Abstract: To understand the potential-dependent kinetics of reactions at the solid–liquid interface, we derive a constant-charge reaction theory for understanding the coupled charge transfer during the chemical bond making/breaking. The charge transfer coefficient (CTC) for reactions at the solid–liquid interface is shown to be linearly proportional to the electrochemical potential change from the initial state to the transition state as well the interface differential capacitance at the constant-charge model, and can be further related to the net dipole change normal to the surface during the reaction. Using the constant-charge theory, the CTC can be explicitly calculated on the basis of the first principles calculations without the need to assume the redox behavior of the elementary reactions and thus provide a unique possibility to evaluate and compare the magnitude of CTC for different reactions across different surfaces. By examining a series of interface reactions and comparing the calculated CTC values, we p...

Sustainable and near ambient DeNOx under lean burn conditions:a revisit to NO reduction on virgin and modified Pd(111) surfaces

Abstract: Catalytic conversion of NO in the presence of H2 and O2 has been studied on Pd(111) surfaces, by using a molecular beam instrument with mass spectrometry detection, as a function of temperature and reactants composition. N2 and H2O are the major products observed, along with NH3 and N2O minor products under all conditions studied. Particular attention has been paid to the influence of O2 addition toward NO dissociation. Although O2-rich compositions were found to inhibit the deNOx activity of the Pd catalyst, some enhancement in NO reduction to N2 was also observed up to a certain O2 content. The reason for this behavior was determined to be the effective consumption of the H2 in the mixture by the added O2 and O atoms from NO dissociation. NO was proven to compete favorably against O2 for the consumption of H2, especially ≤550 K, to produce N2 and H2O. Compared with other elementary reaction steps, a slow decay observed with the 2H + O → H2O step under SS beam oscillation conditions demonstrates its cont...

Microkinetics of oxygenate formation in the Fischer-Tropsch reaction

Abstract: Microkinetics simulations are presented on the intrinsic activity and selectivity of the Fischer–Tropsch reaction with respect to the formation of long chain oxygenated hydrocarbons. Two different chain growth mechanisms are compared: the carbide chain growth mechanism and the CO insertion chain growth mechanism. The microkinetics simulations are based on quantum-chemical data on reaction rate parameters of the elementary reaction steps of the Fischer–Tropsch reaction available in the literature. Because the overall rate constant of chain growth remains too low the CO insertion chain growth mechanism is not found to produce higher hydrocarbons, except for ethylene and acetaldehyde or the corresponding hydrogenated products. According to the carbide mechanism available quantum-chemical data are consistent with high selectivity to long chain oxygenated hydrocarbon production at low temperature. The anomalous initial increase with temperature of the chain growth parameter observed under such conditions is reproduced. It arises from the competition between the apparent rate of C–O bond activation to produce “CHx” monomers to be inserted into the growing hydrocarbon chain and the rate of chain growth termination. The microkinetics simulations data enable analysis of selectivity changes as a function of critical elementary reaction rates such as the rate of activation of the C–O bond of CO, the insertion rate of CO into the growing hydrocarbon chain or the rate constant of methane formation. Simulations show that changes in catalyst site reactivity affect elementary reaction steps differently. This has opposing consequences for oxygenate production selectivity, so an optimizing compromise has to be found. The simulation results are found to be consistent with most experimental data available today. It is concluded that Fischer–Tropsch type catalysis has limited scope to produce long chain oxygenates with high yield, but there is an opportunity to improve the yield of C2 oxygenates.

The mechanism of Menshutkin reaction in gas and solvent phases from the perspective of reaction electronic flux

Abstract: The mechanism of Menshutkin reaction, NH3 + CH3Cl = (CH3-NH3)+ + Cl-, has been thoroughly studied in both gas and solvent (H2O and cyclohexane) phase. It has been found that solvents favor the reaction, both thermody- namically and kinetically. The electronic activity that drives the mechanism of the reaction was identified, fully character- ized, and associated to specific chemical events, bond forming/breaking processes, by means of the reaction elec- tronic flux. This led to a complete picture of the reaction mechanism that was independently confirmed by natural bond-order analysis and the dual descriptor for chemical re- activity and selectivity along the reaction path.

Kinetic modeling of α-hydrogen abstractions from unsaturated and saturated oxygenate compounds by carbon-centered radicals.

Abstract: Hydrogen abstractions are important elementary reactions in a variety of reacting media at high temperatures in which oxygenates and hydrocarbon radicals are present. Accurate kinetic data are obtained from CBS-QB3 ab initio (AI) calculations by using conventional transition-state theory within the high-pressure limit, including corrections for hindered rotation and tunneling. From the obtained results, a group-additive (GA) model is developed that allows the Arrhenius parameters and rate coefficients for abstraction of the a-hydrogen from a wide range of oxygenate compounds to be predicted at temperatures ranging from 300 to 1500 K. From a training set of 60 hydrogen abstractions from oxygenates by carbon-centered radicals, 15 GA values (Delta GAV degrees s) are obtained for both the forward and reverse reactions. Among them, four Delta GAV degrees s refer to primary contributions, and the remaining 11 Delta GAV degrees s refer to secondary ones. The accuracy of the model is further improved by introducing seven corrections for cross-resonance stabilization of the transition state from an additional set of 43 reactions. The determined Delta AV degrees s are validated upon a test set of AI data for 17 reactions. The mean absolute deviation of the pre-exponential factors (log A) and activation energies (Ea) for the forward reaction at 300 K are 0.238 log(m(3) mol(-1) s(-1)) and 1.5 kJ mol(-1), respectively, whereas the mean factor of deviation between the GA-predicted and the AI-calculated rate coefficients is 1.6. In comparison with a compilation of 33 experimental rate coefficients, the between the GA-predicted values and these experimental values is only 2.2. Hence, the constructed GA model can be reliably used in the prediction of the kinetics of a-hydrogen-abstraction reactions between a broad range of oxygenates and oxygenate radicals.