Showing papers on "Elementary reaction published in 2012"
TL;DR: The reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant, and the product angular and internal distributions of such reactions also bear clear signatures as mentioned in this paper.
Abstract: Many gas-phase chemical reactions proceed via reaction intermediates, supported by potential wells. The characteristics of such complex-forming reactions differ drastically from those for direct reactions that involve barriers. For example, the reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant. The product angular and internal distributions of such reactions also bear clear signatures. Specifically, the angular distribution (i.e. differential cross-section) of a complex-forming reaction is often dominated by scattering in the forward and backward directions, and the product rotational state distribution usually peaks near the highest accessible rotational state, while vibrational state distribution often decays monotonically. While the quantum dynamics of direct reactions is well established, our understanding of complex-forming reactions is still far from complete. Given the importance of such react...
207 citations
TL;DR: A set of calculated reaction energies and activation energies for a large number of elementary surface reactions on a series of metal single-crystal surfaces, including surfaces with defects such as steps is introduced and a simple visual query tool for accessing them is developed.
Abstract: Solid catalysts form the backbone of the chemical industry and the hydrocarbon-based energy sector. Most catalysts and processes today are highly optimized, but there is still considerable room for improvements in reactivity and selectivity in order to lower energy consumption and waste production. In addition, the development of sustainable energy solutions is a tremendous challenge to catalysis science and engineering. The ability to store solar energy as a fuel calls for new catalysts, as does the development of a sustainable chemical industry that is based on biomass and other non-fossil building blocks. The development of new catalysts could be accelerated significantly if we had access to systematic data for the activation energies of elementary surface reactions. Once the key parameters that determine the activity or selectivity of a certain process have been established through experiments or calculations, such a database would enable searches for new catalyst leads. Ideally, data would come from detailed, systematic experiments, but it is generally not possible to find such data. Electronic structure calculations provide a powerful alternative. The accuracy is not such that detailed predictions of absolute rates of elementary reaction steps can be made, but for classes of interesting catalysts (such as transition metals) it is possible to create systematic data with sufficient accuracy to predict trends in reactivity. Herein, we introduce such a set of calculated reaction energies and activation energies for a large number of elementary surface reactions on a series of metal single-crystal surfaces, including surfaces with defects such as steps. We also introduce a simple web application (CatApp) for accessing these data. The data will be part of a larger database of surface reaction data that are being developed under the Quantum Materials Informatics Project. The database includes reaction energies for all surface reactions that involve C C, C H, C O, O O, O H, N N, C N, O N, N H splitting for molecules with up to three C, N, or O atoms on close-packed face-centered cubic fcc(111), hexagonal close-packed hcp(0001), and body-centered cubic bcc(110) surfaces, as well as stepped fcc and hcp surfaces. The metals included in the database are Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Re, Rh, Ru, Sc, V. The data have been compiled from previous reports, where details of the calculations can be found. The key point is that the values have all been calculated with the same code (DACAPO), the same exchange-correlation energy functional (GGARPBE), and similar calculational parameters. Therefore one adsorption energy or reaction barrier can be compared to another with some confidence. Gas-phase CO2 and O2 , for which the RPBE functional performs poorly, were corrected as described in Refs. [25] and [26], respectively In cases where there are no calculated data for a given reaction, we use the recently developed scaling relations to provide an estimate. The scaling relations link the adsorption energies of different molecules that contain varying amounts of hydrogen. In a similar fashion, we exploit the fact that transition-state energies are quite generally found to scale with reaction energies. We have developed a simple visual query tool for accessing the data presented above. On our homepage, we maintain a list of hyperlinks to the available versions of the tool, together with a list of references to the scientific data it employs. The tool is a web application implemented in JavaScript, SVG, and HTML, and runs in modern web browsers without any plug-ins. The application can be easily used on computers and portable devices with a touch interface. By running the application, one can choose a surface and an elementary reaction and be presented with a reaction path that reports the reaction and activation energy. If a DFT value is not found for a given reaction and a scaling estimate is used, the value is shown in italic type. In Figure 1 we have shown an example of the use of the application. The energy barrier needed to break the N2 bond on two different surface orientations of ruthenium, Ru(0001) and stepped Ru(0001), are extracted. The plots immediately show the structure dependence of this important step in the Haber–Bosch process (N2+ 3H2!2NH3). Our web application will allow anyone to download data such as that shown in Figure 1, and to quickly explore whether there may be other metals or structures where the N2 bond is broken more readily. All code and data are downloaded when the application is accessed for the first time and is kept in the local storage of the browser. This feature allows the application to be used even when the user has no internet connection. More importantly, it guarantees the user complete privacy, since all queries are performed locally in the browser and not by connecting to our server. The only information that is delivered from the user to our server is an anonymous [*] Dr. J. S. Hummelshoj, Dr. F. Abild-Pedersen, Dr. F. Studt, Dr. T. Bligaard, Prof. J. K. Norskov SUNCAT Center for Interface Science and Catalysis SLAC National Accelerator Laboratory 2575 Sand Hill Road, Menlo Park, CA 94025 (USA)
134 citations
TL;DR: In this article, the effect of reducibility of the support on the mechanism of the reverse water gas shift reaction over Pt/TiO 2 and Pt/Al 2 O 3 catalysts was examined using a differential and fixed bed reactor.
Abstract: In this study, the effect of the reducibility of the support on the mechanism of the reverse water gas shift reaction over Pt/TiO 2 and Pt/Al 2 O 3 catalysts was examined using a differential and fixed bed reactor. The kinetic study showed that the reverse water gas shift reaction using the Pt/TiO 2 and Pt/Al 2 O 3 catalysts were consistent with the redox mechanism. An elementary reaction test and XPS analysis further confirmed that the reverse water gas shift reaction proceeded through the redox mechanism via oxidation and reduction at the Pt sites and reducible support sites on the catalyst surface. In the H 2 TPR and FT-IR experiments, the Pt/TiO 2 was shown to have a new active site located at the metal–support interface. Therefore, the Pt/TiO 2 catalyst produced greater CO 2 conversion and TOFs due to the presence of the new active site, which resulted from the strong metal–support interaction and higher reducibility of the support relative to the Pt/Al 2 O 3 catalyst.
116 citations
TL;DR: In this paper, the mechanism of thermal decomposition of levoglucosan (1,6-anhydro-β-d-glucopyranose) was studied using density functional theory methods.
87 citations
TL;DR: This work investigates how water could be a catalyst in an atmospheric reaction in the laboratory but is slow under atmospheric conditions when taking into account the concentration of the prereactive bimolecular complex.
Abstract: The effect of a single water molecule on the reaction between H2O2 and HO has been investigated by employing MP2 and CCSD(T) theoretical approaches in connection with the aug-cc-PVDZ, aug-cc-PVTZ, and aug-cc-PVQZ basis sets and extrapolation to an ∞ basis set. The reaction without water has two elementary reaction paths that differ from each other in the orientation of the hydrogen atom of the hydroxyl radical moiety. Our computed rate constant, at 298 K, is 1.56 × 10–12 cm3 molecule–1 s–1, in excellent agreement with the suggested value by the NASA/JPL evaluation. The influence of water vapor has been investigated by considering either that H2O2 first forms a complex with water that reacts with hydroxyl radical or that H2O2 reacts with a previously formed H2O·OH complex. With the addition of water, the reaction mechanism becomes much more complex, yielding four different reaction paths. Two pathways do not undergo the oxidation reaction but an exchange reaction where there is an interchange between H2O2·...
84 citations
TL;DR: This study identifies and assigns the signature of the complex decomposition and reaction pathways of riboflavin in the presence of the free radical scavenger Trolox using DNA-wrapped SWCNT sensors dispersed onto an aminopropyltriethoxysilane coated surface.
Abstract: Single-mol. fluorescent microscopy allows semiconducting single-walled carbon nanotubes (SWCNTs) to detect the adsorption and desorption of single adsorbate mols. as a stochastic modulation of emission intensity. In this study, we identify and assign the signature of the complex decompn. and reaction pathways of riboflavin in the presence of the free radical scavenger Trolox using DNA-wrapped SWCNT sensors dispersed onto an aminopropyltriethoxysilane (APTES) coated surface. SWCNT emission is quenched by riboflavin-induced reactive oxygen species (ROS), but increases upon the adsorption of Trolox, which functions as a reductive brightening agent. Riboflavin has two parallel reaction pathways, a Trolox oxidizer and a photosensitizer for singlet oxygen and superoxide generation. The resulting reaction network can be detected in real time in the vicinity of a single SWCNT and can be completely described using elementary reactions and kinetic rate consts. measured independently. The reaction mechanism results in an oscillatory fluorescence response from each SWCNT, allowing for the simultaneous detection of multiple reactants. A series-parallel kinetic model is shown to describe the crit. points of these oscillations, with partition coeffs. on the order of 10-6-10-4 for the reactive oxygen and excited state species. These results highlight the potential for SWCNTs to characterize complex reaction networks at the nanometer scale. [on SciFinder(R)]
60 citations
TL;DR: A new reaction path searching method is developed by combining the recently developed transition state (TS) location method, namely, the constrained Broyden dimer method, with a basin-filling method via bias potentials, which allows the system to walk out from the energy traps at a given reaction direction.
Abstract: To predict the chemical activity of new matter is an ultimate goal in chemistry. The identification of reaction pathways using modern quantum mechanics calculations, however, often requires a high demand in computational power and good chemical intuition on the reaction. Here, a new reaction path searching method is developed by combining our recently developed transition state (TS) location method, namely, the constrained Broyden dimer method, with a basin-filling method via bias potentials, which allows the system to walk out from the energy traps at a given reaction direction. In the new method, the reaction path searching starts from an initial state without the need for preguessing the TS-like or final state structure and can proceed iteratively to the final state by locating all related TSs and intermediates. In each elementary reaction step, a reaction direction, such as a bond breaking, needs to be specified, the information of which is refined and preserved as a normal mode through biased dimer rotation. The method is tested successfully on the Baker reaction system (50 elementary reactions) with good efficiency and stability and is also applied to the potential energy surface exploration of multistep reaction processes in the gas phase and on the surface. The new method can be applied for the computational screening of new catalytic materials with a minimum requirement of chemical intuition.
55 citations
TL;DR: In this article, the Single Event Microkinetic (SEMK) methodology has been applied towards "ethylbenzene dealkylation/xylene isomerization" under industrially relevant conditions.
Abstract: The Single-Event Microkinetic (SEMK) methodology has been applied towards “ethylbenzene dealkylation/xylene isomerization” under industrially relevant conditions. This includes the isomerization of xylenes towards thermodynamic equilibrium, the dealkylation of ethylbenzene as well as a limited amount of xylene transalkylation into toluene and trimethylbenzenes. By accounting for symmetry effects through the calculation of the number of single events and defining elementary reaction families rather than applying product lumping, a huge reduction in the number of adjustable parameters can be achieved without the loss of the molecular detail in the reaction network. In the kinetic model, 37 components, 78 intermediates and a total of 327 elementary reaction steps, classified in families such as (de-)protonation, alkyl shift, dealkylation, transalkylation and hydrogenation, are considered. Only reactant protonation enthalpies and the activation energies of the considered reaction families, are estimated by model regression to experimental data acquired on a bifunctional Pt/H-ZSM-5 catalyst, while the remaining parameters are determined from first principles or retrieved from literature information. The experimental data are adequately described with physically significant parameter values. Dealkylation is found to be energetically most demanding with an activation energy amounting to 198 kJ mol −1 , while alkyl shift and transalkylation reactions are having the lowest activation energies. Entropic effects result in the lowest rate coefficient for transalkylation, however, and rate coefficients of a similar order of magnitude for alkyl shift and dealkylation. The investigated catalyst is shown to have an adequate acid strength for establishing the thermodynamic equilibrium between the xylene isomers.
55 citations
TL;DR: In this article, the effect of water on the bonding characteristics of transition metal surfaces with adsorbates is evaluated by a combination of plane-wave density functional theory calculations in vacuum and properly chosen cluster model calculations with and without an implicit solvation model.
Abstract: It is shown that the effect of water on the bonding characteristics of transition metal surfaces with adsorbates is short-ranged. As a result, adsorption energies in water can be evaluated by a combination of plane-wave density functional theory calculations in vacuum and properly chosen cluster model calculations with and without an implicit solvation model. The scheme is demonstrated for a model C–C cleavage reaction on Pt (111) and for predicting CO frequency shifts on Pd and Pt due to water. We conclude that these shifts originate from water–metal interactions and can be explained by changes in π back-donation. Overall, the results demonstrate that the proposed methodology represents a highly efficient computational approach for approximating the effect of solvents on elementary reaction steps occurring at solid–liquid interfaces of heterogeneous catalysts.
47 citations
TL;DR: In this article, a hot-wire-assisted atomic layer deposition (HW-ALD) has been identified as a successful method to form high quality metallic films using metallocene and NH3.
Abstract: Hot-wire-assisted atomic layer deposition (HW-ALD) has been identified as a successful method to form high quality metallic films using metallocene and NH3. A cobalt film formed by HW-ALD using cobaltocene and NH3 was successfully demonstrated. The authors have elucidated the mechanism of HW-ALD during the precursor feed period and the reducing period. In the case of cobalt, a deposition temperature above 300 °C is needed to avoid an inclusion of carbon impurities. This is because the physisorbed species are involved during the precursor feed period. NH2 radical promotes the dissociation of the carbon–metal bond during the reducing period. This is examined by elucidation of the gas-phase kinetics, estimation of the surface reactions by quantum chemical calculations, and analysis of the exhaust gas using a quadrupole mass spectrometer.
46 citations
TL;DR: First principles‐based kinetic Monte Carlo simulations are performed for the CO oxidation on RuO2(110) under steady‐state reaction conditions and it is found that already small variations in the activation energies lead to distinctly different reaction scenarios on the surface.
Abstract: First principles-based kinetic Monte Carlo (kMC) simulations are performed for the CO oxidation on RuO(2) (110) under steady-state reaction conditions. The simulations include a set of elementary reaction steps with activation energies taken from three different ab initio density functional theory studies. Critical comparison of the simulation results reveals that already small variations in the activation energies lead to distinctly different reaction scenarios on the surface, even to the point where the dominating elementary reaction step is substituted by another one. For a critical assessment of the chosen energy parameters, it is not sufficient to compare kMC simulations only to experimental turnover frequency (TOF) as a function of the reactant feed ratio. More appropriate benchmarks for kMC simulations are the actual distribution of reactants on the catalyst's surface during steady-state reaction, as determined by in situ infrared spectroscopy and in situ scanning tunneling microscopy, and the temperature dependence of TOF in the from of Arrhenius plots.
TL;DR: In this article, the reaction mechanisms for thermal decomposition of cyclohexane in the gas phase have been investigated using quantum chemical calculations and transition-state theory, and three series of reaction schemes containing 38 elementary reactions are proposed.
Abstract: In the present work, the reaction mechanisms for thermal decomposition of cyclohexane in the gas phase have been investigated using quantum chemical calculations and transition-state theory Three series of reaction schemes containing 38 elementary reactions are proposed The geometry optimization and vibrational frequencies of reactants, transition states, and products are determined at the BH&HLYP/cc-pVDZ level, while energies are calculated at the CCSD(T)/cc-pVDZ level The rate constants for the reactions without transition states, including the initial steps of cyclohexane decomposition (C–C bond scission or C–H bond scission), are obtained by the canonical variational transition-state theory (CVT), while the rate constants for the other reactions with saddle-point transition states are obtained by the conventional transition-state theory (TST) in the temperature range of 300–3000 K The rate constants are in good agreement with data available from the literature The kinetic parameters in the modifi
TL;DR: In this article, the extinction limits of methyl butanoate, n-heptane, and methyl decanoate diffusion flames were measured as a function of fuel mole fraction with nitrogen dilution in counterflow with air.
TL;DR: In this paper, the authors compared building and decomposition pathways of two phosphonic acids, amino trimethylene phosphonic acid (ATMP) and 1-hydroxy ethylidene-1,1-diphosphonic acid(HEDP), using elementary reactions and compared to reaction routes published in literature.
TL;DR: The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was determined and the activation energy was established to be 107 kJ mol(-1).
Abstract: Decomposition of nitrous acid in aqueous solution has been studied by stopped flow spectrophotometry to resolve discrepancies in literature values for the rate constants of the decomposition reactions. Under the conditions employed, the rate-limiting reaction step comprises the hydrolysis of NO(2). A simplified rate law based on the known elementary reaction mechanism provides an excellent fit to the experimental data. The rate constant, 1.34 × 10(-6) M(-1) s(-1), is thought to be of higher accuracy than those in the literature as it does not depend on the rate of parallel reaction pathways or on the rate of interphase mass transfer of gaseous reaction products. The activation energy for the simplified rate law was established to be 107 kJ mol(-1). Quantum chemistry calculations indicate that the majority of the large activation energy results from the endothermic nature of the equilibrium 2HNO(2) ⇆ NO + NO(2) + H(2)O. The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was also determined.
TL;DR: In this article, the authors present density functional theory calculations on the direct synthesis of H2O2 from H2 and O2 over an Au12 corner model of a gold nanoparticle.
Abstract: We present density functional theory calculations on the direct synthesis of H2O2 from H2 and O2 over an Au12 corner model of a gold nanoparticle. We first show a simple route for the direct formation of H2O2 over a gold nanocatalyst, by studying the energetics of 20 possible elementary reactions involved in the oxidation of H2 by O2. The unwanted side reaction to H2O is also considered. Next we evaluate the degree of catalyst control and address the factors controlling the activity and the selectivity. By combining well-known energy scaling relations with microkinetic modeling, we show that the rate of H2O2 and H2O formation can be determined from a single descriptor, namely, the binding energy of oxygen (EO). Our model predicts the search direction starting from an Au12 nanocluster for an optimal catalyst in terms of activity and selectivity for direct H2O2 synthesis. Taking also stability considerations into account, we find that binary Au–Pd and Au–Ag alloys are most suited for this reaction.
TL;DR: Three different type of reaction mechanisms with two different types of transition states, leading to two different products are investigated, found to play a crucial role in this reaction.
Abstract: The mechanism of a simple SN2 reaction, viz; OH– + CH3F = CH3OH + F– has been studied within the framework of reaction force and reaction electronic flux. We have computationally investigated three different types of reaction mechanisms with two different types of transition states, leading to two different products. The electronic transfer contribution of the reaction electronic flux was found to play a crucial role in this reaction. Natural bond order analysis and dual descriptor provide additional support for elucidating the mechanism of this reaction.
TL;DR: A computational study using density functional theory is carried out to investigate the reaction mechanism of ethanol steam reforming on Co(0001) surfaces, and it is found that the reaction of formaldehyde yielding formic acid and hydrogen may not be an elementary reaction.
Abstract: A computational study using density functional theory is carried out to investigate the reaction mechanism of ethanol steam reforming on Co(0001) surfaces. The adsorption properties of the reactant, possible intermediates, and products are carefully examined. The reaction pathway and related transition states are also analyzed. According to our calculations, the reforming mechanism primarily consisting of dehydrogenation steps of ethanol, ethoxy, methanol, methoxy, and formic acid, is feasible on Co(0001) surfaces. It is also found that the reaction of formaldehyde yielding formic acid and hydrogen may not be an elementary reaction. The dehydrogenation of ethoxy possesses the highest barrier and is accordingly identified as the rate-determining step.
TL;DR: In this article, a hierarchical multiscale approach is presented to calculate effective rates of reaction for zeolite catalyzed reaction systems, where the first step involves the determination of intrinsic rate coefficients for all elementary reactions by means of quantum chemical calculations combined with transition state theory.
Abstract: A hierarchical multiscale approach is presented to calculate effective rates of reaction for zeolite catalyzed reaction systems. The first step in this approach involves the determination of intrinsic rate coefficients for all elementary reactions by means of quantum chemical calculations combined with transition state theory. The second step is the calculation of adsorption isotherms and diffusion coefficients of all species by means of Monte Carlo and molecular dynamics simulations. The third step comprises a continuum description of a zeolite crystal based on the reaction-diffusion equation. For coupling the intrinsic rate coefficients obtained in the first step to the continuum variables, a model is proposed that calculates the local concentration of reactants at the catalytically active centers from the total concentrations of adsorbed species. The adsorption isotherms and diffusivities are coupled to the continuum variables by analytical theories such as the ideal adsorbed solution theory and the Ma...
TL;DR: In this article, a reduction strategy of hydrogen sulfide/oxygen reaction mechanism is conducted to simplify the detailed mechanism, which is used to track changes in mechanistic pathways of Claus reactions with the reaction progress.
TL;DR: In this article, the isothermal kinetics of sucrose hydrolysis at the acidic ion exchange resin type IR-120 H under conventional (CH) and microwave heating (MWH) were investigated.
Abstract: The isothermal kinetics of sucrose hydrolysis at the acidic ion-exchange resin type IR-120 H under conventional (CH) and microwave heating (MWH) was investigated. Isothermal kinetics curves in the temperature range from 303 to 343 K for both CH and MWH were determined. By application the model-fitting method, it was recognized that the kinetics of sucrose hydrolysis can be described by a first-order chemical reaction for both heating modes. The values of the activation energy ( E a ) and pre-exponential factor (ln A ) for sucrose hydrolysis were found to be lower under MWH than under CH. Application of the differential isoconversional method showed that sucrose hydrolysis was kinetically an elementary reaction. It is found that the increased rate of hydrolysis observed under MWH was not a consequence of overheating. A new explanation of the established effects of microwave heating based on a model of selective energy transfer during the chemical reaction is suggested. The established decreases in the activation energy and in the pre-exponential factor under MWH in comparison to CH is explained by an increase in the energy of the ground vibrational level of the –OH out-of-plane deformation in the sucrose molecule and with a decrease in the anharmonicity factor, which is caused by the selective resonant transfer of energy from the catalyst to the –OH oscillators in the sucrose molecules.
TL;DR: Numerical simulations over pH = 3-12 show that a number of pH- and temperature dependent trends in reaction rates and positions of equilibrium are well described with this simple dimerization model.
Abstract: The first steps in a pH- and temperature-dependent theoretical kinetic model of silicate polymerization and dissolution are examined in this work with a combined ab initio and transition state theory based study of the dimerization of H4SiO4. The role of solvation has been of primary concern in this work, and its influence on theoretical activation energies and pre-exponential factors has been thoroughly benchmarked. Relatively inexpensive MP2/6-31+G(d)//HF/6-31+G(d) calculations of octahydrate clusters, with conductor-like polarizable continuum model corrections obtained in the MP2-level single-point calculations, have been shown to lead to a good description of the limited experimentally determined energetics of dimerization for most elementary reactions. Pre-exponential factors computed from this level of theory are found to be relatively insensitive to the level of theory utilized for geometry optimizations, the number of explicit waters, hindered rotor corrections, and variational effects arising from the minimization of rate constants. Within this framework, a kinetic model of the chemistry of H4SiO4 and H3SiO4−, forming H6Si2O7 and H5Si2O7−, has been compiled. Numerical simulations over pH = 3–12 show that a number of pH- and temperature dependent trends in reaction rates and positions of equilibrium are well described with this simple dimerization model. More specifically to the dimerization process, we obtain dimerization constants, log Kdim, of 1.85 and −7.15 for the formation of H6Si2O7 and H5Si2O7− respectively, which compare well with experimentally determined values of 1.2 and −8.5, respectively.
TL;DR: In this article, the formation of levoglucosan and formaldehyde from anhydroglucose radical from cellulose pyrolysis has been carried out theoretically and the geometries and frequencies of all the stationary points are calculated at the B3LYP/6-31+G(D,P) level based on quantum mechanics.
TL;DR: In this paper, the capabilities of the crossed molecular beam (CMB) method with mass spectrometric detection in the study of multi-channel elementary reactions of relevance in combustion chemistry are illustrated by several examples of reactions involving atomic oxygen and unsaturated hydrocarbons (acetylene, ethylene and allene), as well as hydrocarbon radicals (methyl and allyl).
TL;DR: In this article, a set of experiments were carried out in a continuous fixed-bed reactor to investigate the relative catalytic activities of LaMnO3 and L0.95Pd0.05O3 for the partial oxidation of ethanol to acetaldehyde.
Abstract: A set of experiments was carried out in a continuous fixed-bed reactor to investigate the relative catalytic activities of LaMnO3 and LaMn0.95Pd0.05O3 for the partial oxidation of ethanol to acetaldehyde. Both catalysts were prepared by the sol–gel method. The resultant data have indicated that LaMn0.95Pd0.05O3 is more active than LaMnO3 and that acetaldehyde selectivities of both are close at around 90%. To gain an in-depth understanding of perovskite’s chemistry involved, kinetic analysis of the data has been conducted with the differential method. Accordingly, eight elementary reactions have been proposed by resorting to the Mars–van Krevelen redox cycle. Subsequently, these elementary reactions have been lumped into five steps comprising ethanol adsorption, oxygen adsorption, surface reaction, acetaldehyde desorption, and water desorption. This has rendered it possible to derive a set of rate equations based on the Langmuir–Hinshelwood–Hougen–Watson formalism. The exploration of these rate equations h...
TL;DR: In this paper, the mechanism of DMC synthesis on Cu-exchanged zeolite β has been investigated employing density functional theory (DFT) calculations and a double numerical plus polarization (DNP) basis set.
Abstract: The mechanism of dimethyl carbonate (DMC) synthesis on Cu-exchanged zeolite β has been investigated employing density functional theory (DFT) calculations and a double numerical plus polarization (DNP) basis set. The adsorption energy (ΔE) and decomposition activation energy (Ea) for O2 are −1.84 and 1.72 eV, respectively, suggesting that the decomposition of O2 occurs readily under reaction conditions on the Cu site. The formed O atom further reacts with methanol to form surface-bound (CH3O)(OH)–Cu(I)/β, in which CH3O and OH were coadsorbed on the Cu+ of the catalyst; this process proceeds without an activation barrier and with an energy release of 1.23 eV. The (CH3O)(OH)–Cu(I)/β species then reacts with another methanol molecule and carbon monoxide to produce DMC through two different reaction pathways. In path I, insertion of carbon monoxide into the (CH3O)(OH)–Cu(I)/β leads to the formation of monomethyl carbonate species (CH3OCOOH), which then reacts with methanol to produce DMC and H2O. The activation energies for both steps are 0.97 and 0.65 eV, respectively. In path II, (CH3O)(OH)–Cu(I)/β reacts with methanol first to produce a dimethoxide species ((CH3O)(CH3O)–Cu(I)/β), and the formation of DMC is via the insertion of carbon monoxide into the (CH3O)(CH3O)–Cu(I)/β. The activation energies for these elementary reactions are 0.65 and 0.70 eV, respectively. The topological properties of electron density distributions for all the related stationary points involved in this reaction have also been examined using the atoms in molecule (AIM) theory for the illustration of the bond paths and weak interactions of all the stationary points in the reaction path.
TL;DR: In this article, the most likely reaction pathways and reaction products in the Al-CH4-O2-N2 system were investigated using density functional theory and ab initio calculations.
Abstract: The most likely reaction pathways and reaction products in the Al-CH4-O2-N2 system are investigated using density functional theory and ab initio calculations. The B3LYP functional with extended 6–311+G(3df,2p) basis set as well as the CBS-QB3 composite method are mainly utilised. Theoretical analysis of corresponding reaction rate constants is also performed with the use of simple theoretical models. A critical overview of current knowledge on combustion-relevant reactions with aluminium compounds is given. On the basis of critical comparison of available experimental kinetic data with theoretical calculations, the approximations for rate constants for 44 reversible elementary reactions involving Al-containing species are recommended for use in combustion issues.
TL;DR: In this paper, an experimental study using a flow reactor and a numerical simulation with a detailed kinetic model have been performed to investigate the reaction mechanism of NO formation from pyrrole decomposition.
TL;DR: Using first-principles calculations based on density functional theory, it is shown that the barriers of rate-limiting steps for technologically relevant surface reactions are significantly reduced if concerted reaction mechanisms are taken into account.
Abstract: Any technologically important chemical reaction typically involves a number of different elementary reaction steps consisting of bond-breaking and bond-making processes. Usually, one assumes that such complex chemical reactions occur in a step-wise fashion where one single bond is made or broken at a time. Using first-principles calculations based on density functional theory we show that the barriers of rate-limiting steps for technologically relevant surface reactions are significantly reduced if concerted reaction mechanisms are taken into account.
TL;DR: A chemical model for the camphene ozonolysis, leading to carbonyl final products, is proposed and discussed on the basis of the thermochemical properties and kinetic data obtained at density functional theory levels of calculation, which will contribute to a better understanding of the chemistry of monoterpenes in the atmosphere, as well as the implications for the phenomena of pollution.
Abstract: In this work, a chemical model for the camphene ozonolysis, leading to carbonyl final products, is proposed and discussed on the basis of the thermochemical properties and kinetic data obtained at density functional theory levels of calculation. The mechanism is initiated by the electrophilic attack of ozone to the double bond in camphene leading to a 1,2,3-trioxolane intermediate, which decomposes to peroxy radicals and carbonyl compounds in a total of 10 elementary reactions. The thermodynamic properties (enthalpy and entropies differences) are calculated at 298 K. For the thermochemical evaluation, theoretical calculations are performed with the B3LYP, MPW1PW91, and mPW1K density functionals and the basis sets 6-31G(d), 6-31G(2d,2p), 6-31+G(d,p), and 6-31+G(2d,2p). Eventually, single point calculations adopting the 6-311++G(2d,2p) basis set are performed in order to improve the electronic energies. The enthalpy profiles suggest highly exothermic reactions for the individual steps, with a global enthalpy difference of -179.18 kcal mol(-1), determined at the B3LYP∕6-31+G(2d,2p) level. The Gibbs free energy differences for each step, at 298 K, calculated at the B3LYP∕6-311++G(2d,2p)∕∕B3LYP∕6-31+G(2d,2p) level, are used to estimate the composition of a final product mixture under equilibrium conditions as 58% of camphenilone and 42% of 6,6-dimethyl-ɛ-caprolactone-2,5-methylene. For the reaction kinetics, the bimolecular O(3) + camphene step is assumed to be rate determining in the global mechanism. A saddle point for the ozone addition to the double bond is located and rate constants are determined on the basis of the transition state theory. This saddle point is well represented by a loosely bound structure and corrections for the basis set superposition error (BSSE) are calculated, either by considering the effect over the geometry optimization procedure (here referred as CP1 procedure), or the effect of the BSSE over the electronic energy of a previously optimized geometry, included a posteriori (here referred as CP2). The rate constants, calculated at 298 K from the data obtained at the mPW1K∕6-31+G(d,p), CP1∕B3LYP∕∕6-31+G(2d,2p), and CP2∕B3LYP∕∕6-31+G(2d,2p) levels (3.62 × 10(-18), 1.12 × 10(-18), and 1.39 × 10(-18) cm(3) molecule(-1) s(-1)), are found in good agreement with the available experimental data at the same temperature, 0.9 × 10(-18) cm(3) molecule(-1) s(-1) [R. Atkinson, S. M. Aschmann, and J. Arey, Atmos. Environ. 24, 2647 (1990)]. The importance of the BSSE corrections for the final rate constants must be pointed out. Furthermore, this work will contribute to a better understanding of the chemistry of monoterpenes in the atmosphere, as well as the implications for the phenomena of pollution.