Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

The Transient Method and Elementary Steps in Heterogeneous Catalysis

Abstract: It is possible to consider the local rate of reaction in a fluid phase as a function of state variables, r(C, p, T). We assume that the concentrations of all the components C, which may, for example, be important in initiation and termination are included in the description. The rate must also be slow compared to other phenomena which might create a nonuniform mixture on the scale in which we are interested. If these restrictions are respected, we then can describe the rate in terms of a sequence of elementary steps, providing all the active intermediates are known. For gases the rate parameters (frequency factors, activation energy) for many elementary reactions have been measured; these matters are discussed, for example, in the book by Johnston [l]. The rate r (g-mole/cm3-sec) is in a certain sense a state function; it becomes related to a derivative or other quantities only as it appears in the conservation equations.

Elementary reaction modeling of high-temperature benzene combustion

Abstract: We have developed an elementary reaction mechanism containing 514 reactions without adjusted parameters for the low-pressure flaming rich combustion of benzene. The starting point for the present mechanism is the benzene sub-mechanism of Emdee, Brezinsky, and Glassman. Key features of the mechanism are: accounting for pressure-dependent unimolecular and bimolecular (chemically activated) reactions using QRRK, inclusion of singlet methylene chemistry, and phenyl radical oxidation and pyrolysis reactions. The results are compared to the detailed molecule and free radical profiles measured by Bittner and Howard using a molecular beam mass spectrometer. In general, the present mechanism does a good job of predicting stable species and free radical profiles in the flame. The computed profiles of small free radicals, such as H-atom or OH, match the data quite well. The largest discrepancies between the model and experiment are phenyl radical and phenoxy radical concentrations.

A new perspective on chemical and physical processes: the reaction force

Abstract: The reaction force F(R) of a chemical or physical process is the negative derivative of the system's potential energy V(R) along the reaction coordinate. The features of F(R) – its maxima, minima and zeroes – divide the process into well-defined stages which can, in general, be characterized in terms of changes in structural and/or electronic properties. This has been demonstrated for bond dissociation/formation and for reactions that have activation barriers in both forward and reverse directions. An important aspect of the reaction force is that it naturally and unambiguously divides activation energies into two components, one corresponding to the preparative structural stage of the process and the other to the first phase of the transition to products. It is shown how this can help to elucidate the effect of a solvent or a catalyst upon an activation barrier.

Analysis of the Thermochemistry of NOx Decomposition over CuZSM-5 Based on Quantum Chemical and Statistical Mechanical Calculations

Abstract: CuZSM-5 is the most active catalyst known for the direct decomposition of NOx We have performed first-principles quantum mechanical calculations to evaluate the electronic and structural properties of species adsorbed to Cu sites that might be involved in NOx decomposition Using statistical mechanics, we have calculated ΔU°, ΔH°, and ΔG° of possible elementary reactions in order to evaluate the stability of adsorbates on Cu sites and the ease of their interconversion On the basis of these calculations, we propose a reaction pathway for NOx decomposition This scheme involves only single, isolated copper sites, is internally consistent, and is consistent with experimental observations

CatApp: A Web Application for Surface Chemistry and Heterogeneous Catalysis

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)