1,720,957 research outputs found
Development and Assessment of a Criterion for the Application of Brønsted-Evans-Polanyi Relations for Dissociation Catalytic Reactions at Surfaces
Full text linkWe propose and assess a criterion for the application of Brønsted-Evans-Polanyi (BEP) relations for dissociation reactions at surfaces. A theory-to-theory comparison with density functional theory calculations is presented on different reactions, metal catalysts, and surface terminations. In particular, the activation energies of CH, CO, and trans-COOH dissociation reactions on (100), (110), (111), and (211) surfaces of Ni, Cu, Rh, Pd, Ag, and Pt are considered. We show that both the activation energy and the reaction energy can be decomposed into two contributions that reflect the influence of reactant and products in determining either the activation energy or the reaction energy. We show that the applicability of the BEP relation implies that the reaction energy and activation energy correlate to these two contributions in the range of conditions to be described by the BEP relation. A lack of correlation between these components for the activation energy is related to a change in the character of the transition state (TS) and this turns out to be incompatible with a BEP relation because it results in a change of the slope of the BEP relation. Our analysis reveals that these two contributions follow the same trends for the activation energy and for the reaction energy when the path is not characterized either by the formation of stable intermediates or by the change of the binding mechanism of the reactant. As such, one can assess whether a BEP relation can be applied or not for a set of conditions only by means of thermochemical calculations and without requiring the identification of the TS along the reaction pathway. We provide evidence that this criterion can be successfully applied for the preliminary discrimination of the applicability of the BEP relations. For instance, on the one hand, our analysis provides evidence that the two contributions are fully anticorrelated for the trans-COOH dissociation reactions on different metals and surfaces, thus revealing that the reaction is characterized by a change in the TS character. In this situation, no BEP relation can be used to describe the activation energy trend among the different metals and surfaces in full agreement with our DFT calculations. On the other hand, our criterion reveals that the TS character is not expected to change for CH dissociation reactions both for the same facet, different metals and for same metal, different facets, in good agreement with the DFT calculations of the activation energy. The formation of multiple stable intermediates along the reaction pathways and the change of the binding mechanism of one of the reactants are demonstrated to affect the validity of the criterion. As a whole, our findings make possible an assessment of the applicability of the BEP relation and paves the way toward its use for the exploration of complex reaction networks for different metals and surfaces
A topological model for predicting adsorption energies of polycyclic aromatic hydrocarbons on late-transition metal surfaces
Full text linkFirst-principles simulation of Raman Spectra of Adsorbates on Metal Surfaces
Full text linkA general method is proposed to simulate the Raman spectra of adsorbates on metal surfaces. This method is based on an electrostatic-corrected cluster model with additional charges to compensate the loss of coordination of metal atoms, and an external field added to simulate the surface dipole and to reproduce the charge distribution obtained from periodic calculations. As a result, it is possible to couple the phonon calculation with the Raman tensors computed by this corrected cluster model to simulate the Raman spectra of the adsorbates on metal surfaces. In doing so, it is possible to overcome both the infinite dielectric constant of the ideal metal, which makes calculating Raman spectra with current periodic models impossible, and the inaccuracy in adsorbate-metal interactions described by the cluster model. By means of this method, the relative experimental Raman intensity peaks of ethylene adsorbed on metal surfaces were successfully reproduced. Moreover, the model analysis allowed relating the enhancement of the Raman intensity of both CO and ethylene upon chemisorption on the metal surface to both the gain of charges on C atoms and the polarization of orbitals. As such, the proposed method provides an accurate and efficient way to simulate and interpret Raman spectra of adsorbates on metal surfaces
First-principles assessment of the analogy between gas-phase and gas-solid H-abstraction reactions at graphene edges
No full textDensity functional theory (DFT) methods are used to assess the analogy between the activation energies of hydrogen abstraction reactions in gas phase and at graphene edges. We focus on prototypical hydrogen abstraction reactions by twelve C1-C5 alkyl and alkenyl radicals on both the armchair and zigzag edges of graphene. We find that the energy barriers of abstraction reactions on each edge are only determined by the stability of the abstracting radicals that are characterized by the corresponding C-H bond energy. Such correlation between the abstraction barrier and the radical stability is also shown by our calculations of the reactions in gas phase, thus revealing that the activation energies for the reactions at the gas-solid interface are linearly correlated with the ones for the reactions in the gas phase. As such, our findings provide theoretical underpinnings for the analogy between the reactions at the gas-solid interface and those in the gas phase, thus paving the way towards the estimation of the barriers of H-abstractions at the gas-solid interface of carbonaceous materials from the corresponding gas-phase reaction barriers. Such an approach is expected to play an important role in overcoming the complexity in the development of detailed microkinetic models of processes involving the interaction between gas-phase species and carbonaceous materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A comprehensive kinetic framework for solid carbon deposition and hydrogen production from the pyrolysis of light hydrocarbons streams
Full text linkThermal pyrolysis of hydrocarbons is a promising solution for the industrial production of hydrogen and valuable carbon materials. Pyrolysis reactor design and scale up strongly benefit from predictive chemical kinetic models capable to comprehensively describe the reactivity in the gas phase, including the undesirable formation of amorphous carbon (i.e., soot), as well as the solid carbon deposition mechanism. In this work, a methodology for the determination of rate constants of the heterogeneous growth of pyrocarbon deposit by means of theory-based corrections of analogous gas phase reactions is firstly proposed. Specifically, the theoretical methodology is applied to H-abstraction reactions governing the propagation of superficial active sites. Based on these findings, a detailed pyrocarbon deposition model from the literature is revised and coupled to a state-of-the-art model describing the dynamics of species evolution in the gas phase as well as the molecular growth of polycyclic aromatic hydrocarbons (PAHs) and soot. The model is validated with literature experimental data of pyrocarbon formation from light hydrocarbons feedstocks, covering a large set of operating conditions (pressure, temperature, surface over volume ratio). The comprehensive kinetic framework can reproduce the experimental deposition rates as well as the amount of deposited carbon with high fidelity under varying operative conditions. Moreover, kinetic analyses have been performed for assessing the relevant reaction pathways leading to pyrocarbon deposition from propane and methane feedstocks as well as the competition between carbon deposition and amorphous carbon formation
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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