1,721,222 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
Training set design for machine learning techniques applied to the approximation of computationally intensive first-principles kinetic models
Full text linkWe propose a design procedure for the generation of the training set for Machine Learning algorithms with a specific focus on the approximation of computationally-intensive first-principles kinetic models in catalysis. The procedure is based on the function topology and behavior, by means of the calculation of the discrete gradient, and on the relative importance of the independent variables. We apply the proposed methodology to the tabulation and regression of mean-field and kinetic Monte Carlo models aiming at their coupling with reactor simulations. Our tests – in the context both of mean-field kinetics and kinetic Monte Carlo simulations – show that the procedure is able to design a dataset that requires between 60 and 80% fewer data points to achieve the same approximation accuracy than the one obtained with an evenly distributed grid. This strong reduction in the number of points results in a significant computational gain and a concomitant boost of the approximation efficiency. The Machine Learning algorithms trained with the results of the procedure are then included in both macroscopic reactor models and computational fluid dynamics (CFD) simulations. First, a Plug Flow Reactor is employed to carry out a direct comparison with the solution of the full first-principles kinetic model. The results show an excellent agreement within 0.2% between the models. Then, the CFD simulation of complex tridimensional geometry is carried out by using a tabulated kMC model for CO oxidation on Ruthenium oxide, thus providing a showcase of the capability of the approach in making possible the multiscale simulation of complex chemical reactors
Nature and identity of the active site via structure-dependent microkinetic modeling: An application to WGS and reverse WGS reactions on Rh
Full text linkWe propose a methodology to perform a structure-dependent microkinetic analysis of a catalytic process. The methodology makes it possible to unveil the nature and identity of the active site in a self-consistent manner. The morphology of heterogeneous catalyst nanoparticles as a function of the gas chemical potential is determined using ab initio thermodynamics and Wulff-Kaishew construction methods. The reaction rates are calculated by integrating a microkinetic model which describes the catalytic activity of the crystal facets exposed by the catalyst under reaction conditions. The method is applied for the analysis of the direct and reverse water-gas shift (WGS) reacting systems on a 4% Rh/α-Al2O3 kinetic experiments from the literature. Our findings make it possible to rationalize that far from equilibrium the two different reacting systems not only follow different reaction pathways in agreement with the experimental evidence but also show that the dominant active sites are different for WGS and reverse WGS. Indeed, the WGS reaction occurs mainly on the Rh(111) facet, whereas reverse WGS proceeds on the active sites of Rh(100). As a whole, this methodology makes it possible a concomitant description of the nature of the catalyst material in reaction conditions and of its catalytic consequences in terms of reactivity. As such, it paves the way towards the use of first-principles methods for the interpretation of the experimental evidence in terms of structure-activity relationships
Modeling Morphology and Catalytic Activity of Nanoparticle Ensembles under Reaction Conditions
Full text linkAlthough nanoparticle catalysts obtain different sizes and shapes under reaction conditions, computational modeling in heterogeneous catalysis is usually based on well-defined crystallographic planes. Herein, we combine density functional theory (DFT) calculations with Boltzmann statistics to describe ensembles of nanoparticles obtaining different morphologies under reaction conditions (temperature and gas-phase chemical potential) and their respective distribution of active sites. We apply our methodology on Rh catalytic nanoparticles, and we address the contribution of metastable nanostructures on the overall CO dissociation catalytic activity. Importantly, we demonstrate how catalytic trends can change when accounting for an ensemble of nanoparticles compared to a single, thermodynamically stable nanoparticle. Thus, our work enlightens the impact of statistical representation of catalyst morphology on modeling structure-sensitive reactions
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
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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