1,720,959 research outputs found
Prediction of morphological changes of catalyst materials under reaction conditions by combined: Ab initio thermodynamics and microkinetic modelling
Full text linkIn this article, we couple microkinetic modelling, ab initio thermodynamics and Wulff-Kaishew construction to describe the structural variation of catalyst materials as a function of the chemical potential in the reactor. We focus specifically on experiments of catalytic partial oxidation (CPO) of methane on Rh/α-Al2O3. We employ a detailed structureless microkinetic model to calculate the profiles of the gaseous species molar fractions along the reactor coordinate and to select the most abundant reaction intermediates (MARIs) populating the catalyst surfaces in different zones of the reactor. Then, we calculate the most stable bulk and surface structures of the catalyst under different conditions of the reaction environment with density functional theory (DFT) calculations and ab initio thermodynamics, considering the presence of the MARIs on the catalyst surface in thermodynamic equilibrium with the partial pressures of their reservoirs in the gas phase surrounding the catalyst. Finally, we exploit the Wulff-Kaishew construction method to estimate the three-dimensional shape of the catalyst nanoparticles and the distribution of the active sites along the reactor coordinate. We find that the catalyst drastically modifies its morphology during CPO reaction by undergoing phase transition, in agreement with spectroscopy studies reported in the literature. The framework is also successfully applied for the analysis and interpretation of chemisorption experiments for catalyst characterization. These results demonstrate the crucial importance of rigorously accounting for the structural effect in microkinetic modeling simulations and pave the way towards the development of structure-dependent microkinetic analysis of catalytic processes
Deciphering Size and Shape Effects on the Structure Sensitivity of the CO2 Methanation Reaction on Nickel
No full textThis study advances the understanding of structure sensitivity in CO2 methanation over nickel-based catalysts by highlighting the combined influence of the metal nanoparticle (NP) size and shape on catalytic performance. Density functional theory (DFT) calculations of the metal nanoparticle structure and activity provide the theoretical underpinnings of the experimentally observed structure sensitivity of CO2 methanation over nickel-based catalysts. This is achieved by taking into account the diversity of shapes of metal nanoparticles (NPs) under the reaction conditions and the corresponding distribution of active sites at different metal NP sizes. We built a large ensemble of Ni metal NPs with different shapes and sizes in the range of 0.5-10 nm and quantified the distribution of the potential active sites for each NP. We then computed the reaction rate over each of these active sites on the metal surface to evaluate the activity as a function of the metal NP diameter. Our calculations reveal that the activity at the active sites located at the edge between the Ni(100) and Ni(111) facets largely dominates the overall observed activity. Furthermore, metal NPs can be categorized into families based on their shape, specifically the fraction of exposed Ni(100) facets. The observed maximum in turnover frequency (TOF) for 2-3 nm metal NPs is linked to the dominance of NP families with high Ni(100) fractions. Conversely, experimental conditions favoring NP families with higher Ni(111) fractions result in a hockey stick trend in the TOF. These findings resolve key debates on structure sensitivity in CO2 methanation and offer broader applicability to other structure-sensitive reactions, such as ammonia synthesis, decomposition, and Fischer-Tropsch synthesis, where similar sensitivities have been widely debated
First principles modeling of structure and activity of nanoparticle catalysts under reaction conditions for structure-dependent microkinetic analyses
No full textDOTTORATONon vi è dubbio che l'interpretazione razionale delle relazioni tra la struttura e l'attività nella catalisi eterogenea sia cruciale per l'ingegnerizzazione delle trasformazioni chimiche a livello molecolare. A questo proposito, le analisi multi-scala basate su modelli microcinetici struttura-dipendenti possono essere uno strumento chiave per la comprensione meccanicistica della funzionalità del catalizzatore. Tuttavia, gli effetti della struttura del catalizzatore sull’attività e sulla selettività sono attualmente trascurati nei modelli microcinetici “state-of-the-art”. Pertanto, un “material gap” ostacola l'analisi atomistica dei meccanismi di reazione. Per colmare questa lacuna, è necessaria la modellazione della struttura del catalizzatore in condizioni di reazione.
Questa tesi presenta lo sviluppo e l'applicazione di metodologie per l'analisi delle relazioni tra struttura e attività nella catalisi eterogenea, basate su calcoli di strutture elettroniche ai principi primi. Le metodologie si avvalgono della combinazione di modellazione microcinetica, ab initio thermodynamics, Wulff costruction e Boltzmann statistics. Il metodo ab initio thermodynamics viene applicato per caratterizzare la struttura del bulk e della superficie del catalizzatore. La forma tridimensionale delle nanoparticelle catalitiche e la distribuzione dei siti attivi sono calcolate con i metodi Wulff construction e Boltzmann statistics. Quest’ultimo viene applicato quando le nanoparticelle hanno dimensione ridotta, ed è quindi necessaria una rappresentazione statistica della morfologia del catalizzatore. In questo modo, vengono determinate la struttura e la composizione degli atomi sulla superficie del catalizzatore, e quindi la natura dei siti attivi. In seguito, vengono eseguite analisi microcinetiche sui diversi siti per svelare l'identità dei siti attivi dominanti, che sono quelli che danno un contributo rilevante alla velocità di reazione complessiva.
Le metodologie sviluppate in questa tesi vengono applicate per lo studio di sistemi catalitici importanti a livello industriale. Lo studio dell'ossidazione parziale catalitica di metano su Rh consente di interpretare prove sperimentali che mostrano un cambiamento di selettività accompagnato da una trasformazione morfologica del catalizzatore. Lo studio della reazione di water-gas shift su Rh permette l'interpretazione di esperimenti cinetici che suggeriscono due distinti meccanismi cinetici per le reazioni diretta e inversa. L'analisi delle particelle nanometriche di Rh in condizioni di metanazione svela l'importanza delle nanoparticelle metastabili nel calcolo della velocità di reazione della dissociazione di CO. L'interpretazione di esperimenti di deposizione di NiO in sali fusi consente l'identificazione degli agenti responsabili della formazione di particelle con superfici ad alto indice di Miller, particolarmente adatte per applicazioni di ossidazione parziale.
Nel complesso, questa tesi fornisce un framework efficace per svelare la natura e l'identità dei siti attivi in reazione e rappresenta un passo importante per l’introduzione dell'effetto della struttura del catalizzatore nell'analisi microcinetica dei processi nella catalisi eterogenea.There is no doubt that the rational interpretation of the relationships between the structure and the activity in heterogeneous catalysis is a crucial task in the quest of engineering the chemical transformation at the molecular level. In this respect, multiscale analyses based on structure-dependent microkinetic modeling is acknowledged to be an essential key-tool to achieve a mechanistic understanding of the catalyst functionality. However, the effect of the structure of the catalyst on reactivity and selectivity is at present neglected in state-of-the-art microkinetic modeling with predictive character. As such, a “material gap” hinders the analysis of the underlying mechanisms at the atomic-scale level. To fill this gap, the modeling of the catalyst structure under reaction conditions is required.
This thesis presents the development and the application of methodologies for the analysis of the relationships between structure and activity in heterogeneous catalysis from first principles electronic structure calculations. The methodologies are based on the combination of microkinetic modeling and ab initio thermodynamics with Wulff constructions and Boltzmann statistics at given conditions of chemical potential in the reactor. Ab initio thermodynamics is applied to characterize the bulk and surface structure of the catalyst. The three-dimensional shape of catalyst nanoparticles and the corresponding distribution of active sites are calculated either with the Wulff construction method or Boltzmann statistics, especially at low nanoparticle sizes, where a statistical representation of the catalyst morphology is needed. In doing so, the structure and the composition of the atoms at the surface is determined, and therefore the “nature” of the sites in reaction conditions is fully characterized. Then, microkinetic analyses are performed on the different sites to unravel the “identity” of the dominant active site in reaction, i.e., the sites that give a major contribution to the overall reaction rate.
The methodologies developed are applied for the study of industrially relevant catalytic systems. The study of the CH4 catalytic partial oxidation (CPO) on Rh enables to interpret experimental evidences that show a change of reaction selectivity induced by a drastic morphological transformation of the catalyst occurring during reaction. The application of the methodologies to the water-gas shift reaction (WGS) on Rh allows for the rational interpretation of kinetic experiments, which show two distinct reaction mechanisms for the direct and reverse direction of the reaction. The analysis of nanometric Rh particles under methanation conditions enables to unravel the importance of metastable particles for the reaction rate of the important CO dissociation step. The interpretation of experiments of NiO particle growth in molten salts media allows for the identification of the agents responsible for the formation of high-index particle shapes, particularly suitable for partial oxidation applications.
As a whole, this thesis provides an effective pathway to unraveling the “nature” and the “identity” of the active sites under reaction conditions, and it represents a very important step to explicitly introduce the effect of catalyst structure in the microkinetic analysis of heterogeneous catalytic processes.DIPARTIMENTO DI CHIMICA, MATERIALI E INGEGNERIA CHIMICA "GIULIO NATTA"32NOVA, ISABELLAFRASSOLDATI, ALESSI
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
Structure-dependent microkinetic analysis of WGS and reverse WGS reactions on Rh/Al2O3
No full textLAUREA MAGISTRALEQuesto lavoro di tesi riguarda l’analisi dettagliata della struttura del catalizzatore in condizioni di reazione, superando così il concetto classico di sito attivo statico (“*”) che è prevalente nella modellazione della catalisi eterogena. Questo studio è realizzato andando a svelare a livello atomico l’identità e la natura dei siti attivi adottando un approccio gerarchico ai principi primi.
In particolare, si utilizza una procedura dual-loop. Un modello semi-empirico non struttura-dipendente è adottato per predire l’evoluzione del sistema a livello della macroscala. La morfologia di nanoparticelle eterogenee di catalizzatore in funzione del potenziale chimico nel bulk è determinata tramite metodi ab initio thermodynamics e costruzione di Wulff-Kaishew, basandosi sull’eredità di un precedente lavoro di tesi. Vengono svolti calcoli Density Functional Theory (DFT) per analizzare le specifiche energie libere di Gibbs riscontrate ai siti attivi sulle facce esposte del catalizzatore. Le energie di attivazione degli steps elementari inclusi nel meccanismo globale vengono calcolate per ogni superficie e messe in relazione con le specifiche geometrie e orientazioni locali degli atomi. L’informazione ottenuta al livello della microscala viene poi abbinata alla modellazione globale del sistema nella sua intera dimensionalità, collegando fenomeni che avvengono a scale diverse. L’applicazione di questa metodologia è mostrata nel contesto di sistemi reattivi di water-gas shift (WGS) e reverse WGS su catalizzatore rodio. Questo permette di indicare che la superficie (111) del rodio è il vero centro attivo per la reazione di WGS tramite il meccanismo ossigeno-assistito, mentre la velocità globale della reazione inversa è catalizzata dalle facce cristalline (111) e (311) del rodio e procede per mezzo sia di un meccanismo ossigeno-assistito che tramite formazione di un intermedio carbossilico.
Nel suo insieme, la procedura proposta ha una grande potenzialità dal momento che permette di analizzare localmente un sistema reagente a livello molecolare, tenendo esplicitamente conto della struttura del catalizzatore. In corrispondenza di una certa coordinata di reazione, è possibile determinare quali siano le facce che danno il contributo più significativo all’attività globale e studiare il meccanismo di reazione su ognuna di esse. Nel fare ciò, questo lavoro introduce un’innovazione importante nei modelli microcinetici presenti in letteratura, rendendo possibile comprendere la relazione tra struttura catalitica e attività.This thesis work deals with the detailed analysis of the catalyst structure in reaction conditions, thus overcoming the classical static concept of the active site (“*”), which is prevalent in modelling heterogeneous catalysis. The study is performed by unravelling the identity and nature of the active sites at the atomic scale using a hierarchical first-principles approach.
In particular, a dual-loop methodology is applied. A semi-empirical structureless microkinetic model is adopted to predict the evolution of the system at the macroscale. The morphology of heterogeneous catalyst nanoparticles in dependence of the bulk chemical potential is determined by means of ab initio thermodynamics and Wulff-Kaishew construction methods, basing on the inheritance of a preceding thesis work. Density Functional Theory (DFT) calculations are performed to investigate the peculiar Gibbs free energies experienced at the active sites on the exposed catalyst facets. The activation energies of all the elementary steps included in the overall mechanism are evaluated for each active surface and related to the local peculiar atoms geometries and orientations. The information obtained at the microscale level is then coupled with the overall modelling of the system in its whole dimensionality, bridging the phenomena occurring at different scales. The application of the methodology is shown in the context of water-gas shift (WGS) and reverse WGS reacting systems on rhodium catalyst. It allows to clarify that the flat Rh(111) surface is the true active centre for WGS reaction via oxygen-assisted pathway, whereas the overall rate of the reverse reaction is catalysed by both Rh(111) and Rh(311) crystal facets and proceeds by both oxygen and carboxyl-mediated mechanisms.
As whole, the proposed procedure has a great potential since it gives us the opportunity to locally analyse a real reacting system at the molecular level, explicitly accounting for the catalyst structure. At a certain reaction coordinate, it is possible to determine which are the facets of the catalyst that give predominant contribution to the overall activity and unravel the reaction mechanism active on each of them. In doing so, this work introduces an important innovation in state-of-the-art microkinetic models, making it possible to understand the catalyst structure-activity relation
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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