1,721,054 research outputs found
Maximally resolved anharmonic OH vibrational spectrum of the water/ZnO(101¯0) interface from a high-dimensional neural network potential
No full textUnraveling the atomistic details of solid/liquid interfaces, e.g., by means of vibrational spectroscopy, is of vital importance in numerous applications, from electrochemistry to heterogeneous catalysis. Water-oxide interfaces represent a formidable challenge because a large variety of molecular and dissociated water species are present at the surface. Here, we present a comprehensive theoretical analysis of the anharmonic OH stretching vibrations at the water/ZnO(10 (1) over bar0) interface as a prototypical case. Molecular dynamics simulations employing a reactive high-dimensional neural network potential based on density functional theory calculations have been used to sample the interfacial structures. In the second step, one-dimensional potential energy curves have been generated for a large number of configurations to solve the nuclear Schrodinger equation. We find that (i) the ZnO surface gives rise to OH frequency shifts up to a distance of about 4 angstrom from the surface; (ii) the spectrum contains a number of overlapping signals arising from different chemical species, with the frequencies decreasing in the order v (adsorbed hydroxide) > v (non-adsorbed water) > v (surface hydroxide) > v (adsorbed water); (iii) stretching frequencies are strongly influenced by the hydrogen bond pattern of these interfacial species. Finally, we have been able to identify substantial correlations between the stretching frequencies and hydrogen bond lengths for all species.</p
Oxygen Vacancy Chemistry in Ceria
No full textCerium(IV) oxide (CeO2), ceria, is an active metal oxide used in solid oxide fuel cells and for the purification of exhaust gases in vehicle emissions control. Behind these technically important applications of ceria lies one overriding feature, namely ceria's exceptional reduction-oxidation properties. These are enabled by the duality of the cerium ion which easily toggles between Ce4+ and Ce3+. Here the cerium 4f electrons and oxygen vacancies (missing oxygen ions in the structure) are key players. In this thesis, the nature of ceria's f electrons and oxygen vacancies are in focus, and examined with theoretical calculations. It is shown that for single oxygen vacancies at ceria surfaces, the intimate coupling between geometrical structure and electron localisation gives a multitude of almost degenerate local energy mimima. With many vacancies, the situation becomes even more complex, and not even state-of-the-art quantum-mechanical calculations manage to predict the experimentally observed phenomenon of vacancy clustering. Instead, an alternative set of computer experiments managed to produce stable vacancy chains and trimers consistent with experimental findings from the literature and revealed a new general principle for surface vacancy clustering. The rich surface chemistry of ceria involves not only oxygen vacancies but also other active oxygen species such as superoxide ions (O2−). Experiments have shown that nanocrystalline ceria demonstrates an unusually large oxygen storage capacity (OSC) and an appreciable low-temperature redox activity, which have been ascribed to superoxide species. A mechanism explaining these phenomena is presented. The ceria surface is also known to interact with SOx molecules, which is relevant both in the context of sulfur poisoning of ceria-based catalysts and sulfur recovery from them. In this thesis, the sulfur species and key mechanisms involved are identified
Electronic and structural properties of model catalysts based on cerium oxide
No full textCatalysts based on cerium oxide are ubiquitous in industrial-scale chemical conversion. Here, a thorough study of their fundamental properties is undertaken via a model system ap- proach with the goal of furthering rational design in heterogeneous catalysis. A focus is put on understanding the behavior of oxygen vacancies in cerium oxide with respect to atomic co-ordination and electronic structure perturbations. Utilizing state-of-the-art probing tech- niques, a scalable model system framework is developed that allows for control over both the oxygen vacancy concentration and local co-ordination. High precision of the innova- tive approach facilitated observation of new phases of substoichiometric cerium oxide and lead to a first-of-a-kind investigation of the electronic structure of cerium oxide throughout isostructural transition from CeO2 to Ce2O3. The acquired results advance fundamental understanding of essential properties of cerium oxide that are relevant to its utilization in heterogeneous catalysis and open new pathways for functionalization of cerium oxide-based materials. Furthermore, the methodology developed in the thesis is transferable to other important reducible oxides.
Elektronické a strukturní vlastnosti modelových katalyzátorů na bázi oxidu ceru
Get PDFKatalyzátory na bázi oxidu ceru jsou všudypřítomné v industriálních chemických pro- cesech. V této práci jsou studovány jejich základní vlastnosti skrze modelový přístup s cílem podpořit racionální vývoj v heterogenní katalýze. Práce se zaměřuje na porozumění chování kyslíkových vakancí v rámci atomární koordinace a poruch elektronové struktury. S využitím moderních experimentálních technik je vyvinuta škálovatelná soustava mode- lových systémů, která umožňuje kontrolu koncentrace kyslíkových vakancí a jejich lokální koordinace. Vysoká přesnost tohoto inovativního přístupu umožnila pozorování nových fázi nestoichiometrického oxidu ceru a vedla k první studii elektronové struktury oxidu ceru v průběhu izostrukturního přechodu od CeO2 k Ce2O3. Získané výsledky představují pokrok v porozumění základním vlastnostem oxidu ceru ve vztahu k jeho využití v heterogenní katalýze a otevírají nové cesty k funkcionalizaci materiálů na bázi oxidu ceru. Metodologie vyvinutá v rámci této práce je navíc snadno přenositelná na další důležité reducibilní oxidy. 1Catalysts based on cerium oxide are ubiquitous in industrial-scale chemical conversion. Here, a thorough study of their fundamental properties is undertaken via a model system ap- proach with the goal of furthering rational design in heterogeneous catalysis. A focus is put on understanding the behavior of oxygen vacancies in cerium oxide with respect to atomic co-ordination and electronic structure perturbations. Utilizing state-of-the-art probing tech- niques, a scalable model system framework is developed that allows for control over both the oxygen vacancy concentration and local co-ordination. High precision of the innova- tive approach facilitated observation of new phases of substoichiometric cerium oxide and lead to a first-of-a-kind investigation of the electronic structure of cerium oxide throughout isostructural transition from CeO2 to Ce2O3. The acquired results advance fundamental understanding of essential properties of cerium oxide that are relevant to its utilization in heterogeneous catalysis and open new pathways for functionalization of cerium oxide-based materials. Furthermore, the methodology developed in the thesis is transferable to other important reducible oxides. 1Katedra fyziky povrchů a plazmatuDepartment of Surface and Plasma ScienceMatematicko-fyzikální fakultaFaculty of Mathematics and Physic
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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