1,720,963 research outputs found
Methanol oxidation over shell-core MO<sub>x</sub>/Fe<sub>2</sub>O<sub>3</sub> (M = Mo, V, Nb) catalysts
Full text linkWe present a comparison of Mo, V and Nb oxides as shell materials atop haematite cores used for selective methanol oxidation. While Mo and V both yield high selectivity to formaldehyde, Nb does not. Very different reactivity patterns are seen for Nb, which mainly shows dehydrogenation (to CO) and dehydration (to DME), indicating the lack of a complete shell, while Raman spectroscopy shows that the Mo and V formation process is not followed by NbOx. We suggest this is due to the large differences in mobility within the solid materials during formation, NbOx requiring significantly higher (and deleterious) calcination temperatures to allow sufficient mobility for shell completion.</p
VOx/Fe2O3 shell–core catalysts for the selective oxidation of methanol to formaldehyde
No full textEfficient oxidation catalysts are important in many current industrial processes, including the selective oxidation of methanol to formaldehyde. Vanadium-containing catalysts have been shown to be effective selective oxidation catalysts for certain reactions, and research continues to examine their applicability to other reactions of interest. Several VOx/Fe2O3 shell–core catalysts with varying VOx coverage have been produced to investigate the stability of VOx monolayers and their selectivity for methanol oxidation. Catalyst formation proceeds via a clear progression of distinct surface species produced during catalyst calcination. At 300 °C the selective VOx overlayer has formed; by 500 °C a sandwich layer of FeVO4 arises between the VOx shell and the Fe2O3 core, inhibiting iron cation participation in the catalysis and enhancing catalyst selectivity. The resulting catalysts, comprising a shell–subshell–core system of VOx/FeVO4/Fe2O3, possess good catalytic activity and selectivity to formaldehyde
Ensemble effects on methanol oxidation to formaldehyde on ferric molybdate catalysts
Full text linkThe properties of Mo-doped iron oxide are compared with those of the single oxides of Fe and Mo, and with stoichiometric ferric molybdate for the selective oxidation of methanol. It is found that Mo oxide segregates to the surface of the iron oxide at low loadings, while at higher loadings, but below the stoichiometric ratio, presents layers of ferric molybdate at the surface. The relationship between bulk loading and surface Mo is explored, and it is concluded that the reactivity is dominated by ensemble effects. Simple modelling indicates that four or more Fe cation ensembles are required to combust methanol to CO
2, ensembles of two Mo cations are required for selective oxidation to formaldehyde, whereas it seems that isolated single sites of either Fe or Mo produce CO.</p
Al-doped Fe<sub>2</sub>O<sub>3</sub>as a support for molybdenum oxide methanol oxidation catalysts
Full text linkWe have made high surface area catalysts for the selective oxidation of methanol to formaldehyde. This is done in two ways-(i) by doping haematite with Al ions, to increase the surface area of the material, but which itself is unselective and (ii) by surface coating with Mo which induces high selectivity. Temperature programmed desorption (TPD) of methanol shows little difference in surface chemistry of the doped haematite from the undoped material, with the main products being CO2 and CO, but shifted to somewhat higher desorption temperature. However, when Mo is dosed onto the haematite surface, the chemistry changes completely to show mainly the selective product, formaldehyde, with no CO2 production, and this is little changed up to 10% Al loading. But at 15 wt% Al, the chemistry changes to indicate the presence of a strongly acidic function at the surface, with additional dimethyl ether and CO/CO2 production characteristic of the presence of alumina. Structurally, X-ray diffraction (XRD) shows little change over the range 0-20% Al doping, except for some small lattice contraction, while the surface area increases from around 20 to 100 m2 g-1. Using X-ray absorption spectroscopy (XAS) it is clear that, at 5% loading, the Al is incorporated into the Fe2O3 corundum lattice, which has the same structure as α-alumina. By 10% loading then it appears that the alumina starts to nano-crystallise within the haematite lattice into the γ form. At higher loadings, there is evidence of phase separation into separate Al-doped haematite and γ-alumina. If we add 1 monolayer equivalent of Mo to the surface there is already high selectivity to formaldehyde, but little change in structure, because that monolayer is isolated at the surface. However, when three monolayers equivalent of Mo is added, we then see aluminium molybdate type signatures in the XANES spectra at 5% Al loading and above. These appear to be in a sub-surface layer with Fe molybdate, which we interpret as due to Al substitution into ferric molybdate layers immediately beneath the topmost surface layer of molybdena. It seems like the separate γ-alumina phase is not covered by molybdena and is responsible for the appearance of the acid function products in the TPD. </p
Segregation and reactivity in bi-cationic oxide catalysts
Full text linkThe in
uence of segregation on shell-core selective oxidation catalysts has been investigated
in this thesis in three main areas. Novel shell-core catalysts, namely VOx/Fe2O3
catalysts, are selective to formaldehyde during reaction with methanol, indicating that
the core Fe2O3 is sufficiently segregated, while VOx remains at the surface. Significant
structural understanding has been gained, suggesting VO4 tetrahedra constitute the active
site at the surface. With the soundness of the shell-core model further confirmed by
VOx/Fe2O3, other reactions of interest can be investigated with shell-core catalysts.
While VOx and MoOx/Fe2O3 function well as shell-core catalysts, benefitting from
greater surface area and amenability to analysis, NbOx/Fe2O3 catalysts are unable to
achieve the necessary segregation during calcination, resulting in exposed Fe2O3 at the
surface, worsening their selectivity. This is attributed to the notably higher Tamman
temperature of NbOx which is never reached during calcination, preventing the spreading
of the NbOx across the Fe2O3 required for shell-core formation.
The properties of Al dopants in Fe2O3 cores have also been examined. The addition of
up to 20 wt% Al in Fe2O3 can increase surface area fourfold, enhancing catalytic activity
in turn; however, detrimental effects on catalyst selectivity are seen for higher Al loadings,
indicating a degree of structural disruption. It is now known that Al can only occupy sites
in the Fe2O3 structure at low Al loadings, above which the impetus to phase separate
increases.
Magnetocatalysis has also been investigated using shell-core catalysts based on zincdoped
cobalt ferrite. Clear evidence of selectivity manipulation by applying an external
field during reaction has been obtained. Additionally, strong indications of internal magnetocatalytic
effects have been observed, which are those effects on selectivity caused by
magnetisation changes in the ferrite support.
Overall, these studies have further emphasised the importance of multicomponent catalysts,
and the need to carefully control catalyst speciation
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
- …
