1,720,962 research outputs found
Selective Oxidation of HMF via Catalytic and Photocatalytic Processes Using Metal-Supported Catalysts
No full textIn this study, 5-hydroxymethylfurfural (HMF) oxidation was carried out via both the catalytic and the photocatalytic approach. Special attention was devoted to the preparation of the TiO2-based catalysts, since this oxide has been widely used for catalytic and photocatalytic application in alcohol oxidation reactions. Thus, in the catalytic process, the colloidal heterocoagulation of very stable sols, followed by the spray-freeze-drying (SFD) approach, was successfully applied for the preparation of nanostructured porous TiO2-SiO2 mixed-oxides with high surface areas. The versatility of the process made it possible to encapsulate Pt particles and use this material in the liquid-phase oxidation of HMF. The photocatalytic activity of a commercial titania and a homemade oxide prepared with the microemulsion technique was then compared. The influence of gold, base addition, and oxygen content on product distribution in the photocatalytic process was evaluated
Production photocatalytique de produits chimiques et d’hydrogène à partir de biomasse
Full text linkIn this work, with the aim to tackle several approaches towards sustainable chemistry, two reactions were studied: aerobic photo-oxidation of biomass derived 5-hydroxymethyl-2-furfural (HMF), and anaerobic photo-reforming of glycerol known as a by-product in biodiesel industry, towards production of chemicals and hydrogen. Solar-assisted reactions were performed by means of heterogeneous photocatalysis, in mild conditions such as atmospheric pressure, room temperature and water as a benign solvent. Titanium dioxide (lab-synthesized and commercial) was used as a photo-active catalyst, which surface was modified by introducing different metal (e.g. Au, Au-Cu, Pt) and metal oxide (e.g. NiO) nanoparticles. The prepared materials were characterized by XRD, DRS, BET, TEM, SEM, RAMAN and other techniques. The influence of the support, the size and type of the deposited metal and metal oxide nanoparticles on the photo-catalytic transformation of HMF and glycerol was evaluated. In the case of HMF, the influence of the base addition and the oxygen content on the reaction selectivity was also studied. The effect of the crystalline phase composition and morphology of TiO2 in the glycerol photo-reforming reaction was assessed as well. The surface of the synthesized TiO2 nano-powders was investigated by means of Surface Organometallic Chemistry (SOMC) approach. In particular, the surface was characterized by chemical titration and DRIFT techniques. Furthermore, the SOMC concept allowed preparing of well-dispersed Pt nanoparticles on the TiO2 surface. The photo-catalytic activity of this sample in the glycerol photo-reforming process was tested and compared to that of other Pt-containing catalysts prepared by conventional technics. In view of avoiding the agglomeration and sedimentation of suspended titania powders in water media, thick films of synthesized and commercial TiO2 were deposited on a conductive substrate using screen-printing technique. The prepared electrodes were characterized by profilometry, SEM, XRD, optical, electrochemical and photo-electrochemical methods
Multi-Asset CVaR: Minimizing Downside Risk of Multi-Asset Class Portfolios
No full textInvestors have to search for an equilibrium between portfolio risk and return during their decision-making process. One of the first and most common mathematical models for this problem was formulated by H. Markowitz [1], where the variance of the portfolio
return is used as a risk measure. However, this measure has several serious disadvantages. An alternative measure, Conditional Value-at-risk (CVaR), has become very popular and widely used because it has all properties a risk measure should have. Unfortunately, both optimization frameworks suffer from being sensitive to small variations in the input parameters. One of the widely used and effective approaches for optimization under parameter uncertainty is a robust optimization.
The major objective of this thesis is to compare different frameworks, such as Markowitz and mean-CVaR optimization models, as well as its robust counterparts, for multi-asset class portfolios. The results show that only in the case of non-normal return distributions
there is a difference between Markowitz and mean-CVaR optimal portfolios. The simulation study also shows that robust counterparts have significantly smaller risk in both
models, losing at the same time not so much in portfolio return. It means that portfolio robustification can be a good choice for the conservative investor
Photocatalytic production of chemicals and hydrogen from biomass
No full textLa conversion industrielle durable de la biomasse en tant que matière première renouvelable et abondante devient de plus en plus attrayante pour la synthèse de produits à haute valeur ajoutée pouvant être utilisés dans les secteurs de l’énergie, de l’alimentation, des polymères, de la cosmétique et de la pharmacologie, ainsi que dans la production de produits chimiques fins. Dans le but de traiter plusieurs approches de la chimie durable, nous avons utilisé le 5-hydroxyméthyl-2-furfural (HMF) issu de la biomasse dans un processus de photo-oxydation aérobie. Les produits d'oxydation partielle de cette réaction, en particulier le 2,5-diformylfurane (DFF) et l'acide 2,5-furane-dicarboxylique (AFDC), présentent un grand intérêt pour l'industrie des polymères. Le glycérol en tant que sous-produit majeur de la production de biodiesel a également été soumis à un photo-reformage anaérobie afin d'obtenir de l'hydrogène gazeux et d’autres produits liquides. Ces réactions photo-catalytiques hétérogènes ont été réalisées sous irradiation du soleil dans des conditions douces telles que la pression atmosphérique, la température ambiante et l'eau en tant que solvant bénin. Le dioxyde de titane a été utilisé comme catalyseur photo-actif. Bien que le TiO2 soit le matériau le plus actif, non toxique et peu coûteux, sa large bande interdite ne permet d’utiliser que des irradiations UV qui ne représentent que 5% du spectre solaire. Ainsi, l’introduction de nanoparticules de métaux nobles permet de résoudre le problème de l’absorption de la lumière visible ainsi que de réduire la recombinaison électron-trou dans le semi-conducteur. Le dioxyde de titane a été synthétisé sous forme de nanopoudres en utilisant un système de microémulsion. Des nanoparticules de métaux nobles tels que Au, Ag, Pt, des alliages comme Au-Cu, Au-Ag, et des nanoparticules d'oxyde métallique NiO ont été déposées sur TiO2, qui a été utilisé en tant que support. Ceci a été réalisé soit par un procédé de dépôt-précipitation soit par l'imprégnation à humidité naissante. Des sels de précurseurs métalliques ou des nanoparticules préformées ont été utilisés dans le dernier cas. Les catalyseurs ainsi obtenus ont été caractérisés par DRX, DRS, physisorption de N2, analyse TEM, SEM et RAMAN. Leur activité catalytique a été comparée à celle de poudres de dioxyde de titane disponibles dans le commerce telles que P25 et DT-51. L’influence du support, de la taille et du type de nanoparticules métalliques et d’oxydes a été évalué pour la transformation photo-catalytique de l’HMF et du glycérol. Dans le cas de HMF, l'influence de l'addition de bases et la teneur en oxygène a également été étudiée sur la sélectivité de la réaction. L'effet de la composition de la phase cristalline et de la morphologie de TiO2 a également été abordé pour la réaction de photo-reformage du glycérol. La surface des nanopoudres de TiO2 synthétisées a été étudiée au moyen d'une approche de chimie organométallique de surface (COMS). En particulier, la surface a été caractérisée par les techniques de titrage chimique et DRIFT. En outre, le concept SOMC a permis de préparer des nanoparticules de Pt bien dispersées sur la surface de TiO2. L'activité photo-catalytique de cet échantillon dans le processus de photo-formage du glycérol a été testée et comparée à celle des autres catalyseurs contenant des nanoparticules de Pt préparées par des techniques classiques. Afin d'éviter l'agglomération et la sédimentation de poudres d'oxyde de titane en suspension dans le milieu aqueux, des couches épaisses de TiO2 synthétisé et commercial ont été déposées sur un substrat conducteur en utilisant une technique de sérigraphie. Les électrodes ainsi obtenues ont été caractérisées par profilométrie, SEM, DRX, méthodes optiques, électriques et photoélectrochimiquesSustainable industrial conversion of biomass as renewable and abundant feedstock is becoming increasingly attractive as to synthesize high-value-added products, which can be utilized in energy sector, food, polymer, cosmetics and pharmacological industries, as well as in production of fine chemicals. In this work, with the aim to tackle several approaches towards sustainable chemistry, we used biomass derived 5-hydroxymethyl-2-furfural (HMF) in aerobic photo-oxidation. Partial oxidation products of this reaction, for instance 2,5-diformylfurane (DFF) and 2,5-furane dicarboxylic acid (FDCA), are of particular interest for the polymer industry. Glycerol as a multi-ton by-product of biodiesel production was also subjected to anaerobic photo-reforming in order to obtain hydrogen gas and liquid-phase products. Solar-assisted reactions were performed by means of heterogeneous photocatalysis, in mild conditions such as atmospheric pressure, room temperature and water as a benign solvent. Titanium dioxide was used as a photo-active catalyst. Despite of the fact that TiO2 is the most active, non-toxic and low-cost material, its wide bandgap allows employing only UV irradiation that covers just 5% of the solar spectrum. Thus, the introduction of noble metal nanoparticles makes it possible to embrace the problem of visible light absorption as well as to reduce electron-hole pair recombination in the semiconductor. Titanium dioxide was synthesized in the form of nano-powder using a microemulsion-mediated system. Au, Ag, Pt noble metal nanoparticles, Au-Ag, Au-Cu alloys and core-shell nanoparticles, and NiO metal oxide nanoparticles were deposited on TiO2 support. This was done ether by deposition-precipitation or incipient wetness impregnation method. Metal precursor salts or pre-formed nanoparticles were employed in the latter method. The prepared catalysts were characterized by XRD, DRS, N2 physisorption, TEM, SEM and RAMAN analysis. Their catalytic activity was compared to that of commercially available titanium dioxide powders such as P25 and DT-51. The influence of the support, the size and type of the deposited metal and metal oxide nanoparticles on the photo-catalytic transformation of HMF and glycerol was evaluated. In the case of HMF, the influence of the base addition and the oxygen content on the reaction selectivity was also studied. The effect of the crystalline phase composition and morphology of TiO2 in the glycerol photo-reforming reaction was assessed as well. The surface of the synthesized TiO2 nano-powders was investigated by means of Surface Organometallic Chemistry (SOMC) approach. In particular, the surface was characterized by chemical titration and DRIFT techniques. Furthermore, the SOMC concept allowed preparing of well-dispersed Pt nanoparticles on the TiO2 surface. The photo-catalytic activity of this sample in the glycerol photo-reforming process was tested and compared to that of other Pt-containing catalysts prepared by conventional technics. In view of avoiding the agglomeration and sedimentation of suspended titania powders in water media, thick films of synthesized and commercial TiO2 were deposited on a conductive substrate using screen-printing technique. The prepared electrodes were characterized by profilometry, SEM, XRD, optical, electrical and photo-electrochemical method
Production photocatalytique de produits chimiques et d'hydrogène à partir de biomasse
No full textSustainable industrial conversion of biomass as renewable and abundant feedstock is becoming increasingly attractive as to synthesize high-value-added products, which can be utilized in energy sector, food, polymer, cosmetics and pharmacological industries, as well as in production of fine chemicals. In this work, with the aim to tackle several approaches towards sustainable chemistry, we used biomass derived 5-hydroxymethyl-2-furfural (HMF) in aerobic photo-oxidation. Partial oxidation products of this reaction, for instance 2,5-diformylfurane (DFF) and 2,5-furane dicarboxylic acid (FDCA), are of particular interest for the polymer industry. Glycerol as a multi-ton by-product of biodiesel production was also subjected to anaerobic photo-reforming in order to obtain hydrogen gas and liquid-phase products. Solar-assisted reactions were performed by means of heterogeneous photocatalysis, in mild conditions such as atmospheric pressure, room temperature and water as a benign solvent. Titanium dioxide was used as a photo-active catalyst. Despite of the fact that TiO2 is the most active, non-toxic and low-cost material, its wide bandgap allows employing only UV irradiation that covers just 5% of the solar spectrum. Thus, the introduction of noble metal nanoparticles makes it possible to embrace the problem of visible light absorption as well as to reduce electron-hole pair recombination in the semiconductor. Titanium dioxide was synthesized in the form of nano-powder using a microemulsion-mediated system. Au, Ag, Pt noble metal nanoparticles, Au-Ag, Au-Cu alloys and core-shell nanoparticles, and NiO metal oxide nanoparticles were deposited on TiO2 support. This was done ether by deposition-precipitation or incipient wetness impregnation method. Metal precursor salts or pre-formed nanoparticles were employed in the latter method. The prepared catalysts were characterized by XRD, DRS, N2 physisorption, TEM, SEM and RAMAN analysis. Their catalytic activity was compared to that of commercially available titanium dioxide powders such as P25 and DT-51. The influence of the support, the size and type of the deposited metal and metal oxide nanoparticles on the photo-catalytic transformation of HMF and glycerol was evaluated. In the case of HMF, the influence of the base addition and the oxygen content on the reaction selectivity was also studied. The effect of the crystalline phase composition and morphology of TiO2 in the glycerol photo-reforming reaction was assessed as well. The surface of the synthesized TiO2 nano-powders was investigated by means of Surface Organometallic Chemistry (SOMC) approach. In particular, the surface was characterized by chemical titration and DRIFT techniques. Furthermore, the SOMC concept allowed preparing of well-dispersed Pt nanoparticles on the TiO2 surface. The photo-catalytic activity of this sample in the glycerol photo-reforming process was tested and compared to that of other Pt-containing catalysts prepared by conventional technics. In view of avoiding the agglomeration and sedimentation of suspended titania powders in water media, thick films of synthesized and commercial TiO2 were deposited on a conductive substrate using screen-printing technique. The prepared electrodes were characterized by profilometry, SEM, XRD, optical, electrical and photo-electrochemical methodsLa conversion industrielle durable de la biomasse en tant que matière première renouvelable et abondante devient de plus en plus attrayante pour la synthèse de produits à haute valeur ajoutée pouvant être utilisés dans les secteurs de l’énergie, de l’alimentation, des polymères, de la cosmétique et de la pharmacologie, ainsi que dans la production de produits chimiques fins. Dans le but de traiter plusieurs approches de la chimie durable, nous avons utilisé le 5-hydroxyméthyl-2-furfural (HMF) issu de la biomasse dans un processus de photo-oxydation aérobie. Les produits d'oxydation partielle de cette réaction, en particulier le 2,5-diformylfurane (DFF) et l'acide 2,5-furane-dicarboxylique (AFDC), présentent un grand intérêt pour l'industrie des polymères. Le glycérol en tant que sous-produit majeur de la production de biodiesel a également été soumis à un photo-reformage anaérobie afin d'obtenir de l'hydrogène gazeux et d’autres produits liquides. Ces réactions photo-catalytiques hétérogènes ont été réalisées sous irradiation du soleil dans des conditions douces telles que la pression atmosphérique, la température ambiante et l'eau en tant que solvant bénin. Le dioxyde de titane a été utilisé comme catalyseur photo-actif. Bien que le TiO2 soit le matériau le plus actif, non toxique et peu coûteux, sa large bande interdite ne permet d’utiliser que des irradiations UV qui ne représentent que 5% du spectre solaire. Ainsi, l’introduction de nanoparticules de métaux nobles permet de résoudre le problème de l’absorption de la lumière visible ainsi que de réduire la recombinaison électron-trou dans le semi-conducteur. Le dioxyde de titane a été synthétisé sous forme de nanopoudres en utilisant un système de microémulsion. Des nanoparticules de métaux nobles tels que Au, Ag, Pt, des alliages comme Au-Cu, Au-Ag, et des nanoparticules d'oxyde métallique NiO ont été déposées sur TiO2, qui a été utilisé en tant que support. Ceci a été réalisé soit par un procédé de dépôt-précipitation soit par l'imprégnation à humidité naissante. Des sels de précurseurs métalliques ou des nanoparticules préformées ont été utilisés dans le dernier cas. Les catalyseurs ainsi obtenus ont été caractérisés par DRX, DRS, physisorption de N2, analyse TEM, SEM et RAMAN. Leur activité catalytique a été comparée à celle de poudres de dioxyde de titane disponibles dans le commerce telles que P25 et DT-51. L’influence du support, de la taille et du type de nanoparticules métalliques et d’oxydes a été évalué pour la transformation photo-catalytique de l’HMF et du glycérol. Dans le cas de HMF, l'influence de l'addition de bases et la teneur en oxygène a également été étudiée sur la sélectivité de la réaction. L'effet de la composition de la phase cristalline et de la morphologie de TiO2 a également été abordé pour la réaction de photo-reformage du glycérol. La surface des nanopoudres de TiO2 synthétisées a été étudiée au moyen d'une approche de chimie organométallique de surface (COMS). En particulier, la surface a été caractérisée par les techniques de titrage chimique et DRIFT. En outre, le concept SOMC a permis de préparer des nanoparticules de Pt bien dispersées sur la surface de TiO2. L'activité photo-catalytique de cet échantillon dans le processus de photo-formage du glycérol a été testée et comparée à celle des autres catalyseurs contenant des nanoparticules de Pt préparées par des techniques classiques. Afin d'éviter l'agglomération et la sédimentation de poudres d'oxyde de titane en suspension dans le milieu aqueux, des couches épaisses de TiO2 synthétisé et commercial ont été déposées sur un substrat conducteur en utilisant une technique de sérigraphie. Les électrodes ainsi obtenues ont été caractérisées par profilométrie, SEM, DRX, méthodes optiques, électriques et photoélectrochimique
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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