1,720,964 research outputs found
Effect of ammonia co-feeding on oxygenates over K-Mo2C in the Fischer-Tropsch synthesis
Get PDFThe Fischer-Tropsch (FT) process, producing long chained waxes and transportation fuels, is competing with fuels derived from crude oils and its profitability is therefore dependent on the global oil price. However, increasing the value of synthesized products could render the profitability of the FTS independent of fluctuations in the oil price (which are mostly due to global political trends). One way to achieve this, is to target fine chemicals instead of fuels. At the Catalysis Institute, this has been investigated by adding ammonia to the feed gas stream and obtaining highly valuable amines, amides and nitriles. It has been shown that the so-called nitrogen containing compounds are formed instead of the Fischer-Tropsch typical albeit minor products alcohols, aldehydes and carboxylic acids, i.e. oxygenates. Increasing the oxygenate selectivity was investigated in numerous studies as no commercial FT based process exists which produces oxygenates at a significant yield. Typically, transition metals such as Fe, Co, Rh and Ni are active for the FT synthesis. Based on reaction conditions employed, commercial Fe and Co based catalysts have been shown to produce between 6 and 12 C% oxygenates. Rh has been shown to have a high oxygenate selectivity, but the associated high raw material cost becomes prohibitive for use as a commercial FT catalyst. Catalysts other than the traditionally known FT active transition metals have shown promising results in terms of oxygenate selectivity. Transition metal carbides such as Mo2C, have been investigated under Fischer-Tropsch conditions. While the bare catalyst produces mainly methane and other hydrocarbons, upon promotion with potassium the selectivity shows a significant shift towards oxygenates. This project investigates the use of potassium promoted molybdenum carbide as a catalyst for high oxygenate selectivity in the Fischer-Tropsch synthesis. β-Mo2C was synthesized and subsequently promoted with different levels of potassium and its Fischer-Tropsch synthesis performance was evaluated in a stainless steel fixed bed reactor. The influence of catalyst synthesis protocols, reactor pressure and temperature, feed gas space velocity, and K/Mo wt.% promotion on catalyst activity and selectivity were studied. At a stable CO conversion (±10%) and its related oxygenate selectivity (±35 C%) ammonia was co-fed to the catalyst to study the conversion of oxygenates to nitrogen containing compounds. In summary, an unpromoted β-Mo2C catalyst reached CO conversions to ±40% at the conditions applied. Initial promotion of the catalyst with potassium showed a significant drop in catalyst activity, however, an increase in potassium content did not further decrease catalyst activity. The selectivity towards oxygenates was greatly enhanced from 10 C% up to 42 C% (CO2-free) at similar reaction conditions. Simultaneously, the oxygenate distribution shifted towards higher alcohols. The initial methanol content in the total oxygenate slate was around 60 C%, decreasing to about 20 C% upon potassium promotion. During co-feeding of ammonia, N-containing compounds were observed in the form of nitriles (±9 C%, CO2-free) and small traces of amides (±0.1 C%, CO2-free). Acetonitrile was the most dominating formed N-containing compound (≥58 C%). Upon the co-feeding of ammonia, the oxygenate selectivity decreased by roughly 10 C% points (CO2-free) but did not reach zero. Catalyst activity was slightly affected but recovered with time on stream. A slowly building up blockage appeared after 1-3 hours TOS simultaneously with a decreasing CO2 selectivity, suggesting the reaction with NH3 forming ammonium carbonate. This could however not be confirmed. The benefits of producing N-containing compounds using a potassium promoted β-Mo2C needs to be further investigated, trying to avoid the blockage by suppressing the WGS-activity of the catalyst. It is promising that the activity is hardly affected and that in the short period of time on stream N-containing compounds were observed
Application of Molybdenum Carbide Catalysts for the CO2-assisted Oxidative Dehydrogenation of Ethane
Get PDFThe rising demand for light olefins is at present mainly met via catalytic/thermal dehydrogenation of alkanes at temperatures of up to 900 °C. Under these severe process conditions, competing side reactions and catalyst deactivation via coking are the major challenges. Co-feeding an oxidant significantly decreases the reaction temperature. The oxidative dehydrogenation of ethane to ethylene, using CO2 as the oxidant (CO2-ODH), has earned a lot of interest in the past decade. The use of CO2, a soft oxidant in comparison to O2, prevents the overoxidation reaction of the paraffin to CO2 and allows for improved heat control. Besides that, the coking effect, which is believed to be the main catalyst deactivation pathway during these high temperature processes, could be significantly lowered due to the reverse Boudouard reaction. The most common catalytic materials reported are reducible metal oxides (MOx) due to their redox properties; a key concept to activate the C-H bond of the alkane and subsequently activate CO2. Besides metal oxides, transition metal carbides have also shown to be active for the CO2-ODH, reaching high yields of ethylene. Specifically, molybdenum carbide (MoxCy) has shown to be a highly efficient catalyst for CO2 activation and alkane dehydrogenation, demonstrating its ability to cleave C-H bonds. These characteristics are important in making a MoxCy-based catalyst a serious candidate for the CO2- ODH of light alkanes. This work entails the design of novel MoxCy-based catalysts for application in the CO2-ODH of C2H6. Previous work on MoxCy-based catalysts found that the bulk material has limited activity and selectivity towards producing C2H4 but is significantly improved once dispersed on a support material. The type of support material dictates whether the CO2-ODH reaction takes place, or if one of the major side reactions, the dry-reforming of C2H6 to synthesis gas, is preferred. In this study, MoxCy nanoparticles were prepared via various (novel) synthesis techniques, dispersed on a variety of MOx support materials as well as modified with various promoters. Besides the exploratory nature of this study, gaining knowledge on the activity of the various formulations of MoxCy-based catalysts, the preparation conditions of the carbide materials were investigated. To prepare MoxCy, the precursor samples (in the molybdate or oxide phase) are exposed to a temperature programmed treatment (carburization) in the presence of a carbonaceous and reductive gas mixture. The carbide formation, in terms of crystallite structure, surface composition as well as potential fouling mechanisms is highly dependent on the heating rate, gas mixture, final temperature and precursor composition. Various experiments utilizing in situ characterization techniques, such as in situ X-ray diffraction, X-ray adsorption and Raman spectroscopy as well as online product analysis techniques were employed to gain knowledge on the carburization process, the structural and chemical properties and their effect on the activity of the various prepared catalysts in the CO2-ODH as well as the reverse water-gas-shift reaction. The use of MoxCy-based catalysts in the CO2-ODH reaction has not been thoroughly investigated in literature before and is still a very new topic to the scientific community. The presented research can contribute on various aspects of the use and viability of MoxCy-based catalysts in CO2 utilizing reactions and can be extended to dry-reforming or CO2 hydrogenation to fuels. In terms of catalyst synthesis, the extensive characterization exposing the various possible crystal structures of MoxCy nanoparticles and application of surface sensitive techniques, allowed for a better understanding of the possible active phases responsible for CO2 and alkane activation. Besides the identification of the active phase, the deactivation mechanism for MoxCy-based catalysts in the CO2-ODH reaction is studied in more detail by focusing on the crystal structure and the presence of carbon on the catalyst surface. By varying catalyst compositions as well as reaction conditions, including the use of various co-feeding experiments, an increase in catalytic stability, while maintaining high yields of the desired product from CO2-ODH (ethylene), was achieved
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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