1,721,062 research outputs found
Catalytic Transformation of Ethanol to 1,3-Butadiene over MgO/SiO2 Catalyst
No full textIncreasing concerns regarding global warming, which is caused by growing CO2emissions, have led to efforts focused on discovering alternatives to petroleum for energy and commodity chemical production. (Bio)ethanol has been seen as a platform molecule with increasing production and versatility for upgrading to various high-value fuels and chemicals. Among those high-value chemicals is 1,3-butadiene (1,3-BD), which has demonstrated widespread applications in polymer synthesis and as an organic chemistry intermediate. Its conventional methods of production rely on oil as a feedstock, hence suggesting the need for alternative and more sustainable routes. Interest in the catalytic conversion of ethanol to 1,3-BD, introduced in the 1940s by Lebedev, has been revived and is now focused on the development of selective catalysts, thus minimizing the need for the high cost separation between 1,3-BD and other (by)products, such as C2 and C4 olefins and oxygenates. The main components of the catalyst for this system are MgO and SiO2, where its reactivity and selectivity depend heavily on the method of preparation. This system is still at an early stage of development, with a lot of disagreements on structure of the catalyst, optimum ratio of Mg:Si, reactive intermediates, reaction mechanisms, and kinetics. Reaction mechanism was studied intensively using both theoretical (DFT) and experimental (spectroscopy) methods. Initial screening of the reaction mechanism using DFT with MgO defect site, i.e. kink, demonstrated that aldol condensation is more viable thermodynamically than Prins condensation. In the reaction mechanism, dehydrogenation of ethanol to acetaldehyde, an important reactive intermediate, is shown to be the rate-determining step (RDS) of the reaction. Comparison of the potential energy barrier also shows that acetaldol, the product of acetaldehyde self-aldolisation, dehydration competes with its hydrogenation with an ethanol molecule. This mechanistic study is also supported by comprehensive in-situ DRIFTS. MgO/SiO2 catalyst is synthesized using a wet-kneading method, with equivalent oxide mass ratio and thoroughly characterized with HS-LEIS, DRIFTS, and XRD. Chemical probing was also done with different probe molecules, such as pyridine, NH3, CO2, and methanol. Combination of several reactants and intermediate shows that acetaldehyde is spontaneously transformed to crotonaldehyde under constant reactant flow, while in-situ ethanol DRIFTS requires contribution from the gas-phase ethanol to make 1,3-BD. Furthermore, the crotonaldehyde does not transform to 1,3-BD under inert flow, it requires the presence of ethanol to complete the transformation to 1,3-BD. The resulting catalyst was extensively probed and characterized, revealing a silica-rich surface, where comparison with incipient wetness impregnation catalyst shows a rather Mg-rich surface. Surface silicate that is formed is confirmed by in-situ DRIFTS, where new OH groups were formed. The basicity of the catalyst also varies significantly with different methods of preparation and calcination temperature. All strong, medium, and weak basic sites were found on the catalysts surface. More superior performance, however, is shown to be enforced by lower amount of strong basic sites. Ammonia probing reveals the presence of both open and closed Lewis acid sites (LAS) and limited amount of Brønsted acid sites (BAS). Pyridine, on the other hand, could not identify any BAS, which is due to its larger molecule size. This further demonstrates that the LAS on the catalyst is much more accessible than the BAS.Promotion of the catalyst with transition metal was shown to have a significant enhancement on the reactivity. Since the RDS was determined to be the dehydrogenation of ethanol, transition metal sites lower this barrier, and shift the RDS. Zn and Cu, two very promising ethanol dehydrogenation catalysts, were separately impregnated on the uncalcined wet-kneaded MgO/SiO2 support at low loadings, 2.5 and 1%, respectively. The catalysts were thoroughly characterized using in-situUV-Vis, methanol operando DRIFTS, in-situ XANES and EXAFS, TEM, TPSR, and in-situ DRIFTS. Cu(II) exists as a surface species coordinated in a tetrahedral geometry, where it has 0.8 (or ~1) nearest neighbor, i.e. number of Cu-O-Cu bonds. The transition metal also possesses Cu-O-Mg bond, hinting to formation of solid solution. Similar interaction was also observed for Zn, suggesting the stronger interaction with Mg, instead of Si. This structural change affects the basicity and acidity of the catalyst. Both CO2 and methanol probing with DRIFTS show that the promoted catalysts have less affinity with CO2,while the BAS was eliminated, replaced with another distinct LAS. Redox capability was also modified, shown by the enhanced strength of the redox site in expense of its reduced quantity. During the reaction, Cu(II) is reduced to Cu(0) via an intermediate Cu species, before the catalyst deactivates after long hours of experiment. Zn, on the other hand, maintained its structure even after extensively tested
Destruction of emerging organophosphate contaminants in wastewater using the heterogeneous iron-based photo-Fenton-like process
No full textSmart urea ionic co-crystals with enhanced urease inhibition activity for improved nitrogen cycle management
No full textA smart ionic co-crystal of urea with KCl and ZnCl2has been obtained in two polymorphic modifications via mechanochemical and solution methods and proven to be a very efficient urease inhibitor while, simultaneously, able to provide soil nutrients to complement N supply
Nature and Reactivity of Active Sites in Mn-and Na-promoted WOx/SiO2 Model Catalysts for Oxidative Coupling of Methane (OCM) under Operating Conditions
No full textThe silica (SiO2)-supported, manganese (Mn)- and sodium (Na)-promoted tungsten oxide (WOx) catalyst (Mn-Na2WO4/SiO2; also written as Mn-Na-WOx/SiO2 and Mn2O3-Na2WO4/SiO2) has, for decades, been studied as the most promising catalyst for the production of ethylene (C2H4) via the oxidative coupling of methane (OCM). The moderate activity (~35% CH4 conversion), high C2 selectivity (~80%), and robust thermostability (~1000 hr TOS) has made this catalyst the state-of-the-art catalyst for OCM. However, the techno-economic target for industrial scale application of this OCM catalyst requires that the C2 yield be increased further and that the operating temperature be lowered from the ~800-900oC range. The lack of fundamental understanding of this OCM catalyst system (catalyst structure under working conditions, manganese/sodium promotion mechanism, nature and identity of active phases and sites, and reaction mechanism), however, have hampered the improvement and optimization of this catalyst towards the techno-economic targets. The objectives of the dissertation were to (1) establish the fundamental catalyst structure-activity relationships by application of modern in situ and operando spectroscopy during OCM, combined with kinetic studies and density functional theory (DFT); (2) apply the new fundamental insights to guide rational design of advanced active and selective OCM catalysts functioning at lower temperatures. Various permutations of unpromoted and Na-, Mn- and Na/Mn-promoted SiO2-supported WOx sites were synthesized. The promoters-to W molar ratio was systematically varied to explore their effects on both the structure and properties of the WOx surface sites and the resulting OCM activity and selectivity. The catalysts were characterized in-situ under dehydrated conditions where surface water is not present and under OCM reaction conditions using Raman, UV-vis and IR spectroscopies to determine the molecular and electronic structures of the WOx sites on silica. Experimental findings were complemented with computational insights from VASP DFT calculations to provide additional understanding regarding the structure-activity relationships. Work herein provides the following key pieces of information, for the first time, that will help in the rational design of this OCM catalyst to make it more suitable for industrial application: (1) Dispersed phase WO4 surface sites were identified as the active OCM sites where C-H scission in CH4 takes place. Coordination of these WO4 surface sites to Na-promoter forms Na-WO4 surface sites that are significantly more C2 selective than the unpromoted WO4 surface sites and the crystalline phase Na2WO4. Previously, literature reports had completely missed the presence of these surface sites and had attributed the OCM activity to crystalline phase Na2WO4 which melts above 700oC and hence cannot be the active phase; (2) Mn-promoter was found to be primarily a spectator during OCM, especially at differential conditions where O2 reactant is not limited. Mn-promotion of WO4 surface sites (Mn-WO4) and Mn-promotion of Na-WO4 surface sites (Mn-Na-WO4) does not lead significant changes in the surface kinetics of the parent surface sites. Experimental structural characterization and DFT results further showed that Mn-promotion creates oligomeric MnOx surface sites along with poorly-crystalline nanoparticles (Mn-WO3 and MnWO4), all of which remain spectating during OCM. DFT insights further show that if MnOx oligomers activate CH4, the surface intermediates formed are significantly more stable than those formed from CH4 activation over Na-WO4 surface sites, increasing the likelihood of stable intermediates over-oxidizing to COx due to their higher stability and the resulting slower desorption; (3) the phase of SiO2 support is not relevant in this catalyst and SiO2 is primarily inert. The model catalysts studied herein exhibited ~85% C2 selectivity at ~40% CH4 conversion while the SiO2 was its amorphous phase due to the low Na-promoter concentration. Previously, due to the high Na concentration, researchers found SiO2 to be in its crystalline cristobalite phase as Na induces a phase transformation in SiO2 at elevated temperatures and speculated a critical influence of this crystalline SiO2 phase towards making a more C2 selective OCM catalyst
Natural and Tailored Magnesium-based Materials for Adsorptive Nutrient Recovery and Catalytic Wastewater Treatment
No full textNutrient recovery and water reuse are two important challenges for the 21st century that are closely interlinked in the food-energy-water nexus. Two significant types of water pollution originate from inorganic nutrient encroachment of water systems due to agriculture and the emerging organic contaminants from commonly used products such as pharmaceuticals. Nutrients are lost into the watershed due to the soluble nature of conventional nitrogen and phosphorous fertilizer from runoff. Water systems are further affected by emerging organic contaminants that are ubiquitous due to increasing urbanization and the rise of population. We hypothesize that magnesium based abundant natural minerals and tailored nanomaterials can be utilized in removing inorganic nutrients and emerging organic contaminants from wastewater, to achieve the goals of nutrient recovery and organic contaminant removal. Nutrient capture via struvite recovery has been proven as a highly effective method of capturing NH4+ and PO43- from wastewater with efficiencies exceeding 90% nutrient removal. The objectives of this dissertation were to (1) utilize naturally occurring dolomite and tailored mesoporous MgO nanoparticles as Mg-sources for struvite crystallization with the goals of studying adsorption kinetics, reaction mechanism of adsorption and product characterization (2) using doped MgO nanoparticles as novel heterogeneous catalysts for the photo-Fenton-like oxidation reaction (3) integration of the photo-Fenton-like oxidation reaction and struvite recovery to produce struvite from nutrient-rich wastewater streams containing emerging organic contaminants. The natural mineral dolomite was utilized to remove NH4+ and PO43- from simulated wastewater up to 37% and 94%, as the presence of Ca2+ leads to other phosphate phased being formed in addition to struvite. Temporally-resolved in situ Raman spectroscopy and spectrokinetics revealed a complex surface reconstruction phenomena preceding phosphate adsorption, where surface -OH sites were replaced by -CO3 units, which eventually deplete to form PO43-, HPO42-, H2PO4-, with struvite being formed as the final product. The use of nanostructured MgO as the Mg-source for struvite crystallization was shown to improve surface area normalized rate constants of phosphate adsorption by a factor of 5 compared to microcrystalline MgO, owing to more efficient intraparticle diffusion. Furthermore, the presence of surface dopants Cu, Fe, and Zn were shown to reduce phosphate adsorption rates. As natural periclase is typically contaminated by trace amounts of these transition metals, important kinetic implications were revealed by this work. Finally, the pseudo-second order rate constant values in the temperature range 25℃ - 45℃ were fitted to the Arrhenius equation in order to calculate activation energy of phosphate adsorption on MgO. However, the poor fit of the rate constants revealed that the pseudo-order models do not accurately describe phosphate adsorption on MgO possibly due to the surface reconstruction revealed in the previous spectrokinetic study. Due to the emerging threat of organic contaminants in wastewater that pose both environmental and human health hazards, the Fe-MgO and Cu-MgO nanoparticles were used as heterogeneous photo-Fenton-like catalysts to oxidize model emerging organic compounds salicylic acid and tetracycline which are of pharmaceutical relevance, and the organophosphate paraoxon ethyl, which is an insecticide derivative molecule. The MgO support underwent surface dissolution to increase the pH, which prevented the active Cu and Fe sites from leaching out of the catalyst. While the smaller molecule salicylic acid was completely oxidized to terminal products CO2 and H2O, larger molecules paraoxon ethyl and tetracycline resulted in the production of linear acids as stable intermediate products. The Fe and Cu active sites were shown to undergo reduction due to the Fenton mechanism using diffuse reflectance UV-vis spectroscopy and X-ray photoelectron spectroscopy of the as-synthesized and post-reaction catalysts. The catalyst crystallinity reduced due to the surface dissolution but remained highly active over 5 catalytic cycles. Finally, the catalytic treatment of organic contaminants was coupled with the nutrient recovery process in order to prevent the toxic organic contaminants from incorporating into the struvite, which would have adverse consequences during field application. By using a catalyst loading under the supersaturation limit of struvite formation, the catalytic reaction was shown to proceed without perturbing the N and P nutrients. After catalyst separation, the nutrients were recovered with high removal rates using mesoporous MgO nanoparticles (up to 87% NH4+ and 99% PO43- removal), demonstrating that a tandem oxidation and nutrient recovery process can increase the purity of struvite by pre-treating the organic contaminants
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
Surface analysis insight note
No full textAdventitious carbon contaminations are not only omnipresent and used for charge referencing of XPS spectra but also can alter the apparent presence of the element peaks that span over the large spectral window of binding energies. This Insight note describes the effect of an adventitious contamination layer on Pt and presents, in brief, the approach whereby the component spectra are derived for ion beam cleaned Pt samples that can then utilize linear mathematics to peak fit said spectra thus quantifying the amount of each component including that assigned to the contamination itself of Pt metal
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
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