Due to the environmental problems caused by the steadily increasing usage of fossil fuels, the interest for renewable sources of energy has amplified significantly. Among the several possibilities, hydrogen gas is considered to be one of the most promising fuels forof the future. IfOnce formed from water via photocatalysis it is a desirable, convenient and green improvement in the field of energy. During this work, we have tried to contribute to this important field. 4wt.% Au/TiO2 synthesized by deposition-precipitation with urea was the main photocatalysts used in this project. Other noble metal-loaded (Pt and Ag) titanium dioxide materials were synthesized by deposition precipitation with urea and other methods such as sol gel and sol immobilization. These catalytic systems were studied and their activity compared for hydrogen production from water/methanol mixtures. Sets of monometallic Au, Ag, Pt and bimetallic Au-Pt and Au-Ag supported titanium dioxide were synthesized and tested. Au/TiO2 photocatalysts synthesized by deposition precipitation with urea was were the best in terms of hydrogen production compared to other photocatalysts. In the evaluation of possible sacrificial molecules, isopropanol was less efficient than methanol. Through the formation of bi-metallic Ag-Au/TiO2 and Pt-Au/TiO2 catalysts, the hydrogen production could be further improved. Finally, Ir supported Al2O3 was investigated for the first time as a heterogeneous catalyst for hydrogen production by photocatalytic dehydrogenation of aqueous p-formaldehyde and photoreduction of carbon dioxide.

This thesis presents an integrated study of nanocatalysts for heterogenous catalytic and electrochemical processes using pure ruthenium (Ru) with mixed-phase and platinum-based nanomaterials synthesized by continuous-flow chemistry. There are three major challenges to the application of nanomaterials in heterogenous catalytic reactions and electrocatalytic processes in acidic solution. These challenges are the following: (i) controlling the size, shape and crystallography of nanoparticles to give the best catalytic properties, (ii) scaling these nanoparticles up to a commercial quantity (kg per day) and (iii) making stable nanoparticles that can be used catalytically without degrading in acidic electrolytes. Some crucial limitations of these nanostructured materials in energy conversion and storage applications were overcome by continuous-flow chemistry. By using a continuous-flow reactor, the creation of scalable nanoparticle systems was achieved and their functionality was modified to control the nanoparticles’ physical and chemical characteristics. The nanoparticles were also tested for long-term stability, to make sure these nanoparticles were feasible under realistic working conditions. These nanoparticles are (1) shape- and crystallography-controlled ruthenium (Ru) nanoparticles, (2) size-controlled platinum-metal (Pt-M= nickel (Ni) & copper (Cu)) nanooctahedra (while maintaining morphology) and (3) core-shell platinum@ruthenium (Pt@Ru) nanoparticles where an ultrathin ruthenium shell was templated onto the platinum core. Thus, a complete experimental validation of the formation of a scalable amount of these nanoparticles and their catalytic activity and stability towards the oxygen evolution reaction (OER) in acid medium, hydrolysis of ammonia borane (AB) along with plausible explanations were provided.

The energy consumptions worldwide have increased simultaneously with the growth of the population and of the economy. Nowadays, finding an alternative way to satisfy the energy demand is one of the great challenges for the future of humanity, especially due to the limitation of fossil fuels and their effect on global warming. Hydrogen, as an alternative fuel for the future, is very attractive. Compared to traditional methods, such as the steam reforming of natural gas or coal gasification, photocatalytic water splitting (PWS) is considered to be the most sustainable alternative for producing hydrogen as a future fuel. PWS usually relies on semiconductor material that can transform the absorbed solar photon into photogenerated electrons and holes, creating a photopotential which can drive the electrochemical production of molecular hydrogen from the reduction of water. Despite its promising application, semiconductor-based PWS usually suffers from low carrier mobility and short diffusion length. Furthermore, the recombination of photogenerated electrons and holes might occur, especially if there are no suitable reaction sites available on the surface of the semiconductor. In order to facilitate the catalytic reactions on the surface of the semiconductor, the presence of a cocatalyst is necessary in order to obtain more efficient PWS processes. To this day, noble metals such as Pt, Pd, RuO2 and IrO2 are still the benchmark cocatalysts for PWS. Nevertheless, due to their high cost and limited supply, it is mandatory to develop a suitable strategy and to identify more efficient materials. Therefore, within the framework of this dissertation, novel cocatalysts and strategies that can improve the efficiency of the photocatalytic water splitting processes have been developed. Firstly, we developed a cocatalyst combining noble metals and semiconductors by means of partial galvanic replacement of the Cu2O nanocrystal with Au. The deposition of this cocatalyst on TiO2 was studied for the photocatalytic H2 production in order to explore the synergistic effect of the plasmonic resonance from the Au nanoparticles and pn-junction between Cu2O and TiO2. Additionally, the plasmonic effect of the Au films was also studied and utilized in order to improve the PWS. Secondly, the nanoscaling of cocatalysts was studied in order to improve the efficiency thereof and to reduce the cost of the cocatalyst materials. Moreover, it is sought to explore the quantum size effect on the properties of the cocatalyst and their effect on the photocatalytic reaction. Atomically precise Au and Ni nanoclusters were employed in these studies. Au nanoclusters were studied in relation to the cocatalysts in the photocatalytic water splitting, and Ni nanoclusters were studied in relation to the cocatalysts in the electrocatalytic water oxidation. The results of these studies will provide new insights in relation to the strategy used in order to develop efficient cocatalysts for the purpose of photocatalytic water splitting.

Water electrolysis conjugated with renewable energy sources potentially realizes a sustainable society. Although the current electrolyzers operate at extreme pH to maximize the electrolysis efficiency, near-neutral pH conditions may optimize the overall system operation when conjugated with renewable energy sources. In this context, a study on the electrolysis in the mild conditions is essential. The dissertation investigates the water electrolysis in various conditions, with a particular focus placed on milder conditions, to rationalize and improve its performance. Microkinetic analysis was performed for the cathodic half-reaction in conjugation with mass transport evaluation using various electrode materials. The analysis revealed a significant universal influence of electrolyte properties on the reaction performances at near-neutral pH. Investigation of the associated electrolyte properties (ion size, viscosity and activity/fugacity) rationally optimized the reaction conditions. Together with the separately performed studies on the anodic half-reaction and system configurations, the finding was successfully transferred to electrocatalytic and solar-driven water splitting systems. The presented herein is a fundamental yet crucial aspect of water electrolysis, which can advance the water electrolysis for the future.

Along with many other solar energy conversion processes, research on photocatalytic water splitting to generate hydrogen and oxygen has experienced rapid major development over the past years. Developing an efficient visible-light-responsive photocatalyst has been one of the targets of such research efforts. In this regard, nitride materials, particularly Ta3N5, have been the subject of investigation due to their promising properties. This dissertation focuses on the fundamental parameters involved in the photocatalytic processes targeting overall water splitting using Ta3N5 as a model photocatalyst. The discussion primarily focuses on relevant parameters that are involved in photon absorption, exciton separation, carrier diffusion, carrier transport, and catalytic efficiency. A collection of theoretical and experimental studies of properties associated with Ta3N5 have been utilized to obtain a comprehensive understanding of this material. The fundamental structural and optoelectronic properties of Ta3N5 have been addressed. From the electronic properties, the dielectric constant and effective masses have been calculated. Because of its high dielectric constant and relatively low effective masses, Ta3N5 is promising for photocatalytic reaction applications. Studies of lattice dynamics, optical properties, and band positions have been able to clearly show that the synthesized Ta3N5 is essentially non-stoichiometric and that a truly pure phase of Ta3N5 has never been achieved, even though XRD has shown a pure phase sample. The photophysical properties of Ta3N5, such as the absorption coefficient, carrier mobility, and carrier lifetime, have been experimentally measured by synthesizing Ta3N5 thin films. Very low kinetic properties with very low transport properties and fast carrier recombination explained why overall water splitting has never been achieved with Ta3N5 as a photocatalyst to date. The extent to which the surface states of Ta3N5 photocatalysts affect photocatalytic performance has been investigated. The surface topmost layer is demonstrated to play a critical role in the photocatalytic activity of Ta3N5; further research on the surface properties of Ta3N5 should be conducted to understand and improve charge separation and the resulting photocatalytic activity. Finally, a remarkable improvement in the photocatalytic OER has been achieved with the addition of cobalt as a cocatalyst. There is a trade-off between the optimum contact of hole transfer from bulk Ta3N5 to the surface of the cobalt cocatalyst and providing active sites for the electrochemical reaction. Knowing the characteristics of cobalt on the Ta3N5 surface, further improvement was attempted by adding a noble metal to the CoOx/Ta3N5 photocatalyst system, where a synergetic effect of CoOx and noble metals was observed.

El texto completo de este trabajo no está disponible en el Repositorio Académico UPC por restricciones de la casa editorial donde ha sido publicado. / In the mining industry of Peru, as a consequence of large scale mineral extraction and a dynamic process of ore dressing, the different activities of the operation are neglected. This neglect, in some cases, generates collateral damages that affect the structures, generate extra costs, and result in constant accidents due to rock-fall and delays in the activities of the mining cycle and the ones following it, which seriously harms the Mining Unit and directly affects the workers. In addition, this is directly related to the progress of the daily work and, in turn, it is related to the design of the mine mesh that is prepared to extract the largest possible mineral amounts per shift. This means that, by modifying the design of the mine mesh, the blasting method, and the type of explosive used, the current scenario and the results could be modified. In this respect, a research was conducted, exclusively based on simulations using the JK Simblast software, applying the pre-splitting blast method in the underground mine of the Minera Aurífera Retamas SA company, resulting in a reduction of over break in more than 60% as compared to the mesh used initially. This has an impact on the level of support to be used, and therefore, on the maintenance costs, which were reduced by 18%. Therefore, this article presents the possibility of reducing maintenance costs in underground mining using pre-splitting blasting. Copyright 2019.

Electrochemical water splitting to hydrogen and oxygen is an attractive approach to store and convert intermittent renewable energy sources. Energy efficient, cost effective and durable electrochemical systems are highly required. Firstly, CeOx coated oxygen evolution electrocatalysts were developed to improve the stability. Unique permselectivity of the CeOx layer was disclosed, which helps to prevent dissolution of active metal site. Because oxygen evolution reaction requires a higher overpotential than hydrogen evolution reaction, kinetically facile oxidation of soluble redox ions was proposed as an alternative anodic reaction, in which the oxidized redox ions can be used for succeeding homogeneous reactions, such as treatment of H2S. How to tune the thermodynamics and the diffusion of candidate redox ions is discussed for a desired application. In addition to the anodic reaction, cathodic hydrogen evolution reaction has to be optimized. To maximize hydrogen evolution performance in near-neutral pH buffered conditions, concentration overpotentials from local pH and hydrogen on a Pt cathode are distinguished by mass transport modelling. Finally, stand-alone module was developed to perform solar-driven redox-mediated H2S splitting to H2 and S under natural solar irradiation.

A multi-dimensional upwind fluctuation splitting scheme is developed and implemented for two-dimensional and axisymmetric formulations of the Navier-Stokes equations on unstructured meshes. Key features of the scheme are the compact stencil, full upwinding, and non-linear discretization which allow for second-order accuracy with enforced positivity. Throughout, the fluctuation splitting scheme is compared to a current state-of-the-art finite volume approach, a second-order, dual mesh upwind flux difference splitting scheme (DMFDSFV), and is shown to produce more accurate results using fewer computer resources for a wide range of test cases. The scalar test cases include advected shear, circular advection, non-linear advection with coalescing shock and expansion fans, and advection-diffusion. For all scalar cases the fluctuation splitting scheme is more accurate, and the primary mechanism for the improved fluctuation splitting performance is shown to be the reduced production of artificial dissipation relative to DMFDSFV. The most significant scalar result is for combined advection-diffusion, where the present fluctuation splitting scheme is able to resolve the physical dissipation from the artificial dissipation on a much coarser mesh than DMFDSFV is able to, allowing order-of-magnitude reductions in solution time. Among the inviscid test cases the converging supersonic streams problem is notable in that the fluctuation splitting scheme exhibits superconvergent third-order spatial accuracy. For the inviscid cases of a supersonic diamond airfoil, supersonic slender cone, and incompressible circular bump the fluctuation splitting drag coefficient errors are typically half the DMFDSFV drag errors. However, for the incompressible inviscid sphere the fluctuation splitting drag error is larger than for DMFDSFV. A Blasius flat plate viscous validation case reveals a more accurate vertical-velocity profile for fluctuation splitting, and the reduced artificial dissipation production is shown relative to DMFDSFV. Remarkably the fluctuation splitting scheme shows grid converged skin friction coefficients with only five points in the boundary layer for this case. A viscous Mach 17.6 (perfect gas) cylinder case demonstrates solution monotonicity and heat transfer capability with the fluctuation splitting scheme. While fluctuation splitting is recommended over DMFDSFV, the difference in performance between the schemes is not so great as to obsolete DMFDSFV. The second half of the dissertation develops a local, compact, anisotropic unstructured mesh adaption scheme in conjunction with the multi-dimensional upwind solver, exhibiting a characteristic alignment behavior for scalar problems. This alignment behavior stands in contrast to the curvature clustering nature of the local, anisotropic unstructured adaption strategy based upon a posteriori error estimation that is used for comparison. The characteristic alignment is most pronounced for linear advection, with reduced improvement seen for the more complex non-linear advection and advection-diffusion cases. The adaption strategy is extended to the two-dimensional and axisymmetric Navier-Stokes equations of motion through the concept of fluctuation minimization. The system test case for the adaption strategy is a sting mounted capsule at Mach-10 wind tunnel conditions, considered in both two-dimensional and axisymmetric configurations. For this complex flowfield the adaption results are disappointing since feature alignment does not emerge from the local operations. Aggressive adaption is shown to result in a loss of robustness for the solver, particularly in the bow shock/stagnation point interaction region. Reducing the adaption strength maintains solution robustness but fails to produce significant improvement in the surface heat transfer predictions. / Ph. D.

This thesis studies how the Douglas–Rachford splitting technique can be leveraged for scalable computational optimal transport (OT). By carefully splitting the problem, we derive an algorithm with several advantages. First, the algorithm enjoys global convergence rates comparable to the state-of-the-art while benefiting from accelerated local rates. In contrast to other methods, it does not depend on hyperparameters that can cause numerical instability. This feature is particularly advantageous when low-precision floating points are used or if the data is noisy. Moreover, the updates can efficiently be carried out on GPUs and, therefore, benefit from the high degree of parallelization achieved via GPU computations. Furthermore, we show that the algorithm can be extended to handle a broad family of regularizers and constraints while enjoying the same theoretical and numerical properties. These factors combined result in a fast algorithm that can be applied to large-scale OT problems and regularized versions thereof, which we illustrate in several numerical experiments. In the first part of the main body of the thesis, we present how Douglas-Rachford splitting can be adapted for the unregularized OT problem to derive a fast algorithm. We present two global convergence guarantees for the resulting algorithm: a 1/k-ergodic rate and a linear rate. We also show that the stopping criteria of the algorithm can be computed on the fly with virtually no extra costs. Further, we specify how a GPU kernel can be efficiently implemented to carry out the operations needed for the algorithm. To show that the algorithm is fast, accurate, and robust, we run a series of numerical benchmarks that demonstrate the advantages of our algorithm. We then extend the algorithm to handle regularized OT using sparsity-promoting regularizers. The generalized algorithm will enjoy the same sublinear rate derived for the unregularized formulation. We also complement the global rate with local guarantees, establishing that, under non-degeneracy assumptions on the solution, the algorithm will identify the correct sparsity pattern of the solution in finitely many iterations. When the sparsity pattern is identified, a faster linear rate typically dominates. We also specify how to extend to the GPU implementation and the stopping criteria to handle regularized OT, and we subsequently specify how to backpropagate through the solver. We end this part of the thesis by presenting some numerical results, including performance on quadratically regularized OT and group Lasso regularized OT for domain adaptation, showing a substantial improvement compared to the state-of-the-art. In the last part of the thesis, we provide a more detailed analysis of the local behavior of the algorithm when applied to unregularized OT and quadratically regularized OT. We subsequently outline how to extend this analysis to several other sparsity-promoting regularizers. In the former two cases, we show that the update that constitutes the algorithm converges to a linear operator in finitely many iterations. By analyzing the spectral properties of these linear operators, we gain insights into the local behavior of the algorithm, and specifically, these results suggest how to tune stepsizes to obtain better local rates. / Denna avhandling behandlar hur Douglas–Rachford-splittning kan tillämpas för skalbara beräkningar av optimal transport (OT). Genom en noggrann splittning av problemet härleder vi en algoritm med flera fördelar. För det första åtnjuter algoritmen en global konvergenshastighet som är jämförbara med populära OT-lösare, samtidigt som den drar nytta av accelererade lokalahastigheter. Till skillnad från andra metoder är den inte beroende av hyperparametrar som kan orsaka numerisk instabilitet. Den här egenskapen är särskilt fördelaktig när lågprecisionsaritmetik används eller när data innehåller mycket brus. Uppdateringarna som algoritmen baseras på kan effektivt utföras på GPU:er och dra nytta av dess parallellberäkningar. Vi visar också att algoritmen kan utökas för att hantera en rad regulariseriseringar och bivillkor samtidigt som den åtnjuter liknande teoretiska och numeriska egenskaper. Tillsammans resulterar dessa faktorer i en snabb algoritm som kan tillämpas på storskaliga OT-problem samt flera av dess regulariserade varianter, vilket vi visar i flera numeriska experiment. I den första delen av avhandlingen presenterar vi hur Douglas-Rachford-splittning kan anpassas för det oregulariserade OT-problemet för att härleda en snabb algoritm. För den resulterande algoritmen presenterar vi två globala konvergensgarantier: en 1/k-ergodisk och en linjär konvergenshastighet. Vi presenterar också hur stoppkriterierna för algoritmen kan beräknas utan vidare extra kostnader. Dessutom specificerar vi hur en GPU-kärna kan implementeras för att effektivt utföra de operationer som algoritmen baseras på. För att visa att algoritmen är snabb, exakt och robust utför vi ett flertal numeriska experiment som påvisar flera fördelar över jämförbara algoritmer. Därefter utökar vi algoritmen för att hantera regulariserad OT med s.k. sparsity-promoting regularizers. Den generaliserade algoritmen åtnjuter samma sublinjära konvergenshastighet som härleddes för den oregulariserade fallet. Vi kompletterar också garantierna genom att tillhandahålla lokala garantier genom att fastställa att givet svaga antaganden om lösningen kommer algoritmen att identifiera den korrekta lösningens gleshetsstuktur inom ett begränsat antal iterationer. När glesheten identifierats dominerar typiskt sett en snabbare linjär konvergenshastighet. Vi specificerar också hur man utökar till GPU-kärnan och resultaten av stoppkriterierna för att hantera regulariserad OT, och vi specificerar sedan hur man differentiera genom lösaren. Vi avslutar den här delen av avhandlingen genom att presentera några numeriska resultat för kvadratiskt reglerad OT och group Lasso-reglerad OT för domänanpassning, vilket visar en betydande förbättring jämfört med de mest populära metoderna för dessa tillämpningar. I den sista delen av avhandlingen ger vi en mer detaljerad analys av algoritmens lokala beteende när den tillämpas på oregulerisad OT och kvadratiskt reglerad OT. Vi föreslår även sätt att studera flera andra fallet. I de två första fallen visar vi att uppdateringen som utgör algoritmen konvergerar till en linjär operator inom ett begränsat antal iterationer. Genom att analysera de spektrala egenskaperna hos dessa linjära operatorer får vi ytterligare insikter i algoritmens lokala beteende, och specifikt indikerar dessa resultat hur man kan justera steglängden för att uppnå ännu bättre konvergenshastigheter. / <p>QC 20231123</p>

The purpose of this master thesis is to make use of the category taxonomy of Wikipedia to determine the most reasonable split from the suggestions generated by an independent compound word splitter. The articles a word was found in can be seen as a group of contexts the word can occur in and also different representations of the word, i.e. an article is a representation of the word. Instead of only analysing the data of each single article, the intention is to find more data for each representation/context to perform an analysis on. The idea is to expand each article representing one context by including related articles in the same category. Two perceptions of a ”reasonable split” was studied. The first case was a split consisting of only two parts and the second case of unlimited parts. This approach is well-suited for choosing the correct split out of a several suggestions but unsuitable for identifying compound words. It would more often than not decide to not split a compound word. It is very dependant on the compound words appearing in Wikipedia. / Syftet med detta examensarbete är att utse den rimligaste uppdelningen av ett sammansatt ord genom användning av Wikipedias kategoritaxonomi. Förslag på olika uppdelningar genereras av en oberoende färdig algoritm. Artiklarna som ett ord finns can ses som en grupp av kontexter som ett ord kan förekomma i och olika framställningar av ett ord. Avsikten är att hitta mer data för varje framställning/kontext att utföra en analys på istället för att bara analysera artikeln ordet hittades i. Idéen som ska testas är att expandera varje artikel som representerar en kontext genom att inkludera relaterade artiklar i samma kategori. Två olika synsätt på ”rimliga uppdelningar” studerades. Första fallet var att endast dela upp sammansatta ord i två delar och andra fallet var att dela upp i obestämt antal delar. Metoden visade sig utmärka sig på att välja rätt uppdelning när den väl gjorde ett försök. En stor nackdel var att den ofta valde att inte dela upp sammansättningar trots att den skulle ha gjort det. Metoden är mycket beroende av att sammansättningarna måste finnas i Wikipedia.