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京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21075号 / 工博第4439号 / 新制||工||1690(附属図書館) / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 前 一廣, 教授 吉田 潤一, 教授 長谷部 伸治 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Carbon dioxide (CO2), as a greenhouse gas, has shown to achieve the highest level in history, causes the global warming issue, leading to a 1.2 ℃ increase of the global average temperature. The consumption of fossil fuels is one of the main reasons that cause CO2 emission. Current industrial production of chemicals accounts for 29% of total fossil fuels consumption, which can be the feedstock or raw materials for carbon source, or act as the fuel to generate heat and power. CO2 conversion technologies, e.g., thermo-catalytic reaction and electrochemical reduction, have drawn researchers' attention, since they have the potential to resolve the feedstock and fuel consumption sectors of chemical production at the same time. CO2 conversion technologies use CO2 as the direct carbon source of chemicals and store the intermittent renewable energies as the energy source, which can ultimately achieve a net-zero CO2 emission and produce value-added chemical products. However, there are challenges for a practical application of CO2 conversion technologies. For instance, electrochemical CO2 reduction reaction (ECO2RR) suffers from the low activity and selectivity, while thermocatalytic CO2 conversion, or the CO2 hydrogenation reaction, usually requires harsh reaction conditions and has a low selectivity. Nonetheless, the improvement of developing new promising catalysts remains limited, due to the lack of insights of the reactions. The complex reaction networks and kinetics lead to an elusive reaction mechanism, and various effects, e.g., solvation, potential, structure, and coverage, hinder our fundamental understanding of catalytic processes. Herein, we report the efforts that we have been put in to gain insights of reaction mechanism of CO2 reduction reactions. Bi has shown to reduce CO2 to formic acid (HCOOH), while we have found that, by constituting a Bi-Cu2S heterostructure catalyst, a better catalytic performance was achieved, due to the structural effect of the interface (Chapter 2). However, it is shown that the CO2 electrochemical reduction mechanism on Bi has changed when switching the electrolyte from water to aprotic media, e.g., ionic liquids, and CO was obtained as the main product instead of HCOOH, showing a shift of reaction pathway due to the electrolyte effect (Chapter 3). However, the fundamental understanding of reaction mechanism requires not only the reaction pathways, but the reaction kinetics under reaction conditions, where the lateral or adsorbate-adsorbate interactions play an important role. In this case, we summarized recent advances of applications of machine learning (ML) algorithms for adsorbate-adsorbate interaction model developments to deal with the realistic reaction kinetics (Chapter 4). The lattice based Kinetic Monte Carlo (KMC) has shown promising performances for considering the lateral interactions of surface reactions. We report the mechanistic and KMC kinetic study of CO2 hydrogenation on Cesium promoted Au(111) surface, to gain insights of alkali metal promoting effects under reaction conditions (Chapter 5). To expand the scope, the integration of CO2 reduction with the C-N bond formation provides a promising strategy to produce more value-added product such as urea. Recent studies show that urea can be produced by reducing CO2 and nitrate (NO3-) from wastewater, which mitigate both global warming and nitrate pollution issue. However, the reaction mechanism remains elusive due to the complicated reaction network. Therefore, we employed the first-principles molecular dynamics to reveal the reaction mechanism of C-N coupling and the effect of different reaction conditions including applied potential and electrolyte (Chapter 6). Although recent advances in the computational catalysis field have significantly push forward the understanding of the chemistry nature of heterogeneous catalysis, the gap between theory and experiment remains far beyond bridged due to the complexity nature of the problem in a wide range of time and length scales, hinders the development and discovery of active catalytic materials. Recent advances of narrowing and bridging the complexity gap between theory and experiment with machine learning have been summarized to emphasize the importance of utilizing machine learning for rational catalyst design (Chapter 7). / Doctor of Philosophy / Global warming issue is a rising topic in recent years which has severe impacts on environments. One of the main reasons is the increase level of greenhouse gases that prevent the release of heat that captured from the sun. Carbon dioxide (CO2) is achieving the highest level in history due to the human activities including the consumption of fossil fuels. Therefore, CO2 conversion technologies are needed to tackle reduce the CO2 level in the atmosphere and the emission of CO2 in industries. CO2 conversion technologies, e.g., thermo-catalytic reaction and electrochemical reduction, have drawn researchers' attention, since they have the potential to resolve the feedstock and fuel consumption sectors of chemical production at the same time. However, the complexity of the CO2 conversion processes hinders the development of new technologies. Since the nature of these technologies are heterogeneous catalytic reactions, all reactions are happening at the interface between catalysts and reactants/products, which calls for the understanding of interfacial mechanisms of CO2 reduction reactions. For this type of high degree of freedom problem where many phases including solid-solid, solid-liquid, and solid-gas phases exist, multiscale simulations turn out to be a proper approach since the wide time and length scale that can be covered. Herein, we employed different multiscale modeling methods to tackle various CO2 reduction problems. For electrochemical reduction of CO2, we designed a novel Bi-Cu2S hetero-structured catalyst, which has abundant interfacial sites between Bi and Cu2S, demonstrating the improved catalytic performance of ECO2RR toward formate production. At the same time, it has been found that in non-aqueous solution, the reaction pathway has been switched, where CO is obtained as the final product instead of formate. This effect has been investigated using constant potential calculation method to probe the reaction under reaction condition. For thermo-catalytic reactions, we studied the CO2 hydrogenation on Cesium promoted Au(111) surface using quantum mechanics and kinetic Monte Carlo (KMC) calculations, to gain insights of alkali metal promoting effects under reaction conditions. To expand the scope, the integration of CO2 electroreduction with C-N coupling is a promising strategy for global warming and pollution control, which utilizes the nitrate (NO3-) from wastewater and CO2 to produce high value-added product such as urea. The fundamental investigation of reaction mechanism of C-N coupling has been studied using first principles molecular dynamics.

<p>The reduction of iron oxide from slag is involved in many processes, such as, bath smelting, EAF steelmaking and copper slag cleaning processes, and it is known to occur via gaseous intermediates. Four possible rate determining steps are involved during the reduction. Among them, these two interfacial chemical reactions, gas slag and gas carbon could ultimately limit the enhancement of these processes.</p><p>In this work, the gas slag reaction kinetics in slag cleaning of copper slags has been studied. The dissociation rate of CO2 on the surface of liquid copper slags is measured using an isotope exchange method, where the mass transfer in the gas phase was eliminated by using a sufficiently high gas flowrate.</p><p>It is found that, for slag of the FexO-SiO2-Al2O3-Cu2O system, the apparent rate constant remains fixed with Cu2O content from 1-10 wt pct at higher oxygen potentials. The rate constant becomes approximately 2 times higher after metallic copper is reduced from the slag, this is due to the suspension of small metal drops on the slag surface.</p><p>The effect of temperature in the range from 1200-1450°C on the rate constants was also studied. The activation energy was 190 kJ/mole for slag of composition 60FexO30SiO2-1 0Al2O3. In the presence of Cu metal~10%, the activation energy was reduced to 122 kJ/mole.</p> / Thesis / Master of Engineering (MEngr)

The Oxygen Steelmaking process is used to refine pig iron produced in the blast furnace to produce liquid steel for further refining in secondary steelmaking processes. The main advantages of the process are its autogenous nature, wherein the heat is generated through the refining reactions itself, and the refining is completed in a relatively short time (typically 15-25 mins). Achieving the desired end-point composition of refined steel is essential to avoid re-blows, which lead to delays in downstream processes and an increase in steel production costs. Improving process control through regular monitoring and a better understanding of the process is thus very critical. Multiple reaction interfaces are formed between various phases (slag, metal, gas), at extremely high temperatures and this makes the monitoring of the process through sampling and observation difficult and expensive. Consequently, mathematical modelling has been used as a tool to improve the understanding of the process and propose developments in operation. Numerous models have been developed in the past; however, these models do not address several open questions regarding the detailed reaction mechanisms and the contributions from different reaction zones inside the Basic Oxygen Furnace. The current work aimed to fill this gap. In this work, four prominent reaction zones, namely; impact, slag-metal bulk, cavity, and emulsion zones were identified. A more mechanistic approach involving process variables has been used to decrease the level of empiricism. With regards to the impact and the slag-metal bulk zones, the velocity of flow of metal (or surface-renewal) at the interfaces of these zones are calculated by taking into the momentum induced by the top-jets and bottom-stirring plumes. This study found that these zones contribute negligibly to overall refining in the oxygen steelmaking process. In the case of the emulsion zone, a very rigorous description of all aspects (external and internal decarburization, bloating behavior, and trajectory) pertaining to the life cycle of a single metal droplet in slag has been achieved. The emulsion zone is found to contribute 5 to 75 % of decarburization during various times of blow. The cavity zone model represents the first reported effort to predict the refining behavior of metal droplets that are exposed to oxygen jets within the lance cavities. The model incorporated the mass transfer, reaction equilibria, and kinetics of the reactions. It is predicted that this zone plays a critical role in the removal of silicon and FeO formation in the early part of the blow and removal of carbon throughout the blow. Several significant insights with regards to improvement in the operation of the oxygen steelmaking process are derived from each sub-models. The integration of these models will guide the steelmaker to improve their practices so that they can achieve better consistency in the end-point composition of refined steel and reduce re-blows. / Thesis / Doctor of Philosophy (PhD)

<div>Thermochemical processing using fast-pyrolysis technology has been used to upgrade feedstocks like biomass and natural gas and more recently studied for plastic recycling. This work aims to improve the selectivity to desired products from a pyrolysis process through better catalysts and reactor design.</div><div>Fast-pyrolysis of biomass to fuels is considered a promising technology due to the higher yields to liquid fuel products. However, the process suffers from low carbon efficiency to hydrocarbon products due to carbon losses to biochar, accounting for 25-40 wt.% of the product stream depending on the biomass type. Using a combination of inorganic free-model compounds, biomass pretreatments and mass spectrometric analyses coupled with lab-scale reactor experiments, the char contribution from the lignocellulosic components (cellulose, hemicellulose, and lignin) and mineral content was investigated. The lignocellulosic components were found to follow the order: Lignin > Hemicellulose > Cellulose. Addition of inorganic salts (K, Na and Ca) to cellobiose, a model compound for cellulose, was found to catalyze additional dehydration reactions on primary pyrolysis products (e.g., levoglucosan) to yield secondary products (e.g., 5-HMF), and produce more char. This knowledge of char formation contributors can enable optimization of the bio-refining process sequencing using process system engineering tools and thus achieve higher carbon efficiency for biomass conversion.</div><div>While biomass has been viewed as a future energy source, there is a need for a transition fuel with the lowest possible greenhouse gas (GHG) footprint. Shale gas, consisting primarily of methane, is a potential candidate due to its large availability and high hydrogen to carbon ratio. Recently, single-atom catalysts have been studied as stable and non-coking catalysts for the non-oxidative coupling of methane (NOCM) to higher hydrocarbons (like ethylene). However, lack of post reaction catalyst characterization and rigorous kinetic testing have raised questions on the stability of these materials. This work combines homogenous (Chemkin simulations, gas phase kinetics) and heterogeneous reaction kinetic studies (reaction orders, steady state kinetics), coupled with microscopy (Scanning and Transmission Electron Microscopy (SEM, TEM)) and surface characterization tools (BET, TGA, Raman spectroscopy, CO-IR spectroscopy) to understand the role of the solid materials during NOCM. Post reaction catalyst characterization using transmission electron microscopy (TEM) analysis on the spent samples (CH4 treated at 975 deg C for 3 hours) reveals that the materials containing Pt single atoms (SA) and Pt nanoparticles (NP) are found to sinter to particles approximately 5-7 nm in size. Ethylene hydrogenation experiments, a kinetic probe for surface Pt, shows initial ethane formation rates that are four orders of magnitude lower on the isolated Pt+2 sites, found on Pt SAs, when compared to the rates obtained if all the surface Pt were assumed to be metallic. These results suggest that single atoms are not the active sites. However, under same reaction conditions (50 mL min-1 CH4 flow and 975 deg C), the ethylene formation rates (in mol h-1) on the solid materials are 2-7 times higher than the empty tube rates, indicating that the surface plays a role during NOCM. Addition of incremental amounts of the solid material increases methane conversion, extrapolating to the bare tube conversion at zero loading. This indicates that the solid materials improve the NOCM performance.</div><div>Experiments with pure methane feeds indicate that the solid materials are found to deactivate due to coking on the surface, evidenced by the coke buildup observed using thermogravimetric analysis (TGA) and the initial time-on-stream kinetic results showing rapid methane deactivation. Raman spectroscopy on the spent catalysts indicate at the development of a similar graphite-like surface intermediate under steady state conditions on all the materials. When compared under the same reaction conditions (975 deg C, 60 mL min-1 Pure CH4 with 10% UHP N2 feed, space velocity = 39.6 L h-1 gcat-1), these coked surfaces show a linear dependence for the ethylene formation rate (in mol h-1 gcat-1) with the spent surface area of the material (in m2 gcat-1). This observation is irrespective of the type of the material studied (alpha Al2O3, Davisil SiO2, 1 wt.% Pt/CeO2, Graphene, Graphite, etc.). In conclusion, these results prove that the spent surface area is critical for NOCM.</div><div>Similar experimental setup was used to study the dehydrogenation of methane, ethane, and propane mixture in the gas phase. Initial experiments at 1 bar pressure and reaction temperatures ranging from 650-850 deg C revealed that ethylene and hydrogen are the main gas phase products, with methane acting as a diluting agent under these reaction conditions. These results could enable direct processing of the shale gas without the use of a conventional ethane/propane separation step. These results were further studied by the system engineers using ANSYS ChemkinPro. For practical applications, these experiments were suggested to be performed at much higher operating pressures (~30 bar) and low residence time (~0.2 s), with a quick quenching step added after the reactor to prevent change in the exit stream compositions. A new reaction system was built to experimentally validate these recommendations.</div>

Thesis (MEng)--Stellenbosch University, 2015. / AFRIKAANSE OPSOMMING: Raadpleeg die volteks vir opsomming, asseblief. / ENGLISH ABSTRACT: Please refer to full text for abstract