The focus of this thesis is on C-O bonds activation by transition metal atoms. Lignin is a potential alternative energy resource, but currently is an underused biomass species because of its highly branched structure. To aid in better understanding this species, the oxidative cleavage of the Cβ-O bond in an archetypal arylglycerol β-aryl ether (β–O–4 Linkage) model compound of lignin with late 3d, 4d, and 5d metals was investigated. Methoxyethane was utilized as a model molecule to study the activation of the C-O bond. Binding enthalpies (ΔHb), enthalpy formations (ΔH) and activation enthalpies (ΔH‡) have been studied at 298K to learn the energetic properties in the C-O bond cleavage in methoxyethane. Density functional theory (DFT) has become a common choice for the transition metal containing systems. It is important to select suitable functionals for the target reactions, especially for systems with degeneracies that lead to static correlation effects. A set of 26 density functionals including eight GGA, six meta-GGA, six hybrid-GGA, and six hybrid-meta-GGA were applied in order to investigate the performance of different types of density functionals for transition metal catalyzed C-O bond cleavage. A CR-CCSD(T)/aug-cc-pVTZ was used to calibrate the performance of different density functionals.

abstract: Losses in commercial microwave dielectrics arise from spin excitations in paramagnetic transition metal dopants, at least at reduced temperatures. The magnitude of the loss tangent can be altered by orders of magnitude through the application of an external magnetic field. The goal of this thesis is to produce “smart” dielectrics that can be switched “on” or “off” at small magnetic fields while investigating the influence of transition metal dopants on the dielectric, magnetic, and structural properties. A proof of principle demonstration of a resonator that can switch from a high-Q “on state” to a low-Q “off state” at reduced temperatures is demonstrated in (Al1-xFex)2O3 and La(Al1-xFex)O3. The Fe3+ ions are in a high spin state (S=5/2) and undergo electron paramagnetic resonance absorption transitions that increase the microwave loss of the system. Transitions occur between mJ states with a corresponding change in the angular momentum, J, by ±ħ (i.e., ΔmJ=±1) at small magnetic fields. The paramagnetic ions also have an influence on the dielectric and magnetic properties, which I explore in these systems along with another low loss complex perovskite material, Ca[(Al1-xFex)1/2Nb1/2]O3. I describe what constitutes an optimal microwave loss switchable material induced from EPR transitions and the mechanisms associated with the key properties. As a first step to modeling the properties of high-performance microwave host lattices and ultimately their performance at microwave frequencies, a first-principles approach is used to determine the structural phase stability of various complex perovskites with a range of tolerance factors at 0 K and finite temperatures. By understanding the correct structural phases of these complex perovskites, the temperature coefficient of resonant frequency can be better predicted. A strong understanding of these parameters is expected to open the possibility to produce new types of high-performance switchable filters, time domain MIMO’s, multiplexers, and demultiplexers. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21571号 / 理博第4478号 / 新制||理||1642(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 菅沼 秀夫, 教授 永江 知文, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

In the pursuit of enhancing the electronic properties of transparent p-type semiconductors, this work uses density functional theory to study the effects of doping tin monoxide with nitrogen, antimony, yttrium and lanthanum. An overview of the theoretical concepts and a detailed description of the methods employed are given, including a discussion about the correction scheme for charged defects proposed by Freysoldt and others [Freysoldt 2009]. Analysis of the formation energies of the defects points out that nitrogen substitutes an oxygen atom and does not provide charge carriers. On the other hand, antimony, yttrium, and lanthanum substitute a tin atom and donate n-type carriers. Study of the band structure and density of states indicates that yttrium and lanthanum improves the hole mobility. Present results are in good agreement with available experimental works and help to improve the understanding on how to engineer transparent p-type materials with higher hole mobilities.

Gaseous pollution has become a global issue and its presence above certain limits is hazardous to human health and environment. Detection of such gases is an immediate need and researchers around the world are trying to solve this problem. Metal oxides are being used as sensing materials for a long time, but a high operating temperature limits applications in many areas. On the other hand, two-dimensional (2D) materials with high surface-to-volume ratio and chemical stability are promising candidates in the field of gas sensing. This includes monolayer transition metal dichalcogenides, such as MoS2 and WS2, which are direct band gap materials. While few layer transition metal dichalcogenides are indirect band gap materials, they are easier to synthesize than monolayers. Therefore, it is important to understand whether few layer transition metal dichalcogenides possess the same sensing behavior as the corresponding monolayers. For this reason the first part of this dissertation compares the sensing behavior of monolayer and few layer MoS2 and WS2. Two dimensional hexagonal boron nitride is a highly stable structural analogue of graphene. However, its insulating behavior with large band gap is not suitable for sensing. Recently, monolayer Si2BN has been proposed to exist. As the presence of Si makes this material reactive, the second part of this dissertation addresses its application as sensing material. In the _nal part of this dissertation, in search of a metal free, non-toxic, and earth abundant sensor material, further structural analogues of graphene are considered, namely monolayer C3N, monolayer C3Si, and monolayer C6BN. In particular, different theoretical approaches for studying the sensing performance of materials are compared to each other.

Functional materials are essential to human welfare and to provide foundations for emerging industries. As an alternative route to experimental materials discovery, computational materials designs are playing an increasingly significant role in the whole discovery process. In this work, we use an in-house developed python utility: PAOFLOW, which generates finite basis Hamiltonians from the projection of first principles plane-wave pseudopotential wavefunctions on pseudo atomic orbitals(PAO) for post-process calculation on various properties such as the band structures, density of states, complex dielectric constants, diffusive and anomalous spin and charge transport coefficients. In particular, we calculated the dielectric function of Sr-, Pb-, and Bi-substituted BaSnO3 over wide concentration ranges. Together with some high-throughput experimental study, our result indicates the importance of considering the mixed-valence nature and clustering effects upon substitution of BaSnO3 with Pb and Bi. We also studied two prototype ferroelectric rashba semiconductors, GeTe and SnTe, and found the spin Hall conductivity(SHC) can be large either in ferroelectric or paraelectric structure phase. Upon doping, the polar displacements in GeTe can be sustained up to a critical hole concentration while the tiny distortions in SnTe vanish at a minimal level of doping. Moreover, we investigated the sensitivity of two dimensional group-IV monochalcogenides to external strain and doping, which reveal for the first time giant intrinsic SHC in these materials, providing a new route for the design of highly tunable spintronics devices based on two-dimensional materials.

Thermoelectric materials can convert waste heat into electric power and thus provide a way to reduce the dependence on fossil fuels. Our aim is to model the underlying materials properties and, in particular, the transport as controlled by electrons and lattice vibrations. The goal is to develop an understanding of the thermoelectric properties of selected materials at a fundamental level. The structural, electronic, optical, and phononic properties are studied in order to tune the transport, focusing on KxRhO2, NaxRhO2, PtSb2 and Bi2Se3. The investigations are based on density functional theory as implemented in the all electron linearized augmented plane wave plus local orbitals WIEN2k and pseudo potential Quantum-ESPRESSO codes. The thermoelectric properties are derived from Boltzmann transport theory under the constant relaxation time approximation, using the BoltzTraP code. We will discuss first the changes in the electronic band structure under variation of the cation concentration in layered KxRhO2 in the 2H phase and NaxRhO2 in the 3R phase. We will also study the hydrated phase. The deformations of the RhO6 octahedra turn out to govern the thermoelectric properties, where the high Seebeck coefficient results from ”pudding mold" bands. We investigate the thermoelectric properties of electron and hole doped PtSb2, which is not a layered material but shares “pudding mold" bands. PtSb2 has a high Seebeck coefficient at room temperature, which increases significantly under As alloying by bandgap opening and reduction of the lattice thermal conductivity. Bi2Se3 (bulk and thin film) has a larger bandgap then the well-known thermoelectric material Bi2Te3, which is important at high temperature. The structural stability, electronic structure, and transport properties of one to six quintuple layers of Bi2Se3 will be discussed. We also address the effect of strain on a single quintuple layer by phonon band structures. We will analyze the electronic and transport properties of Tl-doped Bi2Se3 under strain, focusing on the giant Rashba spin splitting (Tl doping breaks the inversion symmetry in Bi2Se3) and its dependence on biaxial tensile and compressive strain.

Materials that are light weight, low cost and have high hydrogen storage capacity are essential for on-board vehicular applications. Some reversible complex hydrides are alanates and amides but they have lower capacity than the DOE target (6.0 wt %) for 2010. High capacity, light weight, reversibility and fast kinetics at lower temperature are the primary desirable aspects for any type of hydrogen storage material. Borohydride complexes as hydrogen storage materials have recently attracted great interest. Understanding the above parameters for designing efficient complex borohydride materials requires modeling across different length and time scales. A direct method lattice dynamics approach using ab initio force constants is utilized to calculate the phonon dispersion curves. This allows us to establish stability of the crystal structure at finite temperatures. Density functional theory (DFT) is used to calculate electronic properties and the direct method lattice dynamics is used to calculate the finite temperature thermodynamic properties. These computational simulations are applied to understand the crystal structure, nature of bonding in the complex borohydrides and mechanistic studies on doping to improve the kinetics and reversibility, and to improve the hydrogen dynamics to lower the decomposition temperature. A combined theoretical and experimental approach can better lead us to designing a suitable complex material for hydrogen storage. To understand the structural, bulk properties and the role of dopants and their synergistic effects on the dehydrogenation and/or reversible rehydrogenation characteristics, these complex hydrides are also studied experimentally in this work.

This thesis is focused on theoretical study of influence of the silanol nest defects on the hydrolysis of zeolite Chabazite under harsh steaming conditions. The motivation of the thesis was a recent experiment proving that the silanol nest defect enhances the hydrolysis of a zeolite. The harsh steaming conditions have been chosen as some important technological processes involving zeolites require high temperatures and have water vapour present. The study was performed by using density functional theory calculations. To investigate the influence of the defect two models were used a reference pristine model and a defected model containing the silanol nest defect. The two models were pure siliceous Chabazite periodical models with supercell containing 36 and 35 Si tetrahedra respectively. A multi-step hydrolysis leading to detachment of a Si(OH)4 cluster from the zeolite, known as total desilication, was calculated for the two models. Multiple possible paths of the hydrolysis were discovered, compared and discussed on both models. Both the most favourable hydrolysis paths of the two model as well as their arithmetic means were compared. The experimentally set expectations that a silanol nest defect enhances the hydrolysis of the zeolite have been met.

The continued release of fossil-fuel derived carbon dioxide (CO₂) emissions into our atmosphere led humanity into a climate and ecological crisis. Converting CO₂ into valuable chemicals and fuels will replace and diminish the need for fossil fuel-derived products. Through the use of a catalyst, CO₂ can be transformed into a commodity chemical by the reverse water gas shift (RWGS) reaction, where CO₂ reacts with renewable hydrogen (H₂) to form carbon monoxide (CO). CO then acts as the source molecule in the Fischer-Tropsch (FT) synthesis to form a range of hydrocarbons to manufacture chemicals and fuels. While the FT synthesis is a mature process, the conversion of CO₂ into CO has yet to be made commercially available due to the constraints associated with high reaction temperature and catalytic stability. Noble metal ruthenium (Ru) has been widely used for the RWGS reaction due to its high catalytic activity, however, several constraints hinder its practical use, associated with its high cost and its susceptibility to deactivation. The doping or bimetallic use of non-noble metals iron (Fe) and cobalt (Co) is an attractive option to lower material cost and tailor the selectivity of the CO₂ conversion towards the RWGS reaction without compromising catalytic activity. Furthermore, employing nanostructured catalysts as nanoparticles is a viable solution to further lower the amount of metal used and utilize the highly active surface area of the catalyst. Dispersing nanoparticles on ionically conductive supports/solid electrolytes which contain species like O²⁻, H⁺, Na⁺, and K⁺, provide an approach to further enhance the reaction. This phenomenon is referred to as metal-support interaction (MSI), allowing for the ions to back spillover from the support and onto the catalyst surface. An in-situ approach referred to as Non-Faradaic Modification of catalytic activity (NEMCA), also known as electrochemical promotion of catalysis (EPOC) is used to in-situ control the movement of ionic species from the solid electrolyte to and away from the catalyst. Both the MSI and EPOC phenomena have been shown to be functionally equivalent, meaning the ionic species act to alter the work function of the catalyst by forming an effective neutral double layer on the surface, which in turn alters the binding energy of the reactant and intermediate species to promote the reaction. The main objective of this work is to develop a catalyst that is highly active and selective to the RWGS reaction at low temperatures (< 400 °C) by employing the MSI and EPOC phenomena to enhance the catalytic conversion. The electrochemical enhancement effect will lower energy requirements and allow the RWGS reaction to take place at moderate temperatures. Catalysts composed of Ru, Fe and Co were synthesized through the polyol synthesis technique and deposited on mixed-ionically conductive and ionically conductive supports to evaluate their performance towards the RWGS reaction and the MSI effect. The nano-structured catalysts are deposited as free-standing nanoparticles on solid electrolytes to in-situ promote the catalytic rate through the EPOC phenomenon. Furthermore, Density Functional Theory (DFT) calculations were performed to correlate theory with experiment and elucidate the role polarization has on the binding energy of reactant and intermediate species. The high dispersion of RuFe nanoparticles on ion-containing supports like samarium-doped ceria (SDC) and yttria-stabilized zirconia (YSZ) led to an increase in the RWGS activity due to the MSI effect. A direct correlation between experimental and DFT modeling was established signifying that polarization affected the binding energy of the CO molecule on the surface of Ru regardless of the type of ionic species in the solid electrolyte. The electrochemical enhancement towards the RWGS reaction has been achieved with iron-oxide (FeOₓ) nanowires on YSZ. The in-situ application of O²⁻ ions from YSZ maintained the most active state of Fe₃O₄ and FeO towards the RWGS reaction and allowed for persistent-promoted state that lasted long after potential application. Finally, the deposition of FeOₓ nanowires on Co₃O₄ resulted in the highest CO₂ conversion towards the RWGS reaction due to the metal-oxide interaction between both metals, signifying a self-sustained electro-promoted state.