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A Study on the Surface State of Catalytic Chemistry Using First-Principles calculation
Get PDFDepartment of ChemistryA catalyst refers to that accelerates a chemical reaction without being consumed during the chemical reactions. This property is usually referred to as the catalytic activity. For a multi-step chemical reaction with multiple possible products, a catalyst can promote the production of a specific chemical compound, which is related to the rate-determining step of the reaction. Heterogeneous catalytic reactions mainly occur on the surface of solid catalysts with accompanying elementary surface chemical processes such as adsorption of reactants from a reaction mixture, surface diffusion and reaction of adsorbed species, and desorption of reaction products. The promotion of a chemical reaction is originated from the high reactivity of surface atoms that facilitates bond breaking and bond rearrangement of adsorbed molecules. In order to increase the yield of a desired reaction, the demand for catalysts showing high activity is growing throughout modern industries. Beyond simple chemical reactions, the complex and multi-step reactions have been important in various fields that require effective catalyst. However, research is needed in many cases to understand the detailed bond rearrangement of the reaction on the catalyst surface or the diffusion or dissolution process of the interface. As the demand for catalyst design or material synthesis increases, the first-principles calculations are required to understand a detailed catalysis.
One area where catalyst is highly required is hydrogen production. As the use of fossil fuels increases, the world faces a climate crisis due to carbon dioxide emissions, and as a promising substitute for fossil fuels, hydrogen has emerged as a clean and renewable energy. In particular, green hydrogen production without CO2 emission can be achieved through electrocatalytic water splitting. In water splitting reactions, the commercial electrocatalysts composed of rare metals such as Ir, Pt and Ru has a problem with their price. Besides, sluggish multi-step oxygen evolution reaction (OER) is another hindrance for efficient water splitting. To overcome these problems, development of cheap electrocatalysts with high performance is necessary. First-principles calculations using density functional theory (DFT) has been used to describe and predict the intrinsic activity of catalysts. The adsorption energies of intermediates determine the activity of the catalyst. The surface electronic structure is highly correlated with intermediate species adsorption energies which determine the catalytic activity.
In Chapter 2, we investigated the influence of cation mixing on the oxygen evolution reaction (OER) activity of LaxSr1-xCoyFe1-yO3 (LSCF) double perovskite in the perspective of surface electronic structure. Based on projected density of states and wavefunction analysis, the minority spin dxy electrons of surface layer metal atoms are significant due to their stability, where the antibonding states between dxy and the lattice oxygen p become occupied when Co atoms with one d electron more than Fe are present. Thus, by additionally considering the dxy band center, surface electronic descriptor (E2p ??? 0.4 Edxy) excellently describes the binding energy of the OER intermediates and the stability against oxygen vacancy formation, which also explains the enhanced OER stability and efficient Fe???Co mixing. Based on the computational analysis, several efficient perovskite electrocatalysts were presented, and it was confirmed that the electronic structure analysis could provide guidance for electrocatalysts design.
Another area in which catalysts play an important role, is the synthesis and etching of low-dimensional carbon materials through the 3d transition metal surface. Graphene is produced from amorphous sp3 carbon through Ni junction, and the surface of diamond is etched under high-temperature conditions. In order to control these interfacial reactions, accurate understanding and mechanism analysis are required, and detailed bond dissociation and forming processes can be identified through first-principles DFT simulations.
In Chapter 3, we analyze the kinetics of dissolution of single crystal diamond (100) and (110), ???D(100)??? and ???D(110)???, into thin films of nickel (Ni) and cobalt (Co). This dissolution occurs at the metal-D(100) or metal-D(110) interface and was studied in the presence and also absence of water vapor at temperatures near 1000 ???. Based on the first-principles calculation, the mechanism of why the diamond surface carbon can diffuse through Ni or Co despite the low activation energy of the reverse reaction were unveiled.ope
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No full textDepartment of Electrical Engineeringclos
Development and methanogenic performance of DIET-active magnetite-embedded anaerobic granular sludge
No full textDepartment of Urban and Environmental Engineering (Environmental Science and Engineering)clos
Development of Disinfection Battery System for Energy-efficient and Self-neutralizing Water Treatment
No full textSchool of Energy and Chemical Engineering (Energy Engineering (Battery Science and Technology))clos
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No full textSchool of Energy and Chemical Engineering (Energy Engineering (Battery Science and Technology))clos
Development of tissue-derived bio-ink with clinical relevance for regenerative medicine
No full textDepartment of Biomedical Engineeringclos
Electrically reconfigurable nonlinear polariton metasurface for difference-frequency-generation
Get PDFDepartment of Electrical EngineeringNonlinear metasurface explains nonlinear optical responses such as polarization, frequency, phase of incident light. In conventional nonlinear optics, crystalline nonlinear materials are used for frequency-mixing and other nonlinear processes. The refractive index of crystalline materials depends on the direction of the light that passes through. Therefore, the orientation of crystalline materials is fixed for momentum conservation. Nonlinear metasurface, which is in subwavelength scale, has been studied for overcoming the limitation of momentum conservation and developed in many applications including quantum photonics, frequency-mixing, and optical encryptions. However, the established nonlinear metasurface has fixed nonlinear optical responses. Electrically reconfigurable nonlienar metasurface is vital for controlling the nonlinear responses over a broadband wavelength range. The conventional methods to control the optical responses electrically are using liquid crystals (LCs) or phase change media (PCM). Recently, the nonlinear metasurface, which was combined a plasmonic nanocavity and voltage tunable intersubband transition in n doped multiple quantum wells, was developed for second-harmonic generation (SHG). Here, we propose electrically reconfigurable nonlinear polaritonic metasurface designed for difference-frequency-generation (DFG) which is another frequency-mixing process. In this work, we used a quantum cascade laser (QCL, ??1=6.1 ??m), a CO2 gas laser (??1=10.6 ??m) for DFG. A magnitude of effective second order susceptibility was 207.336 nm/V, and it was changed from 6.1 ??m to 6.4 ??m. Also, the DFG conversion efficiency of up to 1.6??10-3 % was calculated. This work shows that nonlinear metasurface can generate the light with difference frequency over a broadband wavelength scale by applying external bias voltage.ope
Highly conductive SiCxNy composite anode material with designed buffer matrix
No full textSchool of Energy and Chemical Engineering (Energy Engineering (Battery Science and Technology))clos
Impacts of Physics Parameterization and Surface Boundary Conditions on Poleward-moving Tropical Cyclone Forecasting with a High-resolution Regional Model
No full textDepartment of Urban and Environmental Engineering (Disaster Management Engineering)Recently, the frequency of Tropical Cyclone (TC)s affecting East Asia has been increasing, and the causes that the recent northward extension of TC tracks over the WNP was associated with changes in environmental conditions that were favorable for TC activities. Moreover, intense TCs have become stronger owing to global warming and some TCs maintain their intensity before making landfall, allowing them to cause widespread damage and trigger enormous economic loss. Thus, it is necessary to improve the short-term forecast performance of TCs to mitigate TC hazards and risks in advance. In this study, the possible causes that might significantly affect the TC forecast performances are assumed that 1) model physics settings and 2) surface boundary conditions. The impact of two factors on TC forecast performances were investigated by conducting several sensitivity experiments. To investigate the impact of physics parameterization schemes on the poleward-moving TC forecasts, we simulated six TCs which directly/indirectly affected the South Korea region in recent years using the Weather Research and Forecasting (WRF) model. Three cumulus parameterization schemes (CPSs) of Kain-Fritsch (KF), Betts-Miller-Janji?? (BMJ), modified Tiedtke (TDK), and two cloud microphysics parameterization schemes (MPSs) of WRF-single-moment-microphysics class 6 (WSM6), Predicted Particle Properties (P3) 1-category were selected for the sensitivity experiment. The results showed that there was a significant difference in simulated TC track and intensity performances depending on the physics schemes. Overall, the TC track and intensity spreads tended to be more sensitive to CPSs and MPSs, respectively. Additionally, to investigate the impact of MPSs on poleward-moving TC forecasts, we simulated ten TCs using the WSM3 and WSM6 schemes in the WRF model. The result showed that track errors were prominently reduced by the WSM6 scheme, which realistically captured westward-shifted track during the rapid intensification process. This can be attributed to the improved simulations of TC intensity, size, and associated ??-effect by WSM6 scheme. In contrast, the WSM3 scheme underestimated the above characteristics due to low latent heat release compared to the WSM6 scheme. To investigate the impact of initial condition on poleward-moving TC forecasts, we simulated three TCs that affect the South Korea in recent years. We selected two soil moisture (SM) data for initializing the WRF model. Those were obtained from Global Land Data Assimilation System (GLDAS) Version 2.1 and from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5). The result showed that ERA5 SM data had more wet biases than GLDAS data over the whole simulations except for some areas located south of Mongolia. Also, TC intensity forecast performances were similar in the GL and ERA5 runs, while TC track forecast performances were improved in the GL runs. The differences in track forecasts between the two runs were more considerable during the landfall period. In addition, in ERA5 runs, simulated TCs tended to move westward compared to the GL runs due to the strengthened interaction between simulated TCs and the mid-latitude trough. This study showed that in order to improve the TC track forecast performances during the TC landfall period, it is important to provide the realistic SM data. To investigate the impact of boundary condition on poleward-moving TC, we simulated two TCs consecutively hit South Korea in 2020. The sea surface temperature (SST) data were obtained from the Daily Optimum Interpolation Sea Surface Temperature (OISST) version 2, and HYbrid Coordinate Ocean Model/Navy Coupled Ocean Data Assimilation (HYCOM/NCODAGLBy0.08/expt_93.0). When verified using in-situ observational data, the OISST data did not accurately estimate the changes in SST during each TC???s landfall period compared to the HYCOM data since it has a relatively low temporal resolution. In addition, simulated TC intensities were significantly improved in HY runs, while TC track forecast performances were similar in both runs. Thus, this study showed that the overall TC intensity and forecast performances during the landfall period could be improved when the higher temporal-resolution SST data was prescribed in the model boundary conditions for a better representation of TC-induced SST changes.clos