Browsing by Author "Hiew, Shu Ning"
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Item Atomistic Simulations of Hydrogen Production Kinetics at Novel Fuel Cell Electrodes(2018-10-18) Hiew, Shu NingElectrocatalysts such as platinum (Pt), are used to minimize the overpotential for the hydrogen evolution reaction (HER) during electrochemical water splitting or fuel cell operation. Iron phosphide materials (FexP) were recently identified as one of the more promising transition metal phosphide electrocatalysts for HER that can substitute Pt at a much lower cost. To further understand their activity’s origin, the thermodynamics and kinetics of hydrogen recombination was investigated on six FexP (x=1,2,3) surface facets using density functional theory (DFT). As the differential Gibbs free binding energy of hydrogen (∆G_H) reaches zero, the adsorption of protons on FexP surfaces and the desorption of H2 are both expected to be facile. We analyzed the recombination kinetics of H2 on FexP surfaces for the number of hydrogen coverage that satisfy the necessary criterion ∆GH≈0 kJ/mol. The activation and recombination energies for 2 H* to form H2 were calculated using the Vienna Ab initio Simulation Package (VASP). The energy change of different hydrogen combination from 2 H* to H2 was used to identify the easiest detachable H2 from each surface. The most favourable H2 combination pair for each FexP surface was selected to calculate its activation barrier using the Nudged Elastic Band (NEB) algorithm. Our results show that a stable intermediate, i.e. a surface hydrogen molecule (H2*) is formed on the surface before desorption of H2. Our results also suggest that Fe2P and FeP have better HER kinetics, which we tentatively attribute to their lower metal concentration allowing for higher H mobility.Item Development of a Monte Carlo Algorithm for the Prediction of Acid Site Distribution in Zeolites(2020-05) Hiew, Shu NingZeolites are porous aluminosilicates that afford exceptional benefits as catalysts in the petrochemical industry, or for vehicle emission control, to name a few. The presence of aluminum (Al) in the silicate (SiO4) framework creates a charge defect, which leads to Brønsted acid sites when compensated with a proton. If the defect is compensated by a metal cation, a Lewis acid site is formed. For metal-exchanged zeolites, in particular, the Al distribution and probability of forming paired Al sites in the framework determines the metal speciation and, in turn, the selectivity and activity of the catalyst. This thesis project aims to create an algorithm that can estimate the Al distribution and probability of forming paired Al sites in zeolites based on user-defined parameters: a) Silicon-to-aluminum ratio (SAR), b) simulation temperature, c) number of trials generated for statistical analysis, and d) Al-Al interaction energy. Two different implementations of a Monte Carlo algorithm were tested and evaluated for zeolites with CHA, MFI and MWW framework structures. The traditional (MC1) and modified (MC2) Metropolis Monte Carlo Methods have different means in generating the initial guess for the Al distribution. In MC1, the initial population is random while conforming to Löwenstein’s rule, while in MC2, Löwenstein’s rule is used to generate an initial guess with maximized density of Al site pairs. Even though MC2 has a higher computational cost than MC1, MC2 is better than MC1 because it is less prone to error and has a higher overall efficiency. The final algorithm generates population statistics that are in full agreement with expected results for the provided Al substitution energies and the probability of forming paired Al sites.