Browsing by Author "Chen, Tso-Hsuan"
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Item Ab Initio Molecular Dynamics Study of Pt Clustering on γ-Al2O3 and Sn-Modified γ-Al2O3(Journal of Physical Chemistry C, 2023-10-05) Chen, Tso-Hsuan; Vlachos, Dionisios G.; Caratzoulas, StavrosWe have conducted AIMD free energy simulations to examine the dynamics of Pt atoms and Ptn (n = 2–3) species on dry γ-Al2O3(100), dry γ-Al2O3(110), and wet γ-Al2O3(110) surfaces, with OH coverages corresponding to 500 K (11.8 OH/nm2) and 800 K (5.9 OH/nm2), while varying the Pt and Sn loading. Under the same dry conditions and temperature, comparing the (100) and (110) surface terminations revealed that the interactions between Pt and the surface play a crucial role in determining whether the potential of mean force between reduced Pt atoms is repulsive, as observed on the (100) surface, or if it can support a bound Pt–Pt state, as observed on the (110) surface. The hydration of the (110) surface had a significant impact. At a Pt loading of 0.75 Pt/nm2, with hydration of 5.9 OH/nm2, the energy of the potential of mean force increases. Although a Pt–Pt bound state is still supported, it becomes kinetically less accessible from the dispersed state. At an even higher water loading of 11.8 OH/nm2, the Pt–Pt potential of mean force becomes predominantly repulsive and can no longer sustain the Pt–Pt bound state. Higher Pt loadings of 1.12 Pt atoms/nm2 promote the aggregation of Pt into progressively larger clusters, but high levels of hydration can kinetically impede particle growth. On Sn-modified γ-Al2O3(110), Pt tends to associate with Sn, except at high levels of surface hydration where the potential of mean force between Pt and Sn atoms becomes repulsive. The presence of Sn inhibits the aggregation of Pt particles, and the Pt–Pt potential of mean force becomes increasingly repulsive with higher Sn loading.Item MULTISCALE MODELING OF CONFINEMENT AND DYNAMICS IN HETEROGENEOUS, MULTIPHASE CATALYTIC SYSTEMSChen, Tso-HsuanCatalysis plays a pivotal role in various industrial processes, enabling the efficient conversion of feedstocks into valuable fuels, chemicals, and products. While insights from computations have increased over the past two decade, most computational studies employ static density functional theory (DFT) and microkinetic modeling, focusing on understanding reaction mechanisms. Dynamics, confinement, and entropic effects, stemming from catalyst mobility, solvent reorganization, and micropores as examples, demand different multiscale models, including classical and ab initio molecular dynamics (AIMD), and enhanced sampling approaches for free energy calculations. This thesis focuses on such phenomena and involves four core chapters, each contributing valuable insights into different catalytic systems. Chapter 2 investigates Brønsted acid catalysis in the direct acylation of 2-methyl furan (2-MF) with acetic acid. Utilizing DFT and microkinetic modeling, the role of Brønsted acidity and confinement in the mechanism and kinetics is examined. Surprisingly, stronger Brønsted catalysts do not consistently result in faster reaction rates, and the proposed rate-determining step does not hold for all catalysts. The study emphasizes the profound impact of stabilizing transition states through confinement and co-adsorption of 2-MF on the direct acylation rate. In Chapter 3, ab Initio molecular dynamics (AIMD) free energy simulations are conducted to investigate the stability and dynamics of dispersed Pd atoms and subnanometer clusters on γ-Al2O3. We explore temperature and entropic effects on Pd nucleation on dry γ-Al2O3(100), dry γ-Al2O3(110), and hydrous γ-Al2O3(110) under diverse water coverages and Pd loadings. This research enriches the understanding of static DFT calculations, providing a dynamic perspective. Continuing the exploration of metal clustering on γ-Al2O3, Chapter 4 focuses on Pt clustering on γ-Al2O3 and Sn-modified γ-Al2O3 surfaces. By employing AIMD free energy simulations, the influence of temperature, surface termination, Pt oxidation, and Pt and Sn loading, on Pt clustering is elucidated. Remarkably, the degree of hydration and Sn:Pt ratio significantly impact the Pt dispersion. The findings offer valuable insights into the dynamics and time scales of Pt nucleation or dispersion, guiding the design of dispersed metal catalysts. Lastly, Chapter 5 investigates unexpected kinetic solvent effects on the activity and selectivity of the fructose to HMF reaction within biphasic catalytic systems. By combining fast experimental reaction kinetics, multiscale modeling, in-situ sampling, IR, and 13C-NMR spectroscopy, we find that fructose reacts faster and more selectively in the organic-rich phase due to increased relative abundance of the reactive furanose isomer, the enhanced water-catalyst-substrate interactions driven by nanophase separation, and the higher product stability stemming from preferential solvation. Our findings suggest that the choice of an acid catalyst and the polarity and hydrophobicity of organic solvents can be tailored to fine-tune reactivity and selectivity in the process.