Theoretical Study for Adsorption Behaviors in Catalysis and Energy Applications via Multi-Scale Simulation

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Theoretical Study for Adsorption Behaviors in Catalysis and Energy Applications via Multi-Scale Simulation
Jung, Gwan Yeong
Kwak, Sang Kyu
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Graduate School of UNIST
With increasing energy demands and environmental concerns related to the greenhouse effect, one of the most central themes of research society is the development of sustainable and environmental-friendly energy system. In this regard, catalysts play a pivotal role in producing the fossil-free industrial chemicals (e.g., ammonia, hydrogen, and hydrocarbon) from earth-abundant substances (e.g., nitrogen, water) through (electro-)chemical conversion process. Moreover, energy storage technologies, such as lithium-ion batteries and fuel cells, are equally important for practical applications. However, the current levels of energy conversion and storage technologies are inadequate; that is, more advanced design and fundamental understanding of these processes is now pursued. Adsorption has an important meaning in the (electro-)chemical process in that the energy states of adsorbed reaction intermediates in the mechanistic pathways can determine the reaction mechanisms. Many studies have focused on the adsorption behaviors and corresponding energetics to predict the catalytic activity or battery performance. In this context, molecular simulation approach can be a suitable tool to investigate the adsorption behaviors in the atomistic and molecular level. Particularly, multi-scale molecular simulation methods are useful to properly elucidate the physicochemical phenomena in experimental (or realistic) systems crossing over different temporal and spatial scales. In this dissertation, theoretical studies on adsorption behaviors in catalysis and energy applications have been conducted via multi-scale molecular simulation approach. In Chapter 2, we theoretically demonstrated that atomically dispersed Pt catalysts on carbon nanotube (Pt1/CNT) could catalyze the chlorine evolution reaction (CER) with excellent activity and selectivity. From ab initio Pourbaix diagram, the active adsorbate structures for the Pt1/CNT were initially found. Subsequently, the mechanistic pathways for the CER were thoroughly investigated by combining the experimental (for kinetics) and theoretical data (for thermodynamics). Among Pt–N4 sites, PtN4C12 was identified as the most plausible active site structure for the CER. Moreover, the excellent selectivity of Pt1/CNT was evidenced by the large differences in thermodynamic overpotentials for respective CER and oxygen evolution reaction, which can be determined from the adsorption free energies of the reaction intermediates. We envision that this type of catalysts may be exploited as an alternative CER catalyst instead of precious metal-based mixed metal oxides (MMOs), which have suffered from concomitant generation of oxygen. In Chapter 3, we investigated the thermodynamics of ion adsorption on the amorphous intermediate phases of calcium carbonates. Amorphous calcium carbonate (ACC) have received enormous attentions because their local order in the short-range can affect the subsequent pathways for phase transformation. Using molecular dynamic simulation, we theoretically elucidated the precise role of ion adsorption in controlling the local structures and stability of ACC phases. Starting from the nucleation clusters in aqueous solution, the hydrated and anhydrous forms of ACC were systematically examined by varying the hydration levels and molar composition of additive ions (e.g. Mg2+, Fe2+, Sr2+, and Ba2+). Our results revealed that each ion can exert promoting or inhibiting effect by tuning the local order and stability of ACC phases depending on their hydrophilicity and ionic radii. More importantly, our findings suggested that the thermodynamic spontaneity of the overall phase transition process can be determined by the balance between two opposing factors – endothermic dehydration and exothermic crystallization. Chapters 4 and 5 commonly describe the investigation of adsorption phenomena, related to the next-generation rechargeable batteries such as lithium-sulfur (Li-S) and lithium-oxygen (Li-O2) battery. The adsorption of reaction intermediates for Li-S and Li-O2 battery, which are polysulfides (i.e., Li2Sx, 1"≤" x"≤" 8) and superoxide species (i.e., O2•- or LiO2), respectively, is a decisive step for these systems because these floating reaction intermediates can hamper the cell performance by triggering the unwanted side reactions such as shuttle phenomena of Li2Sx, and electrolyte degradation by O2•- species. In Chapter 4, we investigated the polysulfide adsorption on molecularly designed chemical trap in Li-S battery. A microporous covalent organic framework (COF) net on mesoporous carbon nanotube (CNT) net hybrid architecture was introduced as a new class of chemical trap for polysulfides. Two COFs with different micropore sizes (COF-1 and COF-5) were selected as model systems. Using density functional theory calculation and grand canonical Monte Carlo simulation, the pore-size-enabled selective adsorption of Li2S in COF-1 was theoretically demonstrated. The results also revealed that COF-1 possesses a well-designed micropore size and (boron-mediated) chemical affinity suitable for selective adsorption of Li2S, which can significantly improve the electrochemical performance of Li-S battery. In Chapter 5, we investigated the superoxide adsorption and subsequent disproportionation mechanism in Li-O2 battery. Reactive O2•- species can trigger the side reactions, which are serious hurdles hampering the performance of Li-O2 battery. To resolve this issue, malonic-acid-decorated fullerene (MA-C60) was employed as a superoxide disproportionation chemo-catalyst. Using multi-scale molecular simulation methods including density functional theory and molecular dynamics, we theoretically evidenced the preference to solution mechanism over surface mechanism in the presence of MA-C60 catalyst. Additionally, from the free energy diagram along reaction pathway of the solution mechanism, we identified the beneficial role of MA-C60 to significantly reduce the thermodynamic barrier of the disproportionation step.
Department of Chemical Engineering
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