Molecular Modeling of Nanostructures, Polymer Electrolytes, and Ionic Liquids for Energy, Environmental, and Catalysis Applications

This thesis centers around the structural, electronic, and physicochemical properties of some nanostructures, polymer electrolytes, and ionic liquids for energy, environmental, and catalysis applications. A variety of theoretical tools are employed to enable predictivestudy, design, and development of the systems. Theoretical concepts are developed for rational explanation of reaction mechanism, surface activity, ionic conductivity, formation,and fragmentation of the materials with respect to structure-activity and structure selectivity correlations. The outline of this thesis is as follows: First, the structural stability of isoreticular metal organic frameworks (IRMOFs) confining ionic liquids (ILs) inside their nano-porous cavities is studied. It is shown the IRMOFs are structurally unstable and deforms dramatically from its crystal structure in thepresence of ILs. Simulation results also show that metallic parts of IRMOF-1 mainly interact with anions. These results raise an important question if IL/IRMOF composites aregood candidates for gas storage or separation. Second, interactions of the lithium bis(trifluoromethane sulfonyl)imide (LiTf2N) salt with polyethers are investigated to shed light on the origin of difference in ionic conductivity of the two electrolytes: (1) polyethyleneoxide (PEO) and (2)perfluoropolyether (PFPE). Results indicate that substitution of hydrogens by fluorines leads to less interaction of cation Li+ with the polymers, which provide an atomisticunderstanding of experimental ionic conductivity results.Third, Au25(SR)18 nanoclusters with atomic precision is served as a model system to scrutinize structure-reactivity correlation of thiolate protected gold nanoclusters. In particular, the promotional catalytic effect by imidazolium-based ILs, transition metal ions, and aromatic thiolate ligands are studied. Results show the ILs and the ions cansignificantly enhance activation of the nanoclusters at relatively low temperatures via partial removal of “Au-SR” units from Au25 cluster. Further, guidelines are offered fortailoring nanoclusters’ properties by ligand engineering for specific applications. Fourth, controllable incorporation of a single dopant metal to specific sites of gold nanoparticle is studied. A centrally hollow, Au24 nanoparticle is used to investigate single atom shuttling in/out of the nanoparticle. Using Ag and Cu as the tracers, the pathways ofsingle atom shuttling are mapped out. The results not only demonstrate single-atom level doping via hollow nanoparticles, but also reveal the intriguing atom shuttling behavior. Finally, mechanistic insights into promotional effects of ionic liquids on transesterification of cellulose is provides. Results explains IL anions play the most crucialrole to facilitate the reaction. This study provides molecular insights for experimentalists to optimize reaction conditions of protocols of cellulose modifications using ILs. I believe a key element to research is collaborating with chemists in other fields, as complex problems require input from many chemists with different specialties. I have had theopportunity to collaborate closely with experimental groups at Carnegie Mellon University (Prof. Rongchao Jin), Anhui University (Prof. Zhu), Dalian Institute of Chemical Physics (Prof. Gao Li), and Kanazawa University (Prof. Kenji Takahashi) to develop the third-to-fifth projects mentioned above. Our collaborative works require clearly communicating the intrinsic principles behind the complex results. This involve regular exchanges of different viewpointsto establish guidelines for the development of materials with desired performance. This thesis emphasizes on the computational contribution to the projects. <br

One-Pot Synthesis of Au₁₁(PPh₂Py)₇Br₃ for the Highly Chemoselective Hydrogenation of Nitrobenzaldehyde

In this study, the gold clusters Au₁₁(PPh₃)₇Cl₃ and Au₁₁(PPh₂Py)₇Br₃ (PPh₂Py = diphenyl-2-pyridylphosphine) are synthesized via a one-pot procedure based on the wet chemical reduction method. The Au₁₁(PPh₃)₇Cl₃ cluster is found to be active in the chemoselective hydrogenation of 4-nitrobenzaldehyde in the presence of hydrogen (H₂) and a base (e.g., pyridine). Interestingly, the cluster with the functional ligand PPh₂Py shows similar activity without losing catalytic efficiency in the absence of the base. The structure of the gold clusters and reaction pathway of the catalytic hydrogenation are investigated at the atomic/molecular level via UV–vis spectroscopy, electrospray ionization (ESI) mass spectrometry, and density functional theory (DFT) calculations. It is found that one ligand (PPh₃ or PPh₂Py) removal is the first step to expose the core of the gold clusters to reactants, providing an active site for the catalytic reaction. Then, the H–H bond of the H₂ molecule becomes activated with the aid of either free amine (base) or ligand PPh₂Py which is attached to the gold clusters. This work demonstrates the promise of the functional ligand PPh₂Py in the catalytic hydrogenation to reduce the amount of materials (free base: e.g., pyridine) that ultimately enter the waste stream, thereby providing a more environmentally friendly reaction medium

Au₃₈S₂(SAdm)₂₀ Photocatalyst for One-Step Selective Aerobic Oxidations

We here report a protocol for the synthesis of Au₃₈S₂(SAdm)₂₀ nanoclusters (−SAdm = 1-adamantanethiolate) with a higher production yield (10%) in comparison to previous reported methods. The photosensitizing properties of the gold nanoclusters are investigated for the formations of singlet oxygen (¹O₂) using visible light wavelengths of 532 and 650 nm. The formation of ¹O₂ was detected by 1,3-diphenylisobenzofuran as the chemical trapping probe as well as direct observation of the characteristic ¹O₂ emission (ca. 1276 nm). The efficiency of the ¹O₂ formation using the Au₃₈S₂(SAdm)₂₀ nanoclusters is found to be notably higher than that of Au₂₅(SR)₁₈ nanoclusters. Finally, selective aerobic oxidations of sulfide to sulfoxide and benzylamine to imine in the presence of oxygen (³O₂) and photoexcited Au₃₈S₂(SAdm)₂₀ are well studied. This work demonstrates the promise of Au₃₈S₂(SAdm)₂₀ nanoclusters in the generation of activated singlet oxygen for selective catalytic reactions

Experimental and Mechanistic Understanding of Aldehyde Hydrogenation Using Au₂₅ Nanoclusters with Lewis Acids: Unique Sites for Catalytic Reactions

The catalytic activity of Au₂₅(SR)₁₈ nanoclusters (R = C₂H₄Ph) for the aldehyde hydrogenation reaction in the presence of a base, e.g., ammonia or pyridine, and transition-metal ions M^z+, such as Cu⁺, Cu²⁺, Ni²⁺ and Co²⁺, as a Lewis acid is studied. The addition of a Lewis acid is found to significantly promote the catalytic activity of Au₂₅(SR)₁₈/CeO₂ in the hydrogenation of benzaldehyde and a number of its derivatives. Matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry in conjunction with UV–vis spectroscopy confirm the generation of new species, Au_{25‑<i>n</i>}(SR)_{18‑<i>n</i>} (<i>n</i> = 1–4), in the presence of a Lewis acid. The pathways for the speciation of Au₂₄(SR)₁₇ from its parent Au₂₅(SR)₁₈ nanocluster as well as its structure are investigated via the density functional theory (DFT) method. The adsorption of M^<i>z</i>+ onto a thiolate ligand “SR” of Au₂₅(SR)₁₈, followed by a stepwise detachment of “SR” and a gold atom bonded to “SR” (thus an “Au-SR” unit) is found to be the most likely mechanism for the Au₂₄(SR)₁₇ generation. This in turn exposes the Au₁₃-core of Au₂₄(SR)₁₇ to reactants, providing an active site for the catalytic hydrogenation. DFT calculations indicate that M^z+ is also capable of adsorbing onto the Au₁₃-core surface, producing a possible active metal site of a different kind to catalyze the aldehyde hydrogenation reaction. This study suggests, for the first time, that species with an open metal site like adducts [nanoparticle-M]^{(<i>z</i>‑1)+} or fragments Au_{25‑<i>n</i>}(SR)_{18‑<i>n</i>} function as the catalysts rather than the intact Au₂₅(SR)₁₈

Ultrathin Cobalt Oxide Overlayer Promotes Catalytic Activity of Cobalt Nitride for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) plays a crucial role in various energy devices such as proton-exchange membrane fuel cells (PEMFCs) and metal–air batteries. Owing to the scarcity of the current state-of-the-art Pt-based catalysts, cost-effective Pt-free materials such as transition metal nitrides and their derivatives have gained overwhelming interest as alternatives. In particular, cobalt nitride (CoN) has demonstrated a reasonably high ORR activity. However, the nature of its active phase still remains elusive. Here, we employ density functional theory calculations to study the surface reactivity of rocksalt (RS) and zincblend (ZB) cobalt nitride. The performances of the catalysts terminated by the facets of (100), (110), and (111) are studied for the ORR. We demonstrate that the cobalt nitride surface is highly susceptible to oxidation under ORR conditions. The as-formed oxide overlayer on the facets of CoN_RS(100) and CoN_ZB(110) presents a significant promotional effect in reducing the ORR overpotential, thereby increasing the activity in comparison with those of the pure CoNs. The results of this work rationalize a number of experimental reports in the literature and disclose the nature of the active phase of cobalt nitrides for the ORR. Moreover, they offer guidelines for understanding the activity of other transition metal nitrides and designing efficient catalysts for future generation of PEMFCs

Molecular-like Transformation from PhSe-Protected Au₂₅ to Au₂₃ Nanocluster and Its Application

In this work, we report a new size conversion from [Au₂₅(SePh)₁₈]⁻ to [Au₂₃(SePh)₁₆]⁻ nanoclusters under the reductive condition (NaBH₄). This novel transformation induced by only reductant has not been reported before in the field of gold nanocluster. The conversion process is studied via MALDI mass spectrometry, and UV–vis spectroscopy. These results demonstrate that the [Au₂₃(SePh)₁₆]⁻ nanocluster is directly obtained by pulling out two units of “Au-SeR” from the [Au₂₅(SePh)₁₈]⁻ nanocluster, which is similar to the “small molecular” reaction. In order to further understand this novel conversion, DFT calculations were performed, in which, with addition of two H^– in the [Au₂₅(SeH)₁₈]⁻ model, two Au atoms will depart from the structure of the [Au₂₅(SeH)₁₈]⁻, which is consistent with the experimental results. Furthermore, the as-prepared [Au₂₃(SePh)₁₆]⁻ nanoclusters can be converted into [Au₂₅(PET)₁₈]⁻ nanocluster (PET = SCH₂CH₂Ph) with excess PET under the reductive condition, which is quite remarkable due to a stronger bond of Au–Se in comparison to Au–S of the final product. Interestingly, the number of the PET ligands on the surface of the 25-atoms nanocluster can be controlled by the addition of the reductant. Based on these results, a circularly progressive mechanism of ligand exchange is proposed. This may offer a new approach to synthesis of new gold nanoclusters and also have significant contribution for understanding and further exploration of the mechanism of ligand exchange

Visible Light Gold Nanocluster Photocatalyst: Selective Aerobic Oxidation of Amines to Imines

This work demonstrates the synthesis of an efficient photocatalyst, Au₂₅(PPh₃)₁₀Cl₂(SC₃H₆SiO₃)₅/TiO₂, for selective oxidation of amines to imines. The photocatalyst is prepared via hydrolysis of Au₂₅(PPh₃)₁₀Cl₂[(SC₃H₆Si­(OC₂H₅)₃]₅ nanoclusters in the presence of TiO₂ support. The gold nanoclusters exhibit good photocatalytic activity using visible light and under mild thermal conditions for the selective oxidation with molecular oxygen (O₂). The turnover frequency (TOF) of 4-methylbenzylamine oxidation is found to be 1522 h^–1, which is considerably higher than that conventional gold catalysts. The gold nanoclusters present good recyclability and stability for the oxidation of a wide range of amines. The superior activity of the photocatalyst is associated with its unique electronic structure and framework. The catalytically active sites are deemed to be the exposed gold atoms upon detaching protecting ligands: i.e., PPh₃. The Hammett parameter suggests that the photocatalytic process involves the formation of carbocation intermediate species. Further, Au–H species were confirmed by TEMPO (2,2,6,6-tetramethylpiperidinyloxy) as a trapping agent

Tailoring the Electronic and Catalytic Properties of Au₂₅ Nanoclusters <i>via</i> Ligand Engineering

To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au₂₅(SNap)₁₈]⁻ [TOA]⁺, where SNap = 1-naphthalenethiolate and TOA = tetraoctylammonium. It exhibits distinct differences in electronic and catalytic properties in comparison with the previously reported [Au₂₅(SCH₂CH₂Ph)₁₈]⁻, albeit their skeletons (<i>i.e.</i>, Au₂₅S₁₈ framework) are similar. A red shift by ∼10 nm in the HOMO–LUMO electronic absorption peak wavelength is observed for the aromatic-thiolate-protected nanocluster, which is attributed to its dilated Au₁₃ kernel. The unsupported [Au₂₅(SNap)₁₈]⁻ nanoclusters show high thermal and antioxidation stabilities (<i><i>e.g.</i></i>, at 80 °C in the present of O₂, excess H₂O₂, or TBHP) due to the effects of aromatic ligands on stabilization of the nanocluster’s frontier orbitals (HOMO and LUMO). Furthermore, the catalytic activity of the supported Au₂₅(SR)₁₈/CeO₂ (R = Nap, Ph, CH₂CH₂Ph, and <i>n</i>-C₆H₁₃) is examined in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene. Results show that the activity and selectivity of the catalysts are largely influenced by the chemical nature of the protecting thiolate ligands. This study highlights that the aromatic ligands not only lead to a higher conversion in catalytic reaction but also markedly increase the yield of the heterocoupling product (4-methyl-4′-nitro-1,1′-biphenyl). Through a combined approach of experiment and theory, this study sheds light on the structure–activity relationships of the Au₂₅ nanoclusters and also offers guidelines for tailoring nanocluster properties by ligand engineering for specific applications

Silicon Nanoparticles with Surface Nitrogen: 90% Quantum Yield with Narrow Luminescence Bandwidth and the Ligand Structure Based Energy Law

Silicon nanoparticles (NPs) have been widely accepted as an alternative material for typical quantum dots and commercial organic dyes in light-emitting and bioimaging applications owing to silicon’s intrinsic merits of least toxicity, low cost, and high abundance. However, to date, how to improve Si nanoparticle photoluminescence (PL) performance (such as ultrahigh quantum yield, sharp emission peak, high stability) is still a major issue. Herein, we report surface nitrogen-capped Si NPs with PL quantum yield up to 90% and narrow PL bandwidth (full width at half-maximum (fwhm) ≈ 40 nm), which can compete with commercial dyes and typical quantum dots. Comprehensive studies have been conducted to unveil the influence of particle size, structure, and amount of surface ligand on the PL of Si NPs. Especially, a general ligand-structure-based PL energy law for surface nitrogen-capped Si NPs is identified in both experimental and theoretical analyses, and the underlying PL mechanisms are further discussed

Gold Nanoclusters Promote Electrocatalytic Water Oxidation at the Nanocluster/CoSe₂ Interface

Electrocatalytic water splitting to produce hydrogen comprises the hydrogen and oxygen evolution half reactions (HER and OER), with the latter as the bottleneck process. Thus, enhancing the OER performance and understanding the mechanism are critically important. Herein, we report a strategy for OER enhancement by utilizing gold nanoclusters to form cluster/CoSe₂ composites; the latter exhibit largely enhanced OER activity in alkaline solutions. The Au₂₅/CoSe₂ composite affords a current density of 10 mA cm^–2 at small overpotential of ∼0.43 V (cf. CoSe₂: ∼0.52 V). The ligand and gold cluster size can also tune the catalytic performance of the composites. Based upon XPS analysis and DFT simulations, we attribute the activity enhancement to electronic interactions between nanocluster and CoSe₂, which favors the formation of the important intermediate (OOH) as well as the desorption of oxygen molecules over Au_<i>n</i>/CoSe₂ composites in the process of water oxidation. Such an atomic level understanding may provide some guidelines for design of OER catalysts