Dynamic Acidity in Defective UiO-66

The metal organic framework (MOF) material UiO-66 has emerged as one of the most promising MOF materials due to its thermal and chemical stability and its potential for catalytic applications. Typically, as-synthesised UiO-66 has a relatively high concentration of missing linker defects. The presence of these defects has been correlated with catalytic activity but characterisation of defect structure has proved elusive. We refine a recent experimental determination of defect structure using static and dynamic first principles approaches, which reveals a dynamic and labile acid centre that could be tailored for functional applications in catalysis.Comment: 5 figure

Quantum chemical characterization of Biomolecules in the gas phase and on surfaces of metal oxides

During the four years of my PhD study, I performed systematic studies of the conformations of biomolecules ranging from a small amino acid (e.g. glycine) to a medium-sized nucleoside (e.g. 2’-deoxycytidine). To better account for possible effects brought by explicit environments (e.g. radiation, aqueous solution, and so on), we studied biomolecules in different phases, including neutral and charged species, in the gas phase and solid state, and neutral on solid surface. The work being presented in this thesis is original as: (1) A tool which can automatically generate libraries of conformations for a systematic search of the conformational space of a molecule was developed. When combined with tools developed by our colleagues, our toolbox facilitates a combinatorial computational chemical study of some small biomolecules; (2) A new method which can suppress barriers between different local minima on a molecular potential energy surface (PES) was developed, and with this new deformed PES, a lot of other techniques (e.g. Monte Carlo and simulated annealing) could be adopted to search for the global minima structure in a much more efficient way; (3) We performed a highly accurate study of two conformers of glycine up to the coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) with basis sets up to aug-cc-pVQZ level of theory, and we found that the treatment at the CCSD(T) level of theory is necessary to achieve numerical stability of the relative energies with respect to different basis sets at different geometries; (4) Through a thorough search of the conformational space of 2’-deoxycytidine, we found that its conformations in the gas phase are quite different from those in the solid state, and hopefully this finding could correct some of the previous approaches, in which structural information extracted from solid state experiments was used in computational studies of molecules in the gas phase; (5) Adsorptions of hydrogen, methanol and glycine on different types of solid surfaces (conductive and semiconductive) were studied, and catalytic performances of these surfaces on breaking chemical bonds were discussed. The current thesis not only covers the main applications of computational chemistry tools in the conformational study of biomolecules, it also includes discussions on accuracy and methodology which is involved in these studies. We definitely did not intend to solve all of the problems which people have met in their conformational studies of biomolecules. We just hope that the work being presented here was performed in a much more systematic way, and we hope these studies can give people some insights which might be helpful in their further studies

Microporous metallic scaffolds supported liquid infused icephobic construction.

Ice accretion on component surfaces often causes severe impacts or accidents. Liquid-infused surfaces (LIS) have drawn much attention as icephobic materials for ice mitigation in recent years due to their outstanding icephobicity. However, the durability of LIS constructions remains a big challenge, including mechanical vulnerability and rapid depletion of lubricants. The practical applications of LIS materials are significantly restrained, and the full potential of LIS for ice prevention has yet to be demonstrated. A universal approach was proposed to introduce microporous metallic scaffolds in the LIS construction to increase the applicability and durability, and to prompt the potential of LIS for ice mitigation. Microporous Ni scaffolds were chosen to integrate with polydimethylsiloxane modified by silicone oil addition. The new LIS construction demonstrated significantly improved durability in icing/de-icing cyclic test, and it also offered a solution for the rapid oil depletion by restraining the deformation of the matrix material. Low ice adhesion strength could be maintained via a micro-crack initiation mechanism. The results indicated that the multi-phase LIS construction consisting of microporous Ni scaffolds effectively addressed the shackles of the icephobicity deterioration of LIS materials, confirming a new design strategy for the R&D of icephobic surfaces

Substitutional effect of Ti-based AB2 hydrogen storage alloys: A density functional theory study

Stability of AB2 alloy in Laves phases C14 and C15 were studied by first-principle density functional theory simulations. A range of different combinations of B and C elements in the Ti1−xCxB2 alloys were considered. The formation energies of these alloys generally increase with the unit cell volumes of alloys. The volume also affects the stability of the corresponding metal hydride. We find that the formation energies and the hydrogenation enthalpies of AB2 alloys are likely to be determined by at least three factors: electronegativity, atomic radius and covalent radius. The enthalpies of AB2 hydrides increase with increasing compositionally-averaged electronegativity and volume change upon hydrogenation. However, the enthalpies of AB2 hydrides decrease with increasing compositionally-averaged atomic and covalent radii. This study provides useful insights for future exploration of AB2-type alloys for hydrogen storage applications

Kinetic control of interpenetration in Fe-biphenyl-4,4′-dicarboxylate metal-organic frameworks by coordination and oxidation modulation

Phase control in the self-assembly of metal-organic frameworks (MOFs) is often a case of trial and error; judicious control over a number of synthetic variables is required to select the desired topology and control features such as interpenetration and defectivity. Herein, we present a comprehensive investigation of self-assembly in the Fe-biphenyl-4,4′-dicarboxylate system, demonstrating that coordination modulation can reliably tune between the kinetic product, non-interpenetrated MIL-88D(Fe), and the thermodynamic product, two-fold interpenetrated MIL-126(Fe). Density functional theory simulations reveal that correlated disorder of the terminal anions on the metal clusters results in hydrogen-bonding between adjacent nets in the interpenetrated phase and is the thermodynamic driving force for its formation. Coordination modulation slows self-assembly and therefore selects the thermodynamic product MIL-126(Fe), while offering fine control over defectivity, inducing mesoporosity, but electron microscopy shows MIL-88D(Fe) persists in many samples despite not being evident by diffraction. Interpenetration control is also demonstrated using the 2,2′-bipyridine-5,5′-dicarboxylate linker; it is energetically prohibitive for it to adopt the twisted conformation required to form the interpenetrated phase, although multiple alternative phases are identified due to additional coordination of Fe cations to its N-donors. Finally, we introduce oxidation modulation – the use of metal precursors in different oxidation states to that found in the final MOF – to kinetically control self-assembly. Combining coordination and oxidation modulation allows the synthesis of pristine MIL-126(Fe) with BET surface areas close to the predicted maximum for the first time, suggesting that combining the two may be a powerful methodology for the controlled self-assembly of high-valent MOFs

Predicting vapor liquid equilibria using density functional theory: a case study of argon

Predicting vapor liquid equilibria (VLE) of molecules governed by weak van der Waals (vdW) interactions using the first principles approach is a significant challenge. Due to the poor scaling of the post Hartree-Fock wave function theory with system size/basis functions, the Kohn-Sham density functional theory (DFT) is preferred for systems with a large number of molecules. However, traditional DFT cannot adequately account for medium to long range correlations which are necessary for modeling vdW interactions. Recent developments in DFT such as dispersion corrected models and nonlocal van der Waals functionals have attempted to address this weakness with a varying degree of success. In this work, we predict the VLE of argon and assess the performance of several density functionals and the second order Møller-Plesset perturbation theory (MP2) by determining critical and structural properties via first principles Monte Carlo simulations. PBE-D3, BLYP-D3, and rVV10 functionals were used to compute vapor liquid coexistence curves, while PBE0-D3, M06-2X-D3, and MP2 were used for computing liquid density at a single state point. The performance of the PBE-D3 functional for VLE is superior to other functionals (BLYP-D3 and rVV10). At T = 85 K and P = 1 bar, MP2 performs well for the density and structural features of the first solvation shell in the liquid phase

Design principles for the ultimate gas deliverable capacity material: nonporous to porous deformations without volume change

Understanding the fundamental limits of gas deliverable capacity in porous materials is of critical importance as it informs whether technical targets (e.g., for on-board vehicular storage) are feasible. High-throughput screening studies of rigid materials, for example, have shown they are not able to achieve the original ARPA-E methane storage targets, yet an interesting question remains: what is the upper limit of deliverable capacity in flexible materials? In this work we develop a statistical adsorption model that specifically probes the limit of deliverable capacity in intrinsically flexible materials. The resulting adsorption thermodynamics indicate that a perfectly designed, intrinsically flexible nanoporous material could achieve higher methane deliverable capacity than the best benchmark systems known to date with little to no total volume change. Density functional theory and grand canonical Monte Carlo simulations identify a known metal–organic framework (MOF) that validates key features of the model. Therefore, this work (1) motivates a continued, extensive effort to rationally design a porous material analogous to the adsorption model and (2) calls for continued discovery of additional high deliverable capacity materials that remain hidden from rigid structure screening studies due to nominal non-porosity

Controlling multiple orderings in metal thiocyanate molecular perovskites Ax{Ni[Bi(SCN)6]}

We report four new A-site vacancy ordered thiocyanate double double perovskites, A1–x{Ni[Bi(SCN)6](1–x)/3}, A = K+, NH4+, CH3(NH3)+ (MeNH3+) and C(NH2)3+ (Gua+), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders offer a new route to non-centrosymmetric perovskites which render them as excellent candidates for piezo- and ferroelectric applications

The Interaction of Hydrogen with the van der Waals Crystal γ-InSe

The emergence of the hydrogen economy requires development in the storage, generation and sensing of hydrogen. The indium selenide (γ-InSe) van der Waals (vdW) crystal shows promise for technologies in all three of these areas. For these applications to be realised, the fundamental interactions of InSe with hydrogen must be understood. Here, we present a comprehensive experimental and theoretical study on the interaction of γ-InSe with hydrogen. It is shown that hydrogenation of γ-InSe by a Kaufman ion source results in a marked quenching of the room temperature photoluminescence signal and a modification of the vibrational modes of γ-InSe, which are modelled by density functional theory simulations. Our experimental and theoretical studies indicate that hydrogen is incorporated into the crystal preferentially in its atomic form. This behaviour is qualitatively different from that observed in other vdW crystals, such as transition metal dichalcogenides, where molecular hydrogen is intercalated in the vdW gaps of the crystal, leading to the formation of "bubbles" for hydrogen storage

Non-collinear magnetism in the post-perovskite thiocyanate frameworks CsM(NCS)3

AMX 3 compounds are structurally diverse, a notable example being the post-perovskite structure which adopts a two-dimensional framework with corner-and edge-sharing octahedra. Few molecular post-perovskites are known and of these, none have reported magnetic structures. Here we report the synthesis, structure and magnetic properties of molecular post-perovskites: CsNi(NCS) 3 , a thiocyanate framework, and two new isostructural analogues CsCo(NCS) 3 and CsMn(NCS) 3. Magnetisation measurements show that all three compounds undergo magnetic order. CsNi(NCS) 3 (Curie temperature, T C = 8.5(1) K) and CsCo(NCS) 3 (T C = 6.7(1) K) order as weak ferromagnets. On the other hand, CsMn(NCS) 3 orders as an antiferromagnet (Néel temperature , T N = 16.8(8) K). Neutron diffraction data of CsNi(NCS) 3 and CsMn(NCS) 3 , show that both are non-collinear magnets. These results suggest molecular frameworks are fruitful ground for realising the spin textures required for the next generation of information technology