Understanding the Structure-Function Relationship in Peptide-Enabled High Entropy Alloy Nanocatalysts

The structural complexity in high entropy alloy nanocatalysts (HEAs), afforded by the homogeneous mixing of five or more elements, has resulted in a limited understanding about the origin of their promising electrocatalytic properties. This thesis investigates the structure-function relationship in HEAs using advanced material characterization techniques. At first, a methodology for resolving the atomic-scale structure of peptide-enabled HEAs was developed using high-energy X-ray diffraction (HE-XRD) coupled with atomic pair distribution function (PDF) and reverse Monte Carlo (RMC) simulations, yielding structure models over the length scale of HEAs. Coordination analysis of the structure models revealed a multifunctional interplay of geometric and electronic attributes of surface atoms in HEAs that was responsible for the catalytic activity enhancement during the methanol electrooxidation reaction. Using the methodology for resolving the atomic scale structure of HEAs and peptide sequence engineering, the structure-function relationship of model PtPdAuCoSn HEAs during ethanol electrooxidation reaction (EOR) was studied. Compositional analysis of the PtPdAuCoSn HEA structure models revealed distinct miscibility characteristics that were attributed to the unique biotic-abiotic interactions. Analysis of the structure models identified the rapid dehydrogenation of CH3CHO intermediate into CH3COads in an optimized adsorption configuration as the contributing factor for the high selectivity towards CH3COO- in PtPdAuCoSn HEAs. Armed with these insights, a study was designed for understanding the effect of changing the concentration of Pt in the structure-function relationship of PtPdAuCoSn HEAs using spatiotemporal structural insights from in-situ PDF. The structure models demonstrated a degree of metastability as a function of their corresponding configurational entropy. Analysis of the structure models revealed that high selectivity towards CH3COO- in PtPdAuCoSn HEAs during EOR originates from the enhanced distribution of Pd and Co surface atoms. In summary, this thesis uses atomic PDF and RMC simulations to draw structure-function correlations in HEAs, presenting a path forward for developing strategies for the rational design of HEAs. Through collaborative efforts from theoreticians and experimentalists, the methodology presented here can form the basis for accelerating the discovery of promising HEA configurations for emerging electrocatalytic applications

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified

Activating Methane and Other Small Molecules: Computational study of Zeolites and Actinides

Exploring the catalytic properties and reactivity of actinide complexes towards activation of small molecules is important as human activities have led to the increased distribution of these species in nature. Toward this end, it is important to have a computational protocol for studying these species, in this thesis we provide details on the performance of multiconfigurational pair-density functional theory (MC-PDFT) in actinide chemistry. MC-PDFT and Kohn-Sham Density Functional Theory (KS-DFT) perform well for these species with indications that the former can be used for species with even greater static electron correlation effect. In addition, we study the activity of organometallic trans-uranium complexes towards the electrocatalytic reduction of water. We conclude that, with a guided choice of ligand, neptunium complexes can provide similar reactivity when compared to organometallic uranium complexes.Conversion of methane to methanol has been a major focus of research interest over the years. This is largely due to the abundance of natural gas, of which methane is the major constituent. Copper-exchanged zeolites have been shown to be able to kinetically trap activated methane as strongly-bound methoxy groups, preventing over-oxidation to CO2, CO and HCOOH. In this stepwise process, there are three cycles; an initial activation step to form the copper oxo active site, methane C-H activation and lastly simultaneous desorption of methanol and re -activation of the active site.. We provide detailed description of the pathway for the formation of over oxidation products. It is observed that to ensure high selectivity to methanol and prevent further hydrogen atom abstraction by extra-framework species, the methyl group must be stabilized from the copper-oxo active sites. There is a temperature gradient between the steps in the methane-to-methanol conversion cycle which is an impediment to industrial adoption of this approach for methane-to-methanol conversion. To mitigate this, we have investigated the impact of heterometallic extra-framework motifs on the temperature gradients of each step. Using periodic DFT, we provide detailed descriptions of the mechanistic pathways for each of the three steps. We were subsequently able to design motif(s) with great methane C-H activities as well as the abilities to be formed and regenerated at nearly the same temperatures. We found [Cu-O-Ag] and [Cu-O-Pd] to be potential candidates for isothermal or near-isothermal operations of the methane-to-methanol conversion cycle. Finally, we provide insights to the changes in optical spectra of activated copper-exchanged zeolites, gaining an understanding of the evolution of these systems on a molecular level will provide opportunities to achieve improved reactivity

Polymer-immobilised ionic liquid phase (PIILP) catalysis :supports for molecular and nanoparticles catalysts

PhD ThesisThe Polymer Immobilised Ionic Liquids (PIILs) has been an area of interest recently, particularly, Doherty-Knight group has recently explored and developed this concept, with the aim of designing novel functionalised PIILPs and utilising them as supports to immobilise transition metals catalysts and nanoparticles and then exploring their applications. The second chapter describes the synthesis of tungstate and polyoxotungstate based catalysts for the selective oxidation of sulfides. The polymer immobilised ionic liquids were based on linear pyrrolidinium-modified norbornene-cyclooctene co-polymers prepared by ring opening metathesis polymerisation and the corresponding catalysts were prepared by exchange of the polymer anions with either tungstate or polyoxotungstate. High selectivity for sulfoxide was obtained across a range of aryl-alkyl sulfides using either (WO4@ROMPx or PW12O40@ROMP1) in either acetonitrile or methanol with 2.5 equivalents of hydrogen peroxide for 15 minutes at room temperature. Different catalytic activity was observed based on the nature of the crosslinker whether it is linear (ROMP1) or cyclic (ROMP2). The third chapter describes the synthesis and characterisation of a range heteroatom donor modified polymer immobilised palladium nanoparticles. Three types of polystyrene-based PIILP (amino-, phosphino-, and pyrrolidino-) were prepared via free radical polymerisation and used to support platinum group metal nanoparticles (MNP@R-PIILP; R = CH2NH2, PPh2, CH2Pyrr). All the prepared catalysts have been characterised by a range of techniques including solid-state NMR spectroscopy, SEM, TEM, XRD, XPS, EDX, ICP, TGA and BET analysis. Chapter 4 presents the results of our systematic evaluation of the efficacy of the newly prepared MNP@PIILP (M = Pd, Pt) systems as catalysts for the selective hydrogenation of α, β-unsaturated aldehydes. Our studies have shown that PdNP@PPh2-PIILP catalyses x the hydrogenation of trans-cinnamaldehyde in water with high selectivity for reduction of the C=C double bond to afford dihydrocinnamaldehyde in 76 % selectivity at 96 % conversion under mild conditions and in short reaction times. Notably, the addition of base (K2CO3) to the reaction allows higher selectivities to be obtained (up to 95 % for C=C reduction), however, this results in a decrease in reaction rate (96 to 67.5 %). Chapter 5 explores the use of PdNP@R-PIILP (R = NH2, PPh2) as catalysts for the Suzuki- Miyaura cross-coupling. Interestingly, palladium NPs stabilised by amino-decorated polymer immobilised ionic liquids (PdNP@NH2-PIILP) were shown to be inactive for the Suzuki-Miyaura cross-coupling of aryl bromides with phenyl boronic acid. However, the corresponding PdNP@NH2-PIILP generated by in-situ by reduction of PdCl4@NH2-PIILP was highly active for the Suzuki-Miyaura cross-coupling. Kinetic studies, reaction dilution experiments, mercury poisoning and catalyst loading studies have been employed to investigate the difference between the performance of pre-reduced PdNPs and those generated in-situ.Ministry of Higher Education and Scientific Research-Iraq and Iraqi Cultural Attach

Small Molecule Activation by Transition Metal Complexes: Studies with Quantum Mechanical and Machine Learning Methodologies

One of the largest areas of study in the fields of chemistry and engineering is that of activation of small molecules such as nitrogen, oxygen and methane. Herein we study the activation of such molecules by transition metal compounds using quantum mechanical methods in order to understand the complex chemistry behind these processes. By understanding these processes, we can design and propose novel catalytic species, and through the use of data-driven machine learning methods, we are able to accelerate materials discovery

Beyond Density Functional Theory: the Multiconfigurational Approach to Model Heterogeneous Catalysis

Catalytic processes are crucially important for many practical chemical applications. Heterogeneous catalysts are especially appealing because of their high stability and the relative ease with which they may be recovered and reused. Computational modeling can play an important role in the design of more catalytically active materials through the identification of reaction mechanisms and the opportunity to assess hypothetical catalysts in silico prior to experimental verification. Kohn-Sham density functional theory (KS-DFT) is the most used method in computational catalysis because it is affordable and it gives results of reasonable accuracy in many instances. Furthermore, it can be employed in a “black-box” mode that does not require significant a priori knowledge of the system. However, KS-DFT has some limitations: it suffers from self-interaction error (sometime referred to as delocalization error), but a greater concern is that it provides an intrinsically single-reference description of the electronic structure, and this can be especially problematic for modeling catalysis when transition metals are involved. In this perspective, we highlight some noteworthy applications of KS-DFT to heterogeneous computational catalysis, as well as cases where KS-DFT fails accurately to describe electronic structures and intermediate spin states in open-shell transition metal systems. We next provide an introduction to state-of-the-art multiconfigurational (MC; also referred to as multireference (MR)) methods and their advantages and limitations for modeling heterogeneous catalysis. We focus on specific examples to which MC methods have 2 been applied and discuss the challenges associated with these calculations. We conclude by offering our vision for how the community can make further progress in the development of MC methods for application to heterogeneous catalysis