Solar Hydrogen Generation Using Homogeneous, Heterogeneous and Biphasic Catalytic Systems

Using sunlight to generate hydrogen from water has captivated the attention of researchers as it would prove to be a completely renewable method of fuel production. For several decades, scientists have been trying to achieve ‘totalwater splitting’, which is a highly complex and energetically demanding process. To overcome this barrier, the process is treated separately as two half reactions, namely water oxidation and reduction, with the majority of the research directed at the reduction side which pertains to H2 evolution. Of late, research is also being conducted on using other H2 sources like acids and alcohols which arethermodynamically easier to dehydrogenate. The usual components for photocatalytic hydrogen evolution include a light-absorbing chromophore/photosensitizer, a proton reducing catalyst and a sacrificial donor/ redox mediator which mimics the oxidative half of water photolysis.These reactions have been carried out using both molecular and heterogeneous systems, mainly employing noble metal-based photoharvesters and catalysts, due to their inherent low overpotentials, high activity and stability. To ensurecommercial viability of the hydrogen generation process, recent focus has been on utilizing earth-abundant materials. With this background in mind, this dissertation aims to explore different routes to photocatalytic hydrogengeneration. Both molecular and heterogeneous methods of water reduction were probed, using an earth-abundant Ni-based hexameric cluster and a metal-free nanocarbon, respectively as proton reducing catalysts. Both systems were photodriven using state-of-the-art iridium-based photosensitizers, which are robust and well-known to probe the capability of new catalysts. While the Nibased molecular system achieved up to 30,000 catalyst turnovers, the metal-free nanocarbon was able to outperform platinum catalysts in terms of total hydrogen generation. As an example of replacing synthetic sacrificial agents with naturallyoccurring compounds, this thesis also demonstrates the use of oxalic acid as an electron donor, along with metallosurfactant Ir and Rh complexes in a novel biphasic H2 generation system. Oxidative quenching of the Ir excited state by the Rh, followed by electron donation by oxalic acid via phase transfer is proposed to be the dominant pathway, although further studies need to be conducted to provethis mechanism. Finally, this work contributes to the growing field of alcohol dehydrogenation, using visible light, a rhodium polypyridine catalyst and iodide in an acidic medium. This system was found to generate hydrogen and acetone from isopropyl alcohol, linearly for ~100 h, highlighting the robustness of the catalytic system. <br

Ex-situ dispersion of core-shell nanoparticles of Cu-Pt on an in situ modified carbon surface and their enhanced electrocatalytic activities

Direct dispersion of core-shell nanoparticles on a carbon support (Cu@Pt/C) has been achieved while retaining the essential core-shell features of the nanoparticles by adopting an in situ surface modification-cum-anchoring strategy

Light-Driven Hydrogen Generation from Microemulsions Using Metallosurfactant Catalysts and Oxalic Acid

A unique microemulsion-based photocatalytic water reduction system is demonstrated. Iridium- and rhodium-based metallosurfactants, namely, [Ir­(ppy)₂­(dhpdbpy)]­Cl and [Rh­(dhpdbpy)₂­Cl₂]Cl (where ppy = 2-phenylpyridine and dhpdbpy = 4,4′-diheptadecyl-2,2′-bipyridine), were employed as photosensitizer and proton reducing catalyst, respectively, along with oxalic acid as a sacrificial reductant in a toluene/water biphasic mixture. The addition of 1-octylamine is proposed to initiate the reaction, by coupling with oxalic acid to form an ion pair, which acts as an additional surfactant. Concentration optimizations yielded high activity for both the photosensitizer (240 turnovers, turnover frequency (TOF) = 200 h^–1) and catalyst (400 turnovers, TOF = 230 h^–1), with the system generating hydrogen even after 95 h. Mechanistic insights were provided by gas-phase Raman, electrochemical, and luminescence quenching analysis, suggesting oxidative quenching to be the principle reaction pathway

Improved performance of phosphonated carbon nanotube-polybenzimidazole composite membranes in proton exchange membrane fuel cells

Development of thermally stable polymer electrolyte membranes with higher proton conductivity as well as mechanical stability is a key challenge in commercializing PEM fuel cells operating above 100°C. Polybenzimidazole membranes are one of the promising candidates in this category although with limited mechanical stability and moderate proton conductivity. Here the incorporation of functionalized MWCNT is shown to increase both these key parameters of the polybenzimidazole membranes. Further, formation of a domain like structure after the incorporation of phosphonated MWCNTs (P-MWCNTs) in phosphoric acid doped polybenzimidazole membranes is demonstrated. The enhanced performance has been attributed to the formation of proton conducting networks that formed along the sidewalls of P-MWCNTs with a domain size of 17 nm as estimated from the small angle X-ray scattering measurements. Membrane electrode assembly (MEA) impedance measurements further reveal that the activation energy of oxygen reduction reaction (ORR) reduced for the composite membranes with enhanced proton conductivity. In addition, the mechanical strength measurements reveal a significant improvement in the yield strength and ultimate strength. Also, the mechanical strength of the composite membrane has been increased significantly as indicated by the improvement in the ultimate strength from 65 MPa to 100 MPa for the pristine and composite membranes, respectively. The optimum loading of P-MWCNTs is found to be 1% as inferred from the polarization measurements carried out using pure hydrogen and oxygen. Thus, this study provides a unique opportunity to tune the properties of polymer electrolytes to prepare application oriented hybrid membranes using CNTs with tailor-made functional groups

Photocatalytic Hydrogen Generation System Using a Nickel-Thiolate Hexameric Cluster

We report the use of a nickel-thiolate hexameric cluster, Ni₆(SC₂H₄Ph)₁₂, for photocatalytic hydrogen production from water. The nickel cluster was synthesized ex-situ and characterized by various techniques. Single crystal X-ray analysis, ¹H NMR, 2D COSY, ESI-MS, UV–visible spectroscopy, and TGA provided insight into the structure and confirmed the purity and stability of the cluster. Cyclic voltammetry helped confirm hydrogen evolution reaction (HER) activity of this catalyst. Photoreactions carried out using an iridium photosensitizer, Ir­(F-mppy)₂(dtbbpy)­[PF₆], and TEA as the sacrificial reductant revealed the high activity of the Ni₆ cluster as a water reducing catalyst. High TONs (3750) and TOFs (970 h^–1) were obtained at optimum catalyst concentration (0.025 mM), with low concentrations of catalyst yielding up to 30 000 turnovers. Quenching studies, along with the evidence obtained from the electrochemical analysis, showed that this water reduction system proceeds through a reductive quenching mechanism. Mercury poisoning studies confirmed that no active, metallic colloids were formed during the photocatalytic reaction

Photocatalytic Hydrogen Generation System Using a Nickel-Thiolate Hexameric Cluster

We report the use of a nickel-thiolate hexameric cluster, Ni₆(SC₂H₄Ph)₁₂, for photocatalytic hydrogen production from water. The nickel cluster was synthesized ex-situ and characterized by various techniques. Single crystal X-ray analysis, ¹H NMR, 2D COSY, ESI-MS, UV–visible spectroscopy, and TGA provided insight into the structure and confirmed the purity and stability of the cluster. Cyclic voltammetry helped confirm hydrogen evolution reaction (HER) activity of this catalyst. Photoreactions carried out using an iridium photosensitizer, Ir­(F-mppy)₂(dtbbpy)­[PF₆], and TEA as the sacrificial reductant revealed the high activity of the Ni₆ cluster as a water reducing catalyst. High TONs (3750) and TOFs (970 h^–1) were obtained at optimum catalyst concentration (0.025 mM), with low concentrations of catalyst yielding up to 30 000 turnovers. Quenching studies, along with the evidence obtained from the electrochemical analysis, showed that this water reduction system proceeds through a reductive quenching mechanism. Mercury poisoning studies confirmed that no active, metallic colloids were formed during the photocatalytic reaction

[Ir(N^N^N)(C^N)L]⁺: A New Family of Luminophores Combining Tunability and Enhanced Photostability

The relatively unexplored luminophore architecture [Ir­(N^N^N)­(C^N)­L]⁺ (N^N^N = tridentate polypyridyl ligand, C^N = 2-phenylpyridine derivative, and L = monodentate anionic ligand) offers the stability of tridentate polypyridyl coordination along with the tunability of three independently variable ligands. Here, a new family of these luminophores has been prepared based on the previously reported compound [Ir­(tpy)­(ppy)­Cl]⁺ (tpy = 2,2′:6′,2″-terpyridine and ppy = 2-phenylpyridine). Complexes are obtained as single stereoisomers, and ligand geometry is unambiguously assigned via X-ray crystallography. Electrochemical analysis of the materials reveals facile HOMO modulation through ppy functionalization and alteration of the monodentate ligand’s field strength. Emission reflects similar modulation shifting from orange to greenish-blue upon replacement of chloride with cyanide. Many of the new compounds exhibit impressive room temperature phosphorescence with lifetimes near 3 μs and quantum yields reaching 28.6%. Application of the new luminophores as photosensitizers for photocatalytic hydrogen generation reveals that their photostability in coordinating solvent is enhanced as compared to popular [Ir­(ppy)₂(bpy)]⁺ (bpy = 2,2′-bipyridine) photosensitizers. Yet, the binding of their monodentate ligand emerges as a source of instability during the redox processes of cyclic voltammetry and mass spectrometry. DFT modeling of electronic structure is provided for all compounds to elucidate experimental properties

Mechanistic Insight into the Dehydro-Diels–Alder Reaction of Styrene–Ynes

The Diels–Alder reaction represents one of the most thoroughly studied and well-understood synthetic transformations for the assembly of six-membered rings. Although intramolecular dehydro-Diels–Alder (IMDDA) reactions have previously been employed for the preparation of naphthalene and dihydronaphthalene substrates, low yields and product mixtures have reduced the impact and scope of this reaction. Through the mechanistic studies described within, we have confirmed that the thermal IMDDA reaction of styrene–ynes produces a naphthalene product via loss of hydrogen gas from the initially formed cycloadduct, a tetraenyl intermediate. Alternatively, the dihydronaphthalene product is afforded from the same tetraenyl intermediate via a radical isomerization process. Moreover, we have identified conditions that can be used to achieve efficient, high-yielding, and selective IMDDA reactions of styrene–ynes to form either naphthalene or dihydronaphthalene products. The operational simplicity and retrosynthetic orthogonality of this method for the preparation of naphthalenes and dihydronaphthalenes makes this transformation appealing for the synthesis of medicinal and material targets. The mechanistic studies within may impact the development of other thermal transformations

Evidence for Oxidative Decay of a Ru-Bound Ligand during Catalyzed Water Oxidation

In the evaluation of systems designed for catalytic water oxidation, ceric ammonium nitrate (CAN) is often used as a sacrificial electron acceptor. One of the sources of failure for such systems is oxidative decay of the catalyst in the presence of the strong oxidant CAN (<i>E</i>_ox = +1.71 V). Little progress has been made in understanding the circumstances behind this decay. In this study we show that a 2-(2′-hydroxphenyl) derivative (LH) of 1,10-phenanthroline (phen) in the complex [Ru­(L)­(tpy)]⁺ (tpy = 2,2′;6′,2″-terpyridine) can be oxidized by CAN to a 2-carboxy-phen while still bound to the metal. This complex is, in fact, a very active water oxidation catalyst. The incorporation of a methyl substituent on the phenol ring of LH slows down the oxidative decay and consequently slows down the catalytic oxidation. An analogous system based on bpy (2,2′-bipyridine) instead of phen shows much lower activity under the same conditions. Water molecule association to the Ru center of [Ru­(L)­(tpy)]⁺ and carboxylate donor dissociation were proposed to occur at the trivalent state. The resulting [Ru^III–OH₂] was further oxidized to [Ru^IVO] via a PCET process