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Artificial photosynthesis is broadly defined as the process of solar energy conversion into chemical fuels and represents a promising route towards alleviating the global energy crisis. In this context, the development of photocathodes for the use in photoelectrochemical cells is an attractive approach for the storage of solar energy in the form of a chemical energy carrier (e.g. H$_{2}$ and CO$_{2}$-reduction products from H$_{2}$O and CO$_{2}$). However, molecular catalyst-based photocathodes remain scarcely reported and typically suffer from low efficiencies and/or stabilities due to inadequate strategies for interfacing the molecular component with the light-harvesting material, with benchmark systems continuing to rely on precious metal components. In this thesis, the straightforward preparation of a p-silicon|mesoporous titania|molecular catalyst photocathode assembly that is active towards proton reduction in aqueous media is first established. The mesoporous TiO$_{2}$ scaffold acts as an electron shuttle between the silicon and the catalyst, while also stabilising the silicon from passivation and enabling a high loading of molecular catalysts. When a Ni bis(diphosphine)-based catalyst is anchored on the surface of the electrode, a catalytic onset potential of +0.4 V vs. RHE and a high turnover number of 1 $\times$ 10$^{3}$ was obtained from photoelectrolysis under UV-filtered simulated solar irradiation at 1 Sun after 24 hours. Notwithstanding its aptitude for molecular catalyst immobilisation, the Si|TiO$_{2}$ photoelectrode showed great versatility towards different types of catalysts and pH conditions, highlighting the flexible platform it represents for many potential reductive catalysis transformations. The Si|TiO$_{2}$ scaffold was extended towards solar CO$_{2}$ reduction via the immobilisation of a novel phosphonated cobalt bis(terpyridine) catalyst to achieve the first precious metal-free, CO$_{2}$-reducing molecular hybrid photocathode. Reducing CO$_{2}$ in both organic-water and purely aqueous conditions, the activity of this photocathode was shown to be affected by its environment and reached record turnover numbers for CO production by a molecular photocathode under optimal conditions, maintaining stable activity for more than 24 hours. Critically, in-depth electrochemical and in situ resonance Raman and infrared spectroelectrochemical investigations provided key insights into the nature of the surface-bound Co complex under reducing conditions. While demonstrating the power and precision offered by such in situ spectroelectrochemical techniques, these studies ultimately alluded to a catalytic mechanism that contrasts with that reported for the in-solution (homogeneous) catalyst. Overall, this affords a distinct mechanistic pathway that unlocks an earlier catalytic onset and enables photoelectrochemical activity. Finally, in the context of improving product selectivity in molecular-based CO$_{2}$ reduction, polymers based on the cobalt bis(terpyridine) motif were synthesised and immobilised on inverse opal-type electrodes designed specifically to accommodate large molecules. Rational design of the polymers' co-monomers was aimed towards the provision of an artificial environment for the active complex that would influence product selectivity, which was ultimately demonstrated by the improvement of a H$_{2}$:CO product ratio of 1:2 (molecule) to 1:6 (polymer). Further studies of this all-in-one system included modulating its degree of cross-linkage as well as a CO$_{2}$ reducing demonstration photocathode on a Si|inverse-opal TiO$_{2}$ scaffold.

abstract: The molecular modification of semiconductors has applications in energy conversion and storage, including artificial photosynthesis. In nature, the active sites of enzymes are typically earth-abundant metal centers and the protein provides a unique three-dimensional environment for effecting catalytic transformations. Inspired by this biological architecture, a synthetic methodology using surface-grafted polymers with discrete chemical recognition sites for assembling human-engineered catalysts in three-dimensional environments is presented. The use of polymeric coatings to interface cobalt-containing catalysts with semiconductors for solar fuel production is introduced in Chapter 1. The following three chapters demonstrate the versatility of this modular approach to interface cobalt-containing catalysts with semiconductors for solar fuel production. The catalyst-containing coatings are characterized through a suite of spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection Fourier transform infrared spectroscopy (GATR-FTIR) and x-ray photoelectron (XP) spectroscopy. It is demonstrated that the polymeric interface can be varied to control the surface chemistry and photoelectrochemical response of gallium phosphide (GaP) (100) electrodes by using thin-film coatings comprising surface-immobilized pyridyl or imidazole ligands to coordinate cobaloximes, known catalysts for hydrogen evolution. The polymer grafting chemistry and subsequent cobaloxime attachment is applicable to both the (111)A and (111)B crystal face of the gallium phosphide (GaP) semiconductor, providing insights into the surface connectivity of the hard/soft matter interface and demonstrating the applicability of the UV-induced immobilization of vinyl monomers to a range of GaP crystal indices. Finally, thin-film polypyridine surface coatings provide a molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto GaP. In all constructs, photoelectrochemical measurements confirm the hybrid photocathode uses solar energy to power reductive fuel-forming transformations in aqueous solutions without the use of organic acids, sacrificial chemical reductants, or electrochemical forward biasing. / Dissertation/Thesis / Doctoral Dissertation Biochemistry 2018

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2006. / Härtill 6 uppsatser.

L’objectif principal de ce travail de thèse était de mettre en évidence et de mesurer l’effet co-catalytique de complexes moléculaires organo-métalliques à base de métaux de transition adsorbés sur des semi-conducteurs dopés photo-actifs vis-à-vis des réactions de photo-dissociation de l’eau et de photo-réduction du dioxyde de carbone, en en vue d’applications dans des cellules photochimiques et photo-électrochimiques. Nous avons tout d’abord identifié et sélectionné les matériaux (deux semi-conducteurs photo-actifs et deux co-catalyseurs moléculaires électroactifs) les plus adaptés (les résultats sont présentés dans le chapitre I). Nous avons ensuite conçu, développé et mis au point un bâti expérimental permettant la détection et le dosage en continu des produits de réaction lors des réactions d’intérêt (les résultats sont présentés dans le chapitre II). La détection des produits de réaction se fait à l’aide d’un chromatographe en phase gazeuse couplé au réacteur. Nous avons ensuite élaboré/synthétisé et mesuré les propriétés intrinsèques des matériaux sélectionnés (les résultats sont présentés dans le chapitre III). Finalement, nous avons mis en évidence l’activité co-catalytique des complexes utilisés et mesuré un ensemble d’indicateurs de performance tels que les cinétiques de réaction et les fréquences de « turn-over » (les résultats sont présentés dans le chapitre IV). / The main objective of this research work was to put into evidence the co-catalytic effect of organo-metallic molecular complexes containing transition metals as reactive centers, adsorbed at the surface of doped semiconductors with photo-activity with regard to water photo-dissociation and carbon dioxide photo-reduction, in view of practical applications in photochemistry and photo-electrochemistry. First, appropriate materials (two photoactive semiconductors and two molecular co-catalysts) have been identified and selected (results are presented in chapter I). Then, we have designed, constructed and optimized a specific test bench that can be used for the continuous detection and titration of reaction products (results are presented in chapter II). Product analysis was achieved by coupling a gas-phase chromatograph to the photo-electrochemical reactor. Then, photoactive semiconductors and molecular co-catalysts have been elaborated/synthesized and their intrinsic properties have been measured (results are presented in chapter III). Finally, the co-catalytic activity of molecular complexes has been put into evidence and several performance indicators such as reaction kinetics and turn-over frequency have been measured (results are presented in chapter IV).

Linear alpha olefins (LAOs) are key petrochemical precursors for the synthesis of larger polymers, detergents, plasticizers, and lubricants. Most catalytic ethylene oligomerization processes generate a wide distribution of LAO carbon chain lengths. A major ongoing industrial challenge is to develop homogeneous catalysts that result in selective and tunable ethylene oligomerization to 1-hexene and 1-octene alkenes. Quantum mechanical calculations coupled with rapidly advancing technology have enabled the ability to calculate small molecule systems with high accuracy. Employing computational models to advance from empirical to quantitative prediction of product selectivities has become an active area of exploration. In this work, we demonstrate the development and use of a density-functional theory (DFT) transition-state model that provides highly accurate quantitative prediction of phosphinoamidine (P,N) Cr catalysts for controllable selective ethylene trimerization and tetramerization. This model identified a new family of highly selective catalysts that through computational-based ligand design results in a predictable shift from 1-hexene selectivity to 1-octene. Subsequent experimental ligand synthesis and catalyst testing verified the quantitative computational predictions. DFT calculations also provide key insights to factors controlling catalytic activity and present important design criteria for the development of active Cr-based ethylene oligomerization systems. Non-selective ethylene transformations, referred to as full range processes, provide access to a range of LAOs (C4-C20) that are used to produce polyethylene, surfactants, and other commercial products. During full-range oligomerizations, undesired byproducts degrade the purity of LAOs mostly consisting of branched oligomers. Computational mechanistic investigations reveal the origin of linear versus branched selectivity in Fe-catalyzed ethylene oligomerization reactions.