Molecular Catalysis towards Artificial Photosynthesis

The 21st century is a time of unprecedented uncertainty for the energy sector: a secure, clean, continuous and equally distributed source of energy is fundamental to global economic growth and human development. Nowadays, being able to find a real substitute to fossil fuels represents a fascinating challenge. Among possible alternatives, renewable sources seems to better fit the energetic demand and solar energy is by far the largest exploitable. However, it has to be captured, converted and conveniently stored. Inspired by Nature, artificial photosynthesis is a process aimed at efficiently converting sunlight energy into alternative fuels such as hydrogen or other different reduced form of carbon. This artificial system is characterized by an articulate scheme of events, terminating with redox reactions that need to be efficiently catalysed. The project of this thesis aims to study the development of new catalytic, molecular and Earth-abundant based systems for redox processes in artificial photosynthesis. For our goals, photo-activated systems are preferred in order to better mimic the light-driven activation in an ideal artificial device. Moreover, multi and mono metallic active sites in catalysts structure are considered, inspired by several efficient examples in literature. The work is mainly focused on water oxidation reaction, being still considered the bottleneck of artificial photosynthesis; however also preliminary studies on CO2 reduction have been examined. First, a Cobalt-based oxo cluster, [Co4(μ3-O)4(μ-O2CCH3)4(pyridine)4] has been studied as a molecular catalyst for water oxidation in a light activated system with Ru(bpy)32+ as photosensitizer and S2O82- as sacrificial donor. The species has been characterized through different analytic techniques and tuning electronic substituents properties, structure-activity correlations have been investigated by cyclic voltammetry and laser flash photolysis. Moreover, a synthetic approach to modify the structure of the species has been evaluated in order to design no covalent dyads between the catalyst and the photosensitizer exploiting π−π interactions. Other Cobalt-based species with high nuclearity and totally inorganic ligands (polyoxometalates, POMs) have been studied in water oxidation catalysis. In particular, complexes [Co9(H2O)6(OH)3(PW9O34)3]16-, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(SiW8O31)3}]5- and [{Co4(OH)3PO4}4(PW9O34)4]16- have been investigated with laser flash photolysis and in the photo-activated system. Interesting mechanistic insights have been reached thanks to the analysis of these species. Moreover, during the thesis work a novel single site Copper-based compound with a tetraazacyclotetradecane ligand has been proposed as water oxidation catalyst. In particular, the species has been characterized among the electrochemical system and the catalytic behaviour has been explored by means cyclic voltammetry, electrolysis and photoelectrochemical experiments. With the aim of the development of a sunlight activated water splitting device, for the first time in this thesis work a Copper molecular species has been examined in combination with light. Results seem to be preliminary interesting for further studies on azamacrocyclic Copper-based molecular species. Finally, dealing with the catalysis of CO2 reduction some studies have been performed with a POM-based complex, [Cu(SiW11O39)]6-. Cyclic voltammetry experiments have been run in order to evaluate the possible catalytic activity of the compound in CO2 reduction. The aim of this thesis work is to suggest a method to achieve a better understanding of the analysed topic through optimized experimental conditions and mechanistic insights

Fotosintesi artificiale: complessi molecolari di Cobalto come catalizzatori per l’ossidazione dell’acqua

Water oxidation still represents the bottleneck of artificial photosynthesis, that is light driven water splitting into hydrogen and oxygen. This work is focused on the investigation of molecular catalysts based on Cobalt, using a photoactivated system based on S2O82- as the sacrifical oxidant and Ru(bpy)32+ as the photosynthetizer. The advantage in using such species, stems from the abundance of Cobalt on Earth, its limited cost and its promising catalytic activity. This study analyses catalytic Oxygen production by several Cobalt based molecular species with totally inorganic ligands taken from the polyoxometalate pool, and other polinuclear complexes with organic ligands with different coordination geometries of the metal centres. Moreover, the activity of the hexaaquoion Co(H2O)62+ in the presence of different organic ligands added in situ was hexamined, with the aim of getting informations of the reaction mechanism.openTESI.3687La copia a stampa della tesi e' disponibile presso la biblioteca con la collocazione indicat

Molecular Catalysis towards Artificial Photosynthesis

The 21st century is a time of unprecedented uncertainty for the energy sector: a secure, clean, continuous and equally distributed source of energy is fundamental to global economic growth and human development. Nowadays, being able to find a real substitute to fossil fuels represents a fascinating challenge. Among possible alternatives, renewable sources seems to better fit the energetic demand and solar energy is by far the largest exploitable. However, it has to be captured, converted and conveniently stored. Inspired by Nature, artificial photosynthesis is a process aimed at efficiently converting sunlight energy into alternative fuels such as hydrogen or other different reduced form of carbon. This artificial system is characterized by an articulate scheme of events, terminating with redox reactions that need to be efficiently catalysed. The project of this thesis aims to study the development of new catalytic, molecular and Earth-abundant based systems for redox processes in artificial photosynthesis. For our goals, photo-activated systems are preferred in order to better mimic the light-driven activation in an ideal artificial device. Moreover, multi and mono metallic active sites in catalysts structure are considered, inspired by several efficient examples in literature. The work is mainly focused on water oxidation reaction, being still considered the bottleneck of artificial photosynthesis; however also preliminary studies on CO2 reduction have been examined. First, a Cobalt-based oxo cluster, [Co4(μ3-O)4(μ-O2CCH3)4(pyridine)4] has been studied as a molecular catalyst for water oxidation in a light activated system with Ru(bpy)32+ as photosensitizer and S2O82- as sacrificial donor. The species has been characterized through different analytic techniques and tuning electronic substituents properties, structure-activity correlations have been investigated by cyclic voltammetry and laser flash photolysis. Moreover, a synthetic approach to modify the structure of the species has been evaluated in order to design no covalent dyads between the catalyst and the photosensitizer exploiting π−π interactions. Other Cobalt-based species with high nuclearity and totally inorganic ligands (polyoxometalates, POMs) have been studied in water oxidation catalysis. In particular, complexes [Co9(H2O)6(OH)3(PW9O34)3]16-, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(SiW8O31)3}]5- and [{Co4(OH)3PO4}4(PW9O34)4]16- have been investigated with laser flash photolysis and in the photo-activated system. Interesting mechanistic insights have been reached thanks to the analysis of these species. Moreover, during the thesis work a novel single site Copper-based compound with a tetraazacyclotetradecane ligand has been proposed as water oxidation catalyst. In particular, the species has been characterized among the electrochemical system and the catalytic behaviour has been explored by means cyclic voltammetry, electrolysis and photoelectrochemical experiments. With the aim of the development of a sunlight activated water splitting device, for the first time in this thesis work a Copper molecular species has been examined in combination with light. Results seem to be preliminary interesting for further studies on azamacrocyclic Copper-based molecular species. Finally, dealing with the catalysis of CO2 reduction some studies have been performed with a POM-based complex, [Cu(SiW11O39)]6-. Cyclic voltammetry experiments have been run in order to evaluate the possible catalytic activity of the compound in CO2 reduction. The aim of this thesis work is to suggest a method to achieve a better understanding of the analysed topic through optimized experimental conditions and mechanistic insights.Il 21° secolo appare come un momento di enorme incertezza per il settore energetico: un’energia sicura, pulita, continua ed equamente distribuita risulta necessaria per la crescita economica e lo sviluppo della società umana. Riuscire a trovare un’adatta alternativa ai combustibili fossili costituisce una sfida affascinante per l’avanzamento scientifico. Considerando diverse possibilità, le risorse rinnovabili sembrano essere in grado di rispondere meglio alla richiesta energetica e fra queste, l’energia solare è sicuramente la più sfruttabile, però deve essere raccolta, convertita e conservata. Ispirandosi alla Natura, la fotosintesi artificiale è una soluzione in grado di convertire efficientemente l’energia derivante dalla luce solare in combustibili alternativi come idrogeno o altre forme ridotte di carbonio. Questo sistema artificiale presenta una struttura articolata di eventi, che terminano con reazioni di ossidoriduzione che necessitano un’efficiente catalisi. All’interno del panorama descritto, questo progetto di tesi è quindi focalizzato nello sviluppo di nuovi sistemi molecolari basati su metalli abbondanti sulla superficie terrestre in grado di catalizzare processi redox coinvolti nella fotosintesi artificiale. Lo studio di sistemi foto indotti è stato privilegiato, poiché si avvicina maggiormente all’ attivazione da parte della luce di un ideale sistema artificiale. Inoltre, ispirandosi ai numerosi esempi presenti in letteratura, i catalizzatori considerati sono basati su strutture con centri attivi sia multi che mono metallici. Il lavoro è maggiormente focalizzato sulla reazione di ossidazione dell’acqua, considerata ancora la problematica maggiore nel processo di fotosintesi artificiale, ma sono stati presi in considerazione anche studi preliminari per la catalisi della reazione di riduzione di CO2. Inizialmente, un osso cluster di Cobalto, [Co4(μ3-O)4(μ-O2CCH3)4(pyridine)4] è stato esaminato come catalizzatore molecolare in un sistema foto attivato con Ru(bpy)32+ come fotosensibilizzatore e S2O82- come donatore sacrificale. La specie è stata caratterizzata mediante diverse tecniche analitiche e variando le proprietà elettroniche dei sostituenti, correlazioni fra la struttura e l’attività sono state investigate con voltammetria ciclica e laser flash fotolisi. Inoltre, un approccio sintetico volto alla modifica strutturale del catalizzatore è stato valutato per progettare diadi non covalenti tra la specie stessa e il fotosensibilizzatore sfruttando interazioni π−π. Altre specie ad alta nuclearità, contenenti Cobalto e con leganti totalmente inorganici (poliossometallati, POMs) sono stati valutati per la catalisi di ossidazione dell’acqua. In particolare i complessi [Co9(H2O)6(OH)3(PW9O34)3]16-, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(SiW8O31)3}]5- e [{Co4(OH)3PO4}4(PW9O34)4]16- sono stati investigati nel sistema foto attivato e con laser flash fotolisi. Interessanti informazioni di meccanismo sono state ottenute grazie allo studio di questi composti. Inoltre, durante il lavoro di tesi un nuovo composto basato su un unico atomo di Rame e un legante tetraazaciclotetradecano è stato proposto come catalizzatore per ossidazione dell’acqua. In particolare, la specie è stata caratterizzata nel sistema elettrochimico e la sua attività catalitica è stata valutata mediante voltammetria ciclica, elettrolisi ed esperimenti fotoelettrochimici. Con lo sguardo volto allo sviluppo di un dispositivo per water splitting attivato dalla luce solare, in questa tesi per la prima volta è stata esaminata una specie molecolare di Rame in combinazione con la luce. I risultati ottenuti sembrano aprire la strada a nuove linee di ricerca legate a specie molecolari di Rame con leganti macrociclici azotati. Infine, per quanto riguarda la catalisi della reazione di riduzione di CO2, un complesso di Rame con legante POM è stato selezionato, [Cu(SiW11O39)]6-, ed esperimenti di voltammetria ciclica sono stati effettuati per valutarne l’attività catalitica. Questo lavoro di tesi si propone di indicare un metodo di lavoro per ottenere una migliore comprensione dell’argomento trattato, attraverso l’ottimizzazione delle condizioni sperimentali e approfondimenti riguardanti il meccanismo dei processi in esame

Fotosintesi artificiale: complessi molecolari di Cobalto come catalizzatori per l’ossidazione dell’acqua

Water oxidation still represents the bottleneck of artificial photosynthesis, that is light driven water splitting into hydrogen and oxygen. This work is focused on the investigation of molecular catalysts based on Cobalt, using a photoactivated system based on S2O82- as the sacrifical oxidant and Ru(bpy)32+ as the photosynthetizer. The advantage in using such species, stems from the abundance of Cobalt on Earth, its limited cost and its promising catalytic activity. This study analyses catalytic Oxygen production by several Cobalt based molecular species with totally inorganic ligands taken from the polyoxometalate pool, and other polinuclear complexes with organic ligands with different coordination geometries of the metal centres. Moreover, the activity of the hexaaquoion Co(H2O)62+ in the presence of different organic ligands added in situ was hexamined, with the aim of getting informations of the reaction mechanism

Cobalt based water oxidation catalysis with photogenerated Ru(bpy)⁠3⁠3+: Different kinetics and competent species starting from a molecular polyoxometalate and metal oxide nanoparticles capped with a bisphosphonate alendronate pendant

Polyoxometalates (POMs) are a class of inorganic compounds that have been extensively considered in the development of robust water oxidation catalysts (WOCs). In particular, cobalt based POMs have shown remarkable activity as WOCs under both dark and light activated cycles, although current studies focus on their possible evolution to different forms of cobalt oxides under operating conditions. In this paper, we investigate the activity in water oxidation catalysis of a heptanuclear cobalt based POM [{(B-α-PW⁠9O⁠34)Co⁠3(OH)(H⁠2O)⁠2(O⁠3PC(O)(C⁠3H⁠6NH⁠3)PO⁠3)}⁠2Co]⁠14−, hereafter Co⁠7(Ale)⁠14− and of 5 nm cobalt oxide nanoparticles, Co⁠3O⁠4(Ale), both carrying the same bisphosphonate alendronate ligand. Under the application of an anodic bias in borate buffer pH 8, both species lead to the formation of an active cobalt oxide layer at the working electrode, although in the case of Co⁠7(Ale)⁠14− participation of the POM scaffold in the electrodeposition is evidenced by the presence of W(VI) in the active film. In particular, Co⁠7(Ale)⁠14− and Co⁠3O⁠4(Ale) were investigated in a photocatalytic cycle with Ru(bpy)⁠3⁠2+ (bpy = 2,2′-bipyridine) as the photosensitizer and persulfate as the electron acceptor, where the photogenerated oxidant is Ru(bpy)⁠3⁠3+. Key findings are: (i) ion pairing and aggregate formation of anionic Co⁠7(Ale)⁠14− and Co⁠3O⁠4(Ale) with cationic Ru(bpy)⁠3⁠2+ are limited under photocatalytic regime (20 mM aqueous buffer, pH 8), as confirmed by the solubility product constant K⁠SP = 2.1 ± 0.4 × 10⁠−15 M⁠4 for a Co⁠7(Ale)⁠14−:Ru(bpy)⁠3⁠2+ 1:3 adduct, emission quenching experiments, and bimolecular electron transfer (ET) kinetics; (ii) fast ET to Ru(bpy)⁠3⁠3+, approaching diffusional rates, occurs from Co⁠7(Ale)⁠14− (k⁠ET = 9.4 ± 0.3 × 10⁠8 M⁠−1 s⁠−1) overarching by four orders of magnitude the Co⁠3O⁠4(Ale) nanoparticles; (iii) Co⁠7(Ale)⁠14− enables up to 18 photoinduced ET events in 90 ms, while in the same timeframe, a non-complete bleaching recovery (ET events < 1) results from the Co⁠3O⁠4(Ale) nanoparticle analogs; (iv) photocatalytic oxygen evolution is observed with similar rates for both cobalt based species (TOF per cobalt = 1.2–1.7 × 10⁠−3 s⁠−1); (v) dark O–O bond formation is the likely rate determining step of water oxidation for both systems, as the O⁠2 evolution kinetics do not depend on photoinduced ET rates and oxygenic activity, and light irradiation intensity (λ = 450 nm, 0.4–1.6 × 10⁠−7 einstein s⁠−1); (vi) leaching of Co(II) ions from Co⁠7(Ale)⁠14− in catalytic conditions is estimated below 2% by ⁠31P NMR line broadening experiments and supported by persistent flash photolysis kinetics; (vii) the ultimate fate of Co⁠7(Ale)⁠14− under oxygenic turnover involves a polyoxometalate structural rearrangement, while no evidence of cobalt oxide nanoparticles formation is provided by TEM images; (viii) conversely, Co⁠3O⁠4(Ale) nanoparticles preserve their size along the light driven cycle. Therefore, this study supports the involvement of POM based intermediates in water oxidation catalysis, while future perspectives should be directed toward the identification of such main characters, and of the mechanism of O⁠2 construction

Light-driven water oxidation with a molecular tetra-cobalt(iii) cubane cluster

Photoinduced water oxidation to molecular oxygen takes place in systems made of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as the photosensitizer, [Co4O4(O2CMe)4(py)4] (py = pyridine) as the molecular catalyst and Na2S2O8 as the sacrificial electron acceptor. The photochemical quantum yield of the process reaches the outstanding value of 30% and depends on pH and catalyst concentration. Transient absorption spectroscopy experiments aimed to clarify the first events of the photocatalytic process are also reported

"− Får jag vara med?" : – En kvalitativ studie om inkludering och exkludering inom barnens fria lek

Denna studie syftar till att undersöka fenomen som inkludering och exkludering inom lek i förskolan. Datainsamlingen skedde genom två kvalitativa metoder som bestod dels av pedagogers deltagande i intervjuer och etnografiska observationer på barn under den fria leken. Med hjälp av intervjuer undersöktes förskollärarnas och barnskötarnas uppfattning om och arbete i anknytning till inkludering och exkludering. Den etnografiska ansatsen bidrog till material angående barnens samspel och förhållningsätt i relation till lek. Det insamlade materialet har analyserats och diskuterats utifrån Vygotskijs teori och Corsaros begrepp inom barndomssociologi samt tidigare forskning.   Inkludering och exkludering är ett vanligt fenomen som ofta förekommer i olika kamratkulturer. I studiens resultat blir det synligt att barnen oftast använder sig av tillträdesstrategier för att få tillträde till lek, strategier som även observerats av Corsaro. I resultatet framkommer det att barnen förhandlar om sina statuspositioner för att göra sig själva och andra medvetna om sina möjligheter i den fria leken. Att ha en hög status inom kamratkulturen innebär att barnet har makt, vilket ger möjlighet till att kunna bestämma utifrån närhetsprincipen vilka som ska bli inkluderade samt exkluderade från gemenskap. Majoriteten av pedagogerna har en uppfattning om att exkludering av barn oftast sker i form av sekundär anpassning. Det vill säga att barnen visar att institutionens regler som alla-får-vara-med upprätthålls med vissa undantag i form av villkorat deltagande då barn blir tilldelade en oönskad eller underordnad roll i leken. Resultatet visar även på att pedagogernas närvaro kan ha påverkan på barnens förhandling i leken eftersom pedagogernas auktoritet värderas högre och därmed sker exkludering i mindre grad. Resultat visar också på att pedagogerna har en uppfattning om att språket har en betydelse i barnens lek och kan vara en faktor till varför barnen blir exkluderade från gemenskapen och lekar. I syfte att motverka exkludering och stärka barnens gemenskap och språkutveckling arrangeras olika uppstyrda lekar och samarbetsövningar, men även metoder som ”bra kompis”

Light driven water oxidation by a single site cobalt salophen catalyst

A salophen cobalt(II) complex enables water oxidation at neutral pH in photoactivated sacrificial cycles under visible light, thus confirming the high appeal of earth abundant single site catalysis for artificial photosynthesis