Στην παρούσα ερευνητική εργασία μελετάται η φωτοηλεκτροχημική διάσπαση δύο οργανικών ενώσεων: της χρωστικής Methylene Blue (MB), και της γλυκόζης. Η φωτοκαταλυτική διεργασία πραγματοποιήθηκε με φωτοβόληση υπεριώδους ακτινοβολίας UV (λάμπα Hg, 125 W). Για το σκοπό αυτό χρησιμοποιήθηκαν δύο φωτοκαταλύτες: η νανοκρυσταλλική τιτάνια (TiO2) και το οξείδιο του βολφραμίου (WO3). Η φωτοδιάσπαση των οργανικών ρύπων πραγματοποιήθηκε σε μία φωτοηλεκτροχημική κυψελίδα, η οποία αποτελείται από τα εξής μέρη: 1)Το ηλεκτρόδιο της ανόδου το οποίο φέρει το φωτοκαταλύτη. Στη φωτοάνοδο παράγονται τα ηλεκτρόνια και πραγματοποιόυνται οι αντιδράσεις οξείδωσης. 2)Το ηλεκτρόδιο της καθόδου, το οποίο φέρει τον ηλεκτροκαταλύτη, ο οποίος διευκολύνει τη μεταφορά των ηλεκτρονίων από το ηλεκτρόδιο στο διάλυμα. Στη φωτοκάθοδο πραγματοποιούνται αντιδράσεις αναγωγής. Ως ηλεκτροκαταλύτης χρησιμοποιήθηκε ο λευκόχρυσος ο οποίος είναι ευγενές μέταλλο. 3)Τον ηλεκτρολύτη, ο οποίος ρυθμίζει το pH του διαλύματος και αυξάνει την ιοντική αγωγιμότητα. Ο ηλεκτρολύτης που χρησιμοποιήθηκε είναι το καυστικό νάτριο (NaOH). Όταν ο φωτοκαταλύτης διεγείρεται με ακτινοβολία ενέργειας ίσης ή μεγαλύτερης του ενεργειακού του χάσματος, δημιουργούνται ζεύγη οπών-ηλεκτρονίων. Ένα μέρος των δημιουργούμενων ζευγών φορτίου επανασυνδέονται χάνοντας τη φωτεινή ενέργεια σε θερμότητα. Οι οπές οξειδώνουν το ρύπο απελευθερώνοντας ιόντα υδρογόνου. Τα ηλεκτρόνια ρέουν μέσω του εξωτερικού κυκλώματος στην κάθοδο, όπου αντιδρούν με τα ιόντα υδρογόνου, σχηματίζοντας είτε μοριακό υδρογόνο (υπό αναερόβιες συνθήκες), είτε νερό (υπό αερόβιες συνθήκες). Το πρώτο και σημαντικότερο στάδιο της φωτοκαταλυτικής διεργασίας είναι η προσρόφηση του ρύπου στην επιφάνεια του φωτοκαταλύτη. Ο μηχανισμός προσρόφησης γίνεται σύμφωνα με το κινητικό μοντέλο Langmuir-Hinshelwood. To TiO2 αποτελεί έναν από τους πιο διαδεδομένους και αποδοτικούς φωτοκαταλύτες. Τα υμένια TiO2 εναποτέθηκαν σε υπόστρωμα γυαλιού με τη μέδοδο Doctor Blade. Το WO3 είναι ένας εξίσου αποδοτικός φωτοκαταλύτης με το TiO2, αλλά λιγότερο δημοφιλής. Η εναπόθεση των υμενίων WO3 σε γυάλινα υποστρώματα έγινε με τη μέθοδο του ψεκασμού. Η παρασκευή του αντιηλεκτροδίου, δηλαδή του ηλεκτροδίου της καθόδου, έγινε με τη μέθοδο της ηλεκτροαπόθεσης. Το υψηλό pH του ηλεκτρολύτη (NaOH) είναι απαραίτητο για τη φωτοδιάσπαση οργανικών ενώσεων. Εκτός από την ιοντική αγωγιμότητα που προσφέρει στο φωτοηλεκτροχημικό κελί, επηρεάζει και την επιφανειακή φόρτιση του φωτοκαταλύτη. Η αύξηση της συγκέντρωσης του NaOH, δηλαδή η αύξηση των ιόντων ΟΗ-, είχε σαν αποτέλεσμα την αύξηση της σταθεράς ταχύτητας της φωτοκαταλυτικής αντίδρασης kapp και επομένως ταχύτερη φωτοδιάσπαση του ρύπου. Στα διαλύματα με τις μεγαλύτερες συγκεντρώσεις ηλεκτρολύτη ο χρόνος ημιζωής του ρύπου είναι πολύ μικρότερος συγκριτικά με τα διαλύματα χαμηλής συγκέντρωσης NaOH. Η μέτρηση της συγκέντρωσης των οργανικών ενώσεων στα υδατικά διαλύματα έγινε με φασματοφωτομετρία απορρόφησης ορατού-υπεριώδους (UV/vis). Οι φωτοαποικοδομούμενες ουσίες, λειτουργούν ως «θυσιαστήριες ενώσεις», αφού μειώνουν το ρυθμό επανασύνδεσης ηλεκτρονίων-οπών και επομένως συμβάλλουν στην αύξηση της απόδοσης του συστήματος, ενώ παράλληλα η διάσπασή τους προσφέρει τεράστιο περιβαλλοντικό όφελος. Από τις δύο οργανικές ενωσεις που μελετήθηκαν, μόνο η χρωστική Methylene Blue λειτούργησε αποτελεσματικά ως θυσιαστήρια ένωση. Αντιθέτως η γλυκόζη, που είναι πολύπλοκο μόριο, δεν κατάφερε να διασπαστεί. Τέλος έγινε σύγκριση της δραστικότητας των δύο φωτοκαταλυτών, κατά τη φωτοαποικοδόμηση των οργανικών ρύπων που μελετήθηκαν. Παρατηρήθηκε ότι και το TiO2 και το WO3 είναι εξίσου αποδοτικοί φωτοκαταλύτες. Σε αντίθεση με το TiO2, το WO3 δεν είναι ανθεκτικό στη φωτοδιάβρωση, καθιστώντας αδύνατη την επαναχρησιμοποίησή του σε περισσότερες από δύο φωτοκαταλυτικές διεργασίες. / The present study deals with the photo electrochemical degradation of two organic compounds, namely Methylene Blue and glucose. The photocatalytic process was carried out using UV radiation (Hg lamp, 125 W). The employed photo catalysts were titanium dioxide (TiO2) and tungsten oxide (WO3) deposited in the form of thin films on SnO2:F-coated glass. The photodecomposition of organic wastes was carried out in a photo electrochemical (PEC) cell with the following components: 1) The anode electrode which carries the photo catalyst. The photo anode produces electrodes and oxidation reactions take place there. 2) The cathode electrode, which carries the electro catalyst and facilitates the transfer of electrons from the cathode to the liquid phase. Reduction reactions take place at the cathode. In this study, a noble metal, Pt was used as electro catalyst. Thin films of Pt were obtained by electrodeposition on SnO2:F-coated glass slides. 3) The electrolyte which is added to adjust the Ph in order to increase the ionic conductivity. In this study, an aqueous solution of NaOH has been used as electrolyte. The operation of such a PEC cell is as follows: The absorption of photons by the photocatalyst leads to the creation of electron-hole pairs. The photodegradable substance is oxidized by the holes, liberating hydrogen ions in the aqueous solution. Electrons are transferred through the external circuit towards the cathode, where they reduce hydrogen ions producing hydrogen molecules (in the absence of oxygen). The initial step of the photoelectrocatalytic decomposition is the adsorption of the organic waste on the surface of the photocatalyst according to the Langmuir-Hinshelwood mechanism. TiO2 is among the most successful photo catalysts for heterogeneous photo catalytic degradation of organic wastes. Thin films of nano crystalline titania were deposited on glass substrates using the Doctor Blade method. Another wide band gap semiconductor with that can be used in heterogeneous photo catalysis is WO3. It was also tested and compared with TiO2. The pH value of the electrolyte (NaOH) was found to affect strongly the process. High pH values were required to obtain high OH- concentration because efficient hole scavenging and production of hydrogen radicals is ensured, especially when an organic sacrificial agent is added. As a consequence, the apparent rate constant kapp was increased with increasing NaOH concentrations. The photodegradable substances act like “sacrificial agents” preventing the recombination of electron-hole pairs, which is the main cause for low efficiencies. Of the two organic wastes that have been studied, only MB could be successfully degraded. Finally, the photo catalytic activity of TiO2 and WO3 was compared. Although both catalysts were equally efficient, WO3 is characterized by low stability. .

The preparation of visible light driven photocatalysts for photocatalytic water splitting has been achieved by a CO₂ free, low cost and simple novel method. Combination of peroxide based route with organic free solvent and titanium nitride, carbon free precursor and air and moisture stable, would be useful. Clear red-brown solution of titanium peroxo species was obtained by dissolution of TiN in H₂O₂ and HNO₃ acid at room temperature without stirring. The resultant red brown solution is then used as a titanium solution precursor for yellow amorphous and yellow crystalline TiO₂ synthesis. Visible light photoactivity of the samples was evaluated by photooxidation of methylene blue and photoreduction producing hydrogen from water splitting. The high surface area of yellow amorphous TiO₂ exhibits an interesting property of being both surface adsorbent and photoactive under visible light for photodecolourisation of aqueous solution of methylene blue. However, it might not appropriate for hydrogen production. Nanoparticulate yellow crystalline TiO₂ with defect disorder of Ti³⁺ and oxygen vacancies depending upon synthesis conditions has been characterised by ESR, XPS, CHN analysis and SQUID. Single phase rutile can be produced at low temperature. It is stable at high temperature and the red shift of absorption edge increases with the treatment temperature. Yellow crystalline TiO₂ exhibits an interesting property of being photoactive under visible light. The best photocatalytic performance was observed for 600°C calcination, probably reflecting a compromise between red shift and surface area with changing temperature. Moreover, overall water splitting into hydrogen and oxygen might be obtained by using this material even in air atmosphere. Photoactivity can be improved by testing under anaerobic atmosphere and/or adding sacrificial agent. Quantum efficiency under visible light is still low but comparable to other reports. The maximum efficiency varies from 0.03 % to 0.37 % for hydrogen production and from 0.03 % to 0.12 % for oxygen production, depending on photon energy and sacrificial agents.

The integration of photocatalytic advanced oxidation into solar disinfection is a robust method of improving the microbial and chemical quality of treated water. This study evaluates the performance of photocatalytic solar irradiated batch reactors through an analytical model that reduces treatment parameters by simplifying photoreactor geometry and relating performance to reactor configuration. Accompanying experiments compare the performance of titanium dioxide coated foams of varying pore size to suspended and fixed film configurations through degradation of organic dyes (acid orange 24 and methylene blue), Escherichia coli, and 1,4-dioxane. Results indicate that a catalyst immobilized on a foam support can match the performance of a suspension due to effective mass transport and association between analyte and foam. Additionally, the potential treatment capacity of solar photocatalysis was compared to conventional treatment methods. Results of this comparison stress the fundamental limitation of solar photocatalysis if visible light wavelengths are not harnessed.

The integration of photocatalytic advanced oxidation into solar disinfection is a robust method of improving the microbial and chemical quality of treated water. This study evaluates the performance of photocatalytic solar irradiated batch reactors through an analytical model that reduces treatment parameters by simplifying photoreactor geometry and relating performance to reactor configuration. Accompanying experiments compare the performance of titanium dioxide coated foams of varying pore size to suspended and fixed film configurations through degradation of organic dyes (acid orange 24 and methylene blue), Escherichia coli, and 1,4-dioxane. Results indicate that a catalyst immobilized on a foam support can match the performance of a suspension due to effective mass transport and association between analyte and foam. Additionally, the potential treatment capacity of solar photocatalysis was compared to conventional treatment methods. Results of this comparison stress the fundamental limitation of solar photocatalysis if visible light wavelengths are not harnessed.

The addition of benzyltrimethylsilane to maleic anhydride mediated by TiO2 photocatalysis was initially investigated. The affect of changing the catalyst, the radical trap loading and the substitution of the benzyltrimethylsilane molecule was assessed. Cyclisation precursors based on benzyltrimethylsilane were prepared, but were found not to cyclise via TiO2 photocatalysis. A number of other systems were assessed for their ability to cyclise under TiO2 photocatalysis; tertiary amines, aminomethyltrimethylsilanes, phenoxymethyltrimethylsilanes and phenoxyacetic acids. Phenoxymethyltrimethylsilane and phenoxyacetic acid were found to add effectively to maleic anhydride under TiO2 photocatalysis conditions, however they were unreactive with regards to cyclisation. EPR spectroscopy has been employed to characterise further the reaction of benzylsilanes with maleic anhydride under TiO2 photocatalytic conditions. A number of EPR active species were observed; trapped holes and electrons, which reside within the TiO2 catalyst. In addition, methyl and benzyl radicals were observed and were found to originate from the oxidation of the benzylsilanes by trapped holes in the TiO2 catalyst. However, no radical species were observed from the maleic anhydride. These observations had the following consequences for the currently proposed reaction mechanism for the addition of benzyltrimethylsilane with maleic anhydride under TiO2 photocatalysis.  The observation of the benzyl radical definitely proved that the reactive intermediate was indeed the proposed benzyl radical  The absence of any maleic anhydride EPR active species cast doubt on the role of maleic anhydride as an electron trap. Moreover when maleic anhydride is removed from the reaction system, interstitial Ti3+ species is absent from the EPR spectra, indicating that maleic anhydride is in fact acting as a hole trap.

An increasing global energy demand requires alternative fuel sources. A promising method is artificial photosynthesis. Although, the artificial processes are different from the natural photosynthetic process, the basic principles are the same, i.e. to split water and to convert solar energy into chemical energy. The energy is stored in bonds, which can at a later stage be released upon combustion. The bottleneck in the artificial systems is the water oxidation. The aim of this research has been to develop catalysts for water oxidation that are stable, yet efficient. The molecular catalysts are comprised of organic ligands that ultimately are responsible for the catalyst structure and activity. These ligands are often based on polypyridines or other nitrogen-containing aromatic compounds. This thesis describes the development of molecular ruthenium catalysts and the evaluation of their ability to mediate chemical and photochemical oxidation of water. Previous work from our group has shown that the introduction of negatively charged groups into the ligand frameworks lowers the redox potentials of the metal complexes. This is beneficial as it makes it possible to drive water oxidation with [Ru(bpy)3]3+-type oxidants (bpy = 2,2’-bipyridine), which can be photochemically generated from the corresponding [Ru(bpy)3]2+ complex. Hence, all the designed ligands herein contain negatively charged groups in the coordination site for ruthenium. The first part of this thesis describes the development of two mononuclear ruthenium complexes and the evaluation of these for water oxidation. Both complexes displayed low redox potentials, allowing for water oxidation to be driven either chemically or photochemically using the mild one-electron oxidant [Ru(bpy)3]3+. The second part is a structure–activity relationship study on several analogues of mononuclear ruthenium complexes. The complexes were active for water oxidation and the redox potentials of the analogues displayed a linear relationship with the Hammet σmeta parameter. It was also found that the complexes form high-valent Ru(VI) species, which are responsible for mediating O–O bond formation. The last part of the thesis describes the development of a dinuclear ruthenium complex and the catalytic performance for chemical and photochemical water oxidation. It was found that the complex undergoes O–O bond formation via a bridging peroxide intermediate, i.e. an I2M–type mechanism.

PhD. (Chemistry) / Please refer to full text to view abstract

The harvesting of solar light is one of the main challenges in science. The outstanding optical properties of plasmonic in the visible and near-infrared ranges due to the localized surface plasmon resonance (SPR) has emerged as a promising approach for the solar-tochemical energy conversion. Specifically, it has been demonstrated that the SPR excitation in the visible range in silver (Ag) and gold (Au) nanoparticles can drive and accelerate chemical transformations. This field, coined plasmonic catalysis, enables one to merge catalytic and optical properties in the nanoscale and use visible or near-infrared light as a sustainable energy input to accelerate molecular transformations. In the first part of this thesis. we developed Au branched nanostructures to be employed as plasmonic catalysts. In this case, we aimed at investigating the effect of the sharp tips at their surface over their plasmonic catalytic performance, as it is established that tips can concentrate higher electric field enhancements relative to rounded surfaces as a result of the lightning rod effect, which, in turn, can translate into higher plasmonic catalytic performances. Here, the plasmonic-catalytic performances were tested using the SPR mediated oxidation of paminothiophenol and benzylamine as model transformations. While the Ag and Au nanoparticles support LSPR excitation in the visible and near-infrared ranges, their catalytic properties are limited in terms of versatility. Conversely, metals that are important in catalysis, such as palladium Pd, do not support SPR excitation in the visible or near-infrared range. In the second part of this thesis, we developed multimetallic nanoparticle morphologies, composed of both Au and Pd, that enabled us to marry catalytic and plasmonic component in order to address this challenge. We focused on plasmonic core-catalytic shell structures, in which the shell displayed a branched morphology. Parameters such as shell thickness could be controlled, and structure performance relationships were established towards the methanol electro-oxidation under plasmonic excitation. / O aproveitamento da luz solar é um dos principais desafios da ciência. As excepcionais propriedades óticas plasmônicas nas regiões do visível e do infravermelho próximo, devido a ressonância plasmônica de superfície localizada (SPR), surgiram como uma abordagem promissora para conversão de energia solar em energia química. De maneira mais específica, vem sendo demonstrado que a excitação SPR na região do visível em nanopartículas de prata (Ag) e ouro (Au) podem conduzir e acelerar transformações químicas. Esse campo, chamado catálise plasmônica, permite a fusão de propriedades óticas e catalíticas na nanoescala e a utilização de luz visível ou infravermelha próxima como uma fonte de energia para acelerar transformações moleculares. Na primeira parte desta dissertação, nós desenvolvemos nanoestruturas de ouro ramificadas para serem empregadas em catálise plasmônica. Neste caso, nosso foco era investigar o efeito de pontas afiadas em sua superfície sobre seu desempenho catalítico plasmônico, visto que está bem estabelecido que pontas podem concentrar maiores intensificações de campo elétrico em relação a superfícies arredondadas como resultado do \"efeito para-raios\" o que, por sua vez, pode se traduzir em maiores desempenhos em catálise plasmônica. O desempenho da catálise plasmônica foi testado através da oxidação mediada por SPR do p-aminotiofenol e da benzilamina como reações modelo. Contudo, enquanto nanopartículas de prata e ouro apresentam excitação SPR nas regiões do visível e infravermelho próximo, suas propriedades catalíticas são limitadas em termos de versatilidade. Por outro lado, metais que são importantes em catálise, como o paládio, não apresentam excitação SPR no visível e infravermelho próximo. Por isso, na segunda parte desta dissertação, nós desenvolvemos nanopartículas multimetálicas, compostas de Au e Pd, que nos permitem unir os componentes catalíticos e plasmônicos com o objetivo de enfrentar este desafio. Nós focamos em estruturas do tipo core-shell, com núcleos plasmônicos e cascas catalíticas, na qual a casca apresenta morfologia ramificada. Paramêtros como a espessura da casca puderam ser controlados, e a relação estruturaperformance foi estabelecida através da eletro-oxidação do metanol sobre excitação plasmônica.

Photocatalysis is a process to convert light energy into chemical energies. This advanced process has been extensively applied in different areas, such as water splitting to evolve hydrogen, organic/ inorganic pollutants decomposition, artificial photosynthesis (CO2 reduction), disinfection, heavy metal recovery, organic synthesis and nitrogen fixation (reduction). The difficulty for photocatalysis applied in practical is primarily due to the low quantum yield as for the high recombination of photogenerated charge carriers. Various strategies have been implemented to overcome these challenges. As recently developed advanced materials, two dimensional materials have attracted lots of attentions as for their superiorities such as large specific surface area and high conductivity. These advantages for two dimensional materials make them be promising cocatalysts in enhance catalytic activity. In this thesis, various two dimensional materials (such as MoS2, SnS, BN as well as C3N4) other than graphene were prepared and investigated in the promotion of photocatalytic activity. Specifically, the focus of present work is on two dimensional materials enhanced photocatalysis in environmental remediation, including organic pollutants detoxification as well as bacteria inactivation. It was found that two dimensional materials, including MoS2, SnS, BN, may be excellent candidates as cocatalysts to enhanced visible-light-driven photocatalytic activity. And g-C3N4 as an effective photocatalyst exhibited excellent photocatalytic oxidation activity, and its activity can be further enhanced with surface modification by hydroxyl functional groups (a modification method reported in the thesis). Suggestions for future work were also proposed in this thesis.

Orientador: Leinig Antonio Perazolli / Banca: Marcos Augusto de Lima Nobre / Banca: Rossano Gimenes / Resumo: Neste projeto foram obtidos pós fotocatalíticos de TiO2/SnO2 modificados com Ag2O, utilizando o método dos precursores poliméricos (Pechini). A composição dos óxidos foi TiO2(75%)/SnO2(25%) e o agente modificador Ag2O nas concentrações: 0,01, 0,05, 0,10, 0,50 e 1,00 mol%. Estes óxidos foram analisados utilizando difratometria de Raios X (análise da cristalinidade), área superficial (SBET - análise de area superficial), FEG/MEV (análise da morfologia das partículas), XPS (análise das interações atômicas na superfície) e espectroscopia de refletância difusa (no ultravioleta-visível - UV-Vis - análise do "band-gap" dos óxidos). As atividades fotocatalíticas foram medidas, utilizando um reator montado em nossos laboratórios, realizando a fotodecomposição de soluções de Rodamina B, catalisada pelos óxidos em estudo, em um estudo da descoloração proporcional que foi medida em um espectrofotômetro no comprimento de onda de 300 a 700nm. Os estudos de atividade fotocatalítica foram realizados tanto em lâmpada UV como na luz solar e os resultados foram semelhantes, ou seja, obteve-se um óxido com o mesmo comportamento na luz solar. Estes estudos foram realizados para comparar a mudança na atividade fotocatalítica ao variar a concentração do agente modificador (Ag). Verificou-se que durante o processo de calcinação surgem aglomerados nos óxidos fotocatalisadores, que podem degenerar o efeito da fotoatividade. Obteve-se pós com fotoatividade catalítica, que apresentaram degradação da Rodamina B em 11 minutos, valor próximo aos 9 minutos obtidos para a degradação utilizando P25 (DEGUSSA), que é um óxido comercial amplamente utilizado e estudado / Abstract: In this project, photocatalytical TiO2/SnO2 powders modified with Ag2O were obtained by the polymeric precursor method (Pechini). It were obtained to result in a final oxide composition of: TiO2(75%)/SnO2(25%) and the modifying agent (Ag2O) concentration: 0.01, 0.05, 0.10, 0.50 and 1.00 mol%. This powders were analyzed using X-Ray diffractometry (to analyze the phases formed), superficial area (SBET), by FEG/SEM (to analyze the superficial characteristics), XPS (to analyze the superficial atomic interaction) and diffuse reflectance spectroscopy (on ultraviolet visible - UV-Vis - to analyze the powders band-gap). The photocatalytic activities were measured, using a homemade reactor, during the decomposition of Rodhamine B (RB) solutions, in a study of the proportional discoloration measured in a scanning spectrophotometer in the wave length 300 to 700nm. The RB decomposition studies were carried on both UV-lamp and sun-light and the results were similar, this means that were obtained an oxide with the same behavior on sun-light. This was made to compare the photocatalytic activity when varying the modifier agent concentration (Ag). It were also verified that during the calcination process that some agglomerates evolves and that can degenerate the photocatalytical effect. It were obtained photocatalytical powders, that degenerate RB in 11 minutes, close to the 9 minutes obtained by degradation using P25 (DEGUSSA) that is a commercial oxide widely used and studied / Mestre