Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 92-94). Also available in electronic version.

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Σκοπός της παρούσας εργασίας είναι η κινητική μελέτη της αντίδρασης παραγωγής υδρογόνου μέσω φωτοκαταλυτικής αναμόρφωσης της μεθανόλης και η ανάπτυξη μοντέλου για την περιγραφή του πεδίου της ακτινοβολίας στον πειραματικό φωτοαντιδραστήρα. Τα αποτελέσματα μπορούν να χρησιμοποιηθούν για τον υπολογισμό των κινητικών παραμέτρων της αντίδρασης και τον προσδιορισμό των σχεδιαστικών παραμέτρων που απαιτούνται για την ανάπτυξη και βελτιστοποίηση κατάλληλου φωτοαντιδραστήρα. Η φωτοκαταλυτική διάσπαση του νερού με χρήση ημιαγωγών και ηλιακής ακτινοβολίας αποτελεί μια από τις πλέον υποσχόμενες διεργασίες για τη φωτοχημική μετατροπή και αποθήκευση της ηλιακής ενέργειας. Η αντίδραση μπορεί να λάβει χώρα μέσω διέγερσης ενός ημιαγωγού (π.χ. TiO2) από φωτόνια με ενέργεια ίση ή μεγαλύτερη από το ενεργειακό του χάσμα. Το αποτέλεσμα είναι η προώθηση ενός ηλεκτρονίου από τη ζώνη σθένους (VB) στη ζώνη αγωγιμότητας (CB) του υλικού και η δημιουργία μιας οπής στην ζώνη αγωγιμότητας: (1) Η συνήθης τύχη των φωτοπαραγόμενων φορέων φορτίου είναι η (μη επιθυμητή) επανασύνδεσή τους, που συνοδεύεται από έκλυση της αποθηκευμένης ενέργειας με τη μορφή θερμότητας: (2) Οι φωτοπαραγόμενες οπές και τα ηλεκτρόνια που καταφέρνουν να φθάσουν στην επιφάνεια του ημιαγωγού μπορούν, υπό ορισμένες προϋποθέσεις, να εκκινήσουν αντιδράσεις για την παραγωγή οξυγόνου και υδρογόνου μέσω οξείδωσης και αναγωγής του νερού, αντίστοιχα: (3) (4) Το πρόβλημα είναι ότι ο ρυθμός παραγωγής υδρογόνου είναι πολύ μικρός, κυρίως λόγω της εγγενώς μικρής κβαντικής απόδοσης της διεργασίας, η οποία καθορίζεται από την αντίδραση επανασύνδεσης ηλεκτρονίου-οπής (Εξ.2). Η αντίδραση επανασύνδεσης μπορεί να κατασταλεί παρουσία κατάλληλων “θυσιαζόμενων” ενώσεων στο διάλυμα, οι οποίες αντιδρούν ταχέως και μη αντιστρεπτά με τις φωτοπαραγόμενες οπές. Με τον τρόπο αυτό αυξάνεται ο χρόνος ζωής των ηλεκτρονίων και, επομένως, ο ρυθμός παραγωγής υδρογόνου μέσω της Εξ. 4. Ως θυσιαζόμενες ενώσεις μπορούν να χρησιμοποιηθούν χαμηλού ή “αρνητικού” κόστους οργανικές ενώσεις, όπως παραπροϊόντα και παράγωγα βιομάζας. Οι ενώσεις αυτές οξειδώνονται προοδευτικά από τις οπές προς CO2, με αποτέλεσμα τα φωτοπαραγόμενα ηλεκτρόνια να ανάγουν αποδοτικά το νερό προς παραγωγή Η2. Η συνολική διεργασία μπορεί να περιγραφεί από την ακόλουθη γενική αντίδραση αναμόρφωσης: (5) Χαρακτηριστικά πλεονεκτήματα της μεθόδου αποτελούν ο σχετικά υψηλός ρυθμός παραγωγής υδρογόνου και το γεγονός ότι, σε αντίθεση με τις συνήθεις θερμοκαταλυτικές αντιδράσεις αναμόρφωσης, η αντίδραση πραγματοποιείται σε συνθήκες περιβάλλοντος. Επιπροσθέτως, η παραγωγή υδρογόνου μπορεί να λάβει χώρα με ταυτόχρονη αποικοδόμηση οργανικών ρύπων, με προφανή περιβαλλοντικά οφέλη. Ένα άλλο σημαντικό πρόβλημα που σχετίζεται με τις περιορισμένες εφαρμογές των φωτοκαταλυτικών μεθόδων σε πιλοτική και βιομηχανική κλίμακα οφείλεται στη δυσκολία σχεδιασμού και ανάπτυξης αποδοτικών φωτοαντιδραστήρων. Το πρόβλημα του σχεδιασμού έγκειται στο γεγονός ότι, σε αντίθεση με τους συμβατικούς καταλύτες, η ενεργοποίηση ενός φωτοκαταλύτη δε γίνεται θερμικά αλλά μέσω απορρόφησης φωτονίων κατάλληλης ενέργειας. Επομένως, για την μοντελοποίηση ενός φωτοαντιδραστήρα απαιτείται, εκτός από τη χρήση των συνήθων εξισώσεων για τα ισοζύγια μάζας, θερμότητας και ορμής, μια επιπλέον εξίσωση για την περιγραφή του ισοζυγίου της ενέργειας της ακτινοβολίας στο σύστημα. Η εξίσωση αυτή χρησιμοποιείται για τον υπολογισμό του “τοπικού ογκομετρικού ρυθμού απορρόφησης ενέργειας” (local volumetric rate of energy absorption, LVREA), ο οποίος αποτελεί μια από τις σημαντικότερες σχεδιαστικές παραμέτρους ενός φωτοαντιδραστήρα διότι περιγράφει την ποσότητα των φωτονίων που απορροφούνται ανά μονάδα όγκου σε κάθε σημείο του αντιδραστήρα. Για τον σχεδιασμό του αντιδραστήρα είναι επίσης απαραίτητη και μία έκφραση του ρυθμού της αντίδρασης. Για την εξαγωγή αυτής της έκφρασης απαιτείται η εύρεση του ρυθμού του βήματος ενεργοποίησης μέσω ακτινοβολίας, ο οποίος εκφράζεται συναρτήσει του LVREA. Εφόσον ο ρυθμός αυτός είναι γνωστός μπορεί να εισαχθεί στο κινητικό μοντέλο της αντίδρασης ενώ οι διάφορες κινητικές παράμετροι μπορούν να υπολογιστούν πειραματικά. Μεταξύ των προσεγγίσεων που έχουν προταθεί για τον υπολογισμό του LVRΕA, οι πιο ακριβείς περιλαμβάνουν την αριθμητική επίλυση της εξίσωσης μεταφοράς ακτινοβολίας (radiation transfer equation, RTE). Στην παρούσα εργασία χρησιμοποιείται η μέθοδος των “φασματικών στοιχείων” (spectral elements) για την επίλυση ενός μονοδιάστατου μοντέλου για την περιγραφή του πεδίου της ακτινοβολίας και τον υπολογισμό του LVREA σε έναν πειραματικό αντιδραστήρα, στον οποίο περιέχεται ο φωτοκαταλύτης σε μορφή αιωρήματος. Η αντίδραση που μελετάται είναι η παραγωγή υδρογόνου μέσω της φωτοκαταλυτικής αναμόρφωσης της μεθανόλης (Εξ. 6) σε αιώρημα καταλύτη 0.5%Pt/TiO2, το οποίο ακτινοβολείται με φως στη περιοχή που απορροφά το TiO2. (6) Σύμφωνα με το μοντέλο που αναπτύχθηκε, ο ρυθμός της φωτοκαταλυτικής αντίδρασης εξαρτάται από τη συγκέντρωση του καταλύτη στο αιώρημα, την ειδική ένταση ακτινοβολίας και τη συγκέντρωση του αντιδρώντος στο διάλυμα. Για τον σκοπό αυτό, μελετήθηκε στην παρούσα εργασία η επίδραση των λειτουργικών παραμέτρων της αντίδρασης, όπως η ένταση της προσπίπτουσας ακτινοβολίας (Ι0), η συγκέντρωση του φωτοκαταλύτη (Ccat) και η συγκέντρωση της μεθανόλης, (CMeOH) στο ρυθμό παραγωγής Η2 (rH2). Από τα αποτελέσματα προκύπτει ότι ο ρυθμός παραγωγής υδρογόνου εξαρτάται ισχυρά από τη συγκέντρωση του οργανικού υποστρώματος και αυξάνει κατά σχεδόν δύο τάξεις μεγέθους με αύξηση της CMeOH από 0 σε 1 mol L-1. Επιπλέον, αύξηση του ρυθμού επιτυγχάνεται με αύξηση του Ι0. Τα αποτελέσματα των φωτοκαταλυτικών πειραμάτων μπορούν να χρησιμοποιηθούν για τη μοντελοποίηση του συστήματος και το σχεδιασμό φωτοκαταλυτικού αντιδραστήρα για την παραγωγή υδρογόνου. / Heterogeneous photocatalytic reactions occurring at the surface of illuminated semiconductors, especially TiO2, have been the subject of extensive investigation in the last few years. This is because of the high potential of photocatalytic processes for a wide range of applications, which include mineralization of organic pollutants, disinfection of water and air, production of renewable fuels, and organic syntheses. Although remarkable progress has been made in fundamental research, applications in pilot and industrial scale are still in their infancy. This is mainly due to the lack of efficient solar photocatalysts and the difficulty of designing photoreactors able to integrate maximum light efficiency and mass transfer within one piece of equipment. Regarding photoreactor design, complications arise from the mode of photocatalyst activation, which involves excitation of the semiconductor photocatalyst by photons of appropriate energy. Thus, in addition to the usual equations for mass, heat and momentum balances, photoreactor modelling requires an additional equation to describe the balance of radiation energy in the system. This equation is used to calculate the "local volumetric rate of energy absorption" (LVREA) which describes the amount of photons absorbed per unit volume at each point of the reactor and provides one of the major photoreactor design parameters. The LVREA depends on the characteristics of the incident radiation, the optical properties of the system, the type and concentration of the photocatalyst and the geometry of the reactor. Therefore, calculation of the LVREA requires knowledge of the distribution of the radiation field inside the reactor. Among the various approaches proposed to calculate the LVREA, the most accurate ones are those that solve numerically the “radiation transfer equation” (RTE). This requires the development of a mathematical model that describes the emission model of the radiation source and the radiation field inside the reactor. In the present work, we have developed a one-dimensional spectral element algorithm for the description of the radiation field and the calculation of the LVREA in an experimental photoreactor containing the photocatalyst (Pt/TiO2) in suspension. The target reaction investigated was the photocatalytic reforming of methanol for hydrogen production (CH3OH+H2O→3H2+CO2). The radiation source used was a light emitting diode (LED), which emits radiation at wavelengths (λmax=390 nm) corresponding to the bandgap of TiO2 (3.2 eV). Our results refer to the effect of operating parameters such as incident light intensity (I0), photocatalyst content (CTiO2), and methanol concentration (CMeOH) on the rate of H2 production (rH2). They show that rH2 depends strongly on methanol concentration and increases by almost 2 orders of magnitude when CMeOH is increased from 0 to 1 mol L-1. A substantial enhancement of rH2 is also observed with increasing I0 or CTiO2. Results of photocatalytic experiments and photoreactor modelling are used to extract kinetic parameters for the methanol photoreforming reaction.

Orientador: Maria Valnice Boldrin Zanoni / Banca: Maria Aparecida Zaghete Bertochi / Banca: Germano Tremiliosi Filho / Resumo: A produção de etanol e açúcar a partir de cana-de-açúcar é um atividade de grande importância no Brasil, mas também fonte de grande preocupação ambiental devido ao grande acúmulo de biomassa na forma de bagaço e vinhaça gerados como rejeitos. A biomassa lignocelulósica é constituída de celulose, com estrutura formada de unidades de glicose polimerizadas, deste modo estudos voltados para a conversão de glicose em compostos químicos com maior valor agregado seria relevante para buscar novas formas de aproveitamento destes rejeitos. O objetivo do presente trabalho é investigar a aplicabilidade da técnica de fotoeletrocatálise para promover a oxidação de glicose em meio aquoso utilizando nanotubos de dióxido de titânio (NT-Ti/TiO2) como fotoanodos com vistas a obtenção de compostos com maior valor agregado. Os eletrodos de nanotubos de dióxido de titânio (NT-Ti/TiO2) foram preparados por oxidação eletroquímica de Ti em solução 1,0 mol L-1 de NaH2PO4 + 0,3% (m/m) HF como eletrólito de suporte sob potencial de 20V por 2 h e calcinado a 450 °C. Um método analitico baseado em cromatografia de troca iônica com detecção amperométrica foi desenvolvido para análise de glicose, acido glicônico, acido glicárico, arabitol e arabinose e curvas analíticas foram construídas em concentrações de 1,00 a 20,0x10-6 mol L-1, r = 0,9995, e o método alcançou limites de detecção e determinação ao redor de LD = 0,03x10-6 mol L-1 e LQ = 0,08x10-6 mol L-1 para a determinação de glicose. O ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The production of sugar and ethanol from sugarcane is an activity very important in Brazil, but it is also a source of great environmental concern due to the large accumulation of biomass in form of bagasse and vinasse generated as waste of this process. The lignocellulosic biomass is made up of cellulose, wich structure is made of polymerized glucose units. Thereby studies focused on the conversion of glucose into added value chemical compounds would be relevant to seek new ways to use these waste. This work describes the construction electrodes of titanium dioxide nanotubes (NT-Ti / TiO2) and its application in photoeletrocatalytic oxidation of glucose in aqueous media aimed at converting glucose into compounds with higher added value. The electrodes were prepared by electrochemical oxidation of Ti in solution of 1.0 mol L-1 NaH2PO4 + 0.3% (m/m) HF as supporting electrolyte under a potential of 20V for 2 h and calcined at 450 ° C. An analytical method based on ion exchange chromatography with amperometric detection was developed for glucose, gluconic acid, glucarico acid, arabitol and Arabinose analysis and analytical curves were constructed at concentrations of 1.00 to 20,0x10-5 mol L-1, r = 0.9995, and the method achieved detection limits and determination around 0,03x10-6 mol L-1 and 0,08x10-6 mol L-1 for glucose determination, respectively. The method was used for monitoring the generated products due the oxidation photoeletrocatalytic 10.0 mmol L-1 glucose solution und... (Complete abstract click electronic access below) / Mestre

The emergence of unique or enhanced physical, chemical and optical material properties at the nanoscale underlies the swift rise of nanomaterials science over recent decades. Within this interdisciplinary field, catalysis performed by nanomaterials (i.e. nanocatalysis) is one area where differences between nanoscale and bulk material properties offer particularly attractive opportunities for application. The consequent pursuit of viable nanomaterials with unprecedented catalytic activity has inevitably expanded across the periodic table, whereby a number of highly efficient precious metal, metal oxide and composite nanostructured catalysts have been developed for a wide range of synthetic organic and inorganic transformations. The lanthanide series has not been excluded from this search, but is still underrepresented in catalysis despite some rich chemistry and reactivity which sets these elements apart from many other metals. More recently however, the necessary paradigm shift away from commonly utilized but expensive, potentially toxic precious metal catalysts, and toward more sustainable alternatives, has seen an upsurge in the development of novel nanomaterials for heterogeneous catalysis: the general topic of this doctoral thesis. Heterogeneous nanocatalysis offers distinct advantages over homogeneous catalysis. Catalyst recyclability, ease of separation from reaction mixtures, and minimal product contamination all contribute to the higher overall effectiveness of heterogeneous catalysts relative to their homogeneous counterparts. The use of light as an abundant reagent, both in nanomaterial fabrication and for photocatalysis, is another attractive prospect. This dissertation addresses both points, describing the iterative development and application of photochemically-prepared samarium oxide based nanomaterials for heterogeneous catalysis and photocatalysis. Through a series of related peer-reviewed publications and associated commentary, the evolution of the application-driven design of a nanomaterial which is both efficient and effective for a diversity of heterogeneous catalytic and photocatalytic transformations is presented. Major findings include 1) both colloidal and supported samarium oxide nanoparticles can be prepared photochemically and comprise primarily Sm2O3 but may contain localized mixed valences or dynamic surface oxidation states; 2) colloidal samarium oxide nanoparticles possess high activity for Brønsted acid and oxidative catalysis, but recyclability and overall effectiveness is less than optimal due to a combination of polydispersity and size-dependent catalytic activity; 3) a similarly-prepared “second generation” samarium oxide/titanium dioxide nanocomposite presented several advantages over its predecessor, performing highly efficient and effective pure heterogeneous, dual photoredox-Lewis acid catalysis in two different types of synthetically relevant photocyclizations. Effects of different nanoparticle supports, rare insights into the catalytic mechanisms and behaviour of these nanomaterials‒obtained at the single molecule level by innovative application of Total Internal Reflection Fluorescence Microscopy (TIRFM) to catalysis research‒as well as advances in TIRFM data analysis protocols, are also discussed.

abstract: Intimate coupling of Ti2 photocatalysis and biodegradation (ICPB) offers potential for degrading biorecalcitrant and toxic organic compounds much better than possible with conventional wastewater treatments. This study reports on using a novel sponge-type, Ti2-coated biofilm carrier that shows significant adherence of Ti2 to its exterior and the ability to accumulate biomass in its interior (protected from UV light and free radicals). First, this carrier was tested for ICPB in a continuous-flow photocatalytic circulating-bed biofilm reactor (PCBBR) to mineralize biorecalcitrant organic: 2,4,5-trichlorophenol (TCP). Four mechanisms possibly acting of ICPB were tested separately: TCP adsorption, UV photolysis/photocatalysis, and biodegradation. The carrier exhibited strong TCP adsorption, while photolysis was negligible. Photocatalysis produced TCP-degradation products that could be mineralized and the strong adsorption of TCP to the carrier enhanced biodegradation by relieving toxicity. Validating the ICPB concept, biofilm was protected inside the carriers from UV light and free radicals. ICPB significantly lowered the diversity of the bacterial community, but five genera known to biodegrade chlorinated phenols were markedly enriched. Secondly, decolorization and mineralization of reactive dyes by ICPB were investigated on a refined Ti2-coated biofilm carrier in a PCBBR. Two typical reactive dyes: Reactive Black 5 (RB5) and Reactive Yellow 86 (RY86), showed similar first-order kinetics when being photocatalytically decolorized at low pH (~4-5), which was inhibited at neutral pH in the presence of phosphate or carbonate buffer, presumably due to electrostatic repulsion from negatively charged surface sites on Ti2, radical scavenging by phosphate or carbonate, or both. In the PCBBR, photocatalysis alone with Ti2-coated carriers could remove RB5 and COD by 97% and 47%, respectively. Addition of biofilm inside macroporous carriers maintained a similar RB5 removal efficiency, but COD removal increased to 65%, which is evidence of ICPB despite the low pH. A proposed ICPB pathway for RB5 suggests that a major intermediate, a naphthol derivative, was responsible for most of the residual COD. Finally, three low-temperature sintering methods, called O, D and DN, were compared based on photocatalytic efficiency and Ti2 adherence. The DN method had the best Ti2-coating properties and was a successful carrier for ICPB of RB5 in a PCBBR. / Dissertation/Thesis / M.S. Biological Design 2011

Rapid depletion of fossil fuel stock with a simultaneous rise in greenhouse gas emissions has led to an increase in the need for alternative energy. Cellulose based biofuels, especially bioethanol is a form of alternative energy that has the potential to replace petrol. The first step in cellulosic bioethanol production is the release of fermentable sugars via pre-treatment. Conventionally, physico-chemical and biological pre-treatment methods are energy intensive, environmentally unfavourable and expensive. This study, however reports on the use of a less energy consuming, cheap and environmental friendly alternative; photocatalysis, to produce fermentable sugars from cellulose. To achieve this, a range of photocatalysts were first screened based on their OH radical production rates using coumarin as a probe. TiO2 P25 was the photocatalyst that was found to have the highest OH radical production rate of 35.6 μM/hr, followed by Pt-C3N4 (0.88 μM/hr) and WO3 (0.28 μM/hr). LaCr-SrTiO3, Cr-SrTiO3 and yellow TiO2 did not produce any OH radicals due to their unsuitable electronic structure. P25 was further used for photocatalytic fermentable sugar production from cellulose. Photocatalytic cellulose I breakdown produced 0.04 % fermentable sugars whereas, with cellulose II feedstock the yield increased to 0.2 %. To further improve the yield, membrane bags were deployed which improved the sugar yields to 0.43 % and 0.71 % respectively from cellulose and cellulose II feedstocks. Photonic efficiencies followed the same trends as the sugar yields. Engineering design was further opted to enhance the sugar yields and hence a stacked frame photocatalytic reactor (SFPR) was designed. Various mixer configurations were designed and their mixing regime was determined using COMSOL Multiphysics 5.1 simulations. Amongst the mixers simulated, an 8-blade Rushton impeller was found to be the best configuration due its superior radial mixing profile and higher fluid velocity. The SFPR was then fabricated and operated with the impeller or a plus shaped magnetic bar as the mixer and the sugar yields were determined. Highest sugar yield and photonic efficiency was obtained from the cellulose II-impeller setup and was calculated to be 2.61 % and 9.45 % respectively. Respective lowest yields were obtained with cellulose I-stirrer bar setup and calculated to be 1.71 % and 5.64 %. Furthermore, the effect of H2O2 on fermentable sugar production was also tested. The cellulose II-stirrer bar configuration yielded 3.15 % fermentable sugars with the addition of 0.01 wt% H2O2 to the reaction mixture. The yield improved significantly to 14.1 % when the concentration of H2O2 was increased to 0.1 wt%.

O presente trabalho tem o objetivo de investigar a obtenção de hidrogênio a partir de soluções de etanol em água, por fotocatálise, usando-se catalisadores a base de óxido de zinco (ZnO). Nestes estudos foram empregados cinco catalisadores ZnO, sendo um comercial e os demais preparados através de diferentes metodologias encontradas na literatura. Os catalisadores foram caracterizados por área BET, DRX e FRX, e a investigação preliminar da atividade destes catalisadores foi feita através de ensaios de degradação fotocatalítica de rodamina B em reator slurry em batelada, onde foram avaliadas a taxa de reação e a remoção de corante. Os ensaios para a produção fotocatalítica de hidrogênio foram realizados em um reator de quartzo, operado em batelada com catalisador em suspensão e atmosfera inerte de nitrogênio. A solução foi irradiada por uma série de seis lâmpadas compactas de luz negra. Ao longo dos testes, amostras das fases líquida e gasosa foram coletadas e analisadas para identificação do consumo de etanol e produção de hidrogênio usando-se, respectivamente, Carbono Orgânico Total (TOC) e Cromatografia Gasosa (GC). Resultados preliminares mostraram que os catalisadores ZnO comercial e sintetizado (ZnO Merck e ZnO-B) apresentam atividade fotocatalítica e desempenho similares aos do TiO2 para a degradação da rodamina B. Contudo, estes mesmos catalisadores mostraram-se pouco ativos para a produção fotocatalítica de hidrogênio, com desempenho bastante inferior ao do TiO2 nas mesmas condições. Foi possível observar que o maior rendimento em hidrogênio ocorre para baixas concentrações de catalisador (0,05 gL[elevado a potência menos]1) e elevadas concentrações de etanol, sendo pouco dependente do pH. / This work aims to investigate the hydrogen production from ethanol-water solutions through photocatalysis, using zinc oxide catalysts (ZnO). Five ZnO catalysts were employed in this work; one was a commercial catalyst, while the others were prepared according to different methodologies reported in the literature. The catalysts were characterized by BET, XRD and XRF, and the preliminary investigation of their activity was done by photocatalytic degradation of rhodamine B, through the evaluation of the reaction rate and dye removal. Tests for photocatalytic hydrogen production were carried out in a quartz slurry batch reactor under nitrogen, irradiated by a set of six compact UV light bulbs. During the tests, gas and liquid samples were collected and analyzed in order to identify the consumption of ethanol and hydrogen production using, respectively, Total Organic Carbon (TOC) and Gas Chromatograph (GC). Preliminary results showed that the synthesized and commercial ZnO catalysts (ZnO-B and ZnO Merck) present photocatalytic activity and performance similar to TiO2 for the rhodamine B degradation. However, the ZnO catalysts presented lower performance when compared with TiO2 for hydrogen production, under the same conditions. It was observed that the highest hydrogen yield occurs for low concentrations of catalyst (0.05 gL1) and high concentrations of ethanol, being less dependent on pH.

VIANA NETO, Bartolomeu Cruz. Propriedades estruturais e vibracionais de nanocristais de TiO2 dispersos em matriz vítrea porosa. 2006. 49 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2006. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-11-13T22:08:54Z No. of bitstreams: 1 2006_dis_bcviananeto.pdf: 1287691 bytes, checksum: a818652b8c5c3fe5884464f939ed254d (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-11-13T22:19:20Z (GMT) No. of bitstreams: 1 2006_dis_bcviananeto.pdf: 1287691 bytes, checksum: a818652b8c5c3fe5884464f939ed254d (MD5) / Made available in DSpace on 2014-11-13T22:19:20Z (GMT). No. of bitstreams: 1 2006_dis_bcviananeto.pdf: 1287691 bytes, checksum: a818652b8c5c3fe5884464f939ed254d (MD5) Previous issue date: 2006 / Nanostructured materials are the subject of intense investigation due to their remarkable properties as compared their bulk counterparts. The size-induced phenomena are interesting their own and open unique opportunities not only for using these properties in novel applications but also for improving the current technology. In this scenario integrated chemical systems (ICS) are set apart owing their complexity and the possibility ofcombining diferent nanosystems for getting materials with a designed functionality. For instance, nanocrystal growth in the cavities of a porous host is an attractive ICS because the porous is a restricted environment that can be used as nanoreactors. Besides the such system is very promising for catalysis it also important for studying the size-induced properties of the guest material as well. In this work we report the study of vibrational and structural properties of TiO2 nanocrystals dispersed into a porous vycor glass. We have obtained very small TiO2 nanocrystals in the anatase form. The nanocrystal size is controlled via the mass increment only thus preventing the growth through the coalescence process. The nanocrystal size as monitored through transmission electron microscope and Raman scattering. The Eg lowest frequency mode experiences an upshift and becomes broader as the nanocrystal size decreases. This phenomena is attributed to a size-induced e®ect and we analyzed it based on a phonon con¯nement model. As the nanocrystal size gets smaller the q = 0 selection rule is relaxed and the phonon con¯nement involves large q values contribute to the Raman intensity. The frequency follows the same trend of the phonon dispersion relation for the TiO2 bulk as the nanocrystal size decreases. / Materiais nanoestruturados são objetos de intensa investigação devido as suas notáveis propriedades físicas e químicas e quando comparados em suas formas “bulk". Os fenômenos induzidos pela redução do tamanho são interessantes por si próprio e abrem oportunidades únicas, não somente para o uso dessas propriedades em várias aplicações más também para o aprimoramento da tecnologia corrente. Os sistemas químicos integrados (SQI) são um conjunto à parte com complexidade própria e diferentes possibilidades de combinações de nanosistemas para obtenção de materiais com uma funcionalidade desejada. Por exemplo, o crescimento de nanocristais nas cavidades de um hospedeiro poroso é um SQI atrativo, pois o ambiente dos poros pode ser usado como nanoreatores. Da mesma forma que o sistema pode ser muito promissor para catálise é também importante no estudo das propriedades induzidas pelo tamanho do material hóspede. Neste trabalho, estudamos as propriedades estruturais e vibracionais de nanocristais de TiO2 dispersos dentro dos poros do vidro vycor. Foram estudados nanocristais de TiO2 com tamanhos variando de 3-20 nm na fase anatásio. O tamanho dos nanocristais foi monitorado através de microscopia de transmissão eletrônica e espalhamento Raman. O modo Eg, com freqüência em torno de 144 cm-1, experimenta um deslocamento e alargamento de seu pico com a diminuição do tamanho do nanocristal. Este fenômeno é atribuído ao efeito induzido pelo tamanho e analisamos os resultados experimentais com base no modelo de confinamento de fônons. Quando o tamanho do nanocristal é pequeno, a regra de seleção q ≈ 0 no espalhamento Raman de primeira ordem é relaxada e fônons com grandes valores de q contribuem para a intensidade do pico Raman. A freqüência do pico Raman à medida que o tamanho diminui segue o mesmo comportamento da relação de dispersão de fônons para o TiO2 “bulk".

VIANA NETO, Bartolomeu Cruz. Propriedades estruturais e vibracionais de nanotubos e nanofitas de titanato. 2009. 67 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2009. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-11-13T22:32:43Z No. of bitstreams: 1 2009_tese_bcviananeto.pdf: 2854187 bytes, checksum: 4f6bd432814c19af95dec1be0d243db2 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-11-14T17:50:54Z (GMT) No. of bitstreams: 1 2009_tese_bcviananeto.pdf: 2854187 bytes, checksum: 4f6bd432814c19af95dec1be0d243db2 (MD5) / Made available in DSpace on 2014-11-14T17:50:54Z (GMT). No. of bitstreams: 1 2009_tese_bcviananeto.pdf: 2854187 bytes, checksum: 4f6bd432814c19af95dec1be0d243db2 (MD5) Previous issue date: 2009 / This work reports a study on the structural, morphological and vibrational properties of titanate nanotubes and nanoribbons obtained from hydrothermal treatment of TiO$_2$ in aqueous NaOH solutions. The physical properties of these as-synthesized and heat-treated nanostructures are discussed in comparison with their bulk (Na$_2$Ti$_3$O$_7$ and Na$_2$Ti$_6$O$_{13}$) counterparts. The results obtained from transmission electron microscopy, scanning electron microscopy, atomic emission spectroscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, infrared and Raman spectroscopies allowed us to conclude that the layers of both as-synthesized titanates nanotubes and nanoribbons are isostructural to the Na$_2$Ti$_3$O$_7$ lamellar compound. In the titanate nanotubes the chemical bonds are deformed because of the curvature of walls while in the titanates nanoribbons the layers present structural disorder by size effects. The thermal behavior of titanate nanoribbons is similar to those reported in literature for titanate nanotubes, where structural and morphological changes with increases of temperature are observed and indicated that the nanoribbons, at high temperatures, change to bulk with a phase mixing of Na$_2$Ti$_3$O$_7$ and Na$_2$Ti$_6$O$_{13}$. This is similar to what happens with the bulk Na$_2$Ti$_3$O$_7$ when thermally treated. Thus, we conclude that the chemical composition of both the titanate nanotubes and the titanate nanoribbons is the same, Na$_{2-x}$H$_x$Ti$_3$O$_7$.nH$_2$O (0$leq$x$leq$2). Also, we suggest that Raman spectroscopy can be used for an easy and quick identification of both morphology and structure changes of the nanosized titanates. Furthermore, in this work we report the synthesis, characterization and application of Ce ion-exchanged titanate nanotubes. The physicochemical properties of these nanotubes are discussed in comparison with their pure titanate nanotube counterparts. The transmission electron microscope images showed that the Ce ion-exchanged titanate nanotubes have the same morphology of the pristine nanotubes and their external walls are decorated with cerium oxide nanoparticles. The mechanism of nanoparticle formation is based on the precipitation of Ce ion at the nanotube surface. We observed a shift of the absorption band edge towards the visible region that is attributed to the effects of Ce$^{4+}$ addition (intercalation) and/or the presence of CeO$_2$ nanoparticles decorating the nanotubes surfaces. A red shift of vibrational modes associated with metal ion - oxygen interaction was observed and identified as being due to the effect of Ce addition to the lattice as well as the anchoring of CeO$_2$ nanoparticles to the nanotube wall. We show that this hybrid system is promising for applications in photocatalysis using the blue region of the electromagnetic spectrum and this was demonstrated for photodegradation of reactive blue 19 textile dye using visible light ilumination. / Este trabalho relata o estudo das propriedades estruturais, morfológicas e vibracionais dos nanotubos e das nanofitas de titanato obtidos a partir do tratamento hidrotérmico de TiO2 em solução aquosa de NaOH. As propriedades físicas destas nanoestruturas, como preparadas e tratadas termicamente, são discutidas e comparadas com seus similares sólidos estendidos (Na2Ti3O7 e Na2Ti6O13). Os resultados obtidos por microscopia eletrônica de transmissão, microscopia eletrônica de varredura, espectroscopia de emissão atômica, espectroscopia de energia dispersiva de raios-X, difração de raios-X, espectroscopia de absorção no infravermelho com transformada de Fourier e espectroscopia Raman permitem concluir que as camadas (paredes ou lamelas) de nanotubos e nanofitas de titanato, como preparados, são isoestrutural ao composto lamelar Na2Ti3O7. Nos nanotubos de titanato as ligações químicas são deformadas por causa da curvatura das paredes, enquanto nas nanofitas de titanato as camadas apresentam apenas a desordem estrutural causada pelo efeito do tamanho. O comportamento térmico das nanofitas de titanato é similar ao relatado na literatura para os nanotubos de titanatos, onde mudanças estruturais e morfológicas, com o aumento da temperatura, são observadas e indicam que as nanofitas quando submetidas a altas temperaturas mudam sua morfologia para grandes bastões (sólido estendido) com uma mistura das fases Na2Ti3O7 e Na2Ti6O13. Este resultado é similar ao comportamento do Na2Ti3O7, sólido estendido, quando tratado termicamente. Concluímos que os nanotubos e as nanofitas de titanato têm a mesma composição química Na{2-x}HxTi3O7.nH2O(0·x·2).