Ce travail a été consacré à l'étude cinétique de la chute de surface spécifique de l'anatase et de la cérine lors de calcinations isothermes en présence de chlorure d'hydrogène, de vapeur d'eau et d'oxygène. Il est montre l'importance de relier le préfrittage de ces oxydes à l'adsorption sur ceux-ci des gaz constituant l'atmosphère de calcination. La modélisation de ces phénomènes, complétée par la caractérisation des solides calcines, a permis de dégager les conclusions suivantes : 1) dans le cas de l'anatase calcinée en présence de chlorure d'hydrogène, la chute de surface spécifique est liée a un grossissement de grain, due principalement à une évolution du type murissement d'Ostwald. Le transport de matière se fait à 690 k par l'intermédiaire de dihydroxydichlorure de titane, espèce volatile instable. L'étape limitante du mécanisme de préfrittage est la formation de ce dernier ; 2) dans le cas de la cérine calcinée a 900 k, la chute de surface spécifique peut être expliquée par l'adsorption dissociative de chlorure d'hydrogène, conjointe a la désorption de chlore, la diffusion superficielle des ions hydroxyles ainsi formes et des ions cérium suivie de la désorption d'eau. L'étape limitante du mécanisme de préfrittage est la fixation du chlorure d'hydrogène, soit, dans les premiers instants son apport impose par la procédure expérimentale, puis l'adsorption elle-même.

Theoretical methods can be used to describe surface chemical reactions in detail and with sufficient accuracy. Advances, especially in density functional theory (DFT) method, enable to compare computational results with experiments. Quantum chemical calculations employing ONIOM DFT/B3LYP/6-31G**-MM/UFF cluster method provided in Gaussian 03 are conducted to investigate water adsorption on rutile (110), and water and ammonia adsorption on anatase (001) surfaces of titanium dioxide. Water and ammonia adsorption on anatase (001) surface is studied by also performing PW:DFT-GGA-PW91 periodic DFT method by using VASP code and the results are compared with the results of ONIOM method. The results obtained by means of ONIOM method indicate that dissociative water adsorption on rutile (110) surface is not favorable due to high activation barrier, whereas on anatase (001) surface, it is favorable since molecular and dissociative water adsorption energies are calculated to be -23.9 kcal/mol and -58.12 kcal/mol. Moreover, on anatase (001) surface, dissociative ammonia adsorption is found energetically more favorable than molecular one (-37.17 kcal/mol vs. -23.28 kcal/mol). Thermodynamic functions at specific experimental temperatures for water and ammonia adsorption reactions on anatase (001) surface are also evaluated. The results obtained using periodic DFT method concerning water adsorption on anatase (001) surface indicate that dissociative adsorption is more favorable than molecular one (-32.28 kcal/mol vs. -14.62 kcal/mol) as in ONIOM method. On the same surface molecular ammonia adsorption energy is computed as -25.44 kcal/mol. The vibration frequencies are also computed for optimized geometries of adsorbed molecules. Finally, computed adsorption energy and vibration frequency values are found comparable with the values reported in literature.

Made available in DSpace on 2015-05-14T13:21:41Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 10515395 bytes, checksum: 24068367353a0ab8bc6c91ecaae5abf5 (MD5) Previous issue date: 2014-09-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Titanium dioxide (TiO2) in anatase phase has been the most studied metal oxide in the last two decade due to its potential technological aplication in many areas, such as dyes, (photo)catalysts and solar cells. Computational and experimental methods has been employed to understand the mechanical, electronical and catalytic properties of anatase. In this context, the introduction of dopants and induction of point deffects formation (mainly oxygen vacancies) are responsible for tunning those properties in many situatuions, e.g. increasing the (photo)catalytic efficience. In the present study, we investigated the effect of Ce-doping on the formation of oxygen vacancy and reduced Ti3+ and Ce3+ centers in anatase TiO2 (bulk and (001) surfaces) by means of computational quantum chemical calculations. All calculations were performed at the density functional theory (DFT) level taking into account the periodicity of the systems. The bulk phase was initially studied without dopant, and the Grimme DFT-D2 dispersion potential was reparameterized for octahedral [TiO6] interaction in anatase polymorph within the B3LYP (GTO) approach (hereafter named B3LYP-D*). Several properties were calculated from bulk with B3LYP-D* in good agreement with experimental values from anatase single crystals, such as: Wulff construction of single crystal in thermodynamic equilibrium, elastic constants, infrared and Raman vibrational frequencies, and electronic structure calculations (band structure and density of states). Two DFT approachs were used for mitigate the self-interaction error (SIE) in both reduced and/or Ce-doped systems: the on-site Dudarev DFT+U correction and the hybrid B3LYP (20% HF) functional with plane-wave or Gaussian-type basis set. Only a small local perturbation was associated with the Ce-dopant introduction in the octahedral site (bulk) and Ce(5c) (surface). The Ce-doped systems presented lower oxygen vacancy formation energy than pristine TiO2. The most stable VO configuration in the bulk were in the next neighbors from the reduced [CeO6]/ center, instead the low-coordinated [CeO5] centers. Similarly, the dopant on the subsurface distoted octahedral [CeO6]d site of (001) surface boosted the remotion of O(2c) in the outmost layer of the surface. This behavior was not observed with the dopant on the low-coordinated [CeO5] site. The Ti3+ [3d 1] and Ce3+ [4f 1] midgap states were found up to ~ 1.0 eV bellow de conduction band. These founds are in agreement with experimental evidences of the enhanced facilitation of VO formation in Ce-doped anatase, and superior (photo)catalytic activity when compared with undoped TiO2. In the general way, the vacancy formation energy decreased significantly in the following situations: (i) oxide reduction in the presence of Ce as dopant; (ii) VO in the nearest neighbor sites of the reduced [CeO6]/ octahedra (iii) introduction of two Ce dopants around VO; (iv) VO in the outmost layers plus [CeO6]/ at the subsurface. / Dióxido de titânio (TiO2) na fase anatase tem sido o óxido mais estudado nas últimas décadas devido à sua potencial aplicação tecnológica em diversas áreas, dentre as quais como pigmento, (foto)catalisador e em células solares. Não obstante, métodos experimentais e computacionais têm sido empregados na compreensão de suas propriedades mecânicas, eletrônicas e catalíticas da anatase. Nesse contexto, a introdução de dopantes com formação de defeitos pontuais do tipo vacâncias de oxigênio tem sido responsável por aumentar a eficiência desse material como catalisador sobre muitas reações. No presente estudo, investigamos, por meio de cálculos computacionais, o efeito da dopagem do TiO2 anatase com o lantanídeo Ce sobre a formação de vacâncias de oxigênio e exploramos suas consequências na formação de centros reduzidos Ti3+ e Ce3+, no bulk e na superfície (001). Todos os cálculos foram realizados ao nível da teoria do funcional da densidade (DFT) utilizando códigos computacionais que levam em consideração a periodicidade dos sistemas cristalinos (2D para superfícies ou 3D para bulk). Para uma maior compreensão da anatase sem o dopante, exploramos inicialmente o efeito das interações de dispersão entre os poliedros [TiO6] e reparametrizamos o potencial de Grimme DFT-D2 com funções de base GTO (o qual chamamos B3LYP-D*), a partir do qual várias propriedades estruturais foram obtidas, como construção de Wulff do hábito cristalino sob equilíbrio termodinâmico, propriedades elásticas, vibracionais e eletrônicas. Na presença do dopante e com os sistemas reduzidos, duas principais metodologias foram adotadas na tentativa de reduzir o erro de autointeração da DFT: o uso do funcional híbrido B3LYP e a correção de Dudarev DFT+U. Orbitais do tipo Gaussiana e ondas planas foram utilizadas como funções de base. Apenas uma pequena deformação local ocorreu com a introdução do dopante no sítio octaédrico do Ti no bulk e no sítio Ce(5c) da superfície. Em ambos bulk e superfície (001) a introdução do dopante levou à diminuição da energia de formação de VO. As configurações mais estáveis de VO no bulk ocorreram nos vizinhos próximos ao octaedro reduzido [CeO6]/ em vez daquelas que envolviam a diminuição da coordenação do dopante para [CeO5]. Analogamente, a presença do dopante na subsuperfície da (001) formando o poliedro distorcido [CeO6]d impulcionou a remoção de um O(2c) na camada mais externa da superfície, o que não foi observado quando a dopagem acontecia na última camada de cátions com coordenação [CeO-5]. Assim como no bulk, os estados intermediários no band gap associados com a vacância mais estável na superfície (001) foram referentes ao Ti3+ [3d 1] e Ce3+ [4f 1] localizados em torno de 0.5-1.0 eV abaixo da banda de condução, de acordo com achados experimentais e evidências no aumento da atividade (foto)catalítica dado pela criação destes canais de transferência eletrônica. De modo geral, a energia de formação das vacâncias de oxigênio diminuiu consideravelmente nas seguintes situações: (i) redução do óxido na presença do dopante Ce, (ii) desde que os sítios de criação das VO localizem-se nos primeiros vizinhos ao octaedro [CeO6]/ distorcido por expansão polarônica, (iii) introdução de dois dopantes, em direções opostas, em torno de VO, e (iv) criação de VO nas camadas mais externas da superfície, desde que o dopante esteja na subsuperfície e com coordenação [CeO6].

Nanomaterials have attracted substantial attention in the area of catalysis due to the unique properties they exhibit such as high surface areas, intricate pore networks and unique morphologies. TiO2 has attracted attention as a catalyst since the discovery of its high photocatalytic activity by Fuishima and Honda in 1972. Given its high thermal stability, low cost, low environmental impact, and versatility, TiO2 is a widely used commercial catalyst and catalyst support. TiO2 is used in many applications such as photocatalysis is also an excellent support material for noble metals in a number of oxidative synthesis and pollution-control reactions. Though TiO2 is a widely used catalyst support, currently available commercial titanias often have low surface area and poor thermal and hydrothermal stability. While several methods reported in literature produce materials of higher surface area and more ideal porosity relative to commercially available titanias, these procedures generally involve inherent drawbacks including time-consuming, complicated, and expensive processes that are not industrially viable. Cost-effective, large-scale preparations of stable, high surface area, mesoporous TiO2 need to be developed. The work in this dissertation focuses on (1) producing high surface area stabilized TiO2 supports of controlled pore diameters and (2) the preparation of well dispersed Pt on these supports using industrially viable processes. The effects of dopants Al, La, Si, and Zr on the stability, surface area, and porosity of anatase TiO2 supports were investigated. Results show that dopants increased the surface area and thermal stability of anatase through structural modifications and grain growth inhibition. Stabilized titanias produced by this method demonstrated equivalent or higher thermal stability and surface area compared with pure anatase and previously reported materials after treatment at 400°C and 700°C including 22 mol% Al-TiO2 calcined at 400°C which had a surface area of 479 ± 39 m2/g, a pore volume of 0.46 ± 0.04 cm3/g, and a pore diameter of 2.9 ± 0.2 nm. Ten synthesis variables were examined and optimized using statistically designed experiments (DOEs). Equations were developed to predict the conditions to obtain the highest surface area and pore volume at the desired pore diameter and predict the pore diameter range that may be obtained for aluminum-modified anatase TiO2. Confirmation trials closely matched predicted surface areas, pore volumes, and pore diameters in all but one trial, demonstrating the power of DOEs in identifying and controlling synthesis variables in relatively few experiments. The structure of Al-modified anatase TiO2 was analyzed to determine the mechanism of Al stabilization. Surface Al stabilized TiO2 by lowering anatase surface energy, stabilizing planes of high surface energy which would otherwise join to achieve stabilization. Al in TiO2 lattice vacancies stabilized TiO2 through increasing lattice strain and limiting mass transport necessary for grain growth. Results demonstrate the importance of structure analysis of doped nanomaterials in the development of stabilized catalysts and catalyst supports. An industrially viable, one-pot synthesis of Pt supported on 22 mol% Al-modified anatase is presented. Pt dispersions as high as 54% (one-pot method) and 59% (DI method) have been obtained. Results show that this one-pot method and the DI method using our Al-modified supports are promising syntheses of highly dispersed Pt catalysts and demonstrate that the alumina-stabilized anatase support is superior to other many available anatase supports.

<p>Kao &scaron;to je poznato, RS-2-(4-hlor-o-toliloksi)propionska kiselina (MCPP),&nbsp;(4-hlor-2-metilfenoksi)sirćetna kiselina (MCPA) i 3,6-dihlorpiridin-2-karboksilna kiselina &nbsp;(klopiralid) su herbicidi sa veoma &scaron;irokim spektrom dejstva, a pored toga su rastvorljivi u&nbsp;vodi, te&scaron;ko biorazgradljivi i prema literaturnim podacima su, nažalost, veoma često prisutni&nbsp;herbicidi u pijaćoj vodi. Proces heterogene fotokatalize uz primenu TiO₂i UV zračenja se&nbsp;pokazao kao veoma pogodan način za njihovo uklanjanje iz vode. Međutim, zbog velikog&nbsp;energetskog procepa od 3,2 eV (anataze-oblik), odnosno, 3,0 eV (rutil-oblik), veoma mali&nbsp;udeo bliskih UV zraka iz sunčeve svetlosti (oko 3&minus;4%) biva iskori&scaron;ćen u toku&nbsp;fotokatalitičkog procesa, &scaron;to ukazuje na to da je TiO_2&nbsp;praktično neaktivan u prisustvu&nbsp;sunčeve svetlosti. Na osnovu literaturnih podataka je zapaženo da postoji mogućnost&nbsp;fotorazgradnje pojedinih supstrata u prisustvu TiO_2&nbsp;primenom vidljive svetlosti. Na&nbsp;primeru MCPP je ispitana aktivnost TiO_2&nbsp;Degussa P25 kao fotokatalizatora u prisustvu&nbsp;vidljive svetlosti. Na osnovu refleksionih spektara je utvrđeno da MCPP adsorbovan na&nbsp;TiO_2&nbsp;Degussa P25 apsorbuje vidljivi deo spektra (&lambda; &ge;400 nm). Nastali prelazni kompleks&nbsp;je potvrđen FTIR merenjima. Efikasnost TiO2Degussa P25 primenom vidljive svetlosti je&nbsp;upoređena sa sunčevim i UV zračenjem, kao i direktnom fotolizom u prisustvu pomenutih&nbsp;izvora zračenja. Brzina fotokatalitičke razgradnje MCPP primenom vidljive svetlosti iznosi&nbsp;0,86 &mu;mol dm^&minus;3min^&minus;1, &scaron;to je oko 4 puta brže u poređenju sa direktnom fotolizom. Nadalje&nbsp;je ustanovljena optimalna masena koncentracija katalizatora od oko 8 mg cm^&minus;3, koja je&nbsp;znatno vi&scaron;a u poređenju sa primenom UV zračenja. Razlog je najverovatnije različit&nbsp;mehanizam fotorazgradnje koji se odvija primenom vidljivog i UV zračenja. Naime,&nbsp;prisustvo 2-metil-2-propanola (poznatog hvatača&nbsp;^&bull;OH-radikala) praktično ne utiče na&nbsp;brzinu fotokatalitičke razgradnje MCPP p rimenom vidljive svetlosti, &scaron;to ukazuje da se&nbsp;mehanizam razgradnje MCPP primenom &nbsp;vidljive svetlosti ne odvija posredstvom&nbsp;^&bull;OH-radikala, za razliku od onog uz primenu UV zračenja.</p><p>S obzirom da se katalizator TiO_2&nbsp;Degussa P25 uz primenu vidljive svetlosti nije&nbsp;<br />pokazao kao naročito efikasan kada je u pitanju razgradnja sva tri herbicida i imajući u&nbsp;vidu da se u poslednje vreme iz razloga praktične primene sve vi&scaron;e pribegava procesu dopovanja TiO_2&nbsp;različitim tipovima metala (alkalnih, zemnoalkalnih, prelaznih i dr.) i nemetala (halogenida, halkogenida i dr.), u okviru ove doktorske disertacije je ispitana aktivnost N-TiO₂(sintetisanih mokrim i suvim putem) i TiO_2&nbsp;(rutil) dopovanog sa različitim količinama Fe³⁺-jona (0,13&minus;1,48 at.%) pri razgradnji herbicida MCPP i MCPA primenom vidljive svetlosti. Pored toga je ispitana efikasnost TiO_2&nbsp;(anataze) takođe dopovanog sa različitim količinama Fe³⁺-jona (0,71&minus;1,80 at.%) na primeru MCPP.&nbsp;</p><p>Poredeći N-TiO_2&nbsp;(sintetisan mokrim putem) i N-TiO_2&nbsp;(sintetisani suvim putem), primećeno&nbsp;je da je u drugom slučaju efikasnost katalizatora veća oko 2 puta. Isto tako je zapažena u&nbsp;slučaju MCPP ne&scaron;to veća fotokatalitička aktivnost N-TiO_2&nbsp;(sintetisani suvim putem) u&nbsp;poređenju sa TiO_2&nbsp;(anataze). Kada je u pitanju MCPA aktivnost sva tri katalizatora je&nbsp;veoma slična. Pored toga je zapažena veća efikasnost N-TiO_2&nbsp;(sintetisan mokrim putem) u&nbsp;poređenju sa TiO_2&nbsp;Degussa P25 (oko 1,5 puta) i oko 5 puta u odnosu na direktnu fotolizu,&nbsp;dok su N-TiO_2&nbsp;(sintetisani suvim putem) oko 3 puta efikasniji u poređenju sa TiO_2&nbsp;Degussa P25 i oko 10 puta u &nbsp;poređenju sa direktnom fotolizom. Brzina solarne razgradnje&nbsp;je preko 100 puta manja nego primenom vidljivog i UV zračenja, &scaron;to je posledica različitih&nbsp;intenziteta pomenutih izvora ozračivanja i različitih uslova pri kojima je vr&scaron;ena razgradnja. &nbsp;Ustanovljena je optimalna masena koncentracija N-TiO_2&nbsp;(sintetisan mokrim putem) od&nbsp;4 mg cm^&minus;3.</p><p>Prilikom razgradnje MCPP i MCPA je nađeno da je brzina veća kada se kao&nbsp;katalizator &nbsp;koristi TiO_2&nbsp;(rutil) u poređenju sa Fe-TiO_2&nbsp;i da sa povećanjem količine Fe³⁺-jona fotokatalitička aktivnost uglavnom opada. Kada je kao fotokatalizator kori&scaron;ćen TiO_{2 &nbsp;}<br />(anataze) dopovan različitim količinama Fe³⁺-jona (od 0,71 do 1,80 at.%), razgradnja&nbsp;<br />MCPP je u svim slučajevima znatno sporija u odnosu na TiO_2&nbsp;(anataze).</p><p>S obzirom da su prema literaturnim podacima kinetika i mehanizam fotokatalitičke&nbsp;<br />razgradnje klopiralida nepoznati, ispitana je njegova stabilnost pri različitim&nbsp;eksperimentalnim uslovima. Tokom ispitivanja uticaja pH kako u prisustvu, tako i u&nbsp;odsustvu dnevne svetlosti u intervalu pH od 1,0&minus;9,0, nađeno je da ni u jednom slučaju ne&nbsp;dolazi do razgradnje supstrata u periodu od sedam meseci koliko je proces praćen. Takođe&nbsp;je ispitana kinetika fotokatalitičke razgradnje klopiralida primenom UV i vidljivog&nbsp;zračenja u prisustvu TiO_2&nbsp;Degussa P25, kao i direktna fotoliza primenom oba izvora&nbsp;zračenja. Nađeno je da je brzina fotokatalitičke razgradnje primenom UV zračenja veća&nbsp;oko 5 puta u odnosu na direktnu fotolizu. Za praćenje toka fotokatalitičke razgradnje&nbsp;klopiralida je izabrana pH-vrednost od 3,2. Nadalje je zapaženo da se u ispitivanom opsegu početnih koncentracija supstrata (0,5&ndash;3,0 mmol dm^&minus;3) kinetika fotokatalitičke razgradnje klopiralida može opisati pseudo-prvim redom. Pri ispitivanju uticaja masene koncentracije katalizatora (0,5&ndash;8 mg cm^&minus;3) na brzinu razgradnje klopiralida, ustanovljena je optimalna masena koncentracija primenjenog fotokatalizatora od oko 4 mg cm^&minus;3. Izračunata ukupna &nbsp;prividna energija aktivacije iznosi 7,74 kJ mol^&minus;1. Pored toga, prisustvo kiseonika ubrzava reakciju 2 puta, dok dodatak elektron-akceptora kao &scaron;to su (NH₄)₂S₂O₈, H₂O_2&nbsp;i KBrO_3&nbsp;pokazuje značajan i različit efekat na kinetiku fotokatalitičke razgradnje klopiralida. Pri ispitivanju uticaja etanola, kao hvatača slobodnih radikala, nađeno je da se heterogena fotokataliza odvija uglavnom preko ^&bull;OH-radikala.</p><p>Na osnovu LC&ndash;MS/MS (ESI+) merenja ustanovljeno je prisustvo nekoliko&nbsp;intermedijera: 3,6-dihlor-piridin-2-ol, 3,6-dihlor hidroksipiridin-2-karboksilna kiselina i&nbsp;3,3&#39;,6,6&#39;-tetrahlor-2,4&#39;-bipiridin-2&#39;-karboksilna kiselina. Na osnovu identifikovanih&nbsp;intermedijera, kao i kinetičkih rezultata, predložen je mogućput mehanizma fotokatalitičke&nbsp;razgradnje klopiralida.</p><p>Prilikom ispitivanja uticaja strukture molekula na brzinu razgradnje, konstatovano&nbsp;<br />je da u slučaju klopiralida praktično ne dolazi do fotokatalitičke razgradnje u prisustvu&nbsp;TiO_2&nbsp;(anataze) i N-TiO_2&nbsp;(sintetisani suvim putem) uz primenu vidljive svetlosti, kao i u&nbsp;slučaju TiO_2&nbsp;Degussa P25. Pored toga, primenom TiO_2&nbsp;(rutil) i Fe-TiO_2&nbsp;kao&nbsp;fotokatalizatora, sa povećanjem količine Fe³⁺-jona od 0,13 do 1,27 at.% raste brzina&nbsp;razgradnje klopiralida, ukazujući da strukturne osobine supstrata utiču na brzinu njihove&nbsp;razgradnje.</p> / <p>It is well known that RS-2-(4-chloro-o-tolyloxy)propionic acid (MCPP), (4-chloro-2- methyl-phenoxy)acetic acid (MCPA) and 3,6-dichloropyridine-2-carboxylic &nbsp;acid&nbsp;(clopyralid) are herbicides of wide activity spectrum. They are soluble in water, hardly&nbsp;biodegradable and, unfortunately, often present in drinking water.</p><p>Heterogeneous photocatalysis by application of TiO_2&nbsp;and UV radiation proved to&nbsp;be very suitable for their removal from water. However, due toits large energy gap, i.e.&nbsp;3.2 &nbsp;eV (anatase-form) and 3.0 eV (rutile-form), a very small fraction of sunlight in the near&nbsp;UV range (about 3&ndash;4%) is used during photocatalytic process, which is an indication of&nbsp;TiO_2&nbsp;inactivity in the presence of this light source. Some literature data report on the&nbsp;possibility of photodegradation of certain substrates by visible light in the presence of&nbsp;TiO₂. MCPP served as substrate for testing TiO_2&nbsp;Degussa P25 &nbsp;photocatalytic activity in the&nbsp;presence of visible light. On the basis of reflection spectra it was established that MCPP&nbsp;adsorbed on TiO_2&nbsp;Degussa P25 was absorbing visible spectrum radiation (&lambda; &ge;400 nm).&nbsp;The existence of thus formed &nbsp;charge-transfer complex was confirmed with FTIR analysis.&nbsp;The efficiency of &nbsp;TiO_2&nbsp;Degussa P25 with application of visible light was compared to&nbsp;sunlight and UV radiation, as well to directphotolysis in the presence of these light&nbsp;sources. The rate of MCPP photocatalytic degradation by means of visible light is&nbsp;0.86 &mu;mol dm^{&minus;3&nbsp;}min^&minus;1, which is about 4 times faster than direct photolysis. In addition, the&nbsp;optimal &nbsp;catalyst concentration of about 8 mg cm^&minus;3, much higher than using UV radiation,&nbsp;<br />was established. The reason is, probably, a different mechanism of &nbsp;hotodegradation &nbsp;in the&nbsp;presence of visible and UV irradiation. Namely, the presence of 2-methyl-2-propanol&nbsp;(well-known&nbsp;^&bull;OH radical scavenger) has practicallyno effect on the rate of &nbsp;MCPP&nbsp;photocatalytic degradation using visible light, which points that this degradation&nbsp;mechanism does not involve&nbsp;^&bull;OH radicals, in contrast to that established &nbsp;for UV radiation.</p><p>Since the catalyst TiO_2&nbsp;Degussa P25 with application of visible light was not very&nbsp;<br />efficient in degradation of all three herbicidesand in view that nowadays is very &nbsp;popular&nbsp;doping process of TiO₂ with different types of metals (alkali, alkaline-earth, transition, etc.)&nbsp;and non-metals (halogen, chalcogen, etc.), in the scope of this Ph.D. &nbsp;thesis activities of N-TiO_2&nbsp;(synthesized by wet and dry procedure) and TiO_2&nbsp;(rutile) doped with various amounts&nbsp;of Fe^3+&nbsp;(0.13&ndash;1.48 at.%) in degradation processes of &nbsp;herbicides MCPP and MCPA using&nbsp;visible light were studied. In addition, the efficiency of TiO_2&nbsp;(anatase) doped with various&nbsp;amounts of Fe^3+&nbsp;(0.71&ndash;1.80 at.%) was also tested for MCPP degradation. When comparing&nbsp;N-TiO_2&nbsp;(synthesized by wet procedure) and N-TiO₂ (dry procedure), it was observed that&nbsp;in the latter case the catalyst efficiency was about two times higher. In this case for MCPP&nbsp;was also observed somewhat higher photocatalytic activity of N-TiO₂ (synthesized by dry&nbsp;procedure) in comparison with TiO₂. When activities of all three catalysts towards MCPA&nbsp;are compared, the results are very alike. In addition, higher efficiency of N-TiO_{2 &nbsp;}(wet&nbsp;procedure) comparing to TiO_2&nbsp;Degussa P25 (about 1.5 times) and about 5 times in&nbsp;comparison to direct photolysis were recorded, while N-TiO₂ (dry procedure) was about 3&nbsp;times more efficient than TiO_2&nbsp;Degussa P25 and about 10 times in comparison with direct&nbsp;photolysis. The rate of solar degradation is about 100 times lower than by application of&nbsp;UV and visible radiation, as a consequence of various intensities of the mentioned light&nbsp;sources and different conditions of photodegradation. An optimal concentration of N-TiO₂&nbsp;(wet procedure) of 4 mg cm^&minus;3<br />&nbsp;was established.</p><p>During degradation of MCPP and MCPA it was observed that the rate is higher if&nbsp;TiO₂(rutile) was applied comparing to Fe-TiO_2&nbsp;and with increasing amount of Fe^3+&nbsp;photocatalytic activity mostly decreases. When TiO₂ (anatase) doped with various amounts&nbsp;of Fe^3+&nbsp;(0.71 to 1.80 at.%) was applied for MCPP degradation, the process was much&nbsp;slower than with undoped catalyst.</p><p>Since we have not found relevant literature data on kinetics and mechanism of&nbsp;photocatalytic degradation of clopyralid, its stability in different experimental conditions&nbsp;was tested. In investigating of influences of pH (1.0&ndash;9.0) both in presence and in absence&nbsp;of daylight, in no cases decomposition was observed during seven months experiments.&nbsp;Also, the kinetics of photocatalytic degradation of clopyralid using UV and visible&nbsp;irradiation in the presence of TiO_2&nbsp;Degussa P25 and in direct photolysis by application of&nbsp;both irradiation sources was studied. It was found that the &nbsp;rate of photocatalytic&nbsp;decomposition using UV radiation was 5 times higher comparing to direct photolysis. For&nbsp;clopyralid photocatalytic monitoring a pH value of &nbsp;3.2 was chosen. In addition, in the investigated concentration range (0.5&ndash;3.0 mmol &nbsp;dm^&minus;3) the photocatalytic degradation kinetics of clopyralid in the first stage of the reaction follows approximately a pseudo-first kinetic order. In investigation of influence of catalyst concentration (0.5&ndash;8 mg cm^&minus;3) on the rate of clopyralid degradation the highest reaction rate was observed at 4 mg cm^{&minus;3&nbsp;}of catalyst concentration The apparent activation energy of the reaction being 7.74 kJ mol^&minus;1. The absence of molecular oxygen resulted in a significant decrease (about 2 times) in the rate of clopyralid photodegradation. The effect of the presence of (NH₄)₂S₂O₈, H₂O_2&nbsp;and KBrO₃, acting as electron acceptors along with molecular oxygen affects clopyralid photocatalytic degradation considerably and indifferent ways. By studying the effect of ethanol as a hydroxyl radical scavenger it was shown that the heterogeneous catalysis takes place mainly via ^&bull;OH radicals.</p><p>LC&minus;MS/MS (ESI+) monitoring of the process showed that several pyridine-containing intermediates are formed: 3,6-dichloropyridin-2-ol, 3,6-dichloro&nbsp;hydroxypyridine-2-carboxylic acid and 3,3&#39;,6,6&#39;-tetrachloro-2,4&#39;-bipyridine-2-carboxylic&nbsp;acid. Based on the identified intermediates and overall kinetic results, a probable&nbsp;photocatalytic degradation mechanism was proposed.&nbsp;</p><p>Finally, in the case of clopyralid it was established that practically no degradation&nbsp;<br />occurs in the presence of TiO_2&nbsp;(anatase) and N-TiO₂ (dry procedure) with visible light &nbsp;and&nbsp;also with TiO_2&nbsp;Degussa P25. Besides of that, by using TiO_2&nbsp;(rutile) and Fe-TiO_2&nbsp;as&nbsp;photocatalysts it was noted that increasing the concentration of Fe^3+&nbsp;from 0.13 to 1.27 at.%&nbsp;comes to increasing photodegradation rate of clopyralid. Results indicate that differences&nbsp;in molecular structure of chosen compound, influence obtained photocatalytic activity to a&nbsp;great extent.</p>

Increased demand of power, limited fuel resources and environmental concerns have recently prompted a huge thrust on research areas of alternative energy and photovoltaics have been hailed as energy source for future. Particularly, third generation solar cell configurations like dye-sensitized solar cells and quantum dot Schottky barrier solar cells have drawn more attention because of their ease of processability, cheap cost with decent performance, low payback time and portability. Quantum dots are very attractive materials as sensitizers because of their size dependent bandgap tunability, increased oscillator strength and hence higher absorption coefficient, possibility of multiple exciton generation and photochemical robustness. Hence syntheses of quantum dot based hybrid nanostructures have received huge attention among researchers for using it quantum dot sensitized solar cell configuration. This dissertation can be divided in two parts. In the first part two different methods have been reported to prepare interconnected mesoporous nanostructures of wide band gap semiconductors like TiO2 and ZnO which is very important in providing high surface area for absorption or attachment of the sensitizers. In the second part, methods have been developed to establish direct contacts between quantum dots and wide bandgap substrates without molecular linkers which are expected to increase the electron injection rate from quantum dots to TiO2/ZnO. The entire thesis based on the results and findings obtained from the present investigation is organised as follows: Chapter-I provides a general introduction on the working principle of different type of solar cells and then gives a detailed description of the structure and electronic process of dye sensitised solar cells. Then, benefits of quantum dots as sensitizer over dye molecules has been discussed followed by the modification needed in case of quantum dot sensitized solar cells. Chapter-II deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter-III describes a hybrid sol-gel combustion technique to synthesize large quantities of highly crystalline and phase-pure anatase powder in a single step. Titanium isopropoxide reacts with oleic acid to form a viscous liquid (oxocarboxoalkoxide) which undergoes non-hydrolytic polycondensation to form TiO2 during combustion. Oleylamine takes part in formation of reverse micelle which expands during combustion giving rise to porous interconnected membrane like microstructure of pore size ~5 nm, BET surface area of ~100 m2/g and porosity of ~ 48%. More importantly, this porous powder having a pre-existing network can be used to form thicker film by doctor blade technique from its paste and at the same time is expected to have better transport properties due to its less particulate nature. Chapter-IV presents a general method to prepare mesoporous structure from rod-like morphologies by partial sintering of a green pellet. Material having inherent anisotropy in their crystal structure tends to grow in a particular direction rather undergoing equiaxial growth. For instance, hexagonal ZnO and tetragonal rutile usually grow as rod-shaped particles. A loose compact of these nanorods give nanoporous morphology upon heating. Advantage of this method is the tunability of pore size by tuning the aspect ratio of the nanorods. Preparation of porous TiO2, ZnO and hydroxyapatite has been demonstrated from their corresponding nanorods. Chapter-V deals with a solvothermal based technique that has been developed for in-situ deposition of nanoparticles on any plane or curved surfaces conformally. This has been demonstrated for nanoparticles of FeCo, Au, Co, CdS on substrates like glass, mica, Si, NaCl, Al2O3 M-plane and also conformal coating of Au nanoparticles on polystyrene latex spheres. CdSe on rutile nanorods, ZnO nanorods and CNTs are promising hybrid nanostructures for third generation photovoltaics and their successful preparation has been detailed in the chapter. The mechanism proposed is based on dominant attractive sphere-plate interaction under high temperature and high autogeneous pressure condition which at reduced density and surface tension of the solvent reduces the dispersibility of the nanoparticle and allow effective spreading of the nanoparticles on the substrate. This method is also advantageous for coating of complicated geometry like inner walls of porous structures. Chapter-VI presents a method to coat chalcogenide nanoparticles on mesoporous TiO2 without any molecular linker which can enhance the electron injection rate from the chalcogenide quantum dots to TiO2. CdS, PbS can be easily synthesized through aqueous chemistry. For deposition of these sulfides, the ion layer gas absorption and reaction (IGLAR) method was modified to form uniform dense nanoparticles on anatase and ZnO surfaces. Nitrate salts of corresponding metal ions are dried directly on the semiconductor surface and instead of exposing it to H2S gas, it was treated with a concentrated sulfide solution. This introduces two competitive process i) dissolution of nitrate salt ii) formation of the metal sulfide. This dissolution step was absent when treated with H2S gas (IGLAR) and hence lead to a continuous coating. We have successfully produced CdS-TiO2 and PbS-TiO2 composites using this approach. Photoelectrochemical measurements on CdSTiO2 composites show an overall efficiency of 2.8% which is among the highest values obtained for this system demonstrating the applicability of the method to engineer interfaces to achieve high efficiency solar cells. Chapter-VI explores the combination of strategies of nanocrystal conversion chemistry with previously described sol-gel combustion technique to create dense and uniformly coated QD sensitized TiO2 electrode without compromising heat-treatment routines which is essential for better adhesion and to enhance performance with reduced leakage. Intimate biphasic oxide mixtures of PbO and CdO with TiO2 are first synthesized by nonhydrolytic solgel process with is followed by combustion to produce porous morphology. This powder can be coated as electrode and can sustain high temperature heat treatment routines and finally can be selectively converted to sulfides with Na2S treatment as TiO2 is immune to sulfidation under this condition. Materials at different stages are characterised by XRD, TEM, EDS, UV-Vis and XPS.

Le chauffage de la poudre de dioxyde de titane anatase de haute surface spécifique produit deux phénomènes exothermiques irréversibles : - Une chute de la surface spécifique de l'anatase (sans changement de phase), observée en fonction du temps de calcination à 575°C, - la transformation de phase anatase-rutile, observée en fonction du temps de calcination à une température supérieure à 620°C. L'addition d'éléments cationiques étrangers, ou la présence de vapeur d'eau ou d'oxygène, modifie les vitesses d'évolution de ces phénomènes. En particulier, la présence de vapeur d'eau accélère la chute de surface spécifique de l'anatase. La modélisation de la réduction de surface permet, en se plaçant dans l'hypothèse des régimes purs, de prévoir l'influence théorique des pressions de vapeur d'eau et d'oxygène sur la cinétique de ce phénomène de frittage, et ce, sans aucune hypothèse sur la géométrie des grains élémentaires d'anatase. La comparaison des vitesses expérimentales de chute de surface spécifique, à degré d'avancement constant, effectuée sur des courbes cinétiques issues de calcination d'anatase pure sous atmosphère contrôlée (vapeur d'eau et oxygène), montre que la vitesse de réaction V peut s'écrire : V=K_v P_h₂O^1/2 P_O₂^-1/12. La confrontation des résultats théoriques et expérimentaux indique que l'on est probablement en régime pur d'une réaction élémentaire de diffusion d'ions hydroxyles à la surface de l'anatase.

In this work the electronic and structural properties of anatase TiO2 (100) surface and gold adsorption have been investigated by using the first-principles calculations based on density functional theory (DFT). TiO2 is a wide band-gap material and to this effects it finds numerous applications in technology such as, cleaning of water, self-cleaning, coating, solar cells and so on. Primarily, the relation between the surface energy of the anatase (100)-1x1 phase and the TiO2-layers is examined. After an appropriate atomic layer has been chosen according to the stationary state of the TiO2 slab, the adsorption behavior of the Au atom and in the different combinations are searched for both the surface and the surface which is supported by a single Au atom/atoms. It has been observed that a single Au atom tends to adsorb to the surface which has an impurity of Au atom or atoms. Although, the high metal concentration on the surface have increased the strength of the adsorption, it is indicated that the system gains a metallic property which is believed to cause problems in the applications. In addition, the gold clusters of the dimer (Au2) and the trimer (Au3) have been adsorbed on the surface and their behavior on the surface is investigate. It is observed that the interaction between Au atoms in the atomic cluster each other is stronger than that of gold clusters and the surface.

BaSIC, which stands for Bioclastic and Shape-preserving Inorganic Conversion, is shape-preserving chemical conversion process of biological (or man-made) silica structures for producing complex 3-D microscale structures. This dissertation reports the BaSIC reaction of halide gases (i.e., TiF4, ZrF4, and ZrCl4) with 3-D silica structures, (i.e., diatom frustules, silicified direct-write assembly scaffolds, and Stöber silica spheres) to produce titania and zirconia replicas of the original 3-D structures. The kinetics of reaction of silica with titanium tetrafluoride gas is analyzed by using a novel HTXRD reaction chamber, nitrogen adsorption, and transmission electron microscope (TEM). The crystal structure and the temperature-induced phase transformation (from the room temperature hexagonal R-3c structure to the higher temperature cubic Pm3m structure) of polycrystalline TiOF2 that was synthesized through metathetic reaction of silica with TiF4(g) is reported. Additionally, potential applications of the converted titania diatom frustules (i.e., as a fast micron-sized ethanol sensor, and as a pesticide hydrolyzing agent) are also demonstrated in this work.

L'objet de cette thèse est l'étude par spectroscopie Raman de la taille et la distribution de taille de nanopoudres de TiO2 anatase. L'utilisation de plusieurs techniques de synthèse (chimie douce, microémulsion inverse, synthèse hydrothermale continu) a permis d'obtenir des matériaux dont la taille (de 3 à 20 nm), la forme, la phase sont parfaitement contrôlées et avec une distribution de taille resserrée. L'utilisation des modes optiques à travers le suivi de la raie Eg de l'anatase est la méthode la plus employée pour déterminer la taille des nanocristaux. Cependant, cette raie est entre autres sensible à la non-stoechiométrie. Afin de s'affranchir de ces effets, l'analyse des modes acoustique a été réalisée. Les distributions de taille obtenues par spectroscopie Raman sont comparées avec celles obtenues par MET. L'influence de l'état d'adsorption de la surface des nanocristaux de TiO2 a également été étudiée par spectroscopie Raman in situ.