Metal-organic frameworks (MOFs) have attracted much attention in the past few decades due to their ordered, crystalline nature, synthetic tunability, and porosity. MOFs represent a class of hybrid inorganic-organic materials that have been investigated for their applications in areas such as gas sorption and separation, catalysis, drug delivery, and electron or proton conduction. It has been the goal of my graduate research to investigate MOFs for their ability to transport electrons and store and separate gases for ultimate catalytic applications in alternative energy generation. I aim to provide new insight into the design and development of stable MOFs for such applications. We first investigated a cobalt(III) porphyrin based MOF comprised of Co(II)-carboxylate nodes, designated as CoPIZA, for its electron transport capabilities. Thin films of CoPIZA were formed solvothermally on conductive fluorine-doped tin oxide (FTO) substrates and used for electrochemical characterization. Electrochemistry coupled with spectroscopic analysis of the CoPIZA film revealed reversible reduction of the cobalt centers of the porphyrin linkers with maintenance of the overall framework structure. The mechanism of charge transport throughout the film was facilitated by redox hopping of electrons between the metal centers of the nodes and linkers. The ability to incorporate desired properties, such as pore functionalities or open metal centers, into frameworks makes them attractive for applications in separation of gaseous mixtures, such as CO2/N2 from combustion power plants. To investigate the selective adsorption properties, we performed gas sorption measurements on bulk MOF materials to determine their affinity toward CO2. Two Zn-based MOFs containing 2,5-pyridine dicarboxylate linkers were prepared in our laboratory and contained unsaturated Zn(II) metal centers, which possess a binding site on the metal without an activation procedure to remove bound solvent molecules. These MOFs were compared to the well-known Zn-based MOF-69C containing 1,4-benzene dicarboxylate linkers. Thermodynamic analysis of the gas sorption data revealed that the mechanism of CO2 binding involved the coordinatively unsaturated Zn(II) center. The microporous MOF also demonstrated selectivity for CO2 over N2 under the same conditions. As these materials were able to uptake CO2, their ability to transport electrons was also investigated for ultimate applications in catalysis. Electrochemical impedance spectroscopy was performed on the bulk MOF powders and was coupled with solid-state nuclear magnetic resonance spectroscopy. These results determined that the conduction mechanism proceeded via solvent molecules within the pores of the framework. The catalytic ability toward water oxidation of two MOFs was investigated electrochemically. Initial studies focused on a cobalt-based MOF comprised of 2-pyrimidinolate (pymo) linkers, designated as Co(pymo)2, which was prepared on FTO via drop-casting and used for electrochemical experiments. At applied anodic potentials, the CoII centers of Co(pymo)2 became oxidized to form a Co-oxide species on the electrode surface, which was found to be the active catalysis for water oxidation. Further investigations utilized a notably more stable Zr-based MOF with nickel(II) porphyrin linkers, designated as PCN-224-Ni. PCN-224-Ni was prepared solvothermally on FTO and used directly for electrochemical water oxidation. The mechanism of water oxidation at PCN-224-Ni proceeds via oxidation of the porphyrin macrocycle followed by binding of water to the Ni(II) center. Cooperative proton transfer to the Zr-oxo node facilitated water oxidation with the eventual release of O2. Thorough characterization revealed that PCN-224-Ni retained its structural integrity over the course of electrochemical catalysis. These results have allowed us a deeper understanding of the mechanisms of electron transport and conduction throughout frameworks. Specifically, the incorporation of metalloporphyrin molecules with redox active metal centers coupled with the presence of redox active metal nodes resulted in redox hopping charge transport throughout the MOF. In addition, the presence of solvent molecules in the pores of the framework provided an extended network for charge transport. We have gained insight into the structure-function relationship of MOFs for applications in selective gas sorption, where an unsaturated metal center serves as the binding site for gas molecules. Finally, through selection of the components that comprise the framework, a stable metalloporphyrin MOF was found to be capable of electrochemically facilitating the water oxidation reaction. As a result, we have gained valuable insight into the properties of frameworks necessary for charge transport and stability, which will allow for further improvements in the smart design of MOFs for catalytic applications. / Ph. D.

Designing new materials for CO₂ capture and utilization is one of the most challenging research topics. Metal-organic frameworks (MOFs) are one of the most efficient CO₂ adsorbents, as well as an emerging class of heterogeneous catalysts for CO₂ chemical transformations. Highlighted by their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities, MOFs can serve as highly stable and reusable heterogeneous catalysts and provide a great platform to explore the structure-function relationships for transforming CO₂ into useful chemicals. In this dissertation, we aim to develop a new class of metal-cyclam based robust MOFs as porous materials for CO₂ uptake as well as efficient catalysts for CO₂ chemical transformations, including CO₂ chemical fixation, CO₂ photo- and electroreduction. Chapter 1 introduces the concept and main challenges of CO₂ capture and conversion. The potential of metal-cyclam complexes as molecular catalysts for CO₂ conversion is also mentioned. The current state of the art in designing stable MOFs and azamacrocyclic-based MOFs is briefly discussed. Finally, the strategies, challenges and future outlook of using MOF as catalysts in CO₂ chemical transformation are summarized. Metal-organic frameworks (MOFs) as highly ordered, tunable hybrid materials have shown great promise in photon collection, energy transfer and photocatalytic reactions. In Chapter 2, the fundamental principles of energy transfer in the condensed phase are summarized, and a series of studies in light-harvesting, excited state quenching and photo-excited reactivity occurring within ruthenium-polypyridyl-doped zirconium MOFs are reviewed. The application of MOFs in energy conversion devices such as dye-sensitized solar cells (DSSC) is also discussed. Chapter 3 reports two new robust 3D porous metal-cyclam based Zr-MOFs, VPI-100 (Cu) and VPI-100 (Ni) with potential as heterogeneous catalysts for CO2 chemical fixation. The frameworks are prepared by a modulated synthetic strategy and the structure highlighted by eight-connected Zr₆ clusters and metallocyclams as organic linkers. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO₂ uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zr-based MOFs for the chemical fixation of CO₂ with epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance. A follow-up study of CO₂ chemical fixation using Hf analogs of VPI-100 is presented in Chapter 4. Structural characterization and catalytic performance of Hf-VPI-100 are summarized. Moreover, a detailed comparison of VPI-100 and Hf-VPI-100 is made. In situ powder X-ray diffraction (PXRD), quartz crystal microbalance (QCM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) have been used to probe the interaction between the guest molecules (CO₂/epoxide) and Hf-VPI-100. For CO₂, no specific chemical binding sites in MOFs has been observed and the uptake of CO₂ does not change the crystal structure of Hf-VPI-100. Both QCM and DRIFTs revealed the irreversible binding between the framework and 1,2-epoxybutane. The epoxide uptake per unit cell of VPI-100 MOFs and diffusion coefficients have been calculated by QCM analysis. Transition metal complexes capable of visible light-triggered cytotoxicity are appealing potential candidates for photodynamic therapy (PDT) of cancer. In Chapter 5, two monometallic polyazine complexes, [(Ph₂phen)₂Ru(dpp)]²⁺ and [(Ph₂phen)₂Os(dpp)]⁺ (Ph₂phen = 4,7-diphenyl-1,10-phenanthroline; dpp =2,3-bis(2-pyridyl)pyrazine), were synthesized, characterized and studied as light activated drugs to kill rat malignant glioma F98 cells. Both compounds display strong absorption in visible spectrum, oxygen-mediated DNA and BSA photocleavage and significant photocytotoxicity under blue light irradiation along with negligible activity in the dark. The compounds show approximately five-fold higher cytotoxicity compared the traditional chemotherapeutic drug, cisplatin. Furthermore, [(Ph₂phen)₂Os(dpp)]⁺ shows promising photocytotoxicity in F98 rat malignant glioma cells within the phototherapeutic window with an IC50 value of (86.07±8.48) µM under red light (625 nm) irradiation. In Chapter 6, the mixed-metal supramolecular complex, [(Ph₂phen)₂Ru-(dpp)PtCl₂]²⁺, was found to display significant DNA modification, cell growth inhibition, and toxicity towards F98 malignant glioma cells following visible light irradiation. The design of this complex has a significantly higher potential for membrane permeability than three other FDA-approved anti-cancer agents, including cisplatin, and exhibited a dramatic ten-fold higher uptake by F98 cells than cisplatin in a two-hour window. Based on studies with a rat glioma cell line, the compound has very low cytotoxicity in the dark, but results in substantial cell death upon light treatment. The complex is thus among the first to exhibit all the hallmarks of a very promising new class of PDT agents. / Ph. D. / Increased carbon dioxide (CO₂) emissions have triggered a series of environmental effects, including global warming and ocean acidification. Scientists are trying to develop new materials to capture and convert CO₂ into useful chemical products. However, the main challenge is that CO₂, the gas generated upon burning fossil fuels, has strong C=O bonds that are hard to break. In other words, it is too stable to be easily changed into other compounds. A class of highly porous materials known as metal-organic frameworks (MOFs) possess significant potential for CO₂ adsorption uptake and chemical fixation. MOFs are metal ions or clusters held together by organic linkers to make highly ordered, crystalline 3D structures with tunable porosity and functionality. The design and synthesis of MOFs is similar to playing with Legos at the molecular level; you need to pick the right pieces (metal nodes and linkers) to get your desired structure. In this dissertation, we aim to develop a new class of macrocycle complexes based stable MOFs as porous materials for CO₂ uptake as well as efficient catalysts for CO₂ chemical transformations. We have developed two new stable three dimensional porous frameworks, VPI-100 (Cu) and VPI-100 (Ni) as catalysts for CO₂ chemical fixation. The new 3D robust MOFs named VPI-100 (VPI = Virginia Polytechnic Institute) are assembled by the reaction of zirconium oxo clusters and linkers bearing metal complexes. Using the metal complexes as the linker provides additional metal active sites in the framework that can act as accessible catalytic centers for CO₂ conversion. The VPI-100 MOFs are not only able to convert CO₂ to cyclic carbonates (important industrial chemicals) in high efficiency (~ 98%), but also can be reused for multiple cycles. The heterogeneous catalyst can be easily recovered from the reaction mixture by centrifugation and the active metal centers are earth-abundant transition metals (Cu and Ni), which are cost effective. Additionally, VPI-100 MOFs also show high CO₂ uptake capacity (up to ~10 wt%) at ambient pressure. Since the MOFs can enhance the local concentration of CO₂ around the active catalytic centers located inside the pores of the framework, these materials could be used as catalysts for flow chemistry, which is widely used in industry. We further investigated the CO₂ chemical fixation using Hf analogs of VPI-100. Structural characterization and catalytic performance of Hf-VPI-100 are summarized. Moreover, a detailed comparison of VPI-100 and Hf-VPI-100 is made. Different analytical techniques have been used to further understand the reaction mechanism as well as the interaction between the CO₂/epoxide and the frameworks. These insights would help us to design new MOFs as better catalysts for practical applications.

Heterogeneous electrocatalysis is the usage of solid materials to decrease the amount of energy needed to produce chemicals using electricity. It is of core importance for modern life, as it enables production of chemicals, such as chlorine gas and sodium chlorate, needed for e.g. materials and pharmaceuticals production. Furthermore, as the need to make a transition to usage of renewable energy sources is growing, the importance for electrocatalysis used for electrolytic production of clean fuels, such as hydrogen, is rising. In this thesis, work aimed at understanding and improving electrocatalysts used for these purposes is presented. A main part of the work has been focused on the selectivity between chlorine gas, or sodium chlorate formation, and parasitic oxygen evolution. An activation of anode surface Ti cations by nearby Ru cations is suggested as a reason for the high chlorine selectivity of the “dimensionally stable anode” (DSA), the standard anode used in industrial chlorine and sodium chlorate production. Furthermore, theoretical methods have been used to screen for dopants that can be used to improve the activity and selectivity of DSA, and several promising candidates have been found. Moreover, the connection between the rate of chlorate formation and the rate of parasitic oxygen evolution, as well as the possible catalytic effects of electrolyte contaminants on parasitic oxygen evolution in the chlorate process, have been studied experimentally. Additionally, the properties of a Co-doped DSA have been studied, and it is found that the doping makes the electrode more active for hydrogen evolution. Finally, the hydrogen evolution reaction on both RuO2 and the noble-metal-free electrocatalyst material MoS2 has been studied using a combination of experimental and theoretically calculated X-ray photoelectron chemical shifts. In this way, insight into structural changes accompanying hydrogen evolution on these materials is obtained. / Heterogen elektrokatalys innebär användningen av fasta material för att minska energimängden som krävs för produktion av kemikalier med hjälp av elektricitet. Heterogen elektrokatalys har en central roll i det moderna samhället, eftersom det möjliggör produktionen av kemikalier såsom klorgas och natriumklorat, som i sin tur används för produktion av t ex konstruktionsmaterial och läkemedel. Vikten av användning av elektrokatalys för produktion av förnybara bränslen, såsom vätgas, växer dessutom i takt med att en övergång till användning av förnybar energi blir allt nödvändigare. I denna avhandling presenteras arbete som utförts för att förstå och förbättra sådana elektrokatalysatorer. En stor del av arbetet har varit fokuserat på selektiviteten mellan klorgas och biprodukten syrgas i klor-alkali och kloratprocesserna. Inom ramen för detta arbete har teoretisk modellering av det dominerande anodmaterialet i dessa industriella processer, den så kallade “dimensionsstabila anoden” (DSA), använts för att föreslå en fundamental anledning till att detta material är speciellt klorselektivt. Vi föreslår att klorselektiviteten kan förklaras av en laddningsöverföring från ruteniumkatjoner i materialet till titankatjonerna i anodytan, vilket aktiverar titankatjonerna. Baserat på en bred studie av ett stort antal andra dopämnen föreslår vi dessutom vilka dopämnen som är bäst lämpade för produktion av aktiva och klorselektiva anoder. Med hjälp av experimentella studier föreslår vi dessutom en koppling mellan kloratbildning och oönskad syrgasbildning i kloratprocessen, och vidare har en bred studie av tänkbara elektrolytföroreningar utförts för att öka förståelsen för syrgasbildningen i denna process. Två studier relaterade till elektrokemisk vätgasproduktion har också gjorts. En experimentell studie av Co-dopad DSA har utförts, och detta elektrodmaterial visade sig vara mer aktivt för vätgasutveckling än en standard-DSA. Vidare har en kombination av experimentell och teoretisk röntgenfotoelektronspektroskopi använts för att öka förståelsen för strukturella förändringar som sker i RuO2 och i det ädelmetallfria elektrodmaterialet MoS2 under vätgasutveckling. / <p>QC 20151119</p>

Novel ceramic membrane cells of BaCe₀.₅Zr₀.₃Y₀.₁₆Zn₀.₀₄O[subscript(3-δ)] (BCZYZ), a proton-conducting oxide, have been developed for electrocatalytic ammonia synthesis. Unlike the industrial Haber-Bosch process, in this work an attempt to synthesise ammonia at atmospheric pressure has been made. The membrane cell fabricated by tape casting and solution impregnation comprises of a 200 μm-thick BCZYZ electrolyte and impregnated electrode composites. Electrocatalysts for anode and cathode were investigated. For the anode, the co-impregnation of Ni and CeO₂ provided excellent electrode performance including high catalytic activity, sintering stability and compatibility with the BCZYZ electrolyte. The best composition was the mixture of 25 wt% NiO and 10 wt% CeO₂. A symmetrical cell prepared with this electrode composition revealed low polarisation resistances of 1.0 and 0.45 Ωcm² in humidified 5% H₂/Ar at 400 and 500 °C, respectively. For the cathode, 25 wt% of impregnated Fe oxide provided a satisfactory performance in non-humidified N₂ atmosphere. Significant amounts of ammonia were produced from the single cell with Ni-CeO₂ anode and Fe oxide cathode at 400-500 °C under atmospheric pressure. Ammonia formation rate was enhanced by Pd catalyst addition and electrochemical performance was improved by Ru addition. The highest ammonia formation rate of 4 x 10⁻⁹ mols⁻¹cm⁻² was attained using the cell with a Pd-modified Fe cathode at 450 °C. The formation reaction of ammonia typically consumed around 1-2.5 % of total applied current while most of the applied current was employed in H⁺ reduction. The total current efficiency of around 90-100 % could be obtained from the membrane cells.

This study concerned the preparation, characterisation and evaluation of different proton conductors for the Reverse Water Gas Shift Reaction (RWGS) and their evaluation as electrolytes for Solid Oxide Fuel Cells (SOFC) under H₂ and O₂. Materials with both catalytic and conductive properties are of a great interest, as their use in electrocatalytical systems may be very important. Sr₃CaZr₀.₅Ta₁.₅O₈.₇₅ (SCZT), BaCe₀.₉Y₀.₁O₂.₉₅ (BCY10) and Ba₃Ca₁.₁₈Nb₁.₈₂O₈.₇₃ (BCN18), were the initial materials in this study. Thermogravimetric analysis under different atmospheres (5%H₂/Ar, Ar, 5%CO₂, etc), were performed on SCZT and BCN18; with both materials being shown to be stable under reducing and oxidising conditions. Conductivity measurements by two terminal a.c. impedance were also conducted on SCZT and BCN18 under oxidising and reducing atmospheres. As found in literature, BCN18 showed mixed conductivity; with electronic conductivity at high temperatures and pure ionic conductivity below 550ºC, This behaviour was shown in chapter 3, where the change on conduction process was observed upon different gas feeds. Its maximum conductivity values for the different atmospheres were: 8.50x10⁻⁵ S/cm (Dry 5%H₂/Ar at 200ºC), 4.24x10⁻⁴ S/cm (Wet 5%H₂/Ar at 500ºC) and 4.48x10⁻³ S/cm (Air at 900ºC). SCZT proton conducting behaviour was also measured (wet and dry 5%H₂/Ar). Exhibiting an order of magnitude higher in total conductivity upon hydration of the gas feed (σdry=1.01x10⁻⁶ and σwet=1.07x10⁻⁵ at 450ºC). The doping of barium cerate with Zr and Zn by Tao and Irvine, lead to a more stable and easily sinterable BaCe₀.₅Zr₀.₃Y₀.₁₆Zn₀.₀₄O₃ (BCZYZ). Following this work, the introduction of ZnO as a sintering aid to SCZT and BCN18 resulted in Sr₃CaZr₀.₄Ta₁.₅Zn₀.₁O₈.₇₅ (SCZTZ), and Ba₃(Ca₁.₁₈Nb₁.₇₀Zn₀.₁₂)O₈.₅₅ (BCNZ); with higher final densities (above 90% dense). As with BCN18, BCNZ also exhibited mixed conductivity; with higher total conductivity values than BCN18. A maximum of total conductivity of 1.85x10⁻³ S/cm at 900ºC for BCNZ was measured against 6.99x10⁻⁴ S/cm at 900ºC for BCN18. A change in conductivity process was observed when using air or wet 5%H₂/Ar, achieving a maximum of 3.85x10⁻⁴ S/cm at 400ºC when under wet hydrogen. All materials (as powders) have shown catalytic activity for the reverse water gas shift (RWGS) reaction, with the lowest conversion temperature onset at 400ºC for SCZT and a maximum conversion of CO₂ to CO of 42%, with circa 0.52 and 0.59 mmol/s.m² of CO produced at 900ºC by BCN18 and BCZYZ, respectively. No relation between mechanisms for the RWGS and σ for these materials were expected below 10% conversion, as no correlation was found between their activation energies. BCY10 as shown a partial decomposition when exposed to the RWGS reaction, for what BCZYZ After fuel cell testing under H₂ and O₂ both SCZTZ and BCNZ showed mixed conductivity. SCZTZ under different hydrogen partial pressures, exhibited a behaviour close to a pure proton conductor, however, when exposed to both reducing and oxidising conditions, its behaviour did not follow the theoretical values. On the other hand, BCNZ behaves as a pure ionic conductor below 500ºC; with increasing influence of the electronic conductivity on temperature increase. However, as seen for BCNZ conductivity data from 2 terminal a.c. impedance, below 650ºC wet 5%H₂ exhibited the highest conductivity values. This, in additions to the pure ionic conductive behaviour below 400ºC (from the effective ionic transport number), suggests that BCNZ becomes closer to a pure proton conductor with temperature decrease.

Le réchauffement climatique est dû principalement à l'émission anthropique du dioxyde de carbone (CO2) dans l'atmosphère. Une réduction électrocatalytique et sélective de cette molécule a été proposée au cours de ce projet comme une solution prometteuse pour synthétiser des produits à valeur ajoutée. Une telle réaction requiert l'utilisation de matériaux efficaces et bas coût. Pour ce faire, les travaux de cette thèse ont porté sur la préparation de catalyseurs à base de cuivre dispersés sur différents substrats carbonés tels que le Vulcan XC-72R, les carbones mésoporeux CMK-3 et FDU-15, et des tanins à base d'IS2M pour réduire le CO2 en milieu aqueux. Les matériaux d'électrode ont été préparés à l'aide de la méthode polyol assistée par micro-ondes. Leurs caractérisations physiques et l'analyse élémentaire confirment des compositions atomiques et des taux de charge métallique proches de celles théoriquement envisagées. L'acide formique et le monoxyde de carbone sont les deux produits carbonés issus de la réduction du CO2 (2 bar) réalisée par chronoampérométrie en milieu NaHCO3. La détection et l'identification des produits de réaction ont été effectuées par des méthodes chromatographiques (µ-GC et HPLC), spectrométrique (DEMS) et spectroscopique (RMN). Une sélectivité de la réaction vis-à-vis de HCOOH (62 %) a été obtenue sur une cathode de Cu50Pd50/C. Cette conversion sélective du CO2 en HCOOH s'explique par une conjugaison d'effets électroniques et géométriques dans la structure de surface du catalyseur bimétallique et aussi celui de la texture du substrat carboné. / The anthropogenic emissions of carbon dioxide (CO2) are the major cause of global warming. The selective CO2 reduction reaction (CO2RR) of has been proposed as a promising, convenient and efficient method for sustainable energy conversion systems. The reduction of CO2 to energetically valuable products requires the use of an appropriate electrode material. This study focuses on the preparation of Cu-based electrocatalysts supported on different types of carbon materials such as Vulcan XC-72R, mesoporous carbon CMK-3, mesoporous carbon FDU-15 and tannin based mesoporous carbon IS2M for the CO2RR under mild conditions. Besides, Vulcan XC-72R carbon supported bimetallic copper/palladium alloy materials were prepared for increasing the Faradaic yield. These copper-based catalysts were electrochemically characterized and preparative electrolyses set at constant potential were carried out in order to investigate the reduction products distribution and Faradaic yields as a function of the applied potential and catalyst loading. Chemicals such as HCOOH, CO and H2 issued from the CO2RR, were determined with in-situ and ex-situ complementary (electro)analytical and spectroscopic techniques. The significant difference in the product distribution is probably due to the ensemble (geometry and ligand) effects in the bimetallic CuPd materials, and textural structure of the supporting substrates. Selective CO2 to-HCOOH conversion has been successfully undertaken on Cu50Pd50/C with 62 % Faradaic efficiency.

O dióxido de manganês (MnO2) é um catalisador eficiente de baixo custo utilizado no cátodo de baterias do tipo metal-ar e células a combustível alcalinas, sendo capaz de promover a redução completa de oxigênio pela rota 4e-. No entanto, o dióxido de manganês é um semicondutor e só pode ser utilizado como material eletródico nos dispositivos mencionados se combinado com algum suporte condutor. O suporte condutor mais utilizado para este fim é o carbono em pó. Entretanto, este material não possui estabilidade suficiente nas condições operacionais das células alcalinas, sendo convertido gradativamente em CO2. Uma das possíveis estratégias para tentar minimizar esta deficiência é incrementar a condutividade eletrônica do óxido puro pela dopagem com alguns cátions metálicos. Sendo assim, este trabalho tem como objetivo geral pesquisar de maneira sistemática o efeito da dopagem de dióxido de manganês com alguns cátions metálicos, como o Bi3+e Ce4+ nas propriedades físico-químicas e eletrocatalíticas deste óxido, visando o uso dos mesmos como em cátodos de baterias recarregáveis do tipo Zn-ar. As análises das características morfológicas dos catalisadores por meio de MEV e TEM mostram que os óxidos de manganês são gerados na forma de nano-bastões de 50 a 100 nm de comprimento. Os óxidos puros e dopados com bismuto e cério apresentam estruturas tetragonais típicas, ocorrendo expansão da célula unitária dos óxidos dopados pela troca de íons manganês pelos correspondentes dopantes na rede cristalina de MnO2. Os resultados eletroquímicos sugerem um aumento de condutividade do óxido dopado que possibilita seu uso sem mistura com carbono. Além disso, observa-se que a RRO é catalisada por um mecanismo que envolve a transferência de 4e- nestes materiais com participação de peróxido como intermediário. O óxido de manganês dopado com Bi apresentou promissor desempenho catalítico para a RDO, o que junto com os demais resultados apresentados para a RRO o qualificou a funcionar como o catalisador bifuncional mais promissor de todos os estudados em baterias do tipo metal-ar. Experimentos realizados em mini baterias do tipo Zn-ar demonstraram a total capacidade do catalisador dopado com bismuto operar como catalisador do eletrodo de ar, resultando num desempenho superior ao de um catalisador convencional de MnO2/C. / Manganese dioxide is at the same time an efficient and low-cost material used as cathode catalyst in the air electrode of metal-air and alkaline fuel cells, capable to promote the complete reduction of oxygen thru the 4e- mechanism. However, manganese dioxide is a semiconductor and can be used as electrodic material in the mentioned devices only combined with a conductor support. High surface area carbon powder is the most commonly used material for such purpose. The problem is that carbon suffers from severe instabilities in the experimental conditions that fuel cells and metal-air batteries operates, being gradually converted into CO2. A possible strategy to overcome or at least minimize the low oxide conductivity is by doping this material with some metallic cations. In this sense, the main purpose of this work was the systematic investigation of the physicochemical and electrocatalytic properties of Bi3+ and Ce4+ doped manganese dioxide materials used as cathode catalysts in the air electrode of alkaline type Zn-air batteries. The morphologic characterization performed SEM and TEM revealed that pure as well cation doped MnO2 are formed as poly dispersed nanorods with 50-100 nm length. Both pure and doped materials presented typical tetragonal structures, although a cell expansion was observed in the doped oxides caused by the exchange of some manganese cations by the doping counter parts. Electrochemical results suggest that a material with increased conductivity results from the doping process, allowing it to operate as air catalyst without the use of a carbon support. Besides, it is observed that the oxygen reduction reaction proceeds thru the 4e- mechanism on the doped oxides involving hydrogen peroxide as intermediate. The Bi doped oxide presented the best performance for the oxygen evolution reaction among all catalysts investigated. This result together with the superior performance for the oxygen reduction reaction presented by this material suggest that Bi doped MnO2 is a potential candidate to operate as an air catalyst of rechargeable alkaline metal-air batteries. Experiments conducted in a mini Zn-air battery using Bi doped MnO2 as air catalyst corroborated this observation.

O presente trabalho apresenta um estudo sistemático sobre a adsorção e a eletrooxidação do CO sobre eletrodos monocristalinos de platina. A partir da análise das intensidades das bandas integradas e das freqüências do Pt(111)-CO, apresenta-se uma interpretação dos efeitos de acoplamento dipolo-dipolo e de interconversão do COads.. Assim, sobre a Pt(111) os espectros de FTIR in situ mostram que o aumento na razão da intensidade das bandas integradas ACOB/ACOL e nas freqüências do &nu;COB quando &theta;CO,total diminue é devido à redução do acoplamento dipolo-dipolo entre as moléculas do CO em diferentes sítios e, adicionalmente, à interconversão das formas inclinadas dos COL e COB para a forma do COB. No sentido de explicar esta interconversão, propomos um mechanism baseado nas interações dos orbitais de fronteiras do CO e do metal, associado com a retrodoação de elétrons. Nesse modelo, os deslocamentos das formas inclinadas do COL e do COB em direção à forma do COB são favoráveis provavelmente porque a retrodoação de elétrons, Ptd &rarr; CO2&pi;* (LUMO), aumenta quando &theta;CO,total diminui. Experimentos potenciostáticos sugerem que a cinética de nucleação e crescimento é o melhor modelo para descrever a eletrooxidação do CO. Propomos que no potencial de oxidação, ECO oxi. pode existir uma via muito rápida de formação do precursores oxigenados e que este pode lateralmente colidir com as ilhas de CO, impedindo que ocorra a dissipação das ilhas do COads. no potencial de oxidação, ECO oxi.. Apresentamos a evolução do crescimento e da oxidação de sub-monocamada de CO sobre monocristais de platina facetados. Em baixo grau de recobrimento do CO foi observado que a adsorção dessa molécula ocorre sem ocupação preferencial de sítios quinas ou terraças. Assim, sugerimos que a adsorção é um processo randômico e que depois que as moléculas do CO são adsorvidas estas não apresentam apreciáveis deslocamentos a partir de CO-(111) em direção aos sítios CO-(110). Isto significa que depois da adsorção, as moléculas do CO têm um longo tempo de residência ou que apresentam um coeficiente de difusão muito baixo. Mas, para alto grau de recobrimento por CO, os resultados mostram que é possível que laterais interações desempanham importantes papéis na distribuição de ocupação dos sítios e observamos que durante a eletrooxidação, são liberados simultaneamente sítios quinas e sítios terraços. Quanto à pré-oxidação, foi observado que quatro condições experimentais precisam ser satisfeitas para que ela ocorra sobre os monocristais de platina: (i) alto grau de recobrimento por CO; (ii) que a superfície onde oncorre a oxidação do CO tenha defeitos, como sítios quinas (110); (iii) que a camada do CO seja formada sob potenciais mais negativos do que o potencial de carga total zero do metal; (iv) e que exista pequena quantidade de CO dissolvido. As condições (i) e (ii) precisam ser satisfeitas simultaneamente para promover a pré-oxidação do CO; as condições (iii) e (iv) essencialmente contribuem correspondendo à condição (i). Observamos que a magnitude do pre-pico aumenta com o aumento do grau de recobrimento por CO. Então, isto pode ser indicativo que a pré-oxidação não tem relação com a difusão do CO em superfície porque o aumento do grau de recobrimento reduz a probabilidade de difusão em superfície. O modelo de ilhas comprimidas parece ser mais apropriado para descrever a pré-oxidação do CO. / This work presents a systematic study on the CO adsorption and its oxidation at platinum single crystal electrodes. From analysis of integrated band intensity and band frequency position of the Pt(111)-CO interface in acid, it is presented an interpretation of the dipole-dipole coupling effect and surface site inter-conversions of COads.. Thus, on Pt(111), in situ FTIR data show that the increase in both ratio integrated band intensity ACOB/ACOL and frequency of &nu;COB when &theta;CO,total reduces it is indicative of reduce in dipole-dipole coupling interactions between CO molecules in different surface active sites and a mechanism where the tilted COL and COB in CO pressed adlayer displace or inter-convert in favor of increase of COB concentration. In order to explain that CO interconversion, we propose a mechanism based in frontier molecular orbitals of CO and the orbitals of the metal associated with the electron back bond donation. Thus, the displacement of tilted COL and COB on the surface towards COB is more stable because probably the back bond electron donation, Ptd &rarr; CO2&pi;* (LUMO), increase when &theta;CO,total diminishes. Potentiostatic experiments suggest that the nucleation and growth is the better model to describe the CO oxidation. It is proposed here that close to ECO oxi. might there is a fast pathway toward formation of oxygenated species and it might reach the CO islands by side and this hinder the dissipation of COads. islands at ECO oxi.. We report also time evolution studies of low CO adsorption coverage and oxidative stripping on stepped platinum surfaces. In low CO coverage, it was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. But, in high CO coverage, the results show that it is possible that the lateral interaction might play important role in CO site occupancy and it was observed that during the CO electrooxidation the sites released included both terrace (111) and step (110) orientations. Among the CO oxidation a clear CO preoxidation process also occurs. It was observed four experimental conditions which were verified to be fulfilled to promote CO pre-oxidation on platinum single crystal: (i) the CO coverage is should be higher than minimum threshold; (ii) the surface where CO oxidation take place should have defects, such as (110) steps; (ii) the CO monolayer should be formed at potentials below the potential of zero total charge; (iv) and in a small amount of dissolved CO should be present in the electrolyte solution. In both conditions (i) and (ii) are necessary to take place simultaneously to promote CO pre-oxidation, (iii) and (iv) essentially contribute in fulfilling condition (i). It was verified that the magnitude of pre-peak increases with the amount of CO coverage. Thus, this might indicate that the CO pre-oxidation is not having relationship with the CO diffusion on the surface, because the increase of CO coverage diminishes surface diffusion. A picture model of compressed CO islands seems the most to describe CO pre-oxidation.

O estudo da eletro-oxidação de moléculas orgânicas pequenas, altamente energéticas e ecologicamente sustentáveis, tem ganhado importância no cenário tecnológico atual com o desenvolvimento de atuadores e dispositivos eletrônicos biomiméticos. Ao longo das últimas décadas, avanços no entendimento de mecanismos envolvidos na eletrocatálise destas moléculas levaram luz à existência de comportamento auto-organizado durante as reações destas sobre diversos catalisadores metálicos na ausência e presença de um suporte polimérico. Nesse contexto, a reação de eletro-oxidação de glicose é uma candidata interessante como fonte de elétrons em dispositivos eletrônicos, gerando 24 elétrons em sua oxidação total a CO2. Os objetivos desta tese foram, então, investigar a reação de eletro-oxidação da molécula de glicose, com foco na cinética não-linear sobre diferentes superfícies utilizando Pt, Au, Cu, e polianilina (PANI) (como material de suporte do eletrodo de trabalho de Pt) nas oscilações de potencial observadas. Para isso, foram utilizadas técnicas potencio/galvanodinâmicas, potencio/galvanostáticas, Espectroscopia de Impedância Eletroquímica (EIS) e Nanobalança Eletroquímica de Cristal de Quartzo (EQCN). A reação de eletro-oxidação de glicose se mostrou uma rica fonte de oscilações periódicas de potencial (sobre Pt e Au) e de corrente (sobre Au) em meio alcalino, com oscilações periódicas estáveis, de longa duração e em uma ampla região de parâmetros, porém, a fraca adsorção dessa molécula sobre Cu pode ter sido o principal motivo da ausência de instabilidades neste catalisador. A propriedade das oscilações sobre Au e Pt, em meio alcalino, em apresentar altas amplitudes de potencial tem a capacidade de promover a autolimpeza periódica da superfície do catalisador, prolongando o tempo de vida deste e aumentando sua eficiência. Além disso, a sensibilidade das oscilações de potencial a diferentes adsorbatos permitiu o acesso a informações mecanísticas dificilmente obtidas por demais técnicas eletroquímicas. Estudos nanogravimétricos com a EQCN e eletrodos modificados com PANI mostraram que as variações de massa em condições oscilatórias são determinadas majoritariamente pelo fenômeno de compensação de cargas dentro do filme polimérico e que a oxidação da molécula orgânica é responsável pela variação majoritária de carga no mesmo. O aumento na tolerância destes catalisadores em relação à adsorção de CO e outros intermediários e presença de oscilações de potencial mostra uma mudança no mecanismo reacional em eletrocatalisadores de PANI em comparação a Pt e Au. O processo volumétrico, de aumento e diminuição do volume do suporte polimérico durante as oscilações de potencial, pode ser considerado no edesigne de atuadores auto-oscilantes em dispositivos autônomos. Por fim, a investigação da cinética não-linear na reação de eletro-oxidação de glicose sobre Au foi relatada por este trabalho pela primeira vez na literatura. / The study of the electro-oxidation of small organic molecules, highly energetic and ecologically sustainable, has gained importance in the current technological scenario with the development of actuators and biomimetic electronic devices. Over the last decades, advances in the understanding of the mechanisms involved in the electrocatalysis of these molecules have led light to the existence of self-organized behavior during their reactions on several metal catalysts and metal-polymer composites. More recently, new research has shown that such a phenomenon can be used in the control of the poisoning of catalytic surfaces by self-cleaning the surface, using such oscillatory reactions, increasing the catalyst life time and the potency of practical devices. In this context, the electro-oxidation reaction of glucose is na interesting candidate as electron source in electronic devices, despite the difficulty in its total oxidation, generating 24 electrons in its complete oxidation to CO2. The objectives of this thesis were to investigate the electro-oxidation reaction of the glucose molecule, focusing on the non-linear kinetics on different catalyst surfaces used (Pt, Au and Cu) and the effect of the support for working electrode on potential oscillations. For this, potentio/galvanodynamic, potentio/galvanostatic, Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Quartz Crystal Nanobalance (EQCN) techniques were used. The electro-oxidation reaction of glucose was shown to be a rich source of periodic potential oscillations (over Pt and Au) and of current (over Au) in alkaline medium, with stable periodic oscillations of long duration and in a wide range of parameters, but the low adsorption of this molecule on Cu may have been the main reason for the absence of instabilities in this catalyst. In contrast to the high current densities observed for the oxidation of glucose in alkaline medium over Pt and Au, in acid medium the low glucose activity led to low current densities over Pt and unstable potential oscillations in a more constrained range of parameters. In addition, the nonlinear kinetics of the electro-oxidation reaction of glucose on Au was reported by this work for the first time in the literature. Nanogravimetric studies with EQCN with polyaniline-modified electrodes have shown that the potential oscillations during the electro-oxidation of formic acid are strongly influenced by the process of charge compensation in the polymer film, for both the electrode of Pt coated with PANI (Pt/PANI) and for Pt nanoparticles deposited in this polymer support (Au/PANI/Pt). However, the decrease in the potential values where oscillations are observed and the decrease in their frequency can be explained by the increase in the tolerance of the Pt nanoparticles to the adsorption of CO and other intermediates, as reported by several authors in the literature. And in Pt nanoparticle electrodes supported by films such as PANI, the charge variation is still dominated by the organic oxidation reaction and the mass variation in the surface of the catalyst is dominated by the charge compensation effect within the film. Thus, the decrease of the catalyst affinity by organic adsorption makes the dynamic instabilities kinetics slower, as observed for the Pt, Au and PANIbased electrodes. And the higher tolerance of the catalyst supported on the polymer electrode by strong adsorbates, such as CO, does not prevent the emergence of oscillatory behavior, however, it can direct the oxidation reaction of the organic by a different mechanism pathway.

Recentemente, o mecanismo pelo qual o ácido fórmico é oxidado tem gerado debate na literatura. Há discordância em relação às vias pelas quais o processo ocorre e também qual seria o principal intermediário de uma das vias. Como forma de se obter novas informações sobre este sistema, há trabalhos na literatura explorando diferentes condições experimentais, como por exemplo, a adição de um aditivo. Dentre eles, a hidrazina foi eleita por ser outra molécula de interesse para aplicação em dispositivos de conversão de energia química em elétrica. Assim argumenta-se que a presença da hidrazina não interfere na eletro-oxidação do ácido fórmico e, portanto, gera uma resposta total aditiva da soma das partes individuais. Ao se utilizar o estudo do comportamento complexo de um sistema como metodologia, pode-se encontrar novas informações a respeito deste. Desta forma, foi descoberto que o comportamento ao invés de aditivo seria sinergético e que há mudanças significativas na série temporal do ácido fórmico, como um grande aumento na duração do processo e a alteração de algumas de suas variáveis. Também foi observada uma mudança no comportamento das oscilações potenciostáticas, indicando uma dependência do processo com a superfície utilizada. Foi proposto que a hidrazina atuaria reduzindo o acúmulo de espécies oxigenadas na superfície do eletrodo prevenindo que a série temporal terminasse brevemente. Em seguida empregou-se uma técnica espectrométrica (DEMS) para avaliar a produção de produtos gasosos (CO2) e descobriu-se que, na presença de hidrazina, o ácido fórmico oxida-se de forma facilitada, em potenciais mais baixos. Propôs-se que, além de prevenir o acúmulo de espécies oxigenadas, a hidrazina perturbaria a decomposição do ácido fórmico para a geração de CO, permitindo uma oxidação direta em potenciais mais baixos. Finalmente, para se aprofundar o entendimento dos processos superficiais, utilizou-se a técnica EMSI para se obter uma imagem da superfície. Foi descoberto que a decomposição do ácido fórmico a COads ocorre gerando uma frente reacional que se repete ciclo após ciclo durante a série temporal e que é possível monitorar a variação de adsorbatos por uma mudança na intensidade da imagem. Não foi possível obter dados na presença da hidrazina por conta da presença de bolhas. Como conclusão entende-se neste trabalho que há evidências o suficiente para apontar que a eletro-oxidação do ácido fórmico assistido por hidrazina ocorre, não de forma aditiva, mas sim de forma sinergética. / Recently, the mechanism by which formic acid is oxidized is a matter of debate on the literature. There is disagreement on the pathways that the process may occur as well as which would be the intermediates participating. In this sense, there are some work exploring another aspect of this reaction, such as its behavior facing the addition of an additive. Among them, hydrazine has been chosen as it is another molecule of interest for energy generation devices such as fuel cells. In this fashion, it is argued that the presence of hydrazine would not interfere in the electro-oxidation of formic acid and, therefore, would yield an additive current when being co-oxidized. The complex behavior of a system may display new and relevant information thus this methodology was employed to revisit this system. It was found that the system would behave, instead of the argued additive behavior, synergistically and that there are striking differences on the time-series of formic acid, such as an increase on the duration of the process and the alteration of some of its variables. Also, it was observed a change in the potentiostatic oscillations, showing a dependence of the process with the morphology of the surface employed. It was proposed that hydrazine would act reducing the accumulation of oxygenated species on the surface of the electrode, postponing the end of the time-series. Next, it was employed a spectrometric technique (DEMS) to evaluate the production of gaseous products (CO2) and it was found that, in the presence of hydrazine, formic acid gets oxidized in a more facile way, in lower overpotential values. It was proposed that, besides preventing the accumulation of oxygenated species, hydrazine would disturb the decomposition of formic acid to COads, allowing a direct oxidation in lower overpotentials. Finally, for deepening the understanding of the superficial processes it was employed an imaging technique (EMSI). It was discovered that the decomposition of formic acid to COads there is a reactional front that repeats itself cycle after cycle during the time-series and that it is possible to monitor changes in the coverage of adsorbates by changes in the intensity of the image. It was not possible to obtain data in the presence of hydrazine since it generates many bubbles that disrupt the experiment. As conclusion of this work it is presented the thesis that, with the amount of evidences herein presented, the interaction between formic acid and hydrazine is synergistical rather than additive, as stated on the literature.