Analytical modeling of the root cause of cylinder-to-cylinder variation and the impact on CO2 emission caused by the reduction in engine efficiency <br>

Record high CO2 emissions in the atmosphere and the need to find alternative energy sources to fossil fuels are major global challenges. Conversion of CO2 into useful fuels like methanol and methane can in principle tackle both these environment and energy concerns. One of the routes to convert CO2 into useful fuels is by using supported metal catalyst. Specifically, metal atoms or clusters (few atoms large in size) supported on oxide materials are promising catalysts. Experiments have successfully converted CO2 to products like methanol, using TiO2 supported Cu atoms or clusters. How this catalyst works and how CO2 conversion could be improved is an area of much research. We used a quantum mechanical tool called density functional theory (DFT) to obtain atomic and electronic level insights in the CO2 reduction processes on TiO2 supported metal atoms and clusters. We modeled small Cu clusters on TiO2 surface, which are experimentally synthesizable. Our results show that the interfacial sites in TiO2 supported Cu are able to activate CO2 into a bent configuration that can be further reduced. The Cu dimer was found to be the most reactive for CO2 activation but were unstable catalysts. Following Cu, we also identified other potential metal atoms that can activate CO2. Compared to expensive and rare elements like Pt, Au, and Ir, we found several early and mid transition metals to be potentially active catalysts for CO2 reduction. Because the supported metal atom or cluster is a reactive catalyst, under reaction conditions they tend to undergo aggregation and/or oxidation to form larger less active catalysts. We chose Co, Ni, and Cu group elements to study their catalyst stability under oxidizing reaction conditions. Based on the thermodynamics of Cu oxidation and kinetics of O2 dissociation, we found that TiO2 supported Cu atom or a larger Cu tetramer cluster were the likely species observed in experiments. Our work provides valuable atomic-level insights into improving the CO2 reduction activities and predicts potential catalysts for CO2 reduction to valuable fuels.

Reverse combustion is a process converting CO2 into its different reduced/hydrogenated forms while, ideally, oxygen is being released. Understanding how CO2 is interacting/reacting with vanadium (main component in the CB of m-BiVO4) in different oxidation states was our main goal. In this thesis we have attempted to contribute to the ongoing efforts for overcoming the formidable challenges posed by H2 production and CO2 activation. In the process to prove the role of each metal during a reverse combustion process mediated by m-BiVO4, several strategies were followed to prepare pure monoclinic BiVO4 using different starting materials (Chapter 2). Hydrothermal processes in an autoclave were determined as the most efficient way to obtain m-BiVO4. Photoirradiation experiments were performed in-situ and analyzed by EPR, demonstrating that a photoexcited species was generated. EPR spectra were compared with VO2, which suggested that one electron is being transferred from the VB to the CB in the photoexcitation process, in this case forming a vanadium(IV). This experiment suggested that the reduction process of CO2 is possibly occurring through a one-electron transfer process. Several attempts were made unsuccessfully to prepare a bismuth vanadate-like compound containing only vanadium(IV) in its structure. Bi4V2O10 was obtained where the vanadium atom was present in a lower oxidation state but with different Bi/V ratio than in BiVO4. This species does not present any photo-catalytic activity. Instead, it presented mild reactivity in hydrogen formation from formaldehyde in basic media. Photocatalytic experiments on pure m-BiVO4 in the presence of water and CO2 were performed and methanol was obtained as a product. In this process, vanadium leached out from the structure affording a mixture of V(IV) and V(V). On the surface of the remaining m-BiVO4, Bi2O4−x was deposited as a result of the loss of vanadium. The initial idea behind the preparation of a compound different to BiVO4 was to produce a new photocatalyst that preserves the electronic characteristics of vanadium(V) as well as being a semiconductor (Chapter 3). In addition, a higher oxidation state than the vanadium +5 could provide longer electron-hole recombination times and increase lifetime of the photogenerated electrons. By having a +6-oxidation state, such as provided by a Cr atom, it could give a better chance to improve the reduction of CO2 by facilitating oxygen release. Unfortunately, photochemical activity was not observed under any conditions. On the other hand, both monoclinic and orthorhombic BiOHCrO4 were tested for formic acid thermal decomposition. These two unique crystal structures were analyzed by single crystal XRD. The monoclinic isomer displays a much higher thermal resilience and was chosen for the degradation of formic acid studies. During the process, an active species of BiCrO4 was formed and identified. When using vanadium aryloxide compounds in an oxidation state lower than +5 as possible reagents to reduce CO2, interesting results were obtained (Chapter 4). These compounds were prepared aiming at mimicking the reduction of CO2 as performed by hypothetically formed lower valent vanadium. As presented in chapter 2, during the photoirradiation of BiVO4 a new vanadium species is formed. EPR experiments indicated that it could be V(IV). As a result, while vanadium(IV) showed negligible reduction/interaction with CO2, vanadium(III) aryloxide was a powerful reductant. Experiments attempting to control the electron transfer to CO2 resulted in two different outcomes. Firstly, a two-electron transfer from the metal center to CO2 was obtained affording CO and vanadyl(V) tris-aryloxide. Secondly the introduction of a halogen in the metal coordination sphere of a vanadium(III) compound triggered a radical behavior. The use of a compound of vanadium(II) with polydentate oxygen-donor based ligand still yielded CO. However, an intermediate V-O-V moiety was formed in turn performing radical H atom extraction from the solvent through an unprecedented pattern of reactivity. DFT calculations confirmed the nature of the electronic transfer and the formation of V-O-V that acted as an intermediate for the second CO2 interaction. We successfully arrested the reaction to isolate an intermediate and an unprecedented (ONNO)V(OH)-OCO compound was isolated and fully characterized. This CO2 complex provide the second example of a linearly end on bonded CO2 and the first case of such a bonding mode to a transition element. A further study of the reactivity of the vanadium trivalent state was carried out by modifying the ligand to H2ONOO and secondly, by introducing a Cl atom as in LV-Cl (L = ONNO or ONOO) to enable the formation of derivative such as p-methoxy-phenoxide and methoxide ligands via simple ligand substitution. Unfortunately, the (ONOO)2- ligand quenched the reducing power such that no reaction was observed with CO2. Halogen replacement afforded (ONNO)V(p-methoxy-phenoxide)(THF) which displayed no reactivity with CO2, but once the p-methoxy-phenoxide ligand was replaced by a methoxide group, formaldehyde and formate were formed. The DFT proposed mechanism presented an interesting interaction wherein the cis- position in [V(ONNO)]+ is responsible for the H transfer to occur Finally, we have prepared a heterobimetallic system containing Bi-V atoms (Chapter 6). The oxidation states of Bi and V were +3 and +5 respectively. One pot reaction was the most adequate procedure to obtain the heterobimetallic structure. Trasmetallation on Bi compounds by V atoms was observed when attempting to build the heterobimetallic structure using more rational reaction pathways. Attempts to obtain a heterobimetallic structure in oxidation states different than that presented in m-BiVO4 were unsuccessful. When oxidation states lower than +5 for vanadium (vanadium(III-II)) and +3 in bismuth were used, metallic bismuth and untreatable materials with a mixed-valence vanadium were formed.

Metal-organic frameworks (MOFs) have been extensively studied because of their amazing applications in gas storage, purification, photocatalysis, chemical sensing, and imaging techniques. Ruthenium polypyridyl complexes have been broadly considered as photosensitizers for the conversion of solar energy and photoelectronic materials. With this aspect, we have synthesized three new ruthenium polypyridyl based MOFs ([Ru(H2bpc)Cu(bpc)(Hbpc)2(H2O)]·5H2O (1), [Ru(H2bpc)(Fe(bpc)(Hbpc)2(H2O)2]·6H2O (2) and [Ru(H2bpc)Ni(bpc)(Hbpc)2(H2O)2]·6H2O (3)) from ruthenium(III) chloride, bpc (2,2’- bipyridine-4,4’-dicarboxylic acid) ligand, and 3d M(II) metal ions (M(II)= Cu(II), Fe(II), Ni(II)). These MOFs were synthesized under hydro or solvothermal conditions by using water, ethanol or methanol as solvents. The crystal structures of the new compounds contains zigzag chains of [Ru(bpc)3]n- complex ions linked by Cu, Fe or Ni complex ions individually. Above synthesized crystal structures were characterizing by single-crystal Xray and powder X-ray diffraction strategies, UV-vis and IR spectroscopy. Thermal properties were determining by thermogravimetric analysis. Magnetic properties were also studied.

A crescente emissão do CO2 para a atmosfera, causada pela matriz energética dependente dos combustíveis fósseis tem gerado a necessidade de sistemas que o utilizem como matéria-prima para a produção ou armazenamento de energia. Em vista disso, este trabalho teve como objetivo o estudo do ciclo de estocagem de hidrogênio baseado em etapas eletrocatalíticas da eletro-redução e eletro-oxidação do par CO2/HCOO-. Para o processo de eletro-redução, foram utilizados eletrocatalisadores suportados em pó de carbono formados à base de estanho (Sn/C) e de estanho modificado com cobalto (Co-Sn/C), cobre (Cu-Sn/C) e paládio (Sn-Pd/C). Os materiais foram sintetizados pelo método de impregnação seguido por tratamento térmico e caracterizados fisicamente por Difratometria de Raios X (DRX) e Espectroscopia por energia Dispersiva de Raios X (EDX). Os testes eletroquímicos foram realizados via cronoamperometria (eletrólise) e a quantificação dos íons formato por Cromatografia Líquida de Alta Eficiência (CLAE) e voltametria cíclica (VC). Os resultados obtidos mostraram que os materiais nanoestruturados sintetizados apresentaram estruturas cristalinas, sendo que o estanho apresentou-se na forma de SnO2, mas sofrendo eletro-redução em condições in situ para SnO ou SnOH. Os resultados eletroquímicos mostraram que o Sn/C eletrocatalisa a redução do CO2 para HCOO-, sendo que a quantificação por VC utilizando eletrodos de paládio e platina indicaram correntes de pico crescentes até o potencial de eletrólise de -1,6 V vs. Ag/AgCl/Cl-. Ademais, experimentos de eletrólise evidenciaram o aumento linear da concentração de HCOO- após 6 horas de polarização, indicando alta estabilidade do eletrocatalisador de Sn/C. A atividade eletrocatalítica dos eletrocatalisadores à base de estanho frente a redução de CO2 para HCOO- foi atribuída a dois aspectos: (i) o estanho favorece a adsorção ou interação do CO2 através dos átomos de oxigênio, possibilitando a transferência de prótons e elétrons sem a quebra da ligação C-O e/ou; (ii) a presença de espécies SnOH na superfície, mesmo em baixos potenciais, permite a interação com o CO2 e leva à formação de intermediários adsorvidos reativos, que sofrem a adição de prótons e elétrons para a formação de HCOO-. A eficiência máxima de corrente faradaica para a formação de HCOO- foi de aproximadamente 7 % tendo a reação de desprendimento de hidrogênio (HER) como rota paralela. A investigação da influência da natureza do eletrocatalisador mostrou inatividade do material de Co-Sn/C, mas com aumento da atividade de Cu-Sn/C para a eletro-redução de CO2, quando comparado com Sn/C puro. / With the increase CO2 emissions into atmosphere caused mainly by the energy dependence on fossil fuels, systems for generation or storage of clean energy has been studied to couple CO2 as feedstock. This work proposed a hydrogen storage cycle based on electrocatalytic steps of pair CO2/HCOO-, such electroreduction and electrooxidation. For electroreduction process were used carbon-supported tin-based electrocatalysts (Sn/C) and tin modified with cobalt (Co-Sn/C), copper (Cu-Sn/C) and palladium (Sn-Pd/C). The materials were synthesized by impregnation method followed of thermal treatment, and X Ray Diffraction (XRD) and Energy Dispersive X-ray Spectroscopy (EDS) techniques were used for physical characterization. Electrochemical tests were performed via chronoamperometry (electrolysis) and the quantification of formate ions by High Performance Liquid Chromatography (HPLC) and cyclic voltammetry (CV). Results of synthesized nanostructured materials showed crystalline structures with tin as SnO2 species, but tin oxide suffering electroreduction to SnO or SnOH in situ conditions. Electrochemical results presented that the Sn/C catalyzes the CO2 reduction to HCOO-, with an increase peak current until electrolysis potential of -1.6 V vs. Ag/AgCl/Cl- quantified by CV on palladium and platinum electrodes. Moreover, electrolysis measurements demonstrated the linear increase of HCOO- concentration after polarization for 6 hours, which indicates the high stability of Sn/C electrocatalyst. The electrocatalytic activity of tin-based electrocatalysts for CO2 reduction into HCOO- was attributed to two aspects: (i) tin favors the adsorption or interaction of CO2 through oxygen atoms, which enables the proton and electron transfer without breaking C-O bond and/or; (ii) the presence on surface of SnOH species allows the interaction with CO2 even at low potential, and leads to the formation of reactive intermediates adsorbed that undergo addition of protons and electrons to form HCOO-. Maximum Faradaic efficiency for HCOO- formation was near 7% with Hydrogen Evolution Reaction (HER) as parallel route. Investigation of the influence of the electrocatalyst nature showed inactivity of CO-Sn/C material, but the activity of CO2 electroreduction increased on Cu-Sn/C material as compared to Sn/C pure.

La medición de efectos isotópicos es una herramienta importan­te en el estudio de las transformaciones químicas. El uso de efec­tos isotópicos de átomos ligeros como el deuterio es muy común e incluso aparece en muchos textos básicos de química. Lamen­tablemente, el uso de efectos isotópicos de átomos pesados no ha recibido la misma atención a pesar de su gran utilidad. Este manuscrito sirve como introducción a este tema importante. / The determination of isotope effects is an important tool in the study of chemical transformations. Very common in the liter­ature is the use of deuterium isotope effects, which is typically covered in many textbooks. Unfortunately, heavy atom isotope effects have not received the same attention despite its great rel­evance. This article will serve as an introduction to this very important topic.

This thesis studies the development of a new method for photochemical overall water splitting using a small organic shuttle. In Section 2, BiVO4, was studied to determine the CO2 reduction mechanism and how catalytic activity decays. BiVO4 catalysts were capable of producing a maximum of 200 μmol of methanol per gram of catalyst from CO2 in basic media, and later decomposed by BiVO4. The decay of BiVO4¬ was studied by x-ray diffraction and scanning electron microscopy, demonstrating reversible decomposition of BiVO4. BiVO4 is etched, leeching vanadium into solution, while nanoparticles of bismuth oxide are deposited on the surface of BiVO4. In Section 3, ferrocyanide salts, an aqueous, cheap, and abundant photocatalyst was used for the first time to dehydrogenate aqueous formaldehyde selectively into formate and hydrogen. The catalyst is capable of record turnovers and turnover frequencies for formaldehyde dehydrogenation catalysts. A preliminary mechanism was proposed from experimental and computational data.

Bis(1,3,7,9-tetramethyl-5-mesityldipyrrinato)zinc(II) (ZnDPY) was synthesized in the lab by the McCusker group and a procedure was created to analyze its ability as a photosensitizer, a molecule that provides the energy for the reaction to occur by capturing light energy and turning it into a form that can be used by the photocatalyst. While more work is needed, preliminary steps have been made to create a process that can analyze the amount of carbon monoxide produced by a photocatalytic CO2 reduction reaction with ZnDPY as the photosensitizer. Progress has been made via the setup of a reaction apparatus, targeted gas chromatography (GC) peak separation, and GC calibration. More work will need to be done in order to determine the optimal reaction mix to showcase the sensitizer’s potential.

Microbial electrosynthesis (MES) has recently emerged as a novel biotechnology platform for value-added product generation from waste CO2 stream. Integrating MES technology with renewable energy sources for both CO2 valorization and renewable energy storage is regarded as one type of artificial photosynthesis and a perfect example of circular bioeconomy. However, several challenges remain to be addressed to scale-up MES as a feasible process for chemical production, which include enhanced production rate, reduced energy consumption and excellent resistance to external fluctuations. To fill these knowledge gaps, different in-depth approaches were proposed in this dissertation by optimizing the cathode architecture, CO2 flow rates and utilizing efficient photoelectrode to improve MES performance and stability. A novel cathode design, made of conductive hollow fiber membrane, was developed in this dissertation to improve CO2 availability at MES cathode surface via direct CO2 delivery to chemolithoautotrophs through the pores in the hollow fibers. By modifying the hollow fiber surface with carbon nanotubes (CNTs), higher bioproduct formation was achieved with excellent faradaic efficiencies, which could be attributed to the improved surface area for bacterial adhesion and the reduction of cathodic electron transfer resistance. Since CO2 flow rate from industrial facilities typically varies over time, this hollow-fiber architecture was also applied to test the resistance of MES systems to CO2 flow rate fluctuation. Stepwise increase of CO2 flow rates from 0.3 ml/min to 10 ml/min was tested and the effect of CO2 flow rate fluctuations was evaluated in terms of biochemical generation and microbial community. MES was further integrated with renewable energy supply for both energy storage and CO2 transformation into biofuels and biochemicals. Stable MES photoanode, based on molybdenum-doped bismuth vanadate deposited on fluorine-doped tin oxide glass (FTO/BiVO4/Mo), was prepared for efficient solar energy harvesting and overpotential reduction for oxygen evolution reaction (OER), which contributed to one of the highest solar-to-biochemical conversion efficiencies ever reported for photo-assisted MES systems. The applied nature of this dissertation with fundamental insights is of great importance to bring MES one step closer to full-scale applications and enable MES technology to be economically more viable for renewable energy storage and CO2 valorization.

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