This thesis presents an integrated study of nanocatalysts for heterogenous catalytic and electrochemical processes using pure ruthenium (Ru) with mixed-phase and platinum-based nanomaterials synthesized by continuous-flow chemistry. There are three major challenges to the application of nanomaterials in heterogenous catalytic reactions and electrocatalytic processes in acidic solution. These challenges are the following: (i) controlling the size, shape and crystallography of nanoparticles to give the best catalytic properties, (ii) scaling these nanoparticles up to a commercial quantity (kg per day) and (iii) making stable nanoparticles that can be used catalytically without degrading in acidic electrolytes. Some crucial limitations of these nanostructured materials in energy conversion and storage applications were overcome by continuous-flow chemistry. By using a continuous-flow reactor, the creation of scalable nanoparticle systems was achieved and their functionality was modified to control the nanoparticles’ physical and chemical characteristics. The nanoparticles were also tested for long-term stability, to make sure these nanoparticles were feasible under realistic working conditions. These nanoparticles are (1) shape- and crystallography-controlled ruthenium (Ru) nanoparticles, (2) size-controlled platinum-metal (Pt-M= nickel (Ni) & copper (Cu)) nanooctahedra (while maintaining morphology) and (3) core-shell platinum@ruthenium (Pt@Ru) nanoparticles where an ultrathin ruthenium shell was templated onto the platinum core. Thus, a complete experimental validation of the formation of a scalable amount of these nanoparticles and their catalytic activity and stability towards the oxygen evolution reaction (OER) in acid medium, hydrolysis of ammonia borane (AB) along with plausible explanations were provided.

Over the past decades, since the discovery of a single–site silica-supported catalyst for the alkane metathesis reaction by our group, we have been extensively working on the development of supported catalytic systems for the improved alkane metathesis reaction. During these developments, we understand the reaction mechanism and reached a new perspective for the synthesis of various supported bimetallic systems via the surface organometallic chemistry (SOMC) approach. Recently, with this bi-metallic system, we got a very high TON (10000) in propane metathesis reaction. As these catalysts are very efficient for linear alkanes we thought to apply it for cyclo-alkanes specifically, for cyclo-octane metathesis expecting better activity. Besides, the value of the ring alkanes are higher than the linear alkanes. The current work demonstrates a combination of [(ΞSi−O−)W(Me)5] and [(ΞSi− O−)Ti(Np)3 pre-catalyst with several supports (SiO2-700, SBA-15 and MCM-41) for metathesis of cyclooctane. The catalysts have been synthesized and fully characterized by elemental analysis (EA), FT-IR and NMR spectroscopies. After fully characterization the bi-metallic catalyst was tested for metathesis of cyclooctane with highest ever TON 2500 as compared to that of mono-metallic catalyst where we got 430 TON. Which again corroborates our prediction that bimetallic catalysts are better catalysts than monometallic catalysts.

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A series of niobium-containing nitride and carbides were prepared by a temperature-programmed synthesis method. The catalysts synthesized comprised a monometallic niobium oxynitride and a new bimetallic oxycarbide supported system, Nb-Mo-O-C/Al₂O₃ (Mo/Nb = 1.2; 1.6; 2.0). In the case of the niobium oxynitride, the progress of formation was analyzed by interrupting the synthesis at various stages. The effect of the heating rate on product properties was also investigated. The solid intermediates and the final niobium oxynitride were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental analysis (CHNS), and gas adsorption techniques. The solid state transformation occurred directly from Nb₂O₅ to NbN_xO_y without any suboxide intermediates. The bimetallic supported oxycarbide materials were also characterized by X-ray diffraction (XRD), gas adsorption techniques, X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS). It was found that the electronic properties of the oxycarbide were modified by the interaction with the Al₂O₃ support, and that most of the oxygen atoms were associated with the niobium rather than the molybdenum atom. All of the niobium-containing catalysts were tested in a three-phase trickle-bed reactor for the simultaneous hydrodenitrogenation (HDN) of quinoline and hydrodesulfurization (HDS) of dibenzothiophene. The niobium oxynitride presented low HDS activity and moderate HDN activity, whereas the supported bimetallic oxycarbide was found to be highly active for both, HDN and HDS, demonstrating higher activities than the commercial sulfided Ni-Mo/Al₂O₃ when compared on the basis of active sites. In addition to these studies a comprehensive investigation of the HDN reaction mechanism was carried out over bulk unsupported Mo₂C, NbC, NbMo₂-O-C, and compared with the mechanism over a sulfide catalyst, MoS₂/SiO₂. For this purpose, a comparison of the HDN rate of a series of isomeric amines was performed, and the reaction occurred mainly through a β-elimination mechanism for all catalysts. Temperature programmed desorption of ethylamine was used to investigate the acid properties of the catalytic surfaces, and a good agreement between the specific rate of reaction and the number of Brønsted acid-sites was obtained. Infrared spectroscopy showed that the amines interacted with acidic centers to form adsorbed quartenary ammonium species. The deamination reaction over the carbide and sulfide catalysts probably occurs by a concerted push-pull mechanism involving basic sulfur species and Brønsted-acidic centers. In order to obtain more insight into the mechanism a study of the pyridine HDN network was carried out.All of the catalysts showed the same activity trend: the reactivity of n-pentylamine was high, while those of piperidine and pyridine were relatively low. The carbide catalysts showed higher selectivity towards HDN products than the sulfide catalyst at the same conversion levels. The higher selectivity was related to the higher ratio (r = k₂/k₁) between the rate constants of the two consecutive reactions, hydrogenation of pyridine (k₁) and ring opening of piperidine (k₂). The order of activity of the carbides and sulfide differed considerably depending on the substrate. However, for the pyridine reaction network the similarity in product distribution suggested that a similar surface composition, a carbosulfide, was attained during the reaction. / Ph. D.

The need for green and renewable fuels has led to the investigation of ways to exploit renewable resources. Solar among all the renewables is the most powerful and its conversion into usable energy would help in solving the energy problem our society is facing. Photocatalytic water splitting for hydrogen production is an example of solar energy storage into chemical bonds. The hydrogen produced in this way can then be employed as carbon free fuel creating the “Hydrogen Cycle”. This work investigates the structure and the activity of graphitic carbon nitride (g-C₃N₄), an organic semiconductor that proved a suitable photocatalyst for hydrogen production from water. Synthesised by thermal polycondensation of melamine it is a graphitic like material with a band gap of 2.7 eV which makes it a visible light active catalyst. In a first instance the effect of the synthesis conditions on its structure and morphology are investigated to find the optimum parameters. The temperature of condensation is varied from 450°C up to 650°C and the length from 2.5 h to 15 h. The structural changes are monitored via X-ray diffraction (XRD) and elemental analysis while the effect on the morphology and the band gap of g-C₃N₄ are investigated by mean of scanning electron microscopy and UV-Vis absorption. Subsequently, a study of the crystal structure of the catalyst is carried out. Using structures proposed in the literature, X-ray diffraction and neutron scattering simulations are used to narrow down the number of possible 3D structures. After structural characterisation, the activity of g-C₃N₄ for photocatalytic hydrogen evolution is evaluated. It is confirmed that loading 1 wt.% Pt on its surface significantly increases the hydrogen evolution rate. The attention then focuses on the loading procedures, the reduction pre treatments of the co-catalyst and the reasons of the different performances when different procedures are employed. The catalytic system is characterised by mean of X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and XRD. By investigating the composition and the morphology of the platinum nanoparticles under different conditions, the main factors responsible for the changes in activity of g-C₃N₄ for hydrogen evolution are identified. Additionally, the role of the co catalyst and its interaction with g-C₃N₄ is also elucidated. Finally, taking forward the knowledge acquired on the Pt-g-C₃N₄ system, the effect on the hydrogen evolution rate of alloying platinum with a second metal (Cu, Ag, Ni and Co) is studied. The nanoparticles are characterised by XRD and TEM. A screening of the loading procedures and bimetallic systems is performed to identify the most promising for photocatalytic hydrogen evolution with the aim of bringing them towards further investigation.

Nanopartículas bimetálicas de AuPd têm mostrado excelente atividade catalítica em reações de oxidação. O entendimento dos efeitos da variação da composição e morfologia das nanopartículas bimetálicas em suas propriedades catalíticas é fundamental para a preparação de catalisadores cada vez mais ativos e seletivos. Neste trabalho foram estudadas nanopartículas bimetálicas de AuPd de composição variável suportadas sobre um suporte constituído por nanopartículas de magnetita revestidas por sílica. O efeito da calcinação e da redução com hidrogênio sobre a morfologia e composição das nanopartículas bimetálicas foi acompanhado pelas técnicas de TEM, XEDS, XAS, XRD e XPS. A correlação entre estrutura, composição e atividade catalítica dos catalisadores preparados foi estudada pelo acompanhamento de reações de oxidação de monóxido de carbono e de oxidação de álcool benzílico. As amostras não calcinadas apresentaram segregação metálica em todas as composições estudadas. Após a etapa de calcinação, maior segregação metálica foi encontrada, com a formação de óxido de paládio na superfície das nanopartículas, exceto na amostra mais rica em ouro. O tratamento das amostras oxidadas com hidrogênio foi capaz de reduzir os metais oxidados na superfície das nanopartículas, mas um enriquecimento em paládio na superfície e maior segregação entre ouro e paládio foram observados. Uma melhora na atividade catalítica na oxidação de monóxido de carbono foi observada juntamente com um aumento na composição de paládio, além disso, observou-se uma maior atividade catalítica em relação às nanopartículas não calcinadas para as amostras calcinadas e reduzidas. Para a oxidação de álcool benzílico um aumento na atividade catalítica de até cinco vezes foi observado após a calcinação dos catalisadores, com maior atividade para a amostra de composição Au1Pd2. A queda na atividade catalítica após a redução dos catalisadores mostrou que a presença de óxido de paládio na superfície das nanopartículas é fundamental para que seja observada uma maior atividade catalítica. / AuPd bimetallic nanoparticles have shown excellent catalytic performance in oxidation reactions. Understanding the effects of composition and morphology of the bimetallic nanoparticles in their catalytic properties is key for the preparation of more active and selective catalysts. In this work, we studied AuPd bimetallic nanoparticles supported over silica coated magnetite nanoparticles. The effects of calcination and reduction with hydrogen on the morphology and composition of the bimetallic nanoparticles was investigated by TEM, XEDS, XAS, XRD and XPS. The correlation between structure, composition and catalytic activity of the prepared catalysts was studied by the oxidation of carbon monoxide and benzyl alcohol. The non calcined samples showed metal segregation for all investigated compositions. After the calcination step, greater metal segregation occurred, and the formation of palladium oxide on the surface of the nanoparticles was observed, except for the most gold-rich sample. The treatment of the oxidized samples with hydrogen reduced the metal oxides on the surface of the nanoparticles, but a palladium surface enrichment and greater metal segregation between gold and palladium was observed. The catalytic activity observed for the oxidation of carbon monoxide increased with increasing palladium concentrations. An increase in catalytic activity was observed for the calcined and reduced samples when compared to their non-calcined counterparts. For benzyl alcohol oxidation, a five-fold increase in catalytic activity after calcination was observed, with a peak in catalytic activity for the Au1Pd2 sample. A drop in catalytic activity after the reduction of the oxidized catalysts with hydrogen showed that the presence of palladium oxide on the surface of the nanoparticles is fundamental for an increased catalytic activity.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-09-15T17:59:28Z No. of bitstreams: 1 AugustoSilva.pdf: 4699819 bytes, checksum: cf18a43cf817a3264880d015853561f6 (MD5) / Made available in DSpace on 2017-09-15T17:59:29Z (GMT). No. of bitstreams: 1 AugustoSilva.pdf: 4699819 bytes, checksum: cf18a43cf817a3264880d015853561f6 (MD5) Previous issue date: 2017-08-16 / In this work the theoretical study of the bimetallic clusters of Au-Ag and Au-Cu was carried out, in which such clusters were determined by the addition of Ag and Cu atoms in pure structures of Au clusters, through the GA-DFT joint methodology. The AuCu and Au-Ag structures were described in a classical way using the Gupta potential via the genetic algorithm. The lower energy structures were optimized via DFT using the PBE functional through the SIESTA 3.2 program package. The most stable clusters were those with 13 atoms for Au-Ag bimetallic clusters, Au-Cu bimetallic clusters stabilized with 13, 15 and 20 atoms. Both clusters have a degree of aromaticity, more pronounced in the AuAg atoms, than in the Au-Cu atoms. The Au-Ag clusters adsorb the oxygen more favorably in end-on or bridge, whereas the bimetallic clusters of Au-Cu adsorb the molecular oxygen generally according to the bridge or double bridge model. The adsorption of oxygen in bimetallic clusters is intrinsically related to the ability of oxygen to perform π backdonation for the metallic atoms in the cluster. / Neste trabalho fora realizado o estudo teórico dos clusters bimetálicos de Au-Ag e Au-Cu, em que tais clusters foram determinados através da adição de átomos de Ag e Cu em estruturas puras de clusters de Au, através da metodologia conjunta GA-DFT. As estruturas de Au-Cu e Au-Ag, foram descritas de forma clássica usando o potencial Gupta via algoritmo genético. As estruturas de mais baixa energia foram otimizadas via DFT usando o funcional PBE através do pacote de programas SIESTA 3.2. Os clusters de maior estabilidade foram os com 13 átomos para clusters bimetálicos de Au-Ag, os clusters bimetálicos de Au-Cu estabilizaram com 13, 15 e 20 átomos. Ambos os clusters possuem um certo grau de aromaticidade, mais pronunciada nos átomos de Au-Ag, que nos átomos de Au-Cu. Os clusters de Au-Ag adsorvem o oxigênio de forma mais favorável em end-on ou ponte, enquanto que os clusters bimetálicos de Au-Cu adsorvem o oxigênio molecular em geral segundo o modelo ponte ou dupla ponte. A adsorção do oxigênio nos clusters bimetálicos está intrinsecamente relacionada à habilidade do oxigênio em realizar retrodoação π para os átomos metálicos.

Pt-group metals were some of the first metals to be studied as catalysts for industrial use. The goal of these studies was to ascertain a fundamental understanding of CO oxidation and acetylene cyclotrimerization reactions on Ptgroup metals. A further goal was to determine the optimal conditions for each reaction. CO oxidation on Rh(111),Pt(100), and Pd(100) was scrutinized on various oxide surfaces from chemisorbed to bulk metal oxides. Low pressure reactions on Rh(111) reveal the highest activity was a CO uninhibited surface with <1ML of chemisorbed oxygen. Pt(100) high pressure oxidation revealed that only <1ML oxygen is formed during high pressures reactions. High pressure CO oxidation reactions on Pd(100) show oxygen penetration after CO has been consumed; however, during the highest activity XPS found only chemisorbed species. The cyclotrimerization of acetylene to benzene is another reaction found in industry typically carried out on Pd. The active site is considered to be a 7 atom configuration with 6 atoms surrounding a central atom. By adding relatively catalytically inert Au atoms to the active Pd(111) surface the acetylene coupling activity is enhanced. Cyclization activity is a function of the surface composition and the surface structure. A single Pd atom surrounded by six Au atoms is found to have the highest activity at 300K for acetylene cyclotrimerization.

This thesis reports on the synthesis and characterization of PtNi dendrimer-encapsulated nanoparticles (DENs) containing on average 147 atoms. This is significant because PtNi DENs have not yet been reported. The DENs were prepared by first complexation of Pt²⁺ to the interior tertiary amines of a sixth-generation, hydroxyl-terminated (G6-OH) poly(amidoamine) (PAMAM) dendrimer template, followed by chemical reduction in the presence of free Ni²⁺ to yield PtNi DENs. UV-visible (UV-vis) absorbance measurements exhibit a broad, monotonically decreasing band characteristic of nanoparticle formation. Upon dialysis in both H₂ and O₂ this band is observed to decrease in absorbance. Transmission electron microscopy (TEM) studies indicate that particles have been synthesized and are 1.8 + 0.2 nm before dialysis and 1.9 + 0.2 nm after dialysis under H₂. Results obtained from X-ray photoelectron spectroscopy (XPS) show that Pt is present and the Pt(4f7/2) binding energy is observed at 72.0 eV before dialysis and 71.5 eV after dialysis under H₂. XPS shows that Ni is present and the Ni(2p3/2) binding energy is centered at 857.0 eV before dialysis and 856.6 eV after dialysis under H₂. Finally, oxidative electrochemical stripping is observed at 1.07 V (vs NHE) for PtNi DENs immobilized on glassy carbon electrodes (GCE) and is tentatively assigned to Ni. / text