Catalisadores de platina para reações de produção de hidrogênio

Orientador: Daniela ZanchetTese (doutorado) - Universidade Estadual de Campinas, Instituto de QuímicaResumo: O objetivo do presente trabalho foi o estudo de catalisadores nanoestruturados aplicados à reação de reforma a vapor do glicerol (glycerol steam reforming, GSR). A reação de deslocamento gás-água (water-gas shift, WGS) também foi investigada por ser etapa fundamental no aumento da produção de hidrogênio por esta rota. Os sistemas são catalisadores suportados à base de platina e foram divididos em dois grupos, um contendo óxido de vanádio e o outro, óxido de cério. Os resultados do primeiro grupo evidenciaram o efeito benéfico da adição de vanádio na reação de WGS através da maior conversão de CO. Por outro lado, verificou-se que o aumento do teor de vanádio não impacta na atividade catalítica devido à formação de espécies poliméricas VOx na superfície do suporte que não estão em contato com a Pt. Ainda, a presença do vanádio não favoreceu o aumento da fração de hidrogênio na reação de GSR, pois promoveu a formação de subprodutos, o que resultou no consumo do H2 produzido. O segundo grupo contém catalisadores à base de Pt/CeO2 suportados em sílica produzidos a partir de nanopartículas (NPs) de CeO2 de diferentes tamanhos médios (5 e 9 nm) ou impregnação do precursor de Ce, visando avaliar tanto o efeito da presença da céria quanto o de tamanho/dispersão na atividade catalítica. Os resultados indicaram o aumento expressivo do desempenho nas reações de WGS e GSR na presença da céria, no entanto a comparação entre os catalisadores com os diferentes tamanhos de CeO2 não mostrou alterações de mesma magnitude. A céria se mostrou essencial na reação de GSR por facilitar o desbloqueio dos sítios mais ativos da Pt, liberando-os para interagir com o glicerol através da remoção do CO fortemente ligado como CO2. Utilizando a metodologia de excitação modulada acoplada à espectroscopia no infravermelho por reflectância difusa com transformada de Fourier (modulation-excitation diffuse reflectance infrared Fourier transformed spectroscopy, ME-DRIFTS) em condições reacionais, foi possível identificar a formação de espécies carboxilatos e formiatos como as espécies ativas formadas na superfície dos catalisadores de céria durante a reação de WGS, o que pode contribuir para o melhor entendimento do comportamento destes sistemas nas duas reações de interesse. Desta forma, a sistemática investigação destes sistemas nanoestruturados resultou na identificação de propriedades que determinam seus desempenhos, que devem ser levadas em consideração no desenvolvimento racional de catalisadores mais ativos, estáveis e seletivosAbstract: The present work had as the main goal the study of nanostructured catalysts applied to glycerol steam reforming (GSR) reaction. The water-gas shift (WGS) reaction was also investigated since it is a fundamental step to increase hydrogen production by this approach. The systems are Pt-based supported catalysts and they were divided in two groups, one containing vanadium oxide and the other, cerium dioxide. The results from the first group evidenced a beneficial effect of vanadium addition in WGS reaction, by increasing the CO conversion rates. On the other hand, it was noted that the increment in vanadium loading is not relevant due to the formation of polymeric VOx species on the support surface which are not in close contact with Pt. Moreover, the presence of vanadium did not enhance hydrogen production during GSR reaction since it promoted the formation of lateral products which resulted in the consumption of the produced H2. The second group has silica supported Pt/CeO2 based catalysts which were produced from CeO2 nanoparticles (NPs) with different mean sizes (5 and 9 nm) or by impregnation of Ce precursor, aiming to evaluate both the effect of ceria presence and its size/dispersion on catalytic activity. The results indicated an expressive increase in WGS and GSR performances due to the presence of ceria, however the comparison between the samples with different CeO2 sizes did not show variations with the same magnitude. Ceria demonstrated to be essential on GSR reaction by facilitating the cleaning of the most active Pt sites, releasing them to interact with glycerol through the removal of the strongly bound CO as CO2. By means of the modulation-excitation methodology coupled to diffuse reflectance infrared Fourier transformed spectroscopy (ME-DRIFTS) under reaction conditions, it was possible to identify the formation of carboxylate and formate species as the active intermediate species formed on ceria catalysts surface during WGS reaction, which may contribute to a deeper understanding of the behavior of these systems on both target reactions. Therefore, the systematic investigation of these nanostructured systems led to the recognition of key properties dictating their performance, which should be taken into account for the rational design of more active, stable and selective catalystsDoutoradoQuimica InorganicaDoutora em CiênciasCAPE

Impact of functionalization of ceria nanorods on water-gas shift reaction

Orientador: Daniela ZanchetDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de QuímicaResumo: Nanobastões de céria (CeO2) funcionalizados com grupos amino foram utilizados como suporte em catalisadores de cobre para a reação de deslocamento gás-água (WGS). A funcionalização da superfície do óxido foi realizada visando uma melhor dispersão da fase metálica no suporte, através da interação entre o grupo amino e o precursor Cu2+, com a posterior correlação entre esta modificação e a atividade do catalisador. Utilizou-se o método hidrotérmico para a síntese dos nanobastões, que foram posteriormente funcionalizados com 3-(aminopropil)trimetoxisilano. A adição do precursor Cu2+ ao suporte foi feita via impregnação, seguida de calcinação e redução (ativação do catalisador), etapa na qual se formaram as nanopartículas metálicas (Cu0) suportadas. Comparando os catalisadores com suporte de céria pura e de céria funcionalizada, observou-se que de fato a funcionalização resultou na maior dispersão do Cu2+ na superfície. No entanto, ela causou a menor dispersão do metal (Cu0) após a redução, a diminuição da redutibilidade da céria superficial, a fragmentação dos bastões e o menor desempenho catalítico frente à reação de WGS. Visando a compreensão destes sistemas, verificou-se que a calcinação após a adição de Cu2+ na amostra funcionalizada formou uma camada de SiO2 na superfície da céria, o que diminui a atividade por reduzir as interações Cu-CeO2 (formação de Cu-O-Si), corroborando a grande influência desta interface no desempenho destes catalisadores. Além disso, a menor dispersão de Cu0 na superfície funcionalizada após a redução demonstrou a importância da céria também na estabilização da fase metálica. Desta maneira, a funcionalização da superfície se mostrou uma abordagem interessante no que se refere à dispersão do precursor metálico no suporteAbstract: Amino functionalized ceria nanorods were explored as support on copper catalysts for the water-gas shift (WGS) reaction. The purpose of the design of a functionalized oxide surface was to obtain a better metal phase dispersion on the support provided by amino-Cu2+ interaction, in addition to further correlation between this modification and the catalyst activity. The hydrothermal method was used to synthetize the nanorods, which were subsequently functionalized with 3-(aminopropyl)trimethoxysilane. The Cu2+ precursor was added to the support by impregnation, followed by calcination and reduction (catalyst activation), when the supported metallic (Cu0) nanoparticles were formed. By comparison of the catalysts obtained with pure ceria and functionalized ceria supports it was observed that the functionalization indeed caused a greater Cu2+ dispersion on the oxide surface. However, it gave rise to a lower metal dispersion (Cu0) after reduction step, along with the decrease of surface ceria reducibility, nanorods fragmentation and inferior catalytic performance towards WGS reaction. In order to understand these systems, it was confirmed that the calcination step (after Cu2+ addition) on functionalized sample created a SiO2 layer above ceria surface, therefore lowering the activity due to the decrease of Cu-CeO2 interactions (formation of Cu-O-Si), which corroborated the great influence of Cu-CeO2 interface on the activity. Also, the lower Cu0 dispersion on the functionalized surface after reduction showed the importance of ceria on the metallic phase stabilization. Hence, the surface functionalization demonstrated to be an interesting approach to the dispersion of metal precursor on the catalyst supportMestradoQuimica InorganicaMestra em Químic

Alloying Tungsten Carbide Nanoparticles with Tantalum: Impact on Electrochemical Oxidation Resistance and Hydrogen Evolution Activity

Metal-terminated bimetallic carbide nanoparticles (NPs) of tungsten and tantalum are synthesized in a monodisperse particle size distribution of 2–3 nm. The bimetallic particles feature enhanced electrocatalytic behavior with respect to the monometallic composition. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements indicate that the Ta_0.3W_0.7C NPs consist of a well-mixed random alloy featuring a compressed lattice that favorably impacts stability and catalytic activity. Electrochemical testing shows that the incorporation of 30% tantalum into the tungsten carbide lattice increases the electrochemical oxidation resistance of the NPs. The onset of surface passivation in 0.5 M H₂SO₄ shifted from +0.2 V vs RHE to +0.45 V vs RHE, and the maximum surface oxidation current shifted from +0.4 to +0.75 V vs RHE. The activity toward hydrogen evolution (HER) of the carbon-supported Ta_0.3W_0.7C NPs is preserved relative to the activity of unmodified carbon-supported WC NPs. The increase in electrochemical oxidation resistance is attributed to the presence of surface Ta moieties as determined by X-ray photoelectron spectroscopy (XPS) while the preservation of the HER activity is attributed to the observed lattice compression

Dumbbell-like Au0.5Cu0.5@Fe3O4 nanocrystals: synthesis, characterization, and catalytic activity in CO oxidation

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We report the colloidal synthesis of dumbbell-like Au0.5Cu0.5@Fe3O4 nanocrystals (AuCu@FeOx NCs) and the study of their properties in the CO oxidation reaction. To this aim, the as-prepared NCs were deposited on gamma-alumina and pretreated in an oxidizing environment to remove the organic ligands. A comparison of these NCs with bulk Fe3O4-supported AuCu NCs showed that the nanosized support was far more effective in preventing the sintering of the metal domains, leading thus to a superior catalytic activity. Nanosizing of the support could be thus an effective, general strategy to improve the thermal stability of metallic NCs. On the other hand, the support size did not affect the chemical transformations experienced by the AuCu NCs during the activation step. Independently from the support size, we observed indeed the segregation of Cu from the alloy phase under oxidative conditions as well as the possible incorporation of the Cu atoms in the iron oxide domain.We report the colloidal synthesis of dumbbell-like Au0.5Cu0.5@Fe3O4 nanocrystals (AuCu@FeOx NCs) and the study of their properties in the CO oxidation reaction. To this aim, the as-prepared NCs were deposited on γ-alumina and pretreated in an oxidizing en8422862428632European Union through the EU-ITN network Mag(net)icFun [PITN-GA-2012-290248]CAPES (Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior) [99999.013076/2013-02]CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico) [870359/1997-5, 309373/2014-0]Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Dumbbell-like Au_0.5Cu_0.5@Fe₃O₄ Nanocrystals: Synthesis, Characterization, and Catalytic Activity in CO Oxidation

We report the colloidal synthesis of dumbbell-like Au_0.5Cu_0.5@Fe₃O₄ nanocrystals (AuCu@FeOx NCs) and the study of their properties in the CO oxidation reaction. To this aim, the as-prepared NCs were deposited on γ-alumina and pretreated in an oxidizing environment to remove the organic ligands. A comparison of these NCs with bulk Fe₃O₄-supported AuCu NCs showed that the nanosized support was far more effective in preventing the sintering of the metal domains, leading thus to a superior catalytic activity. Nanosizing of the support could be thus an effective, general strategy to improve the thermal stability of metallic NCs. On the other hand, the support size did not affect the chemical transformations experienced by the AuCu NCs during the activation step. Independently from the support size, we observed indeed the segregation of Cu from the alloy phase under oxidative conditions as well as the possible incorporation of the Cu atoms in the iron oxide domain

Dumbbell-like Au_0.5Cu_0.5@Fe₃O₄ Nanocrystals: Synthesis, Characterization, and Catalytic Activity in CO Oxidation

We report the colloidal synthesis of dumbbell-like Au_0.5Cu_0.5@Fe₃O₄ nanocrystals (AuCu@FeOx NCs) and the study of their properties in the CO oxidation reaction. To this aim, the as-prepared NCs were deposited on γ-alumina and pretreated in an oxidizing environment to remove the organic ligands. A comparison of these NCs with bulk Fe₃O₄-supported AuCu NCs showed that the nanosized support was far more effective in preventing the sintering of the metal domains, leading thus to a superior catalytic activity. Nanosizing of the support could be thus an effective, general strategy to improve the thermal stability of metallic NCs. On the other hand, the support size did not affect the chemical transformations experienced by the AuCu NCs during the activation step. Independently from the support size, we observed indeed the segregation of Cu from the alloy phase under oxidative conditions as well as the possible incorporation of the Cu atoms in the iron oxide domain