No description available.

<p dir="ltr">Metal speciation and structure can be influenced by the deposition method used during synthesis, interactions with the support, and by post-deposition treatments and reaction conditions experienced during its lifetime of carrying out a catalytic reaction. Supported metal particles of different size contain different surface structures and coordination environments, which may not only influence reaction rates but also the interconversion between agglomerated metallic domains and dispersed metal atom or ion sites. Here, we address the influence of post-deposition treatments and support properties on the structural interconversion of Pd and Cu on aluminosilicate chabazite (CHA) zeolites, Pt on gamma-alumina (γ-Al2O3), and Pd on amorphous oxides (γ-Al2O3, La-doped Al2O3, ΘΔ-Al2O3). The fundamental insights from these studies can be used to design catalysts used widely in automotive exhaust aftertreatment systems, including Pd-exchanged zeolites for passive NOx (x = 1,2) adsorbers (PNA), Cu-exchanged zeolites for NOx (x = 1,2) selective catalytic reduction (SCR), Pt/Al2O3 for NH3 oxidation, and Pd/oxides for three-way catalysts (TWC). Incipient wetness impregnation (IWI) and colloidal methods were used to prepare Pd nanoparticles deposited on CHA zeolites with distinct Pd nanoparticle sizes and distributions. These Pd-CHA samples were used to investigate the effects of Pd particle size distribution on structural interconversion between ion-exchanged Pd and agglomerated Pd domains under realistic operating conditions. Smaller Pd nanoparticles had larger fractions of agglomerated Pd that converted to ion-exchanged Pd2+ sites at fixed air treatment temperatures (598–973 K) and H2O pressures (2–6 kPa H2O), consistent with thermodynamic predictions from DFT calculations. Furthermore, the addition of H2O during air treatment of different Pd nanoparticles (2–14 nm) inhibited the formation of ion-exchanged Pd2+ (thermodynamics), but not the rate of redispersion (kinetics). This demonstrates that, regardless of Pd nanoparticle size, water vapor in automotive exhaust streams facilitate metal sintering in PNA applications. Aqueous-phase exchange of Cu on CHA zeolites with varying support properties (i.e., number of paired Al sites in the 6 membered ring) were used to prepare materials with distinct types and numbers of extraframework Cu species (Cu2+, CuOH+). These Cu-CHA materials were used to analyze Cu structural changes before and after exposure to hydrothermal aging conditions. In the absence of H2O, some Cu2+ sites condense to form binuclear Ox-bridged Cu species that can be reduced with H2 to form Cu-hydride sites and reject H2O, leading to a sub-stoichiometric H2 consumption (H2/Cu < 0.5). In the presence of H2O, all nominally isolated Cu2+ species convert to [CuOH]+ structures, which can subsequently be reduced by H2 to form a Cu-hydride and reject H2O, leading to stoichiometric H2 consumption (H2/Cu ~ 0.5). Furthermore, the presence of H2O led to reduction features in H2 temperature programmed reduction (TPR) profiles that were similar among Cu-CHA materials, regardless of the initial Cu2+ speciation, further supporting the proposal that all nominally isolated Cu2+ sites convert to a similar [CuOH]+ motif. This demonstrates how water influences Cu speciation on CHA materials of varying origin or treatment history, aiding in quantifying SCR-active isolated Cu ions and SCR-inactive Cu species (e.g., CuO, CuAl2O4). Pt supported on γ-Al2O3 were prepared with different average Pt particle sizes (2–13 nm) by increasing the temperature of post-deposition air treatment (523–873 K). This suite of materials was interrogated to isolate the effects of Pt particle size on NH3 oxidation rates and selectivities during conditions relevant to NH3 slip applications in diesel exhaust aftertreatment. For all Pt particle sizes, NH3 oxidation rates displayed a hysteresis with temperature, with high rates measured during temperature decreases than during temperature increases. Smaller Pt particles (2 nm) had lower rates (per surface Pt, quantified by CO chemisorption) than larger Pt particles (13 nm), signifying that NH3 oxidation is a structure-sensitive reaction. Furthermore, surfaces of Pt particles restructure under NH3 oxidation reaction conditions, influencing effective Pt oxidation states, surface structures (numbers and types of exposed Pt sites), and surface coverages of intermediates leading to the observed hysteresis in rate. These findings demonstrate that Pt particles undergo dynamic structural changes during reaction, influencing their ability to convert NH3 to environmentally benign products in NH3 slip applications. The influence of treatment conditions, support properties, and initial Pd particle size and distribution on the kinetics of nanoparticle sintering were investigated to identify which material properties allow maintaining high dispersion to maximize metal utilization for three way catalysts (TWC) during the conversion of regulated pollutants (CO, hydrocarbons, NOx). Pd was deposited by IWI methods to generate polydiserse particle size distributions, and using colloidal Pd nanoparticle solutions to generate monodisperse size distributions, onto various supports (γ-Al2O3, La-doped Al2O3, ΘΔ-Al2O3) and subjected to aging under oxidative and reductive conditions relevant for TWC operation. The average Pd particle size for all materials increased with treatment time under both reductive and oxidative environments. For samples prepared with IWI (i.e., log normal distribution of Pd particle sizes), reductive aging treatments led to higher sintering rates than oxidative treatments. In contrast, for samples prepared using colloidal Pd solutions (i.e., normal distribution of Pd particle sizes), oxidative aging treatments led to higher sintering rates than reduction treatments. Furthermore, after the same treatment condition and time, samples prepared with IWI resulted in higher average Pd particle sizes. These results indicate that more monodisperse initial Pd particle size distributions lead to lower sintering rates, providing guidance to design of supported metal TWCs with improved metal utilization during their lifetimes. Here, the combination of synthesis approaches to prepare a suite of model (e.g., powder) supported metal catalysts of varying structure and composition, interrogated using site and structural characterizations and steady-state and transient kinetic measurements, along with predictions from theoretical calculations, enabled unraveling the influence of material properties and gas environments that affect metal speciation, structure, and oxidation state in real-world aftertreatment systems that use more complex catalytic architectures (e.g., layered washcoats) and reactor designs (e.g., monoliths). This approach provides insights into the fundamental thermodynamic and kinetic factors influencing metal restructuring and interconversion under realistic conditions encountered in automotive exhaust aftertreatment applications, and the kinetic and mechanistic factors that underlie complex phenomena (e.g., reaction rate hysteresis) from data measured in the absence of hydrodynamic artifacts. The overall approach used in this work enabled development of synthesis-structure-function relationships on various metal supported catalysts for automotive exhaust aftertreatment applications, which can provide guidance for material design and treatment strategies to form and retain desired metal structures throughout the material lifetime, including synthesis, reaction, and regeneration treatments.</p>

No / Zinc oxide (ZnO) nanoparticles are the mostly used engineered metal oxide nanoparticles in consumer products. This has increased the likelihood of human exposure to this engineered nanoparticle (ENPs) through different routes. At present, the majority of the studies concerning ZnO ENPs toxicity have been conducted using in vitro and in vivo systems. In this study, for the first time we assessed the effect of ZnO ENPs on the major cellular pathways in the lymphocytes of healthy individuals as well as in susceptible patients suffering from lung cancer, chronic obstructive pulmonary disease (COPD) and asthma. Using the differential expression analysis, we observed a significant (P < 0.05) dose-dependent (10, 20 and 40 microg/ml for 6h) increase in the expression of tumour suppressor protein p53 (40, 60 and 110%); Ras p21 (30, 52 and 80%); c-Jun N-terminal kinases; JNKs) (28, 47 and 78%) in lung cancer patient samples treated with ZnO ENPs compared to healthy controls. A similar trend was also seen in COPD patient samples where a significant (P < 0.05) dose-dependent increase in the expression of tumour suppressor protein p53 (26, 45 and 84%), Ras p21 (21, 40 and 77%), JNKs (17, 32 and 69%) was observed after 6h of ZnO ENPs treatment at the aforesaid concentrations. However, the increase in the expression profile of tested protein was not significant in the asthma patients as compared to controls. Our results reiterate the concern about the safety of ZnO ENPs in consumer products and suggest the need for a complete risk assessment of any new ENPs before its use.

Porous, conductive metallic nanostructures are required in several fields, such as energy conversion, low-cost sensors etc. This thesis reports on the development of an electrocatalytically active and conductive membrane for use in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) and fabrication of low-cost substrates for Surface Enhanced Raman Spectroscopy (SERS). One of the main challenges facing large-scale deployment of PEMFCs currently is to fabricate a catalyst layer that minimizes platinum loading, maximizes eletrocatalytically active area, and maximizes tolerance to CO in the feed stream. Modeling the kinetics of platinum catalyzed half cell reactions occurring in a PEMFC using the kinetic theory of gases and incorporating appropriate sticking coefficients provides a revealing insight that there is scope for an order of magnitude increase in maximum current density achievable from PEMFCs. To accomplish this, losses due to concentration polarization in gas diffusion layers, which occur at high current densities, need to be eliminated. A novel catalyst design, based on a porous metallic nanostructure, which aims to overcome the limitations of concentration polarization as well as minimize the amount of platinum loading in PEMFCs is proposed. Fabrication steps involving controlled in-plane fusion of self-assembled arrays of core-shell gold-platinum nanoparticles (Au@Pt) is envisioned. The key steps involved being the development of a facile synthesis route to form Au@Pt nanoparticles with tunable platinum shell thicknesses in the 5 nm size range, the formation of large-scale 2D arrays of Au@Pt nanoparticles using guided self-assembly, and optimization of an RF plasma process to promote in-plane fusion of the nanoparticles to form porous, electrocatalytically active and electrically conductive membranes. This thesis consists of seven chapters. The first chapter provides an introduction into the topic of PEMFCs, some perspective on the current status of research and development of PEMFCs, and an outline of the thesis. The second chapter provides an overview on the methods used, characterization techniques employed and protocols followed for sample preparation. The third chapter describes the modelling of a PEMFC using the Kinetic theory of gases to arrive at an estimate of the maximum feasible current density, based on the kinetics of the electrocatalytic reactions. The fourth chapter presents the development of a simple protocol for synthesizing Au@Pt nanoparticles with control over platinum shell thicknesses from the sub monolayer coverage onwards. The results of spectroscopic and microscopic characterization establish the uniformity of coating and the absence of secondary nucleation. Chapter five describes the formation of a nanoporous, electrocatalytically active membrane by self-assembly to form bilayers of 2D arrays of Au@Pt nanoparticles and subsequent fusion using an RF plasma based process. The evolution of the electrocatalytic activity and electrical conductivity as a function of the duration of RF plasma treatment is monitored for Au@Pt nanoparticles with various extent of platinum coating. Spectroscopic, microscopic, electrical and cyclic voltammetry characterization of the samples at various stages were used to understand the structural evolution with RF plasma treatment duration and discussed. Next durability studies were carried out on the nanoporous, Au@Pt bilayer nanoparticle array with an optimum composition of Pt/Au atomic ratio of 0.88 treated to 16 minutes of argon plasma exposure. After this the novel catalyst membrane design of PEM fuel cell is revisited. Two different techniques are proposed so that the thin, nanoporous, metallic catalyst membrane achieves horizontal electronic resistance equivalent to that of the conventional gas diffusion layer with catalyst layer. The first technique proposes the introduction of gold coated polymeric mesh in between the thin, nanoporous, metallic catalyst membrane and bipolar plate and discusses the advantages. Later the gold coated polymeric mesh is introduced in a conventional membrane electrode assembly and efficiency of the polarization curves probed with and without the introduction of gold coated polymeric mesh. The second technique describes the results of fabrication of a nanoporous metallic membrane using multiple layers of 2D Au@Pt nanoparticle arrays at an optimum composition of Pt/Au atomic ratio of 0.88 to reduce the horizontal electronic resistance. Preliminary studies on the permeability of water through such membranes supported on a porous polycarbonate filter membrane are also presented. In chapter six, a simple reactive inkjet printing process for fabricating SERS active silver nanostructures on paper is presented. The process adapts a simple room temperature protocol, using tannic acid as the reducing agent, developed earlier in our group to fabricate porous silver nanostructures on paper using a commercial office inkjet printer. The results of SERS characterization, spectroscopic and microscopic characterizations of the samples and the comparison of the substrate’s long-term performance with respect to a substrate fabricated using sodium borohydride as the reducing agent is discussed. Preliminary findings on attempts to fabricate a conductive silver network using RF plasma induced fusion area also presented. Chapter seven provides a summary of the results, draws conclusions and a perspective on work required to accomplish the goals of incorporating the porous metallic nanostructures into PEMFCs.

[ES] Teniendo en cuenta el agotamiento de los combustibles fósiles y la creciente concentración de CO2 en la atmósfera, la hidrogenación de CO2 es una forma prometedora de convertir el CO2 en productos químicos y combustibles de carbono de alto valor añadido. Considerando la gran influencia del tamaño de partícula, la composición química, la naturaleza del soporte y las condiciones de operación sobre el comportamiento catalítico de los catalizadores, se han desarrollado una serie de catalizadores para la hidrogenación de CO2 basados en metales abundantes no nobles y polisacáridos naturales como precursores del grafeno. En la presente tesis doctoral, las especies metálicas soportadas sobre una matriz de carbono grafítico defectuosa, con diferentes tamaños de partículas, muestran diferente actividad catalítica y selectividad para la hidrogenación de CO2. Se prepararon, de forma controlada, nanopartículas de aleaciones de Co y Co-Fe soportadas en grafenos dopados con N defectuosos, con una amplia distribución de tamaño de nanopartículas, para la reacción de Sabatier, presentando una selectividad a metano superior al 90% con valores de conversión de CO2 superiores al 85%. En el caso de un solo metal, Co o Fe, y sus aleaciones en forma de "clusters" y pequeñas nanopartículas soportadas en el mismo material, la selectividad de la hidrogenación de CO2 cambia a CO, en lugar de metano, obteniéndose un valor del 98 % y alcanzando una conversión de CO2 del 56%. Conviene resaltar que, los catalizadores basados en "clusters" de aleaciones de metal con una carga de metal incluso por debajo del 0.2 % en peso, exhiben una mayor selectividad y rendimiento que los que tienen nanopartículas de aleaciones de Co-Fe más grandes que varían de 1 a 4 nm y una carga de metal más alta en una composición similar. Siguiendo la línea de investigación de hidrogenación de CO2, se desarrollaron una serie de nanopartículas de aleaciones de Co-Fe soportadas sobre grafenos dopados con N defectuosos con distribución de tamaño de nanopartículas controlada en el rango de 7-17 nm, obteniendo una selectividad hacia hidrocarburos C2+ alrededor del 45% y una conversión del CO2 cercana al 60%. Además, se realizó un estudio comparativo de la actividad catalítica de catalizadores similares basados en Co-Fe con promotores e inhibidores para la hidrogenación de CO2, observando su influencia en la conversión y selectividad de CO2. Finalmente, además de los catalizadores basados en Co-Fe, también se han preparado catalizadores basados en Cu-ZnO mediante un método de dos pasos. Estas nanopartículas de Cu-ZnO soportadas sobre grafeno defectuoso dopado con N exhiben una alta selectividad hacia la conversión de CO2 a metanol. / [CA] Tenint en compte l'esgotament dels combustibles fòssils i la creixent concentració de CO2 en l'atmosfera, la hidrogenació de CO2 és una forma prometedora de convertir el CO2 en productes químics i combustibles de carboni d'alt valor afegit. Considerant la gran influència de la grandària de partícula, la composició química, la naturalesa del suport i les condicions d'operació sobre el comportament catalític dels catalitzadors, s'han desenvolupat una sèrie de catalitzadors per a la hidrogenació de CO2 basats en metalls abundants no nobles i polisacàrids naturals com a precursors del grafé. En la present tesi doctoral, les espècies metàl·liques suportades sobre una matriu de carboni grafític defectuosa, amb diferents grandàries de partícules, mostren diferent activitat catalítica i selectivitat per a la hidrogenació de CO2. Es van preparar, de manera controlada, nanopartícules d'aliatges de Co i Co-Fe suportades en grafens dopats amb N defectuosos, amb una àmplia distribució de grandària de nanopartícules, per a la reacció de Sabatier, presentant una selectivitat a metà superior al 90% amb valors de conversió de CO2 superiors al 85%. En el cas d'un sol metall, Co o Fe, i els seus aliatges en forma de "clústers" i xicotetes nanopartícules suportades en el mateix material, la selectivitat de la hidrogenació de CO2 canvia a CO, en lloc de metà, obtenint-se un valor del 98% i aconseguint una conversió de CO2 del 56%. Convé ressaltar que, els catalitzadors basats en "clústers" d'aliatges de metall amb una càrrega de metall fins i tot per davall del 0.2% en pes, exhibeixen una major selectivitat i rendiment que els que tenen nanopartícules d'aliatges de Co-Fe més grans que varien d'1 a 4 nm i una càrrega de metall més alta en una composició similar. Seguint la línia d'investigació d'hidrogenació de CO2, es van desenvolupar una sèrie de nanopartícules d'aliatges de Co-Fe suportades sobre grafens dopats amb N defectuosos amb distribució de grandària de nanopartícules controlada en el rang de 7-17 nm, obtenint una selectivitat cap a hidrocarburs C2+ al voltant del 45% i una conversió del CO2 pròxima al 60%. A més, es va realitzar un estudi comparatiu de l'activitat catalítica de catalitzadors similars basats en Co-Fe amb promotors i inhibidors per a la hidrogenació de CO2, observant la seua influència en la conversió i selectivitat de CO2. Finalment, a més dels catalitzadors basats en Co-Fe, també s'han preparat catalitzadors basats en Cu-ZnO mitjançant un mètode de dos passos. Aquestes nanopartícules de Cu-ZnO suportades sobre grafé defectuós dopat amb N exhibeixen una alta selectivitat cap a la conversió de CO2 a metanol. / [EN] Considering the depletion of fossil fuels and the increasing atmospheric CO2 concentration, CO2 hydrogenation is a promising way to convert CO2 into value-added carbon-containing chemicals and fuels. Taking into account the significant influences of the particle size, chemical composition, nature of the support, and operation conditions on the catalytic performance of catalysts, a series of catalysts for CO2 hydrogenation have been developed based on the use of abundant non-noble metals and natural polysaccharides as graphene precursors. In the present PhD Thesis, metal species supported on defective graphitic carbon matrix with different particle sizes show different catalytic activity and selectivity for CO2 hydrogenation. Under effective control, Co and Co-Fe alloy nanoparticles wrapped on defective N-doped graphenes with a broad nanoparticle size distribution were prepared and performed for the Sabatier reaction, exhibiting a selectivity to methane over 90 % at CO2 conversion values over 85 %. In the case of single Co or Fe metal and their alloys in the form of clusters and small nanoparticles wrapped on the same support, the selectivity for CO2 hydrogenation shifts to CO, rather than methane, reaching a conversion of 56 % with 98 % CO selectivity. It is worth noting that the metal alloy clusters-based catalysts with the metal loading even below 0.2 wt.% exhibit a higher selectivity and better performance than the ones with larger Co-Fe alloy nanoparticles ranging from 1-4 nm and higher metal loading in a similar composition. Following the research line for CO2 hydrogenation, a series of Co-Fe alloy nanoparticles supported on defective N-doped graphenes with controlled nanoparticle size distribution in the range of 7-17 nm are developed, obtaining a selectivity towards C2+ hydrocarbons about 45% with a CO2 conversion close to 60%. In addition, a comparative catalytic activity of similar Co-Fe-based catalysts with promoters and poison has been studied for CO2 hydrogenation to observe their influence on CO2 conversion and selectivity. Finally, besides Co-Fe-based catalysts, Cu-ZnO-based catalysts have also been prepared by a two-step method. These Cu-ZnO nanoparticles supported on N-doped defective graphene exhibit a high selectivity for CO2 conversion to methanol. / Peng, L. (2021). Metal Nanoparticles Wrapped on Defective Nitrogen-doped Graphitic Carbons as Highly Selective Catalysts for C02 Hydrogenation [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/172329