Influence of total pressure on complete oxidation of methane over Pd/Al2O3 catalysts

Natural gas and biogas, which have methane as their main component, are interesting choices of fuel to reduce the anthropogenic emissions of greenhouse gases. However, as methane is a greenhouse gas, uncombusted methane should be removed from the combustion gases. Emission control catalysts can preferably be used to completely oxidise methane. This thesis aims to examine whether an increased total pressure can be utilized to enhance the methane oxidation reaction over Pd/Al2O3 catalysts. The effects of total pressure are studied by flow-reactor experiments and simulations. The prepared catalyst samples are characterised by N2-physisorption, CO-chemisorption and diffusive reflectance infrared Fourier transform spectroscopy.A multiscale model is developed to simulate the activity of methane oxidation over Pd/Al2O3 where the reaction kinetics are based on first-principles calculations. The results show that the oxidation of methane can be enhanced when the total pressure is increased above atmospheric pressure. However, the effect depends on the gas composition and reaction temperature. In a dry and oxygen rich feed gas composition, the activity benefits from an increased total pressure over the entire examined temperature range. The positive effect is attributed to a high fraction of available under-coordinated palladium and oxygen sites, which can dissociate the increased concentration of methane. When water or carbon dioxide is present in the feed gas these molecules adsorb on the under-coordinated palladium sites and through surface reactions block the palladium atom as adsorbed water, hydroxyl species and bicarbonate. The coverage of hindering species requires a higher temperature to regain available palladium and oxygen sites and the positive total pressure dependence on the oxidation of methane. If the temperature is too low, the simulations predict a negative effect of increased total pressure on the reaction. The multiscale simulations capture the experimental trends and indicate that support effects should be incorporated to the model for a more complete reaction mechanism

Modeling Fixed Bed Membrane Reactors for Hydrogen Production through Steam Reforming Reactions: A Critical Analysis

Membrane reactors for hydrogen production have been extensively studied in the past years due to the interest in developing systems that are adequate for the decentralized production of high-purity hydrogen. Research in this field has been both experimental and theoretical. The aim of this work is two-fold. On the one hand, modeling work on membrane reactors that has been carried out in the past is presented and discussed, along with the constitutive equations used to describe the different phenomena characterizing the behavior of the system. On the other hand, an attempt is made to shed some light on the meaning and usefulness of models developed with different degrees of complexity. The motivation has been that, given the different ways and degrees in which transport models can be simplified, the process is not always straightforward and, in some cases, leads to conceptual inconsistencies that are not easily identifiable or identified

Electrochemical Investigation of High-Performance Dye-Sensitized Solar Cells Based on Molybdenum for Preparation of Counter Electrode

In order to improve the photocurrent conversion efficiency of dye-sensitized solar cells (DSSCs), we studied an alternative conductor for the counter electrode and focused on molybdenum (Mo) instead of conventional fluorine-doped tin oxide (FTO). Because Mo has a similar work function to FTO for band alignment, better formability of platinum (Pt), and a low electric resistance, using a counter electrode made of Mo instead of FTO lead to the enhancement of the catalytic reaction of the redox couple, reduce the interior resistance of the DSSCs, and prevent energy-barrier formation. Using electrical measurements under a 1-sun condition (100 mW/cm(2), AM 1.5), we determined that the fill factor (FF) and photocurrent conversion efficiency (eta) of DSSCs with a Mo electrode were respectively improved by 7.75% and 5.59% with respect to those of DSSCs with an FTO electrode. Moreover, we have investigated the origin of the improved performance through surface morphology analyses such as scanning electron microscopy and electrochemical analyses including cyclic voltammetry and impedance spectroscopy

CO-PrOx over nano-Au/TiO2: Monolithic catalyst performance and empirical kinetic model fitting

In this work, the performance of ceramic monoliths washcoated with Au/TiO2 is studied on CO preferential oxidation (CO-PrOx) reaction in H2-rich environments under a wide range of operating conditions of practical interest. The parameter estimation of a nonlinear kinetic empirical model representing this system is made via genetic algorithms by fitting the model predictions against our laboratory observations. Parameter uncertainty leading to inaccurate predictions is often present when kinetic models with nonlinear rate equations are considered. Here, after the fitting was concluded, a statistical study was conducted to determine the accuracy of the parameter estimation. Activation energies of ca. 30 kJ/mol and 55 kJ/mol were adjusted for CO and H2 oxidations, respectively. The catalyst showed appropriate activity and selectivity values on the CO oxidation on a H2-rich environment. After ca. 45 h on stream the catalyst showed no deactivation. Results show that the model is suitable for reproducing the behavior of the CO-PrOx reactions and it can be used in the design of reactors for hydrogen purification.Peer ReviewedPostprint (author's final draft

Sustainable Carbon as Efficient Support for Metal-Based Nanocatalyst: Applications in Energy Harvesting and Storage

[EN] Sustainable activated carbon can be obtained from the pyrolysis/activation of biomass wastes coming from different origins. Carbon obtained in this way shows interesting properties, such as high surface area, electrical conductivity, thermal and chemical stability, and porosity. These characteristics among others, such as a tailored pore size distribution and the possibility of functionalization, lead to an increased use of activated carbons in catalysis. The use of activated carbons from biomass origins is a step forward in the development of more sustainable processes enhancing material recycling and reuse in the frame of a circular economy. In this article, a perspective of different heterogeneous catalysts based on sustainable activated carbon from biomass origins will be analyzed focusing on their properties and catalytic performance for determined energy-related applications. In this way, the article aims to give the reader a scope of the potential of these tailor-made sustainable materials as a support in heterogeneous catalysis and future developments needed to improve catalyst performance. The selected applications are those related with H2 energy and the production of biomethane for energy through CO2 methanation.This research was funded by the Centro de Desarrollo Tecnologico Industrial-CDTI (ALMAGRID Project-CER-20191006), by the Instituto Valenciano de Competitividad Empresarial-IVACE-FEDER (BIO3 Project-IMDEEA/2019/44) and by the Agencia Valenciana de Investigacion-AVI (REWACER Project INNEST00/19/050).Buaki-Sogo, M.; Zubizarreta Saenz De Zaitegui, L.; García Pellicer, M.; Quijano-Lopez, A. (2020). Sustainable Carbon as Efficient Support for Metal-Based Nanocatalyst: Applications in Energy Harvesting and Storage. 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Investigation of Microstructural and Carbon Deposition Effects in SOFC Anodes Through Modelling and Experiments

The investigation of the SOFC anode microstructural properties affected by microstructural parameters and degradation is the focus of this research. Imaging and image processing techniques are developed to achieve quantification of the anode microstructural information. The analytical and Computational Fluid Dynamics based modelling of the microstructure including the degradation effects developed in this work will enable the microstructure optimisation for achieving performance enhancements

Porous Materials for Environmental Applications

The development of porous materials has attracted the attention of the research community for years. Porosity characteristics have specific impacts on the material properties and materials that are applied in many areas, such as pollutant removal, CO2 capture, energy storage, catalytic oxidation and reduction processes, the conversion of biomass to biofuels, and drug delivery. Examples of porous materials are activated carbons, clays, and zeolites. The aim of this book is to collect the recent advances and progress regarding porous materials and their applications in the environmental area

Dynamic Model-based Analysis of Oxygen Reduction Reaction in Gas Diffusion Electrodes

Weltweit werden ca. 1 % der gesamten elektrischen Energie für die Chlor-Alkali Elektrolyse aufgewendet. Die für die Elektrolyse benötigte elektrische Energie kann um ca. ein Drittel reduziert werden, indem auf Silberkatalysator basiertende Gasdiffusionselektroden -- sog. Sauerstoffverzehrkathoden (englisch: oxygen depolarized cathodes, ODC) -- in den Prozess integriert werden. Ungeachtet ihres großen Energieeinsparpotentials sind die Prozesse und Limitierungen innerhalb der ODC weder verstanden noch quantifiziert. Die Einflüsse der Elektrodenstruktur auf die Leistungsfähigkeit und die Dynamik der ODC sind kaum ergründet. Zur Beantwortung der genannten Forschungsfragen wird das erste Modell einer ODC vorgestellt, das das Phasengleichgewicht der Gas-Flüssigkeitsgrenzfläche und strukturbedingte Inhomogenitäten in der Elektrolytverteilung berücksichtigt. \\ Es wird gezeigt, dass die Stromdichte durch die Verfügbarkeit an gelöstem Sauerstoff im flüssigen Elektrolyten limitiert wird. Bereits im stromlosen Zustand ist nur wenig Sauerstoff in dem Elektrolyten lösbar, der Grund liegt in der hohen Ionenkonzentration und der damit verbundenen niedrigen Wasseraktivität. Durch die Sauerstoffreduktion wird mit zunehmender Stromstärke Wasser verbraucht und Hydroxidionen produziert, die sich nahe der Gas-Flüssigkeitsgrenze anreichern. Dies führt zu einer weiteren Verringerung der Wasseraktivität und somit zu einer weiteren Verschlechterung der Sauerstofflöslichkeit und schließlich zu einer Sauerstoffverarmung. \\ Auf Grundlage dieser Erkenntnisse wird im zweiten Teil der Einfluss des Wassermassentransports und der Wasseraktivität detaillierter untersucht. Dazu werden zwei vergleichbare ODCs in Messaufbauten mit unterschiedlichen Stofftransportbedingungen in der Flüssigphase mithilfe des dynamischen Dreiphasenmodells analysiert. Es wird gezeigt, dass ein konvektiver Elektrolytstrom nicht nur zu einer höheren Elektrodenperformance, sondern auch zu einer weitaus schnelleren Dynamik führt. Beide Effekte sind auf kürzere Diffusionsschichten in der Flüssigphase und somit auf einen schnelleren Wasser- und Ionenmassentransport zurückzuführen. Die Wasseraktivität beeinflusst die Sauerstoffreduktion nur geringfügig direkt, aber signifikant indirekt über die Sauerstofflöslichkeit. \\ Im dritten Teil wird der Einfluss der ODC-Struktur auf die Performance und Dynamik untersucht. Ein modelgestützter Vergleich von ODCs mit unterschiedlichen Binder-Katalysator-Verhältnissen mittels elektrochemischer Impedanzspektroskopie führt zu der Erkenntnis, dass Unterschiede in der Performance im Wesentlichen auf die unterschiedlich ausgeprägte Gas-Flüssigkeitsgrenzschicht zurück zu führen sind, die die Menge an gelöstem Sauerstoff signifikant beeinflusst. Die Impedanzspektren aller untersuchten Elektroden legen den Schluss nahe, dass sich die Dreiphasengrenzfläche inhomogen über die gesamte Elektrodentiefe erstreckt, jedoch scheinen die stärker gefluteten Elektrodenteile elektrochemisch nahezu inaktiv zu sein. \\ Zusammenfassend wird mithilfe der makroskopischen dynamischen Modellierung aufgezeigt, dass zwei Faktoren besonders entscheidend für eine hohe ODC-Performance sind: Eine große Gas-Flüssigkeitsgrenzschicht, die besonders durch die ODC-Struktur beeinflusst werden kann, und ein schneller Wasser- und Ionenmassentransport in der Flüssigphase, der sowohl durch eine konvektive Strömung des Elektrolyten als auch durch die ODC-Struktur begünstigt werden kann. Auf der methodischen Ebene wird demonstriert, dass die dynamische Modellierung ein geeignetes Tool ist, um komplexe Dreiphasensysteme wie das der ODC zu analysieren. Dabei ist besonders die dynamische Ermittlung von Zeitkonstanten zur Separation der sich überlagernden Prozesse unabdingbar