To achieve net-zero emissions targets, a substantial shift away from fossil fuels to renewable energy-based systems is necessary. The development of innovative one-step catalytic routes for directly manufacturing chemicals and fuels, starting from small molecules such as H2O, N2, and CH4, predominantly through sustainable photo-, electro-, and plasma-catalysis, is a strategy to achieve this goal. The challenge involves direct electrification to produce chemicals, fuels, and energy vectors, laying the groundwork for future carbon-neutral chemical production. In this context, the primary objective of this work carried out in the framework of the ERC SCOPE project (Surface-COnfined fast-modulated Plasma for process and Energy intensification in small molecules conversion), was to design an efficient (photo) electrode to be used in various unconventional catalytic processes for the conversion of small molecules. Starting from one of the most widely used photoactive materials, novel photocatalysts based on an ordered array of TiO2 nanotubes on a Ti mesh as support (TiO2NTs/Ti mesh), eventually modified with different metal nanoparticles, have been efficiently developed. These materials, with novel 3D-type structural characteristics, combining a mesoporous structure (due to the ordered arrays of TiO2 nanotubes) with the macro pores of a mesh, guarantee good light harvesting and fast charge transport so improving the photoactivity in the photo (electro)catalytic processes (i.e. water splitting and ethanol photo dehydrogenation). Additionally, the combination of this macro/meso nanostructured catalyst (based on TiO2NTs/Ti mesh) with non-thermal plasma, has been evaluated to investigate the role of the potential synergy between plasma and (photo)catalysis in enhancing overall process efficiency. Most of the research activities were carried out at the laboratory CASPE/INSTM (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina (Dept. Chibiofaram). During the second year, six months were spent at the Department of Chemical Engineering and Chemistry at the Eindhoven University of Technology (Netherlands), under the supervision of Prof. Fausto Gallucci and Dr. Sirui Li, in the research group on Sustainable Process Engineering, The Ph.D. thesis is organized into four main chapters, plus the general conclusions. Chapter 1 explores the general implications and strategies employed to convert small molecules, analysing the latest developments in electro-, photo--, and plasma-catalysis routes, as innovative catalytic technologies implemented thus far. Additionally, attention is given to the main photoactive materials and their potential modifications. Chapter 2 focuses on the experimental techniques employed to synthesize a hierarchical 3D macro-mesoporous structure based on highly ordered arrays of TiO2 nanotubes (NTs) on a Ti mesh, to overcome the main drawbacks of titanium dioxide materials, i.e. the fast charge carriers recombination rate and the large band gap. The chapter provides a comprehensive analysis of the morphological, optical, and photo-electric characteristics of the catalysts. Using electrochemical impedance spectroscopy (EIS), the photoactivity associated with 3D or 2D geometries of the support, with both dark and illuminated conditions, is determined. This study demonstrates the existing correlation between the photoactivity and the specific charge transfer resistances which, in turn, led to a better comprehension of the electronic properties of TiO2 nanotubes. The 3D structure of the nanotubes in the mesh, enhancing light absorption and facilitating faster electron transport along the nanotubes, significantly influences the catalytic performance under illumination increasing the production of H2 and current density in water photo-electrolysis. Chapter 3 deals with the functionalization of the titania nanotubes on Ti mesh with metal nanoparticles with proven LSPR (localized surface plasmon resonance) behaviour (Au, Ag, Cu, Pd). The different deposition procedures are described, followed by an extensive characterization. The effect of the metal on the performances in two different key H2 production reactions (water photo-electrolysis and ethanol photo-dehydrogenation) is discussed. For the ethanol photo-conversion process, two different configurations of the cell, gas-phase, and liquid-phase, are employed, to investigate how gas-phase conditions can influence the performances in terms of productivity and selectivity. The results obtained show a notable enhancement in the performances of the metal-modified electrodes compared to the unmodified titanium dioxide substrate in the water photo-electrolysis reaction. In the ethanol photo-conversion process, the nature of the metal and the presence of an electrolyte influence the type and quantity of products formed at the anode. No acetic acid formation was observed in the gas phase by using an Au-modified TiO2 catalyst, compared to Pd and Ag catalysts, (as opposed to the liquid phase configuration). The results obtained show that, a distinct reaction and, consequently, different selectivity, can be obtained by only varying cell configurations. Finally, chapter 4 investigates the effect of the combination between TiO2NTs/Ti mesh with non-thermal plasma in the reaction of non-oxidative coupling of methane (NOCM). The TiO2NTs/Ti catalysts (also modified with metal nanoparticles) were tested in two different reactors: a conventional tubular DBD (dielectric barrier discharge) plasma reactor and a planar DBD reactor. In addition, the effect of light irradiation was investigated. A photoactive material TiO2, can, in general, be excited by external light radiation to generate localized charges on the surface which, positively interacting with the radicals generated in plasma, may increase the selectivity. A porous meso/macro 3D hierarchical structure, as the titanium dioxide nanotubes on a titanium mesh, increases both conversion and changes selectivity, and the integration of metal plasmonic NPs further enhances the performances. In particular, the gold-modified sample strongly improves the methane conversion in the DBD planar reactor, with a modulation also in the selectivity pushing towards the ethylene production, and this effect is more marked under light irradiation.

New concept 3D-hierarchical nanostructured electrodes for the conversion of small molecules

DE PASQUALE, Luana
2024-04-16

Abstract

To achieve net-zero emissions targets, a substantial shift away from fossil fuels to renewable energy-based systems is necessary. The development of innovative one-step catalytic routes for directly manufacturing chemicals and fuels, starting from small molecules such as H2O, N2, and CH4, predominantly through sustainable photo-, electro-, and plasma-catalysis, is a strategy to achieve this goal. The challenge involves direct electrification to produce chemicals, fuels, and energy vectors, laying the groundwork for future carbon-neutral chemical production. In this context, the primary objective of this work carried out in the framework of the ERC SCOPE project (Surface-COnfined fast-modulated Plasma for process and Energy intensification in small molecules conversion), was to design an efficient (photo) electrode to be used in various unconventional catalytic processes for the conversion of small molecules. Starting from one of the most widely used photoactive materials, novel photocatalysts based on an ordered array of TiO2 nanotubes on a Ti mesh as support (TiO2NTs/Ti mesh), eventually modified with different metal nanoparticles, have been efficiently developed. These materials, with novel 3D-type structural characteristics, combining a mesoporous structure (due to the ordered arrays of TiO2 nanotubes) with the macro pores of a mesh, guarantee good light harvesting and fast charge transport so improving the photoactivity in the photo (electro)catalytic processes (i.e. water splitting and ethanol photo dehydrogenation). Additionally, the combination of this macro/meso nanostructured catalyst (based on TiO2NTs/Ti mesh) with non-thermal plasma, has been evaluated to investigate the role of the potential synergy between plasma and (photo)catalysis in enhancing overall process efficiency. Most of the research activities were carried out at the laboratory CASPE/INSTM (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina (Dept. Chibiofaram). During the second year, six months were spent at the Department of Chemical Engineering and Chemistry at the Eindhoven University of Technology (Netherlands), under the supervision of Prof. Fausto Gallucci and Dr. Sirui Li, in the research group on Sustainable Process Engineering, The Ph.D. thesis is organized into four main chapters, plus the general conclusions. Chapter 1 explores the general implications and strategies employed to convert small molecules, analysing the latest developments in electro-, photo--, and plasma-catalysis routes, as innovative catalytic technologies implemented thus far. Additionally, attention is given to the main photoactive materials and their potential modifications. Chapter 2 focuses on the experimental techniques employed to synthesize a hierarchical 3D macro-mesoporous structure based on highly ordered arrays of TiO2 nanotubes (NTs) on a Ti mesh, to overcome the main drawbacks of titanium dioxide materials, i.e. the fast charge carriers recombination rate and the large band gap. The chapter provides a comprehensive analysis of the morphological, optical, and photo-electric characteristics of the catalysts. Using electrochemical impedance spectroscopy (EIS), the photoactivity associated with 3D or 2D geometries of the support, with both dark and illuminated conditions, is determined. This study demonstrates the existing correlation between the photoactivity and the specific charge transfer resistances which, in turn, led to a better comprehension of the electronic properties of TiO2 nanotubes. The 3D structure of the nanotubes in the mesh, enhancing light absorption and facilitating faster electron transport along the nanotubes, significantly influences the catalytic performance under illumination increasing the production of H2 and current density in water photo-electrolysis. Chapter 3 deals with the functionalization of the titania nanotubes on Ti mesh with metal nanoparticles with proven LSPR (localized surface plasmon resonance) behaviour (Au, Ag, Cu, Pd). The different deposition procedures are described, followed by an extensive characterization. The effect of the metal on the performances in two different key H2 production reactions (water photo-electrolysis and ethanol photo-dehydrogenation) is discussed. For the ethanol photo-conversion process, two different configurations of the cell, gas-phase, and liquid-phase, are employed, to investigate how gas-phase conditions can influence the performances in terms of productivity and selectivity. The results obtained show a notable enhancement in the performances of the metal-modified electrodes compared to the unmodified titanium dioxide substrate in the water photo-electrolysis reaction. In the ethanol photo-conversion process, the nature of the metal and the presence of an electrolyte influence the type and quantity of products formed at the anode. No acetic acid formation was observed in the gas phase by using an Au-modified TiO2 catalyst, compared to Pd and Ag catalysts, (as opposed to the liquid phase configuration). The results obtained show that, a distinct reaction and, consequently, different selectivity, can be obtained by only varying cell configurations. Finally, chapter 4 investigates the effect of the combination between TiO2NTs/Ti mesh with non-thermal plasma in the reaction of non-oxidative coupling of methane (NOCM). The TiO2NTs/Ti catalysts (also modified with metal nanoparticles) were tested in two different reactors: a conventional tubular DBD (dielectric barrier discharge) plasma reactor and a planar DBD reactor. In addition, the effect of light irradiation was investigated. A photoactive material TiO2, can, in general, be excited by external light radiation to generate localized charges on the surface which, positively interacting with the radicals generated in plasma, may increase the selectivity. A porous meso/macro 3D hierarchical structure, as the titanium dioxide nanotubes on a titanium mesh, increases both conversion and changes selectivity, and the integration of metal plasmonic NPs further enhances the performances. In particular, the gold-modified sample strongly improves the methane conversion in the DBD planar reactor, with a modulation also in the selectivity pushing towards the ethylene production, and this effect is more marked under light irradiation.
16-apr-2024
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11570/3295152
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