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This book provides an overview of all new high-silica zeolites which have been discovered between 1975 and 1985. The first part presents some 25 proven recipes for the preparation of high-silica zeolites and describes the characteristics of the materials obtained. This will allow bench-scale production of these materials for scientific research. In the second part, high-silica zeolites with solved structure type are discussed. This part classifies many proprietary materials according to known structure types, and describes the rules and parameters which govern the formation of these materials. In the third part, the formation and characteristics of high-silica zeolites with unknown structure type are discussed. The book contains a wealth of information for all those scientists who incorporate the use of high-silica zeolites in their work.

Materials sciences --- Chemical technology

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If our global society wants to evolve to net zero greenhouse gas emissions, one of the key technological changes that need to be made is the transition from burning fossil-based fuels towards the use of renewable fuels. Hydrogen is such a suitable fuel and can be made from sunlight by splitting water into its constituents: hydrogen (H2) and oxygen (O2).This can be achieved by using renewable electricity from solar panels to perform water electrolysis, a technology well known today. Another way is to integrate both systems and directly perform the water splitting reactions on a photovoltaic material which is coated with a suitable catalyst. Such integrated systems are called photoelectrochemical (PEC) cells.In this PhD thesis, PEC solar hydrogen devices were investigated with an emphasis on upscaling and design of complete PEC cells. In Chapter 2, a general overview of integrated PEC cells is given with an emphasis on cell design and nanostructuring of components. Economic, technical and practical aspects are discussed, pinpointing the major current bottlenecks for solar hydrogen devices to reach the market. In Chapter 3 a first PEC technology based on a particulate TiO2 photoanode, proton conducting membrane and Pt cathode was investigated at larger scales (up to 9cm²) in a dual compartment cell. These obtained results emphasize that upscaling of PEC cells is not trivial and suitable techniques for large scale deposition of membranes and catalyst must be considered. Further, the concept of oxidation of volatile organic components (VOCs) was proposed in this PEC cell. Finally an advanced two-compartment experimental PEC, suitable to characterize a monolithic solar hydrogen device in experimental as well as real-life conditions was designed and constructed.In Chapter 4, the transient phenomena occurring in the aforementioned PEC cell were further investigated. Through collaboration with EPFL, Lausanne, a numerical model was developed based on existing data. Based on results the model assumed the presence of two parallel pathways for charge transport at the photoabsorber−electrolyte interface. The model was experimentally validated and used in combination with a design of experiments (DOE) to predict the transient phenomena in varying operational conditions.A second PEC system was formulated and is based on introducing well-controlled porosity into monolithic Si-based PEC systems. This design was investigated experimentally in Chapter 5. In an attempt to only look at the electrochemical effects, porous monoliths were fabricated without inclusion of photovoltaic materials. Pore diameter and spacing between pores were varied and the effect on Ohmic losses was measured in different conditions. Low Ohmic losses could even be achieved at reduced electrolyte concentrations which have the additional benefit of lowering the stringent requirement for both catalysts as well as encapsulation materials when working at extreme pH values. A simplified electrochemical circuit model suggests Ohmic losses inside the pores start to dominate over surface Ohmic losses at low pore spacing. At lower device thickness, Ohmic losses are expected to decrease further, even at lower porosity.The porous monolithic PEC cell proposed in this thesis is intrinsically scalable. The first steps toward such a device have been made in this work but more research is needed. Currently the effect of pores into photovoltaic materials is being investigated. Also the deposition of membrane materials onto the surface and inside the pores is crucial for product separation. Additionally, this would allow the PEC cells to be operated in air with water vapour as source of water. Such solid state ‘air-PEC cells’ could provide technical and practical solutions to the renewable energy conundrum. A standalone rooftop panel turning air and sunlight into fuel would be a ground-breaking addition to the global renewable energy portfolio.

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Solar water splitting devices are used to produce hydrogen in a sustainable way based on electrolysis of water with oxygen as side-product. It consists of two electrodes, an anode and cathode, between which electrons and ions flow. Reduced protons at the cathode produce hydrogen via the hydrogen evolution reaction (HER), whereas water is oxidized to oxygen at the anode through the oxygen evolution reaction (OER). The device investigated here is made out of one piece and called a monolith in which pores are implemented to reduce ionic losses. The porous monoliths could solve the upscaling difficulties that are currently encountered with state of the art devices. Additionally, these monoliths could operate in gas phase or even in air which contains sufficient water vapour for this purpose. The monolithic structures consist of silicon substrates coated with conducting layers and electrocatalysts on both sides (anode and cathode). From the tested conducting layers, gold has shown to have good conducting properties and to be chemical inert. This is important for the determination of electrocatalysts which perform the reactions. The electrocatalyst should be active, stable and abundantly present. For OER, two electrocatalysts, NiFe and NiMoFe, are tested which are both nickel based. The obtained overpotentials at 10 mA cm^(-2) of NiFe and NiMoFe at an Au sample are 0.264 V and 0.355 V, respectively. A lower overpotential means a higher activity. NiFe shows also better stability. For HER, NiMo and NiMoFe are tested. Overpotentials at 10 mA cm^(-2) of NiMo and NiMoFe are -0.311 V and -0.243 V at an Au sample, respectively. Of these two, NiMoFe has a better stability. To use a solar water splitting device in gas phase a membrane is required for ionic transport and efficient product separation. For nickel based catalysts an anion exchange membrane (AEM) is used. Only if pinholes in the membrane layer are present after depositing AEM on NiMoFe, the formation of nickel redox peaks is possible. Absence of these peaks leads to an unwanted lower activity. Experiments in gas phase with nickel based electrocatalysts were not possible due to fouling of gold particles while fabricating pores. Etching of the particles did not obtain the desired results. Instead, porous Pt/Ir samples were tested in liquid and gas phase. Only in liquid electrolyte an exponential reaction curve was obtained. This indicates that the membrane layer in the pores was insufficient for ionic transport in the gas phase. However, it is worthwhile to further investigate the deposition of the membrane because of the obtained overpotential of 0.441 V for a blank Pt/Ir sample in liquid.