HYDROGEN PRODUCTION FROM CHEMICAL HYDROGEN STORAGE MATERIALS USING CARBON-BASED CATALYSTS

The aim of this thesis is to design new catalytic systems in order to improve the performances of the existing catalysts to convert formic acid (FA) and hydrazine (N2H4 · H2O) into ultra-pure hydrogen in order to be effective in the future hydrogen economy. Different are the issues to solve in order to efficiently employ these chemicals in our energy transition. In particular, efficient carbon-based heterogeneous systems can effectively make enormous difference in the production of hydrogen from liquid carriers. Indeed, metal-based catalysts and especially Pd-based ones offer enhanced activity also at room temperature, but selectivity and stability need to be improved to be industrially applicable. In addition, carbon materials have the advantage of being easily tuned through variation in their structure, for example, changing the surface area and porosity and adding functional groups or generating topological defects. Moreover, their stability in liquid phase reactions makes them auspicious candidates in catalytic processes. For these reasons, the first part of this thesis is dedicated to the design of new catalytic materials with the aim to improve the catalytic behaviour compared to existing Pd-based catalysts for the selective decomposition of FA at mild reaction conditions. In particular, the effect of the metal-support interaction and the geometrical and electronic effect in alloyed catalysts play a fundamental role to enhance the catalytic performance (Chapter 3-5). Chapter 3 is devoted to study the metal-support interaction by doping with O and P functionalities the carbonaceous support. In order to establish the presence of functional groups in the support and their effect on Pd nanoparticles, the obtained samples were then, characterised by Transmission Electron Microscopy (HR-TEM, STEM-HAADF and STEM-EDS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) simulations provided further insights in the interaction of Pd15 cluster with different support surfaces, i.e. pristine graphene (PG), carboxyl doped graphene (G_COOH), hydroxyl doped graphene (G_OH), carbonyl doped graphene (G_CO) and phosphate doped graphene (G_PO3H). The effect of the addition of a second metal to Pd is considered (Rh in Chapter 4 and Au in Chapter 5). In particular, in Chapter 4 the synthesis of PdRh nanoparticles with different Pd:Rh molar ratios was studied. The obtained catalysts were characterised by Transmission Electron Microscopy (TEM) and Inductively coupled plasma optical emission spectroscopy (ICP-OES). For bimetallic catalysts, EDX-STEM analysis of individual nanoparticles was employed to investigate the presence of random-alloyed nanoparticles. Finally, PdRh catalysts were tested in the liquid-phase hydrogenation of muconic acid using formic acid as hydrogen donor. Chapter 5 combines DFT and experimental data to disclose the role of gold in enhancing activity, selectivity and stability of palladium catalyst during the formic acid decomposition. PdAu bimetallic nanoparticles with different Pd:Au molar ratio were synthesised and the obtained catalysts were characterised by using Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP - OES). Finally, Density functional theory (DFT) calculations on Pd15, Au15 and Pd9Au6 clusters supported on a carbon sheet were then simulated to provide atomic level understanding to the beneficial effect of gold observed in the experimental results. In addition, in order to decrease the cost and increase the environmental benignity of the catalyst, the application of metal-free carbon materials in the formic acid dehydrogenation is investigated (Chapter 6). Indeed, Commercial graphite (GP), graphite oxide (GO), and two carbon nanofibers (CNF-PR24-PS and CNF-PR24-LHT) were used as catalysts for the metal-free dehydrogenation reaction of formic acid (FA) in liquid phase. Raman and XPS spectroscopies were employed to characterize the materials and Density Functional Theory (DFT) calculations were utilized to study the role of defects in this reaction. In the final results chapter (Chapter 7), the above reported metal-free carbocatalysts are employed for the hydrazine hydrate dehydrogenation reaction, in order to produce H2 without the presence of CO2. A combination of DFT and experimental studies were used to unravelling the hydrazine hydrate decomposition reaction on metal-free catalysts. The study focuses on commercial graphite and two different carbon nanofibers, Pyrolytically Stripped (CNF-PS) and High Heat-Treated (CNF-HHT), respectively treated at 700 and 3000 °C to increase their intrinsic defects. Finally, Chapter 8 presents a summary of the main findings of this work and different possibilities to continue this research study.