Síntesis de catalizadores para producción de hidrógeno mediante reformado con vapor de compuestos oxigenados
- Autores: Rafael González Gil
- Directores de la Tesis: Luis José Alemany Arrebola (dir. tes.), Izabela Anna Stefanowicz Pieta (dir. tes.)
- Lectura: En la Universidad de Málaga ( España ) en 2025
- Idioma: español
- Tribunal Calificador de la Tesis: Juan Ramón González Velasco (presid.), F. Martín Jiménez (secret.), Mónica García Diéguez (voc.)
- Programa de doctorado: Programa de Doctorado en Química y Tecnologías Químicas. Materiales y Nanotecnología por la Universidad de Málaga
- Menciones:
- Internacional
- Calificación: Sobresaliente Cum Laude
- Materias:
- Enlaces
- Tesis en acceso abierto en: RIUMA
- Resumen
Abstract Hydrogen is widely used in petrochemical industries as a feedstock for the production of chemicals. However, the current hydrogen production method via natural gas steam reforming is unsustainable due to the depletion of natural gas resources and associated pollutant emissions. This Thesis focuses on the steam reforming of oxygenated model compounds, which are the main components of raw bio-oil (a liquid product of biomass fast pyrolysis) and can serve as hydrogen carriers or alternative fuels. The valorization of these compounds through their conversion into hydrogen, an increasingly important energy vector, is proposed. The objective of this work is to compare different routes for hydrogen production to establish the optimal operating conditions for its valorization. Acetone, ethanol and DME/MeOH couple were selected as oxygenated model compounds.
Active catalysts have been developed for the studied processes based on the monometallic Ni/Al2O3 system, which is widely used in industrial natural gas reforming. The system was modified according to the process and the oxygenated compound used, allowing adaptation to the specific conditions of each case. Bimetallic catalysts were prepared by incipient wetness impregnation of noble (RhNi) and transition metals (VNi) on an Al2O3 support, with different metal-to-Ni relative ratios. These catalysts were thoroughly characterized, and their activity, selectivity and stability were evaluated. Additionally, the mechanistic aspects of metal addition to Ni-based catalysts and the interaction and behavior of M-Ni pair were studied, combining activity and spectroscopic data.
Overall, the addition of a second metal to the Ni-based catalyst significantly modified its properties, improving Ni stability and distribution on the support surface, and enhancing catalytic activity by increasing selectivity in the promoted reactions. Acetone and ethanol steam reforming processes were studied with the RhNi bimetallic system, also examining the influence of the support on ethanol reforming due to its sensitivity to carbon deposition on the catalyst surface. Rh incorporation (at a Ni/Rh atomic ratio of 100/1) affects Ni metal size. Moreover, carbon deposited after reaction seems to be dependent not only on the Rh incorporated but also on the properties of alumina support, by reducing the amount of carbon deposited onto catalysts surface after reforming tests. The Ni/Al2O3 system was modified by adding V at different V/Ni ratios to study the steam reforming of the DME/MeOH couple. V incorporation to Ni/Al2O3 increased the relative contribution of NiO, suggesting an interaction between V and Ni that prevents the formation of spinel NiAl2O4. TPR analysis showed a decrease in the signal associated with spinel in bimetallic VNi systems, indicating that V addition modified surface characteristics and prevented undesirable Ni-support interaction Consequently, Ni-O-Ni and V-O-Ni species determined the reaction selectivity in DME-SR.
The catalysts activity in the studied processes was evaluated at atmospheric pressure operating under plug flow conditions across a temperature range of 423 and 973 K (depending on the oxygenated tested). Liquid reactants, in the cases of acetone, ethanol, and methanol-SR, were fed as mixtures with water using a HPLC pump at stoichiometric steam-to-oxygenated molar ratios. Reaction products were characterized using chromatographic techniques, analyzing the gaseous product stream composition. This work demonstrated that a high hydrogen yield and selectivity could be achieved with M-doped Ni/Al2O3 catalyst compared to the typical Ni/Al2O3 system.
Acetone and ethanol can be efficiently reformed into a hydrogen-rich stream. Rh plays a crucial role in acetone steam reforming, allowing complete acetone conversion and promoting H2 production with minimal carbon deposition. Ethanol reforming primarily occurs via ethanol decomposition into acetone, acetaldehyde, and ethylene, followed by steam reforming and decomposition of these intermediates. Ethanol reforming showed higher hydrogen selectivity than acetone-SR. A structured catalyst was designed to analyze its performance at pilot scale in ethanol reforming, producing an H2 stream suitable for feeding a PEM fuel cell. A multimetallic RhCeNi (0.0009:5:9 atomic ratio) in pellet form demonstrated activity in the pilot plant process, where H2 and CO2 were the main components detected with a molar ratio H2/CO2 close to 3.0, achieving 98 % ethanol at 923 K.
Alternatively, DME offers the advantages of easy storage and transportation, with the potential to use existing LPG infrastructure due to similar physical properties. This makes DME not only an H2 source but also an alternative fuel for diesel engines or as a diesel blend. The mechanism of hydrogen recovery via DME steam reforming was further studied, analyzing the reactions and catalyst interactions using FTIR to understand and improve the process. It was shown that water and DME compete for active sites on the catalyst surface, requiring a temperature of 773 K to promote the SR reaction, which proceeds via DME hydrolysis followed by methanol steam reforming. The presence of Lewis acid sites with high acid strength in this catalyst enhances the DME conversion through immediate hydrolysis and direct methanol reforming.
Overall, this Thesis contributes to a better understanding of the processes involved in hydrogen production from different oxygenated compounds with varying properties and characteristics. It aids in the design of catalytic systems for the valorization of raw bio-oil, whose main components were represented and modeled in this work. It also highlights the potential use of these molecules as a H2 carrier for advanced storage, transport, and distribution, supporting the development of decarbonized industry and transport models with hydrogen as a key ally.
