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A mechanochemical synthesis method has been used to synthesize CoRu nanoparticles supported on CeO2 for methane dry reforming. In this work, we study the effect of Ru addition to Co/CeO2-based catalysts and of the synthesis method by screening their catalytic activity, using synchrotron X-ray diffraction (XRD), and operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Ruthenium addition directly impacts the reducibility of cobalt species and results in smaller particle sizes, as demonstrated by H2-temperature programmed reduction and XRD. NAP-XPS shows that Ru modifies the metal-support interaction, as evidenced by the higher Ce3+/Ce ratios for the bimetallic samples and tuning the oxidation state of Ru. The synthesis method also influences the dispersion of Co and Ru on the surface. Mechanochemically-prepared samples (mono- and bimetallic) outperformed the conventionally-synthesized counterparts by reaching higher CH4 and CO2 conversions, resulting in a stable CoRu/CeO2 catalyst for 24 h at 700 °C and yielding an H2/CO ratio close to 1., Peer Reviewed, Postprint (published version)

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts’ nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts., Peer Reviewed, Postprint (published version)

L'objectiu d'aquest projecte final de màster va ser estudiar els impactes adversos de la termo-mecanoquímica aplicada en la preparació de catalitzadors heterogenis per a les reaccions d'oxidació de CO i COPrOx. El procés es va dur a terme en fase gas utilitzant un reactor tubular, on es van analitzar les corbes de conversió i selectivitat de totes dues reaccions. Per monitorar els gasos de sortida del reactor, es va utilitzar l'anàlisi de cromatografia de gasos amb argó com a gas portador inert. Es van realitzar diferents processos de síntesi mecanoquímica del catalitzador, variant la temperatura i el temps de durada del procés de síntesi del Au/CeO_2. Aquests canvis de temperatura es van aconseguir mitjançant l'addició d'energia externa en forma de calor. En el laboratori, es va dissenyar i implementar el sistema de reacció experimental, es va preparar el catalitzador de Au/CeO_2 utilitzant la tècnica mecanoquímica amb un molí de boles, es van realitzar els experiments i es van caracteritzar les mostres. A més, es va dur a terme un estudi econòmic i una breu avaluació de l'impacte mediambiental. L'eficiència de les mostres es va comparar en funció de dos paràmetres principals: la temperatura i el temps de durada de la síntesi mecanoquímica del Au/CeO_2. Els millors resultats es van obtenir en l'oxidació de CO amb la síntesi del catalitzador a temperatura ambient, mentre que en COPrOx es van aconseguir resultats òptims utilitzant mostres escalfades a 50ºC i posteriorment refredades. Aquests troballes destaquen la importància de controlar amb cura els paràmetres de síntesi per a maximitzar l'eficiència dels catalitzadors heterogenis en totes dues reaccions., El objetivo de este proyecto final de máster fue estudiar los impactos adversos de la termo-mecanoquímica aplicada en la preparación de catalizadores heterogéneos para las reacciones de oxidación de CO y COPrOx. El proceso se llevó a cabo en fase gas utilizando un reactor tubular, donde se analizaron las curvas de conversión y selectividad de ambas reacciones. Para monitorear los gases de salida del reactor, se utilizó análisis de cromatografía de gases con argón como gas portador inerte. Se llevaron a cabo diferentes procesos de síntesis del catalizador, variando la temperatura y el tiempo de duración del proceso de síntesis mecanoquímica del Au/CeO_2. Estos cambios de temperatura se lograron mediante la adición de energía externa en forma de calor. En el laboratorio, se diseñó e implementó el sistema de reacción experimental, se preparó el catalizador de Au/CeO_2 utilizando la técnica mecanoquímica con un molino de bolas, se realizaron los experimentos y se caracterizaron las muestras. Además, se llevó a cabo un estudio económico y una breve evaluación del impacto medioambiental. La eficiencia de las muestras se comparó en función de dos parámetros principales: la temperatura y el tiempo de duración de la síntesis mecanoquímica del Au/CeO_2. Los mejores resultados se obtuvieron en la oxidación de CO con la síntesis del catalizador a temperatura ambiente, mientras que en COPrOx se lograron resultados óptimos utilizando muestras calentadas a 50ºC y posteriormente enfriadas. Estos hallazgos resaltan la importancia de controlar cuidadosamente los parámetros de síntesis para maximizar la eficiencia de los catalizadores heterogéneos en ambas reacciones., The objective of this final master's project was to study the adverse impacts of thermo-mechanochemistry applied in the preparation of heterogeneous catalysts for CO oxidation and COPrOx reactions. The process was carried out in the gas phase using a tubular reactor, where the conversion and selectivity curves of both reactions were analyzed. Gas emissions from the reactor were monitored using gas chromatography analysis with argon as an inert carrier gas. Different mechanochemical catalyst synthesis processes were performed, varying the temperature and duration of the Au/CeO_2 catalyst synthesis process. These temperature changes were achieved by adding external energy in the form of heat. In the laboratory, the experimental reaction system was designed and implemented, the Au/CeO_2 catalyst was prepared using the mechanochemical technique with a ball mill, experiments were conducted, and the samples were characterized. Additionally, an economic study and a brief assessment of the environmental impact were carried out. The efficiency of the samples was compared based on two main parameters: the temperature and duration of the Au/CeO_2 mechanochemical synthesis. The best results were obtained in the CO oxidation with catalyst synthesis at room temperature, while in COPrOx, optimal results were achieved using samples heated to 50ºC and subsequently cooled. These findings highlight the importance of carefully controlling the synthesis parameters to maximize the efficiency of heterogeneous catalysts in both reactions.

The optimization of the supported Pd phase for CH4 activation on Pd/CeO2 catalysts has been a matter of great interest in the recent literature, aiming at the design of efficient methane abatement catalysts for Natural Gas fueled Vehicles (NGVs). Under lean conditions, a mixed Pd0/PdO combination has been indicated as exhibiting the best performance, while controversial results have been reported under stoichiometric conditions depending on the support oxide, where either Al2O3 or zeolite-based supports are usually considered. Here, by means of synchrotron-based in situ NAP-XPS and XRD measurements, we follow the evolution of Pd species on Pd/CeO2 samples prepared by dry mechanochemical synthesis (M) under stoichiometric CH4 oxidation feed, unravelling a stable Pd0/Pd2+ arrangement in a close to 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio as the most active palladium state for CH4 activation when excess oxygen is not available, in contrast to what was reported for Pd/alumina materials, where metallic Pd0 nanoparticles showed the highest activity. The combination of NAP-XPS analysis and activity test results highlights the promotional effect of the Pd–Ce interaction, resulting in enhanced oxygen transfer and improved activity and stability of the Pd/CeO2 catalyst prepared by a novel mechanochemical approach even under low O2 content, large excess of water vapor (10 vol%) and high temperature exposure (>700 °C)., Peer Reviewed, Objectius de Desenvolupament Sostenible::13 - Acció per al Clima, Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant, Postprint (published version)

Unlike other engineered ceramic products, alumina (Al2O3) displays interesting mechanical and physical properties, which makes it an ideal candidate for a wide range of uses in different fields and in particular for catalytic applications. However, the manufacturing of ceramic components has still a major drawback in production of highly complex three-dimensional (3D) shapes, microfeatures or structures with tailored porosity. Direct Ink Writing (DIW), also known as robocasting, is a material extrusion Additive Manufacturing technology and is one of such versatile methods with unique flexibility in material and geometry. In this work, a-Al2O3 ceramic materials were designed and produced by DIW to determine the most suitable sintering treatment and ceramic ink composition to design new components for catalytic applications. Several thermal treatments varying sintering temperature and time were tested previously to the preparation of inks with different ceramic loadings, up to 75 wt%. A systematic study of the DIW specimens sintered at the optimal sintering temperature – time combination, in terms of microstructure (density and porosity) and mechanical properties (hardness and indentation fracture toughness), was performed to determine the optimize ceramic loading. Finally, finite element modeling and catalytic experiments conducted for the optimal ceramic ink showed that 3D printed parts with a rectilinear infill pattern and 40% infill density favored catalytic performance., Peer Reviewed, Postprint (author's final draft)

Photocatalysts based on TiO2 and its modification have become popular as promising materials for the sustainable photoproduction of hydrogen. In this work, novel Pt - Cu - TiO2 composites with different TiO2 nanoshapes and modified with biomass-derived reduced graphene oxide (rGO) are reported as promising photocatalysts for the generation of hydrogen from water-alcohol mixtures. The developed photocatalyst has been characterized by X-ray diffraction, X-ray fluorescence spectroscopy, Raman and Fourier-transform infrared spectroscopies, scanning electron microscopy and Brunauer-Emmett-Teller surface area analysis. The partial replacement of platinum with copper definitely lowered the cost of hydrogen photoproduction still keeping its high efficiency. Furthermore, the additional modification of the composite with rGO successfully boosted the amount of generated H2, which was ca. 27 mmol h-1 g-1., Peer Reviewed, Postprint (author's final draft)

Supported Pd/CeO2 catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, operando near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO2 catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO2-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H2-TPR and Raman measurements. Adsorbed peroxide (O22-) species on the catalyst surface could also be detected by Raman spectra. Operando NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdOx and PdII ions in a PdxCe1-xO2-d solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdOx was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the PdxCe1-xO2-d phase, which was created at the Pd–CeO2 interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceri, Peer Reviewed, Postprint (published version)

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed., Peer Reviewed, Postprint (published version)

3D-printed ceria structures have been prepared by robocasting, without using any additive, and impregnated with different amounts of Ni and Ru, characterized and tested for the catalytic decomposition of ammonia in a fixed bed reactor. The best catalytic performance has been achieved with an active phase of 0.5Ni0.1Ru (w/w %). A kinetic expression has been obtained using a crushed catalytic structure, which has been employed in a 1D model to simulate the behaviour of the Ni-Ru impregnated 3D-printed ceria structures. The results have been compared with the experimental data to validate the proposed model. A series of simulations have been performed to determine the relationship between the geometric parameters of the 3D-printed structures and their catalytic performance in the ammonia decomposition, in order to optimize the catalytic structure with the aim of supplying the hydrogen produced to a PEM-type fuel cell., Peer Reviewed, Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant, Postprint (published version)

Ceria-supported Ni-Ru bimetallic catalysts with different metal loadings have been prepared by co-impregnation, characterized and tested in the production of hydrogen from the catalytic decomposition of ammonia. The bimetallic catalysts showed an excellent catalytic performance in long-term stability tests with respect to monometallic Ru/CeO2 and Ni/CeO2 and in multicycle tests under pure ammonia. The best catalytic performance has been obtained over catalysts with 2.4-5 wt.% Ni, 0.4-0.6 wt.% Ru, and a Ni/Ru wt.% ratio of ca. 7. TOFH2 values exceeding 2 s-1 have been obtained, which are among the highest reported for ammonia decomposition at 400 °C. Raman spectroscopy, XRD, HRTEM, XPS, TPR and H2 chemisorption have revealed the existence of an intimate contact between Ni and Ru and CeO2, which is considered the reason of the excellent catalytic activity and stability observed. A kinetic model has been developed using the Langmuir-Hinshelwood-Hougen-Watson approach for the decomposition of ammonia in a fixed bed reactor. The reaction rate expression of the ammonia decomposition on Ni-Ru bimetallics supported on ceria suggests that the dehydrogenation of the ammonia adsorbed on the surface of the catalyst is the limiting step of the reaction and that ammonia decomposition is inhibited by the presence of H2., Peer Reviewed, Postprint (published version)