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Nowadays, Ni@C nanostructured materials are attracting a great deal of attention due to their multiple catalytic or magnetic functionalities. In this article we report on the investigation of the correlation between the microstructure and magnetic properties of Ni nanoparticles embedded in a carbon matrix. The samples were obtained following a two-step procedure that ensures protection against nanoparticle oxidation, and was carried out in the following way: (i) the synthesis of a nickel-imidazole-based metal-organic framework (MOF) by a simple method in an aqueous medium at moderate temperature (95 °C); and (ii) carbonization of the MOF at different temperatures between 400 and 600 °C to obtain a carbon-supported hybrid material, containing Ni nanoparticles with an “artichoke-like” morphology, where a Ni-FCC core is surrounded by “bracts” of Ni-HCP and Ni3C. The average size of the nanoparticle slightly changes from 7 to 10 nm as the carbonization temperature is increased, but the Ni-FCC core diameter ranges from 3 to around 6 nm. We show how the information obtained on the evolution of the magnetic behaviour with carbonization temperature, using X-ray diffraction and electron microscopy, complements each other by providing consistent structural and magnetic characteristics of the investigated Ni@C nanoparticles. In fact, this joint analysis allows us to explain the formation and transformation of different Ni-based crystalline phases along the synthesis process, including Ni3C and Ni with both hexagonal and cubic crystalline structures. The amount of conventional Ni-FCC is below 10 wt% for the sample treated at 400 °C and it can reach up to 50 wt% for that treated at 600 °C. Finally, based on our current findings we propose an explanation for understanding the magnetic properties of Ni@C, in which the Ni-FCC core spins mainly govern the magnetic coupling of the whole system.

Carbon-supported, non-noble metal-based catalysts derived from metal-organic frameworks (MOFs) are attractive alternatives to noble metal-based systems, but typical syntheses of the starting MOFs are not desirable from an environmental and practical perspective (e.g., they rely on non-innocuous organic solvents and long reaction times). Here, we report the preparation of a Ni-based MOF in aqueous medium, at moderate temperature (95 °C) and in a short reaction time (<30 min), which was used as a sacrificial template to access a family of Ni nanoparticle/carbon hybrids that were then tested as catalysts in the reduction of 4-nitrophenol (4-NP). The MOF-derived hybrids exhibited a mesoporous texture, with specific surface areas between 250 and 450 m2 g−1 depending on the carbonization temperature applied to the MOF, as well as high Ni contents (between 36 and 57 wt%). Notwithstanding the latter, the metal was homogeneously distributed throughout the carbon matrix in the hybrid and was quite resistant to extensive agglomeration and sintering, even at temperatures as high as 1000 °C. With increasing carbonization temperature, the Ni component was seen to go through different crystal phases, i.e., Ni3C phase Ni hexagonal close-packed phase Ni face-centered cubic phase. The results of the catalytic tests suggested the former and latter phases to be the most active towards the reduction of 4-NP, with catalytic activity values as high as 0.039 mol4-NP molNi−1 min−1.

[EN] The preparation of porous carbon materials though novel methodologies was investigated in this thesis. It is important that porous carbons are obtained with controlled porosity and morphology, since the latter features will have a strong bearing on their performance in practical applications. One of the explored strategies was the use of graphene oxide (GO) nanosheets as a morphology- and/or structure-directing agent. Thus, carbon xerogels were prepared through hydrothermal carbonization of glucose, where GO was seen to significantly change the typical morphology of hydrothermally carbonized glucose (micrometer-sized spheres), giving rise to xerogels with rough walls covered by a thin (nanometric) coating of hydrothermal carbon. These xerogels were activated with KOH to yield porous carbons with high surface areas and attractive performance when used as adsorbents for pollutants (dyes) from the aqueous phase or CO2 from the gas phase, as well as in their role as electrodes for supercapacitors. Hybrid materials made from nanoparticles of the metal-organic framework ZIF-8 and GO nanosheets were also prepared. The nanosheets were shown to act as lithographic templates that allowed the generation of uniform mesoporosity (~3.4 nm) in the corresponding carbons obtained by pyrolysis and KOH activation. These materials demonstrated an improved performance when used as electrodes in electrochemical charge storage, compared to their counterparts prepared in the absence of GO. Finally, carbon-nickel nanoparticle hybrids were prepared via pyrolysis from a MOFtype metal-organic precursor, which were tested as efficient catalysts for the reduction of nitroarenes., [ES] En esta Tesis se recoge la preparación de materiales de carbono porosos a través de metodologías novedosas. Es importante que estos materiales se obtengan con porosidad y morfología controladas, pues de ellas dependerán significativamente sus prestaciones en muchas aplicaciones. Una de las estrategias seguidas fue el uso de láminas de óxido de grafeno (GO) como agente director de la estructura y/o la morfología del material. De esta forma, se prepararon xerogeles de carbono mediante carbonización hidrotérmica de glucosa, donde el GO sirvió para modificar la morfología típica de la glucosa carbonizada hidrotérmicamente (esferas micrométricas), generando un xerogel de paredes corrugadas y recubiertas de carbón hidrotérmico de pocos nanómetros de espesor. Estos xerogeles fueron activados con KOH, dando lugar a materiales con elevadas áreas superficiales y comportamiento favorable tanto en el uso como adsorbente de contaminantes acuosos (colorantes) y de CO2 en fase gas, así como en sus prestaciones como electrodo en supercondensadores. También se prepararon híbridos de nanopartículas de ZIF-8 y láminas de GO en los cuales el GO actuó como una plantilla litográfica a escala nanométrica que permitió la generación de mesoporosidad uniforme (~3.4 nm) en los correspondientes carbones generados mediante pirólisis y activación con KOH. Estos materiales mostraron un comportamiento electroquímico de carga superior al del material equivalente preparado en ausencia de GO. Por último se prepararon materiales híbridos carbono-nanopartículas de níquel a partir de un precursor organometálico tipo MOF para su uso como catalizador eficiente en la reducción de nitroarenos.

Activated carbon xerogels with a cellular morphology were obtained from hydrothermally carbonized glucose-graphene oxide (GO) hybrids and tested as supercapacitor electrodes. The effect of the chemical activation (using KOH) on the nanometer-scale morphology, local structure, porous texture and surface chemistry of the resulting carbon materials was investigated and correlated with their electrochemical behaviour. The electrochemical performance of the activated xerogels was studied in a three-electrode cell using 1 M H2SO4 as the electrolyte. The results underlined the relevant role played by the xerogel nanomorphology; more specifically, xerogels with cellular structures exhibiting well-connected, continuous and very thin (∼5–15 nm) carbon walls (prepared with lower amounts of activating agent) favored ionic diffusion and electronic conduction compared to materials with broken, thicker walls (obtained from higher amounts of activating agent). The effect of nanomorphology and local structure was also made apparent when the xerogels were used as actual supercapacitor electrodes. Particularly, a symmetric capacitor assembled from a carbon xerogel with very thin walls and relatively high graphitic character delivered a much higher specific capacitance than that of a commercial activated carbon (223 vs 153 F g−1 at 100 mA g−1) as well as a significantly improved retention of capacitance at high current densities.