Resumen de Development of functionalized aerogels for applications as catalyst and hydrides matrix
Urban air pollution is a serious problem in many large cities in the world. In addition, the increasing share of solar and wind in the energetic mix and its fluctuating nature, demands the development of technologies for storage of energy in periods of excess supply, to use it during the periods of excess demand. Direct use of these energy forms on onboard applications is also not possible, and requires an intermediate form of energy storage.
Hydrogen is good as energy vector. Hydrogen based fuel cells are being developed using hydrogen storage systems based on compressed, liquefied and materials-bounded hydrogen. Compared to gas and liquid storage tanks, solid hydrides can store high amounts of hydrogen in small volumes, which is liberated by a reversible chemical reaction by thermolysis.
One of the technological challenges which restrain the development of hydrides for H2 storage is the heat transfer inside the storage tanks. Microwaves seems to be a promising alternative to overcome this obstacle.
Another option for the improving of the kinetic decomposition of hydrides is the nanoconfinement of the hydride inside a micro or mesoporous material. By confining the hydride inside the porous host, hydride particle size is restrained to the pore size, and particle agglomeration and growth process, which could have an adverse effect on the kinetic decomposition, are avoided.
Finally the addition of a catalyst which accelerates the hydride decomposition also looks interesting.
In this thesis aerogels have been developed in order to serve as catalyst support and as matrix for hydrides.
First of all synthesis and supercritical drying of silica aerogels made via a sol–gel process were studied. Tetramethylortosilicate was used as precursor. Hydrolysis and poly-condensation steps were followed by carbon dioxide supercritical drying (T = 45°C; P = 10.5 MPa). The complete supercritical drying step was video recorded in order to study the evolution of the size of the gels, concluding that a noticeable shrinkage only takes place during the decompression of CO2 at the end of the drying process, being the total shrinkage of 3–4%. The mass transfer mechanisms during drying have also been studied through analysis of the evolution transparency of the aerogels along the supercritical drying process. The mass transfer processing during drying was observed to be dominated by convection in the earliest stages, where a direct relationship between drying rate and CO2 flow were found. In the later stages, diffusion of the remaining organic solvent through the alcogel determined the mass transfer process.
The next step was controlling the surface chemistry of silica aerogels by tuning from hydrophilic to hydrophobic functionalization. Tetramethylorthosilicate or a mix of the first one with, trimethylethoxysilane were used as precursors. The hydrolysis and poly-condensation steps were followed by carbon dioxide supercritical drying (T = 45°C; P = 10.5 MPa). The resulting dry hydrophilic aerogels were subjected to a hydrophobic surface treatment. Functionalization was achieved by using supercritical carbon dioxide as solvent media for different silane functionalization reactants: trimethylethoxysilane, octyltrimethoxysilane and chlorotrimethylsilane. Effects of the working pressure and reagent concentration on the functionalization were analyzed using FT-IR spectroscopy and exposing the treated aerogels to saturated moisture conditions in order to study the mass increment during the humidification. Nitrogen adsorption measurements show a considerable drop on the specific areas (13–17%) and on the pore volumes which were reduced by 50% by the functionalization treatment. By modification of the operating pressure and variation of the functionalization agent employed, the degree of functionalization could be gradually increased up to the values of the aerogels synthesized as hydrophobic in the sol–gel phase.
After the synthesis and functionalization the aerogels were ready to be used in different fields. Effective routes to obtain more valuable products require the design of efficient catalysts. These novel structures require the integration of support active sites in a way that preserves their advantages and capabilities. Therefore the development of novel catalytic structures achieved by the integration of metallic nanoparticles evenly distributed in a mesoporous and high-surface aerogel appears as a promising alternative. Thus Pd nanoparticles were embedded on silica aerogel by using three different techniques. In each of them the metal was loaded in the matrix at different steps of the production: the direct synthesis, the wet impregnation and the supercritical impregnation of the previously dried aerogels. The resultant materials were characterized to analyze the differences depending on the applied technique for its impregnation. Atomic absorption, nitrogen physisorption, X-ray diffraction, infrared spectroscopy and transmission electron microscopy where performed. In all the techniques the concentration of metal was varied (from 0.13% to 1.61 wt%) by modifying the concentration of the suspension (Pd-polyvinylpyrrolidone nanoparticles used in the direct synthesis) or of the solution of the metallic precursor (palladium acetylacetonate), both in the organic solvent and the supercritical media. The characterization had generally shown a good distribution of the metallic particles in the matrix, and a negligible effect of the metal on the textural properties. Finally, considerable variations where observed on the silanol groups on the surface of the catalysts. These materials were tested in D-glucose hydrogenation, observing significant differences on the performance of the catalyst depending on the synthesis technique employed.
Another field of study of application of the functionalized aerogels was for the nanoconfinement of hydrogen storage hydrides. Nonoconfinement of these compounds has proven to improve the decomposition kinetics and to reduce their thermolysis temperature. On the contrary, in some cases nanoconfinement could reduce the thermal conductivity, creating significant temperature gradients along the sample. To avoid this disadvantage, a C/SiO2 mesoporous matrix was synthesized by keeping very interesting physical properties (386 m²/g and 1.41 cm³/g), leaving space for the confinement of a hydride and tuning the global dielectric properties of the complex, making it susceptible to microwave heating. This idea was protected by a national patent. Ethane 1,2-diaminoborane was embedded on the matrix (11-31 wt%) by using wet impregnation method. Material characterization and H2 liberation tests by conventional heating and microwaves were performed showing the great potential of this technology. In addition numerical simulation of the device under microwaves was performed to reach better understanding of the process.
Finally the potential commercialization of the product obtained in this last step of the thesis was explore by developing a business model which offers batteries of different powers by using hydrides for hydrogen storage and fuel cell to transform it into electricity.
