Synthesis development of mixed metal oxide aerogels using sol-gel process for catalytic applications

Abstract

Aerogels are nanostructured materials which exhibit high surface areas and an open porosity. The sol-gel process is a prominent technique to synthesize these aerogels, in which a gel is formed from specific precursors and then supercritically dried to obtain mesoporous structures. Almost all types of materials can be synthesized in the sol-gel process. Properties of each single material can be combined so the final material can be improved and tailored for the specific application, designing a suitable and efficient catalyst. Many synthesis processes are energy-intensive compared to the sol-gel process and require several steps or complex reactions to form interconnected nanostructures of mixed metal oxides.[1] This limitation can be addressed by means of a one-pot sol-gel reaction with controlled hydrolysis and condensation using metal alkoxide precursors, water, acid and solvent. This method provides several advantages for the formation of metal-oxide aerogels related to the “green deal” topic. The sol-gel process is a simple method and requires no complex setup. The synthesis is performed at low temperatures and ambient pressures without any hazardous chemicals such as propylene oxide which is commonly used in the sol-gel synthesis for polymerization.[2] Instead simple metal alkoxide precursors are used which react with water to the corresponding metal oxide. The synthesis protocol can be varied to control the nanoscale structure and physicochemical properties. The ability to control the sol-gel process for synthesising aerogels is a powerful tool for designing different materials with specific surface area, pore volume, electronic structures, morphology and electrical conductivity. A new acid-catalyzed sol-gel synthesis route is developed from titanium- and tin alkoxide precursors to obtain TiO2 and SnO2-TiO2 interconnected aerogels with semiconducting properties. Also, inorganic materials such as MoS2 are processed into the structure which could improve the surface area and electron mobility.[3] The chosen materials are preferably used due to their high abundance and low toxicity. Synthesis parameters, e.g. amount of water, type and amount of solvent and pH are studied,which presumably have an impact on the hydrolysis and condensation reaction rate and therefore lead to different structures and morphologies of the resulting network. Metal oxide-based aerogels are promising materials for several applications such as e.g. heterogeneous catalysts, catalyst support in fuel cells and photocatalytic applications such as the hydrogen generation for the sector of electric mobility and aviation with green hydrogen as clean energy carrier. Latest experiments showed promising results using the optimized TiO2 aerogel for the photocatalytic hydrogen generation. [1] S. Han et al., “Hierarchical TiO2-SnO2-graphene aerogels for enhanced lithium storage”, Phys. Chem. Chem. Phys., 2015, 17, 1580. [2] S.A. Lermontov et al., “An approach for highly transparent titania aerogels preparation”, Materials Letters 2018, 215, 19-22. [3] Y.-C. Chang et al., „MoS2@SnO2 core-shell sub-microspheres for high efficient visible-light photodegradation and photocatalytic hydrogen production”, Materials Research Bulletin 2020, 129, 110912.