Ammonia on demand: Titanium Dioxide Aerogels for Photocatalytic Dinitrogen Reduction

Abstract

The growing demand for sustainable and efficient energy solutions has accelerated the search for novel advanced materials with interesting electronic and catalytic properties. Among these materials, titanium dioxide (TiO2) has gathered significant attention due to its exceptional photocatalytic properties.[1] With a band gap typically between 3.0 and 3.5 eV, TiO2 can be excited by the UV-light fraction of the solar spectrum and can catalyze interesting reactions like water splitting and the reduction of elemental nitrogen.[1-3] Due to the outstanding charge carrier separation properties of some TiO2-materials, they can also be used to store electrons and carry out time-shifted photoreactions.[4,5] For photocatalytic applications, nanomaterials, such as nanoparticles, are often favored over bulk materials because of their large surface area and high number of reactive sites.[1] However, the catalytic efficiency of nanoparticles usually decreases when agglomeration occurs, as the incident light can only excite the outer layer of the formed agglomerates.[6] Nanostructured, porous aerogels with large surface areas are a promising approach to tackle this problem.[7] Here the material particles are arranged in a rigid 3-dimensional network that prevents agglomeration and can be designed with sufficient light transmissibility that enables all incorporated particles to be excited upon radiation.[7] The interconnected nature of the particles in aerogels can additionally improve the charge carrier separation in a material.[8,9] Aerogels are synthesized by using the sol-gel method, which allows to directly vary the material parameters by changing the synthesis parameters, followed by supercritical drying to preserve the porous nature of the wet gels.[10] We will demonstrate the development of a titanium dioxide aerogel designed for optimal photocatalytic reduction of nitrogen to ammonia and will discuss the influence of different synthesis parameters on the materials properties. This includes different solvents and solvent-mixtures, catalyst concentrations, and processing parameters. Herein we especially investigated the direct influence of acid concentration and ligand exchange at the titanium alkoxy precursor on the pore structure and crystallinity of the aerogels. We will also show our findings on the underlying mechanisms of the sol-gel synthesis of titania aerogels and their photocatalytic activity.

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