The Dependence of Photocharging TiO2 Aerogels on Surface Area and Charging Conditions and their Activity for the dark N2 and NO3- Reduction Reaction

Abstract

Nanostructuring of semiconductor photocatalysts, such as the generation of mesoporosity, i.e. in aerogels, is an important strategy to improve photocatalytic activities. Aerogels are 3D nanostructured materials of interconnected porous networks with very high porosities, ultra-low densities, and high surface areas, which are higher than those of particulate systems.[1,2] The presumed high amount of reactive sites, a good contact to the electrolyte, and the short diffusion paths for minority charge carriers, as well as the improved charge carrier separation can be advantageous for photocatalytic activity, making TiO2 aerogels an interesting group for photocatalysis.[3-5]

TiO2 aerogels are known to offer an increased density of photoexcited electrons compared to particulate systems.[5] Furthermore, the electrons are trapped close to the surface of TiO2, forming Ti3+ states that exhibit a characteristic dark blue coloration, i.e. a broad absorption at a maximum of 650 nm.[6-8] Various dark reduction reactions can be carried out with these stored electrons, i.e. nitrogen reduction reaction for the production of ammonia or the reduction of metal ions.[8-10] Such sustainable processes using stored electrons can pave the way for energy-efficient and decentralized production of ammonia, which is currently produced by the extremely energy-intensive Haber-Bosch process, which accounts for 2 % of the annual global energy consumption and 1.6 % of the global CO2 emissions.[11]

Herein, we present the electron storage capability and photocatalytic activity of mesoporous TiO2 aerogels with tailored properties, which were prepared via a novel acid-catalyzed sol-gel synthesis followed by supercritical drying.[12,13] The surface area could be tailored over a wide range by using different calcination treatments. A continuous gas flow batch reactor was used for the photocatalytic characterization. The hydrogen evolution activity in water-methanol mixtures decreased with increasing surface area and decreasing crystallinity. At the same time, the electron storage capacity decreased. This was verified by electron quantification experiments using a dark reduction reaction from Pt4+ to Pt0 with detection of the hydrogen evolution peak in the dark. The intensity of this dark hydrogen evolution peak increased with increasing surface area. The dependence of the electron storage capacity on different parameters, i.e. hole scavenger concentration, type of hole scavenger, light intensity, and irradiation time, will also be presented. Furthermore, the ability to reduce nitrogen and nitrate to ammonia in the dark with these stored electrons in our TiO2 aerogels will also be presented.

References: [1] Alwin, S.; Sahaya Shajan, X. Aerogels: Promising Nanostructured Materials for Energy Conversion and Storage Applications. Mater. Renew. Sustain. Energy, 2020, 9, 7. [2] Hüsing, N.; Schubert, U. Aerogels-Airy Materials: Chemistry, Structure, and Properties. Angew. Chemie Int. Ed. 1998, 37, 22. [3] Anderson, M. L.; Stroud, R. M.; Morris, C. A.; Merzbacher, C. I.; Rolison, D. R. Tailoring Advanced Nanoscale Materials Through Synthesis of Composite Aerogel Architectures. Adv. Eng. Mater. 2000, 2, 481. [4] DeSario, P. A.; Pietron, J. J.; Taffa, D. H.; Compton, R.; Schünemann, S.; Marschall, R.; Brintlinger, T. H.; Stroud, R. M.; Wark, M.; Owrutsky, J. C.; Rolison, D. R. Correlating Changes in Electron Lifetime and Mobility on Photocatalytic Activity at Network-Modified TiO2 Aerogels. J. Phys. Chem. C 2015, 119, 17529 [5] Panayotov, D. A.; DeSario, P. A.; Pietron, J. J.; Brintlinger, T. H.; Szymczak, L. C.; Rolison, D. R.; Morris, J. R. Ultraviolet and Visible Photochemistry of Methanol at 3D Mesoporous Networks: TiO2 and Au–TiO2. J. Phys. Chem. C 2013, 117, 15035. [6] Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L. Flash Photolysis Observation of the Absorption Spectra of Trapped Positive Holes and Electrons in Colloidal Titanium Dioxide. J. Phys. Chem. 1984, 88, 709. [7] Mohamed, H. H.; Mendive, C. B.; Dillert, R.; Bahnemann, D. W. Kinetic and Mechanistic Investigations of Multielectron Transfer Reactions Induced by Stored Electrons in TiO2 Nanoparticles: A Stopped Flow Study. J. Phys. Chem. A 2011, 115, 2139 [8] Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. Kinetic and Mechanistic Investigations of the Light Induced Formation of Gold Nanoparticles on the Surface of TiO2. Chem. - A Eur. J. 2012, 18, 4314. [9] Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. Reaction Dynamics of the Transfer of Stored Electrons on TiO2 Nanoparticles: A Stopped Flow Study. J. Photochem. Photobiol. A Chem. 2011, 217, 271 [10] Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. TiO2 Nanoparticles as Electron Pools: Single- and Multi-Step Electron Transfer Processes. J. Photochem. Photobiol. A Chem. 2012, 245, 9. [11] Wang, K.; Smith, D.; Zheng, Y. Electron-Driven Heterogeneous Catalytic Synthesis of Ammonia: Current States and Perspective. Carbon Resour. Convers. 2018, 1, 2. [12] Rose, A.; Hofmann, A.; Voepel, P.; Milow, B.; Marschall, R. Photocatalytic Activity and Electron Storage Capability of TiO2 Aerogels with an Adjustable Surface Area. ACS Appl. Energy Mater. 2022, 5, 14966.