Metal oxides like cerium oxide and titanium oxide are important materials for photocatalysis.[1] Especially gas phase photocatalysis, which has many advantages compared to the so far more intensively investigated liquid phase photocatalysis, holds great promise. Advantages are for example the tunability of reaction parameters like gas flow rate, gas composition and temperature. Furthermore, gaseous reaction environments facilitate the separation of products, reactants and catalysts.[2] However, the efficiency of gas phase photocatalysis is very low so far, suffering two major obstacles
(i) the restricted interface area between the typically employed thin films or powder catalyst and the gaseous reactants and (ii) a limited interaction between catalyst and light. In order to overcome these limitations, investigations on multi-scaled material systems such as three-dimensional nanostructured catalytic aerogels are being driven forward. However, while providing a high reactive surface area, the nanostructure of conventional aerogels drastically limits the gas transport through the pores. Additionally, their high transmittivity lowers the amount of light that can be used to drive the catalytic reaction, limiting photocatalytic efficiency.[3] With respect to that, multi-scaled aeromaterials, i.e. framework structures composed of interconnected hollow microtubes, could offer a promising alternative. For example, aeromaterials based on hexagonal boron nitride have been shown to provide significantly different light transport and gas transport properties, compared to aerogels.[4] Here we present a new strategy for the synthesis of highly porous catalytic aeromaterials based on CeO2 and TiO2. The synthesis is based on a sacrificial template built of zinc oxide tetrapods, which can form accumulated, stable and highly porous macroscopic structures.[5] In a hydrothermal approach it is possible to deposit cerium oxide microcrystals on the template, which merge into a homogeneous film with increasing reaction time. It is also possible to vary the thickness of the cerium oxide film by changing the cerium precursor concentration. After film synthesis the zinc oxide templates can be etched with mild acids leaving an interconnected aero cerium oxide microtube network which is ultralightweight (~50 mg/cm3), highly porous and stable. The additional possibility of modifying aero cerium oxide, for example with metal nanoparticles make this material a promising candidate for applications in gas phase photocatalysis.

References
[1] T. Montini, M. Melchionna. M. Monai, P. Fornasiero, Chemical reviews 2016, 116, 5987–6041.
[2] M. Schreck, M. Niederberger, Chem. Mater. 2019, 31, 597–618.
[3] F. Matter, M. Niederberger,. Advanced science, 2022, 9.
[4] F. Schütt, M. Zapf, S. Signetti et al., Nat Commun. 2020, 11, 1-10.
[5] Y. K. Mishra, R. Adelung, Materials Today, 2018, 21, 631–651.