3D Printed Scaffolds for Monolithic Aerogel Photocatalysts with Complex Geometries

Abstract

clean and renewable energy carrier, produced from sustainable and abundant energy sources, is a promising solution. [2] The combustion of hydrogen does not release any greenhouse gases into our atmosphere. [3] With focus on the photocatalytic production of hydrogen, the challenge is to find the right materials, synthesize them with the appropriate morphology and process them into a form that enables efficient photocatalysis. From a materials point of view, most of the research is dedicated to heterogeneous photocatalysis using semiconducting photo catalysts. [4] Kudo and Miseki compiled a large collection of different photocatalyst materials ranging from various metal oxides to metal (oxy)sulfides and metal (oxy)nitrides. [5] In spite of this immense compositional diversity, the largely available, cheap, stable, and nontoxic titanium dioxide (TiO 2 ) is still one of the most studied photocatalysts, regardless of its activity being limited to ultraviolet (UV) light illumination and its unfavorable fast electron hole recombination. [6] In addition to the materials selection, the morphology of the photocatalyst also plays an important role, because a large surface area, which exposes many adsorption sites to the environment, is crucial. [3] Nanostructures with particle-, [7][8][9] rod-, [10][11][12] tube-, [13][14][15] or sheet-like [16][17][18] morphology provide a large surface-to-volume ratio and thus have been found to be ideal structures for photocatalysis. However, most nanoparticles are used in powder form, which has the disadvantage that such photocatalytic nanostructures tend to agglomerate and that extraction of the photocatalyst from the reaction medium for recycling is challenging. [19] Consequently, processing of the nanoparticles into thin films [20,21] or their immobilization on 3D, photocatalytically nonactive templates such as foams, [22] sponges, [23] mesoporous silica, [24,25] electrospun nanofibers [26][27][28] or hydroxyapatite [29] has been pursued. [3] However, a significant reduction in surface area and number of adsorption sites, both of which are detrimental to photocatalytic activity, is inevitable. [19] A solution to this problem is the fabrication of templatefree, macroscopic, 3D structures entirely made of the photocatalytic material. Examples along these lines include 3D porous g-C 3 N 4 , [30] mesoporous TiO 2 foams, [31] graphene oxide (GO) sponges, [32] porous g-C 3 N 4 monoliths, [33] MoS 2 /rGO aerogels, [34] CN aerogels, [35] or Au-Pt-TiO 2 aerogels. [36] Unfortunately, the Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocata...

The Importance of the Macroscopic Geometry in Gas‐Phase Photocatalysis

“…[ 71 , 72 ] For very low‐density monolithic titania aerogels, however, the penetration depth of light is in the mm range and relatively thick layers of materials are required for adequate light absorption, which in turn renders mass transfer more difficult. [ 59 , 73 ]…”

“…The use of tailor‐made scaffolds with cylindrical macroscopic shape, but with varying internal structure to modify the gas flow, and thus the mass transfer, and to increase the illumination efficiency was explained in Section 5 . [ 73 ] For the second option, when the geometry is changed from cylinder to disk, also the design of the reactor has to be modified. We developed two new reactors that illuminate the disk both from above and below (i.e., from the flat sides), while allowing the gas to flow either in the same direction as the light (along the axis of the disc) as shown in Figure 8b or perpendicular to the light, i.e., along the cross‐section of the disk (Figure 8c ).…”

“…In a recent study, carefully designed 3D printed scaffolds were introduced into cylindrical TiO 2 ‐Pd aerogel monoliths to optimize their photocatalytic performance in a tubular reactor. [ 73 ] It is important to note that in this case, the embedded scaffold did not serve as a support for the photocatalyst, but rather as a geometric constraint. The photocatalyst itself formed a volume‐filling and self‐supporting network in the cavities of the scaffold.…”

“…Hydrogen production rate of TiO 2 -Pd aerogels synthesized with the aid of various scaffold geometries: 1) no scaffold, 2) standard scaffold, scaffold with solid core with 3) circular cross-section, scaffold with solid core with 4) elliptical cross-section, scaffold with 5) solid core with elliptical cross-section and optimized length. Reproduced under the term of the Creative Commons Attribution Licence CC BY-NC 4.0 [73]. Copyright 2021, The Authors, published by Wiley-VCH.…”

Additive‐Free, Gelled Nanoinks as a 3D Printing Toolbox for Hierarchically Structured Bulk Aerogels

“…[ 2 ] An attractive approach to bridge the nanoscopic and macroscopic world is to process nanomaterials into aerogels via gel casting. [ 3,4 ] This method translates the intrinsic properties of nanoparticle dispersions into highly porous bulk solids with immense surface area. However, the restricted range of mold geometries and aerogel's poor subtractive processing abilities limit shape tunability and prevent complex microstructured geometries.…”