# Structured Alumina Substrates for Environmental Catalysis Produced by Stereolithography
## 2 Materials and Methods
### 3D Printing  
An Al\({}_{2}\)O\({}_{3}\)-based photosensitive slurry with a ceramic volume fraction of 30% was developed and employed for this project. Tri(propylene glycol) diacrylate (TPGDA, Allnex, Luxemburg) was used as monomer base to disperse \(\alpha\) alumina particles (Nabalox NO 615-10, Nabaltec, Schwandorf, Germany) previously dried in a vacuum oven (\(-\)1 bar) at 150 \({}^{\circ}\)C for 2 h. Irgacure 819 (BASF, Ludwigshafen, Germany) was employed at 1.1 wt% (with respect to the diacrylate) as photoinitiator. The suspension was obtained via ball mixing for 24 h. Further details can be found in ref. [19; 25].  
Figure 3: Conversion efficiency (\(\eta\)) vs. gas flow speed (Vin) of three additively manufactured structures based on a rotated cube elementary cell (cubic 45) for different struts diameter (ds) and porosities (\(\epsilon\)) in comparison with two state-of-the-art honeycombs (HC) with different cpsi (i.e., 400 and 600). All measurements were performed at high exhaust gas temperatures and under stationary conditions. Thus, the catalyst was operating in the mass transfer limited regime. Data source [16].  
The lattice geometries were 3D printed, with layer thicknesses of 50 \(\upmu\)m, adopting the Digital Light Processing (DLP) technique in the \"bottom-up\" configuration through an industrial ceramic 3D printer specially designed to process highly viscous photosensitive ceramic suspensions (Admaflex 130, Admatec, The Netherlands). A schematic representation of the printing principle is shown in Figure 5.  
The photosensitive ceramic slurry is transported below the build platform by means of a transparent polymeric film. The slurry that is not photopolymerized is recollected after the slice projection and transferred to the material depositor. The light source is positioned below the polymeric foil. The ceramic slurry, when invested by the UV light source, solidifies and attaches to the previous layers. The object then grows upside down attached to the build platform. The benefit of such a system is that a very gentle peel-off force stresses the object after a layer is solidified and the Z-stage moves up. This guarantees the printability of very thin struts without breaking them. Moreover, since the film is used just once, the printing surface is always new and not degraded by its use (e.g., UV irradiation and scratches).  
Figure 4: CAD file of the developed architecture: (**A**) rotated cube unit cell; (**B**) cell replication; (**C**) tubular lattice.  
Figure 5: Schematic representation of the industrial ceramic 3D printer.  
The samples were finally thermally treated to firstly remove the organic binder (i.e., debinding) by heating up the samples to 500 \({}^{\circ}\)C at 1 \({}^{\circ}\)C\(\cdot\)min\({}^{-1}\) for 1 h and subsequently consolidate the ceramic phase (i.e., sintering) at 1600 \({}^{\circ}\)C for 2 h. Debinding and sintering were executed consecutively. The thermal treatment was selected from thermogravimetric (TGA/DSC) measurements. The selected temperature/time profile was implemented in air in a muffle furnace.