# Structured Alumina Substrates for Environmental Catalysis Produced by Stereolithography
## 3 Results and Discussion  
As previously described, some preliminary 3D printing tests were performed to assess the capability of the employed SLA production process. Figure 6 left shows a green body with a struts diameter of 0.20 mm, a cell size of 1 mm, and a wall thickness of 0.55 mm. The printing result is very encouraging, since the component was printed without defects (i.e., cracks or delamination), the struts are intact, and the cells are not occluded. Nevertheless, the part turns out to be extremely fragile and very difficult to handle; thus, the coating with precious metals (washcoating) cannot be properly achieved without damaging the lattice. Moreover, as written previously, the best compromise between surface area and pressure drop (Figure 3) is obtained with a struts diameter of 0.5 mm and a porosity of about 80% (combining this two geometrical characteristics leads to a cell size of approx. 2 mm). The right panel of Figure 6 shows the 3D-printed and sintered lattice in its final configuration ready to be washcoated with precious metals and employed for the conversion tests. The component undergoes a homogeneous volumetric shrinkage of about 12% during the heat treatment (debinding and sintering). Figure 7 shows a traditional honeycomb catalyst (M1 in Table 1) and the 3D-printed lattice (3D-printed in Table 1). Both structures were washcoated with the same catalytically active material. The geometry of both honeycombs (M1 and M2 in Table 1) is the same (400 cpsi), and as shown in the Figure 7, the external dimensions of the tested specimens are equal. Table 1 shows that the three samples were prepared using different washcoating characteristics. M1 and 3D-printed had the same volumetric mass loading, which resulted in an estimated half coating thickness of M1. M2 was thus prepared to obtain the same area loading and coating thickness of the 3D-printed, which resulted in around double the volumetric mass loading, i.e., 124 g\(\cdot\)L\({}^{-1}\) against 70 g\(\cdot\)L\({}^{-1}\). This is the standard loading of 400 cpsi crodierite monoliths for practical applications. Hence, M1 and M2 had around double specific surface compared to the 3D-printed one, while M2 and 3D-printed had the same catalyst mass per unit specific surface. It appears already clear that comparison of the monolithic pieces, honeycomb and 3D-printed, is not trivial when the same catalytically active phase is used. Figure 6: **(left)**: 3D-printed closed-walls lattice, the base cells correspond to a rotated cube with a struts diameter of 0.20 mm, a cell length of 1 mm, and a wall thickness of 0.55 mm. As a reference, a sewing needle with a diameter of about 0.4 mm is placed on the component. **(right)**: 3D-printed and sintered closed-walls lattice in its final configuration, the base cells correspond to a rotated cube with a struts diameter of 0.55 mm and a cell length of 2 mm. Figure 8 shows the NH\({}_{3}\)-SCR results for the three samples obtained at a GHSV of 50,000 h\({}^{-1}\) for NOx conversion and NOx conversion at 10 ppm NH\({}_{3}\) slip. Based on the characteristics mentioned above, M2 exhibited the highest NO conversion on the whole temperature range (200-550 \({}^{\circ}\)C). M1 was characterized by roughly half of the NO conversion of M2 below 300 \({}^{\circ}\)C, in agreement with its reduced (half) catalyst mass (see Table 1). The two samples also produced N\({}_{2}\)O above 400 \({}^{\circ}\)C (not shown), which is due to unselective NH\({}_{3}\) oxidation. At 550 \({}^{\circ}\)C, M1 and M2 produced 20 ppm and 38 ppm, respectively. Under these conditions, the 3D-printed sample was the least active and was less active than sample M1, which was the most similar in terms of active mass load (approx. 70 g-L\({}^{-1}\)). This becomes clear especially above 300 \({}^{\circ}\)C. For example, at 250 \({}^{\circ}\)C, M1 exhibited 48% NOx conversion while the 3D-printed exhibited 44%, and at 300 \({}^{\circ}\)C, the difference increased to 10% conversion.