# Nickel-Based Structured Catalysts for Indirect Internal Reforming of Methane
## 3 Results and Discussion
### Morphological Characterization of Uncoated Foam  
The microstructural characterization of NiCrAl foam was performed using SEM-EDX and FIB-SEM combined systems. The foam was pre-oxidized at high temperature and showed a passivated film on the surface that prevented the internal oxidization [36]. The oxidation behavior of Ni-Cr-Al alloys has been widely investigate [20; 37; 38; 39; 40]. Figure 2 shows the morphology of NiCrAl foam before and after using the FIB milling to expose a cross-section. The elements' concentrations (wt.%) along the oxide/alloy depth-profile were determined by EDX spot-analysis and are reported in Table 2.  
Figure 2a displays pores with size of 500 \(\upmu\)m and a rough surface made of sintered spheres ascribable to the passivated layer. The high roughness of the oxidized layer, due to nano-scale grains grown in island patterns, increases the specific surface area and ensures a good adhesion of the catalyst coating [20; 39]. Figure 2b shows irregular pyramidal-shaped crystallites of \(30\pm 12\)\(\upmu\)m size and two regions in the oxide layer, one inner and one outer. EXD spot analyses (Spectrum 1 and Spectrum 2) revealed different chemical compositions of passivated layer, as reported in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Element** & \multicolumn{4}{c}{**Weight (\%)**} \\ \hline  & **Spectrum 1** & **Spectrum 2** & **Spectrum 3** & **Spectrum 4** \\ \hline O & 2.8 & 3.6 & - & 10.6 \\ Al & 3.3 & 2.2 & 3.8 & 18.4 \\ Cr & 26.9 & 52.3 & 30.0 & 60.4 \\ Ni & 67.0 & 41.9 & 66.2 & 10.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: EDX microanalysis of NiCrAl foam. Figure 2: SEM micrographs of NiCrAl alloy foam (**a**) at low, (**b**) high magnification, (**c**) and after FIB (focused ion beam) milling. From the inner (Spectrum 1) to the outer (Spectrum 2), a significant enrichment of Cr (from 26.9 to 52.3 wt.%) was revealed, while the Ni content decreased from 67.0 to 41.9 wt.%. The Ni content was large in both regions while the Al was almost constant and much lower. In agreement with literature data [41], the irregular pyramidal-shaped grains can be attributed to NiCr\({}_{2}\)O\({}_{4}\)/Cr\({}_{2}\)O\({}_{3}\) mixed oxides, while the larger Ni content in the inner part may be related to a partial formation of NiO. Thus, NiCr\({}_{2}\)O\({}_{4}\) spinel phase can be formed by solid-state reaction between NiO grains and Cr\({}_{2}\)O\({}_{3}\) layer [40; 42]. The presence of surface oxides is further highlighted by Figure 2c, where the oxide/metal alloy cross-section, after FIB milling and polishing steps, is shown. Two different microstructures are clearly displayed, namely, a 1.7 \(\pm\) 0.3 \(\upmu\)m-thick surface oxide layer composed of sintered grains and an underlying dense metallic matrix. A very good adhesion without pores or micro-cracks is displayed. This fact can be likely related to the uniform oxide layer grown during the pre-oxidation process. The chemical composition of the oxide/alloy cross-section was also measured by EDX spot analyses. Moving from the oxide layer (Spectrum 4) toward the metallic substrate (Spectrum 3), the chemical composition sensitively changed. Spectrum 4 revealed a pronounced enrichment of Al and Cr close to the oxide/metal interface and a severe Ni-depletion (10.6 wt.%). Conversely, an increase of Ni (up to 66.2 wt.%) and the disappearance of the oxygen content were revealed in the matrix alloy by Spectrum 3. The chemical analysis of cross-section showed that Al\({}_{2}\)O\({}_{3}\) and Cr\({}_{2}\)O\({}_{3}\) were developed by the internal oxidation of both aluminum and chromium in the alloy, in agreement with previous literature studies [40; 43; 44]. According to [PERSON] and [PERSON] [43], the simultaneous formation of an external layer of Cr\({}_{2}\)O\({}_{3}\) and an inner protective layer of \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) at the oxide/alloy interface is due to the diffusion of atomic oxygen and alloy constituents at high temperature.