Scanning electron micrographs of the prepared spinels show that these solids share the same porous morphology (Figure ). The formation of solids with such porous frameworks a consequence of the selected synthesis method, forming a large amount of gases as bubbles and cavities which are trapped into the final solid material. This is also the source of the very low apparent density of the catalysts. typically given by the physisorption of nitrogen on nonporous or macroporous adsorbents. This shape is the result of unrestricted monolayer-multilayer adsorption up to high values of P/P0. The hysteresis loop is a type H3 loop, usually given by non-rigid aggregates of plate-like particles, but also if the pore network consists of macropores that are not entirely filled with pore condensate. Comparing this information with the SEM images, it is clear that the hysteresis loop is given by the large cavities in the solids' structure. The specific surface area of each sample was calculated using the Brunauer-Emmett-Teller model (BET). The results are reported in Table . The specific surface of the solids directly obtained from the solution combustion step was about double, but it decreased during the calcination step. Nonetheless, the obtained values are comparable with those reported in the literature for spinel oxides synthesized via solution combustion. The exposure to the reaction environment caused a slight increase in the specific surface area of all tested catalysts. This behavior could be due to particle breakage caused by the stirring, which would make the internal porosity of the catalyst (observed by SEM) accessible to the gas, thus leading to an increase of the exposed surface. Energy-dispersive X-ray spectroscopy was used to determine the bulk elemental composition of the cobaltite catalyst samples. In all analyzed catalysts, the metal ions appeared not to be agglomerating, and to be completely dispersed in the structure, meaning that the composition of the catalysts is largely homogeneous throughout the samples. The corresponding EDX maps for O, Mn and Co of MnCo2O4 are reported below in Figure . Quantitative elemental analysis, performed by comparing the intensities of peaks corresponding to the different elements in the EDX spectra, showed that the ratio between the amount of Co and the amount of guest metal M (Mn, Ni, or Zn) in the bulk of the solid was close to the nominal value (Table ). In ZnCo2O4, Co was slightly more abundant than expected.