Fourier transform infrared spectroscopy in attenuated total reflectance mode (FT-IR/ATR) was used to investigate metal-to-oxygen vibrations and to check for the presence of organic compounds on the catalyst surface after reaction. In all measured spectra two intense bands associated with the OB3 and ABO3 vibrations of the spinel lattice (560 and 660 cm -1 for Co3O4) were detected. It is interesting to compare the position of these two peaks in the spectra of the spinels (Figure ). The position of the peaks fit the trend of the normal/inverse configuration and the bond lengths. In general, the wavenumber of a vibrational mode is related to the strength of the bond and to the masses of the atoms involved: the stronger the bond (i.e. the larger the force constant associated to its vibration) and the smaller the reduced mass of the system, the larger the wavenumber. Since Mn has a lower atomic mass than Co, we would expect that MnCo2O4 vibrates at higher wavenumbers than Co3O4, the bond strengths being equal. However, the opposite happens, suggesting that the metal-to-oxygen bond might be weakened by the presence of Mn in the lattice, corroborating the XRD results. The infrared spectra resulting from the analysis of the cobaltites before and after catalytic tests are shown in section S7. No organic compounds seem to be retained by the examined catalysts after the washing. By focusing on 1000-4000 cm -1 region of the spectrum of Co3O4 after the reaction, two additional peaks at 1600 and 3300 cm -1 can be observed, corresponding to the bending and stretching modes of water, respectively.