Figure shows the O2-and CO2-TPD results. The O2-TPD curves cannot be interpreted beyond arbitrary noise, suggesting that no considerable amounts of oxygen had been adsorbed before the temperature ramp. This assumption is backed by Liu et al. , who reported very low mass loss of NiFe2O4 during a combined thermogravimetric analysis and O2-TPD experiment starting from 800 °C only. The CO2-TPD curves differ largely from each other in terms of intensity. The signals in the lower crystallite size sample are much more intense most likely due to the higher surface area. Both curves exhibit two desorption maxima: one low temperature maximum at 72 °C, which is at the same position in both samples and one high temperature maximum at 558 °C and 541 °C, respectively, for the lower and higher crystallite size sample, respectively, possibly indicating a slight difference in adsorption energy.