## 4 Discussion  
As can be seen from the results presented above, the activity of the Au-based catalysts supported on MnO\({}_{2}\) strongly depends on the structure and morphology of the MnO\({}_{2}\) oxide used for the synthesis. It seems that it is not only the differences in surface area and pore volume that are responsible for the high activity of the MnO\({}_{2}\)-NF catalyst. Even if, as could be seen from Table 1, the Au/MnO\({}_{2}\)-NF material has significantly higher surface area than the MnO\({}_{2}\)-Comm catalyst, the Au particle size did not change at all in both cases. This result differs from that observed in the literature. Indeed, [PERSON] et al. [31] studied different Au/MnO\({}_{2}\) materials in 5-HMF oxidation in an organic solvent. They observed that higher surface area was beneficial to obtain smaller Au particle sizes. However, in that case, the Au particle sizes were quite large (between 6 and 20 nm), which could explain the differences in the activity observed by the authors. In our study, all materials have similar Au particle size (about 3 nm), which suggests that only the structure of the support governs the catalytic activity. To better understand the differences, we decided to study in more detail the Mn and Au surface species present in the catalysts. X-ray photoelectron spectroscopy (XPS) is a powerful tool to study the external surface of catalysts, and is able to determine the chemical composition and oxidation states of the components present on the catalyst surface. In the case of the MnO\({}_{2}\) nanoflowers support, one main  
Figure 4: TEM studies of the spent catalysts: Au/MnO\({}_{2}\)-NF (**A**,**B**), Au/MnO\({}_{2}\)-NW (**C**,**D**) and Au/MnO\({}_{2}\)-Comm (**E**,**F**). photoelectron peak at the Mn 2p\({}_{3/2}\) region centred at 642.6 eV, which can be assigned to the presence of Mn\({}^{4+}\) species, was observed. Moreover, after the deposition of small gold nanoparticles, changes in the oxidation state of the Mn species occurred (Table 4). The main peak in the Mn 2p\({}_{3/2}\) region was broadened and shifted to 641.7 eV. This could indicate the partial reduction of Mn\({}^{4+}\) to Mn\({}^{5+}\) (\(\delta<4\)) at the surface. Concerning the O 1s XPS region, three different surface oxygen species could be identified. The binding energy between 529.9-529.5 eV was characteristic of the lattice oxygen (denoted as O\({}_{\rm L}\)), the binding energies between 532.0-531.4 eV were assigned to oxygen vacancies or the surface oxygen ions (denoted as O\({}_{\rm s}\)), and the binding energies around 533.9 eV were characteristic of adsorbed water. In addition, incorporation of Au NPs to the MnO\({}_{2}\) nanoflowers material led to a more significant decrease in the O\({}_{\rm L}\)/O\({}_{\rm s}\) ratio as compared to the pure MnO\({}_{2}\) nanoflowers. This demonstrates that the enrichment of oxygen vacancies or the surface oxygen ions was induced by the partial reduction of Mn\({}^{4+}\) species during the Au NPs deposition step. At the same time, the extent of Au\({}^{5+}\) species increases for the MnO2-NF sample, indicating higher numbers of gold active sites. At the same time, the XPS Au 4f region displayed broad peaks (Figure 5C), suggesting the presence of different oxidation states of gold (Au\({}^{0}\) and Au\({}^{5+}\) species).