## 3 Results
### Catalytic Activity  
All prepared samples and bare supports were tested in the aqueous phase oxidation of furfural to furoic acid. Supports alone showed, already, a significant activity for this reaction (up to 55% furfural conversion in case of MnO\({}_{2}\)-NF). However, the selectivity to furoic acid was much lower than in the case of Au-modified samples (c.a. 80%). It reached only 25% in the case of the MnO\({}_{2}\)-NF sample at a furfural iso-conversion of 55%. It is well established that MnO\({}_{2}\) materials are good catalysts for oxidation reactions. Generally, oxidation of furfural to furoic acid is carried out in the presence of hazardous oxidants [26; 27] or heterogeneous catalysts in a strong alkaline solution [28]. However, at alkaline pH, degradation of furfural can occur, resulting in a drop of selectivity. Furthermore, instead of leading to furoic acid salt, the oxidation reaction can yield mixtures of low and high molecular weight products. The basic medium is also related to degradation of furfural and the formation of other furan derivates (such as 5-HMF) under reaction conditions. Tests in absence of catalyst showed conversion without any detection of oxidation products. In addition, a colour change was noticed in the reaction mixture from yellow to brownish, with the formation of a black precipitate [29; 30]. This precipitate was attributed to the formation of humic acids, which are a mixture of many different acids that can be formed during dehydration, and also arise from the condensation of furfural. The present study showed that working in base-free conditions is possible using highly selective supported gold nanoparticles. Figure 3 presents the results obtained for furfural oxidation with air at 110 \({}^{\circ}\)C. After only 2 h of reaction, Au/MnO\({}_{2}\)-NF catalyst reached 86% of furfural conversion (Table 2, Entry 3). At the same time, Au supported on commercial MnO\({}_{2}\) oxide showed only 69% of conversion. In the case of both homemade catalysts (Au/MnO\({}_{2}\)-NF and Au/MnO\({}_{2}\)-NW), very high selectivity to furoic acid was obtained (more than 80%), and the carbon balance values indicated that no degradation of furfural occurred. The most active Au/MnO\({}_{2}\)-NF catalyst was tested in different reaction conditions, to optimize the furoic acid yield. As could be seen from Table 2, full furfural conversion was reached after 4 h of reaction at 110 \({}^{\circ}\)C (Entry 4). In addition, this catalyst was already active at lower temperatures, and 65% of conversion was obtained at 80 \({}^{\circ}\)C after 4 h of reaction. The best results were obtained at 110 \({}^{\circ}\)C and after 4 h of reaction. In these conditions, furoic acid can be produced with a yield of 82% (Entry 4). The reaction temperature strongly affects the catalytic properties of the supported gold nanoparticles. Indeed, at low temperature, high selectivity could be obtained, whereas high temperature favoured the degradation of furfural. Full conversion was observed at 150 \({}^{\circ}\)C (Entry 7), but only 67% of selectivity to furoic acid and a very low carbon balance, ascribed to the formation of undetected degradation products. Alternatively, for all the tests carried out in the 80-110 \({}^{\circ}\)C temperature range, high values of carbon balance were observed, which suggests that no degradation of furfural took place (Entries 1-6). In addition, ICP analysis of post-reactant mixtures confirmed that no Au leaching occurred. However, a low quantity of Mn\({}^{2+}\) was detected in the solution, indicating low Mn\({}^{2+}\) leaching (12-14 ppm). Taking into account the Mn\({}^{2+}\) leaching, recycling tests were performed. Spent catalysts were also studied by TEM, as reported in the next section. Catalyst reuse was tested on all catalysts. The results are given in Table 3.  
As can be seen, no significant decrease of the activity was observed. Only in the case of the Au/MnO\({}_{2}\)-comm sample was the decrease of furfural conversion about 20%.