## 1 Introduction  
In recent decades, supported gold nanoparticles have been extensively studied because of their unusual catalytic properties [1; 2; 3; 4]. It is well recognized that the size of gold nanoparticles determines both the activity and the selectivity of the final catalyst [4; 5]. It is also considered that the drastic change of the catalytic properties of nanoparticles is due to their new properties (physical and chemical) which are created when their size decreases, mainly due to the significant increase in the number of atoms on the surface [5; 6; 7]. However, when working with supported gold nanoparticles (NPs), the support also plays a very important role. Supported gold nanoparticles should be well dispersed to be able to interact with the support. Appropriate support can increase the dispersion of gold NPs, and hence improve the overall catalytic activity [8]. For Au catalysts supported on reducible supports, such as Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), CeO\({}_{2}\), and MnO\({}_{2}\), the dominant reaction pathway is expected to involve adsorption of a molecular oxygen species on the support, its dissociation at the interface, and then its reaction with the substrate [9]. In the case of Au supported on inert oxides (SiO\({}_{2}\) or Al\({}_{2}\)O\({}_{3}\)), the oxygen supply most likely proceeds via direct dissociative adsorption on the Au particles. The influence of the support on the catalytic activity strongly depends on the type of catalyzed reaction [10]. [PERSON] et al. [11] studied the effect of supported gold catalyst on the catalytic oxidation of glucose. They found that compared to gas phase catalytic oxidation reactions (such as for instance oxidation of CO), in the liquid phase oxidation,the influence of gold particle size is more important than the nature of the support. [PERSON] et al. [12] observed similar results. They have also stated that the size of the Au particles appears to be the most important parameter. However, in these studies, the oxidation was always performed at high pH [12]. Indeed, the majority of works in the literature deal with the oxidation of sugars or of other bio-based compounds at controlled basic pH. At this pH, the substrates were almost always completely converted [13]. Moreover, several papers reported the need for a basic medium to perform the selective oxidation [12; 13; 14; 15]. According to them, the presence of a base allows higher reaction rates, higher feed concentrations, better product solubility and lower adsorption of the products on the catalyst surface. However, oxidation in the liquid phase may occur at the interface between Au particles and the support. In this case, the activation energy would be different, depending on the nature of the support. Metal-oxide support interactions might affect Au clusters electronically and lead to weaker rate dependencies than those observed for unsupported Au nanoparticles. Obviously, the metal-support interactions depend on the nature of the support and of its structure [16]. It would be preferable to study the support effects without using basic conditions. Indeed, in the high pH range, the base used for the reaction governs the mechanism of the reaction. There is no systematic study in the literature which establishes a correlation between the support structure interacting with gold nanoparticles and their catalytic activity and selectivity. Therefore, we attempted to study the catalytic performance of supported gold nanoparticles for the base-free oxidation of furfural, using different manganese oxide structures (MnO\({}_{2}\)) as supports. Firstly, the oxidation of furfural can lead to interesting products, such as 2-furoic [17] or maleic acid [18]. Indeed, 2-furoic acid can undergo a C-H carboxylation with CO\({}_{2}\) to form 2,5-furanodicarboxylic acid (FDCA) [19]. Secondly, MnO\({}_{2}\) is one of the most attractive materials because of its technological importance and possible application in different catalytic and electrochemical processes [20].