# The Effect of Carbon Nanofibers Surface Properties in Hydrogenation and Dehydrogen Reactions
## 1 Introduction  
Carbon nanofibers (CNFs) were observed for the first time in 1889 from the decomposition of an organic gas at high temperature [1]. However, it was not until the last three decades that CNFs attracted considerable attention due to their chemical and physical properties, such as high thermal and electrical conductivity, high mechanical strength and good chemical resistance [2]. Nowadays, CNFs find application in the production of gas sensors and other electronic components, as polymer additive and as drug delivery systems as well [3; 4; 5; 6; 7]. On the other hand, one of the most promising applications of CNFs is in heterogeneous catalysis as support for metal nanoparticles. Their versatility is due to the ease of modification of some physiochemical properties, such as surface area, porosity and surface chemistry [3; 7; 8]. For example, the external surface of CNFs can be functionalised with many O-, N-, or P-containing groups that can improve their dispersion in a broad range of solvents as well as increase the interaction with metal nanoparticles or organic molecules [9; 10; 11; 12; 13]. Many chemical reactions of industrial relevance benefit from the use of CNFs when used as support [14; 15]. Better spreading of the metal on the support surface was observed when CNFs were used, leading to higher activity and selectivity compared to the other supports. The reason for the better spreading was attributed to higher metal-support interaction between the metal and the graphitic surface of the CNFs and to withdrawing properties of the functional groups on the CNFs surface [16]. The selective hydrogenation of \(\alpha\),\(\beta\)-unsaturated aldehydes is an important reaction from an industrial point of view. Cinnamaldehyde, in particular, is one of the most significant fragrancealdehydes, with an estimated production of 1.5 kton/year via base-catalysed condensation of benzaldehyde and acetaldehyde [17]. The products of hydrogenation of the vinyl (C=C) or carbonyl (C=O) group, hydrocinnamaldehyde (HCAL) or cinnamyl alcohol (COH) respectively, are largely used in the production of various fine chemicals for the pharmaceutical and perfumery industry [17; 18]. To selectively hydrogenate either the vinyl or the carbonyl group, several supported noble metal nanoparticles can be used, such as Au, Pd, Pt, Rh, Ru and Ir [19; 20]. Carbon-based supports, in particular, are reported to be very active for this specific reaction [21; 22; 23]. Pd nanoparticles supported on CNFs have shown to be more active and selective towards HCAL than activated carbon due to the high external surface area and the absence of microporosity that can cause mass transfer limitation [24]. [PERSON] et al. extensively studied the metal particle size and surface oxygen content effects for Pt and Ru nanoparticles supported on CNFs [25; 26]. In their work, the authors showed how the absence of oxygen functionalities allows the adsorption of the benzene ring on the CNFs surface, thus increasing the selectivity towards HCAL but decreasing the overall activity. On the other hand, small nanoparticles are more active and selective towards COH due to the metal-benzene ring repulsion that makes the C=C hydrogenation difficult [27]. On the other hand, the reverse reaction (cinnamyl alcohol dehydrogenation), is a typical example of an oxidation reaction of aromatic allylic alcohol [28; 29; 30]. The selective aerobic oxidation of alcohols is an important reaction in organic chemistry and the search for an active, selective and stable heterogeneous catalyst for this type of reactions from both an academic and industrial point of view is still a challenge [31; 32]. The main product derived from the oxidation of cinnamyl alcohol is cinnamaldehyde. As already mentioned, cinnamaldehyde is widely used in the food and perfume industry, and its synthesis still involves chemical platforms derived from fossil fuels such as benzaldehyde and acetaldehyde [17].