# The Effect of Carbon Nanofibers Surface Properties in Hydrogenation and Dehydrogen Reactions
## 3 Results  
### Catalysts Characterisation  
The three commercial supports (CNFs-PS, CNFs-LHT and CNFs-HHT) consist in hollow fishbone fibres with an average diameter of 80 \(\pm\) 30 nm and a specific surface area of around 50 m\({}^{2}\) g\({}^{-1}\). In particular, CNFs-PS are pyrolitically stripped fibre, with a thin chemically vapour deposited (CVD) layer of carbon over a graphitic fishbone core; the pyrolitical stripping step allows to remove any polyaromatic hydrocarbons from the fibre surface, leaving a thin surface layer of amorphous carbon. The CNFs-LHT are fibre treated at a temperature of 1500 \({}^{\circ}\)C to ensure complete carbonisation of any surface CVD carbon. The CNFs-HHT are high temperature treated fibres (3000 \({}^{\circ}\)C) with the surface CVD carbon layer fully graphitised. Finally, the CNFs-PS-OX are the commercial CNFs-PS fibres treated with HNO\({}_{3}\) in order to introduce oxygen functional groups. X-ray photoelectron spectroscopy (XPS) analysis was conducted on fresh and used samples to determine their electronic states and evaluate the metal surface abundance. From the deconvolution of the C 1 s peak, it is possible to measure the concentration of sp\({}^{2}\) (Binding Energy, B.E. ca. 284 eV) and sp\({}^{3}\) (B.E. ca. 285 eV) hybridisation of the CNFs supports [39]. The ratio sp\({}^{2}\)/sp\({}^{3}\), in particular, is a clear indication of the graphitisation order of the CNFs, since graphite is theoretically composed of solely sp\({}^{2}\) hybridised carbons. From Table 1 it is possible to notice that, as expected, increasing the CNFs annealing temperature, the sp\({}^{2}\)/sp\({}^{3}\) ratio increases, and thus the graphitisation order increases as well. In particular, the amount of sp\({}^{2}\) carbon varies from 52% of the less graphitic CNFs-PS-OX to 82% of the most graphitic CNFs-HHT. Figure 1 shows XPS Pd 3d spectra of the fresh samples. The Pd 3d\({}_{5/2}\) component at ca. 335 eV was assigned to metallic Pd [41], while the component at approximately 337 eV to Pd\({}^{2+}\), mainly present as PdO [42]. As expected, the oxygen-functionalised catalyst (Pd/CNFs-PS-OX) displays a high amount of Pd\({}^{2+}\) species. However, no apparent correlation could be detected between the amount of metallic Pd and the modification in morphology through annealing temperature. From Table 2, it could also be noticed that the experimental metal loadings are lower than the values calculated by EDX. This \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & **C sp\({}^{2}\) (\%)** & **C sp\({}^{3}\) (\%)** & **sp\({}^{2}\)/sp\({}^{3}\)** & **I\({}_{\text{D}}\)/I\({}_{\text{G}}\)** \\ \hline CNFs-PS & n.a. & n.a. & n.a. & 0.75 \\ CNFs-LHT & n.a. & n.a. & n.a. & 0.71 \\ CNFs-HHT & n.a. & n.a. & n.a. & 0.11 \\ Pd/CNFs-PS-OX & 51.8 & 34.6 & 1.5 & n.a. \\ Pd/CNFs-PS & 66.7 & 22.4 & 3.0 & n.a. \\ Pd/CNFs-LHT & 72.0 & 17.4 & 4.1 & n.a. \\ Pd/CNFs-HHT & 82.1 & 7.5 & 10.9 & n.a. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: X-ray photoelectron spectroscopy (XPS) and Raman analysis for the evaluation of the graphitisation order. difference is due to the fact that XPS is a surface-sensitive technique, in contrast with the bulk sensitive EDX, and thus cannot detect the presence of metal at a depth greater than ca. 3 nm [43]. The oxygen content was analysed by monitoring the O 1 s peak in the 530-540 eV range and the results are presented in Table 2 and Figure 2. The catalysts which have not been functionalised still present some surface oxygen, in part due to PdO species and in part due to oxygen functionalities formed by exposure of the support to the atmosphere. The oxygen naturally present in the air, in fact, at room temperature can react with carbon atoms of incomplete graphitic layers. As a consequence, increasing the graphitisation order results in a decrease in oxygen content due to the reduction in the number of structural defects that can host functionalisation [44]. Moreover, oxygen can bind to carbon in different ways.