# The Effect of Carbon Nanofibers Surface Properties in Hydrogenation and Dehydrogen Reactions
## 4 Conclusions  
Four carbon nanofiber samples with different surface oxygen content and graphitisation order were used as support for Pd nanoparticles. The catalysts were fully characterised with XPS and Raman to evaluate their surface properties and Pd oxidation state, while TEM and EDX evaluated metal nanoparticles dimension and total metal loading. A more graphitic surface and a lower oxygen content are correlated with an increase in the annealing temperature during the preparation of the CNFs. Moreover, smaller Pd nanoparticles were formed on the low-oxygenated supports. The catalysts were tested in the cinnamaldehyde hydrogenation reaction and the reverse cinnamyl alcohol dehydrogenation. In both cases, a decrease in nanoparticle dimensions and oxygen content resulted in an increase in overall activity, from 2126 to 6776 s\({}^{-1}\) in the CAL hydrogenation and from 2494 to 6144 s\({}^{-1}\) for the COH dehydrogenation. Contrary to what has been reported in the literature, however, we showed an increase in C=O hydrogenation with a decrease in surface oxygen content. We ascribed this effect to the better accessibility of specific peripheral metal atoms when functional groups are not present on the support surface. The most active catalyst (Pd/CNFs-HHT) was tested for multiple consecutive reactions showing good reusability, with a small loss of activity only after the first use (form 98% to 89% of conversion). Finally, we demonstrated how the chemistry surface of CNFs can affect the cinnamyl alcohol dehydrogenation reaction as well, showing that activity increases with  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & **Conversion (\%)** & **TOF\({}_{\text{TOT}}\)** & **TOF\({}_{\text{SURF}}\)** & \multicolumn{3}{c}{**Selectivity at 90\% Conversion (\%)**} \\  & **at 5 Min** & **(s\({}^{-1}\))** & **(s\({}^{-1}\))** & **CAL** & **HCOH** & ** PPE** & **Others** \\ \hline Pd/CNFs-PS-OX & 4 & 680 & 3280 & 52 & 12 & 24 & 12 \\ Pd/CNFs-PS & 2 & 440 & 2494 & 58 & 17 & 15 & 10 \\ Pd/CNFs-LHT & 5 & 720 & 3418 & 63 & 23 & 9 & 5 \\ Pd/CNFs-HHT & 9 & 1360 & 6144 & 70 & 20 & 9 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Catalytic data for the cinnamyl alcohol dehydrogenation reaction. Reaction conditions: 100 \({}^{\circ}\)C, 3 bar N\({}_{2}\), 0.3 mol L\({}^{-1}\) cinnamyl alcohol in p-xylene, substrate: metal molar ratio 1:500. TOF calculated at 5 min of reaction.  
Figure 8: Scheme of the possible products of cinnamyl alcohol (COH) dehydrogenation: cinnamaldehyde (CAL), hydrocinnamaldehyde (HCAL), hydrocinnamyl alcohol (HCOH), 1-phenyl-1-propene (PPE) and 1-phenylpropane (PPR).  
decreasing the nanoparticle dimension, while selectivity towards CAL increases decreasing surface oxygen content.  
The following are available online at [[http://www.mdpi.com/2076-3417/9/23/5061/s1](http://www.mdpi.com/2076-3417/9/23/5061/s1)]([http://www.mdpi.com/2076-3417/9/23/5061/s1](http://www.mdpi.com/2076-3417/9/23/5061/s1)), Figure S1: Time on line conversion of the four catalysts in the cinnamaldehyde hydrogenation reaction. Figure S2: Time on line conversion of the four catalysts in the cinnamyl alcohol dehydrogenation reaction.  
[PERSON] wrote the article, synthesized the catalysts and carried out catalytic evaluation. [PERSON] carried out XPS, Raman and TEM analysis and helped with the interpretation. [PERSON] and [PERSON] designed the experiments and were involved in the writing and editing of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.