# The Effect of Carbon Nanofibers Surface Properties in Hydrogenation and Dehydrogen Reactions
## 3 Results
### Cinnamaldehyde Hydrogenation  
Catalytic CAL hydrogenation tests were conducted with the four different catalysts. The scheme of reaction is presented in Figure 5. The products of hydrogenation of the C=C and C=O groups are HCAL and COH respectively. Further addition of H\({}_{2}\) leads to hydrocinnamyl alcohol (HCOH) or to 1-phenylpropane (PPR) by hydrogenolysis mechanism. The hydrogenation activity was evaluated in terms of Turnover Frequency (TOF) considering both the total Pd content (TOF\({}_{\text{TOT}}\)) and on the surface atoms only (TOF\({}_{\text{SURF}}\)). TOF calculation is reported in the Supplementary Information. Recent studies reported an increase in selectivity towards HCAL with a consequent increase in activity when either Ru, Pt or Pd were supported on low-oxygen content CNFs [24; 26; 45]. In agreement with those studies, highest activity in the CAL hydrogenation was reached when CNFs-HHT was used as support (Table 5), with a TOF 3 times higher than the Pd/CNFs-PS catalyst (from 2126 to 6776 s\({}^{-1}\)). Unexpectedly, however, the support with the highest oxygen content (CNFs-PS-OX) was not the least active. We ascribed the low activity of the Pd/CNFs-PS catalyst to the presence of big metal nanoparticles compared to the other catalysts. It has been previously reported, in fact, that large metal nanoparticles are less active due to a change in the adsorption mode. Moreover, in our case, the increase in activity is not related to the increase in the hydrogenation rate of the C=C bond only. With the decrease of surface oxygen, in fact, the selectivity towards CAL decrease from 80% with the Pd/CNFs-PS-OX to 68% when CNFs-HHT was used as support, with a consequent increase in selectivity mainly towards the HCOH. It is well known that Pd preferentially adsorbs and hydrogenates the C=C double bond. The presence of polar oxygen surface atoms avoids the adsorption of the benzene ring on the carbon surface, thus inhibiting the participation of the peripheral atoms in the hydrogenation. With the increase of annealing temperature, the oxygen content decreases, making the carbon surface available for the adsorption of aromatic groups. With more adsorption sites, the overall hydrogenation activity increases. This, however, greatly enhances the hydrogenation of the carbonylic group due to the better accessibility of the C=O bond rather than the C=C bond, thus increasing the overall production of HCOH (Table 5). It is worth notice how the presence of acidic oxygen functional \begin{table}
\begin{tabular}{c c} \hline
**Catalyst** & **Pd Loading (wt\%)** \\ \hline Pd/CNFs-PS-OX & 1.01 \\ Pd/CNFs-PS & 1.04 \\ Pd/CNFs-LHT & 1.03 \\ Pd/CNFs-HHT & 1.03 \\ \hline \end{tabular}
\end{table} 
Table 4: Pd loading of the four catalysts. Figure 5: Scheme of the possible products of cinnamaldehyde (CAL) hydrogenation: cinnamyl alcohol (COH), hydrocinnamaldehyde (HCAL), hydrocinnamyl alcohol (HCOH), 1-phenyl-1-propene (PPE) and 1-phenylpropane (PPR). groups activates the hydrogenolysis mechanism to form 1-phenylpropane. The plot conversion as a function of time of the four catalysts is reported in Figure S1 in the Supplementary Information. The effect of reaction temperature on the catalytic hydrogenation of CAL was evaluated using CNFs-HHT as support (Table 6). Increasing the temperature from 25 to 100 \({}^{\circ}\)C, the overall activity enhanced by a factor of 16 (from 6776 to 109,862 s\({}^{-1}\)). The reaction temperature seems to affect more the hydrogenation of the C=O bond, with an increase in selectivity towards HCOH from 30 to 44%. Interestingly, COH was produced both at 60 and 100 \({}^{\circ}\)C, confirming the increase in the carbonyl hydrogenation rate at high temperatures. Reusability tests of the Pd/CNFs-HHT catalyst were carried out simply by centrifugation and filtration of the reaction mixture after 2 h of reaction. After the first use, a slight decrease in activity was observed, specifically from 98% to 89% (Figure 6).