## 4 Conclusions  
The effects of the heat treatment of the CBs, Vulcan XC-72 and Ketjen black, on their morphologies, pore volume distributions, and FAOR activity were investigated with the aim of clarifying the high FAOR activity observed for Vulcan embedded into CNFs, as reported in a previous study [13]. Moreover, the different weight ratios of Vulcan XC-72 and Ketjen black embedded in the CNF were also synthesized to clarify the factors affecting the FAOR activity of the CB embedded in the CNF. Pd catalysts supported by Vulcan exhibited a higher FAOR activity after steam activation or carbonization. On the other hand, Pd supported by Vulcan after both carbonization and steam activation exhibited a lower FAOR activity compared to only carbonization or steam activation. Moreover, Pd supported by Ketjen showed no improvement in the FAOR activity, irrespective of the kind of heat treatment. Figure 5: Narrow scan of the Pd3d spectrum for different ratios of (**a**) Pd/VECNF and (**b**) Pd/KECNF, compared to Pd/CNF. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Catalyst Samples** & **Pd Surface Species** & **Binding Energy (eV)** & **Relative Intensity** \\ \hline \multirow{3}{*}{Pd/CNF} & Pd\({}^{0}\) & 334.88 & 0.51 \\  & Pd\({}^{2+}\) & 336.44 & 0.49 \\  & Pd\({}^{4+}\) & & 0.00 \\ \hline \multirow{3}{*}{Pd/KECNF 0.125} & Pd\({}^{0}\) & 334.37 & 0.18 \\  & Pd\({}^{2+}\) & 335.28 & 0.59 \\  & Pd\({}^{4+}\) & 337.05 & 0.23 \\ \hline \multirow{3}{*}{Pd/KECNF 0.25} & Pd\({}^{0}\) & 334.65 & 0.32 \\  & Pd\({}^{2+}\) & 335.71 & 0.46 \\  & Pd\({}^{4+}\) & 337.27 & 0.22 \\ \hline \multirow{3}{*}{Pd/KECNF 0.5} & Pd\({}^{0}\) & 334.56 & 0.26 \\  & Pd\({}^{2+}\) & 335.59 & 0.54 \\  & Pd\({}^{4+}\) & 337.28 & 0.20 \\ \hline \multirow{3}{*}{Pd/VECNF 0.125} & Pd\({}^{0}\) & 334.72 & 0.44 \\  & Pd\({}^{2+}\) & 335.89 & 0.46 \\  & Pd\({}^{4+}\) & 337.51 & 0.10 \\ \hline \multirow{3}{*}{Pd/VECNF 0.25} & Pd\({}^{0}\) & 334.68 & 0.47 \\  & Pd\({}^{2+}\) & 336.13 & 0.45 \\ \cline{1-1}  & Pd\({}^{4+}\) & 337.86 & 0.08 \\ \hline \multirow{3}{*}{Pd/VECNF0.5} & Pd\({}^{0}\) & 335.01 & 0.44 \\ \cline{1-1}  & Pd\({}^{2+}\) & 336.24 & 0.56 \\ \cline{1-1}  & Pd\({}^{4+}\) & & 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Binding energy Pd3d\({}_{(5/2)}\) and relative intensities of surface Pd species from the Pd3d XPS of Pd/CB-embedded carbon nanofiber. On the basis of an analysis of the pore volume distributions, it was suggested that excessive CB heat treatment destroyed the carbon structure and/or pore growth, and Ketjen was more prone to structural damage due to its hollow and porous structure. On the basis of these results, it was found that the appropriate heat treatment of the CB was effective for enhancing the FAOR activity due to the metal-support interactions, but excessive heat treatment caused a decrease in the FAOR activity due to the destruction of the CB microstructure, especially for Pd/C\({}_{\text{k}}\). The effect of the CB on the CNF ratio was also investigated using Vulcan and Ketjen black. Pd/VENF [0.25] and Pd/KECNF [0.25] showed a higher FAOR activity compared to the pure CBs that were only heat-treated. In particular, Pd/KECNF showed the highest FAOR catalytic activity and ECSA value among all the catalysts. These results were attributed to the appearance of the metal-support effect after the appropriate heat treatment of the CB and protection of the CB structure by the CNF. Moreover, the CB/PAN weight ratio which gave rise to the optimal catalytic activity was 0.25, because the amount of CB exposed on the surface of the CNF was the most appropriate. The metal-support interaction of the supported catalyst was confirmed by the XPS data, which showed that a shift in the binding energy promotes COOH oxidation. This study successfully showed that by embedding CB into the CNFs to form the support for the Pd catalysts (both Pd/VENF and Pd/KECNF), the mass activity of the catalyst was improved compared to Pd/CNF and the conventional Pd/C.