## 3 Results and Discussion
### Formic Acid Oxidation Reaction on Carbon Black Embedded in Carbon Nanofibers
The fact that the FAOR activity of Pd/CNF was low suggests that the amount of CB exposed on the CNF surface was also low; this was confirmed by the pore distributions shown in Figure 3 and resulted in the lower supporting effect of the CB when the CB ratio was low, as can be seen in the case of the CB/PAN ratio of 0.125. On the other hand, the CNF structure was damaged at high CB/PAN ratios such as 0.50 because the CNF surface was almost entirely covered by the CB. For these reasons, the optimum CB/PAN ratio was 0.25 under these conditions of heat treatment. If the conditions of the heat treatment, such as the carbonization/steam activation temperature and dew point of the steam, vary, the amount of CB exposed at the CNF surface and the extent of alteration of the carbon microstructure may be significantly different. Maximum mass activity at the peak potential of the CV measurement is crucial to determine the activity of the promising catalyst in actual fuel cell applications [34]. Modification of the catalyst support material possibly contributes to the activity of the catalyst due to its bonding to the catalyst metal. This metal-support interaction modifies both the electronic and geometric properties of the catalyst support, which influences the activity of the catalytic sites on the metal surface and enhances the presence of active sites [17]. In this study, XPS was used to determine the active sites of Pd, thus providing crucial information about the metal-support interaction by observing the electronic modifications of the metal which contribute to the improvement of the FAOR activity. Figure 4 shows the wide scan XPS results for the chemical element contained in the supported Pd catalysts. Figure 5 shows the surface state of Pd3d, which shows the surface composition \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & **Mass Activity at Ep,** & **Peak Potential, Ep** & **Electrochemical Active** \\  & **ip-FAOR (mA mg\({}_{\text{rd}}\)\({}^{-1}\))** & **(V vs. NHE)** & **Surface Area (m\({}^{2}\) g\({}_{\text{rd}}\)\({}^{-1}\))** \\ \hline Pd/VECNF [0.5] & 486 & 0.52 & 38.5 \\ Pd/VECNF [0.25] & 1477 & 0.56 & 35.5 \\ Pd/VECNF [0.125] & 481 & 0.38 & 39.9 \\ Pd/KECNF [0.5] & 945 & 0.52 & 35.8 \\ Pd/KECNF [0.25] & 2022 & 0.62 & 56.9 \\ Pd/KECNF [0.125] & 1096 & 0.60 & 25.1 \\ Pd/CNF [0.0] & 398 & 0.50 & 20.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of catalytic activity and electrochemical active surface area of Pd/VECNF and Pd/KECNF for various CB/PAN ratios. and oxidation state of the metal on the Pd/CNF, Pd/KECNF, and Pd/VECNF catalysts with different mass ratios of Ketjen and Vulcan carbon black to PAN, and indicates a possible electronic interaction [25]. Table 5 shows that the metal state of CB/PAN [0.25] for both Pd/KECNF and Pd/VECNF has the highest relative intensity of Pd metal (Pd\({}^{0}\)) compared to CB/PAN [0.125] and [0.5] which support the fact that CB/PAN [0.25] was the optimum CB/PAN ratio to increase the metal-support interaction. Moreover, the presence of PdO\({}_{2}\) (Pd\({}^{4+}\)), which is a highly active catalyst due to its instability, enhances the FAOR activity [35]. Even though Pd/VECNF [0.25] has a higher Pd metal content compared to Pd/KECNF [0.25], the high relative intensity of Pd\({}^{4+}\) in Pd/KECNF [0.25] contributed to the higher FAOR activity. The optimum ratio of CB/PAN gives a great improvement in the Pd metal state, which improves the interaction between the carbon and Pd. The addition of CB embedded into the CNF improves the active sites of Pd/KECNF and Pd/VECNF in a similar way to the presence of PdO (Pd\({}^{2+}\)) and PdO\({}_{2}\); this is not the case for the Pd/VECNF 0.5 in which the highly oxidized Pd (Pd\({}^{4+}\)) was absent. This state may be due to the concentration of carbon material for Vulcan/PAN at 0.5.