## 3 Results and Discussion
### Effect of Heat Treatment of Carbon Black Supports on the Properties of Pd/C Catalysts

Table 1: Summary of the Brunauer-Emmett-Teller (BET) surface areas, pore volumes, and Pd crystallite sizes of the prepared Pd/CB catalysts. Figure 1: Pore volume distribution for Pd/C for different carbon blacks (CBs) and heat treatments for (a) Pd/C\({}_{\rm v}\) and (b) Pd/C\({}_{\rm k}\). in the water molecules reacted with the surface carbon of the CB resulting in a greater pore production compared to the carbonization process, although the CB was calcined at a temperature lower than the carbonization process. In other words, the intensity of the heat treatment, which indicates the degree of the pore growth and pore production, was higher in the case of steam activation than carbonization. On the other hand, it was found that the total and mesopore volumes of Pd/C\({}_{\rm v}\)-all were the lowest among all the cases. This is due to the excessive growth of the pores that underwent two heat treatment processes. During the carbonization process, some of the mesopores grew and new small mesopores were produced. The newly produced small mesopores and the mesopores grown during the carbonization process were considered to have grown even more in the steam activation process. However, the macropore volume of Pd/C\({}_{\rm v}\)-all was lower than that of Pd/C\({}_{\rm v}\)-act. This fact indicated that the macropores disappeared when the excessive heat treatment was finished. It is known that macropores are formed by aggregates and/or agglomerates [23]. Therefore, it is considered that a part of the aggregates or agglomerates were possibly broken or separated into particles due to the excessive heat treatment, resulting in the macropores disappearing. On the basis of these results, it can be concluded that the BET surface area increased after the heat treatment due to the development of a microporous structure; however, excess heat treatment, such as that used to prepare Pd/C\({}_{\rm v}\)-all, causes splitting of the aggregates or agglomerates. However, for Pd/C\({}_{\rm k}\), Pd/C\({}_{\rm k}\)-all demonstrated the highest pore volume for each type of pore size. Increasing of the macropores in Pd/C\({}_{\rm k}\)-all was due to pore growth of the smaller pores such as the micropores and mesopores. Mesopores in Pd/C\({}_{\rm k}\)-act and Pd/C\({}_{\rm k}\)-all were more prevalent than those in Pd/C\({}_{\rm k}\)-no and Pd/C\({}_{\rm k}\)-carb. The total pore volume increased after the heat treatment and the total pore volume of Pd/C\({}_{\rm k}\)-all was the highest among all the Pd/C\({}_{\rm k}\), although the total pore volume of Pd/C\({}_{\rm v}\)-all was lower than that of Pd/C\({}_{\rm v}\)-act. This difference is due to the hollow structure of Ketjen, which leads to a highly developed pore structure even without heat treatment. During the heat treatment, not only micropore generation, but also pore growth simultaneously occurred. In the case of Vulcan, the effect of the pore growth on the pore distribution was limited because the pore volume was already low without heat treatment; hence the excess heat treatment caused a reduction in the pore volume. On the other hand, in the case of Ketjen, the effect of pore growth, which caused an increase in the pore volume, was significant because there were numerous pores before the heat treatment. Therefore, the total pore volume did not decrease when carbonization and steam activation were conducted as a result of pore growth and a decrease in the pore volume, as the splitting of the aggregates or agglomerates cancel each other out, resulting in the large pores remaining unchanged.