*2.5. Brunauer–Emmett–Teller Analysis*

The BET surface area of activated carbon and the produced catalyst is shown in Table 2. The results reveal that the surface area of activated carbon is 167.4 m2/g, and the surface area of the Fe-C catalyst is 78.7 m2/g. The surface area of the potassium-based promoter catalyst was reduced by 11.1 m2/g because its particles occluded tiny holes of the active sites with their own particles. The high surface area of activated carbon, as compared to the iron catalyst in the iron phase, is greatly disseminated over the surface, forming minute particles of iron oxides. When Fe2O3 is calcined, it can transform into Fe3O4. Surface carbon species may be able to aid this process by acting as an active support [30].

**Table 2.** BET surface area of activated carbon, Fe-C, and Fe-C-K catalyst.


*2.6. Effect of Temperature and Promoter on Downstream Syngas Applications*

2.6.1. Effect of Promoter and Carbon Support

Table 3 shows that adding potassium promoter in an iron carbon-supported catalyst has a substantial impact on the conversion of gasoline, from 36.4 percent (Fe-C) to 72.5 percent (Fe-C-K). A low potassium concentration is thought to be beneficial for promoting the FTS reaction and converting iron carbides to the inactive iron oxide phase. Potassium offers a number of benefits, including increasing the quantity of alkenes in hydrocarbon synthesis, and inhibiting the creation of methane [31]. In general, using a potassium promoter in conjunction with an Fe/Ac-based catalyst reduces the quantity of n-paraffins in FTS products while significantly increasing branching paraffin [32]. When

promoters are added to an iron-based catalyst, the catalyst's physical and chemical properties change, as well as the conversion and selectivity of CO2 and C5+ [33]. The support of carbon helps for the formation of the iron-carbide phase. Due to the iron-carbide phase, the conversion rate of gasoline and diesel was increased. The Figure 6 shows the mechanism of a carbon-supported iron catalyst containing potassium as a promoter.


**Table 3.** Distribution of carbon chain with oxygenates and non-oxygenates.

**Figure 6.** Mechanism of a carbon-supported iron catalyst containing potassium as a promoter.

Potassium was found to be a good promoter that enhanced the water–gas shift reaction, enhancing the selectivity toward olefins, and suppressing the formation of methane. Potassium promoter improves the performance of an iron catalyst by affecting the catalyst phase. In a catalyst, potassium acts as an electron donor when absorbed on the surface of an iron carbide-oxide phase. It inhibits the reduction from α-Fe2O3 to α-Fe3O4, and promotes CO dissociation. In general, a small amount of potassium as a promoter is sufficient to stimulate FTS activity [31–34]. In this study, the results revealed that the use of potassium as a promoter suppressed the formation of methane, and enhanced the C5+ formation.
