*2.4. Thermogravimetric Analysis*

Figure 5 shows the thermal stability of activated carbon (a), as well as the catalysts, Fe-C (b) and Fe-C-K (e). At a temperature of 110–250 ◦C, the water molecules are evaporated [17]. In Figure 5, the evaporation of water molecules occurred between 110–250 ◦C, as shown by Equations (1)–(4). At roughly 450 ◦C, partial thermal degradation of the carbonsupported iron-based catalyst with and without potassium promoters occurs at low and high temperatures, as illustrated in Figure 5b–d,f,g, and shown by Equations (5) and (6). Above 550 ◦C, however, full thermal damage occurs, as shown by Equations (7)–(10). The following steps describe the overall process of thermal degradation of the Fe-C catalyst with and without potassium promoter (Equations (1)–(10)). First, goethite decomposes (Equation (1)) between 200 and 250 degrees Celsius [27,28]. The shape of Fe2O3 (hematite) is then reduced to FeO. In addition, the reaction of iron oxide with CO at 250–300 ◦C produces Fe2O3(magnetite) (Equations (2)–(6)). In the XRD peaks, the production of magnetite was also plainly visible. The creation of CO2 is linked to the reduction of iron oxides to generate the iron carbide carbonaceous material at temperatures above 350 ◦C, as shown by Equations (7)–(10) [29].

$$2\text{FeO(OH)} \rightarrow \text{Fe}\_3\text{O}\_3 + \text{H}\_2\text{O} \tag{1}$$

$$\text{C3FeCO}\_3 \rightarrow \text{Fe}\_3\text{O}\_4 + 2\text{CO}\_2 + \text{CO} \tag{2}$$

$$\text{FeCO}\_3 \rightarrow \text{FeO} + \text{CO}\_2 \tag{3}$$

$$\text{FeC}\_2\text{O}\_4\cdot2\text{H}\_2\text{O} \rightarrow \text{FeC}\_2\text{O}\_4 + 2\text{H}\_2\text{O} \tag{4}$$

$$3\text{FeC}\_2\text{O}\_4 \rightarrow \text{Fe}\_3\text{O}\_4 + 4\text{CO} + 2\text{CO}\_2\tag{5}$$

$$2\text{ 3Fe2O3} + \text{CO} \rightarrow 2\text{Fe3O4} + \text{CO2} \tag{6}$$

$$6\text{FeO}(\text{OH}) + \text{CO} \rightarrow 2\text{Fe}\_3\text{O}\_4 + \text{CO}\_2 + 3\text{H}\_2\text{O} \tag{7}$$

$$\text{FeC}0\_3 \rightarrow \text{FeO} + \text{CO}\_2 \tag{8}$$

$$\text{xFeyO}\_4 + \text{xCO} \rightarrow \text{3Fe}\_2\text{C} + \text{xCO}\_2 \tag{9}$$

xFeO + xCO → FexC + xCO2 (10)

**Figure 5.** TGA analysis of activated carbon (**a**), Fe-C catalyst (**b**), Fe-C LT-FTS (**c**), Fe-C HT-FTS (**d**), Fe-C-K catalyst (**e**), Fe-C-K LT-FTS (**f**), and Fe-C-K HT-FTS (**g**).
