## 3 Results and Discussion  
### Catalyst Characterization  
Figure 3 shows SEM images of cobalt ferrite catalyst granules at different magnifications. The granules have irregular shapes and sizes from 0.8 to 1 mm (Figure 3a). The surface of some granules is mechanically damaged (Figure 3a,b). Probably, longer sintering is necessary to provide stronger granules. Increased magnifications (Figure 3c-e) reveal that granules are composed of smaller particles. The smallest particles have sizes ranging from 2 to 0.5 (Figure 3f).  
Figure 4a shows the EDS spectrum of the catalyst surface. The recorded peaks belong to cobalt, iron, and oxygen atoms. No peaks of other atoms are visible, indicating that the tested material is a mixed Co-Fe oxide. Figure 4b shows the XRF spectrum of the tested catalyst. This spectrum contains only peaks of cobalt and iron. The XRF method is more sensitive than EDS and allows for the detection of trace elements, especially those with high atomic mass. Therefore, the absence of other peaks in the XRF spectrum confirms the high purity of the synthesized catalyst material.  
Both the XRF and EDS techniques also allow the quantitative determination of elemental composition. The intensity of EDS peaks allowed the determination of the mass content of Fe\({}_{2}\)O\({}_{3}\) and CoO, which is 63.4% and 36.6%, respectively. Therefore, the resulting formula of mixed cobalt-iron oxide is Co\({}_{1.18}\)Fe\({}_{1.92}\)O\({}_{4.06}\). In turn, the intensity of XRF peaks allowed the determination of the mass ratio of Co to Fe, which is 24.4 to 43.4. Therefore, the resulting formula is Co\({}_{1.05}\)Fe\({}_{1.96}\)O\({}_{4}\). Both obtained formulas are consistent with each other and correspond well to the general formula of cobalt ferrite, CoFe\({}_{2}\)O\({}_{4}\).  
Figure 3: SEM images of catalyst granules at different magnifications (**a–f**).  
Figure 5a shows the X-ray diffraction pattern of the synthesized catalyst. The narrow reflections indicate the highly crystalline nature of the sample. The obtained diffraction pattern agrees well with the JCPDS #74-2403 pattern, confirming the formation of a single-phase cobalt ferrite without impurity phases. The most intense diffraction peak corresponds to the (311) plane. The other reflections are also characteristic of the standard cubic spinel structure and belong to the (111), (220), (400), (511), and (440) planes (Figure 5).  
The average crystallite size (coherent scattering area, DS) was calculated using the Scherrer formula:  
\[\text{DS}=0.89\ \lambda/\beta\ \cos\theta, \tag{1}\]  
where \(\lambda\) is the X-ray wavelength (here, \(\lambda=1.5406\) A), \(\theta\) is the Bragg angle, and \(\beta\) is the full width at half maximum (FWHM) for the most intense (311) reflection in the XRD pattern [21].  
Rietveld refinement and Scherrer calculations showed that the average crystallite size is about 80 nm. This value is higher than the typical values recorded for as-synthesized cobalt ferrite samples [22]. The observed difference is because the tested catalyst was sintered at a high temperature of 1100 \({}^{\circ}\)C. The lattice parameter was calculated to be 8.38 A. This value is consistent with the literature data regarding the lattice parameter of cobalt ferrite [23; 24].  
The Raman spectrum (Figure 5b) is typical for ferrite spinels [25; 26]. The characteristic peaks were assigned to the corresponding vibrational modes of the metal-oxygen bonds in the spinel lattice (Figure 5b). Therefore, the presented XRD pattern (Figure 5a) and Raman spectrum (Figure 5b) confirm that the synthesized catalyst is a cobalt ferrite spinel.  
Figure 4: (**a**) EDS and (**b**) XRF spectra of the cobalt ferrite catalyst material. The color meanings are as follows: green—background, orange—transition metals, pink—non-metals.  
Figure 5: (**a**) X-ray diffraction pattern and (**b**) Raman spectrum of the tested catalyst.