## 3 Results and Discussion
We believe this is due to the roughness of the corundum material, unlike metal tubes. Therefore, the corundum surface contains more active soot formation centers, resulting in increased hydrogen yield. More about the mechanisms of soot formation on non-porous aluminum oxide is described in [39]. The yield of hydrogen in the nickel tube was less than in the corundum tube by only 2 vol.%. However, the residence time in the nickel tube was higher. Thus, with the same residence time, one should expect an even greater difference in the yield of hydrogen between the corundum and nickel tubes. The same applies to the results obtained for titanium and stainless steel tubes. At the residence time of about 4 s, hydrogen yield in experiments with titanium and stainless steel tubes was 41.51 and 27.75 vol.%, respectively. With a shorter residence time, lower values of the hydrogen yield for titanium and stainless steel tubes should be expected. It should be noted that Ni is often used as a catalyst for the decomposition of methane. This catalyst is often supported by alumina, which is used as a carrier [40]. Due to the use of a nickel catalyst, the temperature of methane decomposition is lowered to 700-800 \({}^{\circ}\)C [4]. However, in this work, at a high temperature of 1100 \({}^{\circ}\)C, metallic nickel is lost to aluminum oxide in the efficiency of methane decomposition. Following [6], the theoretical maximum for the yield of hydrogen at 1100 \({}^{\circ}\)C is about 90%, and it depends on the residence time. At the residence time of about 10 s at 1100 \({}^{\circ}\)C, the yield of hydrogen is about 45%. At the residence time of about 1 s at 1100 \({}^{\circ}\)C, the yield of hydrogen is about 10%. In general, the results of our work are in agreement with the results of the kinetic model [6]. In this work, the obtained yields were slightly higher, which is explained by the presence of a wall that catalyzes the process of methane decomposition. Figure 3 shows SEM images of carbon obtained via the pyrolysis of methane at a process temperature of 1100 \({}^{\circ}\)C and a methane flow rate of 2 L/h in a corundum tube. This carbon was obtained on filter paper. SEM analysis of a sample of carbon obtained via the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tube Material** & \begin{tabular}{c} **The Inner Diameter** \\ **of the Tube, mm** \\ \end{tabular} & \
\begin{tabular}{c} **Methane Flow** \\ **Rate, cm/s** \\ \end{tabular} &
\begin{tabular}{c} **Residence Time in** \\ **the Heating Zone, s** \\ \end{tabular} \\ \hline Nickel & 5.5 & 2.34 & 3.42 \\ \hline Stainless steel & 6 & 1.97 & 4.06 \\ \hline Titanium & 6 & 1.97 & 4.06 \\ \hline Corundum & 4 & 4.42 & 1.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Methane flow rates in the capillary tube, as well as the estimated residence time of methane in the 8 cm heating zone (8 cm is the length of the T-tube along the heated section of the reactor tube). Methane flow rate—2 L/h. pyrolysis of methane in a corundum tube at a temperature of 1100 \({}^{\circ}\)C showed that the carbon particles have a spherical shape, and their average diameter is about 130 nm. Figure 3: SEM images of carbon obtained via pyrolysis of methane at a process temperature of 1100 \({}^{\circ}\)C and a methane flow rate of 2 L/h in a corundum tube, with a resolution of 3 \(\upmu\)m (enlargement \(\times\)50,000) (**A**), 1 \(\upmu\)m (enlargement \(\times\)100,000) (**B**), and 0.5 \(\upmu\)m (enlargement \(\times\)200,000) (**C**). It is worth noting that nickel, titanium, and stainless steel tubes worked at the limit of their capabilities at 1100 \({}^{\circ}\)C. The nickel tube became more ductile at the heating zone after cooling, and the stainless steel and titanium tubes deformed at the heating zone during the experiment. Further exposure at a temperature of 1100 \({}^{\circ}\)C might lead to destruction, which makes it hard to use these tubes at this temperature.