Characterization of carbon nanostructures grown on Fe - based catalyst for methane cracking

The increase in CO2 emissions due to anthropogenic activities, is considered the major cause of the recent global surface temperature rapid increase. In order to prevent a climate escalation, the development of low emitting alternatives is one of the main worldwide objectives. Hydrogen plays a key role for accomplishing such green transition. Among all the thermal H2 production routes, methane cracking is the only one that brings a CO2 - free hydrogen production. Furthermore, as CH4 is also a carbon source, the process allows to obtain high - value carbon materials (e.g., carbon nanotubes, CNTs). Fe, due to its activity and economic aspects, is one of the most promising materials for these purposes. The subject of this work is the characterization of the carbon nanostructures grown during the methane cracking. The study was conducted on post - reaction catalysts, employing two electron microscopes: a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Initially, the effect of the Fe loading was investigated, showing clear differences upon the carbon deposit structure and active phase nanoparticle size. Moreover, through the catalytic tests, the composition with the highest H2 production was identified, therefore, maintaining this Fe content, the addition of copper as a promoter was analyzed. Cu increased the catalyst activity in the first instant of the reaction, but it did not improve CNTs formation and structure. In the second part of the work, the focus was put on the effect of the reduction pretreatment and reaction conditions. Regardless the Cu addition, the absence of the prereduction treatment combined with a lower reaction temperature seemed to cause an increased CNTs formation and a change in the growth mechanism, from tip to base growth. The characterization study also brought to the conclusion that CNTs grew on both Fe3C and 𝛼-Fe, according to a bamboo - shape tangential mechanism.