Conversion of municipal sewage sludge into biogenic multi-walled carbon nanotubes and hydrogen using X-Mo/MgO (X = Co, Fe, Ni) catalysts through pyrolysis-chemical vapor deposition process

Utilization of municipal sewage sludge as a feedstock for synthesizing biogenic multi-walled carbon nanotubes (MWCNTs) and recovering H2 via pyrolysis-chemical vapor deposition (CVD) was explored. The research centered on investigating the impact of X-Mo/MgO catalysts (X = Co, Fe, Ni, and X: Mo: MgO mass ratio of 3:1:6) on the conversion of pyrolysis gases from the sludge at the CVD temperatures of 600–––800 °C. The results demonstrate that the catalyst efficiency is enhanced when CO2 is captured from pyrolysis gas prior to the CVD reactor (here, with a calcium-based sorbent). The three X-Mo/MgO catalysts showed the highest MWCNT and H2 yields at 700 °C compared to other temperatures. The Co-Mo/MgO catalyst exhibited the highest MWCNT production (2.9 wt% per feedstock) and H2 yield (140.7 mL per g of sludge) at 700 °C. The higher yield could be related to the smaller size of the metal particles of the spent catalyst. Furthermore, Co-Mo/MgO at 700 °C grown MWCNTs had more uniform diameter distribution and larger BET surface area. Notably, the produced MWCNTs were doped by N and S heteroatoms due to the presence of these elements in the sludge and the doping extent could be managed by controlling the CVD temperature, opening the pathway for customization of MWCNT properties. The highest doping levels were observed for MWCNTs grown on Fe-Mo/MgO at 600 °C (N − 3.4 at% and S – 2.3 at%) by XPS. According to these results, the pyrolysis-CVD process could be a feasible technique for the conversion of municipal sewage sludge into valuable biogenic MWCNTs and H2. The use of Co-Mo/MgO and Ni-Mo/MgO catalysts exhibited some advantages (e.g., high MWCNTs and H2 yields) compared to Fe-Mo/MgO catalysts.