Gas-phase and catalyst control enabling carbon nanotubes and hydrogen production from organic waste
Year: 2026
Type: dissertation
Abstract: Organic solid wastes such as municipal sewage sludge, animal manure, and lignocellulosic biomass are rich in carbon and hydrogen. Current management practices often focus on reducing waste volume or recovering low-value products. This thesis uses pyrolysis coupled with chemical vapor deposition (CVD) to convert these wastes into carbon nanotubes (CNTs) and hydrogen (H₂). It investigates how feedstock composition, pyrolysis-gas chemistry, catalyst formulation, impurity migration, and catalyst evolution govern CNTs/H₂ formation and carbon-product structure. Municipal sewage sludge was used as a high-impurity feedstock to evaluate the conversion of waste-derived pyrolysis gas into multi-walled carbon nanotubes (MWCNTs) and H₂ over X–Mo/MgO catalysts (X = Co, Fe, or Ni). Limiting CO₂ release was necessary for effective MWCNTs growth. Under the tested conditions, Co–Mo/MgO produced the highest MWCNTs and H₂ yields. The results show that sludge pyrolysis–CVD depends on the availability of hydrocarbon precursors and on the interaction between gas composition and catalyst activity. They also show that changes in metal particles after growth are closely linked to CNTs morphology. Then, the process was applied to pyrolysis gas from animal manure to examine how manure type and multimetallic catalyst composition affect MWCNTs and H₂ co-production. Pyrolysis gas from cow manure produced higher MWCNTs and H₂ yields because it supplied more reactive hydrocarbon precursors, especially C₂H₄ and C₃H₆. Co–Ni–Mo/MgO exhibited smaller spent metal particles, which were associated with higher MWCNTs yields and smaller nanotube diameters. This comparison shows that feedstock type affects catalytic conversion by changing the composition of the pyrolysis gas and the gas environment in the downstream CVD reactor. Biomass pyrolysis–CVD was used as a cleaner reference system to examine how catalyst supports affect the conversion process. For Co–Ni–Mo catalysts, Al₂O₃ showed stronger metal–support interactions, a larger surface area and more uniform metal dispersion than MgO or Al₂O₃–MgO. These properties helped sustain hydrocarbon activation and CNTs growth. The results show that catalyst stability in waste pyrolysis–CVD depends on active metal composition and on how the support controls metal particle formation, evolution, and deactivation. The thesis further compared CaO, NaOH, CuCl₂, CaO–CuCl₂, and NaOH–CuCl₂ purification strategies for treating high-impurity pyrolysis gas from sewage sludge. It examined how these strategies affected MWCNTs and H₂ production. The route combining high-temperature CaO treatment with CuCl₂ scrubbing was the most effective. This route limited the transfer of CO₂, sulfur and chlorine into the CVD reactor. It also reduced catalyst sulfidation and metal particle coarsening. As a result, it increased MWCNTs yield, improved diameter control, enhanced structural ordering and promoted H₂ formation. These results establish gas purification as both a pollutant-removal step and a means of regulating the downstream catalytic-growth environment. Finally, this thesis used pyrolysis gas from sewage sludge to produce carbon products rich in single-walled carbon nanotubes (SWCNTs). CaO–CuCl₂ purification reduced impurity transfer but still generated a gas environment that favored MWCNTs growth. Adding activated carbon downstream moderated the delivery of C₂–C₄ hydrocarbons and volatile organic species to the catalyst bed. This limited non-selective carbon deposition. When the conditioned gas feed was combined with the optimized 3Co–1Mo/MgO catalyst in an Ar atmosphere, the process produced SWCNTs-rich carbon. The product had a mean CNTs diameter of 0.9 ± 0.4 nm and an SWCNTs fraction of approximately 82%. This result demonstrates that waste-derived pyrolysis–CVD can move from MWCNTs/H₂ co-production to enable SWCNTs-enriched nanocarbon synthesis with nanoscale structural control. These results show that waste-derived pyrolysis gases can be conditioned to adjust reactive carbon delivery and the CVD environment for CNTs synthesis and H₂ co-production. This also enables the selective synthesis of SWCNTs. Waste-derived pyrolysis–CVD is governed by the coupling of reactive carbon delivery, impurity migration, gas-phase chemistry, and transition-metal nanoparticle stability, rather than by total carbon supply alone. This analysis provides a basis for organic-waste valorization routes that integrate hydrogen recovery with selective nanocarbon synthesis.
Author: Jintao Lu
Language: English
Subfield: Fuel Technology
Field: Energy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7221402123