Activity and stability of NiCe@SiO₂ multi–yolk–shell nanotube catalyst for tri-reforming of methane

Tri-reforming of methane (TRM) produces syngas by directly utilizing flue gas from a fossil fuel-fired power plant without requiring post-combustion CO₂ separation. In this work, different yolk sizes of a NiCe@SiO₂ multi–yolk–shell nanotube catalyst were prepared and their catalytic properties were evaluated at different oxidizer (CO₂ + H₂O + O₂) to methane (O/M) feed ratios for TRM. The NiCe@SiO₂ multi–yolk–shell nanotube catalyst can exhibit longer stability than the conventional NiCe/SiO₂^Imp catalyst synthesized by impregnation method due to its controlled morphology and synergetic interactions of Ni–Ce and Ni–Si species. At a low O/M feed ratio of 1.0, NiCe@SiO₂ with smaller yolks (< 20 nm) shows higher resistance to carbon deposition than NiCe@SiO₂ with larger yolks due to the facile oxidation of carbon. On the other hand, NiCe@SiO₂ with larger yolks (> 30 nm) presents stable TRM activity at a high O/M feed ratio of 1.1, whereas NiCe@SiO₂ consisting of smaller yolks deactivates. Deactivation of NiCe@SiO₂ with smaller yolks can be explained by the re-oxidation of active Ni species, in which carbon formation and oxidation rates, and Ce³⁺/Ce⁴⁺ redox properties play a crucial role. Our results indicate that the NiCe@SiO₂ multi–yolk–shell nanotube structures can provide high TRM activity, yet their structure should be tuned for stable performance by considering the yolk sizes and interaction of Ni–Ce species.