Kawi’s group investigated ICCU-DRM using Ni-Ca based dual functional materials, focusing on the use of Zr-based materials as the support and Ce as the promotor (https://doi.org/10.1016/j.apcatb.2020.119734). They firstly carried out steady-state DRM. The materials containing ZrO2 and CaO show excellent stability, compared to other materials. The other materials with poor stability are accompanied by severe RWGS reaction, which is supposed to oxidise Ni and reduce the catalytic activity. The authors did TGA analysis to study coke deposition on the surface of the materials. It is noted that the overlap between coke oxidation and the decomposition of CaCO3 limits the detailed understanding. Thus Raman analysis was also carried out to check carbon. In general, the materials having Ca show little coke deposition. This is ascribed to the oxidation of carbon using oxygen from CaCO3. However, without ZrO2 as the support, Ni/Ca is also deactivated quickly due to the sintering of Ni particles, ascribed to the weak interaction between Ni and Ca. The strong interaction between Ni and ZrO2 ensures the stability of Ni particles. Therefore, Ni/Ca@Zr and NiCe/Ca@Zr are the best candidates in terms of directly DRM.
NiCe/Ca@Zr shows a higher yield of H2 and methane conversion compared to Ni/Ca@Zr. Therefore, the addition of Ce can promote ICCU-DRM performance. However, it also results in more carbon formation initially, ascribed to methane decomposition. The oxidation of the deposited carbon by CO2 producing CO is clearly observed during the following carbon capture stage. However, the Ce promoted NiCe/Ca@Zr show less accumulated carbon deposition after 25 cycles of ICCU, than Ni/Ca@Zr. Therefore, Ce-promoted dual functional material has better resistance to the accumulation of inactive carbon. It is noted that the active carbon formed from methane decomposition is re-oxidised during the carbon capture stage. The authors ascribed this to the high mobility of the lattice oxygen from CeO2, and used XPS analysis to support their statement.
“(A) CH4 (solid symbols) and CO2 (hollow symbols) consumption rates (mol s−1 kgNi−1) and (B) H2/CO ratio during DRM over the reduced Ni/Ca, Ni/Zr, Ni/CaZr, Ni/Ca@Zr and NiCe/Ca@Zr. The mass of Ni in each material was calculated based on the ICP results. Experimental conditions: feed: 10 % CO2 + 10 % CH4 + 80 % He; temperature: 720 °C; pressure: 1 bar; GHSV: 60,000 mL h−1 gmat.−1.”