Harnessing the beneficial role of carbon deposit for a superior chemical looping hybrid reforming of methane by flue gas

Suppression of carbon deposition is a widely recognised strategy in high-temperature dry reforming of methane, aimed at ensuring catalyst stability and enabling the simultaneous utilisation of methane and CO₂. However, in this study, we introduce a novel process - chemical looping hybrid reforming (CLHR) - which takes an opposite approach by intentionally enhancing carbon deposition. This in turn, promotes the mass production of H₂ and CO from methane and CO₂, respectively. Specifically, an Fe-rich oxygen carrier (OC) derived from fly ash waste was employed to catalyse the cracking of methane in a fuel reactor, resulting in the production of H₂ with a record-high yield of ∼96 mmol/g_{_OC} and a H₂/CO molar ratio of ∼9.0 at 900 °C. In the subsequent oxidation step, exposure of the reduced OC to a simulated dry flue gas consisting of 16 % CO₂ and 4 % O₂ in N₂ at 900 °C enabled full restoration of its lattice oxygen and catalytic activity, maintaining stable performance over fifty cycles. Consequently, a high-concentration CO stream (∼30 %) with a yield of ∼84 mmol/g_{_OC} was produced. Conversely, the use of flue gas containing steam was found to be detrimental, as it led to the continual growth of carbon nanotubes (CNTs), which progressively encapsulated the reduced OC and ultimately deactivated it. Mechanistically, CNT growth follows the classic vapour-liquid-solid mechanism, in which the presence of metallic Fe⁰ and Fe₃C is essential. Notably, we discovered that steam plays a preferential etching role on the surface of metallic Fe⁰, whereas CO₂ is readily adsorbed on the basic metal oxide interface to remove the amorphous carbon deposited on the CNT walls. These findings provide new insights in harnessing the beneficial role of carbon deposits and advancing the utilisation of methane and CO₂ in a low-emission, efficient, and sustainable manner.