La-promoted CO2 conversion from solid CaCO3: Hydrogen-induced decomposition and methane generation in Ni-supported CaCO3 powders and thin film structures

In calcium looping, captured CO 2 is fixed as solid CaCO 3 , whose decomposition typically requires temperatures around 700 °C. Lowering this decomposition temperature is a key requirement both for utilizing the captured carbon and for regenerating the CaO-based sorbent. This study investigates CH 4 generation from solid-phase calcium carbonate (CaCO 3 ) using Ni-supported composites under a hydrogen atmosphere and examines the effect of lanthanum (La) addition. Gas chromatography confirmed CH 4 formation at 500 °C without external CO 2 supply, while CH 4 was not detected from bare CaCO 3 , suggesting that Ni plays a key role in hydrogen activation. La addition increased CH 4 yield and prolonged the reaction duration. XRD, XPS, and TGA analyses indicated that CaCO 3 decomposition occurs preferentially at the outermost surface region and is associated with atomic hydrogen supplied from Ni surfaces. Similar decomposition of CaCO 3 with a range of 100 nm was observed away from Ni film / CaCO 3 interfaces fabricated by a sputter deposition of Ni on a CaCO 3 (100) substrate. XPS results showed that La species were ultimately converted to La(OH) 3 after the reaction, suggesting that La might have collected CO 2 from the surrounding CaCO 3 and acted as a local reaction site during this transformation. La addition also suggested to suppress carbon deposition on Ni surfaces. These results indicate that CH 4 generation from solid CaCO 3 is confined to the region reached by atomic hydrogen supplied from Ni, and that La addition extends this region by providing local sites for CO 2 transfer.