Exsolved nickel sites for methane decomposition to hydrogen and carbon

Hydrogen production via catalytic decomposition of methane (CDM) offers a CO₂-free route for simultaneous hydrogen and solid carbon generation. However, practical implementation remains limited by catalyst deactivation, metal contamination in the carbon product, and inefficient post-reaction purification. In this work, Ni-promoted (La_0.75Ca_0.25)(Cr_0.5Mn_0.5)O_3-δ (Ni-LCCM) perovskite catalysts prepared via a nitrate-based route were evaluated for CDM. Under thermal CDM at 750 °C without pre-reduction, 050Ni-LCCM achieved a Ni-normalized carbon productivity of up to 11.53 g_C·g_Ni^-1. A non-monotonic dependence of intrinsic carbon productivity on Ni loading was observed, with local maxima for 050Ni-LCCM and 150Ni-LCCM. These two experimental compositions were represented by the 2Ni-LCCM and 6Ni-LCCM models in the density functional theory (DFT) calculations. The 2Ni-LCCM and 6Ni-LCCM models showed lower relative rate-limiting activation barriers than neighboring configurations. Together with structural characterization, these results suggest that methane decomposition activity is strongly influenced by local Ni configuration, Ni reducibility, and site accessibility rather than by total Ni loading alone. Carbon characterization showed the formation of predominantly nanocrystalline graphitic carbon, including mixed carbon nanostructures such as carbon nanotubes, carbon nanofibers, and carbon nano-onions. Post-reaction purification using 5 M HNO₃ reduced the residual inorganic contents to 4.20% and 1.41% for carbon products from 050Ni-LCCM and 150Ni-LCCM, respectively, corresponding to TGA-based carbon contents of approximately 95.8% and 98.6%. Overall, the results demonstrate that Ni-LCCM is an effective catalyst system for CDM, where composition-dependent Ni configurations influence intrinsic carbon productivity while enabling recovery of high-carbon-content solid products after mild acid treatment.