Robust Ni-MOF-derived Ni/Ca5(PO4)3OH catalyst with sustained activity for low-temperature methane pyrolysis to hydrogen and carbon nanotubes

Methane pyrolysis offers a transformative route for converting CH 4 into CO 2 ‑free hydrogen (H 2 ) and valuable carbon nanotubes (CNTs); however, its practical application is constrained by rapid catalyst deactivation and the need for excessively high operating temperatures (600–900 °C). In this study, we introduce a highly active and durable catalyst, Ni‑MOF/hydroxyapatite (Ni‑MOF/HAp, Ca 5 (PO 4 ) 3 OH), derived from a nickel‑based metal‑organic framework (Ni‑MOF) precursor with a remarkably low Ni loading of only ~8 wt%, which enables stable methane decomposition at the considerably lower temperature of 500 °C. Under optimized reaction conditions (CH 4 /N 2 = 12:18 mL·min −1 ), the Ni‑MOF/HAp catalyst delivers a hydrogen yield of 22.4% at this low temperature, substantially outperforming a conventional Ni/HAp reference catalyst, which deactivates rapidly within 165 min. Notably, the Ni‑MOF/HAp catalyst exhibits sustained long‑term stability, maintaining its activity for over 14 h on stream without significant deactivation. Kinetic analysis reveals an apparent activation energy (E a,app ) of only 36.85 kJ·mol −1 for the Ni‑MOF/HAp catalyst, which is substantially lower than that of the conventional Ni/HAp catalyst (60.51 kJ·mol −1 ) and falls well below the typical values reported for conventional Ni‑based systems (>45 kJ·mol −1 ). Microstructural and phase analyses by XRD, SEM, TEM, Raman spectroscopy, and TGA demonstrate that the spent MOF‑derived catalyst largely retains its hierarchical particle morphology, preserves well‑crystallized metallic Ni 0 active sites, and promotes abundant growth of graphitic CNTs, as evidenced by a low I D /I G ratio of 1.103 and a carbon yield of 3.0 gC/gNi. This enhanced catalytic behavior is attributed to three key factors: the unique hierarchical powder architecture of the hydroxyapatite (HAp) support, strong metal‑support interactions originating from in‑situ HAp crystallization around the Ni‑MOF template, and the synergistic role of surface hydroxyl (OH) groups that facilitate C-H bond activation. By linking catalyst microstructure to catalytic performance, this work establishes a rational design paradigm for the production of CO 2 ‑free hydrogen via low‑temperature methane pyrolysis through tailored interfacial metal‑support interactions.