Ce-La ratio-controlled structural transitions in Ni/Ce-La oxide catalysts for direct decomposition of methane

Abstract

Ce–La mixed oxides represent widely applicable catalyst supports owing to their controllable crystal structures, oxygen storage capacity, and electronic properties. In this study, Ni/Ce1-xLax catalysts were systematically synthesized to investigate how Ce–La composition determines lattice structure, redox behavior, and the resulting catalytic performance. Structural analyses by XRD and H2-TPR revealed that moderate La incorporation (x ≤ 0.3) expands the CeO2 fluorite lattice and enhances oxygen vacancy formation, thereby stabilizing highly dispersed Ni0 nanoparticles. XPS and soft XAS confirmed that this composition maximizes the Ce3+ fraction and optimizes the electronic interaction between Ni and the support. By contrast, excessive La content (x ≤ 0.5) induces a structural transition toward perovskite phases (LaNiO3, La2NiO4) and secondary La-based oxides (La2O2CO3, La(OH)3), which embed Ni within the lattice and reduce its accessibility as active sites. These structural differences critically influence the quality of carbon products formed via methane decomposition. The optimally doped Ni/Ce0.7La0.3 catalyst exhibited remarkable performance, producing CO2-free hydrogen together with highly crystalline CNTs featuring narrow diameters, high graphitization, and strong π* transitions.