Carbon-induced metal growth and size-dependent activity in methane splitting on nickel nanoparticle catalysts for COx-free hydrogen production

Catalytic methane splitting (CMS) to hydrogen and elemental carbon is a promising route to produce CO x -free hydrogen, potentially achieving net carbon-negative output when using biogenic methane as the feedstock. On transition metal nanocrystals, especially Ni, CMS proceeds at lower temperatures (<1000 K) compared to (plasma-assisted) methane pyrolysis, additionally leading to higher added-value graphitic carbon nanostructure products. Earlier work has indicated that the size of the metal nanocrystals plays a central role for hydrogen (and carbon) production rates, however, conflicting results have been reported, possibly as a result of challenges associated to the deposition of the carbon products on the catalyst surface as well as phenomena like metal particle growth, fragmentation and deactivation. Herein metal nanocrystal size effects are assessed on the methane splitting reaction rate with a series of model supported Ni catalysts. A maximum methane splitting rate is attained for Ni nanoparticles with a diameter of ∼10 nm at the onset of the reaction. Operando synchrotron X-ray diffraction and activity-specific transmission electron microscopy studies attribute the observed size effects on reaction rate to differences in the relative contribution of Ni nanocrystals larger and smaller than ca. 10 nm. This metal nanoparticle diameter represents a threshold minimum size for the effective nucleation and growth of graphitic carbon filaments, and therefore for sustained hydrogen production, which entails the lifting of Ni nanocrystals away from the oxide support in a carbon tip-growth mechanism. Moreover, operando studies reveal metal nanocrystal growth, which is specifically driven by the earliest methane splitting events on the catalyst surface, and determinant for the metal aggregation extent and intrinsic activity of the actual working catalyst. Density Functional Theory analyses identify *C and *CH 3 adspecies, emerging from CH 4 splitting on neat Ni surfaces, as central mediators to attain favourable energetics for the detachment of Ni adatoms from coordinatively unsaturated sites on the metal nanocrystals and their emission onto the oxide support material, hence responsible for a catalysis-driven Ostwald ripening metal growth mechanism. The results contribute to rationalize the interplay between metal nanocrystal size and carbon-metal dynamics to define an optimal Ni nanoparticle size of ∼10 nm for a maximum hydrogen production rate, providing a blueprint for designing optimized catalysts for low-temperature methane splitting. • CH₄ splitting rate peaks for Ni nanoparticle size of approximately 10 nm. • Small Ni particles fail to grow carbon filaments and become passivated. • Operando XRD and TEM link activity to Ni particle size distribution. • Fast Ni nanocrystal growth occurs early during methane exposure. • DFT shows carbonaceous adspecies mediate Ostwald ripening Ni growth.