Reactive Intermediates or Inert Graphene? Temperature- and Pressure-Determined Evolution of Carbon in the CH₄–Ni(111) System

Atomic-level identification of carbon intermediates under reaction conditions is essential for carbon-related heterogeneous catalysis. Using the in operando technique of near-ambient-pressure X-ray photoelectron spectroscopy, we have identified various carbon intermediates during the thermal decomposition of CH₄ on Ni(111), including *CH, *C₁/Ni₃C, *C_<i>n</i> (<i>n</i> ≥ 2), and clock-reconstructed Ni₂C at different temperature regions (300–900 K). These “reactive” carbon precursors can either react with probing molecules such as O₂ at room temperature or be etched away by CH₄. They can also develop into graphene flakes under controlled conditions: a temperature between 800 and 900 K and a suitable CH₄ pressure (10^–3–10^–1 mbar, depending on temperature). The growth rate of graphene is significantly restrained at higher CH₄ pressures, due to the accelerated etching of its carbon precursors. The identification of in operando carbon intermediates and the control of their evolution have great potential in designing heterogeneous catalysts for the direct conversion of methane. The observed carbon aggregation/etching equilibrium reveals an underlying mechanism in coking prevention and in the fabrication of large-area single-crystal graphene, where the suppression of seeding density and etching up of small grains are required.