Confined Reactivity in the van der Waals Gap beneath Graphene: Supply-Limited Kinetics and Emergent Reaction Pathways

The confinement of molecules within the van der Waals (vdW) gap between a two-dimensional (2D) material and a catalytic substrate offers a promising route toward the development of molecule-selective catalysts with increased reaction rates and access to chemically distinct reaction environments. However, identifying the kinetic limitations and mechanistic consequences of such confined reactions remains challenging. Here, we employ an inverted wedding cake configuration of multilayer graphene on platinum to study the dynamics of graphene etching within the vdW gap by O₂, H₂, and CO using in situ scanning electron microscopy. Under the experimental conditions explored (up to <i>p</i> = 1.4 × 10^-2 Pa and <i>T</i> = 1000 °C), the etching reactions are supply limited for O₂ and H₂. The reaction-limited regime is not observed even for CO, despite its anomalously enhanced transport resulting from a pronounced lifting of the vdW gap. Reactive molecular dynamics simulations reveal that confinement within the vdW gap enables additional CO-mediated etching pathways that are absent on open Pt surfaces. Our results demonstrate that intercalation does not primarily reduce reaction barriers but instead creates a confined, high-chemical-potential nanoreactor in which new reaction pathways can be accessed at comparatively low external pressures.