Mass-transport-limited reaction rates and molecular diffusion in the van der Waals gap beneath graphene

TL;DR

This study investigates how molecular diffusion and mass transport limitations within the van der Waals gap beneath graphene influence reaction rates, revealing confinement effects that can enable otherwise inaccessible catalytic pathways.

Contribution

The paper demonstrates that mass transport within the vdW gap limits reaction rates and shows how confinement can act as a nanoreactor, enabling new reaction pathways, supported by experiments and molecular dynamics simulations.

Findings

Reaction rates are limited by mass transport in the vdW gap.

Enhanced transport for CO lifts the mass-transport limitation.

Confinement enables reaction pathways inaccessible on open surfaces.

Abstract

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. However, identifying the kinetic limitations 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 in the vdW gap by various molecules (O2, H2, and CO), using in situ scanning electron microscopy. Under the experimental conditions explored (up to p = 1.4x10-3 Pa and T = 1000 {\deg}C), the etching reaction rates are limited by mass transport within the confined space. This limitation persists even for CO, despite its anomalously enhanced transport resulting from a significant lifting of the vdW gap. Reactive molecular dynamics simulations further reveal multiple etching pathways for CO, enabled by confinement within the vdW space. Once mass-transport limitations are overcome, the vdW gap acts as an effective nanoreactor, facilitating reaction pathways that would be otherwise inaccessible on a pristine surface without spatial confinement.