Quantum tunnelling driven H$_2$ formation on graphene

TL;DR

This study reveals that quantum tunnelling significantly enhances H$_2$ formation on graphene at low temperatures, challenging prior assumptions about the inefficiency of this process without incident H atoms.

Contribution

The paper introduces a multidimensional tunnelling approach using ring-polymer instanton theory to demonstrate rapid H$_2$ formation on graphene, previously thought unlikely at low temperatures.

Findings

Recombination of adsorbed H atoms is enabled by deep tunnelling.

Reaction rates are increased by tens of orders of magnitude.

A new multidimensional tunnelling pathway for H recombination is identified.

Abstract

It is commonly believed that it is unfavourable for adsorbed H atoms on carbonaceous surfaces to form H$_2$ without the help of incident H atoms. Using ring-polymer instanton theory to describe multidimensional tunnelling effects, combined with ab initio electronic structure calculations, we find that these quantum-mechanical simulations reveal a qualitatively different picture. Recombination of adsorbed H atoms, which was believed to be irrelevant at low temperature due to high barriers, is enabled by deep tunnelling, with reaction rates enhanced by tens of orders of magnitude. Furthermore, we identify a new path for H recombination that proceeds via multidimensional tunnelling, but would have been predicted to be unfeasible by a simple one-dimensional description of the reaction. The results suggest that hydrogen molecule formation at low temperatures are rather fast processes that should not be ignored in experimental settings and natural environments with graphene, graphite and other planar carbon segments.