  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.
