▎ 摘　　要

It is commonly believed that it is unfavorable for adsorbed H atomson carbonaceous surfaces to form H2without the help of incident H atoms. Usingring-polymer instanton theory to describe multidimensional tunnelling effects,combined withab initioelectronic structure calculations, wefind that thesequantum-mechanical simulations reveal a qualitatively different picture. Recombi-nation of adsorbed H atoms, which was believed to be irrelevant at lowtemperature due to high barriers, is enabled by deep tunnelling, with reaction ratesenhanced by tens of orders of magnitude. Furthermore, we identify a new path forH recombination that proceeds via multidimensional tunnelling but would havebeen predicted to be unfeasible by a simple one-dimensional description of thereaction. The results suggest that hydrogen molecule formation at lowtemperatures are rather fast processes that should not be ignored in experimentalsettings and natural environments with graphene, graphite, and other planar carbon segments