Valence states of single Au atoms dictate the catalytic activity of Au 1 /CeO 2 (100).

We tune the valence state of single Au atoms anchored on CeO 2 (100) by treating the catalyst in H 2 at different temperatures and obtain a series of Au 1 /CeO 2 (100). The transition from Au 1 +0.9 to Au 1 +0.3 leads to an enhancement of the CO oxidation activity of Au 1 /CeO 2 (100) by one order of magnitude. This work is of significance for an in-depth understanding of reaction mechanisms and rational design of high-performance catalysts.