Heterogeneous catalysis is inherently complex, and this makes it difficult to trace the reaction and clarify the mechanism. In this study, we investigated the reduction of nitric oxide (NO) by water on Cu(110) in a well-defined environment. Scanning tunnelling microscopy was used to control and image the reaction, and to characterize the product and the intermediate. A one-to-one reaction yields a characteristic NO-water complex, in which water induces partial filling of the empty 2π* orbital of NO, leading to N-O bond weakening. Subsequent reaction of the complex with another water molecule induces further weakening of the N-O bond, leading to bond rupture. We reveal that hydrogen-bond coupling induces back-donation and thus plays a crucial role in N-O bond cleavage; this provides a fundamental insight into the catalytic reduction of NO under ambient conditions.