AbstractIn situ tuning of the electronic structure of active sites is a long-standing challenge. Herein, we propose a strategy by controlling the hydrogen spillover distance to in situ tune the electronic structure. The strategy is demonstrated to be feasible with the assistance of CoO_x/Al₂O₃/Pt catalysts prepared by atomic layer deposition in which CoO_x and Pt nanoparticles are separated by hollow Al₂O₃ nanotubes. The strength of hydrogen spillover from Pt to CoO_x can be precisely tailored by varying the Al₂O₃ thickness. Using CoO_x/Al₂O₃ catalyzed styrene epoxidation as an example, the CoO_x/Al₂O₃/Pt with 7 nm Al₂O₃ layer exhibits greatly enhanced selectivity (from 74.3% to 94.8%) when H₂ is added. The enhanced selectivity is attributed to the introduction of controllable hydrogen spillover, resulting in the reduction of CoO_x during the reaction. Our method is also effective for the epoxidation of styrene derivatives. We anticipate this method is a general strategy for other reactions.