Abstract
To date, the effect of oxidation state on activity remains controversial in whether higher or lower oxidation states benefit the enhancement of catalytic activity. Herein, we discover a volcanic relationship between oxidation state and hydrogen evolution reaction activity based on Os single-atom catalysts. Firstly, a series of Os SACs with oxidation states ranging from + 0.9 to + 2.9 are synthesized via modifying the coordination environments, including Os-N(3)S(1), Os-N(4), Os-S(6), Os-C(3), and Os-C(4)S(2). A volcano-type relation between oxidation states and hydrogen evolution activity emerge with a summit at a moderate experimental oxidation state of + 1.3 (Os-N(3)S(1)). Mechanism studies illustrate that with increasing oxidation states, the adsorption of H atoms on Os is strengthened due to increased energy level and decreased occupancy of anti-bonding states of Os-H bond until the anti-bonding states become empty. Further increasing the oxidation states weakens hydrogen adsorption because of the decreased occupancy of Os-H bonding states. In this work, we emphasize the essential role of oxidation state in manipulating activity, which offers insightful guidance for the rational design of single-atom catalysts.