Ru-O Coordination Number: A Descriptor for Molecular Oxygen Activation in Ru Single-Atom Catalysts

Single-atom catalysts exhibit exceptional performance in O2-driven oxidation reactions. Establishing clear structure-activity relationships is crucial for the rational design of such catalysts. Herein, we correlate the Ru-O coordination number in Ru single-atom catalysts with the efficacy of O2 activation to decipher the catalytic performance in the oxidation of 5-hydroxymethylfurfural (HMF). The Ru-O coordination numbers were controlled by a change in the oxidation state of the manganese oxide support. A low-coordinated Ru single-atom catalysts supported on Mn3O4 allows base-free oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) with 97% yield and an optimized turnover frequency of 38.8 h-1, representing the most efficient catalysts for this reaction. The decrease in Ru-O coordination number leads to an increased electron occupation of the Ru 4d orbital near the Fermi level, facilitating the rate-limiting O2 activation step by the dz2-2p and dxz/dyz-2p orbital hybridizations. Therefore, abundant oxidative radicals were generated to oxidize HMF to FDCA. This work establishes coordination number as a descriptor for guiding the development of advanced single-atom catalysts.