5-Hydroxymethylfurfural (HMF) is an important biomass-based platform chemical, and its selective aerobic oxidation to 2,5-diformylfuran (DFF) remains a formidable challenge. This work reports that activated carbon-supported Ru clusters (Ru/C) efficiently catalyzed HMF oxidation to DFF with a high yield of similar to 96% at 383 K and 2.0 MPa O-2 in toluene. Ru/C exhibited activities and DFF selectivities superior to those of Ru clusters with similar sizes (ca. 2 nm) on oxide supports, including Al2O3, ZSM-5, TiO2, ZrO2, CeO2, MgO, and Mg2AlOx, as a consequence of their different surface acidity-basicity and reducibility, which tend to facilitate degradation and polymerization of HMF and DFF. It was also superior to C-supported Pt, Pd, Rh, and Au at comparable sizes in the HMF oxidation to DFF. The effects of O-2 and HMF concentrations on HMF oxidation were examined on Ru/C, showing near half-order dependence of the activities on them. Kinetic isotopic studies showed marked and no kinetic isotopic effects for two HMF molecules deuterated, respectively, at the methylene (k(C-H)/k(C-D) = 3.73) and hydroxyl (k(O-H)/k(O-D) = 1.09) groups. Taken together, these results are consistent with a Langmuir-Hinshelwood mechanism and a sequence of elementary steps involving kinetically-relevant H-abstraction from adsorbed alcoholate species using adsorbed atomic oxygen species, derived from the quasi-equilibrated dissociation of HMF and O-2, respectively, on Ru surfaces. This reaction mechanism leads to a complex kinetic rate expression that accurately describes the measured HMF oxidation activities in a wide range of HMF and O-2 concentrations. (C) 2013 Elsevier Inc. All rights reserved.