The effects of size and active site structure on catalytic activity were deeply investigated in air oxidation of benzyl alcohol over a series of Ni3Al-LDH (layered double hydroxides)-supported Aun nanocluster (NC) catalysts Au-n/LDH (n = 25, 38, similar to 127), obtained by modified electrostatic adsorption of atomically precise Au(25)Capt(18), Au(38)Capt(24) and Au-similar to 127-Capt (Capt: captopril) NCs followed proper calcination. Detailed characterizations show that the Au-25/LDH, Au-38/LDH and Au-similar to 127/LDH possess ultrafine Au NCs size of 1.2 +/- 0.2, 1.4 +/- 0.3 and 1.8 +/- 0.5 nm, respectively, which are majorly dispersed on the edge sites of the small-sized LDH nanosheets (20-40 nm). All the catalysts exhibit excellent catalytic activities in air oxidation of benzyl alcohol without external base associated with the remarkable size- and active site structure-dependence. The catalyst Au-38/LDH shows the highest activity with TOF of 1884 h(-1), which can be attributable to the ultrafine bi-icosahedron structure Au38NCs and the strongest Au38NCs - Ni-OH (LDH) synergistic effect facilitating the activation of alpha-C-H bond of benzyl alcohol over more negatively charged Au sites upon XPS, CO-DRIFTs and kinetic isotope studies. The findings of the size- and active site structure-dependence of supported Aun cluster catalysts in the oxidation of benzyl alcohol provide important guiding principles for the rational design of supported Au-n cluster catalysts. (C) 2020 Elsevier Inc. All rights reserved.