For conventional palladium-coated catalysts, some active sites of the Pd surface are blocked by a shell and all reactants and products are confined in the same channel for diffusion, which reduces the catalytic activity. To solve these problems, a different type of yolk-shell nanocatalyst were prepared to directly synthesize hydrogen peroxide (H2O2): Pd@HCS, with Pd nanoparticles as core and hollow carbon spheres as shell. The shell pore size of the catalyst Pd@HCS was between 1 and 2 nm, which could be beneficial for the diffusion of the reactant and product molecules. Core Pd nanoparticle size decreased with decreasing number of carbon atoms of the oil phase solvent (isooctane (C-8), n-heptane (C-7), and cyclohexane (C-6)) and surfactant polyethylene glycol monocetyl ether (Brij56) concentration. When Pd nanoparticle size was 5.50 +/- 1.13 nm, the catalyst Pdc6-0.174@HCS had the highest H2O2 selectivity and productivity, which were 94% and 3219 mmol g(Pd)(-1) h(-1), respectively. (C) 2019 Elsevier Inc. All rights reserved.