In this study, we prepared a yolk-shell structured Pd@void@ZrO2 catalyst and tested its activity in direct hydrogen synthesis from hydrogen and oxygen. The catalyst was prepared by first encapsulating the nano Pd crystal with SiO2 and then, the Pd core-silica shell composite (Pd@SiO2) was encapsulated again with ZrOO2 (Pd@SiO2@ZrO2). The yolk-shell structure was obtained by etching the silica layer of Pd@SiO2@ZrOO2 using 0.2 M KOH solution. Compared to the core-shell structured Pd@SiO2, the void space in the yolk-shell structure allowed more gases to access the core Pd surface. In reaction tests, the production rate of gram Pd-specific H2O2 with Pd@void@ZrOO2 was approximately 1.2 times higher than with Pd@SiO2. It was found that the optimum level of halide ion concentration in the reaction medium was between 0.1 and 0.3 mM for Pd@void@ZrOO2 catalyst. (C) 2015 Elsevier B.V. All rights reserved.