The intrinsic catalytic activity of a hybrid gel-derived ZrO2-acetylacetonate (HSGZ) material towards the oxidative degradation of phenanthrene (PHE), in aqueous solution and in the dark, was revealed for the first time. The HSGZ catalyst is a polymeric network of zirconium oxo-clusters on the surface of which part of Zr4+ ions are involved in strong complexation with acetylacetonate (acac) ligands. The HSGZ gave significant PHE degradation rates acting as radicals initiator without any light irradiation at 30 degrees C. Free radicals were formed on the solid surface by the coexistence of Zr(IV)-acac and Zr(III)-acac(center dot) complexes in equilibrium at a given temperature, from which reactive oxygen species were produced in the presence of molecular O-2. A direct evidence of radical's presence on HSGZ solid surface was obtained by EPR spectroscopy. The analysis of the degradation products confirmed that the reaction goes on through the formation of intermediate free radicals, leading to the first ring-opening and to the formation of phthalates as main intermediates. Subsequently, low molecular weight alkanes are produced. Finally, a deep oxidation of the intermediates occurs completing the mineralization process. The HSGZ catalyst showed a good stability under the reaction conditions, retaining its catalytic activity after repeated tests. (C) 2014 Elsevier B.V. All rights reserved.