Palladium catalysts supported on SO4=, Cl-, F-, and Br--doped zirconia were tested for the direct synthesis of hydrogen peroxide under very mild (1 bar and 20 degrees C) and nonexplosive conditions. The catalysts were characterized by thermogravimetric/differential scanning calorimetry analysis, N-2 physisorption, and temperature-programmed reduction before and after catalytic tests to investigate the oxidation state of the metal. The catalytic tests were carried out in different solvents, and the effect of the Pd oxidation state was ascertained. The best catalytic results were observed in methanol, using H-2/O-2 mixtures containing a large excess of oxygen and using the sulfate-dopcd zirconia catalyst. Surface-oxidized Pd-0 catalysts showed high catalytic activity and the highest selectivity. (c) 2006 Elsevier Inc. All rights reserved.