Direct oxidation of hydrogen by gaseous oxygen was studied on Au/SiO2 catalyst without addition of a halogen compound. It was found that Au supported on SiO2 exhibits the high activity to H2O2 formation by direct oxidation of H-2 by O-2. Although the same Au catalyst is used, the H2O2 formation rate is strongly dependent on the support oxide. When the acidic oxides such as SiO2 or zeolite are used for support, reasonably high formation rates of H2O2 were achieved. In particular, the H2O2 formation rate is at a maximum on the SiO2 support. The high H2O2 formation rate observed with the SiO2 supported catalyst can be explained by the low activity for H2O2 decomposition. H2O2 formation rate on Au/SiO2, catalyst increased with increasing hydrogen partial pressure with P-H2(1.05), however, it decreased with oxygen partial pressure with P-O2(-0.505). Therefore, activation of hydrogen on the catalyst seems to be the key step for H2O2 formation by direct oxidation of H-2 by O-2. Effects of additives on Au/SiO2 catalyst were further studied and it was found that the addition of a small amount of Pd is effective for increasing the H2O2 formation rate. The formation rate and the selectivity for H2O2 at initial 2 h were achieved to a value of 3.2 mu mol/h and 30% under the optimized condition of Au-Pd/SiO2 catalyst. (c) 2005 Elsevier B.V. All rights reserved.