Pd-Au (75:25) nano colloid was prepared by using various reductants and it was found that the mono dispersed Pd-Au nano colloid with 4.7 nm in diameter was prepared by reduction with oxalic acid and this Pd-Au nano colloid is highly active for the synthesis of H 2O 2 by direct oxidation of H 2 with O 2. The yield of H 2O 2 increased with increasing reaction pressure and at 1 MPa, H 2O 2 yield achieved a value of 30%. Under these conditions, H 2 conversion is as high as 80%. H 2O 2 yield was further improved by increasing H 2 partial pressure, and the optimum H 2 partial pressure was 10% at 1 MPa because of safety (out of the explosion limit concentration in reactor) and the formation rate of H 2O 2. Because H 2O 2 decomposition is prevented, H 2O 2 yield also increased with decreasing reaction temperature and at 268 K, H 2O 2 yield and production rate are achieved as high as 46% and 25 mmol h -1, respectively, in aqueous solution. H 2O 2 concentration reaches a value of 5.8% in aqueous solution after 20 h at 268 K.
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