Catalysts for direct H2O2 synthesis taking advantage of the high H2 activating ability of Pt: kinetic characteristics of Pt catalysts and new additives for improving H2O2 selectivity

Abstract

To develop efficient catalysts for the direct H₂O₂ synthesis from H₂ and O₂ by taking advantage of the high H₂ activating ability of Pt, kinetic studies of the H₂–O₂ reaction were performed using a Pt-PVP (polyvinylpyrrolidone) colloid and Pt supported on carbon (Pt/C) as catalysts, and new additives were explored. Only 10⁻⁹ N of protons was sufficient to maintain the H₂ reaction rate, suggesting that the dissociatively adsorbed H₂ on Pt was very active. The addition of H⁺ and Br⁻, which is most effective for Pd catalysts, exerted much less influence on Pt, resulting in very low H₂O₂ selectivities. Inorganic and organic iodides as well as some sulfur compounds increased the H₂O₂ yield. The kinetic analysis of the reaction indicated that the H₂O₂ formation selectivity of Pt/C was maximized in combination with H⁺ and p-iodophenol, and was nearly comparable to that of Pd/C combined with H⁺ and NaBr; and indicated that the catalytic activity of Pt/C was higher. However, the much higher H₂O₂ destruction activity of Pt/C compared to that of the Pd catalyst spoiled the total catalytic performance of the Pt catalyst for H₂O₂ formation. Sulfur (Na₂S₂O₃) and phosphine (tris(hydroxymethyl)phosphine) unselectively depressed the H₂–O₂ reaction.