H2O2 direct synthesis under mild conditions on Pd-Au samples: Effect of the morphology and of the composition of the metallic phase

Bimetallic Pd-Au samples supported on silica were prepared by different methods and tested for the direct synthesis of hydrogen peroxide under very mild conditions (room temperature and atmospheric pressure), outside the explosion range and without addition of halides. Further catalytic tests were performed at higher pressure using solvents expanded with CO2. Samples were characterized by N-2 physisorption, metal content analysis, TPR, CO chemisorption, HRTEM combined with X-Ray EDS probe and DR UV-vis measurements. The effect of gold addition to Pd in enhancing the yield of H2O2 was sensitive to the preparation method: the best catalytic results were obtained introducing gold and palladium by incipient wetness co-impregnation. HRTEM and TPR evidenced the presence of a PdAuO phase able to guarantee the availability of less energetic sites, which activate the oxygen molecule without dissociation. Co-impregnated bimetallic PdAu catalysts on different supports were also tested. The productivity follows the trend: PdAuSi approximate to PdAuSZ > PdAuZ >> PdAuCe approximate to PdAuSCe. The origin of the differences among samples can be due to an effect of the acidity of the support on both morphology and size of the bimetallic phase. HRTEM and DR UV-vis spectroscopy revealed bimetallic particles exposing the active less energetic sites on PdAuSi, PdAuSZ and PdAuZ, whereas on ceria the presence of more energetic sites that lead to the formation of water were disclosed. (C) 2014 Elsevier B.V. All rights reserved.