When high metal dispersion has a detrimental effect: Hydrogen peroxide direct synthesis under very mild and nonexplosive conditions catalyzed by Pd supported on silica

摘要

Pd catalysts supported on silica were tested for H _2O _2 direct synthesis under very mild (1 bar, 20 °C, no halide addition) and nonexplosive conditions. Other catalytic tests were performed at higher pressure using a solvent expanded with CO _2. The optimum Pd content is 1.5 wt.%. Calcination is mandatory to preserve catalytic performance and recycle the catalyst. A comparison among Pd samples with similar metal content over different supports was performed under identical experimental conditions, showing that Pd/SiO _2 gives the highest selectivity (constant at 60%). At the same time, the productivity follows the trend: Pd/SiO2>Pd/ZrO2/SO4= >Pd/ZrO2>Pd/CeO2/SO4=>Pd/CeO2. HRTEM and chemisorption results clearly indicate that a high Pd dispersion is not advisable to observe good catalytic performance. On the contrary, the presence of less energetic Pd sites exposed at the surface of big particles of Pd/SiO _2, where oxygen can chemisorb without dissociation, is a necessary condition to produce H _2O _2.