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

Pd catalysts supported on silica were tested for H2O 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 w t.%. 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/ SiO2 gives the highest selectivity (constant at 60%). At the same time, the produc tivity follows the trend: Pd= SiO2> Pd= ZrO2= SO¼4> Pd= ZrO2> Pd= CeO 2= SO¼4> 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 oxyg en can chemisorb without dissociation, is a necessary condition to produce H 2O2