When high metal dispersion has a detrimental effect: hydrogen peroxide direct synthesis under very mild and non explosive conditions catalysed by Pd supported on silica

Pd catalysts supported on silica were tested for H2O2 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 CO2. 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/ SiO2 gives the highest selectivity (constant at 60%). At the same time, the productivity follows the trend: Pd=SiO2 > Pd=ZrO2=SO¼ 4 > Pd=ZrO2 > Pd=CeO2=SO¼ 4 > 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/SiO2, where oxygen can chemisorb without dissociation, is a necessary condition to produce H2O2.