In an effort to determine the active state of supported palladium for the direct formation of H2O2 from H2 and O 2, the catalytic behavior of Pd0/SiO2, PdO/SiO2 and partially reduced PdO/SiO2 was determined. The results obtained in an ethanol slurry, with chloride ions and H 2SO4 being present, showed that the PdO/SiO2 catalyst was almost completely inactive for the formation of H2O 2 at 10 °C. The Pd0/SiO2 catalyst exhibited the highest activity for H2O2 formation, and the PdO/SiO2 material, reduced under very mild conditions, exhibited an intermediate activity. The state of Pd on the three catalysts was characterized by XRD, TEM and XPS methods. Only Pd0 (the metal phase) and PdO were observed on Pd0/SiO2 and PdO/SiO2, respectively. As expected, with the partially reduced PdO/SiO2 catalyst, both Pd0 and PdO phases were evident. The TEM results revealed that the Pd0 particles decorated the larger PdO particles. The results reported here support the role of metallic palladium, rather than the oxide, as the active phase for the direct formation of H2O 2.
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