Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide: Reasons for the Different Effects of p-Elements (P, S) and Medium Modifiers.

The influence of the nature, concentration, and ratio of reaction medium modifiers on the properties of phosphorus- or sulfur-modified palladium catalysts in the direct synthesis of H2O2 has been investigated. The causes of the extreme dependence of productivity and selectivity of the 3%Pd-1.0P/HZSM-5 catalyst on the modifier ratio (H2SO4:KBr) have been established and discussed. It has been shown that the role of KBr in the performance of the 3%Pd-1.0P/HZSM-5 catalyst in direct H2O2 synthesis depends on the nature of the Brønsted acid. Regardless of the nature of the reaction medium modifiers, the 3%Pd-0.25S/NaZSM-5 and 3%Pd-0.5S/NaZSM-5 catalysts exhibit significantly lower H2O2 yields than the 3%Pd-1.0P/HZSM-5 catalyst. The composition of the surface layer of two PdS samples was determined by XPS. The main reasons for the different modifying effects of p-elements have been examined. The transition to sulfur-enriched, low-activity palladium surface sulfides with increasing S:Pd ratio, the presence of sulfur oligomers (Sn) on the Pd particle surface, and likely the higher ionic character of the PdS bond compared to the PdP bond account for the low reactivity of PdS catalysts in the direct synthesis of H2O2 under mild conditions.