Volcano-type relationship between oxidation states and H2O2 yield of Pd/TiO2 catalysts for direct synthesis of H2O2

Abstract

The oxidation state of Pd species has been regarded as significant property to adjust catalytic performance for the direct synthesis of H₂O₂ (DSHP). However, the effect of oxidation state on DSHP remains controversial in whether higher or lower oxidation states benefit the enhancement of H₂O₂ yield. Herein, we discover a volcanic relationship between Pd oxidation state of Pd/TiO₂ catalysts and H₂O₂ yield. A series of Pd/TiO₂ catalysts were synthesized with controllable oxidation state via modifying heat treatment condition. Specifically, the Pd²⁺% has a volcanic relationship with H₂O₂ selectivity, while has an inverted volcanic relationship with H₂ conversion, emerging with a summit of H₂O₂ yield at the moderate experimental oxidation state of Pd (Pd²⁺%=34.55%). Mechanism studies illustrate that the layered Pd oxides on metallic Pd have different oxidation degrees upon diverse heat treatment condition, which accounts for the Pd oxidation state dependent H₂O₂ yield on Pd/TiO₂. The partial oxidation of metal Pd is regarded as the active site of the moderate oxidation state, which effectively hinder the hydrogenation and decomposition of H₂O₂ and first-order reaction to produce H₂O. In this work, we emphasized the important role of oxidation state in regulating catalytic activity and selectivity, which provides guidance for the rational design of Pd-based catalysts towards DSHP.