Hydrophobicity Gradient of Ionic Liquids Modulates Electron Transfer in Pd-Based Catalysts to Enhance Direct Hydrogen Peroxide Synthesis Performance

Direct synthesis of hydrogen peroxide (H2O2) represents a highly promising green alternative to the traditional anthraquinone method. However, this reaction still faces challenges of low selectivity and yield due to O–O bond dissociation. This study leverages the unique charge-modulating effects and hydrophobic properties of imidazole-based ionic liquids (ILs) to modulate the electronic structure of the Pd active component. This forms stable Pd-ILs nanocluster active sites within a hydrophobic three-dimensional microenvironment, enhancing nondissociative adsorption of O2 and facilitating rapid desorption of H2O2 from the Pd crystal surface. Corresponding characterization analyses, including O2-TPD and H2-TPD, reveal that Pd-BmimNTf2/CNT-O exhibits weaker O2 adsorption and faster H2O2 desorption compared to Pd/CNT-O, supporting the enhanced adsorption/desorption properties. DFT calculations further show that electron transfer from Pd to ILs induces slight Pd lattice expansion and a negative shift in the Pd d-band center. This electronic modulation synergistically suppresses O–O bond dissociation and enhances the H2O2 catalytic performance. Furthermore, the electron transfer capacity is positively correlated with the hydrophobicity of ionic liquids. As a result, the synthesized Pd-ILs/CNT-O exhibits favorable catalytic activity in the DSHP reaction, with the most hydrophobic catalyst Pd-BmimNTf2/CNT-O demonstrating outstanding H2O2 yield (18.13 × 103 mmol·gPd–1·h–1) and selectivity (96.92%). This study confirms the first systematic investigation into the impact of hydrophobicity differences among various ionic liquids on the direct synthesis of H2O2, providing crucial evidence for subsequent high-efficiency catalyst design and development.