Electronic regulation and hydrophobicity improvement of the palladium active site by ionic liquid to achieve high selectivity direct synthesis of hydrogen peroxide

Direct synthesis of hydrogen peroxide (H 2 O 2 ) from hydrogen (H 2 ) and oxygen (O 2) is an ideal industrial method to replace the anthraquinone oxidation process for H 2 O 2 production, however, it is still limited by low H 2 O 2 selectivity and productivity. The electronic structure and wettability of the catalyst are crucial factors that affects the catalytic performance. This study demonstrates that binding hydrophobic 1-butyl-3-methylimidazolium-hexafluorophosphate ([Bmim]PF 6 ) ionic liquid on the surface of Pd nanoparticles (Pd NPs) can increase the content of electron-deficient state Pd (Pd 2+ ) at the active site, which exhibits excellent catalytic performance (90.8% H 2 O 2 selectivity and 8828.6 mmol g Pd −1 h −1 H 2 O 2 productivity). The results of XPS, O 2 -TPD and DFT show that the increase of Pd 2+ content effectively inhibits the O-O bond dissociation of O 2 *, OOH*, and HOOH*, thus improving H 2 O 2 selectivity and productivity. Moreover, the hydrophobicity of [Bmim]PF 6 induces a change in the three-dimensional hydrophilic microenvironment of the Pd active site, reducing the energy required for HOOH* desorption from the active site by 0.07 eV (from 0.53 eV to 0.46 eV). This is useful in preventing further hydrogenation and decomposition of HOOH*. TEM characterization suggests that the steric hindrance of [Bmim]PF 6 impedes the aggregation of Pd NPs during the reaction, resulting in different cyclic stability of Pd/OCNT and Pd [Bmim]PF 6 /OCNT.