Reversed Charge Transfer to Modulate the d-Band Center of Pd for Efficient Direct H2O2 Synthesis

The direct hydrogen peroxide (H2O2) product from H2 and O2 is a promising anthraquinone replacement because it is environmentally friendly and has a high atom efficiency. Experimental and theoretical studies have proven that optimizing the adsorption of the critical intermediate OOH* on the metal site significantly promotes the further protonation of this intermediate and inhibits the O–O bond cleavage, thus enhancing the activity and selectivity. Redistributing the charge density of active sites to tuning the d-band center of the metal could effectively modulate the intermediates adsorption, and thus regulate the catalytic efficiency. Herein, we show that a Lewis acid (ZnCl2 solution) induces abundant oxygen vacancies (Ovs) on the TiO2 surface, which results in a reversal of charge transfer from TiO2–Ov support to the Pd atom, generating an electron-rich Pd configuration. Compared with pristine Pd/TiO2, Pd/TiO2–Ov possesses higher H2O2 selectivity and productivity, with values of 80.7% and 186 mol kgcat–1 h–1, respectively. In addition, Pd/TiO2–Ov maintains stability during the six cycles reaction due to its high resistance to the leaching of Pd species. Theoretical calculations reveal that the reversed charge transfer downshifts the d-band center of Pd, which promotes O2 adsorption on the Pd surface and weakens the OOH* intermediates adsorption. Thus, the energy barrier for the hydrogenation of the OOH* intermediate is significantly decreased, and the O–O band cleavage is inhibited. This study reports a reversal of charge transfer tuning the d-band center of the active site for efficient direct H2O2 synthesis, which may provide insight for designing high-performance catalysts.