Crystal refinement of rutile by sonochemical method to achieve high performance Pd catalysts for direct synthesis of hydrogen peroxide

In this study, sonochemically prepared (S) Pd catalysts supported on anatase (Ana) and rutile (Rut) were compared to incipient wetness impregnated (I) Pd catalysts for the direct synthesis of hydrogen peroxide. Pd/Ana-S and Pd/Rut-S achieved higher H₂ conversion than Pd/Ana-I and Pd/Rut-I due to the improvement of metal dispersion by the sonochemical method. Especially, Pd dispersion on Pd/Rut-S was estimated to be six times higher than other catalysts, resulting in the highest H₂O₂ production rate. Through multiple characterizations (TEM, N₂ adsorption-desorption, H₂-TPR and CO-chemisorption), we discovered that many parts of the rutile crystals were cracked during the sonochemical treatment, obtaining rectangle shape rutile crystals and two times higher specific surface area. As a result, the sonochemical method played dual-functionalities as follows: physically cracking rutile crystals and inducing small Pd nucleation by cavitation. In addition, electron-deficient Pd species, beneficial for H₂O₂ selectivity, were dominantly observed on rutile by XPS analysis. This outcome was explained by unstable Ti³⁺ species on anatase favoring the electron-rich metal. Conclusively, it is demonstrated that the sonochemical method and rutile TiO₂ have a substantial synergistic effect on both H₂ conversion and H₂O₂ selectivity for the direct synthesis of hydrogen peroxide.