One-pot mechanochemical ball milling synthesis of the MnOx nanostructures as efficient catalysts for CO2 hydrogenation reactions

Abstract

Here, we report on a one-pot mechanochemical ball milling synthesis of manganese oxide nanostructures synthesized at different milling speeds. The as-synthesized pure oxides and metal (Pt and Cu) doped oxides were tested in the hydrogenation of CO₂ in the gas phase. Our study demonstrates the successful synthesis of the manganese oxide nanoparticles via mechano–chemical synthesis. We discovered that the milling speed could tune the crystal structure and the oxidation state of the manganese, which plays an essential role in the CO₂ hydrogenation evidenced by ex situ XRD and XPS studies. The pure MnO_x milled at 600 rpm showed high catalytic activity (∼20 000 nmol g⁻¹ s⁻¹) at 823 K, which can be attributed to the presence of Mn(II) besides Mn(III) and Mn(IV) on the surface under the reaction conditions. This study illustrates that the milling method is a cost-effective, simple way for the production of both pure, Pt-doped and Cu-loaded manganese nanocatalysts for heterogeneous catalytic reactions. Thus, we studied the Pt incorporation effect for the catalytic activity of MnO_x using different Pt loading methods such as one-pot milling, wet impregnation and size-controlled 5 nm Pt loading via an ultrasonication-assisted method.