Regulation of remote sites to enhance Pt activity in the hydrogenation of sulfur-containing nitroarenes

Abstract

Outer-sphere catalysis, commonly associated with enzyme and homogeneous catalysis, utilizes remote sites to drive reactions and is particularly effective for reactants that are toxic to the catalyst. However, this approach has been rarely explored in heterogeneous catalysis. In this study, we demonstrate the use of remote sites to enhance the activity of Pt nanoparticles (NPs) in the hydrogenation of sulfur-containing nitroarenes. Catalysts with Pt and MoO₃ co-deposited on TiO₂ efficiently catalyze the hydrogenation of 5-nitrobenzothiazole (NBZ) under mild conditions, achieving a conversion rate of 5448 mol_NBZ · mol_Pt⁻¹ · h⁻¹, the highest reported to date, even with ppm levels of Pt. The optimized Pt density (∼3 NPs per 1 × 10⁴ nm²) on the catalyst was found to favor HxMoO₃ mediated sequential H transfer from Pt to Mo and subsequently to the substrate. While higher Pt density may enhance H transfer to Mo, the hydrogenation process becomes limited by the availability of HxMoO₃ for further H transfer to the substrate, revealing the intrinsic reason for the high activity of catalysts with ppm Pt. A direct transfer of intercalated H in HxMoO₃ to the substrate, rather than solvent-mediated proton-coupled electron transfer (PCET) dominating the hydrogenation, was observed and confirmed through solvent isotope kinetic effects and solvent studies. Tuning remote sites on solid catalysts offers a promising strategy for developing catalysts with the minimize the use of precious metals in the hydrogenation of strongly coordinating reactants.