Mechanochemical and Solvent-Free Synthesis of Nitrogen-Doped Mesoporous Silica to Stabilize Ru Catalyst for CO₂ Hydrogenation

A facile, solvent-free mechanochemical method was developed to synthesize nitrogen-doped mesoporous silica as a robust support for ruthenium catalysts in the hydrogenation of CO₂ to formic acid. Comprehensive characterization, including X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), confirmed that nitrogen was effectively incorporated into the silica framework, creating strong anchoring sites for Ru. The optimized catalyst, Ru on N-doped silica, demonstrated outstanding performance, exhibiting a turnover frequency (TOF) of 979 h⁻¹, which is approximately 10-fold higher than that of its Ru on pristine silica. Furthermore, the Ru on N-doped SiO₂ showcased improved catalytic stability, retaining its activity after five recycling tests, whereas the Ru on pristine SiO₂ was rapidly deactivated. Analysis of the spent catalysts revealed that the enhanced catalytic performance originates from doped nitrogen, which stabilizes the Ru active sites by offering strong anchoring sites. The versatility of this mechanochemical method was demonstrated by its successful application to other silica supports, such as SBA-15, KIT-6, and fumed silica. This work establishes a novel and practical methodology for preparing N-doped silica supports capable of significantly enhancing catalytic performance for CO₂ hydrogenation.