Tuning silica acidity with ammonium fluoride: Boosting iridium catalyst performance in furfural hydrogenation

• Ammonium fluoride treatment finely tunes silica surface acidity and porosity via controlled silicon leaching. • Ir/SiO₂ catalysts exhibit enhanced activity and selectivity in furfural hydrogenation. • The Ir-SiO-1.0F catalyst delivers the highest selectivity toward furfuryl alcohol. • XPS and H₂-TPR analyses indicate that the Ir⁰/Ir⁴⁺ ratio governs selectivity through a dual-site mechanism. • Strong correlations established between surface structure, acidity, and catalytic performance. This work reports a strategy to enhance iridium-catalyzed furfural hydrogenation by tuning the acidity of amorphous silica supports via ammonium fluoride treatment. The NH 4 F treatment induced controlled silicon leaching, which increased the pore volume and generated additional Lewis acid sites, thereby promoting superior metal dispersion. Among the catalysts, Ir-SiO-1.0F achieved nearly complete furfural conversion (>95%) with ∼100% selectivity toward furfuryl alcohol, clearly outperforming both unmodified and over-fluorinated systems. Structure–activity correlations revealed that the optimal Ir dispersion (∼40%, particle size ∼3 nm) and a balanced Ir 0 /Ir 4+ ratio are decisive for high selectivity. XPS and NH 3 -TPD measurements demonstrated that acidity and Ir oxidation states act synergistically to favor carbonyl hydrogenation while suppressing ring saturation. Density functional theory (DFT) calculations confirmed the preferential adsorption of furfural on IrO 2 . Dual-site top (perpendicular) adsorption of furfural on the IrO 2 surface enables its selective hydrogenation to furfuryl alcohol, with hydrogen dissociating on metallic iridium. These findings establish NH 4 F-treated silica as a versatile support design and provide a clear framework for tailoring catalysts for the selective upgrading of biomass.