Sum Frequency Generation Vibrational Spectroscopy of 1,3-Butadiene Hydrogenation on 4 nm Pt@SiO₂, Pd@SiO₂, and Rh@SiO₂ Core–Shell Catalysts

1,3-Butadiene (1,3-BD) hydrogenation was performed on 4 nm Pt, Pd, and Rh nanoparticles (NPs) encapsulated in SiO₂ shells at 20, 60, and 100 °C. The core–shells were grown around polyvinylpyrrolidone (PVP) coated NPs (Stöber encapsulation) prepared by colloidal synthesis. Sum frequency generation (SFG) vibrational spectroscopy was performed to correlate surface intermediates observed in situ with reaction selectivity. It is shown that calcination is effective in removing PVP, and the SFG signal can be generated from the metal surface. Using SFG, it is possible to compare the surface vibrational spectrum of Pt@SiO₂ (1,3-BD is hydrogenated through multiple paths and produces butane, 1-butene, and <i>cis</i>/<i>trans</i>-2-butene) to Pd@SiO₂ (1,3-BD favors one path and produces 1-butene and <i>cis</i>/<i>trans</i>-2-butene). In contrast to Pt@SiO₂ and Pd@SiO₂, SFG and kinetic experiments of Rh@SiO₂ show a permanent accumulation of organic material