Tuning the hybridization and charge polarization in metal nanoparticles dispersed over Schiff base functionalized SBA-15 enhances CO2 capture and conversion to formic acid

Abstract

Different Schiff base functionalized SBA-15 materials were synthesized through condensation reactions between 3-aminopropyltriethoxysilane (APTES) and different aldehydes (glutaraldehyde and butyraldehyde) over a mesoporous silica, SBA-15 (APTES-GLU/SBA-15 and APTES-BUT/SBA-15). Both static and dynamic experiments have been used for testing the CO₂ capture efficiency of these materials. The hybridization of the N atom in APTES has been tuned from sp³ to sp² upon condensation facilitating optimum CO₂ capture in the direct synthesis of APTES-GLU/SBA-15. The undesirable oxides of nitrogen have been removed during the synthesis process to improve the CO₂ capture efficiency. These materials were employed as supports for Pd–Ag and Pd–Ni bimetallic systems for the selective conversion of the captured CO₂ to formic acid (FA) in 0.5 M KHCO₃ solution. The Pd–Ni catalyst system exhibited enhanced CO₂ to FA conversion activity compared to other heterogeneous systems, which is ∼4 times better than that of the Pd–Ag system in this study. The X-ray absorption studies over the catalyst material confirmed that the relatively electron-deficient Ni in Pd–Ni compared to Ag in Pd–Ag favoured higher charge polarization between the metals in the Pd–Ni system enhancing the CO₂ to FA conversion. The experimental observations are well supported by the DFT calculations.