Engineering of Hybrid SiO 2 @{N-P-Fe} Catalysts with Double-Ligand for Efficient H 2 Production from HCOOH

Two Fe-based hybrids, [SiO 2 @NP(Ph) 2 /Fe II /PP 3 ] and [SiO 2 @NP(t-Bu) 2 /Fe II /PP 3 ], were synthesized using the double-ligand approach by covalently grafting NP ligands onto the surface of SiO 2 . Both catalytic systems were evaluated for H 2 production through formic acid dehydrogenation (FADH), revealing important efficiency without requiring additional additives and/or co-catalysts. During the continuous addition of FA, [SiO 2 @NP(Ph) 2 /Fe II /PP 3 ] and [SiO 2 @NP(t-Bu) 2 /Fe II /PP 3 ] demonstrated great stability, achieving total TONs = 20,636 and 20,854, respectively. FT-IR and Raman spectroscopy provided insights into the role of NP ligands, such as NP(Ph) 2 and NP(t-Bu) 2 , on the assembly and structural configuration of active hybrid Fe catalysts and their ability to dehydrogenate formic acid. Additional studies, including in situ mapping of the solution potential (E h ) of the catalytic reaction and an Arrhenius study of the activation energy (E a ), revealed a correlation between E a and H 2 production rates: the system [SiO 2 @NP(Ph) 2 /Fe II /PP 3 ] with an E a = 29.4 KJ/mol shows an H 2 production rate of 58 mL-H 2 /min, while [SiO 2 @NP(t-Bu) 2 /Fe II /PP 3 ] with a E a = 50.6 KJ/mol shows an H 2 production rate of 55 mL-H 2 /min. This is the first example of a heterogeneous FADH system where the original strategy of a “double-ligand” has been demonstrated for homogeneous FADH catalytic systems. Herein we demonstrate that we can engineer a decrease in the activation barrier Ea via two synergistic steps: (i) via grafting of the NP ligand onto SiO 2 and (ii) using PP 3 as double ligand. This strategy leads to a boost in the H 2 production efficiency of [SiO 2 @NP(Ph) 2 /Fe II /PP 3 ] as a heterogeneous catalyst, which for the first time has been shown to be able to outperform the parental reference/homogenous catalyst [Fe II /PP 3 ].