Effects of Surfaces and Confinement on Formic Acid Dehydrogenation Catalyzed by an Immobilized Ru–H Complex: Insights from Molecular Simulation and Neutron Scattering

Formic acid is a potential liquid hydrogen carrier (LHC) for storage and transport of hydrogen, which requires the rapid and efficient recovery of hydrogen from formic acid (through catalysis). Various homogeneous complexes fulfill all of the desired criteria for a formic acid dehydrogenation catalyst. Immobilization of such complexes on solid supports is a common strategy for their application industrially in flow reactors. However, the properties of the support can change the reactivity of the catalyst, often reducing the catalytic activity in ways that are difficult to predict. Here we examine the effect of supports on the dehydrogenation of formic acid using H₂Ru(PPh₃)₂(PPh₂)₂N–C₃H₆–Si(OEt)₃ (1) through a combination of kinetic measurements, molecular dynamics (MD) simulations, and small-angle neutron scattering (SANS). Immobilization of 1 on all supports decreases the rate of formic acid conversion in the order SiO₂ > Al₂O₃ > ZnO > SBA-TMS > SBA-15. Combination of MD simulations and SANS shows that high affinity of formic acid for the surfaces and in particular when confined in the pores results in a high local FA concentration, which inhibits the catalytic rate. Collapse of the Ru complex onto the surface/pore wall may also contribute to this overall inhibition.