Real-time observation of the exchange process between H2O and NO in the metal–organic framework Ni-MOF-74

Abstract

Molecular exchange is a common step occurring in many technological processes such as competitive adsorption, chemical separation, capture, delivery, and release. However, the underlying principle is not fully understood, especially in nanoconfined environments where the energetics and kinetics of such processes can deviate from that on flat surfaces. In this paper, we unravel the mechanism of a molecular exchange process by studying the displacement of NO by H₂O in Ni-MOF-74 in real-time using in situ infrared spectroscopy combined with ab initio calculations. We show that weakly bound H₂O gradually displaces strongly bound NO on the metal sites by first weakening the M–N bond through forming H-bond and then moving the NO away so that it eventually desorbs. Interestingly, we further find that additional water facilitates this exchange by significantly lowering the kinetic barrier associated with this process as well as the overall energy of the final state. Although our study focuses on Ni-MOF-74, we believe that our finding and explanation of unexpected exchange phenomena—where strongly adsorbed molecules are apparently easily displaced by much weaker bound H₂O—is applicable to a much larger group of frameworks and will be helpful in designing and improving MOFs for real-world applications where humidity is often present.