Constructing Mimic-Enzyme Catalyst: Polyoxometalates Regulating Carrier Dynamics of Metal–Organic Frameworks to Promote Photocatalytic Nitrogen Fixation

Abstract

In nature, nitrogenase can achieve efficient environmental fixation of nitrogen (N2) molecules through π-backbonding mechanism, which inspires us to simulate the N2 fixation process of nitrogenase to overcome the bottleneck in existing artificial synthetic ammonia (NH3), so as to achieve efficient artificial conversion of N2 to NH3 under mild conditions. Herein, we highlight a strategy of constructing a mimic-enzyme catalyst for photocatalytic N2 fixation based on metal–organic frameworks (MOFs) and polyoxometalates (POMs) in which POMs can efficiently regulate carrier dynamics of MOFs to simulate the π-backbonding mechanism of nitrogenase. Oxygen-rich group POMs with strong electronegativity attract electrons from transition-metal atom in MOFs, thus reducing the 3d orbitals’ electron density of transition-metal atom and enhancing unoccupied d-orbitals in favor of adsorbing N2. Simultaneously, the photoexcited electrons in MOFs are efficiently transferred by POMs into the N–N π* antibonding system to activate N2 via π-backbonding mechanism. Taking MIL-88A (C12H6O13Fe) and PMo10V2 (H5PMo10V2O40) as an example, we have obtained a significantly enhanced NH3 production rate of 50.82 μmol g–1 h–1, which is enhanced by 6 times for MIL-88A and 14 times for PMo10V2. Our results make an important contribution to further guidance for artificial N2 fixation under mild conditions.

Photosynthetic System Based on a Polyoxometalate-Based Dehydrated Metal–Organic Framework for Nitrogen Fixation

“…After integrating with the SiW 9 Fe 3 materials, SiW 9 Fe 3 @d-UiO-66 exhibits a polyhedral morphology like d-UiO-66 (Figures a,b and S6). The high-resolution transmission electron microscopy image observed the distributed POM clusters in such a SiW 9 Fe 3 @d-UiO-66 composite, which may enhance the charge transfer of charge carriers (Figure c). ,,, The element mapping and energy-dispersive X-ray spectroscopy (EDX) images of SiW 9 Fe 3 @d-UiO-66 (Figures d and S7) also reveal that SiW 9 Fe 3 clusters are dispersed in the d-UiO-66 host. The SiW 9 Fe 3 @d-UiO-66 composite material was evaluated via atomic force microscopy (AFM).…”

“…The second weight loss at around 100–400 °C includes solvent molecules, hydroxyl ligands, and part organic ligands (20.5%). The third weight loss at around 500–800 °C is the loss of organic ligands of the UiO-66 framework, resulting in the formation of ZrO 2 materials (Figure b) . Motivated by the above considerations, UiO-66 was tested in the range of 100–300 °C.…”

“…The pH of the sample solution was adjusted to 2 using hydrochloric acid for NMR measurement. Finally, DMSO- d 6 (0.6 mL) was injected into the solution …”

“…It has been demonstrated through theoretical and experimental findings that linker defects result in unsaturated coordination (UC) metal sites in MOFs that have been proved as active sites for enhancing the unoccupied d-orbitals to facilitate nitrogen adsorption and dissociation. − Various functional guests or active species (e.g., metal nanoparticles, perovskite quantum dots, enzymes, polyoxometalates (POMs), etc.) can be encapsulated into MOFs, leading to the development of novel host–guest functional materials. − Compared with other guest materials, POMs as a family of discrete metal-oxo clusters can undergo reversible multielectron redox transformations while maintaining stable structures. − Owing to its remarkable tunability, the energy level of POMs can be modulated at the molecular level through changing the element composition . The nanometer-sized, semiconductor-like features and thermal stability grant POMs with workability in the integration with MOFs.…”

“…The nanometer-sized, semiconductor-like features and thermal stability grant POMs with workability in the integration with MOFs. The encapsulation of POM guests has greatly enhanced the charge separation behaviors of composites (POM@MOFs), which is profitable to promote the photocatalytic reaction. ,, By virtue of the integrated merits of defect MOFs and POMs, POM@MOFs exhibit an architecture and functionality comparable to those of nitrogenase.…”