N‐Heterocyclic Carbene‐Stabilized Ultrasmall Gold Nanoclusters in a Metal‐Organic Framework for Photocatalytic CO₂Reduction

Abstract

Ultrafine gold nanoclusters (Au‐NCs) are susceptible to migrate and aggregate, even in the porosity of many crystalline solids. N‐heterocyclic carbenes (NHCs) are a class of structurally diverse ligands for the stabilization of Au‐NCs in homogeneous chemistry, showing catalytic reactivity in CO2 activation. Herein, for the first time, we demonstrate a heterogeneous nucleation approach to stabilize ultrasmall and highly dispersed gold nanoclusters in an NHC‐functionalized porous matrix. The sizes of gold nanoclusters are tunable from 1.3 nm to 1.8 nm based on the interpenetration of the metal‐organic framework (MOF) topology. Control experiments using amine or imidazolium‐functionalized MOFs afforded the aggregation of Au species. The resultant Au‐NC@MOF composite exhibits a steady and excellent activity in photocatalytic CO2 reduction, superior to control mixtures without NHC‐ligand stabilization. Mechanistic studies reveal the synergistic catalytic effect of MOFs and Au‐NCs through the MOF‐NHC‐Au covalent‐bonding bridges.

Functionalized MOF‐Based Photocatalysts for CO2 Reduction

“…UiO‐68‐NH 2 with functionalized modification of N‐heterocyclic carbene (NHC) was synthesized by ligand exchange (PSE) method. The ultra‐small uniformly distributed Au nanoparticles (NCs) were also introduced in the MOF interstices by heterogeneous nucleation method in Figure 10a [84] . The Au‐NC@UiO‐68‐NHC retained the topology of UiO‐68, as evidenced by powder X‐ray diffraction (PXRD) (Figure 10b).…”

“…(e) Charge density difference (CDD) under the ground state (left) and the excited state (right) between Au NCs (gold), UiO‐68‐NHC (top), and UiO‐68‐NH 2 (bottom). Electrons and holes are represented by green and blue ellipses, respectively [84] . Copyright 2021, Wiley‐VCH.…”

Heteroatom‐Doped Ag25 Nanoclusters Encapsulated in Metal–Organic Frameworks for Photocatalytic Hydrogen Production

“…Moreover, MOFs are highly tunable in light harvesting from UV to near‐infrared regions, which might be simultaneously photoexcited with metal NCs to constitute a heterojunction photocatalytic system for promoted charge separation. Despite a few reports on the stabilization of metal NCs with MOFs for photocatalysis, [5a,7b] the formation of heterojunction or related mechanisms for enhanced photocatalysis has not yet been observed in these composites.…”