Modulating Coordination Environment of Single-Atom Catalysts and Their Proximity to Photosensitive Units for Boosting MOF Photocatalysis

Abstract

Well-organized photosensitive units and catalytic sites in proximity are crucial for improving charge separation efficiency and boosting photocatalysis. Herein, a general and facile strategy for the construction of high-loading (>4 wt %) single-atom catalysts (SACs) with a tunable coordination microenvironment has been developed on the basis of metal–organic frameworks (MOFs). The neighboring −O/OH x groups from a Zr6-oxo cluster in the MOFs provide lone-pair electrons and charge balance to immobilize the extraneous single metal atoms. The well-accessible and atomically dispersed metal sites possess close proximity to the photosensitive units (i.e., linkers), which greatly accelerates charge transfer and thereby promotes the redox reaction. The coordination environment of the representative single-atom Ni sites significantly modulates the electronic state and the proton activation barrier toward hydrogen production. As a result, the optimized Ni1–S/MOF with a unique Ni­(I) microenvironment presents excellent photocatalytic H2 production activity, up to 270 fold of the pristine MOF and far surpassing the other Ni1–X/MOF counterparts. This work unambiguously demonstrates the great advantage of MOFs in the fabrication of high-content SACs with variable microenvironments that are in close proximity to photosensitive linkers, thereby facilitating the electron transfer and promoting photocatalysis.

Research Progress and Application of Single-Atom Catalysts: A Review

“…According to different interactions, they are divided into different types of catalysts, mainly including metal single atoms anchored on the surface of metal oxides, metals, carbon-based materials, metal-organic framework materials (MOFs), covalent organic framework (COF) materials, and composite materials. The construction of a single-atom catalyst with a controllable coordination environment and close to the photoactive site in the MOF material can effectively promote the charge transfer during the photocatalytic reaction and enhance the photocatalytic performance [11][12][13][14][15][16][17].…”

Dual Role of Missing-Linker Defects Terminated by Acetate Ligands in a Zirconium-Based MOF in Promoting Photocatalytic Hydrogen Peroxide Production

“…Defect engineering, which can generate missing-linker defects in the MOF structure by using modulators, has the potential to solve these two problems with UiO-66-NH 2 . Previous computational and experimental studies have demonstrated that the introduction of missing-linker defects in UiO-66-NH 2 is an effective approach to enhance photocatalytic activity in hydrogen production and CO 2 reduction. − In the photoexcitation of UiO-66-NH 2 , an energy is required to transfer electrons from the excited 2-aminoterephthalate linkers to the unoccupied d orbitals of the zirconium–oxo clusters (Δ E LCCT ) as well as to excite the linkers (Δ E Abs ). ,, By introducing missing-linker defects into the UiO-66-NH 2 structure, we can decrease the value of Δ E LCCT by lowering the level of the unoccupied d orbitals of zirconium . On the other hand, it is widely known that hydrophobic catalysts are effective in suppressing H 2 O 2 decomposition due to rapid separation of H 2 O 2 and catalyst particles. , Acetic acid, which is an agent for creating missing-linker defects, is spontaneously coordinated to missing-linker defects on zirconium–oxo clusters during the synthesis.…”

Single-Atom Metal Oxide Sites as Traps for Charge Separation in the Zirconium-Based Metal–Organic Framework NDC–NU-1000

“…Frequently, these reactive sites consist of metal atoms that are undercoordinated and, thus, can either exhibit catalytic properties themselves or can be further modified via a variety of synthetic transformations to support other metal-based species or organic ligand(s). , In the context of photocatalysis, the well-defined distances between light-sensing chromophores and catalytic sites are highly conducive toward precise characterization of the photoinduced charge and energy transfer in MOFs. Among some common approaches in the field are porphyrin-based linkers, which can support a single metal atom and allow reactants to approach axially, − encapsulation of noble metal single atoms, , and precise defect engineering, which allows for positioning of the catalytic sites in an optimal fashion; zirconium-based MOFs are particularly attractive for this approach as a result of the rich variety of linker coordination motifs and potential catalyst grafting sites available to the Zr 6 O 4 (μ 3 -OH) 4 -derived node. , …”