Metal-organic frameworks as photocatalysts in energetic and environmental applications

Abstract

Metal-organic frameworks (MOFs) are an exciting new class of porous materials with great potential for photocatalytic applications in the environmental and energy sectors. MOFs provide significant advantages over more traditional materials when used as photocatalysts due to their high surface area, adaptable topologies, and functional ability. In this article, we summarize current developments in the use of MOFs as photocatalysts for a variety of applications, such as CO₂ reduction, water splitting, pollutant degradation, and hydrogen production. We discuss the fundamental properties of MOFs that make them ideal for photocatalytic applications, as well as strategies for improving their performance. The opportunities and challenges presented by this rapidly expanding field are also highlighted.

Keywords: Metal-organic frameworks; photocatalysis; pollutant degradation; solar energy; solar fuel production.

Figure 1

(a). The number of photocatalytic MOF publications per year (2014–2022). (b) Publications percent of MOFs for photocatalytic applications areas. Data obtained from the Web of Science as of April 2023.

Figure 2

Approaches for photocatalytic activity in MOFs: a) the organic linker harvests the light and LMCT is promoted; b) the MOF is used as a container of a light absorbing catalyst; c) charge transfer occurs between the MOF and the encapsulated catalyst [51].

Figure 3

Scheme for the preparation of a MOF [53].

Figure 4

A schematic view of MOF synthesis, properties, and applications.

Figure 5

The schematic representation of MOF acting as a photocatalyst in an aqueous solution [57].

Figure 6

The experiment studied the degradation of R6G (a dye) in a solution under various conditions, comparing the results with and without light irradiation. The different conditions tested were as follows: (a) R6G/UV light without a catalyst (denoted as —■—a). (b) R6G in the dark with compound 1. (c) R6G in the dark with compound 2. (d) R6G with compound 1 under UV light. (e) R6G with compound 2 under UV light [81].

Figure 7

Crystal structures of Cd (II) complexes (1–4) and the photocatalytic degradation of MB and MO solution under UV-light irradiation 1–4 [82].

Figure 8

Mechanism of excitation pathways for photocatalytic reduction of Cr(VI) over NH₂–MIL-88B (Fe) [94].

Figure 9

(a) Photocatalytic performance of various MOFs activated under daylight [108].

Figure 10

Schematic representation of Fe-based MILs for degradation of tetracycline [129].

Figure 11

The photocatalytic water-splitting process and its working principle [36].

Figure 12

Common semiconductors and their band gap energy [36].

Figure 13

(a) Metalation in UiO-66(Zr) to obtain multimetallic clusters (b) photocatalytic activity of multimetallic clusters in water splitting [36,132].

Figure 14

Schematic representation of NU-1000 that has been modified by adding NiS_x functionality by atomic layer deposition. In the photocatalytic reaction, a pyrene-based linker acts as a UV sensitizer, transferring an electron to the NiSx-functionalized node, which then reduces protons or water to generate H₂ [134].

Figure 15

Schematic illustration of photocatalytic CO₂ reduction over MOFs [154].

Figure 16

(a) The structure of a porphyrin-Co unit that is capable of CO₂ adsorption (b) CO₂ activation energy barrier [157].

Figure 17

ZrPP-1-Co structure and CO time courses from visible-light-irradiated CO₂ photoreduction with ZrPP-1-M catalysts [158].

Figure 18

Schmatic crystal structures of photocatalytic MOFs (a) MIL-101(Fe) (b) Cd-TBAPy (c) Bi-mna [169].

Figure 19

(a) Perspective view of Cu(I) coordination environment (b) The CuI chains are connected via the BPEA ligand, (c) View of 2D sheets (d) the stacking of the 2D layers that the layers are arranged on top of each other to form a 3D structure. (e) The plot of (αhv)² versus photon energy (hv) which may indicate that the material has photovoltaic properties with band gap (Eg = 2.13eV). (f) (a) Comparative photocatalytic H₂ evolution over 1, BPEA ligand, and CuI [176].

Figure 20

(a) Structure of MOF-253-Pt. (b) Photocatalytic H₂ production of MOF-253, MOF-253-Pt and Pt(bpydc)Cl₂. (c) Photocatalytic H₂ production mechanism of MOF-253-Pt [144].