Solar energy is an alternative, sustainable energy source for mankind. Finding a convenient way to convert sunlight energy into chemical energy is a key step towards realizing large-scale solar energy utilization. Owing to their structural regularity and synthetic tunability, metal-organic frameworks (MOFs) provide an interesting platform to hierarchically organize light-harvesting antennae and catalytic centers to achieve solar energy conversion. Such photo-driven catalytic processes not only play a critical role in the solar to chemical energy conversion scheme, but also provide a novel methodology for the synthesis of fine chemicals. In this review, we summarize the fundamental principles of energy transfer and photocatalysis and provide an overview of the latest progress in energy transfer, light-harvesting, photocatalytic proton and CO2 reduction, and water oxidation using MOFs. The applications of MOFs in organic photocatalysis and degradation of model organic pollutants are also discussed.

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Simply mixing a Cu(II) salt and 6,6'-dihydroxy-2,2'-bipyridine (H2L) in a basic aqueous solution afforded a highly active water oxidation catalyst (WOC). Cyclic voltammetry of the solution at pH = 12-14 shows irreversible catalytic current with an onset potential of ~0.8 V versus NHE. Catalytic oxygen evolution takes place in controlled potential electrolysis at a relatively low overpotential of 640 mV. Experimental and computational studies suggest that the L ligand participates in electron transfer processes to facilitate the oxidation of the Cu center to lead to an active WOC with low overpotential, akin to the use of the tyrosine radical by Photosystem II to oxidize the CaMn4 center for water oxidation.

The rational design of highly efficient, low-cost, and durable electrocatalysts to replace platinum-based electrodes for oxygen reduction reaction (ORR) is highly desirable. Although atomically dispersed supported metal catalysts often exhibit excellent catalytic performance with maximized atom efficiency, the fabrication of single-atom catalysts remains a great challenge because of their easy aggregation. Herein, a simple ionothermal method was developed to fabricate atomically dispersed Fe−N x species on porous porphyrinic triazine-based frameworks (FeSAs/PTF) with high Fe loading up to 8.3 wt %, resulting in highly reactive and stable single-atom ORR catalysts for the first time. Owing to the high density of single-atom Fe−N 4 active sites, highly hierarchical porosity, and good conductivity, the as-prepared catalyst FeSAs/PTF-600 exhibited highly efficient activity, methanol tolerance, and superstability for oxygen reduction reaction (ORR) under both alkaline and acidic conditions. This work will bring new inspiration to the design of highly efficient noble-metal-free catalysts at the atomic scale for energy conversion.