Ternary Metalation in a Copper-Covalent Organic Framework for Tandem Photocatalytic CO Reduction with High Selectivity.

Developing multifunctional photocatalysts with efficient synergistic effects for selective photocatalytic CO reduction is sought-after yet challenging. Herein, a tandem photocatalytic strategy has been developed through sequential post-synthetic metalation (SPSM) of variable Ru/Co-sites into a trinuclear Cu-based COF, Cu-BPY-COF, to construct a series of tri-metallized covalent-organic frameworks (COFs), Cu-BPY-COF(Ru/Co)-X (X = 1, 2, or 3, corresponding to the Ru:Co molar ratios of 1/39.6, 1/19.6, or 1/12.7, respectively), for photocatalytic CO methanation in high selectivity. The introduction of a small fraction of Ru-sites confers the framework with enhanced visible-light response, while the Co-sites immobilized in the bipyridine units significantly boost the generation of CO to enrich its concentration around the active trinuclear Cu-sites, which enables dramatically improved photocatalytic CO reduction activity toward CH. Significantly, the optimized Cu-BPY-COF(Ru/Co)-2 showcases exceptional photocatalytic activity with a CH evolution rate of 31.5 µmol g h and selectivity of 95%, 15.0 times higher than that of Cu-BPY-COF(Ru) merely introducing Ru-photosensitizer. The underlying photocatalytic mechanism has been studied to unveil how the ternary Ru/Co/Cu-sites collaborate to result in highly efficient and selective CO-to-CH reduction, shedding light on the design of highly selective CO methanation photocatalysts by leveraging the synergistic effect of multiple active centers within heterometallic covalent-organic frameworks (HM-COFs).