Engineering Single-Atom Sites with the Irving-Williams Series for the Simultaneous Co-photocatalytic CO 2 Reduction and CH 3 CHO Oxidation.

The bonding effects between 3d transition-metal single sites and supports originate from crystal field stabilization energy (CFSE). The 3d transition-metal atoms of the spontaneous geometrical distortions, that is the Jahn-Teller effect, can alter CFSE, thereby leading to the Irving-Williams series. However, engineering single-atom sites (SASs) using the Irving-Williams series as an ideal guideline has not been reported to date. Herein, alkynyl-linked covalent phenanthroline frameworks (CPFs) with phenanthroline units are developed to anchor the desired 3d single metal ions from d 5 to d 10 (Mn 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+ ). The Irving-Williams series was employed to accurately predict the bonding effects between 3d transition-metal atoms and phenanthroline units. To verify this, theoretical calculations and experimental results reveal that Cu-SASs/CPFs exhibits higher stability and faster charge-transfer efficiency, far surpassing other metal-SASs/CPFs. As expected, Cu-SASs/CPFs demonstrates a high photoreduction of CO 2 -to-CO activity (~30.3 μmol ⋅ g -1  ⋅ h -1 ) and an exceptional photooxidation of CH 3 CHO-to-CH 3 COOH activity (~24.7 μmol ⋅ g -1  ⋅ h -1 ). Interestingly, the generated *O 2 - is derived from the process of CO 2 reduction, thereby triggering a CH 3 CHO oxidation reaction. This work provides a novel design concept for designing SASs by the Irving-Williams to regulate the catalytic performances.