Downsize Effect on Photocatalytic Syngas Production of a 2D Metal Organic Framework—Tuning the Selectivity From HER to CO ₂ RR via Dimensional Modulation and Mechanical Exfoliation of 2D Metal‐Organic Layers

Tuning the selectivity from hydrogen evolution reaction (HER) to CO 2 reduction reaction (CO 2 RR) is a challenge in heterogeneous photocatalytic CO 2 reduction by water as the proton source. The chemical inertness of CO 2 and multiple proton‐coupled electron transfer for CO 2 RR favor HER over CO 2 RR. To favor CO 2 RR, here we show the effect of downsizing the 2D Co‐containing metal–organic framework (MOF) [Co(pdc)(H 2 O) 2 ]·H 2 O ( 2D ) (pdc 2− = 2,5‐pyridinedicarboxylate) on its photocatalytic CO 2 RR performance. MOF 2D has been downsized in two ways: covalent bond breaking, leading to the 1D MOF [Co(pdc)(H 2 O) 2 ]·4H 2 O ( 1D ), and supramolecular bond breaking of the metal‐organic layers (MOLs) by liquid phase exfoliation ( 2D‐ex ). Structural analysis reveals that the Co II centers have an octahedral geometry in 2D and a square pyramidal geometry in 1D , resulting in open metal sites. X‐ray photoelectron spectroscopy (XPS) indicates that acetonitrile molecules are attached to the metal centers in 2D‐ex , replacing some coordinated water molecules. Photocatalytic activity shows that these materials produce syngas with a tunable range of CO:H 2 ratio depending on the structure. Thus, 2D produces a higher proportion of H 2 , whereas 1D and 2D‐ex show a CO‐enriched syngas. Theoretical studies show the relationship between the photocatalytic activity of these MOF and adsorption energies of CO 2 .