Trade-Off of Metal Sites in Fe–Ni Bimetal Metal–Organic Frameworks for Efficient CO2 Photoreduction with Nearly 100% CO Selectivity

This work disclosed the trade-off effect of two metal sites, which display distinct, key functionalities in naturally occurring and artificial catalysts for developing an advanced CO2 reduction system. To exploit the metal-organic frameworks (MOFs) as advanced catalysts, we prepared a series of Prussian blue analogues (FeNix PBAs) of tunable Ni/Fe molar ratio without changing the oxidation state of Fe and Ni for use as a photocatalyst in the CO2 reduction reaction (CRR). The FeNi0.66 PBA gives a superior CO yield rate (14.28 mmol·g-1·h-1) with nearly 100% CO selectivity, but the PBA would be basically CRR-inactive without either Ni or Fe. Experimental and calculation studies demonstrate that Fe and Ni display distinct functionalities. Specifically, Fe is an efficient mediator that boosts the electron transfer both from the photosensitizer to FeNix PBA and from FeNix PBA to CO2, and Ni serves as the active site for CO2 adsorption and reduction. Intriguingly, when there is already sufficient Ni in the catalyst, further increase of the Ni content gives marginal gains in the CO2 adsorption affinity that cannot offset the weakened electron transfer due to the Ni excess. The findings can help advance the design of bimetallic MOF catalysts that mimic naturally occurring bimetallic catalysts.