Construction of New Active Sites: Cu Substitution Enabled Surface Frustrated Lewis Pairs over Calcium Hydroxyapatite for CO2 Hydrogenation

Calcium hydroxyphosphate, Ca-10(PO4)(6)(OH)(2), is commonly known as hydroxyapatite (HAP). The acidic calcium and basic phosphate/hydroxide sites in HAP can be modified via isomorphous substitution of calcium and/or hydroxide ions to enable a cornucopia of catalyzed reactions. Herein, isomorphic substitution of Ca2+ ions by Cu2+ ions especially at very low levels of exchange created new analogs of molecular surface frustrated Lewis pairs (SFLPs) in CuxCa10-x(PO4)(6)(OH)(2), thereby boosting its performance metrics in heterogeneous CO2 photocatalytic hydrogenation. In situ Fourier transform infrared spectroscopy characterization and density functional theory calculations provided fundamental insights into the catalytically active SFLPs defined as proximal Lewis acidic Cu2+ and Lewis basic OH-. The photocatalytic pathway proceeds through a formate reaction intermediate, which is generated by the reaction of CO2 with heterolytically dissociated H-2 on the SFLPs. Given the wealth of information thus uncovered, it is highly likely that this work will spur the further development of similar classes of materials, leading to the advancement and, ultimately, large-scale application of photocatalytic CO2 reduction technologies.

Automated summary

The study explored the use of isomorphic substitution to create new analogs of molecular surface frustrated Lewis pairs (SFLPs) in CuxCa10-x(PO4)6(OH)2, enhancing its performance in heterogeneous CO2 photocatalytic hydrogenation. The characterized SFLPs were found to have proximal Lewis acidic Cu2+ and Lewis basic OH- characteristics, and the photocatalytic pathway proceeds through a formate reaction intermediate.