Construction of New Active Sites: Cu Substitution Enabled Surface Frustrated Lewis Pairs over Calcium Hydroxyapatite for CO$_{2}$ Hydrogenation

Abstract

Calcium hydroxyphosphate, Ca$_{10}$(PO$_{4}$)$_{6}$(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 Ca$^{2+}$ ions by Cu$^{2+}$ ions especially at very low levels of exchange created new analogs of molecular surface frustrated Lewis pairs (SFLPs) in Cu$_{x}$Ca$_{10-x}$(PO$_{4}$)$_{6}$(OH)$_{2}$, thereby boosting its performance metrics in heterogeneous CO$_{2}$ 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 Cu$^{2+}$ and Lewis basic OH$^{-}$. The photocatalytic pathway proceeds through a formate reaction intermediate, which is generated by the reaction of CO$_{2}$ 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 CO$_{2}$ reduction technologies