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

Jiuli Guo,Geoffrey A. Ozin,Yan Liang,Joel Y. Y. Loh,Rui Song,Ying Dai,Lu Wang,Nazir P. Kherani,Christian Kübel,Wu Wang

doi:10.1002/advs.202101382

Abstract

Calcium hydroxyphosphate, Ca10(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 H2 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.

Highlights

In recent years, excessive CO2 emissions caused by fossil fuel combustion and automobile exhaust emissions have seriously disturbed the natural carbon cycle and exacerbated global warming
The synthesis of 0 and 0.5 mol% Cu-HAP is accomplished by the co-precipitation method. 0.5 mol% Cu-HAP displayed the very similar powder X-ray diffraction (PXRD) pattern of hexagonal HAP but a small shift to lower 2θ diffraction angles relative to those of 0 mol% Cu-HAP (Figure S1, Supporting Information), which signals the substitution of the larger ionic radius Ca2+ (1.0 Å) by smaller Cu2+ (0.73 Å).[5]
This conclusion is further supported by representative scanning transmission electron microscopy imaging in combination with energy-dispersive X-ray spectroscopy images of 0.5 mol%

Summary

Introduction

Excessive CO2 emissions caused by fossil fuel combustion and automobile exhaust emissions have seriously disturbed the natural carbon cycle and exacerbated global warming. Kübel Technical University Darmstadt (TUDa) Department of Materials & Earth Sciences Alarich-Weiss-Straße 2, Darmstadt 64287, Germany. To understand if the Cu2+ in HAP structure has any relevance in the reaction, it became apparent that low levels of exchange (0 and 0.5 mol%) Cu-HAP might be advantageous to avoid CuO in flow CO2 conversion to products. These results revealed a CO production rate of 215 μmol gcat−1 h−1 with no CH4 being detected for 0.5 mol% Cu-HAP, which is around 61 orders photoactivity enhancement of the reverse water gas shift (RWGS) reaction compared to the pristine 0 mol% Cu-HAP. Even more, was the new Cu2+/OH− SFLPs in 0.5 mol% Cu-HAP, composed of coordinately unsaturated copper sites, adjacent to an oxygen vacancy and a hydroxide group, enable the heterolysis of H2 and reaction with CO2 to form CO through a formate reaction intermediate driven photocatalytically

Results and Discussion

Conclusion

Data Availability Statement