Phonon resonance enabled Cu(I) valence pinning in hydroxyapatite for photothermal CO₂ hydrogenation

Cu-based catalysts attract considerable attention because of their exceptional CO₂ photo-/electro-/thermo-reduction capacities, where Cu(I) is generally treated as the active species. However, a significant problem hindering the large-scale applications of Cu-based catalysts is the inactivation of Cu(I) via irreversible redox to Cu(II)/Cu(0). This study proposes a Cu(I) valence pinning method based on hydroxyapatite (HAP). Experimental and theoretical studies demonstrate that the phonon resonance among the Cu(I) ions, their adjacent heteroatoms, and the intermediates adsorbed at the Cu(I) sites yields Cu(I) valence electrons in their lowest energy states. Thus, Cu(I) is stabilized, stable, and efficient photothermal CO₂ hydrogenation is promoted. However, because of the change of Cu(I) coordination environment during the CO₂ hydrogenation reaction, Cu(I) ions migrate into the bulk phase, leading to activity attenuation. Nevertheless, Cu(I) ions can be pulled out to the surface of HAP under the oxidative humid air condition, and the catalytic activity can be easily recovered. Thus, we propose a simple cyclic reaction/regeneration process. This enables the Ca₅(FeCuCe)₅-HAP catalyst to achieve the CO yield of 402.8 mmol g^-1 h^-1 and a CO₂ conversion rate of 27.7%, which is close to the thermodynamic equilibrium. This catalyst also displays a selectivity of approximately 100% and cycle stability of 156 h at 500 °C under a pressure of 1 atmospheric. Our study provides a viable method for the scale applications of Cu(I) based catalysts in the "negative carbon" industries.