Reconstruction of Highly‐Defective MgO and Exceptional Photochemical Activity on CO₂ Upgrade in Aqueous Solution

Abstract Defects on metal oxide have attracted extensive attention in photo‐/electrocatalytic CO₂ reduction. Herein, porous MgO nanosheets with abundant oxygen vacancies (V_os) and three‐coordinated oxygen atoms (O_3c) at corners are reported, which reconstruct into defective MgCO₃·3H₂O exposing rich surface unsaturated ‐OH groups and vacancies to initiate photocatalytic CO₂ reduction to CO and CH₄. In consecutive 7‐cycle tests (each run for 6 h) in pure water, CO₂ conversion keeps stable. The total production of CH₄ and CO attains ≈367 µmol g_cata⁻¹ h⁻¹. The selectivity of CH₄ gradually increases from ≈3.1% (1^st run) to ≈24.5% (4^th run) and then remains unchanged under UV‐light irradiation. With triethanolamine (3.3 vol.%) as the sacrificial agent, the total production of CO and CH₄ production rapidly increases to ≈28 000 µmol g_cata⁻¹ in 2 h reaction. Photoluminescence spectra reveal that V_os induces the formation of donor bands to promote charge carrier seperation. A series of trace spectra and theoretical analysis indicate Mg‐V_o sites in the derived MgCO₃·3H₂O are active centers, which play a crucial role in modulating CO₂ adsorption and triggering photoreduction reactions. These intriguing results on defective alkaline earth oxides as potential photocatalysts in CO₂ conversion may spur some exciting and novel findings in this field.