Atomic-level understanding of the active sites and transformation mechanisms under realistic working conditions is a prerequisite for rational design of high-performance photocatalysts. Here, by using correlated scanning fluorescence X-ray microscopy and environmental transmission electron microscopy at atmospheric pressure, in operando, we directly observe that the (110) facet of a single Cu₂O photocatalyst particle is photocatalytically active for CO₂ reduction to methanol while the (100) facet is inert. The oxidation state of the active sites changes from Cu(i) towards Cu(ii) due to CO₂ and H₂O co-adsorption and changes back to Cu(i) after CO₂ conversion under visible light illumination. The Cu₂O photocatalyst oxidizes water as it reduces CO₂. Concomitantly, the crystal lattice expands due to CO₂ adsorption then reverts after CO₂ conversion. The internal quantum yield for unassisted wireless photocatalytic reduction of CO₂ to methanol using Cu₂O crystals is ~72%.

在实际工作条件下对活性位点和转化机理的原子级了解是合理设计高性能光催化剂的先决条件。在这里，通过在大气压力下使用相关的扫描荧光X射线显微镜和环境透射电子显微镜，在操作中，我们直接观察到单个Cu ₂ O光催化剂颗粒的（110）小面具有光催化活性，可将CO ₂还原为甲醇，而（100）方面是惰性的。活性位点的氧化态由于CO ₂和H ₂ O的共吸附而从Cu（i）变为Cu （ii），然后又变回Cu（i）），然后在可见光照射下进行CO ₂转化。Cu ₂ O光催化剂在还原CO _2时将水氧化。伴随地，由于CO ₂吸附，晶格膨胀，然后在CO ₂转化后恢复。使用Cu ₂ O晶体进行无辅助的无线辅助光催化将CO ₂还原为甲醇的内部量子产率为〜72 ％。