Charge Separation and/or Hot Spots: Clarification of Efficient CO ₂ Reduction over Ru–Ni Nanoparticles Compared to Photocatalysis on Ru–Ni–ZrO ₂ Composites

The true reaction pathway and the catalytic role responsible for it remain uncertain in photocatalysis, where charge separation, hot spots, and energetic modulation of the ground and excited states are involved. Herein, a Ru atom was embedded in a ∼1.6 nm Ni nanoparticle dispersed on ZrO₂. The ¹³CO₂ photoreduction rate to ¹³CH₄ showed a negligible change of ∼26 μmol h⁻¹ gcat⁻¹ upon adding Ru (1.0 wt %) to the Ni (10 wt %)-ZrO₂ photocatalyst under a 295 K water bath for cooling irradiated by UV-visible light at 568 mW cm⁻². By contrast, the rate using the Ru-Ni-ZrO₂ catalyst exceeded that of Ni-ZrO₂ by >2.7× (>7.9 mmol h⁻¹ gcat⁻¹) when the 295 K water bath for catalysts was not applied. Extended X-ray absorption fine structure analysis revealed that the hot spot Ni temperature increased from 295 to 399 ± 29 K under 654 mW cm⁻² irradiation, even with the 295 K ethylene glycol bath for cooling, confirming that multiple hydrogenations occurred on Ni for COH species transferred from the O vacancy sites at the ZrO₂ surface. In the absence of the 295 K water bath, CO₂ was directly adsorbed on the RuNi₂ site, and its dissociation into CO and O species proceeded with a low activation energy of 0.45 eV, enabling a photothermal pathway, as supported by DFT and FTIR analyses. These results highlight the importance of monitoring the hot spot temperature in photocatalysts to identify the active site combined with redox sites on the semiconductor, driven by charge separation under UV-visible light irradiation.