Reactivity and Selectivity of Photocatalytic Nitrate Reduction Reaction to Ammonia: Pt, Pd, Ir, and Rh Single Atoms on p-Type TiO2 Nanotubes

Abstract We present a systematic study of noble metal (Pt, Pd, Ir, and Rh) single-atom cocatalysts decorated on p-type and quantum-confined TiO2 nanotubes for photocatalytic nitrate reduction to ammonia. All single-atom cocatalysts were prepared through a reactive deposition using different metal precursors, resulting in a single-atom oxidation state (δ+) of δ ≈ 2. At a similar loading concentration (0.2–0.3 at%) and in the absence of any sacrificial agent, Pd single atoms exhibit the highest element-specific activity for ammonia production, while Pt single atoms display a prominent photocatalytic activity with a strong tendency of nitrite reduction toward ammonia. Rh single atoms achieve the highest ammonia selectivity, whereas Ir single atoms predominantly produce nitrite due to inhibited subsequent reduction. Comparisons with single atoms deposited on conventional TiO2 (P25) and with nanoparticles supported on the nanotubes reveal that both the semiconductor properties (quantum confinement effect and p-type conductivity) and the speciation of the cocatalyst (single atom) can significantly enhance activity and selectivity. These findings aid in establishing design principles for different single-atom catalysts toward efficient and selective photocatalytic nitrogen valorization.