Synergy of Oxygen Vacancies and Ni Single Atoms Toward Efficient Urea Photosynthesis from CO2 and N2

The urchin-like Ni₁/TiO_{2- x} catalyst with atomically dispersed Ni and Ov dual sites enables efficient urea synthesis by synergistically activating N₂ and CO₂. The Ni sites facilitate N₂ adsorption, while Ov activates CO₂. The CO₂ reduction product (i.e., CO) can migrate to Ni sites, coupling with *N₂ to form *NCON*, converting to urea. This dual-site mechanism lowers barriers and boosts kinetics, offering a green urea synthesis route.

Abstract

The direct photochemical synthesis of urea under mild conditions presents considerable promise. Nevertheless, the photocatalytic urea synthesis is severely restricted by the co-adsorption and activation of inert CO₂ and N₂ molecules. Herein, an ingenious design of a high-performance Ni₁/TiO_{2- X} catalyst by strategically coupling Ni single atoms (Ni SAs) and oxygen vacancies (Ov) onto urchin-like TiO₂ architecture. By virtue of the unique synergistic catalysis between atomically dispersed Ni species and precisely engineered Ov sites, the Ni₁/TiO_{2- X} catalyst achieves a remarkable urea production rate of 15.73 µmol g_cat. ⁻¹h⁻¹. Further mechanistic studies reveal that the atomically dispersed Ni sites facilitate N₂ adsorption and activation, generating *N₂ species, while the adjacent Ov sites activate CO₂ to form *CO intermediates. More intriguingly, the *CO species can migrate from the Ov site to the nearby Ni active centers, where they spontaneously undergo thermodynamic coupling with *N₂ to form a “tower-like” urea precursor (*NCON* intermediate), subsequently converting to urea. The present work establishes a dual-active-site mechanism, comprising isolated Ni centers and adjacent Ov sites, which synergistically lowers the activation barrier for urea photosynthesis and accelerates reaction kinetics, and pioneers a transition strategy for the environmentally friendly and efficient synthesis of high-value products.

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