The synthesis of ammonia has been traditionally accomplished through the Haber-Bosch process that usually employs iron-based catalysts to fixate nitrogen (N₂) with hydrogen (H₂) under rigorous reaction conditions (i.e., 15-25 MPa and 673-873 K), consuming more than 1% of global energy supply.  Photocatalysis, in which semiconductor directly harvests solar energy to generate electron-hole pairs, may supply energetic electrons for N₂ activation and thus offer an appealing approach to the fixation of N₂ to ammonia.  However, it remains a grand challenge to develop highly active photocatalysts for N₂ fixation, as N₂ molecule exhibits ultrahigh stability due to the extremely large bond energy of 940.95 kJ mol^-1.  Most recently, the research group led by Prof. XIONG Yujie at the University of Science and Technology of China in collaboration with Prof. WU Xiaojun (theoretical simulation) has discovered that doping can refine the defect state of catalyst to significantly improve the activity for N₂ activation, based on the defect engineering on metal oxide photocatalysts.  This work has been published in Journal of the American Chemical Society (J. Am. Chem. Soc. DOI: 10.1021/jacs.8b02076).

Given the ultrahigh chemical stability of N₂ molecule, in terms of thermodynamics, the molecular activation is typically considered as the prerequisite for N₂ reduction.  It have been reported that defect engineering can introduce coordinately unsaturated sites for N₂ chemisorption. The localized electrons are transferred from catalytic sites into the anti-bonding π-orbitals of N₂, facilitating the activation and bond dissociation.  However, the overall performance of defect-engineered photocatalysts is still limited by some bottlenecks.  Firstly, the molecular adsorption and electron transfer should be further modulated to improve the activity for N_2­ activation.  Secondly, the relaxation of photoexcited electrons from conduction band to defect band, which causes the energy loss of electrons, should be alleviated.

Illustration for the light-driven N₂ fixation to ammonia using Mo-doped W₁₈O₄₉ ultrathin nanowires as photocatalyst (Courtesy of XIONG Yujie and Journal of the American Chemical Society)

To address the grand challenge, Xiong research group has doped Mo element into the defect sites on W₁₈O₄₉ catalyst, achieving the significant enhancement for light-driven N₂ fixation to ammonia.  Combining synchrotron radiation-based X-ray spectroscopy characterizations, in-situ infrared spectroscopy detection and theoretical simulations, the researchers have revealed the refinement of defect states through Mo doping.  The doped Mo species elevate defect-band center towards the Fermi level to alleviate the energy relaxation of electrons so that more energetic electrons can be preserved for N₂ reduction.  Moreover, the substitution of Mo species for W atoms can provide heterogeneous Mo-W dimer sites for the chemisorption of N₂ molecules, at which the two N atoms are more polarized with larger charge difference and adsorption energy.  Meanwhile, Mo doping enhances the metal-oxygen covalency, in which the improved O-2p character in metal d-orbitals can facilitate the electron transfer from the active sites to N₂ molecules.  As a result, the multi-synergetic effects induced by subtle Mo doping enable a substantially enhanced performance for photocatalytic N₂ fixation.  This work provides fresh insights into the design of photocatalyst lattice for N₂ fixation, and reaffirms the versatility of subtle electronic structure modulation in tuning catalytic activity.

Profs. Li Song, Junfa Zhu and Zeming Qi made important contributions to the synchrotron radiation-based X-ray spectroscopy, photoelectron spectroscopy characterizations and in-situ diffuse reflectance infrared Fourier-transform spectroscopy characterizations in this work, respectively.  This work was financially supported by the 973 Program, NSFC and CAS Key Research Program of Frontier Sciences.

Publication link: https://pubs.acs.org/doi/10.1021/jacs.8b02076