With the rapid development of global industry, urea-rich wastewater has become the main pollution source of water eutrophication. Through natural decomposition, it might be converted to nitrate and toxic ammonia, posing a threat to human health.

Chundong Wang's team from Huazhong University of Science and Technology and Yujie Xiong's team from University of Science and Technology of China jointly designed a highly efficient dual-active site catalyst to convert urea into non-toxic nitrogen and carbon dioxide through urea oxide reaction (UOR).

The study was published in Research, the first Science Partner Journal recently launched by the American Association for the Advancement of Science (AAAS) in collaboration with the China Association for Science and Technology (CAST). 

While a few UOR catalysts already exist, the urea degradation of those catalysts are rarely reported. More importantly, in the commonly used inorganic materials, however, their rigid crystalline structures largely limit the tunability of active sites. Furthermore, key fundamental issues concerning the active sites (e.g., genuine origin of intermediates and catalytic activity) have not been elucidated so far, which calls for determination of the active intermediate species at the atomic level during the UOR process but is hindered by the complexity of catalyst surface structures.

The team met the challenge by creating a new catalyst by using Metal-organic frameworks (MOFs) with homogeneous microenvironment as platform to regulate active sites. The optimal NiMn0.14-BDC catalyst exhibits a low voltage of 1.317 V to deliver a current density of 10 mA cm-2, which outperforms monometallic Ni- and Mn-MOFs. Moreover, the urea degradation over the NiMn0.14-BDC supported on nickel foam (NiMn0.14-BDC/NF) reaches 97.65%, 96.71% and 81.87% in 0.0033 M, 0.033 M and 0.33 M urea solution, respectively, demonstrating that the catalyst possesses excellent potential for wastewater treatment. Based on the homogeneous microenvironment of NiMn-BDC, the theoretical model was further constructed to uncover the synergistic effect of bimetallic Ni-Mn centers in the UOR process. The Ni sites favor the urea absorption and the Mn sites serve as the highly energetic sites for generating key intermediates, thereby reducing the energy requirements for UOR. The authors hope that, this new insight of tuning active sits in MOF platform can offer train of thought for designing UOR catalysts at molecular level.    

Original source:

Xu, X., Deng, Q., Chen, H. C., Humayun, M., Duan, D., Zhang, X., ... & Xiong, Y. (2022). Metal-Organic Frameworks Offering Tunable Binary Active Sites toward Highly Efficient Urea Oxidation Electrolysis. Research, 2022. (https://doi.org/10.34133/2022/9837109)