Urea oxidation reaction (UOR) is known as a central reaction in the area of clean energy storage and conversion, urea fuel cells, de-ureation of wastewater, artificial kidney and etc. However, UOR suffers from its sluggish reaction kinetics in the conventional UOR mechanisms, which greatly impedes its practical applications. A research team (Peng group) from Fudan University, for the first time, has reported a novel lattice-oxygen-involved UOR mechanism, which has faster reaction kinetics than the conventional UOR mechanisms. Their research was published on October 11th, 2019 in Angewandte Chemie, International Edition.

Previous studies showed that nickel oxides exhibited higher catalytic activity for UOR than other metals and metal oxides in alkaline media, where Ni³⁺ sites were identified as the catalytic active sites. Besides, due to the unfavorably strong binding of Ni³⁺ sites with the *COO intermediates, the CO₂ desorption step was proven as the rate-determining step (RDS). Breakthroughs in facilitating the CO₂ desorption in RDS are desired to develop high-performance UOR catalysts.

Combining density functional theory studies, 18O isotope-labeling mass spectroscopy and in-situ infrared spectroscopy, we find that lattice oxygen is directly involved in transforming *CO to CO₂ and accelerating the UOR rate. The resultant Ni⁴⁺ catalyst on glassy carbon electrode exhibits a high current density of 264 mA/cm² at 1.6 V versus reversible hydrogen electrode, and the turnover frequency of Ni⁴⁺ active sites towards UOR is 5-times higher than that of Ni³⁺ active sites.

The electronic structure of Ni⁴⁺ (NiClOH-derived, NiClO-D) and Ni³⁺ (Ni(OH)^2- derived, NiOH-D) catalysts were probed and confirmed using X-ray absorption fine structure spectroscopy measurements at 1W1B-XAFS station of Beijing Synchrotron Radiation Facility (BSRF). The higher oxidation state of Ni active sites in Ni⁴⁺ catalyst and its endowing enhanced Ni-O covalency, trigger the activation of lattice oxygen to couple with *CO intermediate, and thus enable a lattice-oxygen-involved reaction pathway. The increased oxidation state of metal sites not only governs the catalytic activity but also the reaction mechanism, where lattice oxygen can be activated and promote new reaction pathways.

Article:

Longsheng Zhang, Liping Wang, Haiping Lin, Yunxia Liu, Jinyu Ye, Yunzhou Wen, Ao Chen, Lie, Wang, Fenglou, Ni, Zhiyou Zhou, Shigang Sun, Youyong Li*, Bo Zhang* and Huisheng Peng*. A lattice-oxygen-involved reaction pathway boosting urea oxidation. Angew. Chem. Int. Ed., 2019, 58, 16820.