Construction of Surface Synergetic Oxygen Vacancies on CuMn₂O₄ Spinel for Enhancing NO Reduction with CO

The effectiveness of surface synergetic oxygen vacancy (SSOV) on a catalyst has been proposed in the selective reduction of NO to N-2 by CO. In this work, we prepared fresh CuMn2O4 spinel catalyst using the freeze-assisted sol-gel method, and then engineered SSOVs through CO pretreatment (CO-CuMn2O4) at 250 degrees C. The catalytic performance of the CO-CuMn2O4 catalyst showed significant improvement, attributed to the presence of SSOVs, in comparison to that of the fresh CuMn2O4 sample. Additionally, our findings elucidated the limited reactivity of surface oxygen vacancies (SOVs) on a single metal oxide, emphasizing the crucial role played by SSOVs. Experimental results, including NO temperature-programmed desorption-mass spectrometry and in situ diffuse reflectance infrared Fourier transform spectroscopy, provided further insights by suggesting that SSOVs facilitate the formation of N2O and its subsequent decomposition into N-2. Density functional theory calculations have unveiled the pivotal role of SSOV in stabilizing the nitrogen atom derived from gaseous NO, facilitating the NO + CO -> N* + CO2 reaction. Notably, the energy barrier for this process is only 0.54 eV, which is the rate-determining step of the NO + CO reaction. In stark contrast, this reaction scarcely occurs on the SOVs of single CuO and Mn2O3 surfaces. Furthermore, the presence of SSOVs considerably lowers the energy barrier for the conversion of N2O to N-2, with a minimal barrier of 0.12 eV. In contrast, the reduction of N2O by CO without SSOV assistance necessitates a significantly higher energy barrier of 2.77 eV. Extending our investigation, we engineered SSOVs on the CuFe2O4 spinel catalyst and observed similar SSOV-mediated effects in the NO + CO reaction. Our research offers a comprehensive understanding of atomic-level role of SSOV, thereby offering valuable insights for the design of efficient NO + CO catalysts.