Effects of doping Mn, Cu and Fe oxides on polyhedron CeO₂ catalyst during NH₃-SCR reaction

Background: Ceria (CeO₂) was considered as one of the most promising catalysts due to its oxygen storage capacity and redox property. However, the low activity of CeO₂ catalyst in SCR reaction limited its further application on industrial processes. Methods: Transition metal (Mn, Cu and Fe) oxides were impregnated on nanopolyhedron CeO₂ (CeO₂-NP) catalysts to investigate the SCR performance. Significant findings: The Mn-CeO₂-NP catalyst displayed the highest NO_x conversion (ca. 70% conversion at 300 °C), followed by the Cu-CeO₂-NP catalyst (ca. 65% conversion at 300 °C) and then the Fe-CeO₂-NP catalyst (ca. 45% conversion at 300 °C). The results of H₂-TPR showed that Mn-CeO₂-NP and Cu-CeO₂-NP catalysts were more efficient in reducing hydrogen at lower temperature than that of Fe-CeO₂-NP catalysts. Based on XPS measurements, it was determined that the O_α/(O_α+O_β) ratio was Mn-CeO₂-NP (ca. 65%)> Fe-CeO₂-NP (ca. 59%)> Cu-CeO₂-NP (ca. 55%) catalyst. It was also shown by Raman spectroscopy that the Mn-CeO₂-NP catalyst had a higher concentration of oxygen vacancies than that of the Cu-CeO₂-NP and Fe-CeO₂-NP catalysts. Meanwhile, more Brønsted acid sites, Lewis acid sites, nitrate and nitrites species were found in the Mn-CeO₂-NP catalyst as seen by in situ DRIFT spectra.