Catalytic NO Oxidation to NO₂ under Industrial Nitric Acid Conditions: An Operando IR Spectroscopic Study

The increasing global demand for food is driving the need for more efficient and sustainable production of nitrogen-based fertilizers. Nitric acid is an important raw material for nitrate fertilizers and is mainly produced through the Ostwald process. Although this process is well established, some of its steps are still energy intensive. One important stage is the oxidation of NO to NO₂, which currently occurs mainly in the gas phase before the absorption step. Using a catalyst for this reaction could increase the reaction rate, reduce the required residence time, and improve the overall efficiency of nitric acid production. This study was carried out within the collaboration between the University of Messina and Casale and focuses on the catalytic oxidation of NO under conditions relevant to nitric acid production. CeO₂ supported noble metals and high entropy oxides were investigated as potential catalysts. Their activity and stability were evaluated in the 300–400 °C range using high NO feeds under both dry and wet conditions. The influence of temperature, pressure, and space velocity was also examined. Operando IR spectroscopy was used to follow the reaction under working conditions and to identify gas phase species and surface intermediates involved in NO oxidation. Among the CeO₂-supported noble metals, Ru/CeO₂ showed the best performance, reaching approximately 76% NO conversion at 350 °C, followed by Pd/CeO₂ and Ag/CeO₂. The presence of water caused only a moderate decrease in activity, while increasing the pressure to 4 bar significantly enhanced NO conversion. High entropy spinel oxides also showed promising behaviour, with the multicomponent spinel reaching about 74% conversion at 300 °C and maintaining stable activity in long term tests. A combination of XRD, BET, Raman spectroscopy, H₂-TPR, electron microscopy, In-Situ IR, and thermal analysis was used to relate catalytic performance to structural and surface properties. Overall, this work demonstrates that Ru/CeO₂ and high entropy oxide catalysts can promote NO oxidation under industrial nitric acid production conditions.