Decomposition behavior of ammonium nitrate on ceria catalysts and its role in the NH3-SCR reaction

Abstract

NH₄NO₃ species are proved to be important and stable intermediates in NH₃-SCR reactions on ceria catalysts. The decomposition behaviors of NH₄NO₃ were studied by Diffuse Reflectance Infrared Fourier Transformed Spectra (DRIFTS), Thermo Gravimetry/Differential Scanning Calorimetry (TG/DSC) and Mass Spectrometry (MS). MnO_x–CeO₂ catalysts synthesized by a co-precipitation method show 90% NO_x conversion at 120–180 °C and ZrPO₄–CeO₂ catalysts were prepared by loading zirconium phosphate on CeO₂, which present over 90% NO_x conversion at 250–450 °C. Decomposition of NH₄NO₃ on the ceria catalyst is determined by both the redox properties and acidity of the catalyst, which further influence the catalyst's temperature window. For the decomposition of NH₄NO₃ on the MnO_x–CeO₂ catalyst with higher redox properties but much lower acidity, nitrate species can react with NO to generate nitrite, then react with gaseous NH₃ to form NH₄NO₂ and finally decompose into N₂ + H₂O at 120 °C, in which process NH₄NO₃ can be considered as the important intermediate of the NH₃-SCR reaction at 120–180 °C. At temperatures below 90 °C, NH₄NO₃ on MnO_x–CeO₂ can be a poison blocking the active sites. While on the ZrPO₄–CeO₂ catalyst with lower redox properties but higher acidity, two distinct sites for NH₃ and NO_x adsorption lead to the strong interaction between ads-NH₄⁺ and nitrates, resulting in the decomposition temperature of NH₄NO₃ elevating to 250 °C, which is also the threshold of the catalyst's temperature window. At temperatures over 250 °C, NH₄NO₃ is an intermediate on the ZrPO₄–CeO₂ catalyst in participating the NH₃-SCR reaction.