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

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). MnOx-CeO2 catalysts synthesized by a co-precipitation method show 90% NOx conversion at 120-180 degrees C and ZrPO4-CeO2 catalysts were prepared by loading zirconium phosphate on CeO2, which present over 90% NOx conversion at 250-450 degrees 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 MnOx-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 N-2 + H2O at 120 degrees C, in which process NH4NO3 can be considered as the important intermediate of the NH3-SCR reaction at 120-180 degrees C. At temperatures below 90 degrees C, NH4NO3 on MnOx-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 NOx adsorption lead to the strong interaction between ads-NH4+ and nitrates, resulting in the decomposition temperature of NH4NO3 elevating to 250 degrees C, which is also the threshold of the catalyst's temperature window. At temperatures over 250 degrees C, NH4NO3 is an intermediate on the ZrPO4-CeO2 catalyst in participating the NH3-SCR reaction.