Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions

Abstract

In this study, the ammonia-SCR process was investigated using 2.5wt.% Cu/BEA, 2.6wt.% Cu/SAPO-34 and 3.1wt.% Cu/SSZ-13. Several reactions such as NO oxidation, ammonia oxidation, standard SCR, fast SCR and NO2 SCR were studied to understand the effect of zeolite type. It was found that the small-pore zeolites/silicoaluminophosphates with CHA structure (Cu/SAPO-34 and Cu/SSZ-13) exhibited higher SCR activity at 150°C and lower selectivity toward N2O formation during standard SCR conditions than Cu/BEA. However, formation of ammonium nitrate species during fast SCR conditions at 150°C occurred over Cu/CHA catalysts, which resulted in a gradual decrease of the NOx conversion. Such blocking was also observed over Cu/BEA, albeit to a minor extent. The ammonium nitrate formation and its decomposition temperature regimes resulted in that the Cu/BEA was catalytically more active at lower temperature than either Cu/SAPO-34 or Cu/SSZ-13 during fast SCR conditions. Additionally, our results show that the ammonium nitrate species were more stable on the small-pore zeolites than on Cu/BEA. Comparing the two Cu/CHA catalysts, Cu/SAPO-34 and Cu/SSZ-13, it was found that ammonia oxidation at high temperatures and ammonia SCR at 150°C was higher on Cu/SAPO-34. Further, TPR experiments showed that Cu in Cu/SAPO-34 is more easily reduced compared to Cu/SSZ-13. This can facilitate the redox processes and can thereby be a reason for the higher activity at 150°C for Cu/SAPO-34. In addition, Cu/SAPO-34 forms less N2O and this might be a result of the formation of more stable ammonium nitrates. To conclude, the choice of the type of zeolite/silicoaluminophosphates affects the activity and selectivity of the different steps in the SCR process.