Mechanistic investigation of hydrothermal aging of Cu-Beta for ammonia SCR

Abstract

The selective catalytic reduction of NOx with NH3 over a Cu-BEA catalyst was studied after hydrothermal aging between 500 and 900°C. The corresponding catalyst was characterized using XPS and XRD techniques in the aging interval of 500, 700 and 800°C. No structural changes during the aging process were observed. However, the oxidation state of copper changed during aging and more Cu2+ was formed. We suggest that one of the deactivation mechanisms is the decrease of the Cu+ species. The NO oxidation and NH3 oxidation activity was decreased with increasing aging temperature. Further, we observed that the ammonia oxidation was decreased faster compared to the SCR reactions at low aging temperatures. The experiments from the calorimeter as well as from the ammonia TPD investigations indicate a trend towards more weakly bound ammonia with higher aging temperatures. From the results of the SCR experiments using different NO2/NOx ratios and ammonia oxidation experiments we suggest that most of the N2O is coming from side reactions of the SCR mechanism and not from reactions between NH3 and O2 alone. Interestingly, we observe that after the 900°C aging a quite large activity remained for the case with 75% NO2/NOx ratio. The N2O production shows a maximum at 200°C, but increases again at higher temperatures. However, the N2O formed at low temperature is decreased after hydrothermal aging while the high temperature N2O is increased. We propose that the different reactions examined in this work do not all occur on the same type of sites, since we observe different aging trends for some of the reactions.