The role and activity of various adsorbed ammonia species on Cu/SAPO-34 catalyst during passive-SCR process

Abstract

In this work, the adsorption and reaction performance of various adsorbed ammonia species during the passive-SCR process were investigated by temperature-programmed desorption (TPD), temperature-fixed surface reaction (TFSR), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) and kinetic tests. The NH₃-TPD and DRIFTS-NH₃ adsorption results showed that the number of weak and strong acid sites in the Cu/SAPO-34 sample increased due to Cu species incorporation, which formed new Lewis acid sites. Further, a decrease in the number of moderate acid sites resulted from the replacement of hydroxyls by Cu²⁺. The TFSR results revealed that the adsorbed NH₃ species presented different SCR activity, which could be divided into active NH₃ species and inactive NH₃ species. In the low temperature range, a large amount inactive NH₃ resulted in lower activity and resulted in a long equilibrium process. However, the performance of inactive NH₃ was distinctly different at high temperatures. Furthermore, the in situ DRIFTS results illustrated the activity difference and migration between the two adsorbed NH₃ species. It was proved that the NH₃ migration rate from the Brønsted acid sites to Lewis acid sites was slower than the SCR rate at low temperatures, which might determine the SCR reaction rates. However, at high temperatures, the NH₃ migration rate was faster than the SCR rate of active NH₃. In other words, NH₃ migration from the Brønsted acid sites to Lewis acid sites may be the rate determining step of passive-SCR lean period at low temperatures, while NH₃ adsorption possibly was the rate determining step of the passive-SCR rich period at high temperatures. On the basis of the above results, the present work gives an insight into the potential of Cu/SAPO-34 for passive-SCR applications in the future.