Origins of Bi-modal NO conversion behavior in NH3-SCR over Cu-chabazite revealed by mass transfer and surface kinetics analysis

Abstract

A multi-site kinetic model for the NH3-SCR (Selective Catalytic Reduction) on a commercial Cu-Chabazite washcoated monolith catalyst was developed and validated using synthetic gas bench data. The model was used to demonstrate the overall controlling regimes in the reactor at temperature and space velocity ranges of 120–550 °C and 30,000–80,000 h−1, respectively. This was performed via characteristic times analysis and via selectively removing external and internal mass transport phenomena from the model. Latter resulted in a bi-modal NO conversion with a conversion trench in 230–430 °C range where main mass transfer limitations were found. Bi-modal NO conversion profile was due to the competition between NH3 adsorption/desorption reactions and Standard SCR for surface NH3 species and it can be masked depending on the ratio of Standard SCR rates to the rates of adsorption and desorption in the presence of mass transfer limitations which have positive effects towards NO conversion.