Study on the kinetics of the adsorption and desorption of NH3 on Fe/HBEA zeolite

In this work, a Fe/HBEA zeolite (Si/Al: 12.5), representing an effective catalyst for the NH3-SCR process, was physico-chemically characterized and investigated regarding the kinetics of the adsorption and desorption of NH3. The sample was evaluated by N-2 physisorption, Fe-57 Moessbauer and DRUV-Vis spectroscopy, while the kinetics was investigated by temperature-programmed desorption of NH3 (TPD) including different adsorption temperatures. It was shown that the NH3 chemisorption results in weakly and strongly bonded molecular ammonia as well as ammonium species. A kinetic mean field model was developed implying two different types of adsorbates reflecting low (<ca. 200 degrees C) and high temperature desorption of NH3 (>ca. 200 degrees C). Kinetic parameters and surface coverages were obtained from numeric fits of the TPD curves, whereas pre-exponential factors of adsorption were deduced from the kinetic gas theory. As a result, the activation energy for the NHx adsorbate decomposition in the low temperature regime, which is assigned to single and double bonded ammonium species was determined to be 106 kJ mol(-1). The NH3 desorption at higher temperatures referred to an activation energy of 133 kJ mol(-1) predominately related to NH3 coordinated to Lewis acid surface sites and to some extent to stabilized NH4+ species. For validation of the kinetic model, experiments were simulated including NH3 adsorption at different temperatures, subsequent flushing with N-2 and final TPD. Additionally, the consistency of the activation energies with the thermodynamic data was checked using differential scanning calorimetry and a van't Hoff approach.