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

Abstract

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 N₂ physisorption, ⁵⁷Fe Möessbauer 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 °C) and high temperature desorption of NH₃ (>ca. 200 °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 NH_x 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 N₂ 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.