Simulation of NH3 Temperature-Programmed Desorption Curves Using an ab Initio Force Field

Temperature-programmed desorption curves for ammonia (NH3-TPD) were predicted successfully using grand canonical ensemble Monte Carlo simulation methods and force field parameters derived from quantum mechanical ab initio data. This approach provides a means to study the relationship between the structure and acidity of zeolites at the molecular level. Analysis of the predicted NH3-TPD curves reveals that zeolite pore size is a critical factor influencing the curve shape. The so-called weak and strong acids of zeolites, which give rise to two peaks in the TPD curve, are roughly related to the interactions among ammonia molecules in the pores and to the interactions between ammonia molecules and the zeolite pore walls, respectively. However, the NH3-TPD curves may still show a double peak feature without any strong acid centers if the pore is such a size that additional strain is put on the NH3 hydrogen-bond network.