Heat of adsorption for NH3, NO2 and NO on Cu-Beta zeolite using microcalorimeter for NH3 SCR applications

Microcalorimetry is a powerful technique with which to measure the heat of adsorption (ΔH), producing values that are very important when developing kinetic models. The method provides a way of determining these parameters independently. For kinetic models describing NH3 SCR it is critical to be able to accurately describe the storage of ammonia and NOx in order to simulate rapid transients occurring in the experiments. The objective of our study is to measure the heat of adsorption of NH3, NO2 and NO on Cu-Beta. An ammonia TPD experiment was conducted at 150 °C using the microcalorimeter, resulting in the observation of an exotherm when introducing ammonia due to adsorption. This resulted in an average heat of adsorption of −100 kJ/mol. A good reproducibility was found when using a second sample, resulting in −97 kJ/mol. In order to investigate the coverage dependence of the heat of adsorption, an ammonia stepwise experiment was conducted. First, the catalyst was exposed to NH3 at 500 °C, resulting in the adsorption of strongly bound ammonia and obtaining a heat of adsorption of −110 kJ/mol. Thereafter, the catalyst was cooled in Ar and at 400 °C, NH3 was again introduced. Due to that the temperature is lower the ammonia that adsorbed was weaker. The procedure was repeated at 300, 200 and 100 °C, resulting in a coverage dependent activation energy for ammonia desorption (if assuming zero activation for adsorption) according to the following formula:Edesorption,NH3=120.0(1−0.38θNH3)Edesorption,NH3=120.0(1−0.38θNH3)where θNH3θNH3 is the coverage of ammonia on the surface. The NO and NO2 adsorption and desorption were investigated using NO and NO2 TPD experiments, respectively. For the NO2 TPD experiment, approximately three NO2 were stored for each NO produced, corresponding to the disproportionation mechanism. This resulted in ΔH of −65 kJ/mol per NO2 consumed. The NO TPD experiment resulted in that only small amounts of NO was adsorbed.