Heat of adsorption for NH 3, NO 2 and NO on Cu-Beta zeolite using microcalorimeter for NH 3 SCR applications

Abstract

Microcalorimetry is a powerful technique with which to measure the heat of adsorption (Δ H), producing values that are very important when developing kinetic models. The method provides a way of determining these parameters independently. For kinetic models describing NH 3 SCR it is critical to be able to accurately describe the storage of ammonia and NO x in order to simulate rapid transients occurring in the experiments. The objective of our study is to measure the heat of adsorption of NH 3, NO 2 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 NH 3 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, NH 3 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: E d e s o r p t i o n , N H 3 = 120.0 ( 1 − 0.38 θ N H 3 ) where θ N H 3 is the coverage of ammonia on the surface. The NO and NO 2 adsorption and desorption were investigated using NO and NO 2 TPD experiments, respectively. For the NO 2 TPD experiment, approximately three NO 2 were stored for each NO produced, corresponding to the disproportionation mechanism. This resulted in Δ H of −65 kJ/mol per NO 2 consumed. The NO TPD experiment resulted in that only small amounts of NO was adsorbed.