Detailed Kinetic Modeling of NH₃ and H₂O Adsorption, and NH₃ Oxidation over Cu-ZSM-5

Abstract

The NH 3 and H 2 O adsorption and desorption, and the NH 3 oxidation was studied using detailed kinetic modeling and flow reactor experiments. Ammonia storage and ammonia oxidation are important for the NH 3 SCR application. In this study, both ammonia storage and oxidation are investigated, with and without the presence of water. Four sites were included in the model. On each copper atom was one active site introduced, denoted S1a, where NH 3 , H 2 O, NO 2 and O 2 can adsorb. However, electron paramagnetic resonance studies (EPR) and also DFT calculations in the literature suggest that [Cu(NH 3 ) 4 ] 2+ complex are formed in copper zeolites. We therefore introduced three additional sites (S1b) that ammonia can adsorb on in order to add up to the four ammonia adsorbed per copper atom. It was important to separate between S1a and S1b since it is not possible for four NO 2 to adsorb per copper and also in order to describe the ammonia TPD and SCR reactions simultaneously. The Cu-ZSM-5 catalyst also contains Bro ¨nsted acid sites (S2), and in order to account for the large amount stored at ambient temperature, sites for weakly bound species (S3) were included as well. The Bro ¨nsted sites were investigated using NH 3 and H 2 O TPD experiments on H-ZSM-5. Water and ammonia TPD experiments on Cu-ZSM-5 were also used in the model development and the model was able to describe the experiment well. An NH 3 TPD experiment with storage performed in the presence of water was used for model validation, and the model was able to predict the experimental results adequately. The model was then extended to include steps for oxygen adsorption, desorption, dissociation, recombination and two summary steps for ammonia oxidation. Ammonia oxidation in both the presence and absence of water was used in the model development. The resulting model was able to predict ammonia storage, desorption and oxidation accurately, both in the presence and absence of water.

Kinetic modeling of NH3-SCR over a supported Cu zeolite catalyst using axial species distribution measurements

“…An alternative explanation for the coverage dependent activation energy is that in zeolites there is a range of acidity for the sites, and a coverage dependent barrier reflects this inhomogeneity. A coverage dependent activation energy for NH 3 desorption has been used in many kinetic models [16,19,20,32] and is in line with microcalorimetric experiments by Wilken et al [23]. Micro-calorimetry experiments on Cu-BEA powder from the same batch as is used in this study resulted in an adsorption heat of 112.75 kJ/mol with a coverage-dependence ˛ of 0.39.…”

“…The reaction heat associated with NH 3 SCR is small and, therefore, the simulations were considered isothermal, which also was the case in several earlier NH 3 SCR models [16][17][18][19][20][21][22].…”

“…The amount of adsorbed and released ammonia was calculated and used to fit the number of active sites, which is a crucial parameter for the kinetic model. Adsorption was considered to be a non-activated step and the activation barrier was set to zero, as done in several other studies [19]. The pre-exponential factors for adsorption and desorption were fitted to reproduce the shape of the associated data in Fig.…”

“…The model has a lower ammonia storage compared to the experiment and also there is a larger tailing of the ammonia concentration after turning it off. The reason is that the NH 3 adsorption preceding the TPD experiments was made at 40 • C, and at this low adsorption temperature it is possible that physisorbed ammonia is available on the catalyst, which was added in a kinetic model by Sjövall et al [19] for SCR over Cu/ZSM-5. Since the activity of the catalyst was modeled at 200 • C and higher, the physisorbed ammonia was not taken into consideration in this study.…”

“…These models include side reactions, several possible routes for NO x reduction, such as "Standard SCR", "Fast SCR" and "NO 2 SCR" [17], or those focused on the nature and dynamics of the active sites [18][19][20][21]. Catalysts for NO x -selective reduction are practically coated onto a monolithic support and this system behaves like a plug flow reactor along which chemical species concentrations evolve according to http reactions, adsorption and release.…”