Title : Towards a transient multi-site kinetic model of cu chabazite for NH3 SCR linked to the active site hydrothermal aging kinetics
Abstract:
Cu-chabazites (CHA) are widely used as catalysts for NOx abatement using NH3-SCR (Selective Catalytic Reduction) technology within the aftertreatment systems (ATS) of the diesel engines. The design and calibration of ATS are usually accomplished with the help of engine dynamometer and vehicle tests that span a large operation region of the engine. However, such tests are very long, labor intensive and expensive. An alternative is to utilize mathematical models based on the physico-chemical phenomena occurring in the reactors which make up the ATS. In order to describe the hydrothermal aging effects of a multi-active site containing Cu-CHA catalysts on the NH3-SCR performance, a multi-site transient kinetic model is required. For this, one needs to describe the hydrothermal aging kinetics effects on the active site concentrations and link these with the multi-site kinetic model. However, there is no such model in the literature for the Cu-CHA. For this purpose, a multi-site kinetic model on a commercial Cu-Chabazite washcoated monolith was developed using laboratory scale data and the model was validated using engine dynamometer tests. The hydrothermal aging effects on the active sites were measured using an aging protocol consisting of aging the catalyst for certain time at a certain temperature followed by the measurement of NH3 adsorption, desorption and TPD. NH3-TPD curves were deconvoluted into three sites consisting of Cu1, Cu2 and Brönsted sites with peak centres at 317, 456 and 526, oC, respectively along with sites associated with weakly bound NH3 desorbed during the isothermal desorption at 110 oC before the TPD for the degreened catalyst. The changes in the concentration of these sites were modelled using first order kinetics after hydrothermal aging at 650-800 oC upto 45h. The total NH3 storage and release did not change with hydrothermal aging at 650 oC after 45h but decreased after aging at higher temperatures. The concentration of Cu1 sites decreased by 6% after aging at 650 oC for 0.7h then started increasing after an aging time of 3h and after 45h it reached to a value 28% higher as compared to its concentration after degreening. The concentration of the Cu2 sites, on the contrary, initially increased with aging time upto 0.7h reaching a value 12% higher than the degreened concentration, then decreased with aging time resulting in a drop of 23% as compared to the degreened value. The concentration of the Brönsted sites exponentially decreased with aging time reaching a plateau of 85% lower than the degreened concentration at 36h. Hydrothermal aging kinetics of the active sites were linked to the multi-site kinetic model through the addition of site migration reactions for each active site. The effects of both aging time and temperature on the NH3 adsorption, desorption, oxidation and NH3 to NO reaction were successfully simulated in the 150-600 oC range. This study is the first step towards the development of a multi-site, multi-reaction, kinetic model of the NH3-SCR process for NOx reduction which is able to simulate the effects of hydrothermal aging according to both aging time and temperature.
