An Aging Model of NH3 Storage Sites for Predicting Kinetics of NH3 Adsorption, Desorption and Oxidation over Hydrothermally Aged Cu-Chabazite

Selmi Erim Bozbag,Barkın Özener,Deniz Şanlı,Can Erkey,Gökhan Hisar

doi:10.3390/catal10040411

Abstract

A unified transient kinetic model which can predict the adsorption, desorption and oxidation kinetics of NH3 over hydrothermally aged Cu-chabazite was developed. The model takes into account the variation of fractional coverages of NH3 storage sites due to hydrothermal aging. In order to determine the fractional coverage of these sites, the catalyst was aged for various times at a certain temperature followed by NH3 adsorption, desorption and temperature-programmed desorption (TPD) experiments. TPD profiles were deconvoluted mainly into three peaks with centres at 317, 456 and 526 °C, respectively. Hydrothermal aging resulted in the progressive increase in the intensity of the peak at 317 °C and decrease in the intensity of the peaks at 456 and 526 °C, along with decreased NH3 oxidation at high temperatures. A model for hydrothermal aging kinetics of the fractional coverage of storage sites was developed using three reactions with appropriate rate expressions with parameters regressed from experimental data. The model was then incorporated into a multi-site kinetic model for the degreened Cu-Chabazite by the addition of aging reactions on each storage site. The effects of both aging time and temperature on the kinetics NH3 adsorption, desorption and oxidation were successfully predicted in the 155-540 °C range. This study is the first step towards the development of a hydrothermal aging-unified kinetic model of NH3-Selective Catalytic Reduction over Cu-chabazite.

Highlights

An engine aftertreatment system (ATS) of a heavy-duty vehicle comprises an NH3 -SelectiveCatalytic Reduction (NH3 -SCR) reactor in which NOx and NH3 react to form N2 and H2 O via a variety of reactions including Standard, Fast and NO2 -SCR
hydrothermal aging (HA) kinetics model successfully captured the changes in the fractional coverages of the NH3 storage sites, which could be identified with their NH3 -temperature-programmed desorption (TPD) peak centres observed at 317, 456 and 526 ◦ C, namely ZCu1, ZCu2 and ZB, respectively, for the degreened catalyst
The fractional coverages of ZCu2 and ZCu1 initially increased and decreased, respectively, with HA time after isothermal HA at 650 ◦ C, which was well represented by the HA kinetics model

Summary

Introduction

An engine aftertreatment system (ATS) of a heavy-duty vehicle comprises an NH3 -SelectiveCatalytic Reduction (NH3 -SCR) reactor in which NOx and NH3 react to form N2 and H2 O via a variety of reactions including Standard, Fast and NO2 -SCR. Much-speculated upcoming Euro emission regulations will impose more stringent NOx emission standards than before and restrict CO2 emissions. The latter would require increasing the combustion efficiency of the engine by lowering engine outlet temperatures, which would lower SCR efficiencies. The design and calibration of SCR reactors is usually carried out using data from engine dynamometer and vehicle tests that scan a large operation region of the engine. Such tests are very long, Catalysts 2020, 10, 411; doi:10.3390/catal10040411 www.mdpi.com/journal/catalysts

Methods

Results

Conclusion