(27b) In Situ-Drifts Study of Selective Catalytic Reduction by NH3 Over a Cu-Exchanged SAPO-34 Small Pore Zeolite Catalyst

NH₃ Selective Catalytic Reduction (NH₃-SCR) is a widely applied technology to efficiently treat NO_x emissions from heavy duty diesel vehicles. Copper-exchanged chabazite (CHA) catalysts exhibit both superior activity and hydrothermal stability for this reaction in comparison with Cu-BEA and Cu-ZSM-5, and have therefore been recently adapted for on-vehicle application. In this study, the intrinsic mechanism of the SCR reaction over a Cu-exchanged chabazite catalyst was studied by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), coupled with a mass spectrometer to measure inlet and outlet gas concentrations. The evolution of the surface intermediates, as well as the reactivity of NH₃ with surface NO_x species and NO_x with surface NH₃ species, was evaluated. The presentation will include evaluating surface species and rates associated with the different SCR reactions (standard, fast and NO₂). As an example of the data to be shown, when exposing the catalyst to NO₂ after being saturated with NH₃, at low temperature, all the Lewis acid-adsorbed NH₃ was consumed by NO₂ and formed an intermediate, which is assigned as NH₄NO₃. However, 40% of total amount of NH₃ adsorbed on Brønsted acid sites wasretained and did not react away. NO + O₂ was inactive with pre-adsorbed NH₃ on CHA at 100^oC and no nitrate or nitrite species were observed on the surface by DRIFTS. However, at 200^oC, NO oxidation was activated, resulting in the consumption of NH₃ and formation of N₂ in the outlet. These examples and the data to be presented thus provide some mechanistic evidence on the enhanced performance of NO_x reduction over copper-exchanged CHA zeolites.