277890 In Situ-Drifts Study of Selective Catalytic Reduction by NH3 Over a Cu-Exchanged SAPO-34 Small Pore Zeolite Catalyst

Monday, October 29, 2012: 8:50 AM
320 (Convention Center )
Di Wang, Li Zhang and William Epling, Chemical and Biomolecular Engineering, University of Houston, Houston, TX

NH3 Selective Catalytic Reduction (NH3-SCR) is a widely applied technology to efficiently treat NOx 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 NOx species and NOx 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 was retained and did not react away. NO + O2 was inactive with pre-adsorbed NH3 on CHA at 100oC and no nitrate or nitrite species were observed on the surface by DRIFTS. However, at 200oC, 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 NOx reduction over copper-exchanged CHA zeolites.

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