In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR

Abstract

The influence of the zeolite framework structure on the formation of N₂O during ammonia-SCR of NO_x was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA). The evolution of surface species during the SCR reaction at different temperatures was monitored with step-response experiments using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at different reaction conditions. Also, density functional theory (DFT) calculations were performed to assist the interpretation of the experimental results. The DRIFTS results indicate that NO⁺ and nitrate species are the main products formed during NO oxidation, and NO appears to adsorb on both Cu-Lewis and Al-Lewis acid sites. The DFT calculations for NO adsorption on the SSZ-13 sample reveal adsorption at Brønsted acid sites with similar adsorption energies but with a slight difference in NO⁺ stretching vibrations in the DRIFT spectra. Within the standard SCR reaction, in the O–H stretching region, the number of NH₃ molecules adsorbed on the Brønsted acid sites is higher for the small-pore size sample compared to the medium- and large-pore zeolites. The obtained DRIFTS results for nitrate species are supported by DFT calculations by simulating the IR spectra of mobile and framework bound nitrate species, which both have a signature at 1604 cm⁻¹ associated with the O–N bond on NO₃⁻. It is revealed that N₂O is produced in a higher amount at lower temperatures for all three samples irrespective of the NO/NO₂ ratio. Furthermore, the obtained results from both DRIFTS studies and flow reactor experiments show the higher formation of N₂O for the large-pore zeolite compared to the medium- and small-pore zeolite.