Molecular Aspects of the Mechanism of NH3-SCR on Tungsten-Loaded Ceria from Modulated Excitation DRIFTS

Abstract

Tungsten-loaded ceria is a potential alternative catalyst for the selective catalytic reduction of NO_x by NH₃ (NH₃–SCR). In this work, we carried out operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments using concentration modulation excitation (ME) coupled with phase sensitive detection (PSD) analysis to study key transient adsorbates on a W(10 wt%)/CeO₂ catalyst. During NO oxidation, NO₂ produced by NO oxidation is adsorbed on the catalyst, and the adsorbed NO₂ (NO_2ad) undergoes disproportionation to yield NO⁺ and NO₃⁻, accompanied by consumption of the bridged OH groups on the CeO₂ surface. During NH₃-SCR, NH₃ adsorbed on a Ce⁴⁺ Lewis acid site (L-NH₃) reacts with NO to yield Ce³⁺ site, N₂, H₂O, and H⁺ (Brønsted acid site). A portion of the initially adsorbed L-NH₃ desorbs and subsequently re-adsorbs onto the newly created Brønsted acid site, generating what we term B-NH₃. Consequently, the Ce³⁺ species undergoes oxidation, facilitated by O₂ and H⁺, resulting in the regeneration of Ce⁴⁺ species as well as the formation of H₂O. This oxidation process coincides with the depletion of the tentatively formed Brønsted acid site (B-NH₃).