Mechanistic Study on NH ₃ -SCR Over Chromium-Exchanged Zeolite Using Modulation-Excitation XAS, Raman Spectroscopy, and DFT Calculations

The reaction mechanism of the selective catalytic reduction of nitric oxide (NO) using NH3 (NH3-SCR) over Cr-exchanged β zeolite (Cr-β) was investigated. In situ Raman spectroscopy showed a consumption of [Cr(O)2]+ species in the reduction-half cycle at 300 °C (RHC, 0.1 vol % NH3 + 0.1 vol % NO). In the subsequent oxidation-half cycle (OHC, 10 vol % O2, 300 °C), [Cr(O)(OH)2]+ species was generated whose fraction halved under SCR conditions (0.1 vol % NO + 0.1 vol % NH3 + 10 vol % O2, 300 °C). Modulated-excitation X-ray absorption spectroscopy (ME-XAS) measurements provided the X-ray absorption near edge structure (XANES) spectra of kinetically distinct species and their relative concentrations, revealing that the OHC is faster than the RHC. Transition state and vibrational analyses based on density functional theory (DFT) calculations support the experimental findings that the isolated Cr species serve as the main catalytic centers for NH3-SCR by Cr-β.