<i>In Situ</i> Spectroscopic Studies of the Redox Catalytic Cycle in NH<sub>3</sub>–SCR over Chromium-Exchanged Zeolites
Shunsaku Yasumura, Yucheng Qian, Takashi Toyao, Zen Maeno, Ken‐ichi Shimizu
Abstract
In situ infrared (IR) and Cr K-edge X-ray absorption spectroscopy (XAS) experiments as well as ex situ electron paramagnetic resonance (EPR) were performed to clarify the reduction/oxidation half-cycles in the NH3-based selective catalytic reduction (SCR) of nitrogen oxide (NO) (NH3–SCR) over a Cr-exchanged β zeolite (Crβ). The XAS results revealed that the high-valent Cr (Cr (V), Cr(VI)) species were reduced by NH3 + NO into Cr(III) species, which were subsequently oxidized by O2 to recover the high-valent Cr species. Further, the EPR-active species, Cr(III) and Cr(V), were observed after the NH3 + NO and O2 treatments, respectively, employing the ex situ EPR measurements. Thus, combining the results of in situ IR with online mass spectrometry analysis (operando IR), a steady-state NH3–SCR mechanism was proposed, as follows: the Cr(V) species was reduced by NO to afford Cr(III) and NO+ species on the zeolite. NO+ reacted with NH3 to produce N2 and water, and the Cr(III) species was reoxidized by O2 to regenerate the Cr(V) species.