Cu-ZSM-5 Catalyst Impregnated with Mn–Co Oxide for the Selected Catalytic Reduction of NO: Physicochemical Property–Catalytic Activity Relationship and In Situ DRIFTS Study for the Reaction Mechanism

A series of MnCo/Cu-ZSM-5 (MnCo/Cu-Z) catalysts with different Mn/Co ratios for the selective catalytic reduction (SCR) of NO with NH3 were prepared by the combination of ion exchange and impregnation. The physicochemical properties of the catalysts were investigated by N2 adsorption/desorption, X-ray diffraction (XRD), temperature-programmed reduction with hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of NOx (NOx-TPD) and NH3 (NH3-TPD), and the diffuse reflectance infrared Fourier transform (DRIFT) technique was employed for the detection of intermediate species and the study of the mechanism. The introduction of Mn and Co boosts the catalytic activity of the Cu-Z catalyst at a temperature below 200 °C, and the optimum activity was obtained over the Mn1Co2/Cu-Z catalyst. The enrichment of the high-valent metal ion on the catalyst surface and the improvement in the reducibility of metal oxide are responsible for the elevation in the catalytic activity of MnCo/Cu-Z. The bridged and bidentate modes are the prevailing nitrate species on the Cu-Z and MnCo/Z catalysts, respectively, and both of them were detected over the MnCo/Cu-Z catalyst. Furthermore, the DRIFT spectroscopy (DRIFTS) results indicate that, at 150 °C, the bridged nitrate can react with the adsorbed ammonia species, but the bidentate nitrate fails to do. The SCR reaction over the Cu-Z catalyst follows the Eley–Rideal and Langmuir–Hinshelwood mechanisms simultaneously at 150 °C, whereas an overwhelming dominance of the Eley–Rideal mechanism is identified for the MnCo/Z catalyst. As to the SCR mechanism on the MnCo/Cu-Z catalyst, it combines the features of Cu/Z and Mn1Co2/Z.