MnM₂O₄ microspheres (M = Co, Cu, Ni) for selective catalytic reduction of NO with NH₃: Comparative study on catalytic activity and reaction mechanism via in-situ diffuse reflectance infrared Fourier transform spectroscopy

MnM₂O₄ microspheres (M = Co, Cu, Ni) were successfully prepared by a hydrothermal method. The catalytic activities of the as-prepared MnM₂O₄ microspheres were comparatively investigated by the selective catalytic reduction (SCR) of NO with NH₃, and their reaction mechanisms were studied by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The results showed that MnCo₂O₄ presented the best low-temperature catalytic activity (nearly 100% NO_x conversion in a wide temperature window from 120 to 330 °C under a high gas hourly space velocity (GHSV) of 36,000 h⁻¹), the best N₂ selectivity (100% N₂ selectivity in the temperature range from 60 to 360 °C), and excellent water resistance. The high catalytic activity of MnCo₂O₄ was mainly attributed to the large quantity of surface acid sites with dominant Brønsted acid sites rather than Lewis acid sites. In addition, larger S_BET and higher surface Mn⁴⁺ concentration also had positive effect on its catalytic activity. Contrastively, the least number of surface acid sites with dominant Lewis acid sites rather than Brønsted acid sites over MnCu₂O₄ resulted in its worst low-temperature catalytic activity. The catalytic activity of MnNi₂O₄ was in the middle. In-situ DRIFT revealed that the NH₃-SCR of NO over MnCo₂O₄ and MnNi₂O₄ followed both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. For MnCu₂O₄, the reaction was dominated by E-R mechanism.