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

Abstract

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% NOx conversion in a wide temperature window from 120 to 330°C under a high gas hourly space velocity (GHSV) of 36,000h−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 SBET 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.