Contrasting Effects of Potassium Addition on M3O4(M = Co, Fe, and Mn) Oxides during Direct NO Decomposition Catalysis

While the promotional effect of potassium on Co3O4NO decomposition catalytic performance is established in the literature, it remains unknown if K is also a promoter of NO decomposition over similar simple first-row transition metal spinels like Mn(3)O(4)and Fe3O4. Thus, potassium was impregnated (0.9-3.0 wt.%) on Co3O4, Mn3O4, and Fe(3)O(4)and evaluated for NO decomposition reactivity from 400-650 degrees C. The activity of Co(3)O(4)was strongly dependent on the amount of potassium present, with a maximum of similar to 0.18 [(mu mol NO to N-2) g(-1)s(-1)] at 0.9 wt.% K. Without potassium, Fe(3)O(4)exhibited deactivation with time-on-stream due to a non-catalytic chemical reaction with NO forming alpha-Fe2O3(hematite), which is inactive for NO decomposition. Potassium addition led to some stabilization of Fe3O4, however, gamma-Fe2O3(maghemite) and a potassium-iron mixed oxide were also formed, and catalytic activity was only observed at 650 degrees C and was similar to 50x lower than 0.9 wt.% K on Co3O4. The addition of K to Mn(3)O(4)led to formation of potassium-manganese mixed oxide phases, which became more prevalent after reaction and were nearly inactive for NO decomposition. Characterization of fresh and spent catalysts by scanning electron microscopy and energy dispersive X-ray analysis (SEM/EDX), in situ NO adsorption Fourier transform infrared spectroscopy, temperature programmed desorption techniques, X-ray powder diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) revealed the unique potassium promotion of Co(3)O(4)for NO decomposition arises not only from modification of the interaction of the catalyst surface with NOx(increased potassium-nitrite formation), but also from an improved ability to desorb oxygen as product O(2)while maintaining the integrity and purity of the spinel phase.