| Summary: | While the promotional effect of potassium on Co<sub>3</sub>O<sub>4</sub> NO 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<sub>3</sub>O<sub>4</sub> and Fe<sub>3</sub>O<sub>4</sub>. Thus, potassium was impregnated (0.9–3.0 wt.%) on Co<sub>3</sub>O<sub>4</sub>, Mn<sub>3</sub>O<sub>4</sub>, and Fe<sub>3</sub>O<sub>4</sub> and evaluated for NO decomposition reactivity from 400–650 °C. The activity of Co<sub>3</sub>O<sub>4</sub> was strongly dependent on the amount of potassium present, with a maximum of ~0.18 [(µmol NO to N<sub>2</sub>) g<sup>−1</sup> s<sup>−1</sup>] at 0.9 wt.% K. Without potassium, Fe<sub>3</sub>O<sub>4</sub> exhibited deactivation with time-on-stream due to a non-catalytic chemical reaction with NO forming α-Fe<sub>2</sub>O<sub>3</sub> (hematite), which is inactive for NO decomposition. Potassium addition led to some stabilization of Fe<sub>3</sub>O<sub>4</sub>, however, γ-Fe<sub>2</sub>O<sub>3</sub> (maghemite) and a potassium–iron mixed oxide were also formed, and catalytic activity was only observed at 650 °C and was ~50× lower than 0.9 wt.% K on Co<sub>3</sub>O<sub>4</sub>. The addition of K to Mn<sub>3</sub>O<sub>4</sub> 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<sub>3</sub>O<sub>4</sub> for NO decomposition arises not only from modification of the interaction of the catalyst surface with NO<sub>x</sub> (increased potassium-nitrite formation), but also from an improved ability to desorb oxygen as product O<sub>2</sub> while maintaining the integrity and purity of the spinel phase.
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