Effects of K doping on ZnCoO spinel catalysts for low-temperature catalytic NO decomposition in the presence of inhibitors.

The recent development of NH as a fuel has led to significant emissions of NO, a major greenhouse gas. Direct catalytic NO decomposition (de-NO) is a promising technology for NO emissions control because it effectively decomposes NO at low temperatures without requiring reducing agents or producing other pollutants. In marine applications, to improve the flame properties, NH is often mixed with marine diesel oil, which contains sulfur. Although alkali-metal-doped Co-based catalysts show high de-NO activity at low temperatures, their performance in the presence of SO has not been evaluated. In this study, we examined the effects of alkali metals on the ZnCoO spinel-structured catalyst for low-temperature de-NO in the absence and presence of inhibitors (O, HO, and SO). Incorporating alkali metals (Na, K, Rb, and Cs) significantly enhanced the catalytic activity of the ZnCoO spinel catalyst. Specifically, adding 1 wt% K reduced the light-off temperature from 271.7 °C to ∼150.0 °C at 60,000 h gas hourly space velocity in the absence of inhibitors. The enhanced electronic properties resulting from the addition of alkali metals led to weakening of Co-O bonds, promoting the regeneration of active sites and thus enhancing catalytic activity. K served as sacrificial sites for sulfur adsorption, delaying deactivation of the 1K-ZnCoO spinel catalyst by SO. The formation of bulk sulfates, surface sulfites, and CoSO was responsible for the deactivation by SO. Mechanisms for the deactivation by SO and the promotion of SO resistance by K in the ZnCoO spinel catalyst are proposed.