Alkali Poisoning Of Mn/Ce-ZrO₂ For Low-Temperature Selective Catalytic Reduction

Selective catalytic reduction of NOx by NH3 (NH3-SCR) is an efficient technology for eliminating NOx from emissions in stationary sources. And catalyst is the core of the NH3-SCR technology. However, fly ash in flue gas contains numerous alkali metals, which easily poison the catalysts. Thus, it is necessary to research catalysts that have good resistance to alkali metal.In this paper, Mn/Ce-ZrO2 was prepared by co-precipitation and impregnation method. And alkali metal was doped by impregnation method to simulate alkali poisoning. The effects of alkali metal types and alkali metal content on the SCR activity were researched. In order to improve the resistance to alkali metal, the catalyst was modified with Co and Sn, and the effects of the modifier, modification methods and modifier content on the resistance to alkali metal were researched. The results are as follows:Alkali inhibits the SCR activity of Mn/Ce-ZrO2, and the inhibition of K is stronger than Na. The effect of inhibition increases with the increasing alkali metal content, and the effect of inhibition offers upgrade firstly than descending latter tendency in the temperature of 80～240℃. When the molar ratio of K/Mn was 0.1,0.2,0.3, the NO conversion rate decreased by 40.23%, 73.30%,87.62% respectively at 120℃,140℃, and 140℃, and the inhibition to activity was the most serious at the above temperature.Compared with the catalyst modified with Sn, the catalyst modified with Co shows better SCR activity and better resistance to alkali metal. When the molar ratio of Co/(Ce+Zr+Co) was 0.05 to 0.3, the SCR activity of the catalyst increased, and the NO conversion rate was 100% in the range of 120～240℃. When the doping of Co was 0.1, the resistance to alkali metal was the best, and the NO conversion rate increased by 50% compared to unmodified catalyst at 240℃ after K poisoning. Both Co modified support and modified the active component can improve the SCR activity. When the reaction temperature was higher than 100℃, the catalyst NO conversion rate was maintained above 95%. Catalyst with Co as a modifier to support has a much better resistance to alkali metal than that with Co as a modifer to active component.After alkali doping, the surface area, pore volume, and number of surface acid sites decrease, which inhibit the absorption and activation of the reaction gas on the surface of the catalyst. After Co is added as support, the catalyst has a lower crystallinity, a larger surface area, a richer acid sites, and a better oxidation-reduction ability, which result in a better SCR activity. And after alkali metal poisoning, the surface area decreased slightly, oxidation-reduction ability is further improved, and the acid sites are still more, which lead to a better resistance to alkali metal. After Co is added as active component, the surface area is decreased, and after alkali metal doping, the surface area decrease furtherly. Thus, Mn/Ce-ZrO2 modified with Co as support has a better resistance to alkali metal than that modified with Co as active component. The catalyst has an important application value on the elimination of nitrogen oxides.