NH3 inhibits mercury oxidation over low-temperature MnOx/TiO2 SCR catalyst

Manganese oxide/titanium dioxide (MnOx/TiO2, or MnTi) catalyst prepared by an ultrasound-assisted impregnation method was employed for simultaneous nitrogen oxide (NO) reduction and elemental mercury (Hg0) oxidation under a simulated selective catalytic reduction (SCR) atmosphere. An NO reduction rate of 83% and Hg0 oxidation rate of 96% were simultaneously achieved at a relatively low temperature of 200 °C. Ammonia (NH3) as a reducing agent in the NO-SCR process was systematically investigated for its effect on Hg0 oxidation. The results showed that NH3 inhibited Hg0 oxidation over the low-temperature MnTi catalysis process. In the presence of NH3, competitive adsorption between NH3 and Hg0 on the MnTi catalyst was partly responsible for the weakening Hg0 oxidation. NH3 consumed surface oxygen (O2) on the MnTi catalyst, which also resulted in lower Hg0 oxidation rate, especially in the absence of gas phase O2 in the flue gas. More importantly, oxidized mercury was reduced by NH3 to form Hg0 with the aid of NO, i.e., the combined presence of NO and NH3 induced the reduction of oxidized mercury, and hence partially offset Hg0 oxidation. The effects of individual SCR gases, such as O2, NO, and the combinations, on Hg0 oxidation were also elaborated.