Abstract: Mn/TiO₂ has good low temperature NH₃ selective catalytic reduction (SCR) activity for NO_x. The presence of alkali metals in the flue gas can physically and chemically poison the catalyst leading to toxic deactivation of the Mn/TiO₂ catalyst. This study investigated the mechanism of K-poisoning in Mn/TiO₂ low temperature SCR catalysts by preparing K-poisoning Mn/TiO₂ catalysts using exposed 101 surface TiO₂ as a carrier. It was found that the denitrification efficiency of the Mn/TiO₂ catalyst decreased with increasing K-poisoning concentration. Experimental characterisation and DFT calculations showed that the NH₃-SCR reaction on the surface of the fresh Mn/TiO₂ catalyst was controlled by both E-R and L-H mechanisms. K adsorption led to a reduction in the catalyst specific surface area, a decrease in the ratio of Mn⁴⁺ and chemisorbed oxygen on the catalyst surface and a decrease in the number of acidic sites on the surface, resulting in a decrease in denitrification activity; at the same time, K was more likely to adsorb near the Mn top site as well as the bridging O site, resulting in the activation of NO adsorption was severely curtailed and the adsorption of NH₃ was weakened, making the L-H mechanism blocked and the E-R mechanism the main control.