Mn/TiO2　catalysts　has　been　shown　excellent　selective　catalyst　reduction　(SCR)　activity　at　low　temperature.　However,　the　exhaust　gases　in　some　plant　contained　alkali　metal　compounds　after　dust　removal,　which　showed　a　serious　poisoning　effect　on　activity　of　Mn/TiO2　catalyst.　Our　previous　work　showed　that　the　poisoning　effect　of　potassium　on　catalyst　was　most　serious　among　these　alkali　elements.　In　the　study,　in　situ　diffuse　reflectance　infrared　transform　spectroscopy　(DRIFT)　and　density　function　theory　plus　dispersion　(DFT-D)　calculation　were　used　to　investigate　the　mechanism　how　the　potassium　species　have　a　negative　impact　on　Mn/TiO2　catalytic.　The　results　indicated　that　there　were　two　reasons　for　that　alkali　metal　elements　make　the　Mn/TiO2　catalyst　inactive.　The　first　one　is　that　the　amount　of　NH3　adsorption　was　on　the　Lewis　acid　sites,　which　was　main　active　substances　for　the　reduction　of　NO.　The　second　one　is　that　the　electrons　tend　to　occupy　high　energy　states　under　effect　of　potassium,　which　resulted　in　decrease　of　oxidative　capacity　of　catalyst.　(C)　2017　Elsevier　B.V.　All　rights　reserved.