Heteropoly　acids　(HPAs)　with　the　Keggin　structure　have　been　widely　used　in　NOx　removal.　Two　kinds　of　catalysts　(those　with　and　without　the　Keggin　structure)　are　prepared　for　studying　the　effect　of　the　Keggin　structure　on　the　NH3-SCR　reaction.　A　series　of　in　situ　diffuse　reflectance　infrared　Fourier　transform　spectroscopy　(DRIFT)　analyses　are　conducted　to　investigate　the　surface-adsorbed　species　on　the　catalysts　during　the　SCR　reaction.　The　mechanism　for　enhancing　low-temperature　activity　of　the　catalysts　is　proposed.　Furthermore,　the　effect　of　NH4+　in　the　Keggin　structure　is　also　investigated.　Results　indicate　that　both　the　Langmuir-Hinshelwood　(L-H)　and　Eley-Rideal　(E-R)　mechanisms　occurred　in　the　NH3-SCR　reaction　over　the　catalyst　with　the　Keggin　structure　(Cat-A);　in　addition,　when　more　acid　sites　are　provided,　NOx　species　activity　is　improved　and　more　NH4+　ions　participate　in　reaction　over　Cat-A,　thus　promoting　SCR　activity.