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Manganese oxide-supported palygorskite (MnO_x/PG) catalysts are considered highly efficient for low-temperature SCR of NO_x. However, the MnO_x/PG catalyst tends to be poisoned by SO₂. The effect of SO₂ on activity of the SO₂-pretreated poisoning catalysts under ammonia-free conditions was explored. It was determined that the MnO_x/PG catalyst tends to be considerably deactivated by SO₂ in the absence of ammonia and that water-washed regeneration can completely recover activity of the deactivated catalyst. Based on these results and characterizations of the catalysts, a reasonable mechanism for the deactivation of MnO_x/PG catalyst by SO₂ was proposed in this study. SO₂ easily oxidized to SO₃ on the surface of the catalyst, leading to the formation of polysulfuric acid, wrapping of the active component and blocking the micropores. The deactivation of the MnO_x/PG catalyst is initially caused by the formation of polysulfuric rather than the deposition of ammonia sulfate, which occurs later.