The operation of a purification apparatus for flowing gases comprises passing the gas stream through a filter body having a catalyst, a reducing agent being added to the gas before entering the catalyst. The flow direction (1) of the gas according to the treating stretch in the catalyst of the filter body (2), reducing agent added to the flowing gas, and contact with the catalyst of the filter body (2) are maintained. Also claimed is the purification apparatus comprising a feed device for the reducing agent, which is added to the gas while flowing through the filter body (2).
