A cleaning kit for removing process impurities carried on the surface of a NO.sub.x reduction catalyst which is installed in the path of a flue gas flow exiting from a fossil fuel burning facility including: a reagent supply grill; a source of supply of liquid cleaning reagent; a reagent collection basin; and a recirculating structure. The reagent supply grid is adapted to be selectively positioned above a portion of a catalyst layer. The source of supply of liquid cleaning reagent adapted to be in communication with the supply grid, and the reagent collection basin being adapted to be selectively positioned below the portion of such catalyst layer to catch cleaning reagent therein after such reagent passes through such portion of the catalyst layer, the recirculating structure recirculates at least a portion of such reagent from the collecting basin for recirculating through such supply grid for further cleaning of such portion of the catalyst layer.