Abstract: A catalyst comprising carbonaceous materials such as activated carbon, etc. impregnated with bromine, iodine, chlorine or their compounds, is impregnated with a promoter containing a specific metal, and brought in contact with a flue gas in the presence of ammonia, whereby nitrogen oxides and sulfur oxides contained in the flue gas are removed with a very high percent removal. The removal can be carried out at a temperature much lower than that of the conventional process.

Abstract: A sensor element for detecting smoke and/or reducing gases, which comprises a shaped body formed from a material consisting essentially of:A. a complex metal oxide having a perovskite type crystal structure and represented by the general formula ABO.sub.3-.delta. wherein A is at least one element selected from the group consisting of yttrium, a rare earth element having an atomic number of from 57 to 71 and an alkaline earth metal, B is at least one element selected from the group consisting of transition metals having atomic numbers from 21 to 30, O is oxygen and .delta. is a nonstoichiometric parameter which may vary from 0 to 0.25 andB. at least one metal oxide selected from the group consisting of CdO, In.sub.2 O.sub.3, SnO, Tl.sub.2 O.sub.3 and PbO.