The present invention concerns the selective removal of nitrogen oxides (NOx) from gasses. In particular, the invention concerns a process, a catalyst and the use of a catalyst for the selective removal of nitrogen oxides in the presence of ammonia from gases containing a significant amount of alkali metal and/or alkali-earth compounds which process comprises using a catalyst combined of (i) a formed porous superacidic support, said superacidic support having an Hammett acidity stronger than Ho=−12, and (ii) a metal oxide catalytic component deposited on said superacidic support selected from the group consisting of oxides of Fe, Cu, V, Cr, Mn, and any mixtures thereof.