The invention relates to a process for preparing a catalyst based on a zeolite of AFX structural type and on at least one transition metal, comprising at least the following steps: i) mixing, in an aqueous medium, of at least one source of silicon in oxide form SiO2, of at least one source of aluminium in oxide form Al2O3, of an organic nitrogen-comprising compound R, of at least one source of at least one alkali metal and/or alkaline-earth metal M until a homogeneous precursor gel is obtained; ii) hydrothermal treatment of said precursor gel to obtain a crystallized solid phase, iii) at least one ion exchange with a transition metal; iv) heat treatment.
The invention also relates to the catalyst capable of being obtained or directly obtained by the process and to the process for the selective reduction of NOx employing the catalyst.