A structure made from catalyst for use in the separation of nitrogen oxides from combustion exhaust gases which contain dust. The catalytic structure has plates with spaces therebetween to carry a flow of the exhaust gas therethrough. The catalyst is constructed of individual ceramic plates. The abrasion or wear resistance of the plates is chosen such that the dust in the exhaust gases flowing through the spaces abrades the plate surface at a rate which is at least equivalent to the deactivation of the catalyst surface by action of the nitrogen oxides, which constantly provides a fresh catalyst surface for reaction with the nitrogen oxides. The movement of the exhaust gas through the catalytic structure carries the dust and abraded material along with it and out of the structure.