Process for removing the nitrogen oxides from the exhaust gases using the selective catalytic reduction with ammonia, without formation of dinitrogen oxide

摘要

The selective catalytic reduction (SCR) of nitrogen oxides (NOx) to ammonia comprises the use of a zeolite catalyst partially exchanged with copper, with a silicon/aluminum (Si/Al) ratio of 0.8-20, and which does not generate nitrogen protoxide. The zeolite has a structural arrangement of SiO4 and AlO4 tetrahedra which give rise to small cavities through the windows of 6 tetrahedra and to big cavities accessible through windows formed of at least 8 tetrahedra. The exchange rate with copper is 5-60%. The catalyst has by two waves of hydrogen consumption in a temperature program, the first wave corresponding to less than 800 degrees C and the second one to above 800 degrees C. The hydrogen consumption corresponds to the peak of the second wave being sensibly equal to half mole of hydrogen per mole of copper in the solid, which comprises two peaks (P1 and P2 respectively) resolved by Gaussian deconvolution, the ratio A1/A2 of the areas corresponding respectively to peaks P1 and P2 being greater than 1.5. Independent claims are included for: (A) a mixed faujasite of formula Cu(x)M'(y)M(z)FAU can be used, in which: FAU represents the cubic faujasite network with an Si/Al ratio of 0.8-20; M' is a metal of valency 2, 3 or 4 of ionic diameter 0.16-0.28 nm; M is H+, Na+ or K+, preferably Ca2+ or lanthanide ion, or a mixture of lanthanide ions; and x, y and z are the respective theoretical exchange rates in copper II ions, M' and M ions expressed in % of the total exchange capacity of the faujasite, and are given by: x is greater than or equal to 5 and less than or equal to 60; y is greater than or equal to 20 and less than or equal to 70; and z is less than or equal to 100-x-y, where the mixed faujasite has the same characteristics as the above zeolite; and (B) the production of a zeolite, comprising: (i) selecting a zeolite of Si/Al ratio 0.8-20, where SiO4 and AlO4 tetrahedra are disposed in such a way as to give rise to small cavities through the windows of 6 tetrahedra and to big cavities accessible through windows formed of at least 8 tetrahedra; (ii) subjecting the zeolite to an ion exchange by contacting a solution of a metallic element of valency 2, 3 or 4 of ionic diameter 0.16-0.28 nm, in well defined conditions such that an exchange rate of 20-60% is achieved; (iii) calcining the resulting product to 350-650 degrees C; (iv) subjecting the calcination product to an ion exchange with a copper salt solution in well defined conditions such that an exchange rate of 5-60% is achieved; and (v) calcining the resulting product to 350-650 degrees C.