Irregular influence of alkali metals on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia

Abstract

SCR activity of Cu-SAPO-34 catalyst was reduced by alkali metal ions. The alkali metals ions (Li+, Na+ and K+) have shown irregular influences on Cu-SAPO-34. The order of poisoning strengths under 400 °C was found to be: Na+ > K+ > Li+, which is not consistent with the basicities of their corresponding metals. Experimental results and calculations showed that the alkali metal ions readily replace H+ and Cu2+/Cu+ ions. These exchanges result in the loss of Brønsted acid sites and migration of isolated Cu2+ ions in Cu-SAPO-34, which decrease the NH3-SCR activity. Both the basicity and ion diameter will affect the exchanging behavior of an alkali ion. Na+ and Li+ ions will influence both H+ and Cu2+/Cu+ ions but K+ ions only preferably replace the H+. We hypothesize that K+ cannot enter into a small ring (6-membered ring) to replace a Cu2+/Cu+ ion because of its large ion diameter. The displaced Cu2+/Cu+ ions will transfer to adjacent unbonded Al site to form a CuAlO2 species.