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.

Keywords: Alkali metal; Cu species; Cu-SAPO-34; Migration.