Irregular influence of alkali metals on Cu-SAPO-34 catalyst for selective catalytic reduction of NO with ammonia.

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 Cu/Cu ions. These exchanges result in the loss of Brønsted acid sites and migration of isolated Cu ions in Cu-SAPO-34, which decrease the NH-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 Cu/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 Cu/Cu ion because of its large ion diameter. The displaced Cu/Cu ions will transfer to adjacent unbonded Al site to form a CuAlO species.