Migration of Cu species in Cu/SAPO-34 during hydrothermal aging

In the present work, we report the migration of copper species in Cu/SAPO-34 during hydrothermal aging and its role in selective catalytic reduction (SCR) of NOx. Two Cu/SAPO-34 catalysts, prepared by ion-exchange and precipitation methods, were hydrothermally aged at 700 °C for 48 h and characterized in detail. The aged ion-exchanged catalyst exhibited little deterioration in NH3-SCR activity; however, the catalytic activity of the precipitated catalyst, which showed inferior activity for the fresh sample, was markedly improved after hydrothermal treatment and became comparable to that of the ion-exchanged sample. A detailed characterization of the Cu species before, during, and after the hydrothermal treatment using a combination of experimental techniques clearly demonstrated that the copper species in the precipitated sample, which initially existed as CuO clusters on the external surface of SAPO-34, migrated to the ion-exchanged sites as isolated ions after aging. The change in copper oxidation states and coordination environment during the aging process was probed by in situ X-ray absorption spectroscopy, and a possible mechanism of Cu migration involving metallic Cu species was proposed.