In situXAS and IR studies on Cu:SAPO-5 and Cu:SAPO-11: the contributory role of monomeric linear copper(i) species in the selective catalytic reduction of NOx by propene

Abstract

Cu:SAPO-5 and Cu:SAPO-11 were prepared by conventional and hydrothermal ion exchange. Copper incorporation is increased six-fold by hydrothermal ion exchange relative to conventional methods. In all cases, the amount of copper taken up by SAPO-11 is superior to uptake in SAPO-5. Copper is divalent and in tetragonally-distorted octahedral environments in the as-prepared samples independent of the method of incorporation for both systems. The local structures about the metal and the valence states associated with the different steps in the selective catalytic reduction of NO_x in the presence of propene (SCR-HC) have been investigated using X-ray absorption spectroscopy (XAS). For both the Cu:SAPO-5 and Cu:SAPO-11 systems, heating in helium partially autoreduces copper(II) to copper(I). Following activation in oxygen, propene causes further reduction to copper(I) in all four samples as shown by the evolution of an intense pre-edge diagnostic feature. XANES analysis reveal this to be characteristic of monomeric linear two coordinate copper(I) species. This is a prime example of a pre-edge peak with such a high intensity being observed in the solid state. This is supported by IR where peaks attributed to bidentate copper were observed for Cu:SAPO-11/HT. For all four samples NO_x partially reoxidises the copper(I) formed in the helium and propene steps. Ion exchanged Cu:SAPO-5 and Cu:SAPO-11 exhibit low activity in reducing NO_x by propene in an oxygen rich environment. The role of the copper ion during NO adsorption was studied using in situ infra red spectroscopy. The activity of copper exchanged materials is governed by both the degree of reducibility of copper(II) and the ease of reversing the valence states with the structural characteristics of the parent materials playing a crucial role.