Selective catalytic reduction of NOx over microporous CuAPO-5: structural characterisation by XAS and XRD

Abstract

The effects of copper source, copper content, Al : P ratio and silicon content for the hydrothermal synthesis of CuAPO-5 using tetraethylammonium hydroxide as template have been investigated. The copper to phosphorus ratio in the synthesis gel was varied from 0.04 to 0.2, and copper(II) oxide and copper(II) acetate were used as copper sources. The samples were characterised by XAS (XANES, EXAFS), XRD (X-ray diffraction), TGA (thermogravimetric analysis), SEM (scanning electron microscopy) and elemental analysis. Rietveld refinements of the structure of CuAPO-5 were carried out on synchrotron X-ray powder diffraction data. The catalytic properties of CuAPO-5 in selectively reducing NOx in the presence of hydrocarbons (SCR-HC) were studied. The results from EXAFS show that copper is present in a distorted octahedral environment, with 4 or 5 Cu–O distances at 1.95 Å and 1 or 2 Cu–Al/P distances at about 3.14 Å for the as-synthesised samples. The calcined samples still show the same four Cu–O distances at 1.95 Å, but with the composite peak at 3.14 Å being significantly reduced. The turquoise colour of the as-synthesised CuAPO-5 is consistent with the copper atoms being complexed by water molecules within the pores as the hexaaquacopper(II) complex. The extraframework complex cation is electrostatically bound to the anionic phosphate sites of the framework. The colour changes to olive green on calcination, indicating tetrahedrally coordinated framework copper. The selective catalytic reduction of NOx by propene is catalysed by CuAPO-5 to the extent of 18% conversion. The Rietveld analysis show that calcined CuAPO-5 belongs to the P6/mcc spacegroup in which Al and P are indistinguishable.