Copper and tin incorporated microporous aluminium phosphate AIPO4-5 studied by x-ray absorption spectroscopy : including the Rietveld structure refinement of tin-containing AIPO4-5

Abstract

X-ray absorption spectroscopy (XAS) and x-ray powder diffraction (XRD) have been used to study the metal substituted microporous aluminophoshates CuAPO-5 (Papers 1 and 2) and SnAPO-5 (Paper 3). Also investigated were the copper-impregnated materials Cu:AlPO4-5 (Paper 2) and Cu:SnAPO-5 (Paper 3) in which copper is introduced using the incipient wetness method. The activities of these matherials in the selective catalytic reduction of NOx in the presence of ammonia (Paper 1) and hydrocarbons (Paper 2) have been measured. Although the materials transpired to be relatively poor catalysts, the catalytic measurements are of interest because they form part of the overall charaterisation of copper-environments and hence contribute to rationalising catalytic activity in terms of structural and electronic features.

XAS (XANES and EXAFS) studies on CuAPO-5 derived from copper (II) oxide have shown that copper is sited within the AlPO4-5 framework. In the as-synthesised sample the local environment is a tetragonally distorted Cu(0)6 octahedreon in which the axial bonds are possibly hydroxyl groups. Upon calcination at 550°C the environment changes to a non-tetragonally distorted octahedron for which the environment changes to a non-tetragonally distorted octahedron for which the coordination to the framework is increased by two long bonds to framework oxygens. For CuAPO-5 derived from copper (II) acetate, the XAS-results show that this is not a source for framework substitution. Copper is present as Cu(0)4 -groups which are attached to the AlPO4-5 framework through two Cu...Al distances. A combined EXAFS and powder x-ray diffraction study on SnAPO-5 shows that tin substitutes into the AlPO4-5 framework. It is shown that tin is five-coordinate in a trigonal bipyramid with its second axial corner protruding into the extra-framework area. From Rietveld refinements it is concluded that the overall framework is not affected by tin substitution into the framework or by the incorporation of copper.

The peak at ca. 3.1 Å in the Fourier transformed EXAFS is a convenient indicator of framework substitution in that it is known that metal framework sites are surrounded by four T-atoms at this distance. For copper-containing materials, colour is valuable when interpreting the EXAFS results because a turquoise colour is characteristic of a tetragonally distortet evironment. The two axial bonds (Cu-O) of the distorted octahedral environment are also shown to be an indicator of framework siting. The results show that this shell is often not revealed in the Fourier transformed EXAFS, due to a combination of high Debye-Waller factors and destructive interference from higher shells. However, when copper interacts with framework oxygen at these positions the shell does to appear in the Fourier transform, consistent with the relatively low Debye-Waller factor.

EXAFS studies on Cu:AlPO4-5 and CuAPO-5 show that copper is attached to the AlPO4-5 framework. Copper interacts with the tin-site rather than the regular T(Al/P)-site. Cu:SnAPO-5 contains significantly less copper than Cu:AlPO4-5, showing that the presence of tin seems to prevent the uptake of large amounts of copper.