Identification of copper species present in Cu-ZSM-5 catalysts for NOx reduction

The structure of Cu-ZSM-5 catalysts that show activity for direct NO decomp osition and selective catalytic reduction of NOx by hydrocarbons has been i nvestigated by a multitude of modern surface analysis and spectroscopy tech niques including X-ray photoelectron spectroscopy, thermogravimetric analys is, and in situ Fourier transform infrared spectroscopy. A series of four c atalysts were prepared by exchange of Na-ZSM-5 with dilute copper acetate, and the copper loading was controlled by variation of the solution pH, Unde rexchanged catalysts contained isolated Cu2+OH-(H2O) species and as the cop per loading was increased Cu2+ ions incorporated into the zeolite lattice a ppeared. The sites at which the latter two copper species were located were fundamentally different. The Cu2+OH-(H2O) moieties were bound to two latti ce oxygen ions and associated with one aluminum framework species. In contr ast, the Cu2+ ions were probably bound to four lattice oxygen ions and asso ciated with two framework aluminum ions. Once the Cu-ZSM-5 samples attained high levels of exchange, the development of [Cu(mu-OH)(2)Cu](n)2+OH-(H2O) species along with a small concentration of Cu(OH)(2) was observed. On acti vation in helium to 500 degrees C the Cu2+OH-(H2O) species transformed into Cu2+O- and Cu+ moieties, whereas the Cu2+ ions were apparently unaffected by this treatment (apart from the loss of ligated water molecules). Calcina tion of the precursors resulted in the formation of Cu2+O2- and a one-dimen sional CuO species. Temperature-programmed desorption studies revealed that oxygen was removed from the latter two species at 407 and 575 degrees C, r espectively. (C) 1999 Academic Press.