Nature, distribution and reactivity of copper species in over-exchanged Cu-ZSM-5 catalysts: an XPS/XAES study

Abstract

Cu-ZSM-5 catalysts prepared by “over-exchange” (degree of exchange = 130%) and by impregnation have been studied by X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger spectroscopy (XAES). In both preparations, the external surface region of the catalysts is highly enriched in copper. In the over-exchanged zeolite, the observation of high XPS binding energies and unexpectedly low Auger kinetic energies shows that copper is present as isolated ions or small clusters. In the impregnated material large aggregates of metallic copper at the external zeolite surface are produced by reduction in hydrogen, which are not dispersed by treatment in oxygen or NO. In the exchanged materials, the copper becomes more evenly distributed across the zeolite crystal as a result of calcination or reductive treatment. Changing the composition of the atmosphere to which the exchanged Cu-ZSM-5 is exposed shows that there is ready conversion between Cu(II) and Cu(I). Copper is predominantly in the +2 oxidation state under conditions relevant to selective catalytic reduction of NO in lean-burn engine exhaust gas purification.