X-ray photoelectron spectroscopy analysis of platinum- and/or cobalt-loaded zeolites relevant for selective catalytic reduction of NOx

Bimetallic Pt-Co and the corresponding monometallic Pt and Co samples, exch anged with Na-mordenite, Na-ZSM5 and K-ferrierite zeolites, have been studi ed by photoelectron spectroscopy. The oxidation state of the elements and t heir abundance at the surface were studied by exposing the samples, in the pretreatment chamber of the spectrometer, to several different reaction env ironments that resemble the conditions encountered in the selective catalyt ic reduction (SCR) of exhaust gases. In calcined Go-zeolites, Co2+ ions occ upy exchanged positions in the structure and also coordinate to lattice oxy gen with a high dispersion. Incorporation of Pt2+ in Go-containing samples influences the electronic state of cobalt in both the calcined and reduced states. Upon reduction in hydrogen at 623 K, highly dispersed bimetallic Pt -Co clusters are generated, which coexist with Co2+ and Pt2+ ions in both e xchanged positions and coordinated to lattice oxygen. Exposure to the SCR r eaction mixture of CH4 + NO + O-2 + H2O leads to reoxidation of cobalt in t he bimetallic Pt-Co clusters, forming Co2+ and Co(OH)(2) on the zeolite sur face, while Pt migrates towards the outer surface, yielding larger Pt parti cles. Copyright (C) 1999 John Wiley & Sons, Ltd.