XPS Analysis of Platinum and /or Cobalt-loaded Zeolites relevant for SCR Reaction

BOIX, ALICIA V.; GARCÍA FIERRO, JOSÉ LUIS

SURFACE AND INTERFACE ANALYSIS

Jonh Wiley & Sons, Ltd.

Lugar: Londres; Año: 1999 vol. 27 p. 1107 - 1113

0142-2421

Bimetallic Pt-Co and the corresponding monometallic Pt- and Co-samples, exchanged with Na-mordenite, Na-ZSM5 and K-ferrierite zeolites, have been studied by photoelectron spectroscopy. The oxidation state of the elements and their abundance at the surface were studied by exposing the samples, in the pretreatment chamber of the spectrometer, to several different reaction environments that resemble the conditions encountered in the catalytic selective reduction (SCR) of exhaust gases. In calcined Co-zeolites, Co2+ ions occupy exchanged positions in the structure and also coordinate to lattice oxygen with a high dispersion. Incorporation of Pt2+ in Co-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 PtCo clusters are generated, which coexist with Co2+ and Pt2+ ions in both exchanged and coordinated to lattice oxygen positions. Exposure to the SCR reaction mixture: CH4+NO+O2+H2O leads to reoxidation of cobalt in the bimetallic Pt-Co clusters, forming Co2+ and Co(OH)2 on the zeolite surface, while Pt migrates towards the outer surface, yielding larger Pt particles.