Effects of oxidation-reduction and oxychlorination-reduction cycles on CO adsorption by Pt-Sn/Al2O3 catalysts

PtSn/Al2O3 catalysts containing 0.30 wt% Pt and 0, 0.15, 0.30, and 0.45 wt% Sn and prepared from Cl-free precursors have been studied by CO chemisorpt ion and FTIR of adsorbed CO after each cycle in a series of six oxychlorina tion-reduction cycles or six oxidation-reduction cycles followed by oxychlo rination-reduction. Spectra of CO on each catalyst directly after oxychlori nation are also reported. After oxychlorination catalysts contained exposed Pt sites present as Pt-o (covered with O-adatoms), Pt(II), Pt(IV) oxide, PtCl2 (forming PtCl2CO and PtCl2(CO)(2) with CO), and PtOxCly, the relative proportions of these speci es varying with Sn content. Tin hindered both catalyst sintering during oxi dation and redispersion during oxychlorination. The IR results for oxidised catalyst after subsequent reduction were compatible with Pt-o dispersed ov er a Sn(II)-modified alumina surface. However, addition of chlorine promote d greater intimacy between Pt and Sn with the latter blocking low coordinat ion Pt sites and reducing the size of exposed ensembles of Pt atoms. Two Pt(0.3%)-Sn(0.3%)/Al2O3 catalysts prepared from tin(TI) oxalate and tin (II) tartrate gave different results emphasising the sensitivity of catalys t character to preparation procedure. (C) 1999 Academic Press.