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 chemisorption and FTIR of adsorbed CO after each cycle in a series of six oxychlorination-reduction cycles or six oxidation-reduction cycles followed by oxychlorination-reduction. Spectra of CO on each catalyst directly after oxychlorination 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 species varying with Sn content. Tin hindered both catalyst sintering during oxidation and redispersion during oxychlorination. The IR results for oxidised catalyst after subsequent reduction were compatible with Pt-o dispersed over a Sn(II)-modified alumina surface. However, addition of chlorine promoted greater intimacy between Pt and Sn with the latter blocking low coordination 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 catalyst character to preparation procedure. (C) 1999 Academic Press.