Influence of oxychlorination treatment on the surface and bulk properties of a Pt-Sn/Al2O3 catalyst

A Pt(3%)-Sn(4.5%)/Al2O3 catalyst has been studied after treatments prior to reduction involving calcination in air at 673 R or 823 K, or oxychlorination in air + 1,2-dichloropropane at 823 K. Bulk phases present were characterised by XRD and surface character was probed by IR of adsorbed CO and by CO chemisorption. Comparative IR and chemisorption results for Pt(3%)/Al2O3 are also presented. Pt-Sn/Al2O3 was used to catalyse heptane reforming reactions. Catalyst prepared from a chlorine-free precursor and calcined at 673 K contained well-dispersed SnO2 and PtO2 but no Pt degrees, whereas calcination at 823 K gave Pt degrees particles covered with a layer of O-adatoms. After reduction both catalysts contained Pt degrees particles, the surfaces of which, although exposing arrays of Pt degrees atoms, were partly covered in Sn degrees. No alloy was formed. Tin, probably as Sn2+, was also spread over the alumina support surface. The spreading of Sn over both Pt and alumina was greater after the higher temperature calcination pretreatment followed by reduction. Oxychlorination also gave Pt degrees particles with surface O-adatoms and probably surface Cl together with segregated crystalline SnO2. However, in marked contrast to the Cl-free catalysts, subsequent reduction gave 1:1 PtSn alloy particles with excess Sn spread over the alumina. Alloy surfaces did not exhibit large ensembles of exposed Pt degrees atoms. Alloy formation reduced catalyst activity but induced stability with respect to both activity and selectivity. Both alloy formation, and coking with time for catalysts with arrays of exposed Pt degrees promoted isomerisation reactions, but decreased aromatisation, hydrogenolysis and C-5 cyclisation. Selectivity changes accompanying alloy formation are discussed and the results are primarily attributed to geometric ensemble effects. (C) 1999 Elsevier Science B.V. All rights reserved.