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 oxychlorinati on in air + 1,2-dichloropropane at 823 K. Bulk phases present were characte rised by XRD and surface character was probed by IR of adsorbed CO and by C O chemisorption. Comparative IR and chemisorption results for Pt(3%)/Al2O3 are also presented. Pt-Sn/Al2O3 was used to catalyse heptane reforming reac tions. Catalyst prepared from a chlorine-free precursor and calcined at 673 K contained well-dispersed SnO2 and PtO2 but no Pt degrees, whereas calcin ation at 823 K gave Pt degrees particles covered with a layer of O-adatoms. After reduction both catalysts contained Pt degrees particles, the surface s of which, although exposing arrays of Pt degrees atoms, were partly cover ed in Sn degrees. No alloy was formed. Tin, probably as Sn2+, was also spre ad over the alumina support surface. The spreading of Sn over both Pt and a lumina was greater after the higher temperature calcination pretreatment fo llowed by reduction. Oxychlorination also gave Pt degrees particles with su rface O-adatoms and probably surface Cl together with segregated crystallin e SnO2. However, in marked contrast to the Cl-free catalysts, subsequent re duction gave 1:1 PtSn alloy particles with excess Sn spread over the alumin a. Alloy surfaces did not exhibit large ensembles of exposed Pt degrees ato ms. Alloy formation reduced catalyst activity but induced stability with re spect to both activity and selectivity. Both alloy formation, and coking wi th time for catalysts with arrays of exposed Pt degrees promoted isomerisat ion reactions, but decreased aromatisation, hydrogenolysis and C-5 cyclisat ion. Selectivity changes accompanying alloy formation are discussed and the results are primarily attributed to geometric ensemble effects. (C) 1999 E lsevier Science B.V. All rights reserved.