J. Llorca et al., PLATINUM-TIN CATALYSTS SUPPORTED ON SILICA HIGHLY SELECTIVE FOR N-HEXANE DEHYDROGENATION, Journal of catalysis, 166(1), 1997, pp. 44-52
Silica-supported Pt-Sn catalysts were prepared by two-step impregnatio
n from [PtCl2(PPh(3))(2)] and SnCl2 solutions of appropriate concentra
tions to yield Pt/Sn atomic ratios ranging from 0.2 to 5.0. In these s
ystems, the presence of true Pt-Sn alloys was confirmed by X-ray diffr
action, transmission electron microscopy, energy dispersive X-ray anal
ysis and electron nanodiffraction. Pt and PtSn ahoy phases were found
on catalysts with Pt/Sn > 1, PtSn alloy alone on the catalyst with Pt/
Sn = 1 and PtSn and PtSn2 alloys, together with Sn in the catalysts wi
th Pt/Sn < 1. All these catalysts were tested in the skeletal reaction
s of n-hexane at 753 K and atmospheric pressure. The selectivity of Pt
changed significantly when alloyed with tin. For Sn-rich compositions
a segregation of tin toward the catalyst surface was shown by photoel
ectron spectroscopy, and high hydrogenolysis selectivity and fast deac
tivation were observed. In contrast, Pt-rich catalysts, in which a wel
l defined PtSn alloy was observed, were much more stable and exhibited
high selectivity to dehydrogenation reaction while maintaining low co
nversions to benzene and hydrogenolysis products. This selectivity pat
tern can be interpreted in terms of a change in adsorption properties
due to differences in the number of adjacent Pt atoms required for the
various reaction pathways. (C) 1997 Academic Press.