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Abstract:
Pt black samples in a fresh state, deactivated by coking and treated with H2S were studied with electron spectroscopy. Their catalytic properties were tested in skeletal reactions of n-hexane. The Pt 4f XPS peaks show that Pt is predominantly in the metallic state after the treatments. After sulfidation, a minor amount of PtS can be detected in the difference spectrum (Pt unsulfided - Pt sulfided) only. The growth of the O 1s intensity after sulfidation can partly be attributed to the sulfate component. C 1s shows, as a rule, residual carbon in low amounts on sulfided samples. The S 2p band shows 4–8 at.-% S with respect to the total surface in two valence states: sulfate and sulfide, the latter including also minor amounts of organic sulfur which could arise from the sulfidation of hydrocarbonaceous residues. UPS demonstrated chemical Pt—S interaction even after O2/H2 regeneration when the Fermi-edge Pt intensity rose to a height almost equal to that of a regenerated sulfur-free Pt. ISS shows that sulfur - as opposed to carbon and oxygen - is a surface component. Sputtering effect of He+ ions used in ISS can remove sulfate while a large part of sulfide is removed during O2/H2 regeneration and/or during n-hexane test runs. Both sulfidation and carbonization strongly decrease the overall catalytic activity. The selectivity of Pt with S present mainly as sulfate was similar to that of carbonized Pt, mainly producing hexenes. Pt mainly containing sulfide, produced initially mainly methylcyclopentane, as reported also for the sulfided single crystal catalyst. Carbonized Pt could be fully regenerated by O2/H2 treatment at 600 K but the same treatment - even repeated - restored the overall activity of a sulfided Pt only partially. We believe that sulfide acts mainly as a bonding modifier whereas sulfate can be a structural modifier influencing the selectivity similarly as coke or coke precursors.
