SULFUR ADSORBED ON PT CATALYST - ITS CHEMICAL-STATE AND EFFECT ON CATALYTIC PROPERTIES AS STUDIED BY ELECTRON-SPECTROSCOPY AND N-HEXANE TEST REACTIONS

Pt black samples in a fresh state, deactivated by coking and treated w ith H2S were studied with electron spectroscopy. Their catalytic prope rties were tested in skeletal reactions of n-hexane. The Pt 4f XPS pea ks show that Pt is predominantly in the metallic state after the treat ments. After sulfidation, a minor amount of PtS can be detected in the difference spectrum (Pt unsulfided - Pt sulfided) only. The growth of the O 1 s intensity after sulfidation can partly be attributed to the sulfate component. C 1s shows, as a rule, residual carbon in low amou nts 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 l atter including also minor amounts of organic sulfur which could arise from the sulfidation of hydrocarbonaceous residues. UPS demonstrated chemical Pt-S interaction even after O-2/H-2 regeneration when the Fer mi-edge Pt intensity rose to a height almost equal to that of a regene rated sulfur-free Pt. ISS shows that sulfur - as opposed to carbon and oxygen - is a surface component. Sputtering effect of He+ ions used i n ISS can remove sulfate while a large part of sulfide is removed duri ng O-2/H-2 regeneration and/or during n-hexane test runs. Both sulfida tion and carbonization strongly decrease the overall catalytic activit y. The selectivity of Pt with S present mainly as sulfate was similar to that of carbonized Pt, mainly producing hexenes. Pt mainly containi ng sulfide, produced initially mainly methylcyclopentane, as reported also for the sulfided single crystal catalyst. Carbonized Pt could be fully regenerated by O-2/H-2 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 selectivi ty similarly as coke or coke precursors.