SULFIDED PT CATALYSTS - MONITORING SURFACE CHEMICAL-STATE AND CATALYTIC PROPERTIES IN N-HEXANE REACTIONS

The amount and valence state of sulfur in sulfided Pt catalysts was mo nitored, together with parallel studies of the catalytic properties of these catalysts in order to clarify the catalytic effect of S in diff erent chemical states on reactions of low-temperature n-hexane reformi ng. Pt black and 6% Pt/SiO2 (EUROPT-1) were sulfided by H-2/H2S. X-ray photoelectron spectroscopy (XPS) detected both sulfate and sulfide. T he chemical state of Pt corresponded to metallic Pt with low amounts o f S containing surface adlayer and/or PtS present. This could be obser ved with Pt black and also with EUROPT. With S present, both catalysts lost ca 95% of their activity in n-hexane transformation at 603 K, n- hexenes being the predominant products. Prolonged runs with n-hexane a s well as regenerations using O-2 and H-2 removed most sulfidic S from Pt black and about half of the original activity came back. Isomers a nd methylcyclopentane were produced in this state with high selectivit ies. Some reactivation of EUROPT (where no sulfate could be detected) occurred within one single run. A uniform scheme is proposed to descri be the effects of sulfate and sulfide. Accordingly, sulfate acts as a structural modifier occupying a fraction of the active surface. Sulfid e in small amounts, in turn, has a bonding modifier effect and poisons selectively benzene and fragmentation, enhancing (within the reduced activity) the selectivity of Cs-cyclization and, possibly, also that o f isomerization. (C) 1998 Academic Press.