High-resolution electron microscopic, spectroscopic, and catalytic studies of intentionally sulfided Pt/ZrO2–SO4 catalysts

Pt (3 wt%)/sulfated zirconia (Pt/SZ) was treated with H2S/H2 (10% v/v mixture) in the “fresh” (dried) as well as in the “used” state (after calcination and use in an autoclave run under 20 bar H2 and 300 Torr n-hexane). Morphological and structural changes as well as the chemical state of sulfur were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM). XPS detected sulfate (BE ∼169.5 eV) as the only chemical form of S in the unsulfided catalysts. A sulfide S 2p component (BE ∼162.5 eV) appeared in the S 2p region after sulfidation of both samples. Its amount correlated with the Pt content. Almost all “sulfide” components disappeared from the X-ray photoelectron spectra quasi in situ hydrogen treatment. Ion scattering spectroscopy (ISS) uncovered partly “embedded” Pt in sulfided catalysts and increased the sulfur signal. The H2S treatment almost completely removed surface carbon impurities. Simulation of the Pt 4f peaks by artificial composite spectra of metallic Pt (Pt0), PtOads, and PtS showed that Pt was present as a mixture of Pt0 in close interaction with support oxygens (and/or with an adsorbed O overlayer) and PtS after sulfidation. Using HRTEM several small flat Pt0 particles were detected in unsulfided catalysts. After sulfidation distinct flat Pt0 and PtS particles coexisted. Sulfided catalysts were inactive in benzene and cyclohexene hydrogenation but hydrogen treatment at 573 K reactivated the catalyst for the latter reaction. Thus, the platinum surface accommodated some deactivating chemisorbed sulfur; more S did not, however, form, a compact surface overlayer but appeared as a crystalline PtS phase with discernible lattice distances. Separate PtS particles seen by HRTEM were produced either by total sulfidation of small Pt0 particles or by migration of surface PtS entities from larger Pt crystallites to the support.