Oxychlorination Redispersion of Pt Catalysts: Surface Species and Pt-Support Interactions Characterized by X-ray Absorption and FT-IR Spectroscopy

摘要

To help elucidate the oxychlorination redispersion reaction mechanism, the surface species formed on the surface of γ-Al 2 O 3 was characterized by X-ray absorption spectroscopy (XAS). The efficacy of redispersion was assessed by the Pt–Pt coordination number (CN Pt–Pt ) of redispersed, and then reduced samples. A nearly fully redispersed complex (Pt rd52 ) was prepared by treating a sintered model Pt/γ-Al 2 O 3 catalyst at 520 °C, Air/EDC (ethylene dichloride) of 30, and WHSV (Weight Hourly Space Velocity) of 0.07 h ?1 for 16 h. For investigating temperature effects, samples treated at 460 (Pt rd46 ) and 560 °C (Pt rd56 ) were also prepared for comparison. It was found that, while an octahedral resembling Pt(O s ) 3–4 (O–Cl) 2–3 (O s represents support oxygen or hydroxyl oxygen) complex was formed on γ-Al 2 O 3 of Pt rd52 , less O–Cl ligands were formed on the redispersed complexes, Pt rd46 and Pt rd56 . A negative correlation of CN Pt–Pt with CN Pt–Cl* (Cl* represents the Cl atom in O–Cl ligand) for these three samples further suggested that the formation of Pt–O–Cl played a key role in the redispersion process. Pt–O–Cl could be formed in the reaction of reactive Cl? and PtO 2 . At an operation temperature of lower-than-optimal temperatures of 520 °C, less Cl 2 dissociation and less O–Cl ligands were formed. On the other hand, higher temperatures may facilitate Cl 2 dissociation, but reduce the equilibrium conversion of HCl to Cl 2 , leading to increased HCl reaction with Pt (PtO 2 ) clusters to form Pt–Cl (Cl is the atom bonded directly to Pt), and decreased formation of Pt–O–Cl.