Abstract
To help elucidate the oxychlorination redispersion reaction mechanism, the surface species formed on the surface of γ-Al2O3 was characterized by X-ray absorption spectroscopy (XAS). The efficacy of redispersion was assessed by the Pt–Pt coordination number (CNPt–Pt) of redispersed, and then reduced samples. A nearly fully redispersed complex (Ptrd52) was prepared by treating a sintered model Pt/γ-Al2O3 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 (Ptrd46) and 560 °C (Ptrd56) were also prepared for comparison. It was found that, while an octahedral resembling Pt(Os)3–4(O–Cl)2–3 (Os represents support oxygen or hydroxyl oxygen) complex was formed on γ-Al2O3 of Ptrd52, less O–Cl ligands were formed on the redispersed complexes, Ptrd46 and Ptrd56. A negative correlation of CNPt–Pt with CNPt–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 PtO2. At an operation temperature of lower-than-optimal temperatures of 520 °C, less Cl2 dissociation and less O–Cl ligands were formed. On the other hand, higher temperatures may facilitate Cl2 dissociation, but reduce the equilibrium conversion of HCl to Cl2, leading to increased HCl reaction with Pt (PtO2) clusters to form Pt–Cl (Cl is the atom bonded directly to Pt), and decreased formation of Pt–O–Cl.
Highlights
Notwithstanding a number of investigations to prevent sintering, supported-metal catalyst deactivation owing to metal sintering is inevitable [1], and redispersion of sintered metal clusters is the key in catalyst rejuvenation
In order to separate the contributions from different shells and to identify ligands of the redispersed complexes, phase- and amplitude-corrected Fourier transform was performed to the EXAFS function, x(k)
O–Cl ligand concomitant with the disruption of Pt clusters in the redispersion process, as was evidenced by EXAFS, suggested that chlorine formed from the reaction of HCl with oxygen plays the main role in the redispersion process
Summary
Notwithstanding a number of investigations to prevent sintering, supported-metal catalyst deactivation owing to metal sintering is inevitable [1], and redispersion of sintered metal clusters is the key in catalyst rejuvenation. Based on scientific and patent literature surveys, halogen acid gas, oxygen, chlorine, and nitric oxide can be used as agents to redisperse sintered Pt catalysts [1,2,3,4,5,6,7,8,9]. Oxychlorination, which uses oxygen- and chlorine-containing compounds as the dispersion agent, is the most common method employed in the industry to redisperse noble metals on non-reducible metal oxides, such as silica, alumina, zeolite, and cordierite [1,4,10,11]. Catalysts 2019, 9, 362; doi:10.3390/catal9040362 www.mdpi.com/journal/catalysts (X-ray Absorption Spectroscopy) and TEM (Transmission Electron Microscopy), a rapid and effective redispersion of Pt particles supported on ceria-based oxide catalysts three-way catalysts (TWCs) has been developed by Nagai et al [13,14].
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