Performance, structure, and mechanism of CeO _2 in HCl oxidation to Cl 2

摘要

Experimental and theoretical studies reveal performance descriptors and provide molecular-level understanding of HCl oxidation over CeO _2. Steady-state kinetics and characterization indicate that CeO _2 attains a significant activity level, which is associated with the presence of oxygen vacancies. Calcination of CeO _2 at 1173 K prior to reaction maximizes both the number of vacancies and the structural stability of the catalyst. X-ray diffraction and electron microscopy of samples exposed to reaction feeds with different O _2/HCl ratios provide evidence that CeO _2 does not suffer from bulk chlorination in O 2-rich feeds (O 2/HCl ≥ 0.75), while it does form chlorinated phases in stoichiometric or sub-stoichiometric feeds (O _2/HCl ≤ 0.25). Quantitative analysis of the chlorine uptake by thermogravimetry and X-ray photoelectron spectroscopy indicates that chlorination under O _2-rich conditions is limited to few surface and subsurface layers of CeO _2 particles, in line with the high energy computed for the transfer of Cl from surface to subsurface positions. Exposure of chlorinated samples to a Deacon mixture with excess oxygen rapidly restores the original activity levels, highlighting the dynamic response of CeO _2 outermost layers to feeds of different composition. Density functional theory simulations reveal that Cl activation from vacancy positions to surface Ce atoms is the most energy-demanding step, although chlorine-oxygen competition for the available active sites may render re-oxidation as the rate-determining step. The substantial and remarkably stable Cl _2 production and the lower cost of CeO _2 make it an attractive alternative to RuO _2 for catalytic chlorine recycling in industry.