Performance, structure, and mechanism of CeO2 in HCl oxidation to Cl2

Experimental and theoretical studies reveal performance descriptors and provide molecular-level understanding of HCl oxidation over CeO2. Steady-state kinetics and characterization indicate that CeO2 attains a significant activity level, which is associated with the presence of oxygen vacancies. Calcination of CeO2 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 CeO2 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 CeO2 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 CeO2 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 CeO2 make it an attractive alternative to RuO2 for catalytic chlorine recycling in industry. (C) 2011 Elsevier Inc. All rights reserved.