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Free keywords:
CeO2, dopants, conductivity, XRD, PGAA, DFT, HCl oxidation, CO oxidation
Abstract:
The role of trivalent (La, Sm, Gd, Y) and tetravalent (Hf, Zr, Ti) dopants was investigated on the catalytic, structural and electronic properties of ceria. Promoted ceria catalysts were synthesized by co-precipitation with ammonia, and tested in HCl and CO oxidation. Ceria catalysts exhibit a medium high reactivity and excellent stability in HCl oxidation. The intrinsic reactivity of ceria in HCl oxidation can be improved by a factor of 2 when doping with Hf and Zr in appropriate quantities, whereas trivalent dopants are detrimental. Although both oxidation reactions rely on the existence of oxygen vacancies, the order of reactivity in HCl and CO oxidation is not completely parallel. The effect of promoters on the electronic conductivity and the vacancy formation energy were studied by contactless conductivity experiments using the microwave cavity perturbation technique and by DFT calculations. Furthermore, transport properties were also assessed based on theoretical calculations. The oxygen vacancy formation energy follows well the conductivity (polaron mobility) order (trivalent > tetravalent > undoped) observed in inert and oxidizing conditions. This implies that none of these properties correlates with the reactivity. On the other hand, reducing conditions strongly enhanced the conductivity of Hf and Zr doped ceria. In HCl oxidation only the balanced reduction of both Cl and O vacancy formation energies allows for an enhanced reactivity. Promoters give rise to lattice contraction/expansion modifying vacancy formation energies, adsorption properties and surface coverages.
