Promoted Ceria: A Structural, Catalytic, and Computational Study

Abstract

The role of trivalent (La, Sm, Gd, and Y) and tetravalent (Hf, Zr, and Ti) dopants in the catalytic, structural, and electronic properties of ceria was investigated. Promoted ceria catalysts were synthesized by coprecipitation 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 effects 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 density functional theory calculations. Furthermore, transport properties were also assessed on the basis of theoretical calculations. The order of oxygen vacancy formation energy follows well the order of conductivity (polaron mobility) (trivalent > tetravalent > undoped) observed under 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.

Electrospun ceria-based nanofibers for the facile assessment of catalyst morphological stability under harsh HCl oxidation reaction conditions

“…If a catalytic reaction mechanism involves formation and curing of oxygen vacancies in the catalyst, then the rate of the reaction may depend on the number of the volatile oxygen species available, which should lead to a correlation between the reaction rate and the catalyst's OSC. The suggested mechanisms of the HCl oxidation reaction over ceria involve formation of oxygen vacancies in the catalyst [17,53]. In support of this theory, the study [17] establishes the correlation between HCl oxidation activity of differently treated ceria samples and their ultimate reducibility by hydrogen at 573 K (as opposed to the correlation to their surface area).…”

“…In support of this theory, the study [17] establishes the correlation between HCl oxidation activity of differently treated ceria samples and their ultimate reducibility by hydrogen at 573 K (as opposed to the correlation to their surface area). Teschner and co-workers [53] offer further theoretical insights into the role of oxygen vacancies in the HCl oxidation over ceria: according to the calculations, lower energy of oxygen vacancy formation in the catalyst will facilitate the Deacon reaction if such energy decrease is accompanied by lower energy of chlorine vacancy formation. Calculations of the vacancies formation energies for different doped ceria samples are in good agreement with their HCl oxidation performance: tetravalent metal oxides (Zr, Hf) are beneficial, whereas trivalent metal oxides (La, Sm) are detrimental.…”

Unraveling the structure sensitivity in methanol conversion on CeO2: A DFT+U study

“…Perdew-Becke-Ernzerhof (PBE) [45] functional was used in combination with an effective U term, U eff , of 4.5 eV (this term is defined as the difference between the Coulomb, U, and the exchange, J, terms) [46], as it has been previously proven to provide satisfactory results [47][48][49][50]. The purpose of adding this Hubbard term U is to diminish the self-interaction error and to properly localize the Ce 4f states.…”

Effect of metal ions doping (M = Ti4+, Sn4+) on the catalytic performance of MnO /CeO2 catalyst for low temperature selective catalytic reduction of NO with NH3

“…One of the efficient methods to restrain this effect and improve the property of CeO 2 support is incorporating foreign metal cations into the lattice of CeO 2 [15][16][17]. It has been reported that doping of Ti or Sn in CeO 2 can modify the catalyst surface area and insertion of doping ions improve the lattice oxygen mobility, resulting in a better redox quality [18]. Solid solutions of Ce 1−x Ti x O 2 and Ce 1−x Sn x O 2 have been reported by Baidya et al [19] to show high OSC.…”