oxygen defects

A 3.7% Rh/CeO 2 catalyst was prepared by wet impregnation of a low-surface-area ceria (23 m 2 g −1) with an aqueous solution of rhodium nitrate. This catalyst was reduced in a flow of H 2 at temperatures T R ranging from 200 to 1000 • C. The number of reduced centres of ceria in Rh/CeO 2 catalysts was measured by in situ reoxidation of these centres at T OX by pulses of water according to the following reaction: Reduced centre + H 2 O → Oxidised centre + H 2. The amount of hydrogen, Q H , evolved in the reaction allows us to calculate the number of reduced centres of the support since the noble metal would not be oxidised. Q H remains relatively small for T R not exceeding 500 • C, then increases rapidly to reach a pseudo-plateau at about 850 • C. At 900–1000 • C, the oxide composition would be close to CeO 1.75. Contrasting with the effect of T R , relatively small variations are recorded with T OX for a catalyst reduced at 850 • C: a maximum value of Q H is reached for T OX close to 850 • C, but 74% of this maximum yield in H 2 is already obtained when the reaction with steam is carried out at 100 • C. Oxidation by steam was also carried out on a catalyst prereduced in deuterium. It was shown that the hydrogen formed comes from the decomposition of water (H 2) and not from the storage of deuterium in the support during the reduction stage. In situ characterisation by XRD showed that there is a deep perturbation of the cubic structure at 850 • C, followed by a reduction of CeO 2 into Ce 2 O 3 (hexagonal) at higher temperatures (900–1000 • C). A treatment in water restores the cubic structure of CeO 2. The support having been previously reduced at 900 • C, there is virtually no sintering of the ceria crystallites upon these high-temperature treatments in hydrogen and steam.

Hydroxyapatite (HAp) has structural features that define its basic physical properties, which have an important role at the surface, and it is one of the most used materials in bone implants. In this work, we present a density functional modeling (DFT) study of HAp both as bulk and with special HAp models with various defects, especially oxygen vacancies in HAp surface layers, which can also determine photocatalytic properties, confirmed experimentally. The first-principles calculations of bulk and modified HAp were carried out using local basis (AIMPRO) and plane-wave (VASP) codes. Data obtained are analyzed using both approaches, and compared.