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We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer La2–xAxCoO4±δ(A = Sr, Ca; 0 ≤ x≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co K-edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co2.5+independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co2.9+. The Co mixed-valence state along the two series is fluctuating between two valence states, Co2.4+as in La2CoO4.2and Co2.9+as in LaSrCoO3.91, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co2+and low-spin Co3+. Furthermore, there is no quenching of the orbital contribution for the high-spin Co2+, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the abplane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the caxis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the clattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.

The effect of substituting iron and zinc for cobalt in CaBaCo4O7was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo4O7decreases with increasing the content of either Fe or Zn. The samples CaBaCo3ZnO7and CaBaCo4–xFexO7with x≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo4–xFexO7(0.5 ≤ x≤ 2) samples are antiferromagnetically ordered following a new propagation vector k= (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo3ZnO7sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe3+or Zn2+doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co2+/Co3+valence state or, alternatively, Co2+and Co3+ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO4tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo4O7, revealing the occurrence of local magneto-elastic coupling.

CVD-grown multiwalled carbon nanotubes were purified by applying four different treatments with increasing oxidation severity. The growing severity of the treatment results in progressive oxygen functionalization of the surface along with introduction of an increasing quantity of defects on the carbon nanotube walls. Iridium–N-heterocyclic carbene complexes were covalently anchored to those oxidized surfaces through their surface carboxylic acids viaacetyl linkers. The carbon nanotube-based iridium–NHC hybrid materials developed are active in the hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source but with rather different activity. The catalytic activity of the hybrid catalysts is strongly influenced by the type and amount of oxygenated functionalization resulting from the treatment applied to the support, being the most active and the most oxidized material.