Structural, physical and chemical properties of nanostructured nickel-substituted ceria oxides under reducing and oxidizing conditions

Abstract

This work reports the synthesis of nanostructured Ce1-xNixO2-δ (x = 0.05, 0.1, 0.15 and 0.2) oxides prepared by cation complexation route and with the main objective of studying their redox properties using a combination of electron microscopy, synchrotron radiation X-ray diffraction (SR-XRD) and X-ray absorption near-edge spectroscopy (XANES). The Ce1-xNixO2-δ series of nanopowders maintain the cubic crystal structure (Fm3m space group) of pure ceria, with an average crystallite size of 5-7 nm indicated by XRD patterns and confirmed by transmission electron microscopy. In situ SR-XRD and XANES carried out under reducing (5% H2/He; 5% CO/He) and oxidizing (21%O2/N2) atmospheres at temperatures up to 500 °C show a Ni solubility limit close to 15 at.% in air at room temperature, decreasing to about 10 at.% after exposure to 5% H2/He atmosphere at 500 °C. At room temperature in air, the effect of Ni on the lattice parameter of Ce1-xNixO2-δ is negligible, whereas a marked expansion of the lattice is observed at 500 °C in reducing conditions. This is shown by XANES to be correlated with the reduction of up to 25% of Ce 4+ cations to the much larger Ce , possibly accompanied by the formation of oxygen vacancies. The redox ability of the Ce 4+ /Ce 3+ couple in nanocrystalline Ni-doped ceria is greatly enhanced in comparison to pure ceria or achieved by using other dopants (e.g. Gd, Tb or Pr), where it is limited to less than 5% of Ce cations.

Nickel Speciation and Methane Dry Reforming Performance of Ni/CexZr1–xO2 Prepared by Different Synthesis Methods

“…It is suggested that the incorporation of nickel into the CZ support introduced defects into the CZ lattice, which increased the oxygen storage capacity of the support and thus the H 2 consumption during TPR. 89 For both bulk synthesis techniques (CP and CS), the total H 2 consumption increased with increasing nickel content. At 2 wt % Ni content, the bulk synthesis produced more surface oxygen species compared to the surface synthesis methods, and CS samples had the highest surface oxygen concentration.…”

“…The H 2 consumption assigned to CZ were also higher in bulk synthesis samples (Table S3). It is suggested that the incorporation of nickel into the CZ support introduced defects into the CZ lattice, which increased the oxygen storage capacity of the support and thus the H 2 consumption during TPR . For both bulk synthesis techniques (CP and CS), the total H 2 consumption increased with increasing nickel content.…”

Highly Cr-Substituted CeO2Nanoparticles Synthesized Using a Non-equilibrium Supercritical Hydrothermal Process: High Oxygen Storage Capacity Materials Designed for a Low-Temperature Bitumen Upgrading Process

“…The ionic radius of the Cr 3+ (CN: VI, 61.5 Å) is much smaller than that of the Ce 4+ (CN: VIII, 97 Å) . The charge balance becomes unstable, resulting in the formation of an oxygen vacancy. − Therefore, the crystal lattice shrinks while being strained as much chromium is substituted, thereby further affecting crystal size and morphology.…”

Facile Synthesis and Characterization of Nanostructured Transition Metal/Ceria Solid Solutions (TMxCe1–xO2−δ, TM = Mn, Ni, Co, or Fe) for CO Oxidation

“…The products ( np -TM x Ce 1– x O 2−δ ) with 10 mol % doping all displayed the cubic CeO 2 phase, as shown in Figure . On the basis of Rietveld refinement analysis, the fitting lattice parameters of these samples were 5.4086, 5.4102, and 5.4127 Å for Fe, Co, and Ni doping, respectively (Table S7), and all these values are comparable to the previous reports. − From these results, the mole fractions of the transition metal ions substituted into the ceria lattice were calculated as Fe 0.09 Ce 0.91 O 2−δ , Co 0.08 Ce 0.92 O 2−δ , and Ni 0.08 Ce 0.92 O 2−δ . ICP and EDS quantitative analyses also indicated consistent compositions, confirming the successful synthesis of homogeneous np -Fe 0.09 Ce 0.91 O 2−δ , np -Co 0.08 Ce 0.92 O 2−δ , and np -Ni 0.08 Ce 0.92 O 2−δ solid solutions.…”