Abstract
The cerium(iii) hydroxide chloride Ce(OH)2Cl crystallises directly as a polycrystalline powder from a solution of CeCl3·7H2O in poly(ethylene) glycol (Mn = 400) heated at 240 °C and is found to be isostructural with La(OH)2Cl, as determined from high-resolution synchrotron powder X-ray diffraction (P21/m, a = 6.2868(2) Å, b = 3.94950(3) Å, c = 6.8740(3) Å, β = 113.5120(5)°). Replacement of a proportion of the cerium chloride in synthesis by a second lanthanide chloride yields a set of materials Ce1-xLnx(OH)2Cl for Ln = La, Pr, Gd, Tb. For La the maximum value of x is 0.2, with an isotropic expansion of the unit cell, but for the other lanthanides a wider composition range is possible, and the lattice parameters show an isotropic contraction with increasing x. Thermal decomposition of the hydroxide chlorides at 700 °C yields mixed-oxides Ce1-xLnxO2-δ that all have cubic fluorite structures with either expanded (Ln = La, Gd) or contracted (Ln = Pr, Tb) unit cells compared to CeO2. Scanning electron microscopy shows a shape memory effect in crystal morphology upon decomposition, with clusters of anisotropic sub-micron crystallites being seen in the precursor and oxide products. The Pr- and Tb-substituted oxides contain the substituent in a mixture of +3 and +4 oxidation states, as seen by X-ray absorption near edge structure spectroscopy at the lanthanide LIII edges. The mixed oxide materials are examined using temperature programmed reduction in 10%H2 in N2, which reveals redox properties suitable for heterogeneous catalysis, with the Pr-substituted materials showing the greatest reducibility at lower temperature.
Highlights
Cerium dioxide is well-known for its widespread applications in catalysis, where it commonly plays the role of a redox-active support, making use of its easy and reversible release and uptake of oxygen.[1]
The thermal decomposition of crystalline precursors provides another convenient approach for formation of ceria, and it has been commonly observed that the crystallite morphology of the ceria product reflects the morphology of the precursor salt.[21]
In this paper we describe a convenient solvothermal route to bulk, polycrystalline samples of Ce(OH)2Cl, and show how replacement of the cerium for other lanthanides is possible, before studying their thermal decomposition to give substituted cerium oxides, Ce1−xLnxO2−δ, with four examples of substituents chosen to illustrate the scope for substitutional chemistry
Summary
Cerium dioxide is well-known for its widespread applications in catalysis, where it commonly plays the role of a redox-active support, making use of its easy and reversible release and uptake of oxygen.[1]. The thermal decomposition of crystalline precursors provides another convenient approach for formation of ceria, and it has been commonly observed that the crystallite morphology of the ceria product reflects the morphology of the precursor salt.[21] This ‘shape memory effect’ has the potential for forming ceria crystallites with unusual surface reactivity, while the inclusion of substituent cations in the precursor salt provides a way of achieving a homogeneous elemental distribution in the oxide. Kim et al prepared the material using a hexamethylenetetramine solution route in the form of powders and as films.[30] In this paper we describe a convenient solvothermal route to bulk, polycrystalline samples of Ce(OH)2Cl, and show how replacement of the cerium for other lanthanides is possible, before studying their thermal decomposition to give substituted cerium oxides, Ce1−xLnxO2−δ, with four examples of substituents chosen to illustrate the scope for substitutional chemistry
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