Metal oxide materials for high temperature CO₂ sorption studies

In recent years, a number of novel ceramic oxide materials have emerged that are capable of absorbing CO2 at high temperatures (>500C) while remaining stable over a large number of cycles and a wide range of temperatures [1]. The most promising are been considered for carbon capture applications - specifically, for use in combustion chambers and the smoke stacks of power plants where combustion gases which contain primarily a mixture of CO2 and N2 at high temperature. Compared to other CO2 sequestration technologies, these ceramics have some advantages (eg. chemisorption at high temperatures) and disadvantages (eg. limited kinetics over time) [3]. Examples of oxides already known to show significant CO2 absorption include Li5AlO4, Li6Zr2O7, Na2ZrO3 and Ba4Sb2O9. The phase formations and structural evolution of these metal oxides have been studied under environmental conditions mimicing those found in combustion chambers and power plants, over the temperature range 873-1173 K. CO2 absorption by these materials is believed to proceed through a layering effect of the sorbent material, explained through a core-shell model (see figure). Each phase is represented as a layer covering a particle, with the outermost layer exposed and allowed to react with the environment. Detailed studies into the mechanism of CO2 absorption and the material layers will shed more information that can be used to fine tune the materials to increase their CO2 absorption capacity. Previous work has focused on the identification of phases ex situ and studies of their practical absorption capacity and kinetics. The new work we will present here uses a combination of a x-ray spectroscopy, x-ray and neutron diffraction, to understand both how the sorption process works and how the structural evolution of the phases affects the CO2 sorption of the materials over time in-situ.

Transformation from pyrochlore to fluorite by diffraction and X-ray spectroscopy

We have studied the long-range average and local structures in a number of zirconium containing materials of the type A2B2O7 ( A = Ln or Y; B = Zr, Hf or Sn) using synchrotron X-ray and neutron powder diffraction and X-ray absorption spectroscopy. Studies of the system Gd2-xTbxZr2O7 include neutron diffraction data, obtained at λ ≍ 0.497 Å to minimise absorption, not only provide evidence for independent ordering of the anion and cation sublattices, but also suggest that the disorder transition across the pyrochlore-defect fluorite boundary of Ln2Zr2O7 is rather gradual. In general we observe that while the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at specific compositions corresponding to a critical ionic radius ratio of the A and B cations (rA/rB) x ~ 1.0-1.2, X-ray absorption near-edge structure (XANES) results reveal a gradual structural evolution across the compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. Where possible the experimental results were supplemented by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures were predicted, and the trends agree well with the experimental XANES results. X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the defect fluorites.

Perovskites in low dimensional multi-layer structure types

This study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi₅PbTi₃O₁₄Cl [1] with charge balancing by removing Pb²⁺ for Bi³⁺. A statistical 1:2 distribution of M³⁺ and Ti⁴⁺ across all three perovskite layers was found in Bi₆Ti₂MO₁₄Cl, M = Cr³⁺, Mn³⁺, Fe³⁺, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi₅PbTi₃O₁₄Cl) and pronounced spin-glass behavior below T_irr(0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi₆Ti₂FeO₁₄Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi₃Sr₂Nb₂O₁₁Br, based on Bi₃Pb₂Nb₂O₁₁Cl [2], was synthesized by simultaneously replacing Pb²⁺ with Sr²⁺ and Cl^- with Br^-. Inter-layer mismatch prevented the formation of Bi₃Sr₂Nb₂O₁₁Cl and Bi₃Pb₂Nb₂O₁₁Br. Sr²⁺ doping reduces the impact of the stereochemically active 6s² lone pair found on Pb²⁺ and Bi³⁺, resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br^-. X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi₃Sr₂Nb₂O₁₁Br compared to Bi₃Pb₂Nb₂O₁₁Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.