Cerium based oxides as oxygen selective sorbents

To reduce global warming, increasing attention has been directed at energy generation technologies that possess a low carbon footprint. The aim of this work was to develop oxygen selective sorbents that produce high purity oxygen (>99 %) – for use as a lower energy alternative to current air separation methods. CeO2 based oxides were investigated as prospective candidates due to their known behaviour for reversible oxygen uptake and perfect selectivity that results as only oxygen can enter the lattice with variations in oxygen partial pressure. Increasing the reversible oxygen uptake is clearly a desirable attribute that was achieved in this project by generating oxygen vacancy defects through the addition of dopants. Materials with a high number of defects have been associated with higher oxygen uptakes. Investigations into the effect of Ce salt and precipitating agent showed the smallest crystallites were produced from Ce4+ and urea. Scanning electron (SEM) and transmission electron microscopy (TEM) revealed Ce3+ and urea formed rod shaped particles and the X-ray diffraction (XRD) reflections of the precursor matched those of Ce2O(CO3)2·H2O. In-situ XRD investigations showed that the transformation of Ce2O(CO3)2·H2O to CeO2 occurred through an unidentified intermediate. TEM- energy dispersive X-ray spectroscopy (EDX) of a Tb-CeO2 material with rod morphology showed that Tb segregated on the rod tips in regions of ~>20 nm in size. The Ce4+ and urea method was selected for the synthesis of all doped mixed metal oxides prepared subsequently, as part of this study. A series of Tb-CeO2 and Pr-CeO2 materials were synthesised to determine which would possess the highest oxygen uptake and facilitate the lowest operation temperature. Through N2-air-N2 gas switching experiments carried out using thermal gravimetric analysis (TGA) the oxygen uptakes were measured. At 600 °C the uptake of Tb-CeO2 with %Tb=10, 20, 30 and 40 was 17, 46, 75 μmol.g-1 and 100 μmol. g-1, respectively and the uptake for %Pr=5, 10, 15 and 20 was 12, 33, 55 μmol.g-1 and 95 μmol.g-1. For all materials the uptake increased with increasing %Tb and %Pr, and both possessed higher uptakes than pure CeO2 (<5 μmol.g-1). The Pr-CeO2 series exhibited the highest uptakes at the lowest temperature and, so, were doped with Cu to further enhance its uptake. Doping with Cu was shown to decrease the oxygen uptake at 420 °C and increase it at 600 °C. By alternating between air and nitrogen atmospheres, for 20 cycles at 600 °C, all these materials demonstrated good stability. Further investigations into the feasibility of Tb-CeO2 as sorbent materials were undertaken via oxygen chemisorption studies. Results supported those already determined using TGA and showed at 400, 500 and 600 °C the oxygen uptake increased with increasing %Tb. Isotherms also supported that these materials exhibit reversible oxygen uptake and high O2/N2 selectivity. To draw links between the oxygen uptake and the structural properties of the materials, the relative Ce3+ and oxygen vacancy defect concentrations were investigated using electron energy loss spectroscopy (EELS). It was predicted that the lattice strain energy due to cation size miss-match is minimised in the Tb-CeO2 materials as Tb preferentially adopts the +3 oxidation state when doped into CeO2. The Pr cations may adopt the +4 oxidation state in CeO2, so are more susceptible to reduction. Special attention was paid to the sampling method as it was observed that the Ce3+ and vacancy concentration were higher when spectra were obtained from a single crystallite compared to a collection of crystallites. EELS measurements showed that the relative Ce3+ (Ce IM5/IM4) and vacancy concentrations (O IB/IC) increased with increasing %Tb. Raman spectroscopy also showed the presence of a defect mode in the Tb-CeO2 materials not seen in the CeO2. Investigations into the valence state and the reducibility of the Pr cation in CeO2 indicated, through Raman, UV-vis spectroscopy and XRD measurements, that Pr adopts both the +3 and +4 oxidation state. Based on EELS analysis, the Ce3+ concentration in the Pr-CeO2 series did not vary with %Pr. A stability study investigating the reducibility of the Pr4+ cation was carried out by continually irradiating Ce0.8Pr0.2O2 and calculating the variation in Ce3+ and vacancy concentration as a function of time. After 240 min of beam exposure the O fine structure features broadened but there was no change in the Ce IM5/IM4 ratio. No observable differences in the Pr white lines were evident implying the Pr cations were already in the +3 oxidation state. The observed increase in vacancy concentration was therefore attributed to the slow reduction of Ce4+. It appears that any differences in the Ce IM5/IM4 ratio were less than that observable. Structural and bulk defects that were attributed to the addition of Pr and Cu to CeO2 were investigated using neutron diffraction. To account for anion Frenkel defects that may be present, refinement was carried out by constraining tetrahedral oxygen anions to move from the 8c into the 48i sites of the Fm"3" ̅m space group of CeO2. The refined oxygen occupancies were found to decrease with the addition of Pr and Cu showing these cations introduce oxygen vacancies. It was observed that these became available as oxygen adsorption sites after a minimum analysis temperature was reached. At 420 °C the oxygen uptake of all Ce0.85-xPr0.15CuxO2-δ and Ce0.8-xPr0.2CuxO2-δ materials (13–24 μmol.g-1) were lower than for Ce0.85Pr0.15O2 (31 μmol.g-1) and higher oxygen uptakes were measured at 600 °C. Uptakes increased with %Pr and %Cu showing that both cations promote the oxygen uptake of CeO2. Despite the presence of a segregated CuO phase in the neutron diffraction pattern of C e0.7Pr0.15Cu0.15O2-δ, this was not detrimental to the oxygen uptake; the uptake of Ce0.7Pr0.15Cu0.15O2-δ (86 μmol.g-1) was higher than Ce0.75Pr0.15Cu0.1O2-δ (76 μmol.g-1) and Ce0.8Pr0.15Cu0.05O2-δ (62 μmol.g-1). As the uptake ability of these materials can be attributed to a switch in oxidation state under oxidising and reducing conditions, the extent of lattice contraction and expansion of Ce0.65Pr0.2Cu0.15O2-δ was determined in a N2-air-N2 gas switching experiment using in-situ XRD. The lattice parameter was found to increase under a flow of nitrogen with increasing temperature as cations were reduced. When the gas was switched to air a decrease in the lattice parameter occurred as cations were re-oxidised. It was also observed that the kinetics of oxidation were faster than that of reduction. Investigations into the surface chemistry of Ce0.8Pr0.2O2-δ, Ce0.75Pr0.2Cu0.05O2-δ and Ce0.65Pr0.2Cu0.15O2-δ were carried out via methanol adsorption and desorption using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Addition of Pr to CeO2 resulted in a shift of the bridging methoxy mode to a higher wavenumber that indicated the surface of Pr-CeO2 contained a higher quantity of uncoordinated cation/+3 sites. Compared to CeO2 and Ce0.8Pr0.2O2-δ, the surface of Ce0.75Pr0.2Cu0.05O2-δ was found to be dominated by reduced cations/vacancies. Further shifts were not observed for Ce0.65Pr0.2Cu0.15O2-δ indicating that there was no further increase in the sub-surface vacancy concentration due to the presence of segregated particles attributed to CuO from SEM-EDX analysis. Temperature programmed desorption (TPD) showed the Cu containing materials had a higher catalytic activity toward methanol oxidation as formates were present at 25 °C for both Ce0.75Pr0.2Cu0.05O2-δ and Ce0.65Pr0.2Cu0.15O2-δ, at 125 °C on Ce0.8Pr0.2O2-δ and at 175 °C on CeO2. By monitoring the gases evolved during TPD by mass spectrometry (MS), formates adsorbed on CeO2 and Pr-CeO2 were converted via a dehydrogenation mechanism to H2, CO and CO2. The highly basic/nucleophilic surface of the Cu materials did not form H2 and resulted in the simultaneous evolution of CO and CO2. Oxygen isotopic exchange experiments were carried out to determine the oxygen mobility through the lattice by measuring the 18O2 (P36), 16O18O (P34) and 16O2 (P32) response using MS as the temperature was increased. As indicated by a decrease in the lattice oxygen 16O2 (P32) evolution temperature, the lattice oxygen mobility was higher for Ce0.8Pr0.2O2-δ, compared to CeO2. The temperature of 16O2 (P32) evolution was further lowered for Ce0.75Pr0.2Cu0.05O2-δ as dopants located within the bulk of the particle increase the bulk vacancy concentration. By increasing the Cu content from 5 % to 15 % Cu, the lattice mobility did not change and was attributed to Cu segregation that did not further increase the bulk defects. An increase in the surface oxygen mobility was observed for Ce0.65Pr0.2Cu0.15O2-δ as shown by a decrease in the 16O18O (P34) evolution temperature indicating vacancies are located within the first few atomic layers. Despite segregation, the highest uptake of 77 μmol.g-1 was obtained for the Ce0.65Pr0.2Cu0.15O2-δ material followed by 61 μmol.g-1 and 59 μmol.g-1 for Ce0.75Pr0.2Cu0.05O2-δ and Ce0.8Pr0.2O2-δ, respectively. It was concluded that the observed reversible, selective and oxygen storage ability of CeO2 based materials can be attributed to their structural properties and defect nature which results from doping. Consequently these materials may have applications as oxygen selective sorbents or membranes for air separation