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Sodium zirconate (sodium zirconium oxide; Na2ZrO3) is a widely investigated carbon dioxide (CO2) sorbent. Since it was first discussed in the 1960s, Na2ZrO3 has been reported to adopt monoclinic, hexagonal, and cubic structures, and it is widely believed that the CO2 capture performance of Na2ZrO3 is related to its crystal structure. Researchers have relied on the differences in the relative intensities of two peaks (2 ∼16.2° and 38.7°) in the powder X-ray diffraction (PXRD) pattern to determine the phase of this compound. However, to date, a defined crystal structure of hexagonal Na2ZrO3 has remained elusive. Our findings show that the current literature discussion on the structure of Na2ZrO3 is misleading. With the use of 3D electron diffraction (3D ED), and PXRD, we prove that hexagonal Na2ZrO3 does not exist. The so-called hexagonal Na2ZrO3 is actually Na2ZrO3 with three different types of disorder. Furthermore, the two PXRD peaks (2 ∼16.2° and 38.7°) cannot be used to distinguish the different phases of Na2ZrO3, as the change in the PXRD pattern is related to the extent of structure disorder. Finally, we also show that the CO2 uptake properties of Na2ZrO3 are not related to the differences in crystal structures, but rather to the Na+ site occupancy differences in different Na2ZrO3 samples. In order to further develop applications of Na2ZrO3, as well as other mixed-metal oxides, their structures, and the existence of any disorder, need be understood using the methods shown in this study. [ABSTRACT FROM AUTHOR]

The CO2 sorption properties of sodium hafnium oxide (Na2HfO3) were investigated in this study. Na2HfO3 was synthesized by solid-state synthesis using Na2CO3 and HfO2 as starting materials. The solid-state synthesized Na2HfO3 appeared structurally similar to other mixed metal oxides such as Na2ZrO3, but stacking disorder appeared to be common in Na2HfO3. The synthesis conditions, including the Na : Hf ratio (between 0.5 and 1.5 : 1), synthesis temperature, time and heating rate, were investigated to optimize CO2 sorption properties of Na2HfO3. The Na2HfO3 sorbent showed comparable CO2 uptake capacity, reaction rate and excellent cycling stability compared to other metal oxide sorbents. Na2HfO3 with Na : Hf = 1 : 1 and 1.25 : 1 showed the highest CO2 uptake among all Na2HfO3 samples obtained, with a CO2 uptake capacity of around 15 wt% (at 650–800 °C). The CO2 uptake rate of NHO-1 and NHO-1.25 was fast with over 80% of the equilibrium uptake reached within 250 s. Na2HfO3 remained stable even after 100 cycles with less than 3% difference in the CO2 uptake capacity between the 1st and 100th cycles. We performed kinetic analysis on the CO2 sorption data and found that the Avrami–Erofeev model fitted the kinetic data best among the kinetic models used. Apart from sorbent optimization, we showed that 3D-printing of Na2HfO3 : HfO2 mixtures can be used to produce structured Na2HfO3 sorbents with a slightly improved CO2 uptake rate and the same CO2 uptake capacity as the powder-based solid-state synthesized Na2HfO3 sorbent. [ABSTRACT FROM AUTHOR]

For the first time, the use of stannate-based sorbents was investigated as high temperature CO2 sorption to evaluate their potential to contribute towards reducing carbon emissions. The sorption capacity and kinetics of commercial tin oxide, sodium, potassium and calcium stannates and lab synthesised potassium stannates were tested using thermogravimetric analysis. Commercial K2SnO3 was found to possess the largest CO2 uptake capacity (2.77 mmol CO2/g or 12.2 wt%) at 700 °C, which is among the highest for potassium sorbents, but the CO2 desorption was not successful. On the contrary, the in-house synthesised K-stannate (K-B) using facile solid-state synthesis outperformed the other sorbents, resulting in a CO2 uptake of 7.3 wt% after 5 min, an adsorption rate (0.016 mg/s) one order of magnitude higher than the other stannates, and stability after 40 cycles. The XRD and XPS analyses showed that K-B contains a mixture of K2SnO3 (76%) and K4SnO4 (21%), while the Scherrer crystal sizes confirmed good resistance to sintering for the potassium stannates. Among the apparent kinetic model tested, the pseudo-second order model was the most suitable to predict the CO2 sorption process of K-B, indicating that chemical adsorption is dominant, while film-diffusion resistance and intra-particle diffusion resistance governed the sorption process in K-B. In summary, this work shows that solid-state synthesised potassium stannate could be an effective sorbent for high temperature separation, and additional work is required to further elucidate its potential. [ABSTRACT FROM AUTHOR]

Carbon capture is an important and effective approach to control the emission of CO2 from point sources such as fossil fuel power plants, industrial furnaces and cement plants into the atmosphere. For an efficient CO2 capture operation, many aspects of the CO2 capture steps need to be carefully considered. Currently the most mature CO2 capture technology is liquid amine scrubbing. Alternatively, solid sorbents can be used to effectively capture CO2 while alleviating the disadvantages associated with liquid amine sorbents. In this review, we critically assess solid metal oxide CO2 sorbents, especially oxides of group 1 (Li, Na and K) and group 2 (Mg, Ca, Sr and Ba) metals, for capturing CO2 at moderate to high temperatures. In particular, we focus on the recent advances in developing synthetic metal oxide sorbents, and the correlation between the design, synthetic approaches and their cyclic CO2 capture performance, which are characterised by CO2 uptake capacity, rate of carbonation and cyclic stability. The state-of-the-art, challenges, opportunities and future research directions for these metal oxide sorbents are discussed. By devoting more research effort to address the issues identified, there can be great potential to utilise Group 1 and 2 metal oxides as cost-effective, highly efficient sorbents for CO2 capture in a variety of carbon capture applications. [ABSTRACT FROM AUTHOR]

Engineering the building blocks in metal-organic materials is an effective strategy for tuning their dynamical properties and can affect their response to external guest molecules. Tailoring the interaction and diffusion of molecules into these structures is highly important, particularly for applications related to gas separation. Herein, we report a vanadium-based hybrid ultramicroporous material, VOFFIVE-1-Ni, with temperature-dependent dynamical properties and a strong affinity to effectively capture and separate carbon dioxide (CO 2 ) from methane (CH 4 ). VOFFIVE-1-Ni exhibits a CO 2 uptake of 12.08 wt % (2.75 mmol g -1 ), a negligible CH 4 uptake at 293 K (0.5 bar), and an excellent CO 2 -over-CH 4 uptake ratio of 2280, far exceeding that of similar materials. The material also exhibits a favorable CO 2 enthalpy of adsorption below -50 kJ mol -1 , as well as fast CO 2 adsorption rates (90% uptake reached within 20 s) that render the hydrolytically stable VOFFIVE-1-Ni a promising sorbent for applications such as biogas upgrading.