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The flexibility of the zeolite Rho framework offers great potential for tunable molecular sieving. The fully copper‐exchanged form of Rho and mixed Cu,H‐ and Cu,Na‐forms have been prepared. EPR spectroscopy reveals that Cu2+ ions are present in the dehydrated forms and Rietveld refinement shows these prefer S6R sites, away from the d8r windows that control diffusion. Fully exchanged Cu‐Rho remains in an open form upon dehydration, the d8r windows remain nearly circular and the occupancy of window sites is low, so that it adsorbs CO2 rapidly at room temperature. Breakthrough tests with 10 % CO2/40 % CH4 mixtures show that Cu4.9‐Rho is able to produce pure methane, albeit with a relatively low capacity at this pCO2 due to the weak interaction of CO2 with Cu cations. This is in strong contrast to Na‐Rho, where cations in narrow elliptical window sites enable CO2 to be adsorbed with high selectivity and uptake but too slowly to enable the production of pure methane in similar breakthrough experiments. A series of Cu,Na‐Rho materials was prepared to improve uptake and selectivity compared to Cu‐Rho, and kinetics compared to Na‐Rho. Remarkably, Cu,Na‐Rho with >2 Cu cations per unit cell exhibited exsolution, due to the preference of Na cations for narrow S8R sites in distorted Rho and of Cu cations for S6R sites in the centric, open form of Rho. The exsolved Cu,Na‐Rho showed improved performance in CO2/CH4 breakthrough tests, producing pure CH4 with improved uptake and CO2/CH4 selectivity compared to that of Cu4.9‐Rho. [ABSTRACT FROM AUTHOR]

With hydrogen being seen as a key renewable energy vector, the search for materials exhibiting fast hydrogen transport becomes ever more important. Not only do hydrogen storage materials require high mobility of hydrogen in the solid state, but the efficiency of electrochemical devices is also largely determined by fast ionic transport. Although the heavy alkaline-earth hydrides are of limited interest for their hydrogen storage potential, owing to low gravimetric densities, their ionic nature may prove useful in new electrochemical applications, especially as an ionically conducting electrolyte material. Here we show that barium hydride shows fast pure ionic transport of hydride ions (H−) in the high-temperature, high-symmetry phase. Although some conductivity studies have been reported on related materials previously, the nature of the charge carriers has not been determined. BaH2 gives rise to hydride ion conductivity of 0.2 S cm−1 at 630 °C. This is an order of magnitude larger than that of state-of-the-art proton-conducting perovskites or oxide ion conductors at this temperature. These results suggest that the alkaline-earth hydrides form an important new family of materials, with potential use in a number of applications, such as separation membranes, electrochemical reactors and so on. [ABSTRACT FROM AUTHOR]

Doped strontium titanates have been widely studied as potential anode materials in solid oxide fuel cells (SOFCs). The high n-type conductivity that can be achieved in these materials makes them well suited for use as the electronically conductive component in SOFC anodes. This makes them a potential alternative to nickel, the presence of which can be a major cause of degradation due to coking, sulfur poisoning and low tolerance to redox cycling. Here anode performance results are presented for an A-site deficient strontium titanate co-doped with lanthanum and calcium on the perovskite A-site, La0.20Sr0.25Ca0.45TiO3 (LSCTA-). LSCTA- anodes and LSM cathodes were screen printed on 160 µm thick 6-ScSZ electrolyte supports. The LSCTA- anode backbone showed poor electrode performance, but its conductivity was sufficient to keep ohmic losses low. Upon impregnation with combinations of ceria and nickel, ohmic losses and polarization impedances are significantly reduced, resulting in a drastic improvement in anode performance. Unexpectedly, the performance of cells impregnated with both ceria and nickel showed an improvement upon redox cycling. A stable area specific resistance of 0.37 Ωcm² was achieved after 20 redox cycles and 250 hours of operation at 900°C in H2 with 8% H2O, showing excellent redox stability. [ABSTRACT FROM AUTHOR]

Abstract: The structure of calcium nitride hydride and its deuterided form has been re-examined at room temperature and studied at high temperature using neutron powder diffraction and thermal analysis. When synthesised at 600°C, a mixture of both ordered and disordered Ca2ND0.90H0.10 phases results. The disordered phase is the minor component and has a primitive rocksalt structure (spacegroup Fm3m) with no ordering of D/N on the anion sites and the ordered phase is best described using the rhombohedral spacegroup R-3m with D and N arranged in alternate layers in (111) planes. This mixture of ordered and disordered phases exists up to 580°C, at which the loss of deuterium yields Ca2ND0.85 with the disappearance of the disordered phase. In the new ordered phase there exists a similar content of vacancies on both anion sites; to achieve this balance, a little N transfers onto the D site, whereas there is no indication of D transferring onto the N-sites. These observations are thought to indicate that the D/N ordering is difficult to achieve with fully occupied anion sites. It has previously been reported that Ca2ND has an ordered cubic cell with alternating D and N sites in the [100] directions ; however, for the samples studied herein, there were clearly two coexisting phases with apparent broadening/splitting of the primitive peaks but not for the ordered peaks. The rhombohedral phase was in fact metrically cubic; however, all the observed peaks were consistent with the rhombohedral unit cell with no peaks requiring the larger ordered cubic unit cell to be utilised. Furthermore this rhombohedral cell displays the same form of N–D ordering as the Sr and Ba analogues, which are metrically rhombohedral. [Copyright &y& Elsevier]

The flexibility of the zeolite Rho framework offers great potential for tunable molecular sieving. The fully copper-exchanged form of Rho and mixed Cu,H- and Cu,Na-forms have been prepared. EPR spectroscopy reveals that Cu 2+ ions are present in the dehydrated forms and Rietveld refinement shows these prefer S6R sites, away from the d8r windows that control diffusion. Fully exchanged Cu-Rho remains in an open form upon dehydration, the d8r windows remain nearly circular and the occupancy of window sites is low, so that it adsorbs CO 2 rapidly at room temperature. Breakthrough tests with 10 % CO 2 /40 % CH 4 mixtures show that Cu 4.9 -Rho is able to produce pure methane, albeit with a relatively low capacity at this p CO2 due to the weak interaction of CO 2 with Cu cations. This is in strong contrast to Na-Rho, where cations in narrow elliptical window sites enable CO 2 to be adsorbed with high selectivity and uptake but too slowly to enable the production of pure methane in similar breakthrough experiments. A series of Cu,Na-Rho materials was prepared to improve uptake and selectivity compared to Cu-Rho, and kinetics compared to Na-Rho. Remarkably, Cu,Na-Rho with >2 Cu cations per unit cell exhibited exsolution, due to the preference of Na cations for narrow S8R sites in distorted Rho and of Cu cations for S6R sites in the centric, open form of Rho. The exsolved Cu,Na-Rho showed improved performance in CO 2 /CH 4 breakthrough tests, producing pure CH 4 with improved uptake and CO 2 /CH 4 selectivity compared to that of Cu 4.9 -Rho., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Zeolites with flexible structures that adapt to coordinate extraframework cations when dehydrated show a rich variety of gas adsorption behavior and can be tuned to optimize kinetics and selectivity. Merlinoite zeolite (topology type MER) with Si/Al = 3.8 has been prepared in Na, K, and Cs forms and its structural response to dehydration measured: the unit cell volumes decrease by 9.8%, 7.7%, and 7.1% for Na-, K-, and Cs-MER, respectively. Na-MER adopts Immm symmetry, while K- and Cs-MER display P 4 2 / nmc symmetry, the difference attributed to the preferred locations of the smaller and larger cations. Their performance in CO 2 adsorption has been measured by single-component isotherms and by mixed gas (CO 2 /CH 4 /He) breakthrough experiments. The differing behavior of the cation forms can be related to structural changes during CO 2 uptake measured by variable-pressure PXRD. All show a "breathing" transition from narrow to wide pore forms. Na- and Cs-MER show non-Type I isotherms and kinetically-limited CO 2 adsorption and delivery of pure CH 4 in CO 2 /CH 4 separation. However, K-MER shows good uptake of CO 2 (3.5 mmol g -1 at 1 bar and 298 K), rapid adsorption and desorption kinetics, and promising CO 2 /CH 4 separation. Furthermore, the narrow-to-wide pore transition occurs rapidly and at very low p CO 2 via a "triggered" opening. This has the consequence that whereas no CH 4 is adsorbed from a pure stream, addition of low levels of CO 2 can result in pore opening and uptake of both CO 2 and CH 4 , although in a continuous stream the CH 4 is replaced selectively by CO 2 . This observed cation size-dependent adsorption behavior derives from a fine energetic balance between different framework configurations in these cation-controlled molecular sieves.

With hydrogen being seen as a key renewable energy vector, the search for materials exhibiting fast hydrogen transport becomes ever more important. Not only do hydrogen storage materials require high mobility of hydrogen in the solid state, but the efficiency of electrochemical devices is also largely determined by fast ionic transport. Although the heavy alkaline-earth hydrides are of limited interest for their hydrogen storage potential, owing to low gravimetric densities, their ionic nature may prove useful in new electrochemical applications, especially as an ionically conducting electrolyte material. Here we show that barium hydride shows fast pure ionic transport of hydride ions (H(-)) in the high-temperature, high-symmetry phase. Although some conductivity studies have been reported on related materials previously, the nature of the charge carriers has not been determined. BaH2 gives rise to hydride ion conductivity of 0.2 S cm(-1) at 630 °C. This is an order of magnitude larger than that of state-of-the-art proton-conducting perovskites or oxide ion conductors at this temperature. These results suggest that the alkaline-earth hydrides form an important new family of materials, with potential use in a number of applications, such as separation membranes, electrochemical reactors and so on.