C.S. Swamy

MAGNESIUM AND MAGNESIUM ALLOY HYDRIDES

Abstract This review addresses the question of using magnesium and magnesium-based alloys as “rechargeable hydrogen storage” media. The effect of addition of organic compounds as well as other metals (both transition and non-transition elements) on the sorption characteristics of magnesium is considered in detail. The current status of information in this field is reviewed with a view to evaluating the potential of these materials for use as hydrogen storage media in vehicular applications.

Effect of interlayer anions on the physicochemical properties of zinc–aluminium hydrotalcite-like compounds

Zinc–aluminium hydrotalcite-like compounds (ZnAlAn-–HT) with a Zn/Al atomic ratio 2.0 and An- = CO2-3, Cl-, NO-3 and SO2-4, were synthesized by coprecipitation under low supersaturation. Their physicochemical properties were studied using powder X-ray diffraction (PXRD), infrared (IR) and laser Raman (LR) spectra, thermogravimetry (TG), differential scanning calorimetry (DSC), evolved gas analysis (EGA), 27Al MAS NMR, BET surface area and pore-size determination. The PXRD of the synthesized samples showed that the crystallinity was affected by the nature of the anions present in the interlayer space. The IR and LR studies revealed that except the NO-3 ion, the symmetry of these interlayer anions was reduced upon intercalation. The TG, DSC and EGA results showed two or three stages of weight loss corresponding to the removal of the interlayer water, structural water and the anion, respectively. The activation energy, Ea, for the decomposition process was found to decrease in the order ZnAlCO3–HT>ZnAlSO4–HT>ZnAlCl–HT>ZnAlNO3–HT. Formation of a pentacoordinated Al (AlV) in addition to the octahedral (AlVI) and tetrahedral Al (AlIV) was the special feature noticed in the 27Al MAS NMR of the calcined samples. Thermal calcination around 500 °C resulted in the formation of non-stoichiometric ZnO whose crystallinity decreased in the order ZnAlNO3–CHT>ZnAlCl–CHT>ZnAlSO4–CHT>ZnAlCO3–HT while their extent of solid solubility was found to be the reverse. The crystallinity of the calcined samples was also correlated with surface area and pore-size determination.

Effect of composition on the physicochemical properties of nickel aluminium hydrotalcites

Ni-Al hydrotalcite compounds with the structural formula [Ni1−xAlx(OH)2]x+ [(CO3)x/2 ·mH2O]x− were synthesized in the composition range Al/Ni+Al (x)=0.20–0.50 by sequential precipitation. The samples were characterized by X-ray diffraction, infrared absorption, thermogravimetry-differential scanning calorimetry-evolved gas analysis, transmission electron microscopy and BET surface-area measurements. The crystallinity and the lattice parameters of the samples are significantly affected with respect to composition. Hydrothermal treatments performed on the aged samples enhanced the crystallinity. The nitrate ion is also present along with carbonate ion as the charge-balancing anion in the interlayer space. The thermal stability of these materials increased with decrease in Ni/Al atomic ratio. Thermal calcination of these compounds yielded non-stoichiometric NiO whose crystallinity and thermal stability depends on the composition as well as on calcination temperature.

Synthesis and physicochemical properties of cobalt aluminium hydrotalcites

Cobalt aluminium hydrotalcites with different compositions were prepared by a coprecipitation method under low supersaturation conditions. The compounds were characterized by X-ray diffraction (XRD), infrared absorption (IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and BET surface area measurements. XRD and IR studies revealed that all the compounds are single-phase crystallized under a hydrotalcite-like network. Hydrothermal treatments given to the aged sample increased the crystallinity of the samples. TG studies showed two stages of weight loss, the first due to the removal of interlayer water and the second ascribed to the removal of water molecules from the brucite sheet and CO2 from the interlayer carbonate anion, whose transition temperature depends on the Co/Al atomic ratio. Thermal calcination of these materials results in the formation of high surface area non-stoichiometric spinel phase whose crystallinity increases with increase in the calcination temperature attributed to the sintering of the particles.

Studies on the thermal characteristics of hydrides of Mg, Mg2Ni, Mg2Cu and Mg2Ni1−xMx (M = Fe, Co, Cu or Zn; 0 < × < 1) alloys

X-ray results on the alloys of the composition Mg2Ni1−xMx (M = Fe, Co, Cu or Zn; 0 < × < 1) suggest a Mg2Ni-type structure. The alloys, Mg2Ni0.75M0.25(M = Fe, Co, Cu or Zn) upon hydriding lead to the formation of quarternary hydrides while on dehydriding yield the starting ternary alloys except for copper containing alloys which show multi-phase regions and follow a different path way for the hydriding-dehydriding process. Thermal studies (TG-DTA) on the hydrides indicate the amount of hydrogen evolved as well as the desorption temperatures. The thermodynamic quantities, namely the enthalpies and entropies of formation of the hydrides were deduced from the DTA peak maximum temperature data. The kinetic parameters such as activation energies, reaction rates and orders of the reaction for the decomposition of the hydrides formed from Mg, Mg2Ni and Mg2Cu alloys were evaluated from the DTA data. As to the modified Mg2Ni system, the copper substituted alloys show lower thermal stability and also presents some interesting properties. Hence, it is considered as one of the promising ternary combinations (Mg-Ni-Cu) for hydrogen storage purposes.

Effect of trivalent cation on the physicochemical properties of cobalt containing anionic clays

A series of cobalt containing hydrotalcites having the general formula CoMCO3-HT where M = Al, Fe and Cr were synthesized by sequential and coprecipitation methods. The samples were primarily characterized by X-ray diffraction (XRD), infrared absorption studies, transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDAX), thermogravimetry-differential scanning calorimetry (TG-DSC) and BET surface area measurements. The lattice parameters, FTIR stretching wavelengths for O–H bonds and thermal stability depended on the ionic radius of the trivalent M cation. Thermal calcination of these materials yielded a non-stoichiometric spinel-type phase whose crystallinity was also affected by the nature of the trivalent cation.

Catalytic activity and XPS investigation of dalofossite oxides, CuMO2 (M=Al, Cr or Fe)

Catalytic decomposition of N2O has been carried out on oxides having dalofossite structure CuMO2; (M=Al, Cr or Fe) at 50 and 200 torr initial pressure of N2O in the temperature range 380–480 °C. CuFeO2 showed higher activity while CuCrO2 showed low conversion in the temperature range studied. The variation in activity has been explained based on the presence of mixed valence copper and the concentration of copper on the surface as evinced from X-ray photoelectron spectroscopy.

Surface properties of LaNi5: A reinvestigation

Abstract Although a number of surface analysis reports are available on the LaNi5 system, yet a clear picture of the actual species involved and their consequences on the activation and deactivation processes has not been reached. Moreover, differences of opinion have been seen among the researchers. The present results distinctly indicate that the air exposed samples are completely contaminated with a variety of oxygenated species (such as oxides, hydroxides and carbonates) of the constituent elements, hitherto not reported by others. The protective nature of such species in regard to the durability of the alloy and the formation of supported-metal type species upon activation accounts for its excellent behaviour. The relative higher stability of LaNi5 over CaNi5 under cyclic operating conditions has been explained based on surface energy criteria of the constituent elements rather than the deterioration initiated by hydrogen and/or gaseous impurities alone. In addition, the specific importance of X-ray induced Auger electron spectroscopy (XAES) technique in conjunction with X-ray photoelectron spectroscopy (XPS) has been highlighted in the identification of different species.

Catalytic behaviour of NiAl2O4 spinel upon hydrogen treatment

The effect of reduction on the catalytic activity and selectivity of nickel aluminate spinel has been reported using the 2-propanol decomposition reaction. The unreduced catalyst showed only dehydration activity. When reduced in hydrogen at different temperatures starting from 200 °C, dehydrogenation activity was observed. After reduction at 450 °C, the activity completely transformed into dehydrogenation, while the spinel structure remained unchanged. The unreduced and reduced catalysts were characterized by X-ray diffraction, temperature-programmed reduction, diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The mechanism for the decomposition is discussed in terms of the coordination of aluminium ion and reducibility of nickel.

Surface characterization and catalytic activity of Ln2Ti2O7 (Ln=Y, Sm, Gd and Tb)

The surface of the catalyst, Ln2Ti2O7 where Ln=Y, Sm, Gd and Tb was analysed by X-ray photoelectron spectroscopy. Terbium is found to be present in 3+ and 4+ states while Y, Sm and Gd are present in the 3+ state. The catalytic activity of these oxides was tested using decomposition of N2O as the test reaction and the catalytic activity was correlated to the surface properties.