Density Functional Theory Study of the Adsorption of Oxygen and Hydrogen on 3d Transition Metal Surfaces with Varying Magnetic Ordering

ABSTRACT We have employed density functional theory (DFT) calculations to investigate the adsorption of molecular oxygen and hydrogen on 3d transition metal (TM) surfaces with varying ordered magnetic structures in the bulk, namely ferromagnetic Fe(110), Co(0001), Ni(111) and diamagnetic Cu(111). The trend observed in the energies of adsorption was compared with the magnetic moment of the cell using the d-band centre model of chemisorption and the Stoner model of magnetic energy. As the gap between the d-band centre and the Fermi level of the TM decreases, more antibonding orbitals are present above the Fermi level and thus unoccupied, leading to stronger binding. Correspondingly, the shift in the d-band centre decreases the density of states (DOS) at the Fermi level giving rise to the ordered magnetic structure. Keywords: d-Band centre, chemisorption, Hedvall effect, magnetism.

Reactivity of Carbonates in Superheated Steam under Atmospheric Pressure

The effect of superheated steam on the decomposition of CaCO3 and MnCO3 and on the solid state reactions of ZnCO3-FeOOH and MnCO3-CaCO3 was investigated. A newly developed apparatus for the experiments under 1 atm of pure water vapor was used. CaCO3 decomposed at 800 oC in the superheated steam to form the single phase of CaO. On the other hand, the decomposition was uncompleted in air. CaCO3 transformed into CaO via Ca (OH)2 in superheated steam. During the transformation of carbonate into hydroxide, the crystal lattice is temporarily disordered to make it active, leading lower decomposition temperature of CaCO3. MnCO3 decomposed to form γ-Mn2O3 at above 1000 oC in air, whereas γ-Mn2O3 was obtained at 800 oC in the superheated steam. The solid state reaction in the steam was suppressed for the mixture of ZnCO3 and FeOOH. This seemed to be due to the large difference in decomposition temperature between ZnCO3 and FeOOH. MnCO3 reacted with CaCO3 to form CaMn2O4 at 800 oC in the superheated steam. However, a higher temperature of 1000 oC was required to cause the reaction in air. The low-temperature transformation of MnCO3 and CaCO3 in the superheated steam would affect the reaction. It was concluded that the reactivity of carbonate in super heated steam was promoted by the Hedvall effect, which was caused by the formation of intermediate phase such as hydroxide.

Ionic conductivity and solid phase transitions in Na2SO4–Cs2SO4 system

The ac conductivity and DSC calorimetry data reveal interesting solid state interactions and solid phase transition behavior in the Na2SO4–Cs2SO4 binary system. The β → α solid transition of NaCsSO4 is reported for the first time. These data indicate preferential reaction between Cs2SO4 and the high temperature phase I of Na2SO4, exemplifying the Hedvall effect, to form the compound NaCsSO4 which further interacts with excess Na2SO4 or excess Cs2SO4 to form 1:1 association complexes, viz. NaCsSO4:Na2SO4 and NaCsSO4:Cs2SO4. These complexes appear to dissociate at low temperatures reminiscent of spinodal-type decomposition behavior. The reaction enthalpies of NaCsSO4, β → α transition of NaCsSO4 and complex associations are reported.The report contains two unexpected results: (i) a subdued higher ionic conductivity relative to its Rb2SO4 analogue and (ii) a conductivity drop accompanying the high temperature phase transition which is in strong contrast to all the other Na2SO4 – mixed alkali compositions.