NH3/Ir(100): Electronic structure and dehydrogenation

Density functional theory was employed to compute the geometric and electronic structures and the stepwise dehydrogenation of NH₃ on Ir(100). The NH₃, NH₂ and NH were found to bind predominately on top, bridge and four-fold hollow sites with adsorption energies to be 0.82 eV, 3.26 eV and 4.85 eV respectively. The mixings of 3a₁(₃NH)∼5dz²/6s(Ir)$3a_1\left({\text{NH}}_3\right)\sim 5{d_z}^2/6s\left(\text{Ir}\right)$, 3a₁/1b₁(NH₂) ∼ 5d_yz(Ir) and 3a₁ + 1a₂(NH) ∼ 5d_xz(Ir) and 2a₁(NH)∼5dz²(Ir)$2a_1\left(\text{NH}\right)\sim 5{d_z}^2\left(\text{Ir}\right)$ are responsible for the adsorption of NH₃, NH₂ and NH respectively. The reaction barriers are 0.88 eV, 0.97 eV, 0.94 eV and 0.63 eV for the first, second, third N–H bond breakings and N₂ formation respectively. The competition between decomposition and desorption for NH₃ was demonstrated, NH₂ was found to be the most stable surface species, its accumulation in practice would suppress the first N–H bond scission and thus make it to be the rate-determining step.     

Investigation on the structure and electrochemical properties of the laser sintered La0.7Mg0.3Ni3.5 hydrogen storage alloys

The structure and electrochemical properties of the _{La0.7Mg0.3Ni3.5}${\mathrm{La}}_{0.7}{\mathrm{Mg}}_{0.3}{\mathrm{Ni}}_{3.5}$ alloys laser sintered at different powers were investigated. It is found that all alloys contain three phases _La3MgNi14${\mathrm{La}}_3{\mathrm{MgNi}}_{14}$ with the _Ce2Ni7${\mathrm{Ce}}_2{\mathrm{Ni}}_7$ structure, _LaNi5${\mathrm{LaNi}}_5$ and _LaMgNi4${\mathrm{LaMgNi}}_4$. The abundance of the main phase _La3MgNi14${\mathrm{La}}_3{\mathrm{MgNi}}_{14}$ is 43, 68 and 63 wt%, respectively, when sintering power varies from 1000 to 1200 and 1400 W. The laser sintered _{La0.7Mg0.3Ni3.5}${\mathrm{La}}_{0.7}{\mathrm{Mg}}_{0.3}{\mathrm{Ni}}_{3.5}$ alloys can be activated to their maximum discharge capacity within three cycles. The discharge capacities of those alloys prepared by laser sintering at 1000, 1200 and 1400 W are 324.6, 352.8 and 340.5 mAh/g, respectively. The _{La0.7Mg0.3Ni3.5}${\mathrm{La}}_{0.7}{\mathrm{Mg}}_{0.3}{\mathrm{Ni}}_{3.5}$ alloy laser sintered at 1200 W has a best cyclic stability (_S100=58.4%)$(S_{100}=58.4\%)$ and high-rate dischargeability (_HRD800=79.4%)$({\mathrm{HRD}}_{800}=79.4\%)$ due to the high amount of the main phase _La3MgNi14${\mathrm{La}}_3{\mathrm{MgNi}}_{14}$.