Methane Activation by MH⁺ (M = Os, Ir, and Pt) and Comparisons to the Congeners of MH⁺ (M = Fe, Co, Ni and Ru, Rh, Pd)

The mechanism of ligated-transition-metal- [MH⁺ (M = Os, Ir, and Pt)] catalyzed methane activation has been computed at the B3LYP level of density functional theory. The B3LYP energies of important species on the potential energy surfaces were compared to CCSD­(T) single-point energy calculations. Newer kinetic and dispersion-corrected methods such as M05-2X provide significantly better descriptions of the bonding interactions. The reactions take place more easily along the low-spin potential energy surface. The minimum-energy pathway proceeds as MH⁺ + CH₄ → M­(H)₂(CH₃)⁺ → TS → MH­(CH₂)­(H₂)⁺ → MH­(CH₂)⁺ + H₂. The ground states are ⁵Π, ⁴Σ^–, and ¹Σ⁺ for OsH⁺, IrH⁺, and PtH⁺, respectively. The energy level differences of the reactants between the high- and low-spin states gradually become smaller from OsH⁺ to PtH⁺, being 30.66, 9.17, and 0.09 kcal/mol, respectively. The CH bond can be readily activated by MH⁺ (M = Os, Ir, and Pt) with a negligible barrier in the low-spin state; thus, OsH⁺, IrH⁺, and PtH⁺ are likely to be excellent mediators for the activition of the CH bond of methane. H₂ elimination is quite facile without barriers in the presence of excess reactants. The products of the reactions of MH⁺ (M = Os, Ir, and Pt) + methane are all carbene complexes MH­(CH₂)⁺. The exothermicities of the reactions are 3.99, 15.66, and 12.14 kcal/mol, respectively. The results for MH⁺ (M = Os, Ir, and Pt) are compared with those for the first- and second-row congeners, and the differences in behavior and mechanism are discussed

Reactions of CO, H₂O, CO₂, and H₂ on the Clean and Precovered Fe(110) Surfaces – A DFT Investigation

The reactions of CO and H₂O on the clean Fe(110) surface as well as surfaces with 0.25 monolayer O, OH, and H precoverage have been computed on the basis of density functional theory (GGA-PBE). Under the considerations of the reductive nature of CO as reactant and H₂ as product as well as the oxidative nature of CO₂ and H₂O, we have studied the potential activity of metallic iron in the water-gas shift reaction. On the clean surface, CO oxidation following the redox mechanism has a similar barrier as CO dissociation; however, CO dissociation is much more favorable thermodynamically. Furthermore, surfaces with 0.25 monolayer O, OH, and H precoverage promote CO hydrogenation, while they suppress CO oxidation and dissociation. On the surfaces with different CO and H₂O ratios, CO hydrogenation is promoted. On all of these surfaces, COOH formation is not favorable. Considering the reverse reaction, CO₂ dissociation is much favorable kinetically and thermodynamically on all of these surfaces, and CO₂ hydrogenation should be favorable. Finally, metallic iron is not an appropriate catalyst for the water-gas shift reaction

Mechanisms of H₂O and CO₂ Formation from Surface Oxygen Reduction on Co(0001)

Surface O removal by H and CO on Co(0001) has been studied using periodic density functional method (revised Perdew–Burke–Ernzerhof ; RPBE) and ab initio atomistic thermodynamics. On the basis of the quantitative agreement in the H₂O formation barrier between experiment (1.34 ± 0.07 eV) and theory (1.32 eV), H₂O formation undergoes a consecutive hydrogenation process [O + 2H → OH + H → H₂O], while the barrier of H₂O formation from OH disproportionation [2OH → H₂O + O] is much lower (0.72 eV). The computed desorption temperatures of H₂ and H₂O under ultrahigh vacuum conditions agree perfectly with the experiment. Surface O removal by CO has a high barrier (1.41 eV) and is strongly endothermic (0.94 eV). Precovered O and OH species do not significantly affect the barriers of H₂O and CO₂ formation. All of these results indicate that the present RPBE method and the larger surface model are more suitable for studying cobalt systems

The RALTCT system and the operation environment.

<p>(a) The RALTCT system consists of four basic components: (1) the ultrasound (US) machine for two-dimensional (2D) US image display, (2) a five degrees-of-freedom (DOF) robot to manipulate the needle, (3) a PC-based surgical workstation that integrates the surgical navigation software and supervisory control of the robot, and (4) an electro-magnetic (EM) tracking system to record the position and orientation of the US probe. (b) The operation environment (top view).</p

Results of the 6-cm-diameter artificial tumor experiment.

<p>Results of the 6-cm-diameter artificial tumor experiment.</p

The local enlarged map.

<p>The local enlarged map.</p

Three-dimensional scenes from the surgical navigation software for the first needle surgical planning.

<p>Three-dimensional scenes from the surgical navigation software for the first needle surgical planning.</p

Schematic diagram of multiple-needle surgical planning environment.

<p>(a) The abdominal operation environment. (b) The information of target points and insertion order.</p

The boundaries of needle 1’s CFRW.

<p>(a) The 2D CFRW for needle 1 with z = 150. (b) The 3D CFRW for needle 1 with z = 150, 160, 170 mm.</p

Optimal needle-insertion trajectories for needle 1 and needle 2.

<p>Optimal needle-insertion trajectories for needle 1 and needle 2.</p