Paper & Patents

In Pt-transition metal (TM) alloy catalysts, the electron transfer from the TM to Pt is retarded owing to the inevitable oxidation of the TM surface by oxygen. In addition, acidic electrolytes such as those employed in fuel cells accelerate the dissolution of the surface TM oxide, which leads to catalyst degradation. Herein, we propose a novel synthesis strategy that selectively modifies the electronic structure of surface Co atoms with N-containing polymers, resulting in highly active and durable PtCo nanoparticle catalysts useful for the oxygen reduction reaction (ORR). The polymer, which is functionalized on carbon black, selectively interacts with the Co precursor, resulting in Co–N bond formation on the PtCo nanoparticle surface. Electron transfer from Co to Pt in the PtCo nanoparticles modified by the polymer is enhanced by the increase in the difference in electronegativity between Pt and Co compared with that in bare PtCo nanoparticles with the TM surface oxides. In addition, the dissolution of Co and Pt is prevented by the selective passivation of surface Co atoms and the decrease in the O-binding energy of surface Pt atoms. As a result, the catalytic activity and durability of PtCo nanoparticles for the ORR are significantly improved by the electronic ensemble effects. The proposed organic/inorganic hybrid concept will provide new insights into the tuning of nanomaterials consisting of heterogeneous metallic elements for various electrochemical and chemical applications.

Pt nanoparticles (NPs) in a proton exchange membrane fuel cell as a catalyst for an oxygen reduction reaction (ORR) fairly overbind oxygen and/or hydroxyl to their surfaces, causing a large overpotential and thus low catalytic activity. Realizing Pt-based core-shell NPs (CSNPs) is perhaps a workaround for the weak binding of oxygen and/or hydroxyl without a shortage of sufficient oxygen molecule dissociation on the surface. Towards the end, we theoretically examined the catalytic activity of NPs using density functional theory; each NP consists of one of 12 different 3d-5d transition metal cores (groups 8-11) and a Pt shell. The calculation results evidently suggest the enhancement of catalytic activity of CSNPs in particular when 3d transition metal cores are in use. The revealed trends in activity change upon the core metal were discussed with respect to the thermodynamic and electronic structural aspects of the NPs in comparison with the general d-band model. The disparity between the CSNP and the corresponding bilayer catalyst, which is the so-called size effect, was remarkable; therefore, it perhaps opens up the possibility of size-determined catalytic activity. Finally, the overpotential for all CSNPs was evaluated in an attempt to choose promising combinations of CSNP materials.

A scale-bridging technique was used to investigate the effect of the elastic properties of β-Ti alloys on the stress distribution around the femoral stem of an artificial hip joint with a simplified geometry when under an external loading. The anisotropic elastic constants of single-crystalline β-Ti alloys (TN1: Ti-18.75 at% Nb, TN2: Ti-37.5 at% Nb, and TN3: Ti-43.75 at% Nb) were calculated using an ab-initio technique that was based on density functional theory calculation. The single-crystalline elastic constants calculated via the ab-initio technique were used to calculate the elastic constants of polycrystal β-Ti alloys using an elastic selfconsistent scheme. Finite element analysis based on the elastic constants of polycrystalline β-Ti alloys for a femoral stem was conducted to calculate the above-mentioned stress distribution. The model system consisting of a TN1 alloy exhibited a relatively high level of von Mises stress on the surface of cancellous and cortical bones compared to model systems consisting of TN2, TN3 alloys and commercial biomaterials (Ti-6Al-4V alloy and 316STS). The thickness of the cancellous bone between the femoral stem and the cortical bone affected the stress concentration on the surface of the cortical bone.

Carbon-supported Mo2C nanoparticles were synthesized and used as catalysts for the deoxygenation of oleic acid and soybean oil to produce diesel-range hydrocarbons. Various carbon materials, such as reduced graphene oxide (RGO), glassy spherical carbon (SC), activated carbon (AC), and mesoporous carbon (MC), were used as supports to determine the effects of RGO in the deoxygenation reactions. The effects of the flow rate, Mo content of the catalyst, and the structure of the carbon support, on the conversion and product selectivity were investigated. The morphology analysis revealed that Mo2C nanoparticles were well-dispersed onto the RGO (Mo2C/RGO). Under moderate reaction condition (T = 350 ºC, P = 5.0 MPa, H2/oil ratio = 4.5, LHSV = 2 h-1), oleic acid was efficiently deoxygenated using the Mo2C/RGO catalyst, which produced hydrocarbons with ≥ 85% yield and ≥ 90% hydrocarbon selectivity. This value was much higher than those obtained using the Mo2C/SC, Mo2C/AC, and Mo2C/MC catalysts (yields = 18.5-50.3%) under identical condition. The higher catalytic activity of the RGO-supported catalyst originated from its large pore size, which facilitated transport of the reactants, and uniform deposition of the Mo2C nanoparticles on the RGO surface. Even over a short contact time (LHSV = 8 h-1) and using natural triglyceride as a reactant, the Mo2C/RGO catalyst exhibited ≥ 40% yield of hydrocarbons, whereas a commercial CoMoSx/Al2O3 catalyst produced ≤ 10% yield under identical conditions. The Mo2C/RGO catalyst was highly selective towards C−O bond scission in hydroxyl group, which produced water and hydrocarbons without truncating the carbon skeleton of the starting material. Mo2C/RGO exhibited a prolonged catalyst lifetime for the deoxygenation of soybean oil (13% decrease in conversion after 6 h), compared to the commercial CoMoS/Al2O3 catalyst (42% decrease).

Oxidation of Co at the surface poses a major problem in the cyclable use of CoPt, a cost-effective catalyst for proton exchange membrane fuel cells. This can be alleviated by attaching a ligand selectively to Co-sites to stop its oxidation without compromising the catalytic activity. Here, we present a comparative analysis of adsorption of NH3 on the (0001) surface of Co in the HCP structure and (111) surfaces of Pt and CoPt alloy in the FCC structure, using first-principles density functional theoretical calculations. While NH3 binds more strongly with the Pt surface than with the Co surface, we find that its binding with the Co atom is stronger than that with the Pt atom on the surface of the CoPt alloy. Our analysis of the charge density and electronic structure shows how this originates from (a) the electron transfer from the minority spin d-band of Co to Pt, and (b) shift in the energy of d-bands and the magnetic moments of Co atoms on the surface of the CoPt alloy relative to those on the (0001) surface of Co. Hybridization of the d-states of Co in CoPt with pz states of N in NH3 used to stop Co oxidation also results in improving the charge transfer from Co to Pt that is relevant to the catalytic activity of CoPt. We finally present the analysis of how the interaction of NH3 with the CoPt surface can be tuned with strain.

Organic-inorganic perovskites are promising materials for improving the efficiency of solar cells, but there are still uncovered issues on the understanding of their electronic band structures. Using first principles calculations we investigate the electronic band features of organo-lead iodide perovskites and present the efficient model to predict the band gap variation based on the orbital interaction scheme. The orbital interaction between Pb and I atoms can be controlled through the structural modification such as the change in lattice constant and the deviation of I atoms from cubic symmetry sites. The increase of the lattice constant and the positional distortion of I atoms from the cubic symmetry sites lead to the increase of the band gap. With our findings, puzzling band gap variation behaviors in previous experiments and simulations can be understood and we suggest a pathway to precisely control their band gap. Our study can serve as the design rule for band gap engineering for various kinds of organic-inorganic hybrid perovskites.

Inspired by the controversy over tensile deformation modes of single-crystalline 〈110〉/{111} Au nanowires, we investigated the dependency of the deformation mode on diameters of nanowires using the molecular dynamics technique. A new criterion for assessing the preferred deformation mode—slip or twin propagation—of nanowires as a function of nanowire diameter is presented. The results demonstrate the size-dependent transition, from superplastic deformation mediated by twin propagation to the rupture by localized slips in deformed region as the nanowire diameter decreases. Moreover, the criterion was successfully applied to explain the superplastic deformation of Cu nanowires.

The effects of oxygen vacancies on the atomic and electronic structures in crystalline Zn2SnO4 were examined by ab-initio calculations using screened hybrid density functional. The formation energy and the electronic structure indicated that the neutral oxygen vacancy was stable in the n-type region with forming deep level in the bandgap. The ionization energy of oxygen vacancy was calculated to be around 2.0 eV, which can be excited by visible photon energy. The atomic movements and the Bader charge analyses around the neutral oxygen vacancy showed that Sn played a dominant role in changing the electronic properties by forming the pair of Sn2+-VO0, which was accompanied by the reduction of the charge state of Sn due to its multiple oxidation states. On the contrary, the electronic state of Zn hardly changed despite of its more predominant atomic shifts than Sn around the oxygen vacancy.

Catalysts in commercial proton exchange membrane fuel cells must be optimized for activity and cost, but improvements are hampered for Pt nanoparticles by low activity resulting from extensive OH adsorption. A model system clearly elucidating the impact of OH coverage on Pt nanoparticles was therefore developed. Using first principles calculations, free energies due to changing OH coverage in the oxygen reduction reaction were predicted as functions of electrode potential and local surface structure. Free energy contour plots that considered surface OH coverage were developed. Therefrom, we theoretically predicted the optimum range of operating potentials, yielding design guidelines for catalytic surfaces.

The creep properties of an 18Cr9Ni3CuNbVN austenitic stainless steel have been investigated at temperatures ranging from 923 to 1,073 K and stresses from 120 to 250 MPa. The rupture lives ranged from 10 to 20,105 hours. The stress dependence of the minimum creep rate obeyed a power law, with stress exponents ranging from 6 to 8.6. The activation energy was determined to be 460 to 485 kJ/mol/K. The microstructural evolution during the creep test was investigated. The V-rich Z-phase and metallic Cu precipitates began to precipitate in the middle of the creep deformation, resulting in an increase of the creep strength, while only Nb-based MX precipitates were present from the beginning of the high-temperature creep test.