Spin polarization effects on O2 dissociation from heme-O 2 adduct

We consider singlet and triplet iron-porphyrin-O2 (FeP-O 2) adducts to investigate spin polarization effects on the FeP-O 2 interaction using ab initio calculations based on density functional theory (DFT). The presence of the imidazole (Im) ligand induces spin polarization from O2 to Fe in the triplet (Im)FeP-O2 adduct. The O-O bond of the triplet (Im)FeP-O2 is weaker than that of the triplet FeP-O2 because of this spin polarization effects. Our results suggest that magnetization of heme or heme-based nanomaterials may be utilized as cathode electrode catalysts in polymer electrolyte fuel cells (PEFCs). ©2005 The Japan Society of Applied Physics

Hydrogenase-based nanomaterials as anode electrode catalyst in polymer electrolyte fuel cells

We consider hydrogenase-based nanomaterials for possible use as anode electrode catalysts in polymer electrolyte fuel cells (PEFCs). We choose Fe-only hydrogenase component of Desulfovibrio desulfuricans (DdHase) as a hydrogenase complex, and investigate its catalytic activity for H2 dissociation using ab initio calculations based on density functional theory (DFT). We found two possible H-H bond cleavage pathways, which are heterolytic and possess low activation barriers. Moreover, the H2 dissociation can be promoted by inducing spin polarization of the H2 adduct. We report that hydrogenase or hydrogenase-based nanomaterials can manipulate to exhibit the catalytic activity equivalent to the well-known platinum catalyst. © 2004 Elsevier Ltd. All rights reserved

Behavior of hydrogen atom at Nafion-Pt interface

We present a mechanism depicting how a hydrogen atom migrates from a platinum catalyst to a Nafion membrane using ab initio calculations based on density functional theory (DFT). The H atom initially adsorbed on Pt is extracted by the sulfonate group (–SO3−), which is a part of the side chain of Nafion. We found partial electron transfer from the H atom to the Pt, which directly binds to the –SO3− group, through this reaction. The electron-poor H atom conducts in Nafion, and the partial electron transfer from the H atom results in the generation of electric power in polymer electrolyte fuel cells (PEFCs)

Temperature dependence of the spectral profile of the Yosida-Kondo resonance for a single magnetic atom adsorbed on a metal surface

The Kondo temperature TK, for a system consisting of a magnetic atom adsorbed on a non-magnetic metal surface, is usually determined experimentally from the corresponding energy width of the dI/dV(V) spectra, at the vicinity of the Fermi level EF. However, the dI/dV(V) spectra could exhibit symmetric or asymmetric, dip or peak structures around V = 0, depending on the Fano parameter q (which gives us a measure of the spatial extent of the wavefunction function for the localized orbital-how far it protrudes-out of the surface, for a particular adsorbate-metal system). Determining the corresponding Kondo temperature TK for a magnetic adsorbate on a metal surface system is non-trivial and not so straightforward. Here, we propose another means to experimentally determine TK, and to check that the resonance observed experimentally does have the temperature dependence characteristic of the Kondo effect, i.e., by measuring the peak height of the second-derivative of the dI/dV(V) spectra at the vicinity of EF-d2I/dV2(V). We also provide a brief history of the Kondo effect and the events leading up to this present study. © 2005 Elsevier B.V. All rights reserved

Behavior of hydrogen atom at Nafion-Pt interface

We present a mechanism depicting how a hydrogen atom migrates from a platinum catalyst to a Nafion membrane using ab initio calculations based on density functional theory (DFT). The H atom initially adsorbed on Pt is extracted by the sulfonate group (-SO3-), which is a part of the side chain of Nafion. We found partial electron transfer from the H atom to the Pt, which directly binds to the -SO3- group, through this reaction. The electron-poor H atom conducts in Nafion, and the partial electron transfer from the H atom results in the generation of electric power in polymer electrolyte fuel cells (PEFCs). © 2005 Elsevier Ltd. All rights reserved

Bis(histidine)/Bis(imidazole) Heme complex - Polymer electrolyte fuel cell application as an alternative cathode electrode catalyst

We focus on a variant of the heme protein, i.e., the recently discovered neuroglobin (NHb) and cytoglobin (CHb), looking for clues with regard to possible applications in polymer electrolyte fuel cells (PEFCs), in a quest to design nanomaterials that would replace the expensive platinum catalysts used in PEFC electrodes. The active heme sites of these two new members of the globin superfamily display a functionally-relevant heme iron atom, whose sixth coordination site is occupied by an endogenous (internal) protein ligand, in the absence of exogenous (external) ligands, such as O2, CO, and NO. Density Functional Theory (DFT)-based calculation results indicate a possible means of controlling the adsorption and desorption on adsorbed O2 through the endogenous protein ligand occupying the sixth coordination site of the central iron atom in the heme structure. © 2005 The Surface Science Society of Japan

Ab initio study of H2 desorption from magnesium hydride MgH 2 cluster

In the present study, we investigate H2 desorption from a magnesium hydride MgH2 cluster by performing total energy calculations based on the density functional theory (DFT). We calculate the potential energy surface (PES) for the H2 desorption from a (MgH 2), cluster. According to the total atomic charges of the Mg and H atoms at the initial (MgH2)5 cluster, the Mg-H bond is ionic-like. Because of this, the activation barrier for the H2 desorption is considerably high (3.30 eV). Moreover, the structural relaxation for the system, allowing for only the two Mg atoms closest to the two desorbing H atoms, does not largely affect the potential energy of the system. The results indicate that the ionic-like Mg-H interaction is related to the thermodynamical stability of MgH2. © 2004 The Physical Society of Japan

Dynamical cluster approximation in disordered systems with magnetic impurities

We present a theoretical method to study diluted magnetic semiconductors, beyond the single-site approximation, extending the dynamical cluster approximation (DCA). By the method, we can consider the nonlocal correlations due to disorder and discuss effects of the direct exchange interaction between magnetic impurities on the properties of the itinerant carriers. We apply the method to a three-dimensional cubic lattice system with random localized spins. We show that the strong antiferromagnetic superexchange interaction between nearest-neighbor sites suppresses the polarization of the carrier spins. ©2004 The Physical Society of Japan

Ab initio study of cyclohexane dehydrogenation with a transition metal (Pt, Pd, Ni and Cu) atom

To investigate cyclohexane dehydrogenation by a transition metal M (M: Pt, Pd, Ni and Cu) atom, we perform total energy calculations, based on the density functional theory. We consider the process where cyclohexane approaches the M atom. Upon interacting with the M atom, the M atom draws an H atom from the cyclohexane, forming an H-M bond. With a broken C-H bond, the dehydrogenated cyclohexane separates from the M atom. Of the M elements we investigated, a Pt atom exhibited the highest reactivity. In breaking the C-H bond of the cyclohexane, the σ donation dominates for a Pd and Cu atom as compared with the Pt atom, and the π back-donation dominates for a Ni atom as compared with the Pt atom. The results indicate that the excess charge transfer requires more energy for breaking the C-H bond of the cyclohexane with the Pd, Ni and Cu atom. ©2004 The Physical Society of Japan

Hydrogen atom quantum migration on platinum

We have reexamined hydrogen-platinum systems, giving emphasis on the atomic migration on the metal surface. The ideal (111) surface was firstly revisited, with the model potential energy surface for the motion of hydrogen created from calculations using a slab of platinum atoms fixed at the bulk separation obtained from experimental measurements. Calculated hydrogen hopping states in an adiabatic approximation are presented, with the lowest of these found at only 30 meV above the ground state. Effects of the presence of vacancies on the atomic migration are discussed from a potential energy surface constructed using a similar model. © 2006 The Surface Science Society of Japan