Muneyuki Tsuda

Comparative study of O2 dissociation on various metalloporphyrins.

We investigate O(2) interaction with various metalloporphyrins (MnP, FeP, CoP, and NiP) using ab initio calculations based on density-functional theory. We discuss the trends in the activation barriers for the O-O bond cleavage in relation to the geometric, vibrational, electronic, and energetic properties of the systems. Whether the lowest unoccupied molecular orbital-highest occupied molecular orbital (LUMO-HOMO) levels of the metalloporphyrins involve the corresponding metal centers depends on the d orbital occupancies of the metals. We found that activation barriers for the O(2) dissociation can be mainly determined from the LUMO-HOMO characters of the metalloporphyrins, and consequently the FeP is the best catalyst with respect to the O(2) interaction from adsorption to dissociation.

Ab Initio Study of H2 Desorption from Magnesium Hydride MgH2 Cluster

In the present study, we investigate H 2 desorption from a magnesium hydride MgH 2 cluster by performing total energy calculations based on the density functional theory (DFT). We calculate the potential energy surface (PES) for the H 2 desorption from a (MgH 2 ) 5 cluster. According to the total atomic charges of the Mg and H atoms at the initial (MgH 2 ) 5 cluster, the Mg–H bond is ionic-like. Because of this, the activation barrier for the H 2 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 MgH 2 .

Proton Transfer to Oxygen Adsorbed on Pt: How to Initiate Oxygen Reduction Reaction

We investigate the proton transfer to oxygen adsorbed on a platinum catalyst using ab initio density functional calculations. The proton tends to preferentially migrate to O 2 as compared to O beca...

Magnetized/charged MgH2-based hydrogen storage materials

We propose two methods to facilitate the Mg–H bond cleavage of magnesium hydride MgH2. We found via density functional calculations that Mn (or Fe), when inserted between two MgH2, exhibits a much higher catalytic activity than the other 3d transition metal elements because of the induced spin polarization of the MgH2. We also found that an ionized MgH2 (MgH2+) has considerably softer Mg–H bonds. We can thus significantly reduce the H2 desorption temperatures from MgH2 to practical levels, ∼400K, by forming an alloy hydride with Mn (or Fe), and/or ionizing the MgH2.

Cyclohexane dehydrogenation catalyst design based on spin polarization effects

We investigate and discuss spin polarization effects on cyclohexane (C6H12) dehydrogenation using a Ni atom as a test catalyst, by performing total energy calculations based on the density functional theory (DFT). We compare the results with those of the well known catalyst Pt. We consider the process where cyclohexane approaches a transition metal M (M: Ni and Pt), and determine the reaction paths from the calculated potential energy surfaces (PESs) for singlet cyclohexane/M and triplet cyclohexane/Ni systems. Unlike the singlet cyclohexane/Ni, no energy is required to separate cyclohexyl intermediate (C6H11) from the H–Ni system for the triplet cyclohexane/Ni. Our results suggest that the catalytic reactivity of spin-polarized Ni becomes close to that of Pt, which is considered to be, up to now, the best catalyst for cyclohexane dehydrogenation.

Spin Polarization Effects on O2 Dissociation from Heme-O2 Adduct

We consider singlet and triplet iron-porphyrin-O2 (FeP-O2) adducts to investigate spin polarization effects on the FeP-O2 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).

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.

H2 Dissociative Adsorption on Strained/CO-Precovered Pt

We report the effects of straining and CO precover on H2 interaction with Pt determined by ab initio density functional calculations. Compressive (tensile) strain prevents (promotes) H2 dissociation and subsequent H adsorption. On the other hand, a CO neighbor prevents only H2 dissociation. Therefore, the catalytic activities for H2 dissociative adsorption are the best for realizing tensile-strained Pt, and the worst for realizing compressive-strained and CO-precovered Pt. We suggest that straining and/or CO precover can be used as a catalytic tailor for H2 dissociative adsorption.

First Principles Interpretation of Cyclohexane Dehydrogenation Process Using Pt

In this present study, we investigate cyclohexane (C6H12) dehydrogenation process using a Pt cluster by performing total energy calculations based on density functional theory (DFT). We consider the process where cyclohexane initially approaches a Pt3 cluster. After interacting with the Pt cluster, the Pt cluster draws an H atom from the cyclohexane, and an H–Pt bond is formed. With a C–H bond broken, the cyclohexyl intermediate (C6H11) desorbs from the Pt cluster. Since the most stable structures of C6H10 and C6H9 are obtained by removing H atoms from the adjacent C atoms of the cyclohexane, we suggest that cyclohexane dehydrogenation process involves the sequential cleavage of one H atom at a time from the cyclohexane.

Change of magnetic properties of benzenes in multiple-decked sandwich clusters: Mnn(C6H6)n+1 (n = 1, 2)

Local magnetic moments in benzenes of a multiple-decked sandwich cluster, namely, Mnn(C6H6)n+1 (n = 1,2) are investigated using density functional theory. A significant increase in the magnetic moments of benzenes was found in the 2pz orbitals. From the values of the local magnetic moments of Mn(C6H6)2 and Mn2(C6H6)3, we find that the local magnetic moment of benzene in the centre of the chain is larger than that at the edge position. These results indicate a promising possibility of changing the magnetic properties of benzenes using a magnetic material.