Kenichi Koizumi

Atom-Scale Reaction Pathways and Free-Energy Landscapes in Oxygen Plasma Etching of Graphene

We report first-principles molecular dynamics calculations combined with rare events sampling techniques that clarify atom-scale mechanisms of oxygen plasma etching of graphene. The obtained reaction pathways and associated free-energy landscapes show that the etching proceeds near vacancies via a two-step mechanism, formation of precursor lactone structures and the subsequent exclusive CO2 desorption. We find that atomic oxygen among the plasma components is most efficient for etching, providing a guidline in tuning the plasma conditions.

Reducing the Cost and Preserving the Reactivity in Noble‐Metal‐Based Catalysts: Oxidation of CO by Pt and Al–Pt Alloy Clusters Supported on Graphene

The oxidation mechanisms of CO to CO2 on graphene-supported Pt and Pt-Al alloy clusters are elucidated by reactive dynamical simulations. The general mechanism evidenced is a Langmuir-Hinshelwood (LH) pathway in which O2 is adsorbed on the cluster prior to the CO oxidation. The adsorbed O2 dissociates into two atomic oxygen atoms thus promoting the CO oxidation. Auxiliary simulations on alloy clusters in which other metals (Al, Co, Cr, Cu, Fe, Ni) replace a Pt atom have pointed to the aluminum doped cluster as a special case. In the nanoalloy, the reaction mechanism for CO oxidation is still a LH pathway with an activation barrier sufficiently low to be overcome at room temperature, thus preserving the catalyst efficiency. This provides a generalizable strategy for the design of efficient, yet sustainable, Pt-based catalysts at reduced cost.

Hybrid-DFT study on electronic structures of the active site of sweet potato purple acid phosphatase: the origin of stronger antiferromagnetic couplings than other purple acid phosphatases.

The electronic structure and magnetic interactions of the active site of sweet potato purple acid phosphatase (PAP) were investigated by using UHF, pure DFT (UBLYP), and hybrid DFT methods (UB3LYP and UB2LYP). PAP catalyzes the hydrolysis of a phosphate ester under acidic conditions and contains a binuclear metal center. Sweet potato PAP provides stronger antiferromagnetic coupling than other PAPs. UB3LYP showed reasonably good agreement with the experimental magnetic coupling, indicating that this stronger antiferromagnetic coupling is caused by a mu-oxo bridge in the Fe(III)-Mn(II) binuclear metal center, which is the origin of the asymmetric spin delocalization. The type of bridging ligand is essential for the reaction mechanism, because the bridging ligand is suggested to function as a nucleophile in the reaction. Analyses of the natural orbital and spin density distributions implied the asymmetric spin delocalization on the bridging oxygen. The mechanism and the pathway of the antiferromagnetic coupling between Fe(III) and Mn(II) were discussed, using chemical indices introduced with the occupation numbers of singly occupied natural orbitals.

Simple but Efficient Method for Inhibiting Sintering and Aggregation of Catalytic Pt Nanoclusters on Metal-Oxide Supports.

A simple and efficient method to inhibit aggregation of Pt clusters supported on metal oxide was developed, preserving the accessible clusters surface where catalytically active sites are located even at relatively high temperatures up to 700 K. The key idea was the inclusion of transition metal atoms such as Ni into the Pt clusters, thus anchoring the clusters through formation of strong chemical bonds with oxygen atoms of the metal-oxide support. To elucidate the efficiency of the method, first-principles molecular dynamics enhanced with free-energy sampling methods were used. These virtual experiments showed how doped Ni atoms, having a stronger affinity to O than Pt, anchor the Pt clusters tightly to the metal-oxide supports and inhibit their tendency to aggregate on the support.

Steric effect in the endothermic Penning ionization reaction of tert-butyl bromide with Kr(3P)

The steric opacity function (the dependence of reaction cross section upon mutual molecular orientation) for the endothermic Penning ionization channel of Kr(3P)+(CH3)3CBr reaction was determined by using an oriented tert-butyl bromide molecular beam at 0.1 eV average collision energy. A remarkably large steric effect was observed in contrast with the Ar(3P)+CH3Br reaction. We find that the ionization cross section is maximum at sideways and it becomes “zero” at the tert-butyl end. The Br end is found to be unfavorable as compared with sideways approaches. The stereo selectivity observed in the present work is discussed in terms of the electron exchange mechanism, in which the shielding effect by bulky nonreactive tert-butyl group as well as the smearing effect in impact parameter are taken into account.

Theoretical design of a novel copper doped gold cluster supported on graphene utilizing ab initio molecular dynamics simulations

Ab initio molecular dynamics simulations have been used to inspect the adsorption of O2 to a small gold-copper alloy cluster supported on graphene. The exposed Cu atom in this cluster acts as a crucial attractive site for the approaching of O2 and consequently widens the reaction channel for the adsorption process. Conversely, a pure Au cluster on the same graphene support is inactive for the O2 adsorption because the corresponding reaction channel for the adsorption is very narrow. These results clearly indicate that doping a different metal to the Au cluster is a way to enhance the oxygen adsorption and to promote catalytic reactions.