Kohei Sakata

Identification of sources of lead in the atmosphere by chemical speciation using X-ray absorption near-edge structure (XANES) spectroscopy

Sources of Pb pollution in the local atmosphere together with Pb species, major ions, and heavy metal concentrations in a size-fractionated aerosol sample from Higashi-Hiroshima (Japan) have been determined by X-ray absorption near-edge structure (XANES) spectroscopy, ion chromatography, and ICP-MS/AES, respectively. About 80% of total Pb was concentrated in fine aerosol particles. Lead species in the coarse aerosol particles were PbC2O4, 2PbCO3 Pb(OH)2, and Pb(NO3)2, whereas Pb species in the fine aerosol particles were PbC2O4, PbSO4, and Pb(NO3)2. Chemical speciation and abundance data suggested that the source of Pb in the fine aerosol particles was different from that of the coarse ones. The dominant sources of Pb in the fine aerosol particles were judged to be fly ash from a municipal solid waste incinerator and heavy oil combustion. For the coarse aerosol particles, road dust was considered to be the main Pb source. In addition to Pb species, elemental concentrations in the aerosols were also determined. The results suggested that Pb species in size-fractionated aerosols can be used to identify the origin of aerosol particles in the atmosphere as an alternative to Pb isotope ratio measurement.

DFT calculations for Au adsorption onto a reduced TiO2 (110) surface with the coexistence of Cl

Residual chlorines, which originate from HAuCl4, enhance the aggregation of gold (Au) nanoparticles and clusters, preventing the generation of highly active supported Au catalysts. However, the detailed mechanism of residual-chlorine-promoted aggregation of Au is unknown. Herein to investigate this mechanism, density functional theory (DFT) calculations of Au and Cl adsorption onto a reduced rutile TiO2 (110) surface were performed using a generalised gradient approximation Perdew, Burke, and Ernzerhof formula (GGA–PBE) functional and plane-wave basis. Although both Au and Cl atoms prefer to mono-absorb onto oxygen defect sites, Cl atoms have a stronger absorption onto a reduced TiO2 (110) surface, abbreviated as rTiO2 (110) in the following, than Au atoms. Additionally, co-adsorption of a Cl atom and a Au atom or Au nanorod onto a rTiO2 surface was investigated; Cl adsorption onto an oxygen defect site weakens the interaction between a Au atom or Au nanorod and rTiO2 (110) surface. The calculation results suggest that the depletion of interaction between Au and rTiO2 surface is due to strong interaction between Cl atoms at oxygen defect sites and neighbouring bridging oxygen (OB) atoms.

DFT calculations for aerobic oxidation of alcohols over neutral Au6 cluster

Lately, it was found that the Au nanoclusters stabilised by poly(N-vinyl-2-pyrrolidone) [PVP; (C6H9ON)n], abbreviated to Au:PVP, can oxidise p-hydroxybenzyl alcohol selectively into the corresponding aldehyde in water without degradation. This observation indicates that Au cluster can exhibit high catalytic activity without any metal oxide supports. From previous works, it was found that the anionic Au clusters played an important role for the activation of oxygen molecule on the Au clusters. However, the catalytic activity of neutral Au clusters for the aerobic oxidation reaction of alcohol is not still investigated in detail. In order to examine the catalytic activity of neutral Au clusters, the aerobic oxidation of p-hydroxybenzyl alcohol to the corresponding aldehyde catalysed by Au6 has been investigated quantum chemically using density functional theory with the PBE0 functional. Possible reaction pathways are investigated taking account of full structure relaxation of the model systems. From the calculation results, it was found that the formations of both a hydroperoxyl anion and a hydride were the important steps for the aerobic oxidation of p-hydroxybenzyl alcohol over Au6 cluster.

Theoretical investigation for isomerization of allylic alcohols over Au6 cluster

Transformation of allylic alcohols to corresponding saturated carbonyl compounds is one of the important reactions for industrial processes. Lately, Au-supported catalysts exhibit the catalytic activity for the isomerization of allylic alcohols to saturated aldehydes. However, the detail catalytic mechanism of this reaction was not elucidated in detail. Thus, theoretical calculations were carried out for the isomerization of 2-hexen-1-ol over isolated Au6 cluster in order to elucidate the reaction over Au catalysts. From these calculation results, it was found that the rate determining step of the reaction process was the hydrogen elimination from OH group of allylic alcohol, and the substrate was converted to 1-hexen-1-ol on Au6 cluster. Finally, it was also confirmed that 1-hexen-1-ol was converted to the corresponding aldehyde, and its activation barrier was much smaller than that of the deprotonation from OH group of allylic alcohol.

DFT Calculations of the Heterojunction Effect for Precious Metal Cluster Catalysts

Recently, catalytic reactions mediated by quasi-heterogeneous catalysts, also known as polymer-stabilized nanosize metal cluster catalysts, have attracted considerable attention. It is well known that the heterojunction between metal clusters and metal oxide supports is an important factor for the activities of heterogeneous catalysts, such as metal oxide-supported Au catalysts. However, as quasi-heterogeneous catalysts lack metal oxide supports, here, we investigated the effects of introducing heteroatoms into monometal clusters and the interaction between stabilizing polymers and metal clusters using the density functional theory. Based on the calculation results, we concluded that charge transfer interactions between heteroatoms in these model metal cluster systems play an important role for the activities of quasi-heterogeneous catalysts.