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628 results on '"Kuwahara, Yasutaka"'

201. Pd–Cu Alloy Nanoparticles Confined within Mesoporous Hollow Carbon Spheres for the Hydrogenation of CO2to Formate

Author

Yang, Guoxiang, Kuwahara, Yasutaka, Mori, Kohsuke, Louis, Catherine, and Yamashita, Hiromi

Abstract

It is intriguing yet challenging to synthesize high-performance heterogeneous catalysts for carbon dioxide (CO2) hydrogenation reactions. The excellent catalytic performance of heterogeneous catalysts has attracted widespread attention recently. Herein, a series of Pd–Cu alloy nanoparticles confined within mesoporous hollow carbon spheres (MHCS) were prepared and tested as catalysts for CO2hydrogenation to formate. The optimized Pd2Cu14–N@MHCS catalyst, which was obtained with about 1 wt % Pd loading and a Pd/Cu molar ratio of 2:14 by the coreduction method, exhibited high activity for CO2hydrogenation to formate, showing a high turnover number (TON) of 1432 at 100 °C after 24 h. Scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS) analyses revealed that when the Pd/Cu molar ratio was 2:14, the Pd atoms were well isolated by the Cu atoms via a Pd–Cu alloy. Density functional theory (DFT) calculations demonstrated that the PdCu2(111) surface enhanced not only the dissociation of H2but also the attack of the C atoms of the adsorbed HCO3–on Cu atoms by the dissociated H atoms on Pd atoms. This was attributed to an electron transfer from Cu to Pd in the alloy.

Published
2021
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229. Construction of Hybrid MoS2Phase Coupled with SiC Heterojunctions with Promoted Photocatalytic Activity for 4-Nitrophenol Degradation

Author

Zhang, Yuxiao, Kuwahara, Yasutaka, Mori, Kohsuke, and Yamashita, Hiromi

Abstract

Phase engineering has been recognized as a promising method for boosting the catalytic activity of molybdenum sulfide (MoS2) in the field of electrocatalysts and photocatalysts. The metallic 1T-MoS2exhibits much higher catalytic activity than natural semiconducting 2H-MoS2but suffers from harsh synthetic conditions and metastable physical/chemical properties. The hybrid 1T/2H phase MoS2shows higher catalytic activity than the 2H-MoS2and exhibits better stability than the 1T-MoS2, which is more favorable than the 2H-MoS2in the photocatalytic reactions. In this study, we report a hydrothermal synthesis of the hybrid 1T/2H-MoS2phase coupled with SiC as a heterojunction photocatalyst for 4-nitrophenol (4-NP) degradation. SiC acts as a counterpart of the heterojunction structure and a morphology modifier, which dramatically promotes the reaction rate and visible light responsibility, providing new candidates and strategies in photocatalysis.

Published
2020
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230. RuPd Alloy Nanoparticles Supported on Plasmonic HxMoO3–y for Efficient Photocatalytic Reduction of p‐Nitrophenol.

Author

Yin, Haibo, Kuwahara, Yasutaka, Mori, Kohsuke, and Yamashita, Hiromi

Subjects
*PHOTOREDUCTION, *SURFACE plasmon resonance, *ALLOYS, *SEWAGE, *MOLYBDENUM oxides
Abstract

p‐Nitrophenol (PNP) has aroused much concern due to its high toxicity in industrial wastewater and drinking water. Herein, we designed a new plasmonic RuPd/HxMoO3–y catalyst synthesized by a simple impregnation method, followed by H2 atmosphere reduction. The synthesized RuPd/HxMoO3–y hybrid, having RuPd alloy nanoparticles (NPs) anchored on HxMoO3–y, namely hydrogen‐doped molybdenum oxide support, shows a strong localized surface plasmon resonance (LSPR) absorption peak at around 639 nm. Photocatalytic activity of Ru/HxMoO3–y hybrid for the reduction of PNP to p‐aminophenol (PAP) under visible light irradiation can be dramatically improved by alloying with Pd atoms. More importantly, a reasonable mechanism for PNP reduction is proposed. We find that synergetic effect between HxMoO3–y and RuPd alloy NPs of RuPd/HxMoO3–y plays a vital role in photocatalytic reduction, in which RuPd alloy NPs act as the active species and plasmonic HxMoO3–y acts as electron‐donation centre. [ABSTRACT FROM AUTHOR]

Published
2019
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231. PdAg nanoparticles supported on resorcinol-formaldehyde polymers containing amine groups: the promotional effect of phenylamine moieties on CO2 transformation to formic acid.

Author

Masuda, Shinya, Mori, Kohsuke, Kuwahara, Yasutaka, and Yamashita, Hiromi

Abstract

The transformation of CO2 to formic acid (FA, HCOOH) is one of the most promising means of developing economically-viable CO2-mediated energy cycles, owing to the many advantages of FA as a feedstock, hydrogen source and fuel source for fuel cells. In the present study, we synthesized a series of catalysts consisting of PdAg nanoparticles supported on resorcinol-formaldehyde polymers having various amine contents (PdAg/amine-RF). These catalysts exhibited significant activity during CO2 hydrogenation to produce FA, especially when the support with the highest amine concentration was employed. Analyses of active Pd species using X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) demonstrated the formation of Pd–N bonds in these materials, and oxidized nitrogen, phenylmethanimine, phenylimine and phenylamine moieties were identified based on N 1s XPS data. Density functional theory calculations showed that the adsorption energy of bicarbonate ions on the PdAg alloy is much higher in close proximity to phenylamine molecules, while the activation energy of the rate determining step is significantly lower. These predictions are in good agreement with the experimental results, which demonstrate that the reaction order associated with the bicarbonate concentration is decreased with increasing nitrogen concentration in the catalyst. A significant correlation was observed between the turnover number and the nitrogen content (attributed to the presence of phenylamine moieties) estimated from both XPS and elemental analysis, indicating that phenylamine groups made the most significant contribution toward the promotion of this reaction. [ABSTRACT FROM AUTHOR]

Published
2019
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232. Hollow titanosilicate nanospheres encapsulating PdAu alloy nanoparticles as reusable high-performance catalysts for a H2O2-mediated one-pot oxidation reaction.

Author

Kuwahara, Yasutaka, Matsumura, Ryo, and Yamashita, Hiromi

Abstract

A tandem reaction combining direct H2O2 production from H2 and O2 and a subsequent oxidation reaction with H2O2 in a single reaction vessel offers an energy-saving and cost-effective oxidation route for the synthesis of targeted chemicals. Herein, we report the synthesis of hollow titanosilicate nanospheres encapsulating Pd and PdAu alloy nanoparticles (NPs) for the H2O2-mediated one-pot oxidation reaction. The yolk–shell nanostructured catalyst (Pd@Ti-HMSS), composed of a Pd NP core encapsulated within a hollow mesoporous titanosilicate shell having isolated Ti atoms, is fabricated by a facile etching and re-assembly process using SiO2 spheres as a template and two different kinds of Si alkoxides as precursors. The synthesized catalyst shows superior catalytic activity in the one-pot oxidation reaction of sulfide under a co-flow of H2 and O2, and far outperforms the prototype titanosilicate-supported Pd NP catalysts. Detailed structural characterization and kinetic analysis reveal that the increased catalytic efficiency is attributed to efficient mass transfer of reactants and the associated improved reactivity with in situ generated H2O2 over Ti sites. A further enhancement in activity is achievable by encapsulating PdAu alloy NPs, which provide a faster H2O2 production rate than monometallic Pd NPs. Owing to the presence of a protective silicate shell, the catalyst shows good stability and reusability over multiple catalytic cycles without significant loss of activity. [ABSTRACT FROM AUTHOR]

Published
2019
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233. Defect Engineering of MoS2 and Its Impacts on Electrocatalytic and Photocatalytic Behavior in Hydrogen Evolution Reactions.

Author

Zhang, Yuxiao, Kuwahara, Yasutaka, Mori, Kohsuke, and Yamashita, Hiromi

Subjects
*PHOTOCATALYSIS, *ELECTROCATALYSIS, *HYDROGEN evolution reactions, *MOLYBDENUM disulfide, *ELECTRON-transfer catalysis
Abstract

Molybdenum disulfide (MoS2) has been regarded as a favorable photocatalytic co‐catalyst and efficient hydrogen evolution reaction (HER) electrocatalyst alternative to expensive noble‐metals catalysts, owing to earth‐abundance, proper band gap, high surface area, and fast electron transfer ability. In order to achieve a higher catalytic efficiency, defects strategies such as phase engineering and vacancy introduction are considered as promising methods for natural 2H‐MoS2 to increase its active sites and promote electron transfer rate. In this study, we report a new two‐step defect engineering process to generate vacancies‐rich hybrid‐phase MoS2 and to introduce Ru particles at the same time, which includes hydrothermal reaction and a subsequent hydrogen reduction. Compositional and structural properties of the synthesized defects‐rich MoS2 are investigated by XRD, XPS, XAFS and Raman measurements, and the electrochemical hydrogen evolution reaction performance, as well as photocatalytic hydrogen evolution performance in the ammonia borane dehydrogenation are evaluated. Both catalytic activities are boosted with the increase of defects concentrations in MoS2, which ascertains that the defects engineering is a promising route to promote catalytic performance of MoS2. Having major defects: Defect engineering of MoS2 to improve electrocatalytic/photocatalytic performances for hydrogen production was examined. [ABSTRACT FROM AUTHOR]

Published
2019
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234. A mini-review on carbon and silicon-based yolk-shell structured catalysts for CO2conversion: Design, advantages, and challenges

Author

Yang, Guoxiang, Wang, Qi, Kuwahara, Yasutaka, Wang, Longyang, Deng, Hao, and Yamashita, Hiromi

Abstract

Especially in the context of current greenhouse effect and energy shortage, the development of various catalysts to catalyze CO2resource utilization is an attractive field. Recently, the yolk-shell catalyst solves the problems of coking and sintering and further provides confinement effects as well as improved stability. In this mini-review, the development status of carbon, silicon-based yolk-shell structured catalysts for hydrogenation of CO2to formate/formic acid, and CO2dry reforming of methane are introduced. The latest research progress of catalysts and active metals commonly used in these reactions is reviewed, as well as how to provide superior performance in these reactions. In addition, the properties of customized yolk-shell structure and the effect of pore size change on CO2hydrogenation and dry reforming reaction will be briefly described. Furthermore, the effect of space restriction and how to enhance the performance of the yolk-shell structure nanoreactor are also discussed. We firmly believe that this mini-review will stimulate more research in the catalysis community, especially to advance the design of yolk-shell catalytic materials, and ultimately achieve sustainable development of the carbon cycle.

Published
2023
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