称号及び氏名

博士（工学） Mai Phuong Tu

学位授与の日付

2017 年 9 月 25 日論「Development of Electrocatalysts toward Complete Oxidation ofEthanol

文

名（エタノールの完全酸化を目指した電極触媒の開発）」
論文審査委員

主査

井上 博史

副査

長岡

副査

松岡 雅也

勉

論文要旨
Direct alcohol fuel cells (DAFCs) have increasingly attracted attention in their applications toalternative power sources for portable electric devices to lithium ion batteries. Among alcohols,ethanol is more attractive than methanol because it has lower toxicity and higher energy density.
Ethanol, moreover, is a carbon-neutral fuel, and can be easily produced in great quantity byfermentation of sugar-containing raw materials. However, ethanol oxidation reaction (EOR) has verysluggish kinetics, and the C-C bond in an ethanol molecule is hard to break, which inhibits complete
oxidation to CO2. Pt-based binary anode catalysts have demonstrated high catalytic activity for EOR.
Many studies have proved PtSn alloy and Pt/SnOx were the most promising catalysts for EOR. In
these catalysts Pt facilitated the adsorption and dissociation of ethanol, while Sn provided OHspecies for the oxidation of CO intermediates strongly adsorbed on Pt, which is known as abi-functional effect. The main product, however, was not CO2, but acetaldehyde and acetic acid, thepartial EOR products, which were unfavorable for the operation of direct ethanol fuel cells (DEFCs).
Another binary catalyst with a composition of PtRh was effective for the C-C bond cleavage or CO2
formation, although the EOR activity was low. Recently, ternary Pt-Sn-Rh catalysts prepared byvarious methods have been reported to be very effective for EOR because they had lower onsetpotential for EOR current density (Eonset) and higher EOR current density (jEOR), and also facilitatedthe C-C bond cleavage. It was proved that all three constituents were needed for the cleavage of theC-C bond in ethanol to form CO2 with keeping EOR activity. However, there have been only a few
researches about atomic arrangement at active sites so far. Considering the previous results, I built ahypothesis in which Pt and Sn (or SnOx) sites should be separated by Rh atoms for selective CO2formation.
The purpose of this study is to verify my hypothesis, and establish how to prepare atomic
arrangement-controlled Pt/Rh/SnO2 nanoparticle-loaded carbon black (Pt/Rh/SnO2/C) catalyst. For
this purpose, I first developed two ways to deposit Rh and SnO2 on a polycrystalline Pt model
electrode where the Pt and SnO2 sites were separated by Rh atoms. Then I characterized the atomic
arrangement-controlled Pt/Rh/SnO2 electrodes, and evaluated their EOR activity and CO2 selectivity
by electrochemical measurements and in situ infrared reflection-adsorption spectroscopy (IRAS),respectively. Next, toward the application to DEFCs, I developed how to prepare the atomic
arrangement-controlled Pt/Rh/SnO2/C nanoparticle catalyst, and evaluated its EOR activity and CO2
selectivity by the electrochemical and spectroscopic measurements.
This thesis consists of five chapters, and the content of each chapter is as follows.
Chapter 1 includes discussion of the research background knowledge as well as the purpose andsummary of the current research work.
In chapter 2, the atomic arrangement-controlled Pt/Rh/SnO2 electrode was prepared bydepositing SnO2 on a Pt model electrode, followed by depositing Rh after CO adsorption and thenCO stripping at a specific potential, which is named a limited CO stripping method. Sn wasdeposited on a Pt electrode by potentiostatic method, and oxidized to SnO2 in air. The coverage ofSnO2 on Pt electrode was 0.23. After the limited CO stripping, only CO molecules adsorbed on Pt
sites adjacent to SnO2 were removed, and Rh was deposited by galvanic replacement with atomic
hydrogen adsorbed on the exposed Pt sites, which is named an H-upd method (controlledPt/Rh/SnO2(h)). For comparison, Rh was randomly deposited by the H-upd method without thelimited CO stripping (normal Pt/Rh/SnO2(h)). The Rh contents of the normal and controlled
Pt/Rh/SnO2(h) electrodes were 0.09 and 0.04, respectively. The jEOR value of the controlled
Pt/Rh/SnO2(h) electrode at 0.6 V (vs. RHE) was similar to that of the Pt/SnO2 electrode, and ca. 3
times as high as that of the Pt electrode. However, the jEOR value of the normal Pt/Rh/SnO2(h)electrode was ca. 1.5 times as high as that of the Pt/SnO2 electrode. The Eonset value for the Pt/SnO2
and both Pt/Rh/SnO2(h) electrodes was 0.1 V and 0.2 V lower than that for the Pt electrode,respectively. The in situ IRAS exhibited that for the normal Pt/Rh/SnO2(h) electrode, the formationof acetic acid was predominant over that of CO2, whereas the controlled Pt/Rh/SnO2(h) electrodewith lower EOR activity was superior in CO2 selectivity to the normal one.
In order to increase the EOR performance, the Sn coverage was enhanced and Rh was depositedby potentiostatic method instead of the H-upd method after the limited CO stripping (controlledPt/Rh/SnO2(p)). The controlled Pt/Rh/SnO2(p) electrode had the Sn coverage of 0.42 and the Rh
content of 0.28. The jEOR value of this electrode at 0.6 V was ca. 4 times as high as that of the Ptelectrode. The Eonset value for the Rh/Sn/Pt electrode was 0.2 V lower than that for the Pt electrode.
The in situ IRAS exhibited that the controlled Pt/Rh/SnO2(p) electrode greatly suppressed theformation of acetic acid, leading to the selective CO2 formation.
In chapter 3, another atomic arrangement-controlled Pt/Rh/SnOx electrode was prepared bydepositing Rh on a Pt model electrode, followed by depositing Sn only on Rh (tandem Pt/Rh/SnOx).
The deposition of Rh and Sn was performed by the H-upd method. For comparison, Sn wasrandomly deposited on Rh-deposited Pt (Pt/Rh) electrode by the H-upd method (randomPt/Rh/SnOx). The deposited Sn was considerably oxidized to SnO and SnO2 in air. The covarages ofSnOx and Rh for the random Pt/Rh/SnOx were 0.36 and 0.22, respectively, and those for the tandemPt/Rh/SnOx were 0.07 and 0.28, respectively. The Pt/Rh electrode exhibited the similar Eonset andjEOR to the Pt electrode. However, both Pt/Rh/SnOx electrodes were 5 times as high in jEOR as the Pt
and Pt/Rh electrodes. The Eonset values of these two ternary catalysts were about 0.1 V lower than thePt and Pt/Rh electrodes. In situ IRAS exhibited that the tandem Pt/Rh/SnOx electrode selectively
formed CO2 without the formation of acetic acid, whereas the random Pt/Rh/SnOx electrode still
formed acetic acid. The latter suggests SnO2 was deposited on not only Rh but also Pt surface.
In chapter 4, based on findings obtained in chapter 2, the atomic arrangement-controlled
Pt/Rh/SnO2/C was prepared by depositing Rh on Pt/SnO2(3:1) nanoparticle-loaded carbon black(Pt/SnO2/C) by H2 reduction method after the limited CO stripping (controlled Pt/Rh/SnO2/C). For
comparison, Rh was randomly deposited on Pt/SnO2/C by the same method without the limited COstripping (normal Pt/Rh/SnO2/C). The size of nanoparticles for the normal and controlledPt/Rh/SnO2/C was estimated as 2.8 nm by X-ray diffraction spectroscopy. The Rh contents of thenormal and controlled Pt/Rh/SnO2/C were 5.5 and 5.0 at.%, respectively. The jEOR at 0.6 V for thecontrolled Pt/Rh/SnO2/C electrode was ca. 2 and 4 times as high as that for the Pt/SnO2/C andnormal Pt/Rh/SnO2/C, respectively. The Eonset for the controlled Pt/Rh/SnO2/C electrode was themost negative among three catalysts. The in situ IRAS exhibited that for the normal Pt/Rh/SnO2/C
electrode acetaldehyde and acetic acid were mainly produced, whereas for the controlled
Pt/Rh/SnO2/C electrode the main products were CO2 and acetaldehyde, and their production started
at 0.2 V. In the latter the amount of CO2 increased with potential, but that of acetaldehyde did notdepend on potential. Thus I succeeded the preparation of the atomic arrangement-controlled
Pt/Rh/SnO2/C with high CO2 selectivity and high EOR activity.
Chapter 5 finally concludes this thesis by summarizing the previous chapters.

審査結果の要旨
本論文は、直接型エタノール燃料電池のアノード反応であるエタノールの CO2 への酸化反応を選択的に行わせるための Pt/Rh/SnOx 触媒の構造制御に関する研究成果をまとめたものであり、以下の成果を得ている。
（１） SnO2 修飾 Pt 電極表面に CO を吸着させた後、0.5 V vs. RHE の電位を印加して SnO2に隣接する Pt に吸着した CO のみを酸化除去した。その後、Rh を析出させることにより Rh を Pt と SnO2 の間に配置させることに成功した。このような Pt/Rh/SnO2
触媒のエタノール酸化活性は Pt や Pt/SnO2 触媒より高いことを明らかにした。
また、
in situ 赤外反射-吸収分光法（IRAS）より、Rh の析出位置を制御した触媒では、完全酸化生成物である CO2 が選択的に生成することを明らかにした。
（２） Rh 修飾 Pt 電極上に 0.5 V vs. RHE の電位を印加しながら CO を電解液に吹き込むと、CO は Pt にのみ吸着することを見出した。これを利用して、Rh 上にのみ Sn を析出させることに成功した（tandem Pt/Rh/SnOx）。tandem Pt/Rh/SnOx 触媒のエタノール酸化活性は Pt や Pt/Rh 触媒の約 5 倍であることを明らかにした。また、in situ IRAS により、tandem Pt/Rh/SnOx 触媒では、CO2 が選択的に生成することを明らかにした。
（３） Pt/SnO2 ナノ粒子担持カーボン（Pt/SnO2/C）上に CO を吸着させた後、0.5 V vs. RHEの電位を印加して Sn に隣接した Pt に吸着した CO のみを選択的に酸化除去した。
その後、水素還元法により Sn に隣接した Pt にのみ Rh を析出させることに成功した。
この触媒のエタノール酸化活性は Pt/SnO2/C 触媒の約 2 倍であることを明らかにした。また、in situ IRAS により、ナノ粒子触媒の場合にも、CO2 が選択的に生成することを明らかにした。
以上の研究成果は、エタノールの完全酸化触媒の開発にきわめて有益な知見を与えるものであり、燃料電池やその関連分野の学術的・産業的な発展に貢献するところ大である。
また、申請者が自立して研究活動を行うのに必要な能力と学識を有することを証したものである。
学位論文審査委員会は、本論文の審査ならびに最終試験の結果から、博士（工学）の学位を授与することを適当と認める。