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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2
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Effect of Thermal Aging on Local Structure and Three-Way Catalysis of Cu/AlO Hiroshi Yoshida, Taiki Hirakawa, Haruka Oyama, Ren Nakashima, Satoshi Hinokuma, and Masato Machida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01848 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019
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The Journal of Physical Chemistry
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Effect of Thermal Aging on Local Structure and Three-way Catalysis of Cu/Al2O3 Hiroshi Yoshida,†‡ Taiki Hirakawa,§ Haruka Oyama,§ Ren Nakashima,§ Satoshi Hinokuma,†‡ and Masato Machida*†‡
†
Division of Materials Science and Chemistry, Faculty of Advanced Science and
Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan
‡ Unit
of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, 1-30
Goryo-Ohara, Nishikyo, Kyoto 615-8245, Japan
§ Department
of Applied Chemistry and Biochemistry, Graduate School of Science and
Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto, 860-8555, Japan
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ABSTRACT
Cu supported on Al2O3, prepared by impregnation, was thermally aged at different temperatures, and the influence of thermal aging on the local structure, redox behavior of Cu, and catalytic activity for a stoichiometric NO-CO-C3H6-O2 reaction was investigated. Crystalline CuO was mainly formed on Al2O3 after thermal aging at 89&& °C, whereas aging at higher temperatures induced Cu2+ incorporation into tetrahedral (Td-Cu2+) rather than octahedral (Oh-Cu2+) sites of -Al2O3. Despite its lower surface area, thermally aged Cu/Al2O3 with Td-Cu2+ sites showed a higher catalytic performance for the stoichiometric NO-CO-C3H6-O2 reaction compared with the as-prepared catalyst, especially for NO reduction. Td-Cu2+ was reduced to Td-Cu+ during the reaction with CO and/or C3H6, and NO could be reduced in subsequent reoxidation of Td-Cu+ to TdCu2+ by NO. This redox behavior is more probable on Td-Cu rather than crystalline CuO, resulting in enhancement of NO reduction during the three-way catalyst reaction.
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The Journal of Physical Chemistry
INTRODUCTION Three-way catalysts (TWCs) comprising nanoparticles of platinum-group metals (PGMs) are the most widely used for controlling automotive emissions due to their excellent performance. Despite their scarcity, the use of PGMs continues to increase with the increase in gasoline-fueled automobiles around the world. The development of alternative catalytic materials has therefore gained wide attention, and abundant non-PGMs are expected to become potential substitutes. TWCs must simultaneously remove nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (HC), and the catalytic activities of materials comprising non-PGMs were investigated for several reactions that may occur on a TWC, such as CO oxidation,1–12 CONO reaction,13–28 and NO reduction with hydrocarbons.29–35 Cu is a promising candidate among non-PGMs due to its excellent activity for CO oxidation, which is comparable to that of a Pt catalyst.3,4,6–12 For NO reduction via the CO-NO reaction, the activities of various metals including Co,13 Ni,14,21 Cu,15–18,22–28 and Ag19,20 were also reported, but these were much lower than those of PGMs. We recently reported the importance of local Cu structures on Cr-Cu embedded CeO2 surfaces with respect to the chemical states, thermal stability, and catalytic activities for CO oxidation and the CO-NO reaction.36–38 These studies revealed that an atomically dispersed Cu site enhances CO adsorption and oxidation, which is highly effective for the above-mentioned reactions with high turnover.38 Therefore, the catalytic performance can be drastically improved by controlling the local structure of active Cu sites when synthesizing nonPGM TWCs. There are numerous reports on Cu-containing catalysts for TWC reactions, some of which focus on controlling local Cu structures using composite oxides.39–43 Glisenti et al. synthesized Cu-doped perovskite oxides, LaCo1
