J. Phys. Chem. C 2009, 113, 12799–12805
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Origin of the High Activity of Porous Carbon-Coated Platinum Nanoparticles for Aerobic Oxidation of Alcohols Yun Hau Ng,† Shigeru Ikeda,*,† Yoshihiro Morita,† Takashi Harada,† Keita Ikeue,‡ and Michio Matsumura† Research Center for Solar Energy Chemistry, Osaka UniVersity, 1-3 Machikaneyama, Toyonaka 560-8531, Japan, and Department of Nano Science and Technology, Graduate School of Science and Technology, Kumamoto UniVersity, 2-39-1 Kurokami, Kumamoto 860-8555, Japan ReceiVed: April 9, 2009; ReVised Manuscript ReceiVed: May 11, 2009
In an attempt to clarify the origin of the high activity and durability for aerobic oxidation of alcohols over a platinum (Pt)-carbon composite, i.e., Pt nanoparticles embedded in microporous carbon (nPt@hC), the catalytic reaction mechanism and microstructure of Pt nanoparticles were investigated in detail. By means of kinetic analyses, catalytic oxidation on nPt@hC was found to proceed through the formation of Pt-alcoholates, the β-hydride elimination to form Pt-hydride species (Pt-H), and oxidation of Pt-H with molecular oxygen. It was also revealed that the β-hydride elimination step was a rate-determining step in this reaction. These findings and results of structural studies indicate that the achievement of high catalytic activity on nPt@hC is due to stabilization of its transition state of a positively charged carbocationic component by the electronrich carbon matrix surrounding Pt nanoparticles, leading to lowering activation energy. Moreover, detailed investigation of the surface characteristics of Pt nanoparticles in nPt@hC after catalytic reactions by using various analytical methods revealed that the durability of nPt@hC for aerobic oxidation of alcohols is due to the suppression of aggregation of Pt nanoparticles and prevention of chemical poisoning of Pt surfaces. Introduction Liquid-phase oxidation of alcohols into corresponding aldehydes and ketones is a crucial transformation in the field of fine chemical and pharmaceutical industries. Traditional noncatalytic methods often employ stoichiometric quantities of highvalent metal oxidants, such as chromates, permanganate, and mineral acids, and are frequently used in halogenated organic solvents.1–3 However, isolation and disposal of enormous amounts of metal salts and harmful solvents are a serious drawback economically and environmentally. Owing to increasing environmental concerns, the replacement of these toxic oxidants with molecular oxygen (O2), a clean and inexpensive oxidant, is of great importance. Homogeneously catalyzed oxidation by means of complexes of Pd, Cu, Ru, V, and Co with use of O2 has been reported.4–8 However, most of these systems require the presence of other additives such as bases and co-oxidants,9 and the difficulty in recovery of these catalysts hampers their further applications in green chemistry of catalysis. Heterogeneous catalysts are useful for industrial applications due to their facile handling, recovery, and reusability. Hence, many examples of alcohol oxidation over supported and/or immobilized metal catalysts have been reported.10–19 Among these catalysts, Pt supported on activated carbon (Pt/AC) has attracted much attention because it works efficiently under mild conditions.17–19 A major bottleneck for large-scale applications of Pt metalcatalyzed alcohol oxidation is the rapid catalyst deactivation. Two major deactivation pathways have been reported: one is chemical poisoning of the Pt surface induced by adsorption of * To whom correspondence should be addressed. Phone/fax: +81-6-68506696/6699. E-mail:
[email protected]. † Osaka University. ‡ Kumamoto University.
intermediates or byproduct, and the other is overoxidation of Pt with O2 that results in the formation of platinum oxides on its surface.16,18 Although both pathways lead to a decrease in the number of active catalytic sites, the predominant pathway of deactivation is still controversial. Mallat and Baiker studied extensively the process of Pt chemical poisoning in detail and proposed that chemical poisoning is the primary cause of deactivation.14 In contrast, Vleeming et al. suggested that overoxidation of Pt is dominant for deactivation of the reaction.19 We previously reported the fabrication of carbon-encapsulated Pt nanoparticles (nPt@hC) based on a titanium(IV) oxide (TiO2) photocatalytic reaction.20–23 In the nanocomposite, Pt nanoparticles are embedded inside the framework of porous carbon, leading to strong resistance toward sintering and leaching.22 Compared to commercial Pt/AC, nPt@hC was found to be highly active for aerobic oxidation of alcohols under atmospheric pressure of O2 at 60 °C.23 Moreover, the nanocomposite exhibited high reusability, while the activity of Pt/AC in the reuse examination dropped to 20% of its original level under the same reaction conditions. These remarkable catalytic functions that are different from the functions of conventional Pt catalysts aroused our interest in obtaining a better understanding of the essential cause of these properties. In this study, therefore, we investigated in detail the origin of the high activity and durability of nPt@hC for aerobic oxidation of alcohols in relation to the reaction mechanism and structural features. Experimental Section Catalyst Preparation. The nPt@hC sample was synthesized by using a previously reported procedure.20–23 To 500 cm3 of aqueous solution containing 200 mg of phenol and 7.7 µmol of hexachloroplatinic acid (H2PtCl6) was added 500 mg of anatase TiO2 powder (Ishihara ST-21, average particle size 20 nm, BET surface area 50 m2 g-1). The suspension was then evacuated
10.1021/jp903561q CCC: $40.75 2009 American Chemical Society Published on Web 06/12/2009
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J. Phys. Chem. C, Vol. 113, No. 29, 2009
several times in a Pyrex inner-irradiation-type vessel connected to a closed gas circulation and evacuation system to ensure complete air removal. Photoirradiation was carried out for 5 h with a high-pressure Hg lamp (450 W) through a Pyrex water jacket (cutoff
