Oxidation of benzyl alcohol over nanoporous AuâCeO2 catalysts

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Kinetics, Catalysis, and Reaction Engineering 2

Oxidation of benzyl alcohol over nanoporous Au–CeO catalysts prepared from amorphous alloys and effect of alloying Au with amorphous alloys Ai Nozaki, Tasuku Yasuoka, Yasutaka Kuwahara, Tetsutaro Ohmichi, Kohsuke Mori, Takeshi Nagase, Hiroyuki Y Yasuda, and Hiromi Yamashita Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00927 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Industrial & Engineering Chemistry Research

Oxidation of benzyl alcohol over nanoporous Au–CeO2 catalysts prepared from amorphous alloys and effect of alloying Au with amorphous alloys Ai Nozaki,†,⊥ Tasuku Yasuoka,† Yasutaka Kuwahara,†,‡ Tetsutaro Ohmichi, † Kohsuke Mori,†,‡,§ Takeshi Nagase,†,ǁ Hiroyuki Y. Yasuda,† and Hiromi Yamashita* †,‡

†

Divisions of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.

‡

Unit of Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan.

§

JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.

ǁ

Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan.

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ABSTRACT

Au–CeO2 catalysts are prepared from amorphous Ce–Al alloys by the de-alloying method, and their catalytic performances are examined in the oxidation of benzyl alcohol to benzaldehyde using O2 as an oxidant. The catalytic activity of the Au–CeO2 catalysts is improved using an amorphous alloy as a precursor, and further improved activity is attained by directly alloying Au with Ce–Al rather than by depositing Au on pre-formed CeO2. Consequently, Au–CeO2 prepared directly from an amorphous Au–Ce–Al alloy exhibits the highest activity among the prepared samples, despite its low surface area. Detailed characterization by means of XPS analysis indicates that most of the Au species in Au–CeO2 prepared from the amorphous Au– Ce–Al alloy exist in a +1 oxidation state (Au+). The presence of Au+ facilitates the migration of oxygen atoms through the lattice, which likely contributes to the high catalytic activity of the Au–CeO2 catalyst.

KEYWORDS

Au catalyst, nanoporous metal oxide, amorphous alloy, CeO2, oxidation reaction.


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Introduction

CeO2 is a promising support material for catalytic oxidation reactions owing to its redox property and high oxygen storage capacity, which derives from the rapid and reversible switching between Ce3+ and Ce4+ oxidation states.1-8 Over the past few decades, many methods for the preparation of CeO2 have been reported, such as gas phase method, calcination of cerium hydroxide, and so on. Among these, CeO2 prepared by the de-alloying method shows a high surface area and excellent oxygen storage capacity compared with those of CeO2 prepared by the conventional methods.9-10 In the de-alloying method, nanoporous materials with high surface areas can be formed by the selective extraction of a specific component from a multi-component alloy by treatment with acidic or basic solutions. The formed nanoporous materials exhibit disordered atomic arrangements and large amount of low-coordinated atoms, leading to surface steps and kinks, which result in high catalytic performances of resulting CeO2.11-13

The disordered atomic arrangements of amorphous alloys differ completely from those of crystalline alloys. Consequently, a significant number of research has been dedicated to the unique properties of amorphous alloys, such as their chemical properties (high corrosion resistance and high surface activity), mechanical properties (high strength and high toughness),



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and magnetic properties (low coercive force and high magnetic permeability).14-16 Of these, we have focused on the chemical properties17-25 using an amorphous alloy as a catalyst precursor.

Recently, we reported that the atomic arrangement of a precursor dramatically affects the structure and catalytic properties of the resultant nanoporous metal catalysts. We found that using an amorphous alloy as the precursor for a nanoporous metal catalyst leads to a finer structure than that formed using the corresponding crystalline alloy owing to the high corrosion resistance of the amorphous alloy.26-30 It is expected that CeO2 with a fine porous structure would be obtained using an amorphous alloy as a precursor as well. Aluminium (Al) has known to be an element which can form amorphous alloy with Ce, albeit within a limited range of composition (7%–10% Ce)31 and Al can be easily extracted from the alloy by chemical treatment in basic solutions.

CeO2 has been extensively studied as a support material for Au, and it has been reported that Au/CeO2 shows excellent catalytic properties in various reactions. Au was generally considered to be catalytically inactive. However, since the discovery of the oxidation activity of Au nanoparticles by Haruta in the 1980s,32-33 their high catalytic activity has been extensively investigated.34-46 Recent studies have reported that metal oxides can play an important role in oxidation reactions when used as a support for Au nanoparticles47-50 because oxygen molecules can be adsorbed and dissociated at the peripheral regions of the interfacial


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surfaces between the Au and the metal oxides. Furthermore, the type, structure, and electronic state of the metal oxide affect the dissociative adsorption energy of oxygen over the Au/metal oxide catalysts.51

In this study, Au–CeO2 catalysts were prepared from amorphous Ce–Al alloys by the dealloying method, and the effect of the atomic arrangement of the precursor alloy on the catalytic activity of Au–CeO2 was investigated. Two methods were used to prepare the Au–CeO2 catalysts. In the first method, nanoporous CeO2 was prepared from amorphous Ce–Al alloys by immersion in NaOH solution to extract the Al moieties, and Au was then deposited on the prepared nanoporous CeO2. These catalysts are termed Au/CeO2 hereafter. In the second method, the catalysts were prepared directly from amorphous Au–Ce–Al alloys to allow closer contact of Au with CeO2. These catalysts are termed AuCeO2 hereafter. The catalytic performances of the prepared catalysts were examined in the oxidation of benzyl alcohol to benzaldehyde using O2 as an oxidant. It was proven that a fine porous AuCeO2 catalyst obtained by the selective leaching of Al moieties from an amorphous Au–Ce–Al alloy performs effectively for the oxidation of benzyl alcohol to benzaldehyde compared to CeO2-supported Au nanoparticle catalysts prepared by the conventional technique. We have demonstrated that amorphous Au–Ce–Al alloys can act as promising precursors for fabricating fine porous AuCeO2 catalysts with high catalytic activity.



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Experimental

Catalyst preparation.

Scheme 1 presents the procedure for the preparation of CeO2 and AuCeO2 catalysts. According to the literature, amorphous Ce–Al alloys can be prepared within a composition range of 7%–10% Ce.31 Master ingots of Ce8Al92 (and Au0.06Ce8Al92) alloy were prepared from the mixture of pure Ce and Al (and Au) lumps using arc-melt technique in a highly purified Ar atmosphere. Amorphous Ce8Al92 and Au0.06Ce8Al92 alloys (denoted as a-CeAl and a-AuCeAl, respectively) in the form of ribbons (1 mm wide, 10-20 μm thick) were produced from master ingots through liquid quenching by single roller melt-spinning method using a copper wheel. a-CeAl and a-AuCeAl were stored in a vacuum desiccator until use. The Al moieties were selectively extracted from a-CeAl and a-AuCeAl by immersion in 1 M NaOH aqueous solution at 343 K for 4 h (intended for the precursor of a crystalline alloy) or 6 h (intended for the precursor of an amorphous alloy), followed by washing with distilled water and drying to fabricate nanoporous CeO2 or Au–CeO2 (denoted as a-CeO2 and a-AuCeO2).

For the preparation of crystalline Ce–Al and Au–Ce–Al alloys, a-CeAl and a-AuCeAl were heated at 573 K under vacuum, and the products were denoted as c-CeAl and c-AuCeAl, respectively. The nanoporous CeO2 and AuCeO2 samples prepared from c-CeAl and c-AuCeAl, followed by treatment with NaOH solution are denoted as c-CeO2 and c-AuCeO2, respectively. 6 ACS Paragon Plus Environment

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Au/a-CeO2 and Au/c-CeO2 (Au loading: 1 wt%) were synthesized by depositing Au on the as-synthesized a-CeO2 and c-CeO2 by a deposition precipitation method using urea and HAuCl4 as an Au precursor according to a previously published method.52 For comparison, Au was deposited on JRC-CEO-2 (reference catalyst, The Catalysis Society of Japan) by the same method to give Au/JRC-CEO-2 as a standard catalyst.

Catalyst characterization.

The crystallinity of the samples was analysed using X-ray diffraction (XRD; Rigaku, Ultima IV). Differential thermal analysis (DTA) for determining the crystallization temperature of aCeAl were performed using a TG-DTA (RIGAKU TG8120) from RT to 1073 K at a heating rate of 2 K min-1 under an N2 flow of 50 mL min-1. The surface morphology of the samples was observed using field emission-scanning electron microscopy (FE-SEM; JEOL, JSM-6500). Prior to FE-SEM observations, the sample surfaces were coated with gold/palladium using an ion-sputtering instrument (Sanyu SC-701 MkII). TEM micrographs were obtained with a Hitachi Hf-2000 FE-TEM operated at 200 kV. The surface atomic ratio was analysed by energy dispersive X-ray spectrometry (EDX, EDAX Ltd. DX-4). The surface areas of the samples were estimated by the Brunauer–Emmett–Teller (BET) method using krypton and nitrogen physisorption isotherms obtained at 77 K (MicrotracBEL Corp. BEL-SORP max). X-ray photoelectron spectroscopy (XPS) was performed to evaluate the Au and Ce compositions at



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the sample surface. Ce L3-edge and Au L3-edge X-ray absorption fine structure (XAFS) spectra were obtained at the BL01B beamline of the SPring-8, Japan Synchrotron Radiation Research Institute (JASRI) (prop. no. 2017A1057 and 2017A1063). The spectra were recorded in transmission (Ce L3-edge) and fluorescence (Au L3-edge) mode at 298 K. Extended X-ray absorption fine structure (EXAFS) data were examined using an analysis program (Rigaku, REX2000). Fourier transformations of k3-weighted EXAFS oscillations were performed in the range of 3 < k (Å-1) < 12 to obtain the radial structure function.

Catalytic reactions.

The oxidation of benzyl alcohol was performed in a quartz reaction vessel at 373 K in the presence of atmospheric oxygen. The relevant Au–CeO2 catalyst (10 mg, 1 wt% Au) and K2CO3 (0.5 mmol) were placed in a quartz reaction vessel. A mixture of benzyl alcohol (0.5 mmol) and toluene (5 mL) was then injected to initiate the reaction. During the reaction, the reaction mixture was magnetically stirred at 373 K in the presence of atmospheric oxygen. At appropriate intervals, a portion of the reaction mixture was withdrawn and analysed using a gas chromatograph (Shimadzu, GC-2014) equipped with a flame ionization detector. Quantification of organic substrate and products were performed using biphenyl as an internal standard.


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Results and discussion

Nanoporous CeO2 and Au–CeO2 prepared from Ce–Al and Au–Ce–Al amorphous alloy.

The crystallinities of a-CeAl and a-AuCeAl prepared by the liquid quenching method were analysed using X-ray diffraction (XRD). a-CeAl and a-AuCeAl exhibit only a broad peak in the 2θ range of 35°–40° and no sharp peaks (Figure 1a and 1c) because amorphous alloys do not have a long range order.

The crystallization temperature of a-CeAl was evaluated using differential thermal analysis (DTA). An exothermic peak assignable to the crystallization of a-CeAl is observed at approximately 460 K (Figure S1). Therefore, to prepare the crystalline alloys (c-CeAl and cAuCeAl), a-CeAl, and a-AuCeAl were heated at 573 K under vacuum. After heating at 573 K, sharp peaks assigned to Al and Ce3Al11 intermetallic compounds are observed without broad peaks (Figure 1b and 1d), showing that c-CeAl and c-AuCeAl are crystalline alloys.

For the selective extraction of the Al moieties, a-CeAl, a-AuCeAl, c-CeAl, and c-AuCeAl were immersed in a 1 M NaOH aqueous solution. After the NaOH treatment, some peaks assigned to fluorite-like structure of CeO2 are observed in the XRD patterns of all samples (Figure 2a–d). It is assumed that the Al is selectively extracted by NaOH treatment and the residual Ce is easily oxidized by exposure to air during the drying process. Peaks assigned to Au cannot be detected because the Au particles are small and highly dispersed. The crystallite 9 ACS Paragon Plus Environment


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diameters of CeO2 were estimated to be 4.0, 4.6, 3.8, and 6.2 nm for Au/a-CeO2, Au/c-CeO2, a-AuCeO2, and c-AuCeO2, respectively, which were calculated from the half-height widths of the peaks. The crystallite size of a-AuCeO2 is small, which suggests that a-AuCeO2 may have low crystallinity or small ligament sizes.

The atomic ratios of the samples were evaluated by energy-dispersive X-ray spectroscopy (EDX) analysis (Table 1). The Al/Ce ratios for a-CeAl and a-AuCeAl are ca. 10, which is almost the same as the initial ratio of the raw metal ingots. After NaOH treatment, the Al concentration significantly decreases (Al/Ce: ca. 0.1). These results indicate a successful selective extraction of Al moieties from the Ce–Al and Au–Ce–Al alloys

The surface areas of the Au–CeO2 catalysts were estimated by the BET method using N2 physisorption isotherms obtained at 77 K (Table 1). The surface areas of a-CeAl and a-AuCeAl are quite low (

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