Elsevier

Journal of Catalysis

Volume 330, October 2015, Pages 497-506
Journal of Catalysis

The effect of the Au loading on the liquid-phase aerobic oxidation of ethanol over Au/TiO2 catalysts prepared by pulsed laser ablation

https://doi.org/10.1016/j.jcat.2015.07.033Get rights and content

Highlights

  • Gold nanoparticles (NPs) were synthesized by pulsed laser ablation in water (PLAL).

  • The ligand-free Au NPs were efficiently deposited on TiO2 (P25) without any post-treatment.

  • Au loadings up to 10 wt% were obtained while maintaining the initial particle size of 7.8 nm.

  • Au NPs occupy the surface oxygen vacancies of TiO2 with increasing loading up to 4 wt%.

  • A further increase in loading beyond 4 wt% results in additional weakly adsorbed Au NPs.

  • High catalytic reactivity requires efficient pinning of the Au NPs to the TiO2 surface.

Abstract

Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO2 (P25) without any post-treatment yielding catalysts with Au loadings up to 10 wt%. Regardless of the loading, the Au NPs had a mean diameter of 8 nm before and after deposition. The ligand-free Au NPs strongly bind to TiO2 surface oxygen vacancies and maintain a homogeneous distribution with loadings up to 4 wt%, while a further increase in Au content up to 10 wt% results in additional weakly adsorbed Au NPs. The catalytic tests of the Au/TiO2 samples in the selective oxidation of ethanol in the liquid phase identified an optimal loading of 4 wt% resulting in the highest yield of acetic acid, which is ascribed to the homogeneous Au distribution and the adequate occupation of surface oxygen vacancies by strongly bound Au NPs without significant Au sintering during reaction.

Introduction

Highly active catalysts are obtained when supporting gold nanoparticles (NPs) on metal oxides, and the extensive exploration of Au-based catalysts in the past two decades has expanded their range of applications to reactions such as low-temperature CO oxidation [1], [2], [3], water–gas shift reaction [4], direct synthesis of hydrogen peroxide [5], acetylene hydrochlorination [6], and alcohol oxidation [7], [8], [9], [10], [11]. In the field of alcohol oxidation, supported Au catalysts became highly attractive as deactivation is a less critical issue for Au relative to Pd or Pt [12], [13], [14] benefitting from its higher resistance to oxygen poisoning. Moreover, Au NPs showed superior selectivity in preferentially oxidizing functional groups without forming by-products originating from Csingle bondC bond scission or Cdouble bondC bond isomerization [15], [16], [17]. Generally, the particle size [18], [19], [20], oxidation state [21], [22] and surrounding chemical environment affected by capping agents or ions [10], [23], [24], [25] are discussed in the literature as major factors influencing the reactivity of Au NPs. However, little attention was paid to the influence of the Au loading on the catalytic properties considering that the surface chemistry of the supports may influence the catalytic process as well [26], [27], [28] and that the support properties may vary as a function of the Au loading. Au/TiO2 is a highly efficient catalyst in the oxidation of alcohols, and the origin of its catalytic activity has been ascribed to the presence of perimeter sites activating molecular oxygen [29], [30]. As a reducible oxide with variable oxidation states (Ti4+ and Ti3+), TiO2 contains surface defects such as oxygen vacancies, which can influence the catalytic properties and stabilize deposited NPs. In many cases Au NPs supported on carbon materials were less active in the absence of base in alcohol oxidation compared with Au supported on oxides indicating a positive effect of the oxide support [25], [31], [32], [33], [34].

The preparation methods can significantly influence the properties of Au-based catalysts in the oxidation of alcohols [10], [35]. The commonly adopted methods include wet impregnation, deposition–precipitation and sol immobilization, where calcination is essential to activate the catalyst by decomposing the precursors or removing the protecting agents. High-temperature treatments can result in metal–support interactions, sintering of the Au NPs, and changes in the surface properties of the support. Contrary to the tedious and operation-dependent classic chemical methods, pulsed laser ablation in liquid (PLAL) method provides a facile approach for the fabrication of Au NPs free from additional ligands introduced by the chemical precursors and free of surfactants [36], [37], [38]. These Au NPs fabricated via PLAL are purely electrostatically stabilized due to their high surface charge density [39] and are ready for deposition on desired supports by simple mixing without additional post-treatment [36], [38], [40]. The as-prepared catalysts preserve the intrinsic characteristics of the Au NPs and the support to the maximum extent without modifications related to the preparation method. Moreover, without a protecting layer hindering the deposition onto a support, Au NPs can be easily deposited at room temperature onto TiO2 with high loadings while maintaining the initial particle size, which is rather difficult to achieve with conventional chemical methods. Accordingly, PLAL-assisted preparation of supported Au catalysts also offers the opportunity to systematically investigate the effect of the Au loading over a wide range excluding both primary particle size changes and surface ligand effects during catalyst preparation and catalytic reaction.

The selective oxidation of ethanol under base-free conditions was chosen as a model reaction to investigate the catalytic properties of Au/TiO2 catalysts prepared by PLAL. The aerobic oxidation of ethanol generates various products including acetaldehyde, acetic acid, ethyl acetate and CO2. 5 wt% aqueous ethanol and synthetic air were used in our study to simulate aqueous bioethanol [41] obtained by fermentation of biomass and to have good comparison with previous studies [42], [43], [44], [45], [46]. Au/TiO2 catalysts prepared by PLAL with Au loadings from 1 wt% to 10 wt% were systematically characterized and applied in the oxidation of ethanol in the liquid phase to study the influence of Au loading, Au particle size, degree of Au NP distribution on the support and reversible surface poisoning.

Section snippets

Catalyst preparation

Au NPs were synthesized using the PLAL method. A nanosecond Nd:YAG laser (Rofin-Sinar, RS-Marker 100D) with a fundamental wavelength of 1064 nm was focused into a flow chamber which was continuously flushed with a 0.1 mmol/L aqueous NaCl solution at a flow rate of 45 mL/min. The emitted laser pulses had a duration of 40 ns and a pulse energy of 8 mJ at a repetition rate of 5 kHz. The generated Au NPs were centrifuged for 3.2 h at 5000 rpm (3000 G-force) using a Hettich Universal 32 centrifuge.

Effect of Au loading on the properties of the Au/TiO2 catalysts

The surface plasmon resonance (SPR) is an important spectroscopic feature of Au NPs caused by electronic intraband transitions resulting in the absorption of light in the UV–VIS region. Generally, the position and shape of the SPR band can be influenced by the size, shape, and dielectric environment of the NPs [48]. Freshly synthesized Au/TiO2 catalysts with various gold loadings were characterized by diffuse reflectance UV–VIS spectroscopy. For all the prepared Au/TiO2 catalysts with different

Discussion

One feature of our preparation method is that it involves no post-treatment of samples such as calcination, which could introduce surface vacancies and induce metal–support interactions changing the surface properties of the support. As suggested by XRD and TEM measurements of the freshly prepared Au/TiO2 catalysts, supported Au NPs retained their original size distribution of the prepared Au colloid without aggregation. Similar crystallite and particle sizes of the Au NPs indicate that the NPs

Conclusions

Ligand-free gold nanoparticles fabricated by pulsed laser ablation in liquid were successfully supported on TiO2 (P25) at room temperature with various Au loadings ranging from 1 wt% up to 10 wt% while retaining the initial gold nanoparticle diameter at all loadings due to the chosen preparation method. The activity of the TiO2-supported Au catalyst in the selective oxidation of ethanol was found to depend on the Au loading. Hereby, optimal performance with respect to both conversion and yield

Acknowledgments

The authors are grateful to Dr. Thomas Reinecke at Ruhr-University Bochum for the XRD measurements and analyses. W.D. thanks the IMPRS-SurMat for a research grant. W.D. and S.R. contributed equally to this work. This work was supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG). Additionally, the authors thank the German Ministry of Research and Education (BMBF) for funding within the young investigator competition NanoMatFutur (Project

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