While gold has long been regarded as a poorly active catalyst, it is now widely investigated in the field of catalysis and gas sensing. Indeed, it exhibits surprisingly high catalytic activity when deposited as nanoparticles (NPs) on base metal oxides, carbon materials or organic polymers. Especially, gold nanoparticles catalysts with 2 to 10 nm diameters are active for many reactions, such as CO oxidation at a temperature as low as – 70°C. The catalytic performance of supported gold NPs depends on the kind of support materials, the size of gold NPs, and the gold/metal oxide interface structure[1].

A variety of preparation methods (more than 10) have been developed to obtain gold NPs with homogeneous dispersions on supports. Classical chemical methods need a calcination step in order to (i) reduce Au³⁺ ions deposited from a precursor (HAuCl₄ is the most popular one) by means of impregnation or deposition–precipitation techniques; (ii) remove organic ligands such as polyvinyl pyrrolidone or polyvinyl alcohol, which prevent the aggregation of gold NPs in the sol-immobilisation method;(iii) crystallize the metal oxide support in coprecipitation method. During the calcination step, the deposited gold particles generally grow to larger ones, so a precise control of the NPs size is difficult with these methods. On the other side, physical methods (PVD, Cathodic Arc Plasma Deposition) are cleaner and allow precise size distribution of the gold NPs on supporting materials, but they needs specific and expansive devices.

In this work, an original chemical method to prepare gold NPs deposited on a metal oxide is presented. No calcination is required. The gold precursor is HAuCl₄. The oxide used as supporting material must have a lamellar structure. In this work, we choose an Aurivillius phase (Bi_3.25La_0.75Ti₃O₁₂).The preparation consists in two steps. First, lithium is intercalated in the oxide structure, using n-butyl-lithium. After washing and drying at room temperature, a stable lithiated compound is obtained, in which some metallic cations have been reduced at a lower oxidation state [2]. Then, the lithiated powder is mixed with a gold precursor in aqueous solution. Gold ions are directly reduced near the support surfaces, without any other reducing agent. and the nanoparticles are formed only atthe oxide surface and they are well dispersed (figure 1). In addition, the NPs formation is accompanied by a partial delamination of the oxide grains which are separated in nanoplatelets (figures 2 and 3).

The materials have been characterized by electronic microscopies (HRSEM, TEM) at each step of the preparation process. The effect of the gold concentration has been investigated and the kinetics of the NPs deposition has been studied by UV-VIS spectroscopy (figure 4), using the gold NPs localized plasmon surface resonance property.

[1] Takei, T. et al; Heterogeneous Catalysis by Gold, in Advances In Catalysis, 55, 1-126, 2012, doi: 10.1016/B978-0-12-385516-9.00001-6

[2] Chevallier, V. et al., Exfoliated nanoplatelets of an Aurivillius phase, Bi_3.25La_0.75Ti₃O₁₂: Characterisation by X-ray diffraction and by high-resolution electron microscopy, J. of Solid State Chemistry, 181, 439–449, 2008   doi:10.1016/j.jssc.2007.12.012

Figures:

Figure 1: SEM image of Au nanoparticles on BLT crystals

Figure 2: TEM image showing the Au nanoparticles well dispersed on a nanoplatelet

Figure 3: TEM image of one Au nanoparticle on the surface of a BLT crystal

Figure 4: UV-VIS absorption spectra at different reaction times

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EMC Abstracts - https://emc-proceedings.com/abstract/preparation-and-structural-characterization-of-au-nanoparticles-supported-on-metal-oxide-nanoplatelets-for-catalysis-by-a-new-two-step-method/