Stabilizing Gold Nanoparticles by Solid Supports

ZHEN MA AND SHENG DAI

1.1 Introduction

Catalysis by nanostructured materials has attracted tremendous interest recently. Nanostructured catalysts may have interesting catalytic properties associated with their small sizes and geometric/electronic structures. In particular, Haruta and co-workers found that gold nanoparticles finely dispersed on some metal oxide supports have excellent activities in low-temperature CO oxidation. This finding has been followed by thousands of studies on supported gold catalysts and their catalytic applications in environmental catalysis and chemical synthesis.

Gold nanoparticles may be synthesized via a traditional colloidal chemistry approach, in which AuCl4- ions are reduced by sodium citrate, tetrakis(hydroxymethyl)phosphonium chloride (THPC) or sodium borohydride (NaBH4), and the formed gold nanoparticles can be stabilized by polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), polydiallyldimethylammonium chloride (PDDA), or cetyltrimethylammonium bromide (CTAB). These gold nanoparticles (colloids) can be used either directly as catalysts in liquid phase or deposited onto solid supports.

Alternatively, gold nanoparticles can be formed on solid supports by loading a gold precursor (usually a gold salt or complex) onto solid supports followed by reduction or calcination. During the reduction or calcination process, the gold cations are reduced into gold atoms that aggregate into gold nanoparticles. The extent of agglomeration during this stage depends on many factors such as the ambient, temperature, and duration of the process as well as the nature of solid supports.

Supported metal catalysts are usually composed of metal nanoparticles and solid supports. Solid supports may provide a platform for dispersing and stabilizing gold nanoparticles so as to expose more surface gold atoms to the reactants, thus increasing catalytic activity. They may tune the oxidation state of gold by charge transferring or by mediating the reducing degree of gold precursors upon calcination or reduction. Some supports may undergo phase transformation or structural collapse under high temperatures, thus aggravating the sintering of gold nanoparticles on these supports or leading to the encapsulation of gold nanoparticles by these supports. Figure 1.1(c) shows a schematic diagram illustrating the phase transformation of a support at high temperatures. Besides the facets related to metal–support interactions mentioned above, solid supports may participate in catalysis by adsorbing and activating reactants as well as supplying active oxygen. They may also, of course, strongly adsorb some reaction intermediates or products, leading to catalyst deactivation.

Although solid supports can disperse gold nanoparticles, the sintering of gold nanoparticles at elevated temperatures is often inevitable because of their low melting points and high surface free energies. Figure 1.1(a) and Figure 1.1(b) show two models (crystalline migration and atom migration) proposed for the sintering of metal nanoparticles on supports. In the first model, entire metal crystallites migrate, collide, and coalesce on the support surface. In the second model, metal atoms migrate from one crystallite to another via the surface or the gas phase, making big crystallite bigger and small crystallites smaller. The phase transformation or structural collapse of supports under elevated temperatures, as shown in Figure 1.1(c), may exacerbate the sintering or encapsulation of gold nanoparticles.

Because the catalytic activities of supported gold catalysts often decrease sharply as the gold nanoparticle agglomerate under elevated temperatures but a high temperatures is often encountered during the calcination, operation, and regeneration of catalysts, it is necessary to enhance the thermal stability of supported gold nanoparticles. This can be achieved by improving synthesis details, e.g., thoroughly washing away residual chloride that may facilitate sintering. However, a more versatile way is to tune the structural environment surrounding gold nanoparticles, e.g., by strengthening the metal-support interaction and by designing sturdy inorganic shells that encapsulate gold nanoparticles. These strategies rely on synthesis and modification of catalytic materials.

Most of the publications relevant to gold catalysis deal with the conventional synthesis, characterization, and applications of supported gold nanoparticles, as well as the elucidation of the nature of active sites and reaction mechanisms. Only a small portion of publications have addressed the thermal stability and stabilization of gold nanoparticles on solid supports. The Dai group at the US Oak Ridge National Laboratory has been interested in designing new-structured gold nanocatalysts with enhanced properties, including catalytic activity, stability on stream, and thermal stability. It is known from these studies that catalytic performance and thermal stability of supported gold catalysts depend critically on their composition and catalyst structure. Below we first summarize some recent advances in the stabilization of gold nanoparticles by solid...