The electrochemistry of gold: II the electrocatalytic behaviour of the metal in aqueous media

Abstract

Although it is the noblest and most inert of metals, and is a very weak chemisorber, gold displays a very wide range of electrocatalytic activity -especially in base. Such unexpected behaviour is rationalized here in terms of the unusual properties of nanocluster, microparticles or quantum dots. Such species, present as defects at the metal surface, undergo oxidation at unusually low potentials (significantly lower than that of bulk gold) in a process known as premonolayer oxidation. The resulting low coverage (or incipient) oxide species are assumed to be the mediators for oxidation reactions while unoxidized active gold atoms, present at lower potentials, often act as the mediators for reduction. Electrocatalysis on noble metals is quite often confined to the double layer region and only low coverage metal atoms at active sites are involved. The enhanced activity of gold in base is readily rationalized in terms of a super-Nernstian E/pH effect. The unusual properties of metal nanoclusters may also be used to rationalize the surprisingly high level of catalytic activity of oxide-supported gold microparticles as reported recently by Haruta and co-workers.

Probing the surface oxidation of chemically synthesised gold nanospheres and nanorods

“…However, numerous studies have shown that gold is not as inert as its d 10 configuration suggests which accounts for its pronounced catalytic and electrocatalytic activity [6,[17][18][19][20][21][22][23][24]. This has been attributed to active sites on the surface that consists of atoms or clusters of atoms that have low co-ordination number and have the ability to partake in electrocatalytic reactions [21,22,25,26]. Recent work by Scholz has demonstrated in the case of gold that active or defect sites are located on the asperities of an electrode surface which are the loci of partially filled d orbitals that can stabilise free radical intermediates [27,28].…”

“…The electrooxidation of peroxide on gold also generates oxygen; however, in this system, it appears that this is insufficient to remove CTAB from the gold rods. The incipient hydrous oxide adatom mediator (IHOAM) model [22,40] of electrocatalysis predicts that reactions are mediated by a surface-confined redox couple based on the oxidation of active surface sites (M*) to hydrous oxide species (OA1 process). The gold spheres show a clear oxidation process indicating that the carboxylic acid group allows H 2 O 2 to interact with the surface.…”

Scope for new applications for gold arising from the electrocatalytic behaviour of its metastable surface states

“…However, it has been known for quite some time that even macroscopic gold displays remarkable electrocatalytic activity, especially in base, for certain Faradaic processes, e.g. aldehyde oxidation (3); a 2-part review of the electrocatalytic properties of gold in aqueous media was published recently (4,5). The unexpectedly high catalytic behaviour of gold surfaces was highlighted by Haruta and co-workers (6) who observed that oxidesupported gold nanoparticles display unrivalled catalytic activity for certain processes such as carbon monoxide oxidation.…”

Synthesis, processing, assembly and activation of core-shell structured gold nanoparticle catalysts

“…The understanding of the synthesis, processing, assembly and activation is geared towards the exploration of nanostructured catalysts. For more comprehensive overviews of gold nanoparticles in catalysis, readers are referred to several recent reviews and articles, including those in earlier issues of Gold Bulletin (3)(4)(8)(9)(10)(11)(12)(13).…”