Model catalysts of supported Au nanoparticles and mass-selected clusters

Abstract

In surface science, much effort has gone into obtaining a deeper understanding of the size-selectivity of nanocatalysts. In this article, electronic and chemical properties of various model catalysts consisting of Au are reported. Au supported by oxide surfaces becomes inert towards chemisorption and oxidation as the particle size became smaller than a critical size (2-3 nm). The inertness of these small Au nanoparticles is due to the electron-deficient nature of smaller Au nanoparticles, which is a result of metal-substrate charge transfer. Properties of Au clusters smaller than ∼20 atoms were shown to be non-scalable, i.e., every atom can drastically change the chemical properties of the clusters. Moreover, clusters with the same size can show dissimilar properties on various substrates. These recent endeavours show that the activity of a catalyst can be tuned by varying the substrate or by varying the cluster size on an atom-by-atom basis.

Catalysis by clusters with precise numbers of atoms

“…Because of the reduced size of clusters, and the fact that a considerable fraction of their atoms are located at the cluster/support interface, these effects are likely to be more pronounced in cluster catalysts than typical nanoparticle catalysts, leading to modification of their structural and electronic character. Furthermore, a wide range of techniques including X-ray photoelectron spectroscopy (XPS) 21,22 , scanning tunnelling microscopy (STM) [23][24][25] , (scanning) transmission electron microscopy ((S)TEM) 26 , atomic force microscopy (AFM) 27 , and synchrotron methods [28][29][30] are available to characterize deposited clusters, and can be used to gain knowledge of their unique properties and interaction with support materials. Moreover, elaborate reactionsFischer-Tropsch, partial oxidation, polymerization 31 and dehydrogenation 32 processes, for example -that have complex reactants and products, and would be difficult to investigate in the gas phase, can be studied with deposited clusters.…”

Tailored Nanoparticles for Clean Technology–Achieving Size and Shape Control

“…The first route relies on classical surface science ultrahigh-vacuum (UHV) techniques. In this case, metal nanoparticles are created on flat supports, either via gas-phase delivery and supportassisted self-assembly [57] of metal atoms by evaporation of the metal [58][59][60] or by the deposition of size-selected clusters formed in the gas phase using various methodologies [61][62][63][64].…”

Reprint of “Mass-selected supported cluster catalysts: Size effects on CO oxidation activity, electronic structure, and thermal stability of Pd /alumina (n≤ 30) model catalysts”

“…Ganteför and co-workers developed another instrument combining cluster deposition with STM and used this for a number of studies of Au n deposited on several supports [85][86][87][88]. This work has proceeded in collaboration with Kim and co-workers in Korea, recently focusing on gold chemistry under ambient conditions [32,[89][90][91]. Watanabe and co-workers have an instrument that allows STM, XPS, and high pressure reactivity studies of size-selected model catalysts [92] and have reported studies of CO oxidation over Pt n /TiO 2 (110) [11,93] and correlations with core level binding energies.…”

“…Understanding the effects of particle size is complicated by the fact that there is typically a broad distribution of sizes present, and it is generally not possible to vary size independently, without also changing other properties such as metal loading or support structure. Consider gold nanocatalysts, which have received tremendous attention over the past decade, using a wide variety of approaches [17,[29][30][31][32][33]. Much of the original work focused on the properties of Au nanoparticles in the 2-5 nm size range, because these were the obvious species detectable in conventional electron microscopy [34][35][36][37].…”