**Acknowledgements**

*Gold Nanoparticles - Reaching New Heights*

MCM-41 Aerosol-silica

Hexagonal mesoporous silica (HMS)

**Support Au loading** 

**(wt%)**

Au-Co SBA-15 2 <5 0 [105] Au-Pd Silica fume 5.9–77.3 4–12 150–250 [106] Au-Pt Silica — 3.5–4.6 280 [107] Au-Sn SiO2 2.6–3.0 — 200 [108]

SBA-15 0.6–2.5 2.9–5.0 Au-Ti 150

Stober silica 1 5 (Au-M) 300 [111]

2.8–3.4 0.5–6 Au-Fe 150

**Au size (nm) Temperature (°C) Ref.**

Au-Ce 50

Au-Ce 200 Au-Ti 200

[110]

[112]

5 3–8 >400 [109]

**2. Conclusions**

**Bimetallic catalyst**

Au-Co Au-Fe

Au-Ti Au-Ce

Au-Co Au-Fe Au-Cu

Au-Fe Au-Ce Au-Ti

**Table 2.**

the corresponding transition metal complexes.

tions for in situ reduction of Au(III).

The synthesis of stable Au NPs and metal oxide support introduces new possibilities for catalysis since their properties are neither those of the bulk materials or of the isolated atoms. Optimum activity for, in particular, CO oxidation is practical for gold clusters in the 2–5 nm range. Furthermore, these gold nanoparticles can have a strong interaction with the support and an electron relocation may happen within either directions. These interactions can result in the formation of stabilized surface charged species, which act as Lewis acids comparable to as of salts in solution or in

*Au-M bimetallic catalysts with different types of silica supports for the CO oxidation reaction.*

Mono- and bimetallic gold nanocatalysts have been made over a variety of metal oxides supports. In particular, silica is being considered a suitable support for Au NPs using appropriate synthesis methods. Au NPs can be supported over a series of different mesoporous silicas, such as SBA-15, SBA-16, MCM-41, and MCM-48. These silica-supported gold nanocatalysts are found to exhibit exceptional catalytic properties. Most common methods for the modification of silica support is either by direct or post-grafting functionalization with amine or thiol groups, where these groups show dual performances, as ligand for improving the interaction between the metal precursors and the silica surface; and mild reductants under basic condi-

Several active silica-supported gold nanocatalysts have been prepared by functionalizing the mesoporous silica support by direct or subsequent incorporation of the gold precursor. Major problems related to these processes include particle size, gold loading, uniform dispersion Au NPs, and ligand removal during calcination, which contribute to the aggregation of Au NPs, metal leaching, and low stability. Finally, it is precise to say that silica-supported gold nanocatalyst has become a hot topic of research for CO oxidation; conversely, there are still many challenges ahead for the improvement of silica-supported gold nanocatalyst filling the main necessities of any catalyst such as easy and low-cost synthesis method, high activity, selectivity, and greater stability at lower temperature. Furthermore, the proof of identity for the active gold species is still a challenging job for CO oxidation reac-

**42**

tions catalyzed by gold.

The authors acknowledge the support from the Ministry of Higher Education, Saudi Arabia, in the establishment of the Center of Research Excellence in Petroleum Refining & Petrochemicals at King Fahd University of Petroleum & Minerals (KFUPM).
