*1.1.4 Chemical vapor deposition*

*Gold Nanoparticles - Reaching New Heights*

has been revealed that, Au NPs catalyst over mesoporous silica not only converts CO to CO2 at 50% level at −20°C but also stable against sintering up to at least 200°C. Dai et al. [62] described a novel method for the synthesis, characterization, and catalytic behavior of small and well-dispersed Au NPs on Au/Cab-O-Sil fumed SiO2 and Au/MOx/SiO2 catalysts using Au(en)2Cl3 (en = ethylenediamine) as the precursor. It has been found that, these Au/SiO2 catalysts are extremely active for CO oxidation below 0°C. Pretreating of as-synthesized Au/SiO2 in H2-Ar at 150°C and in O2-He at 500°C is found to be very cooperative for high activity with optimum gold loading of 1.1 and 2.5 wt%. Furthermore, the post-treatment of calcined (and activated) Au/SiO2 in different media motivates the activity in CO oxidation. Moreover, the addition of metal oxide dopants also has been used to tune the catalytic activity. Wu et al. investigated the nature of Au species over Au/SiO2 catalyst after oxidative and reductive pretreatments and also their role in room temperature CO oxidation using operando diffuse reflectance infrared spectroscopy (DRIFT) coupled with quadruple mass spectrometry (QMS) [63]. It has been observed that, the oxidative pretreatment of catalyst leads to a cationic Au species, which is inactive for CO oxidation at rt. However, in situ reduction of the cationic Au during CO oxidation leads to the formation of Au(0) species, which is active for CO oxidation. It is recognized that the reductive pretreatment results in a Au(0) species, which have sturdier interaction with the support and thus are more active for CO oxidation than those on oxidatively treated catalyst. It is also shown that, water has two positive effects in CO oxidation reaction on Au/SiO2: activating O2 species and

*Light-off curves of the Au catalysts supported on SBA-15, synthesized in the solutions of Au(en)2Cl3 with different pH values. Reproduced with permission, copyright 2018, American Chemical Society.*

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**Figure 6.**

supporting the reduction of Au species.

To elucidate the effect of metal oxide support on the catalytic activity of gold for CO oxidation, Okumura et al. have deposited gold on SiO2, Al2O3, and TiO2 with high dispersion by chemical vapor deposition (CVD) of an organo-gold complex [64]. The results show that orders of TOF values for CO oxidation at 0°C are similar among Au/Al2O3, Au/SiO2, and Au/TiO2. Further, it was demonstrated that, the deposition of gold particles on the support with strong interaction is a major key controlling factor for the evolution of catalytic activity for CO oxidation at temperature 0°C (and not at −70°C) and the nature of the support is not a dominant factor.

Similarly, silica-supported Au NPs of size 1.4 nm were prepared by organometallic chemical vapor deposition method (Au/SiO2-CVD) by Claus et al. [65]. Furthermore, the synthesized catalysts show a notable activity for CO oxidation at low temperature.

Okumura et al. have deposited gold on Al2O3, SiO2, MCM-41, TiO2, SiO2-Al2O3, and active carbon (AC) support by gas-phase grafting (GG) of an organo-gold complex with high dispersion to display the effect of support in CO oxidation [66]. It has been found that, order of TOF values for CO oxidation at 0 °C are similar among Au/Al2O3, Au/SiO2, and Au/TiO2, this clearly shows the deposition of Au NPs on the supports with strong interaction which play important role in catalytic activity. Whereas semi-conductive or reducible nature of the support is not a presiding factor for CO oxidation at 0 °C. Au/SiO2-Al2O3 and Au/AC shows lower catalytic activities due to acidic and nonmetal-oxide supports respectively.

Veith et al. reported a new way to prepare small size Au NPs (2.5 nm) over a fumed silica support, using the physical vapor deposition technique of magnetron sputtering [67]. These Au/SiO2 catalysts are found to be structurally stable when heated in air to 500°C for several weeks or during a CO oxidation reaction. However, under these annealing conditions, traditional Au/TiO2 catalysts rapidly sinter to form large 13.9 nm gold clusters, resulting in a fivefold decrease in activity. The authors witnessed that the stability of Au/SiO2 is usually accredited to the absence of residual impurities (ensured by the halide-free production method) and a strong bond between gold and defects at the silica surface (about 3 eV per bond) is calculated from density functional theory (DFT) calculations. Properties that make the material worthy of study include the ability to easily reactivate the catalyst, thermal stability, and the unique gold-support interactions.
