*1.1.3 Synthesis of Au/SiO2 using cationic gold complex [Au(en)2]Cl3 (en = ethylenediamine)*

Dai et al. reported a unique deposition-precipitation (DP) method for the preparation of highly active Au catalysts supported over mesoporous silica (SBA-15) using a gold cationic complex precursor [Au(en)2] 3+ via a wet chemical process [60] (**Figure 5**). This new DP procedure comprises the use a cationic gold precursor instead of anionic AuCl4 <sup>−</sup>, which is facilitated by ion-exchange route. The subsequent mesoporous catalyst is found to be extremely active for CO oxidation reaction at room temperature and even below 0°C. Its catalytic activity is found to be much greater than that of silica-supported Au catalysts formerly prepared through solution techniques.

In addition, pH of the gold precursor solution founds to play a key role in determining the catalytic activity through the regulation of [Au(en)2] 3+ deprotonation reaction and the surface interaction of silica with the gold precursor (**Figure 6**). It is also observed that these mesoporous gold silica catalyst are highly resistant toward sintering because of the stabilization of Au NPs within mesopores. The authors projected that this synthesis strategy of silica-supported gold catalysts is entirely solution-based and can be applied to prepare gold catalysts supported over different kinds of silica materials (e.g., silica particles and microporous zeolites).

Gies et al. and coworkers have deposited Au NPs around 3 nm particles inside the channels of mesoporous silica-TiO2-MCM-48 using deposition techniques [61]. It

*TEM images [(a) dark field, (b) bright field and (c) size distribution histogram] of the Au catalyst supported on SBA-15 (synthesized at pH of 9.6 and reduced at 150 °C).*

#### **Figure 6.**

*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.*

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 supporting the reduction of Au species.

**37**

*1.1.6 New methods*

over SiO2/Si(100) by Ar+

*Silica-Supported Gold Nanocatalyst for CO Oxidation DOI: http://dx.doi.org/10.5772/intechopen.80620*

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

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,

McFarland et al. examined the reactivity of gold clusters (8–22 nm diameter) supported on different metal oxides TiO2, ZnO, ZrO2, and SiO2 in a continuous flow reactor [68]. Synthesis involves the encapsulation of gold clusters within diblock copolymer [polystyrene81,000-block-poly(2-vinylpyridine)14,200] in toluene solution, impregnated onto the bulk supports, and reduced by calcination at temperature 300°C. TiO2 > ZrO2 > ZnO order of support-dependent sintering was observed in air at 300°C. Au nanoclusters on TiO2 found to exhibit the highest activity for CO

Guczi et al. [69] have reported a new method for the preparation of Au NPs sizes of about 50–100 × 20–30 × 2–7 nm along with spheres of 5–10 nm diameter

of 10 nm thickness. Photoemission spectra show that during size reduction, Au 5d

ion implantation of a bulk-like Au/SiO2/Si(100) thin film

thermal stability, and the unique gold-support interactions.

oxidation compared to other metal oxide supports.

*1.1.5 Synthesis by dispersion of gold colloids or pre-synthesized AuNPs*

*1.1.4 Chemical vapor deposition*

low temperature.
