**1. Introduction**

Since the revelation of the earliest inspiring studies [1], catalysis by Au NPs has been in the limelight of chemical research and it has been recognized as a vital tool for a wide range of transformations such as selective oxidations and hydrogenations of organic substrates [2–6], water-gas shift reaction [7–9], acetylene hydrochlorination [10, 11], direct synthesis of hydrogen peroxide [12], reduction of NO to N2 [13, 14], and the addition of nucleophiles to acetylenes [15]. Particularly, the aerobic oxidation of CO by gold [16] has been a subject of constant exploration, where it plays a crucial role for industry and the environment [17].

Carbon monoxide is a colorless and odorless gas [18]. Due to its high affinity with hemoglobin, it is enormously toxic for humans and animals. Its long-term exposure should not exceed 25 ppm in 8 h, where more and more obvious harmful effects are observed above this limit up to the lethal concentration of around 650–700 ppm. Massive amount of CO is continuously being emitted (1.09 billion tons in 2000), frequently from transportation, power plants, and industrial and domestic doings [19]. CO is also an originator of ground-level ozone, which can cause several severe respiratory problems. Also, it is not well soluble in water (23 mLCO(g) L<sup>−</sup><sup>1</sup> H2O(l)), which also limits its removal from air by means of aqueous treatments. So its oxidation into CO2 is a key solution for CO reduction in air

depollution managements. To achieve CO elimination, it is necessary to design catalysts, which permits the oxidation reaction with a sufficient rate.

Au NPs supported over metal oxides demonstrate high catalytic activity for CO oxidation under mild conditions and even below room temperature [20]. Concerning the structure-activity relationship, Au NPs size has been witnessed to play a key role in this transformation, with a finest activity observed for gold clusters in the size range of 2–5 nm [21]. The nature of the support and the way the Au NPs is being prepared and interacted are significant [22]. Till the beginning of the twentieth century, scarce oxides such as hopcalite have been identified to oxidize CO at ambient temperature. Hopcalite is composed of a binary amorphous Mn–Cu oxide, whose texture will get stabilized by the addition of CaO, Al2O3, or bentonite clay [23]. Later, several oxides have been investigated and higher catalytic activity are being observed for Au NPs supported on various reducible metal oxides such as TiO2, [24] Co3O4, [25] CeO2, [26], and Fe2O3 [27]. However, silica is an inert, inexpensive, and convenient support that can be shaped into a host of attractive and varied morphologies [28]. Silica-based mesoporous supports have been acclaimed in precise for high surface area, high accessibility of the catalytically active center, and reduced particle sintering [29]. However, it is still challenging to make properly sized, well-dispersed, and uniform Au NPs placed within the nanopores of the material without conceding mass transfer and morphological properties [30]. Gold alloy (bimetallic nanocatalysts) along with other metals such as Pd, Cu, and Ag, also found to exhibit high and exceptional catalytic activity compared to Au alone.

There are few reviews on different facets of gold catalysis [31–37]. This chapter will offer an overview of the different possible methodologies available for the synthesis of silica supported especially mono and bimetallic gold nanocatalyst for CO oxidation. The key advantages or limits for the proposed methods are shortly discussed; in some cases, the most important features of the presented methodologies are highlighted from an industrial point of view.

### **1.1 Methods to synthesize gold nanocatalyst supported on silica for CO oxidation**

#### *1.1.1 Post-synthetic treatment of silica before gold loading*

Schuth et al. reported the method for the preparation of an active gold catalyst for CO oxidation, supported on silica, made by a novel solution technique [38]. The surface of SBA-15 was functionalized by treating with positively charged groups, and [AuCl4] <sup>−</sup> species into the channel structure through ion exchange. Later on reduction with NaBH4, resulted a highly dispersed Au NPs in the channels system of the mesoporous host. This composite material showed a reaction rate of 2.7 × 10<sup>−</sup><sup>4</sup> mmolg<sup>−</sup><sup>1</sup> cats<sup>−</sup><sup>1</sup> for CO oxidation. The Au NPs to some extent get stabilized by the pore system of SBA-15. Still, the interaction is relatively weak and sintering was observed around 100°C, which was confirmed from TEM images of Au/SBA-15. The authors conclude that a specific interaction between gold and the support is not compulsory for the generation of very active gold-based catalysts.

Datye et al. demonstrated a novel silica geometry, which allows to trap nanoparticles and limit the mobility of species that lead to thermal sintering [39]. Mesoporous silica was prepared by two different methods, spontaneous selfassembly of amphiphilic molecules (batch synthesis) and evaporation-induced selfassembly (aerosol synthesis). To deposit gold inside the pores of mesoporous silica, firstly silica surface was functionalized with organic amine then treated with Au precursor. These reduced catalysts were tested for CO oxidation with 20% CO and 10% O2 from room temperature up to 400°C. Results showed that the hexagonal structure inside aerosol silica shells support to contain the Au NPs within the pore

**29**

**Figure 1.**

*copyright 2018, American Chemical Society.*

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

growth of the Au particles.

to gas phase species as related to straight pores in MCM-41.

CO oxidation, reaching a very high activity values as 7.0 × 10<sup>−</sup><sup>3</sup>

structure and protect the active part from gas phase poisons, allowing facile access

Datye et al. further investigated the role of pore size and structure in controlling the thermal sintering of Au NPs on mesoporous silica [40]. The phenomenon of sintering of Au NPs is dependent on pore size, pore wall thickness (strength of pores), and pore connectivity. Au was placed on mesoporous silica samples with a varied range of pore sizes and pore arrangements (2-D hexagonal, 3-D hexagonal, and cubic) (**Figure 1**). Later, all catalysts were reduced at 200°C in flow of H2 and then used for CO oxidation at temperatures ranging from 25 to 400°C. Among all samples, SBA-15 with the thickest pore walls was found most active at regulatory

Mou et al. have prepared the Au NPs embedded within the mesoporous silica's and used as catalysts for CO oxidation [41]. The silane APTS (H2N(CH2)3- Si(OMe)3) was used for the surface-functionalization of mesoporous silica such as MCM-41, MCM-48, and SBA-15. The catalysts were activated by calcinations and later reduced with H2 reduction at 600°C. The catalysts were found to be active for

*Au particle size distributions after reduction at 200°C for 2 h in flowing hydrogen. Pore sizes of the samples are calculated using BJH theory. Average Au particle sizes are number averages. Reproduced with permission,* 

mmolg<sup>−</sup><sup>1</sup>

cats<sup>−</sup><sup>1</sup> at *Silica-Supported Gold Nanocatalyst for CO Oxidation DOI: http://dx.doi.org/10.5772/intechopen.80620*

*Gold Nanoparticles - Reaching New Heights*

depollution managements. To achieve CO elimination, it is necessary to design

Au NPs supported over metal oxides demonstrate high catalytic activity for CO oxidation under mild conditions and even below room temperature [20]. Concerning the structure-activity relationship, Au NPs size has been witnessed to play a key role in this transformation, with a finest activity observed for gold clusters in the size range of 2–5 nm [21]. The nature of the support and the way the Au NPs is being prepared and interacted are significant [22]. Till the beginning of the twentieth century, scarce oxides such as hopcalite have been identified to oxidize CO at ambient temperature. Hopcalite is composed of a binary amorphous Mn–Cu oxide, whose texture will get stabilized by the addition of CaO, Al2O3, or bentonite clay [23]. Later, several oxides have been investigated and higher catalytic activity are being observed for Au NPs supported on various reducible metal oxides such as TiO2, [24] Co3O4, [25] CeO2, [26], and Fe2O3 [27]. However, silica is an inert, inexpensive, and convenient support that can be shaped into a host of attractive and varied morphologies [28]. Silica-based mesoporous supports have been acclaimed in precise for high surface area, high accessibility of the catalytically active center, and reduced particle sintering [29]. However, it is still challenging to make properly sized, well-dispersed, and uniform Au NPs placed within the nanopores of the material without conceding mass transfer and morphological properties [30]. Gold alloy (bimetallic nanocatalysts) along with other metals such as Pd, Cu, and Ag, also found to exhibit high and exceptional catalytic activity compared to Au alone. There are few reviews on different facets of gold catalysis [31–37]. This chapter will offer an overview of the different possible methodologies available for the synthesis of silica supported especially mono and bimetallic gold nanocatalyst for CO oxidation. The key advantages or limits for the proposed methods are shortly discussed; in some cases, the most important features of the presented methodolo-

**1.1 Methods to synthesize gold nanocatalyst supported on silica for CO oxidation**

Schuth et al. reported the method for the preparation of an active gold catalyst for CO oxidation, supported on silica, made by a novel solution technique [38]. The surface of SBA-15 was functionalized by treating with positively charged

Later on reduction with NaBH4, resulted a highly dispersed Au NPs in the channels system of the mesoporous host. This composite material showed a reaction rate of

by the pore system of SBA-15. Still, the interaction is relatively weak and sintering was observed around 100°C, which was confirmed from TEM images of Au/SBA-15. The authors conclude that a specific interaction between gold and the support is not

Datye et al. demonstrated a novel silica geometry, which allows to trap nanoparticles and limit the mobility of species that lead to thermal sintering [39]. Mesoporous silica was prepared by two different methods, spontaneous selfassembly of amphiphilic molecules (batch synthesis) and evaporation-induced selfassembly (aerosol synthesis). To deposit gold inside the pores of mesoporous silica, firstly silica surface was functionalized with organic amine then treated with Au precursor. These reduced catalysts were tested for CO oxidation with 20% CO and 10% O2 from room temperature up to 400°C. Results showed that the hexagonal structure inside aerosol silica shells support to contain the Au NPs within the pore

compulsory for the generation of very active gold-based catalysts.

<sup>−</sup> species into the channel structure through ion exchange.

for CO oxidation. The Au NPs to some extent get stabilized

catalysts, which permits the oxidation reaction with a sufficient rate.

gies are highlighted from an industrial point of view.

*1.1.1 Post-synthetic treatment of silica before gold loading*

**28**

groups, and [AuCl4]

mmolg<sup>−</sup><sup>1</sup>

cats<sup>−</sup><sup>1</sup>

2.7 × 10<sup>−</sup><sup>4</sup>

structure and protect the active part from gas phase poisons, allowing facile access to gas phase species as related to straight pores in MCM-41.

Datye et al. further investigated the role of pore size and structure in controlling the thermal sintering of Au NPs on mesoporous silica [40]. The phenomenon of sintering of Au NPs is dependent on pore size, pore wall thickness (strength of pores), and pore connectivity. Au was placed on mesoporous silica samples with a varied range of pore sizes and pore arrangements (2-D hexagonal, 3-D hexagonal, and cubic) (**Figure 1**). Later, all catalysts were reduced at 200°C in flow of H2 and then used for CO oxidation at temperatures ranging from 25 to 400°C. Among all samples, SBA-15 with the thickest pore walls was found most active at regulatory growth of the Au particles.

Mou et al. have prepared the Au NPs embedded within the mesoporous silica's and used as catalysts for CO oxidation [41]. The silane APTS (H2N(CH2)3- Si(OMe)3) was used for the surface-functionalization of mesoporous silica such as MCM-41, MCM-48, and SBA-15. The catalysts were activated by calcinations and later reduced with H2 reduction at 600°C. The catalysts were found to be active for CO oxidation, reaching a very high activity values as 7.0 × 10<sup>−</sup><sup>3</sup> mmolg<sup>−</sup><sup>1</sup> cats<sup>−</sup><sup>1</sup> at

**Figure 1.**

*Au particle size distributions after reduction at 200°C for 2 h in flowing hydrogen. Pore sizes of the samples are calculated using BJH theory. Average Au particle sizes are number averages. Reproduced with permission, copyright 2018, American Chemical Society.*

80°C as compared to the results from catalysts made with the standard precipitation-deposition method. Studies also recommend that the CO conversion increases with decreasing size of Au NPs.

Rombi et al. [42] have functionalized SBA-15 with new mercaptopropyltrimethoxysilane (MPS) and used for the preparation of supported gold nanocatalyst for the low-temperature CO oxidation reaction. It has been observed that, the presence of organic residues and the size of the Au NPs intensely affect catalytic activity. High temperature calcination (300–560°C) in air followed by treatment (600°C) under H2 atmosphere only leads to the formation of homogeneously small size Au spherical nanocrystals dispersed inside the SBA-15 channels with average crystal sizes of about 6.5 nm. The catalysts were first calcined at 560°C and then reduced under H2/He 600°C. Though, regardless of their large particle size, remarkable catalytic activity for CO oxidation was observed, while the as-made catalysts were found to be inactive for CO oxidation at low temperatures. Gold was found mostly in metallic Au(0) state, due to the strong interaction between the mercapto functional groups and Au, positively charged Au<sup>δ</sup><sup>+</sup> species also exist.

SBA-15 functionalization with 3-MPS has been used by Ferino et al. to make a supported gold catalyst for the low-temperature CO oxidation [43]. Catalytic runs were studied by varying different reaction parameters such as atmospheric pressure, temperature 40–150°C, and different thermal treatments of the sample prior to reaction. The catalyst was pretreated toward different thermal treatments, that is, calcination in air, treatment in H2 atmosphere, and calcination followed by H2 treatment. It has been observed that, the pre-treatment conditions sturdily affect not only the gold particle size but also the nature of the Au surface species. A substantial catalytic activity for CO oxidation was observed for the catalysts treated at 600°C in H2/He atmosphere, only after the removal of functionalizing agent by an earlier high-temperature calcination (**Table 1**).

Rombi et al. further slightly modified the procedure for the preparation of silicasupported gold catalyst for CO oxidation by functionalizing the silica surface with 3-MPS, anchoring gold using HAuCl4 solution, and later reducing it with sodium citrate [44]. Before the catalytic runs, the Au/SiO2–SH catalyst was submitted to different thermal treatments. It has been achieved a notable CO conversion when the catalyst was calcined in air at 560°C and afterward treated in H2/He at 600°C or directly treated in H2/He at 600°C.
