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

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 oxidation compared to other metal oxide supports.

### *1.1.6 New methods*

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 over SiO2/Si(100) by Ar+ ion implantation of a bulk-like Au/SiO2/Si(100) thin film of 10 nm thickness. Photoemission spectra show that during size reduction, Au 5d

valence band of Au NPs shows a change in the valence band density state, and hence the catalytic activity in the CO oxidation increases. Moreover, it has been observed that, at high-temperature catalytic tests, both size and valence band of the Au NPs get returned to its typical bulk gold, which consequently curbed its catalytic activity.

Margitfalvi et al. reported a novel process for the preparation of silica-supported nanosized gold catalysts using HAuCl4 gold precursor and ammonia solution. This gold catalyst showed relatively high catalytic activity in CO oxidation in a broad temperature region [70]. It was identified that, uniform well deposition of gold over silica support takes place because of the electrostatic interaction between the silica surface and the positively charged gold amine complexes resulting due to the being of ammonia solution, which avoids the growth of particle during hydrogen treatment at 350°C; this subsequently results in a highly dispersed gold particles over the silica surface. Margitfalvi et al. also observed that the presence of moisture in reactant mixture increases the catalytic activity of Au/SiO2 catalysts, while TOS study reveals the unstability of Au/SiO2 catalysts.

Coperet et al. and coworkers, described a novel route for the direct preparation of Au NP (1.8 nm) on partially dehydroxylated silica at 700°C [SiO2-(700)] by surface organometallic chemistry [71]. Method involves the controlled formation of well-defined and dispersed Au NPs functionalized over passivated silica surface with a AuI complex, {Au[N(SiMe3)2]}4, followed by mild reduction under H2 at 300°C (**Figure 7**). This methodology results in a very efficient tailor-made gold

**Figure 7.**

*(A) Schematic route for the preparation of AuNP on passivated silica by SOMC (2.79 wt% Au). (B) (a) HR-TEM image and (b) size distribution histogram of AuNPs over passivated silica by SOMC.*

**39**

**Figure 8.**

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

but also for oxidation of CO.

of 626 m2

g<sup>−</sup><sup>1</sup>

catalyst, suitable not only for liquid phase aerobic epoxidation of trans-stilbene

Recently, we have described a facile route for the synthesis of Au NPs of different average sizes (ranging from 1 to 2, 3 to 5, and 11 to 13 nm) on to the fibrous silica nanospheres (KCC-1) either by immobilizing pre-made Au NPs on amine-functionalized KCC-1 (Au/KCC-1-NH2) or by grafting HAuCl4 on Au/KCC-1-NH2 followed by reduction by NaBH4, CO, and citrate [72]. It's been important to note that size and the location of the Au NPs over the support were found to influence by the preparation method (**Figure 8**). These catalysts were then tested for the oxidation of CO. The catalytic activity of the Au NPs over KCC-1 was proved to be size dependent (**Figure 9**). Recently, Fan et al. have prepared an exceptional pollen-structured hierarchically meso-/macroporous silica spheres (PHMSs) created from unique rape pollen grains and a triblock copolymer poly(ethylene oxide-block-propylene oxide-block-ethylene oxide) (P123) as templates [73]. The PHMSs are enclosed of mesoporous walls, which create a macroporous structure repeating the net-like morphology of the pollen grain exine. This newly structured catalyst found to exhibit a high BET surface area

and uniform mesopores of average pore size less than 5 nm. Au NPs

supported on PHMSs (Au 6.8 wt%) show a higher CO conversion at an ambient temperature (30°C), compared to those supported on typical mesoporous silica SBA-15. Furthermore, the effects of silica support on the catalytic activity have been discovered from the calculation of diffusion coefficients and simulation of gas diffusion in both PHMSs and SBA-15. Catalytic performance for CO oxidation proves that the hierarchical PHMSs facilitate better gas diffusion than the nonhierarchical SBA-15.

*HR-TEM images and size histogram of Au/KCC-1-NH2-a1 (a–c), Au/KCC-1-NH2-a2 (d–f), and Au/KCC-1-*

*NH2-b1 (g–i). Reproduced with permission, copyright 2018, Wiley.*

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

*Gold Nanoparticles - Reaching New Heights*

study reveals the unstability of Au/SiO2 catalysts.

valence band of Au NPs shows a change in the valence band density state, and hence the catalytic activity in the CO oxidation increases. Moreover, it has been observed that, at high-temperature catalytic tests, both size and valence band of the Au NPs get returned to its typical bulk gold, which consequently curbed its catalytic activity. Margitfalvi et al. reported a novel process for the preparation of silica-supported nanosized gold catalysts using HAuCl4 gold precursor and ammonia solution. This gold catalyst showed relatively high catalytic activity in CO oxidation in a broad temperature region [70]. It was identified that, uniform well deposition of gold over silica support takes place because of the electrostatic interaction between the silica surface and the positively charged gold amine complexes resulting due to the being of ammonia solution, which avoids the growth of particle during hydrogen treatment at 350°C; this subsequently results in a highly dispersed gold particles over the silica surface. Margitfalvi et al. also observed that the presence of moisture in reactant mixture increases the catalytic activity of Au/SiO2 catalysts, while TOS

Coperet et al. and coworkers, described a novel route for the direct preparation of Au NP (1.8 nm) on partially dehydroxylated silica at 700°C [SiO2-(700)] by surface organometallic chemistry [71]. Method involves the controlled formation of well-defined and dispersed Au NPs functionalized over passivated silica surface with a AuI complex, {Au[N(SiMe3)2]}4, followed by mild reduction under H2 at 300°C (**Figure 7**). This methodology results in a very efficient tailor-made gold

*(A) Schematic route for the preparation of AuNP on passivated silica by SOMC (2.79 wt% Au). (B) (a) HR-TEM image and (b) size distribution histogram of AuNPs over passivated silica* 

**38**

**Figure 7.**

*by SOMC.*

catalyst, suitable not only for liquid phase aerobic epoxidation of trans-stilbene but also for oxidation of CO.

Recently, we have described a facile route for the synthesis of Au NPs of different average sizes (ranging from 1 to 2, 3 to 5, and 11 to 13 nm) on to the fibrous silica nanospheres (KCC-1) either by immobilizing pre-made Au NPs on amine-functionalized KCC-1 (Au/KCC-1-NH2) or by grafting HAuCl4 on Au/KCC-1-NH2 followed by reduction by NaBH4, CO, and citrate [72]. It's been important to note that size and the location of the Au NPs over the support were found to influence by the preparation method (**Figure 8**). These catalysts were then tested for the oxidation of CO. The catalytic activity of the Au NPs over KCC-1 was proved to be size dependent (**Figure 9**).

Recently, Fan et al. have prepared an exceptional pollen-structured hierarchically meso-/macroporous silica spheres (PHMSs) created from unique rape pollen grains and a triblock copolymer poly(ethylene oxide-block-propylene oxide-block-ethylene oxide) (P123) as templates [73]. The PHMSs are enclosed of mesoporous walls, which create a macroporous structure repeating the net-like morphology of the pollen grain exine. This newly structured catalyst found to exhibit a high BET surface area of 626 m2 g<sup>−</sup><sup>1</sup> and uniform mesopores of average pore size less than 5 nm. Au NPs supported on PHMSs (Au 6.8 wt%) show a higher CO conversion at an ambient temperature (30°C), compared to those supported on typical mesoporous silica SBA-15. Furthermore, the effects of silica support on the catalytic activity have been discovered from the calculation of diffusion coefficients and simulation of gas diffusion in both PHMSs and SBA-15. Catalytic performance for CO oxidation proves that the hierarchical PHMSs facilitate better gas diffusion than the nonhierarchical SBA-15.

#### **Figure 8.**

*HR-TEM images and size histogram of Au/KCC-1-NH2-a1 (a–c), Au/KCC-1-NH2-a2 (d–f), and Au/KCC-1- NH2-b1 (g–i). Reproduced with permission, copyright 2018, Wiley.*

**Figure 9.**

*Turn over frequency (TOF S<sup>−</sup><sup>1</sup> ) for CO oxidation over Au/KCC-1-NH2-a1, Au/KCC-1-NH2-a2 and Au/KCC-1-NH2-b1 after H2 treatment at 300°C for 1 h. Reproduced with permission, copyright 2018, Wiley.*

Yang et al. reported an improved strategy for the preparation of SBA-15 supported metal nanoparticles [74]. Supported Au nanoparticle catalyst Au/SBA-15 exhibited significantly higher activities in the oxidation of CO by molecular oxygen after adding polyvinylpyrrolidone (PVP) to the reaction system. Earlier, it has been reported that, PVP could strongly interact with the soluble Au NPs in water solvent, leading to high activity in oxidation reaction [75–76].

Recently, Behm et al. have developed an improved catalytic system for the stability of metal oxide-supported Au catalysts for low-temperature CO oxidation reaction [77]. Effect of mesoporous silica SBA-15 substrate on the catalytic activity and stability of Au/TiO2 catalysts has been explored, where the catalyst system composed of a SBA-15 support surface modified by a monolayer of TiOx and Au NPs on top. Later, TiOx surface layers were systematically increased without changing the Au loading and particle size. Kinetic measurements were calculated at three different temperatures (30, 80, and 180°C). It was noted that catalyst surface modification with Ti has distinct effect, the performance of these catalysts increases significantly with Ti concentration and also with reaction temperature.
