*1.1.2 Direct synthesis*

Lin et al. have studied CO oxidation over Au/SiO2 and Au/TiO2 catalysts [45]. It has been found that, after a high-temperature reduction (HTR) at 500°C, TiO2 supported Au became very active for CO oxidation at 40°C, an order of magnitude


**31**

**Figure 2.**

*with permission, copyright 2018, Springer.*

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

impurity has not yet been established.

more active than Au/SiO2. However a low-temperature reduction (LTR) at 200°C made an Au/TiO2 catalyst with very low activity. It has been observed that, a HTR step followed by calcination at 400°C and a LTR step (HTR/C/LTR) provided the most active Au/TiO2 catalyst of all, which at 40°C 100-fold more active than a typical 2% Pd/Al2O3 catalyst and also found to be stable above temperature 127°C, while a sharp reduction in activity was observed with the other Au/TiO2 (HTR) sample. Besides, the activity of the Au/SiO2 catalyst was ten-fold higher than the Au/TiO2 (LTR) sample but ten-fold lower than the Au/TiO2 (HTR/C/LTR) catalyst. Later, Cl analysis disclosed that the Au/TiO2 (LTR) sample retained around 50% of the Cl from the Au precursor, while the other three catalysts retained only 16%, which gave a final Cl/Au ratio of 0.5; subsequently, the low activity of the Au/TiO2 (LTR) sample may be due to its high chloride content. However, the inhibitive role of Cl

Au catalysts supported over mesoporous silica and titania supports were made by Overbury et al. and tested for the oxidation of CO [46]. Two methods were used for the preparation of silica-supported catalysts using triamine as complexing ligands, which lead to mesoporous silica with wormhole and hexagonal structures. The use of triamine ligands is the essential for the synthesis of uniformly sized 2–3 nm Au MPs in the silica pores. While over mesoporous titania, high gold dispersions were obtained without using functional ligand. It was noticed that, Au supported on titania showed a much higher activity for CO oxidation, even the Au particle sizes were essentially equal on the titania and the wormhole silica support. The results advise that the presence of small size Au NPs (2–3 nm) alone is not very satisfactory to achieve high activity in CO oxidation (**Figure 2**). Instead, the support also impacts the activity through other possible ways such as stabilization of active sub-nanometer particles, formation of active oxygen-containing reactant intermediates (such as hydroxyls or O2), or stabilization of optimal Au structures. Effect of moisture on the catalytic activity for CO oxidation over three gold catalysts prepared over TiO2, Al2O3, and SiO2 supports were investigated by Date et al. [47]. A varied range of concentrations was studied, from about 0.1 to 6000 ppm H2O.

*CO conversion (points) is shown as function of temperature for different supported Au catalysts. Reproduced* 

**Table 1.** *Catalytic performance of the Au/SBA-15-SH sample for the low-temperature CO oxidation reaction.* *Gold Nanoparticles - Reaching New Heights*

with decreasing size of Au NPs.

tional groups and Au, positively charged Au<sup>δ</sup><sup>+</sup>

an earlier high-temperature calcination (**Table 1**).

directly treated in H2/He at 600°C.

*1.1.2 Direct synthesis*

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

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 func-

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

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

Lin et al. have studied CO oxidation over Au/SiO2 and Au/TiO2 catalysts [45]. It has been found that, after a high-temperature reduction (HTR) at 500°C, TiO2 supported Au became very active for CO oxidation at 40°C, an order of magnitude

*Catalytic performance of the Au/SBA-15-SH sample for the low-temperature CO oxidation reaction.*

species also exist.

**30**

**Table 1.**

more active than Au/SiO2. However a low-temperature reduction (LTR) at 200°C made an Au/TiO2 catalyst with very low activity. It has been observed that, a HTR step followed by calcination at 400°C and a LTR step (HTR/C/LTR) provided the most active Au/TiO2 catalyst of all, which at 40°C 100-fold more active than a typical 2% Pd/Al2O3 catalyst and also found to be stable above temperature 127°C, while a sharp reduction in activity was observed with the other Au/TiO2 (HTR) sample. Besides, the activity of the Au/SiO2 catalyst was ten-fold higher than the Au/TiO2 (LTR) sample but ten-fold lower than the Au/TiO2 (HTR/C/LTR) catalyst. Later, Cl analysis disclosed that the Au/TiO2 (LTR) sample retained around 50% of the Cl from the Au precursor, while the other three catalysts retained only 16%, which gave a final Cl/Au ratio of 0.5; subsequently, the low activity of the Au/TiO2 (LTR) sample may be due to its high chloride content. However, the inhibitive role of Cl impurity has not yet been established.

Au catalysts supported over mesoporous silica and titania supports were made by Overbury et al. and tested for the oxidation of CO [46]. Two methods were used for the preparation of silica-supported catalysts using triamine as complexing ligands, which lead to mesoporous silica with wormhole and hexagonal structures. The use of triamine ligands is the essential for the synthesis of uniformly sized 2–3 nm Au MPs in the silica pores. While over mesoporous titania, high gold dispersions were obtained without using functional ligand. It was noticed that, Au supported on titania showed a much higher activity for CO oxidation, even the Au particle sizes were essentially equal on the titania and the wormhole silica support. The results advise that the presence of small size Au NPs (2–3 nm) alone is not very satisfactory to achieve high activity in CO oxidation (**Figure 2**). Instead, the support also impacts the activity through other possible ways such as stabilization of active sub-nanometer particles, formation of active oxygen-containing reactant intermediates (such as hydroxyls or O2), or stabilization of optimal Au structures.

Effect of moisture on the catalytic activity for CO oxidation over three gold catalysts prepared over TiO2, Al2O3, and SiO2 supports were investigated by Date et al. [47]. A varied range of concentrations was studied, from about 0.1 to 6000 ppm H2O.

#### **Figure 2.**

*CO conversion (points) is shown as function of temperature for different supported Au catalysts. Reproduced with permission, copyright 2018, Springer.*

It has been observed that, the degree of rate of improvement rest on the type of support used: high for insulating Al2O3 and SiO2 and moderate for semiconducting TiO2. The effect of moisture becomes notable only above about 200 ppm H2O for Au/Al2O3, while the activity for Au/SiO2 decreases significantly with a decrease in moisture to about 0.3 ppm. While the catalytic activity of Au/TiO2 at about 3000 ppm H2O is very high and reaches full CO conversion (100%). The authors also proposed as a mechanism model that moisture improves the catalytic activities for no less than two orders of magnitude and the effect of moisture depends over the type of metal oxides used. Moisture plays dual role in the reaction: one is the activation of oxygen and other is the decomposition of carbonate.

Dai et al. demonstrated the surface sol-gel process (SSP) for the modification of silica mesopores surfaces and the tuning of mesopore diameters [48]. The procedure involves the preparation of one or multilayers of titanium oxide over SBA-15 [49]. This layer-by layer approach controls the mesopore diameters with monolayer precision (**Figure 3**). Ultra-small Au NPs were effectively prepared on surfacemodified SBA-15 via a deposition-precipitation method without the restriction of surface isoelectric point (**Figure 3**). These developed materials are found very active catalysts for CO oxidation.

Corma et al. presented a modified sol-gel approach for the preparation of Au NPs embedded in silica, and Au NPs are capped with two different thiol molecules, 1-dodecanethiol (DT) and 3-mercaptopropyltrimethoxysilane (MPMS), which lead to high-surface area Au/SiO2 catalysts [50]. The synthesis comprises the formation of a three-component metal-organic-inorganic structure collected of Au NPs capped with alkanethiols and partly functionalized with alkoxysilane groups, and polymerized with tetraethyl orthosilicate (TEOS). Upon calcination, the material becomes highly active catalyst for the CO oxidation at low temperatures, the same as that obtained with gold on TiO2. This clearly specifies that it is possible to attain highly active gold-silica catalysts by a liquid-based method, as long as accessible small gold particles and strong metal-support interactions exist.

The structure and oxidation state of gold clusters of different sizes supported over various metal oxides (Al2O3, TiO2, and SiO2) were studied to different CO oxidation conditions that were investigated by Bokhoven et al. using in situ X-ray absorption spectroscopy at the Au L3 edge [51]. During catalysis, only phase detected in all catalysts was Au0 with no other oxidation states. For the most active catalyst with small size gold particles (Au/Al2O3), difference in the electronic structure of the gold clusters in changing reaction conditions was observed by XANES spectroscopy and attributed to the adsorption of CO on the metallic gold clusters.

#### **Figure 3.**

*Pore-size distribution as function of the number of TiO2 layers (left), Z-contrast TEM image of ultra-small Au NPs on ordered mesoporous materials (right). The bright spots (0.8–1.0 nm) correspond to Au NPs. Reproduced with permission, copyright 2018, American Chemical Society.*

**33**

**Figure 4.**

*2018, Elsevier.*

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

ment of the catalytic activity (**Figure 4**).

Further FEFF8 calculations confirmed that the changes in the XANES signature

of CO. For Au/Al2O3, Au-O backscattering was also seen in the EXAFS, suggesting weak cluster-support interactions. Nevertheless for Au/SiO2 and Au/TiO2, the structure of the gold clusters remained unaffected throughout all experiments. It is well recognized that several-supported metal particles are subjected to the geometric/electronic variations when treated in oxidizing atmosphere at high temperatures [52–53]. Recently, Huang et al. examined the effect of oxygen treatment on the catalytic performance of Au/SiO2 catalysts toward CO oxidation [54]. The Au/SiO2 catalysts were synthesized using deposition-precipitation practice. It was observed that, the as-prepared Au/SiO2 catalyst executes a poor catalytic activity, which is because of the relatively large size of Au particles. After treating the catalyst in O2 at temperatures higher than 800°C efficiently enhances the catalytic activity with the agglomeration of Au particles. However, TEM results reveal the coexistence of uniformly well-dispersed ultrafine Au particles over the surface. XPS results disclose that after the oxygen pretreatment at temperatures above 800°C, the Au 4f binding energy moves to higher binding energy. Remarkably, dealing of the Au/SiO2 catalyst in He at 800°C also displays the similar geometric and electronic structure changes of Au particles. Therefore, the enhancement effect as that in O2, indicates that the Au-O2 interactions at high temperatures do not contribute greatly

to the progress of catalytic activity. The authors proposed that evaporationdeposition mechanism of gold particles treated at high temperatures, which accounts for the formation of ultrafine Au particles, is responsible for the improve-

*The CO conversion of Au/SiO2-O2-800 and Au/SiO2-He-800 catalysts. Reproduced with permission, copyright* 

orbitals

of Au/Al2O3 could be due to the back donation of d-electrons into the π∗

*Gold Nanoparticles - Reaching New Heights*

catalysts for CO oxidation.

It has been observed that, the degree of rate of improvement rest on the type of support used: high for insulating Al2O3 and SiO2 and moderate for semiconducting TiO2. The effect of moisture becomes notable only above about 200 ppm H2O for Au/Al2O3, while the activity for Au/SiO2 decreases significantly with a decrease in moisture to about 0.3 ppm. While the catalytic activity of Au/TiO2 at about 3000 ppm H2O is very high and reaches full CO conversion (100%). The authors also proposed as a mechanism model that moisture improves the catalytic activities for no less than two orders of magnitude and the effect of moisture depends over the type of metal oxides used. Moisture plays dual role in the reaction: one is the

Dai et al. demonstrated the surface sol-gel process (SSP) for the modification of silica mesopores surfaces and the tuning of mesopore diameters [48]. The procedure involves the preparation of one or multilayers of titanium oxide over SBA-15 [49]. This layer-by layer approach controls the mesopore diameters with monolayer precision (**Figure 3**). Ultra-small Au NPs were effectively prepared on surfacemodified SBA-15 via a deposition-precipitation method without the restriction of surface isoelectric point (**Figure 3**). These developed materials are found very active

Corma et al. presented a modified sol-gel approach for the preparation of Au NPs embedded in silica, and Au NPs are capped with two different thiol molecules, 1-dodecanethiol (DT) and 3-mercaptopropyltrimethoxysilane (MPMS), which lead to high-surface area Au/SiO2 catalysts [50]. The synthesis comprises the formation of a three-component metal-organic-inorganic structure collected of Au NPs capped with alkanethiols and partly functionalized with alkoxysilane groups, and polymerized with tetraethyl orthosilicate (TEOS). Upon calcination, the material becomes highly active catalyst for the CO oxidation at low temperatures, the same as that obtained with gold on TiO2. This clearly specifies that it is possible to attain highly active gold-silica catalysts by a liquid-based method, as long as accessible

The structure and oxidation state of gold clusters of different sizes supported over various metal oxides (Al2O3, TiO2, and SiO2) were studied to different CO oxidation conditions that were investigated by Bokhoven et al. using in situ X-ray absorption spectroscopy at the Au L3 edge [51]. During catalysis, only phase detected in all catalysts was Au0 with no other oxidation states. For the most active catalyst with small size gold particles (Au/Al2O3), difference in the electronic structure of the gold clusters in changing reaction conditions was observed by XANES spectroscopy and attributed to the adsorption of CO on the metallic gold clusters.

*Pore-size distribution as function of the number of TiO2 layers (left), Z-contrast TEM image of ultra-small Au NPs on ordered mesoporous materials (right). The bright spots (0.8–1.0 nm) correspond to Au NPs. Reproduced* 

*with permission, copyright 2018, American Chemical Society.*

activation of oxygen and other is the decomposition of carbonate.

small gold particles and strong metal-support interactions exist.

**32**

**Figure 3.**

Further FEFF8 calculations confirmed that the changes in the XANES signature of Au/Al2O3 could be due to the back donation of d-electrons into the π∗ orbitals of CO. For Au/Al2O3, Au-O backscattering was also seen in the EXAFS, suggesting weak cluster-support interactions. Nevertheless for Au/SiO2 and Au/TiO2, the structure of the gold clusters remained unaffected throughout all experiments.

It is well recognized that several-supported metal particles are subjected to the geometric/electronic variations when treated in oxidizing atmosphere at high temperatures [52–53]. Recently, Huang et al. examined the effect of oxygen treatment on the catalytic performance of Au/SiO2 catalysts toward CO oxidation [54]. The Au/SiO2 catalysts were synthesized using deposition-precipitation practice. It was observed that, the as-prepared Au/SiO2 catalyst executes a poor catalytic activity, which is because of the relatively large size of Au particles. After treating the catalyst in O2 at temperatures higher than 800°C efficiently enhances the catalytic activity with the agglomeration of Au particles. However, TEM results reveal the coexistence of uniformly well-dispersed ultrafine Au particles over the surface. XPS results disclose that after the oxygen pretreatment at temperatures above 800°C, the Au 4f binding energy moves to higher binding energy. Remarkably, dealing of the Au/SiO2 catalyst in He at 800°C also displays the similar geometric and electronic structure changes of Au particles. Therefore, the enhancement effect as that in O2, indicates that the Au-O2 interactions at high temperatures do not contribute greatly to the progress of catalytic activity. The authors proposed that evaporationdeposition mechanism of gold particles treated at high temperatures, which accounts for the formation of ultrafine Au particles, is responsible for the improvement of the catalytic activity (**Figure 4**).

**Figure 4.**

*The CO conversion of Au/SiO2-O2-800 and Au/SiO2-He-800 catalysts. Reproduced with permission, copyright 2018, Elsevier.*

MCM-41 was fused with Au NPs by Mokhonoana et al. [55] using ethylenediamine through a modified deposition-precipitation method, where ethylenediamine operated as both base and complexing agent for the Au(III) species. Synthesis involves the mixing aqueous solution of HAuCl4 separately with both the as-synthesized MCM-41 (still containing CTABr) and the calcined MCM-41 for 1 h. pH was adjusted to 10 using 1 M ethylenediamine solution. Resulting slurry was agitated at room temperature for 13 h, and the recovered solids were filtered, washed with warm water to eliminate the chloride ions, dried and calcined at 500°C for 12 h (for the as-synthesized MCM-41) or 400°C for 4 h (for the calcined MCM-41).

It has been observed that, subsequent catalysts with (4 and 5% nominal Au loading, respectively), having average Au NPs of 12 and 10 nm, respectively, holds the ordered structure and high surface area of the MCM-41 material. Nevertheless, calcination at 500°C results in aggregation and migration of Au NPs to the surface. Still, both catalysts show good activity in the CO oxidation at T > 250°C.

Huang et al. investigated that, NaOH additive to Au/SiO2 catalyst considerably improves the catalytic activity of inert in catalyzing CO oxidation at temperatures below 150°C, while Au/NaOH/SiO2 catalyst with a NaOH:Au atomic ratio of 6 is active at room temperature [56]. It has been witnessed that both particle size distribution and the electronic structure of Au NPs remain equal in Au/SiO2 and Au/NaOH/SiO2 catalysts, where hydroxyls excite the activation of O2 on "inert" Au NPs, which benefits to catalyze CO oxidation even at room temperature. Further, density functional theory (DFT) calculation results also proves the defining role of COOH in hydroxyls-induced activation of O2 on the Au(111) surface.

The impact of pretreatments effect for gold supported over hexagonal mesoporous silica (HMS) with He, O2, and H2 on the physicochemical and catalytic properties has been studied by Pestryakov et al. [57]. It has been investigated that, gold supported on mesoporous silica forms different states such as Au3+ and Au<sup>+</sup> ions, neutral, partly charged gold clusters, and metal NPs of varied sizes. It is noted that as-prepared catalyst with Au3+ ions does not show any catalytic activity in CO oxidation. While reduced pretreated catalysts increases the catalytic activity and its oxidative treatments deactivate the catalyst. Catalytic tests demonstrate that, reduced samples contain several regions with various catalytic behaviors at 20–200°C, 200–400°C, and >400°C; this is because of the co-existence of gold active sites of varied forms. Further, a comparative analysis using XPS, UV-visible spectroscopy, FTIR and catalytic data shows that Au<sup>δ</sup><sup>+</sup> n clusters or/and Au<sup>+</sup> ions are responsible for the activity in the low-temperature region (i.e. <200°C); while neutral Aun clusters are found to be active in the temperature range 200–400°C; and Au NPs catalyze the high-temperature CO oxidation.

Vinod et al. have proposed and confirmed the theory of active sites. They have examined the role of interfaces for the CO oxidation reaction for trisoctahedral (TOH) Au NPs adorned with nano oxides of CeO2 and TiO2 encapsulated in porous silica system [58]. The TEM images display that size of TOH morphology of the Au NPs is ∼70 nm, which shows the high index facets with unvarying defect sites. Its atomic model shows the existence of {221} and {331} high index planes, which carries {111} terrace and mono atomic {110} step atoms as reported earlier [59]. Further, line profile analysis of HRTEM image from the surface of the TOH particle reveals the existence of step-terrace geometry. These oxides adorned and silica-encapsulated system were found to display substantial activity and stability for CO oxidation at room temperature; nevertheless, the Au particle size was above the optimum range. This is due to the conservation of morphology and thereby the active centers due to encapsulation.

**35**

**Figure 5.**

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

*(en = ethylenediamine)*

instead of anionic AuCl4

tion techniques.

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

mining the catalytic activity through the regulation of [Au(en)2]

kinds of silica materials (e.g., silica particles and microporous zeolites).

15) using a gold cationic complex precursor [Au(en)2]

Dai et al. reported a unique deposition-precipitation (DP) method for the preparation of highly active Au catalysts supported over mesoporous silica (SBA-

[60] (**Figure 5**). This new DP procedure comprises the use a cationic gold precursor

quent 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 solu-

In addition, pH of the gold precursor solution founds to play a key role in deter-

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

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).*

<sup>−</sup>, which is facilitated by ion-exchange route. The subse-

3+ via a wet chemical process

3+ deprotonation

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

*Gold Nanoparticles - Reaching New Heights*

MCM-41).

Au<sup>+</sup>

clusters or/and Au<sup>+</sup>

active centers due to encapsulation.

oxidation.

MCM-41 was fused with Au NPs by Mokhonoana et al. [55] using ethylenediamine through a modified deposition-precipitation method, where ethylenediamine operated as both base and complexing agent for the Au(III) species. Synthesis involves the mixing aqueous solution of HAuCl4 separately with both the as-synthesized MCM-41 (still containing CTABr) and the calcined MCM-41 for 1 h. pH was adjusted to 10 using 1 M ethylenediamine solution. Resulting slurry was agitated at room temperature for 13 h, and the recovered solids were filtered, washed with warm water to eliminate the chloride ions, dried and calcined at 500°C for 12 h (for the as-synthesized MCM-41) or 400°C for 4 h (for the calcined

It has been observed that, subsequent catalysts with (4 and 5% nominal Au loading, respectively), having average Au NPs of 12 and 10 nm, respectively, holds the ordered structure and high surface area of the MCM-41 material. Nevertheless, calcination at 500°C results in aggregation and migration of Au NPs to the surface.

Huang et al. investigated that, NaOH additive to Au/SiO2 catalyst considerably improves the catalytic activity of inert in catalyzing CO oxidation at temperatures below 150°C, while Au/NaOH/SiO2 catalyst with a NaOH:Au atomic ratio of 6 is active at room temperature [56]. It has been witnessed that both particle size distri-

 ions, neutral, partly charged gold clusters, and metal NPs of varied sizes. It is noted that as-prepared catalyst with Au3+ ions does not show any catalytic activity in CO oxidation. While reduced pretreated catalysts increases the catalytic activity and its oxidative treatments deactivate the catalyst. Catalytic tests demonstrate that, reduced samples contain several regions with various catalytic behaviors at 20–200°C, 200–400°C, and >400°C; this is because of the co-existence of gold active sites of varied forms. Further, a comparative analysis using XPS, UV-visible spectroscopy, FTIR and catalytic data shows that Au<sup>δ</sup><sup>+</sup>

region (i.e. <200°C); while neutral Aun clusters are found to be active in the temperature range 200–400°C; and Au NPs catalyze the high-temperature CO

Vinod et al. have proposed and confirmed the theory of active sites. They have examined the role of interfaces for the CO oxidation reaction for trisoctahedral (TOH) Au NPs adorned with nano oxides of CeO2 and TiO2 encapsulated in porous silica system [58]. The TEM images display that size of TOH morphology of the Au NPs is ∼70 nm, which shows the high index facets with unvarying defect sites. Its atomic model shows the existence of {221} and {331} high index planes, which carries {111} terrace and mono atomic {110} step atoms as reported earlier [59]. Further, line profile analysis of HRTEM image from the surface of the TOH particle reveals the existence of step-terrace geometry. These oxides adorned and silica-encapsulated system were found to display substantial activity and stability for CO oxidation at room temperature; nevertheless, the Au particle size was above the optimum range. This is due to the conservation of morphology and thereby the

ions are responsible for the activity in the low-temperature

n

Still, both catalysts show good activity in the CO oxidation at T > 250°C.

bution and the electronic structure of Au NPs remain equal in Au/SiO2 and Au/NaOH/SiO2 catalysts, where hydroxyls excite the activation of O2 on "inert" Au NPs, which benefits to catalyze CO oxidation even at room temperature. Further, density functional theory (DFT) calculation results also proves the defining role of COOH in hydroxyls-induced activation of O2 on the Au(111) surface. The impact of pretreatments effect for gold supported over hexagonal mesoporous silica (HMS) with He, O2, and H2 on the physicochemical and catalytic properties has been studied by Pestryakov et al. [57]. It has been investigated that, gold supported on mesoporous silica forms different states such as Au3+ and

**34**
