Mechanism of gold nanoparticles anchored onto silica

In order to take a view of what happened during the calcination process, the in situ FTIR spectra are carried out during a simulation calcination procedure. The temperature increases from room temperature to 300^oC at 1^oC/min under air and the chosen spectra are shown in Figure 4.8.

Figure 4.8 Operando FTIR spectra over Au/SS5 sample along with temperature rising from room temperature to 300^oC by 1^oC/min under air (a) and the comparison of Au/SS5 sample at room temperature and 300^oC (b). The last black line in picture (a) is the spectrum of fresh Au/SS5-C after 4 h calcination at 300^oC in air.

3800 3600 3400 3200 3000 2800 2600

1

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The multiple peaks ranging from 600-1400 cm^-1 are belonged to the characteristic of silica structure and do not change much at the very beginning of temperature rising. Three peaks located at 2918, 2947 and 2983 cm^-1 are observed and due to the C-H sp³ stretch ^[43-45] that generated from the residual TEOS or PVA compounds. The large peaks locating at 3000-3700 cm^-1 and 1400-2100 cm^-1 are resulted from the Si-OH stretching ^[44] and water species in the sample, respectively. Along with the raising of temperature, the intensity of Si-OH bond and water species largely reduce at first and then gently decrease with time. The organic compounds are steadily removed from room temperature to 300^oC as shown in Figure 4.8(b). Two small peaks around 2950 cm^-1 still exist until 300^oC. For the Au/SS5-C sample after calcined at 300^oC for 4 h, the IR spectra (the last black spectrum in Figure 4.8) displays no peak of C-H stretching evidences the totally remove of reactants utilized during the preparation of Au-NPs and silica.

A new peak at 3735 cm^-1 that is attributed to isolated Si-OH vibration ^[46] is present as well as the hydrate species decreases, indicating the lost of hydrate species from the silica structure.

The decrease amount of hydrate species in silica structure together with the slightly increase of Si-O and Si-O-Si species (900-1200 cm^-1) demonstrating the variation of silica structure during calcination.

Figure 4.9 Au 4f XPS spectra of Au/SS1-C and Au/SS5-C.

The valence states and components surface ratio over Au/SS1-C and Au/SS5-C are characterized by X-ray photoelectron spectroscopy (XPS) technique. Figure 4.9 shows the Au 4f spectra of the two samples. Both the samples possess two overlapped peaks that can be fitted

90 88 86 84 82 80

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by four peaks of o two states-one at about 84 eV- binding energy of Au 4f_7/2 that corresponds to Au⁰ species, the other with higher binding energy presenting the existence of Au^δ+ species. The binding energy of Au 4f5/2 is about 3.65 eV higher than that of Au 4f7/2. Both the two calcined samples possess large amount of metallic Au species and minority of Au^δ+ species. The surface ratios between tested elements are listed in Table 4.2 as below. It is surprisingly that the Au/Si ratio in Au/SS1-C is much lower than in Au/SS5-C. Since that the SS5 is non-porous and constructed by spheres much larger than the Au-NPs, it is believed that the Au-NPs are only highly dispersed on the surface of SS5. In this case, the atom ratio between different gold species revealed from XPS ought to be the same value of the real ratio of entire sample.

Meanwhile, there is chance that the real atom ratio between Au and Si is larger than measured since the large inner part of the compact Stöber silica may not be directly detected by the machine. For SS1, the much smaller silica globules are more facile for the XPS scan over the Si component. From this point of view, the measured Au/Si ratio over Au/SS1-C should be somewhat lower than over Au/SS5-C. No matter how, the SS5 is considered better for Au-NPs coating for both Au-NPs dispersing and protecting.

Table 4.2 XPS data of Au/SS1-C and Au/SS5-C samples

Sample Au 4f7/2 (eV) Surface gold species Atom ratio Au/Si Si/O

Au/SS1-C 83.7 Au⁰ (80.1%)

0.5% 59.7%

84.5 Au^δ+ (19.9%)

Au/SS5-C 83.7 Au⁰ (87.2%)

0.9% 62.5%

85.2 Au^δ+ (12.8%)

Based on the results from in situ FTIR and XPS, a model is put forward for understanding the preparation mechanism of supported Au-NPs on Stöber silica. After coating process, the Au-NPs are deposited on the surface of silica. The Au-NPs are dispersed and protected by PVA, a part of which is linked to the surface of silica globules by hydroxyl species (the first step of Scheme 4.2).

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Scheme 4.2 Mechanism of Au-NPs anchored onto silica during calcination.

At the beginning of the calcinations, only absorbed water in the sample is quickly removed.

After the temperature becomes higher, hydrate species either in silica structure or PVA layer protecting the Au-NPs are also started to be removed as water. Along with the temperature further raises, the PVA layer begins to be removed steadily by a combustion process. The emerged Au-NPs without covering of PVA are thus anchored on silica surface by the residual hydroxyl species as shown in the second step of Scheme 4.2. During the total calcination that includes temperature increase and maintenance, the PVA is finally burned out and the Au-NPs with edges and corner are exposed and fixed on the surface of Stöber silica. Minority of the gold species after calcination changes into partial oxidative stated Au^δ+ species. The morphology of Stöber silica is mainly maintained as spheres as initial.

In fact, the Au-NPs coating globular silica can be both viewed as a basic model for the core-shell structures and the supported nano-gold material. The core-shell structure with this simple synthesis method can also be achieved by increasing the gold loading amounts as previous report ^[22]. Besides, this small Au-NPs coating silica provides a path for synthesizing gold catalysts that can be used in the inorganic heterogeneous catalysis field. By this simple method, the Au-NPs can be well dispersed on the surface of silica and separated from each other and on the other hand can be facilitate for the understanding of the gold-support interaction.

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4.4 Conclusion

In the present work, we provided a strategy to synthesize Stöber silica coated by very small and homogeneous colloidal Au-NPs, in which the size of Au-NPs is controlled as the initial colloid (2-3 nm). This method is facile and inexpensive without either surfactant or catalytic- poisonous compound, thus providing an available path for this material utilized in the inorganic heterogeneous catalysis field. The Stöber silica with diameter much larger than the Au-NPs facilitated the high dispersion of Au-NPs and anti-clustering during calcination. The Au-NPs protected by PVA were bonded by silica, while the calcination process help the Au-NPs fixed on the silica surface by the residual hydroxyl species after combined removing of hydroxyl species in both PVA and silica structure. The satisfied part is that small nanoparticles still exist even after calcined at 300^oC for 4 h. The Au-NPs coated Stöber silica is very stable under temperature lower than 300^oC by this simple method, which can also be expended into SiO₂@Au structures (by increasing the content of Au), thus providing the perspective for the series of materials used in the inorganic heterogeneous catalysis field.

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Chapter V Impacts of metal addition into Au/SS5-C applied as