1. Introduction

Nowadays, gold nanoparticles (AuNPs) due to their unique properties such as optical, mechanical, electrical and catalytic activity play crucial role for industries ^{[1, 2, 3]}. Catalytic activity of AuNPs is well known since 1980’s when the work of Haruta has been published ^[4]. Catalysts based on AuNPs and supported on metal oxides have attracted researcher’s attention because of their high catalytic activity for various oxidation and reduction reactions under mild conditions ^[5]. For instance, authors ^[6] used AuNPs supported on ceria oxide for oxidation of carbon monoxide at low temperature. A catalytic property of gold nanoparticles deposited on aluminum oxide depends on stabilizing method of AuNPs and their dimensions. There are many researches devoted to investigation of catalytic properties of gold nanoparticles which are deposited on different supporting agents such as non-reducible (Al₂O₃, SiO₂ etc.) and reducible oxides (Fe₂O₃, CeO₂, TiO₂, MnO_x etc.) ^{[7, 8, 9]}. AuNPs can also be deposited at the poly(ethyleneterephlate) surface by ion-beam irradiation ^[10].

In the present work we stabilized AuNPs with poly(N-vinylpyrrolidone) having different molecular weights, determined their sizes, prepared aqueous solutions of AuNPs, immobilized them on aluminum oxide by impregnation method and studied the catalytic activity.

2. Experimental

Standard aqueous solution of tetrachloauric acid HAuCl₄ with concentration 100mg•L^-1 was purchased from Sigma-Aldrich. Poly(N-vinylpyrrolidone) (PVP) with different molecular weights 10•10³, 40•10³, 350•10³ and aluminum oxide were also purchased from Sigma-Aldrich.

Absorption spectra of AuNPs were determined at room temperature by UV-Vis spectroscopy (Specord 210 plus BU, Germany). The sizes of AuNPs were determined with the help of DLS device Malvern Zetasizer Nano ZS90 (UK). Concentration of Au in supernatant was determined by ion-coupled plasma atomic emission spectroscopy ICP-AES “Optima 5100DV” (Perkin Elmer, USA). Transmission electron microscopy (TEM) image was recorded on a JEOL JEM–1011LA (Japan).

PVP stabilized AuNPs were obtained by “one-pot” synthetic protocol ^[11]. Aqueous solutions of HAuCl₄ (5mL), 0.5M KOH (4mL), and 4% PVP (5mL) were mixed, stirred and heated up to 100C during several minutes. As a result the colored solutions of AuNPs stabilized by PVP (PVP-AuNPs) were obtained as shown in Figure 1.

PVP-AuNPs were supported on Al₂O₃ by impregnation method. For this 0.5g of Al₂O₃ was added to 5mL of PVP-AuNPs and stirred during 5 hours. After the precipitate was separated by preparative centrifuge “Eppendorf 5810R” (Germany) at 10•10³rpm, then it was washed out 5 times with distilled water. The content of Au in supernatant was determined by the ICP-AES. The precipitate was dried at 50°C. The powders of Al₂O₃ with supported PVP-AuNPs (Al₂O₃/PVP-AuNPs) were used as catalysts for decomposition of H₂O₂ (Figure 2).

Figure 1. Samples of AuNPs stabilized by PVP with M_n = 10•10³ (1), 40•10³ (2) and 350•10³ (3)

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Figure 2. Samples of Al₂O₃ with immobilized PVP-AuNPs. M_n = 40•10³ (1) and 350•10³ (2)

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3. Results and Discussion

Visible spectra of freshly prepared PVP-AuNPs show the absorption bands with maxima at 520-550nm corresponding to so-called “plasmon resonance” spectra that are specific for AuNPs ^[12].

Figure 3 represents the size distribution of AuNPs stabilized by PVP with various molecular weights. In dependence of the molecular weights of PVP the size distribution of PVP-AuNPs is varied from 10 to 25nm.

TEM image of AuNPs-PVP deposited onto Al₂O₃ reveals that there are no big agglomerates of the gold nanoparticles (Figure 4). TEM micrograph clearly shows the attached to the surface of Al₂O₃ gold nanoparticles with average size varying from 10 to 30nm. TEM results are in good agreement with hydrodynamic sizes of AuNPs measured by DLS.

Figure 3. Size distribution of AuNPs stabilized by PVP with M_n = 10•10³ (A) 40•10³ (B) and 350•10³ (C)

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Figure 4. TEM image of AuNPs-PVP-40•10³ supported on Al₂O₃

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The catalytic activity of AuNPs depends on the size of gold nanoparticles, e.g. the smaller AuNPs size the higher catalytic activity ^[3]. However, in our case the catalytic activity of AuNPs also depends on the amount of gold nanoparticles deposited on the surface of Al₂O₃. We have found that the concentration of Au nanoparticles deposited on Al₂O₃ is very small and equal to 0.06-0.1%. Despite of this fact, as illustrated in Figure 6, the rate of H₂O₂ decomposition changes exponentially while Al₂O₃ itself and PVP supported onto Al₂O₃ do not show any catalytic activity in decomposition of H₂O₂.

Figure 5. Time dependent decomposition of hydrogen peroxide in the presence of gold nanocatalysts

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It should be noted that in the presence of Al₂O₃/AuNPs-PVP the rate of H₂O₂ decomposition exhibits an induction period that depends on the molecular weight of PVP and changes in the following order PVP-350•10³ > PVP-40•10³ > PVP-10•10³. As seen from Figure 5, decomposition of H₂O₂ in the presence of Al₂O₃/AuNPs-PVP-350•10³ starts after 1h while the same reaction starts immediately in the presence of Al₂O₃/AuNPs-PVP-10•10³. This is probably explained by less accessibility of gold nanoparticles to substrate due to surrounding of catalytic centers by high molecular weight PVP.

4. Conclusion

Poly(N-vinylpyrrolidone) protected gold nanoparticles were prepared by “one-pot” method and impregnated on the surface of Al₂O₃. The average size of AuNPs stabilized by PVP is varied from 10 to 25nm. The amount of deposited onto Al₂O₃ gold nanoparticles is extremely low and in the range of 0.06-0.1%. TEM results show that the average size of gold nanoparticles attached to the surface of Al₂O₃ is varied from 10 to 30nm. The catalytic activity of Al₂O₃/AuNPs-PVP nanocatalysts increases exponentially with induction period of time in dependence of molecular weight of PVP.

Acknowledgements

Authors thank JSC “National R&D holding company “Parasat” for financial support.

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