Elsevier

Solid State Sciences

Volume 10, Issue 7, July 2008, Pages 908-914
Solid State Sciences

Formation of TEM- and XRD-undetectable gold clusters accompanying big gold particles on TiO2–SiO2 supports

https://doi.org/10.1016/j.solidstatesciences.2007.10.037Get rights and content

Abstract

Sol–gel method allows obtaining complex Ti–Si oxide supports possessing the effect of strong metal–support interaction and favouring the stabilisation of gold in the form of small charged gold clusters. TEM and XRD measurements show the samples containing big gold particles with average size more than 90 nm. However, Rietveld XRD refinement revealed a very low (≤5%) relative amount of large particles on mixed oxide supports. Highly dispersed gold species undetectable by XRD and TEM on TiSi supports provide the main contribution to total amount of gold due to extremely high surface area of these mixed oxides. The combination of DRS, Rietveld XRD analysis and XPS methods was effective for registration and characterisation of these dispersed gold species. The formation of such gold species should be taken into account when studying the nature of gold active sites, as they can be the participants of the catalytic process instead of TEM-detectable metal particles or along with them.

Introduction

Supported gold is known as an active catalyst for low-temperature CO oxidation, aldehyde and hydrocarbon synthesis, water-gas shift reaction [1], [2], [3], [4], [5] and some other processes. Electronic state of gold particles and character of their interaction with the support surface play a key role in catalytic properties of gold. It was determined that only dispersed gold particles (<5 nm) display catalytic activity in these reactions. Simple and mixed Ti, Cu and Fe oxides are generally used as supports for gold catalysts. Until now, it is not clear which specific gold species (cations, clusters, metal nanoparticles, etc.) are active in catalysis. Because it was shown that only highly dispersed gold species are active in catalysis, the registration and the characterisation of highly dispersed gold species becomes a point of great attention. According to the most studies, formation of active dispersed particles of gold depends strongly on the method of the catalyst preparation. In the case of impregnation method gold particles are too big and not active in catalytic reactions, while co-precipitation gives active highly dispersed gold clusters [1], [2].

In contrast, authors of Ref. [5] reported the activity of big gold particles supported on mixed silica–titania supports prepared by sol–gel method in the process of hydrocarbon synthesis. Samples of Au supported on mixed Ti–Si oxides contained metal particles more than 100 nm in diameter and exhibited catalytic activity in conversion of CO + H2 to alkanes, alkenes and alcohols. These results are intriguing as now extremely high catalytic activity of gold is concerned solely with highly dispersed particles. Some other authors also reported high catalytic activity attributed to big gold particles [6].

In principle, large Au particles (even bulk Au) can be active in some catalytic processes, for example in selective oxidation of alcohols [7]. However, this group of reactions occurs at high temperature (500–700 °C). At lower temperatures, the activity of large Au particles, as far as we know, was not observed. But we cannot exclude the activity of big gold particles, especially for poorly studied (for gold catalysts) reactions, such as conversion of CO + H2 occurring at relatively high temperatures (200–400 °C).

The aim of the present paper is to study structural and electronic properties of gold deposited on the mixed silica–titania supports prepared by sol–gel method. The combination of XPS, XRD and UV–vis spectroscopies permits to detect highly dispersed gold species invisible by TEM along with big Au particles. Estimation of the ratio of big/dispersed Au allows understanding the nature of active gold species in Au catalysts supported on the mixed silica–titania supports. Moreover, the mixed Ti–Si oxides might be effective supports for gold catalysts in other processes.

Section snippets

Sample preparation

Silicon and titanium oxides as well as mixed TiO2–SiO2 oxides with 5 wt.% (TiSi5) and 10 wt.% (TiSi10) of titania in titania–silica were prepared by the sol–gel method. Alkoxides tetraethoxysilane and titanium tetraisopropoxide were used in the synthesis. Silica was prepared using HCl as hydrolysis catalyst and 2-propanol as solvent. The obtained gel was dried at 60 °C for 24 h and calcined at 500 °C for 4 h in an air flow. Titania was synthesised by adjusting the pH with concentrated HNO3. The

Surface area and pore characteristics

Specific surface area (SBET) of pure SiO2 and TiO2 supports (TiSi) was 426 and 40 m2/g, respectively (Table 1). Specific surface area drastically increased up to 800 and 1154 m2/g for corresponding mixed oxides (Table 1). This synergetic effect is a proof of very good contact between SiO2 and TiO2 components in mixed oxides prepared by sol–gel method. Mixed oxides do not represent a mechanical mixture of two oxides but a new chemical compound.

Pore radius for TiO2 is 3.5 nm. For all other supports

Conclusions

Specific surface area of SiO2, TiO2 and mixed SiO2–TiO2 supports produced by sol–gel method drastically increased for mixed oxides (800 and 1154 m2/g) compared with single oxide supports (426 and 40 m2/g) manifesting good contact of components for mixed oxides.

Rietveld XRD refinement procedure has shown that majority of gold (∼80%) supported on TiO2 are present in the form of large gold particles as observed in TEM and XRD patterns. However, only 1/3 of gold supported on SiO2 was registered by

Acknowledgements

Authors thank R.I. Conde, F. Ruiz M, M.E. Aparicio, E. Flores, A. Quiroz and L. López for technical assistance in experimental work. This work was supported by CONACYT grant No 42568-Q.

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