AuPd bimetallic nanoparticles on TiO2: XRD, TEM, in situ EXAFS studies and catalytic activity in CO oxidation

Dedicated to Professor Renato Ugo on the occasion of his 65th birthday
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Abstract

Monometallic Au, Pd and bimetallic AuPd nanoparticles were prepared from Au(III), Pd(II) precursor ions in aqueous media with reduction by a mixture of Na-citrate and tannin producing stable metal sols of narrow size distribution. The nanoparticles of 4–7 nm in diameter as indicated by transmission electron microscopy were adsorbed on TiO2. In situ X-ray absorption spectroscopy (XAS) at Au LIII and Pd K-edges and X-ray diffraction (XRD) techniques evidenced the presence of bimetallic particles in the supported AuPd/TiO2 sample. The high amount of organic residues of the samples was removed by calcination at 400 °C proved by temperature programmed oxidation (TPO). After reduction in H2 the TEM, XRD and the CO chemisorption showed some enlargement in the metal particle sizes. The catalytic activity of the bimetallic AuPd/TiO2 in the CO oxidation revealed a slight synergistic effect compared to the activity of monometallic analogous referred to the estimated surface area of Au and Pd in the bimetallic sample.

Tannin stabilized bimetallic AuPd nanoparticles of narrow size distribution were adsorbed on TiO2 support from metal sol. The catalytic activity of the calcined/reduced AuPd/TiO2 in CO oxidation revealed a slight synergistic effect compared to the activity of monometallic analogous referred to the Au and Pd surface area in the bimetallic sample.

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Introduction

It is of great interest to study of gold-based nanoparticles due to their diverse range of physical [1], [2] and catalytic properties [3], [4], [5], [6]. Among the different parameters, which affect their activity/selectivity, the particle size, the nature of oxide supports (e.g. α-Fe2O3 [7], TiO2 [8], SiO2 [9] or ZrO2 [10]) as well as the interaction between gold particles and the supporting oxide [11] can be mentioned. Particularly high activity of the gold nanoparticles in the CO oxidation is achieved by depositing gold on transition metal oxide of variable oxidation state. These oxides alone are generally also active catalysts providing activated oxygen to the CO oxidation. However, a combination with Au nanoparticles produces spectacularly high activity. According to one of the explanations this is due to the increased number of oxygen vacancies responsible for O2 activation in the oxides in vicinity of the gold/oxide interface [12]. The size of the Au particles being below 5 nm, plays a decisive role in the dramatic enhancements in the activity [13]. The formation and stabilization of such highly dispersed gold needs generally special preparation techniques.

Palladium on different supports is a well-known oxidation catalyst, on which the chemisorbed CO reacts with the subsurface oxygen. Combination of gold and palladium is an interesting possibility to achieve a catalyst with higher activity and to better understand the nature of the outstanding activity of gold containing catalysts. The exclusive effect of the co-operation of the two metals has been studied on inert silica support in CO oxidation because the promotional effect of the gold/silica interface was regarded negligible [14]. On silica Pd presented much higher activity than Au and alloying them the activity of Pd decreased significantly.

The co-operation of Au and Pd on an active TiO2 support interacting strongly with Au is an intriguing study. For easier interpretation similar size of the components in the bimetallic and the monometallic references is required. This is why the method of adsorption from sols have been used for the preparation of the supported bi- and monometallic nanoparticles that provides an efficient control of particle size and size distribution [15], [16].

In the study of heterogeneous catalysts X-ray absorption spectroscopy (XAS) is now widely recognized to describe the local order around metal elements [17], [18], [19] even for nanometer scale metallic clusters [20], [21]. In the present paper we report in situ XAS characterization of a TiO2 supported bimetallic AuPd system correlated with X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM), CO chemisorption and with the catalytic activity measured in the CO oxidation.

Section snippets

Sample preparation

For sol preparation HAuCl4×3H2O and PdCl2 were used as precursors, tannin and tri-Na-citrate-2-hydrate as reducing agent and stabilizer simultaneously, distilled water served as solvent. The aqueous solution of the Au and Pd precursors was mixed with the solution of tannin and sodium-citrate, whose pH was set to 7.5 by adding 1 wt.% sodium carbonate solution. The sol formation was allowed to proceed at 65 °C under vigorous stirring. During the reduction process the color of the solutions changed

Results

In the preparation of Au–Pd bimetallic as well as gold and palladium nanoparticles with narrow size distribution the parameters were changed to find the optimal conditions. In Table 1 the particle size of the different metal sols produced in selected way by two to three parallel preparations and measured by TEM are presented. As can be seen the size in the three types of sols is quite similar to each other and varies between 4.2 and 7.3 nm. On the other hand, the reproducibility of sol

Discussion

According to the literature data activity of gold can be increased by addition of noble metals [33], [34], [35], [36], [37]. Regarding the Pd–Au bimetallic system, several studies have already been published [14], [29], [38], [39], [40]. For example, Liu et al. [38] have characterized the system by TEM, XRD and XPS to elucidate the formation of bimetallic colloids. XPS data indicated that the constituent elements were in metallic state and that palladium atoms were concentrated on the surface

Conclusions

Monometallic Au, Pd and bimetallic AuPd nanoparticles were prepared by sol technique and deposited on TiO2 support. The stable metal sol of narrow size distribution was characterized by TEM. After removing the stabilizer from the supported samples by TPO the original particle size (4–7 nm) increased to 9–13 nm in diameter.

Presence of bimetallic particles was confirmed by in situ EXAFS at Au LIII and Pd K-edges as well as XRD experiment.

In the CO oxidation reaction the catalytic activity of the

Acknowledgements

The authors are indebted to the National Science and Research Fund (OTKA grant #T-034920) for financial support and to the COST D15/005/99 and D15/016/00 projects.

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