Elsevier

Journal of Catalysis

Volume 233, Issue 1, 1 July 2005, Pages 186-197
Journal of Catalysis

A novel efficient Au–Ag alloy catalyst system: preparation, activity, and characterization

https://doi.org/10.1016/j.jcat.2005.04.028Get rights and content

Abstract

We present a novel efficient catalyst, Au–Ag alloy nanoparticles supported on mesoporous aluminosilicate. The catalysts were applied to the low-temperature CO oxidation reaction. The sample was prepared in one pot, in which the formation of nanoparticles was coupled in aqueous solution with the construction of mesoporous structure. Both XRD and TEM characterizations show that the alloy particles are much larger than the monometallic gold particles and become even bigger with an increase in the amount of Ag. We shall demonstrate that such large particles with an average particle size of about 20–30 nm exhibit exceptionally high activity for CO oxidation at low temperatures. Moreover, the activity varies with the Au/Ag molar ratios and attains the best conversion when Au/Ag is 3:1. The presence of excess H2 deactivates the alloy activity completely at room temperature. UV–vis and EXAFS confirm the Au–Ag alloy formation. XPS results show that the alloy catalysts are in the metallic state, and they have a greater tendency to lose electrons than do the monometallic catalysts. EPR results show there is an O2 species on the catalyst surface, and the intensity of the O2 species becomes the strongest at Au/Ag = 3:1. The catalytic activity coincides with the magnitude of O2 EPR signal intensities. Based on the spectroscopic study and catalytic activity measurements, a reaction mechanism has been proposed.

Introduction

Supported gold catalysts have been extensively investigated for low-temperature CO oxidation since Haruta's pioneering work [1], [2], [3], [4], [5]. It has been found that the catalytic activity of gold is remarkably sensitive to the size of the gold particle [6], [7], [8], [9], [10], the preparation methods [11], [12], [13], [14], and the nature of the support [15], [16], [17], [18], [19]. Therefore, most of the reported works focused on the tuning of the particle size, modification of the support, and the pretreatment conditions [20], [21], [22]. Both experimental works and theoretical calculations show that the adsorption and activation of O2 are the key steps in this reaction [23], [24], [25], [26]. For active supports, such as Fe2O3, and TiO2, the oxygen activation occurred on the support surface and the CO oxidation reaction occurred at the periphery between the support and the gold nanoparticles [2], [16]. Thus, the requirement for very small gold nanoparticles may arise mainly from larger contact peripherals. However, in the case of inert supports, such as SiO2, the adsorption of both CO and O2 has to be carried out on the gold surface. Then the size of the gold nanoparticle plays a paramount role in this reaction [27], [28].

Conceivably, an alternative way to modify the gold-based catalysts is to search for a second metal that can form an alloy with gold and possesses stronger affinity with O2 than gold. That is, where two different metal atoms are in intimate proximity to each other, as in an alloy, the activated O2 can easily react with the activated CO at a neighboring gold atom to give the product CO2. Some success along this line has recently been reported. Häkkinen et al. [22] have confirmed that doping Au with Sr significantly changes the bonding and activation of O2 compared with that in the pure gold, resulting in an enhanced activity for CO oxidation. However, their soft-landing method is not suitable for the practical preparation of a large amount of catalyst. Guczi et al. [29], [30] investigated the Au–Pd bimetallic system for CO oxidation. They found that when supported on SiO2, the activity of bimetallic catalyst was inferior to that of monometallic Pd/SiO2 catalyst. When supported on TiO2, the bimetallic catalyst exhibited a slightly synergistic effect. This may due to the fact that Pd adsorbs O2 very strongly and weakens the role of gold. Baiker et al. [31] used amorphous metal alloy as the precursor for the preparation of Au–Ag/ZrO2 and found that the alloy catalyst shows good activity and stability for CO oxidation. However, because Au/ZrO2 itself is a very active catalyst, the alloying of gold with silver did not seem to have a significant promoting effect.

It is known that the electron transfer from metal to O2 is a key factor for the chemisorption of oxygen on a metal surface [32], [33]. Electron transfer is difficult on a Au(111) surface, since the gold surface has a high work function [34]. Relative to gold, both Cu and Ag have a larger electron-donating ability. It is known that the adsorption of O2 occurs most easily on Cu, and next on Ag, but not on Au. On the other hand, both gold and copper are able to adsorb CO, but silver is not [34], [35]. Thus, combining gold with silver may be an alternative avenue to achieving a catalyst with higher activity for CO oxidation.

In our earlier work, we developed a simple one-pot method to incorporate surfactant-protected gold particles into mesoporous MCM-41 [36]. Because the gold particles obtained with this method have a large size of about 7–8 nm, the catalytic activity is not so high. More recently, Au–Ag alloy nanoparticles supported on mesoporous aluminosilicate were prepared by this one-pot synthesis method, with the use of hexadecyltrimethylammonium bromide (CTAB) both as a stabilizing agent for nanoparticles and as a template for the formation of mesoporous structure [37]. The alloy catalyst exhibited exceptionally high activity in low-temperature (250 K) CO oxidation. Although monometallic Au@MCM-41 and Ag@MCM-41 show no activity at this temperature, the Au–Ag alloy system shows a strongly synergistic effect in high catalytic activity.

Our previous communication was a brief report on catalytic activities of the Au–Ag alloy nanocatalyst [37]. A fundamental understanding from detailed characterizations of the catalytic system was not available up to now. In this work, we prepared a series of Au–Ag alloy catalysts supported on MCM-41 to study the variations of catalytic activities with respect to changing temperature and composition. Many characterization techniques were used to study the catalyst system, such as nitrogen adsorption, XRD, XPS, EXAFS, UV–vis, and EPR spectroscopy. Based on these detailed studies, we then discuss the origin of the unique synergistic effect in the catalysis of CO oxidation.

Section snippets

Preparation of catalysts

To synthesize the gold–silver alloy nanoparticles supported on mesoporous aluminosilicate (denoted AuAg@ MCM) in one pot, the first step is to prepare the alloy Au–Ag nanoparticles in aqueous solution. A proper amount of HAuCl4 (Aldrich) and AgNO3 (Acros) was added into an aqueous solution of quaternary ammonium surfactant C16TMAB (Acros) to form a clear yellow solution. Then, NaBH4 solution was added dropwise, and a dark-red solution was formed. The Au–Ag alloy nanoparticle solution was then

Physical properties of catalysts

Table 1 lists the BET surface area, pore volume, and average pore size of the catalysts with different Au/Ag molar ratios. The BET surface areas of the catalysts were between 800 and 900 m2/g, and the pore size was between 2.3 and 2.4 nm. The pore volume was relatively large compared with traditional MCM-41 material. This arose from the fast neutralization procedure used in this work, which led to the formation of relatively nanosized particles of the support material, ranging from 50 to 100

Formation of Au–Ag alloy nanoparticles on the mesoporous support

The direct preparation of Au–Ag alloy on mesoporous support by traditional deposition methods with the use of two precursors, such as AgNO3 and HAuCl4, is difficult [41], [45]. However, recent developments in the preparation of gold–silver colloids in aqueous solution [44], [45] provide an efficient approach to preparing the supported Au–Ag alloy nanoparticles. In the present work, we combine the formation of Au–Ag alloy nanoparticles with the construction of mesoporous structure in one pot. In

Conclusion

In this work, Au–Ag alloy nanoparticles on a mesoporous support were prepared by a novel one-pot method. The mesoporous support helps the dispersion of the nanoparticles of Au–Ag alloy, and the mesopores facilitate the transport of molecules. Although thus formed alloy nanoparticles are relatively large, they exhibited exceptionally high activity for CO oxidation at low temperatures. Compared with monometallic Au or Ag catalysts, the alloy catalysts showed an unusual activity profile with the

Acknowledgments

This work was supported by a grant from the Ministry of Education of Taiwan through the Academy Excellence Program. T.-S.L. acknowledges partial support from the NSF and PRF. We also acknowledge technical assistance from Prof. H.P. Lin, Mr. C.C. Huang, and M.L. Lin.

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