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Topics in Catalysis Vol. 34, Nos. 1–4, May 2005 ( 2005) DOI: 10.1007/s11244-005-3786-4
The nucleation, growth, and stability of oxide-supported metal clusters W.T. Wallace, B.K. Min, and D.W. Goodman* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012
The optimization of oxide-supported metal clusters as heterogeneous catalysts requires a detailed understanding of the metal cluster–oxide interface. Model catalysts, prepared by deposition of a catalytically active metal onto a thin film oxide support, closely mimic real-world catalysts, yet are amenable to study using surface sensitive techniques. Surface science methods applied to model catalysts, combined with the use of in situ high-pressure reaction studies, have provided a wealth of information about cluster structure and reactivity. STM capabilities for imaging individual particles under reaction temperatures and pressures offer a new approach for studying supported cluster catalysts on a particle-by-particle basis. This article describes recent work in our laboratories using variable temperature STM to investigate the role of the support and its defects in the nucleation and stabilization of metal clusters. KEY WORDS: metal clusters; metal-oxide surfaces; nucleation; bimetallic clusters; gold; silver; palladium; defects.
1. Introduction The extensive use of metal clusters dispersed on metal-oxide supports as catalysts has motivated a plethora of studies [1–11]. An atomic level understanding, however, is a formidable challenge given the complexity of technical catalysts and the conditions under which the catalysts typically operate. Numerous relevant studies have been carried out using single crystal metal surfaces as model catalysts, facilitating the use of ultrahigh vacuum surface analytical techniques [12, 13]. However, single crystals are incapable of modeling certain properties of supported metal clusters, e.g. those related to limited dimensions, where the cluster size and the oxide support are important. Also, ultrahigh vacuum environments preclude studies of the influence of reactant environments on surface intermediates, composition, and morphology. To more realistically model technical catalysts, simplified planar supported cluster catalysts have been synthesized by nucleating metal clusters onto a thin oxide film grown on a refractory metal substrate [13]. These thin oxide films are structurally and electronically similar to the corresponding bulk oxides, yet are thin enough to permit the use of electron spectroscopic techniques [14]. Support defects are important as reaction and metal cluster nucleation sites. For example, for small, sizeselected clusters, Heiz and coworkers have reported that oxygen vacancies (point defects) on MgO are essential for the nucleation of active Au clusters [15, 16]. Furthermore, theoretical studies have shown that electron transfer from defects to the cluster facilitate CO oxidation. Also, Heiz and Pacchioni, in studies of acetylene polymerization on Pd, have suggested that trapped electrons at oxygen vacancies more efficiently *To whom correspondence should be addressed. E-mail:
[email protected]
activate single Pd atoms for reaction than do lowcoordinated oxygen atoms [17]. Jennison and Bogicevic [18, 19] and a brief review discussing oxygen vacancies by Pacchioni [20], summarize recent relevant theoretical studies related to the metal cluster-oxide interface. The role of defects in the nucleation and stability of larger clusters has been studied recently in some detail for several systems [9, 11, 21–26]. For copper and vanadium on thin film alumina, Wiltner et al. found that metal clusters, rather than preferentially decorating steps or defects, nucleate at one of two superstructures [22]. The growth of bimetallic CoAPd clusters on alumina was studied by Freund and coworkers who noted that Pd clusters tend to nucleate at domain boundaries (line defects) of an alumina film [11]. Co clusters, on the other hand, nucleate at point defects at room temperature and at line defects at higher temperatures. When deposited subsequently to Co, Pd nucleates only on the Co particles, forming a core-shell structure. Due to a significant metal-support interaction, Co deposited subsequently to Pd decorates primarily Pd clusters and point defects. In this review, we discuss recent work from our laboratory addressing the role of defects in cluster nucleation and as reactive sites.
2. Metal–oxide surfaces 2.1. SiO2 film production The recipe for the synthesis of the SiO2 thin films used in these studies has been described in detail [27– 31]. In order to achieve a high quality, ordered thin film, a Mo(112) single crystal was first oxidized in 1 · 10)7 Torr O2 at 800 K for 5 min to produce a p(2 · 3)-O structure. The SiO2 film was prepared sufficiently thick to produce the structural and elec1022-5528/05/0500–0017/0 2005 Springer Science+Business Media, Inc.
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tronic properties of bulk SiO2, yet thin enough to allow the use of the full array of surface spectroscopies as well as STM. The Si-covered surface was then oxidized and annealed in O2 (1 · 10)7 Torr) at 1150 K for 30 min. This step forms a highly-ordered SiO2 thin film that exhibits an atomically resolved
STM image and a sharp LEED pattern with c(2 · 2) periodicity (figures 1 and 2). The formation of a stoichiometric film was confirmed by the absence of Si0 or Si2+ AES features. 2.2. TiO2 surfaces The STM and electron spectroscopic measurements described herein were carried out on TiO2(110) single crystals, although TiO2 thin films have been synthesized in our laboratories [32–34]. The TiO2(110) surface was cleaned by several cycles of Ar+ sputtering and anneals to 700–1000 K. An image of a TiO2(110) surface (50 · 50 nm) before metal deposition is shown in figure 3(a); an atomically resolved image is shown in figure 3(b). As seen in the enlarged image, the TiO2 surface consists of numerous extended defects, namely step edges with very small terraces. The bond distances derived from the high-resolution STM image agree with those of a (1 · 1) TiO2(110) unit cell.
3. Au
Figure 1. STM image (100 · 100 nm) of a bare SiO2 thin film prepared using the methods described in the text. The tunneling parameters are: Us ¼ )1.7 V and I ¼ 0.18 nA. Wide terraces with low point defect densities can be seen under these conditions.
Figure 2. LEED pattern obtained from a sample such as that shown in figure 1. A sharp c(2 · 2) array can be seen. The sharpness of the lattice spots is a strong indicator of both the long range and short range (point defect density) order of the thin film.
During the last decade following the pioneering work of Haruta and coworkers, numerous studies of Au as a catalyst have been reported [35–37]. In their studies, Haruta and coworkers found that small Au clusters (
