Understanding silica-supported metal catalysts: Pd/silica as a case study
Introduction
The study of metal particles on oxide supports is of importance in heterogeneous catalysis because the size and nature of the interaction of a metal particle with an oxide support are critical in determining catalytic activity and selectivity [1], [2], [3]. It is well-known that metals on reducible oxides such as TiO2 [4], [5] exhibit a strong metal–support interaction (SMSI). On the other hand, irreducible oxides like SiO2 are assumed to be relatively inert. However, in certain cases, silica has been shown to exhibit a metal–support interaction following a high temperature treatment [6], [7], [8].
Oxidation and reduction at elevated temperatures are essential steps for the preparation of supported, high surface area catalysts; however, these treatments can cause morphological changes of the dispersed metal particles arising from sintering and/or metal–support interactions. Therefore, it is of considerable importance to investigate and define optimal conditions for catalyst preparation, pretreatment and activation [9]. Depending on the particular metal–oxide system, various morphological changes resulting from a metal–support interaction have been reported, namely sintering [10], [11], [12], [13], [14], encapsulation [15], [16], inter-diffusion [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], and alloy formation [28], [29]. In particular, silicide formation from metals supported on silica has received considerable attention because of the importance of the metal–silica interface to numerous technologies. For example, studies of metal–oxide–semiconductor (MOS) structures are directly related to many aspects of semiconductor technology including the design of MOS devices. In addition, metallization is important for creating contact layers and durable electrically conducting vias on insulating substrates for semiconductor devices [30], [31]. Furthermore, silicide formation between metals and SiO2 in a catalyst has been shown to alter catalytic activity and selectivity [32], [33]. For instance, it has been shown that Pd-silicide, formed during the high temperature reduction of Pd/SiO2, dramatically increases selectivity for the isomerization of neopentane [33].
However, in spite of the numerous studies on metals supported on SiO2 at elevated temperatures, there are still controversial and unresolved issues regarding the nature of the metal–support interaction in silica-supported catalysts, namely:
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the nature of the metal–support interaction between metals and SiO2;
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the morphological changes that occur during the high temperature reduction of metals supported on SiO2;
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the role of oxygen vacancies in the inter-diffusion of metals into SiO2;
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the extent to which silicides are formed by the direct interaction between metals and SiO2;
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the role of the silicon substrate, frequently used to prepare SiO2 thin films, in metal silicide formation;
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the composition, if formed, of metal silicides;
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the mechanism of silicide formation between metals and SiO2.
In the first section of this review, the effect of high temperature reduction of technical catalysts consisting of metals supported on high surface area SiO2 will be discussed. In the second section, related results obtained for model catalyst systems will be described. Finally, recent studies from our laboratories for model SiO2-supported Pd catalysts, consisting of SiO2 thin films prepared on a Mo substrate, will be addressed and serve to illustrate how this particular model preparation circumvents the complications frequently encountered when using a silicon substrate to synthesize a thin film silica support.
Section snippets
Experimental
Details of the ultrahigh vacuum chamber, equipped with scanning tunneling microscope (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) with a base pressure of 5×10−10 mbar, have been published elsewhere [34]. Briefly, this apparatus is equipped with a double-pass cylindrical mirror analyzer, reverse view LEED optics, and a room temperature STM (Omicron). Typically, the STM images were acquired in the constant current mode
High surface area, silica-supported metal catalysts
Reduction at elevated temperature is one of the important steps in the synthesis of supported metal catalysts; however, heating a supported metal catalyst can cause morphological changes in the metal particles depending upon the particular metal–oxide system. Chang et al. [9] have shown that various metal–support interactions are operative for Pd catalysts on various supports, e.g. SiO2, Al2O3, and TiO2, using a combination of temperature-programmed reduction and adsorption methods. Hydrogen
Conclusions
It has been shown that metals supported on SiO2 exhibit a metal–support interaction that varies from weak to strong, depending upon the metal, after high temperature treatments. A strong metal–support interaction, manifested as encapsulation, inter-diffusion, and alloying, can alter the catalytic property significantly. In general, sintering is favored in those systems that show a weak metal–support interaction whereas encapsulation or/and inter-diffusion and alloy formation occurs for the
Acknowledgements
The authors acknowledge with pleasure the support of this work by the Department of Energy, Office of Basic Energy Science, and Division of Chemical Science, and the Robert A. Welch Foundation.
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Present address: Halliburton Energy Services, Duncan, OK, USA.