Opportunities for Tailoring Catalytic Properties Through Metal-Support Interactions
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Opportunities for Tailoring Catalytic Properties Through Metal-Support Interactions M. Cargnello • P. Fornasiero • R. J. Gorte
Received: 16 July 2012 / Accepted: 21 July 2012 / Published online: 7 August 2012 Ó Springer Science+Business Media, LLC 2012
Abstract The oxides used as supports for metal catalysts can be used to modify the catalyst properties. In this paper, we discuss three relatively new ways for optimizing the oxide–metal interactions and show examples where these methods have been used to improve catalytic performance. Opportunities still exist for using each of these approaches to produce materials with improved catalytic performance. Keywords Heterogeneous catalysis Nanostructure Colloidal synthesis
1 Introduction Interactions between metal catalysts and oxide supports have been known to be important for many years. Although oxide supports are primarily used with metal catalysts to maintain metal dispersion, some early work by Schwab and coworkers [1] reported large changes for metal catalysts supported on semiconducting oxides and suggested that electron transfer between the oxide and the metal phases, similar to what occurs in Schottky barriers, was responsible for modifying the properties of the metal. Because electron M. Cargnello Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA P. Fornasiero Department of Chemical and Pharmaceutical Sciences, ICCOMCNR, Consortium INSTM, University of Trieste, 34127 Trieste, Italy R. J. Gorte (&) Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA e-mail: [email protected]
densities in metals are so much larger than electron densities in semiconducting oxides, such long-range electron transfer is no longer believed to be important, at least for particles larger than about 2 nm; however, there clearly are a number of ways that contact between an oxide and a metal can influence rates and selectivities for reactions, dramatically in some cases. Therefore, understanding and controlling oxide–metal interactions provides an opportunity for improved catalytic performance. For reducible oxides, such as ceria, the role of the support is at least partially to transfer oxygen to or from the metal. Evidence for this has been presented for ceria-supported metals in the water–gas-shift (WGS) [2], methanesteam-reforming [3], and CO2-reforming [4] reactions. Reduced ceria has been shown to be oxidized by steam or CO2 [4], with the oxidized ceria then being reduced by transfer of its oxygen to the metal [5]. Oxygen transfer appears to be important as well for methane combustion over Pd on ceria and other reducible supports [6, 7]. The most active phase for methane oxidation is PdO; however, PdO transforms to metallic Pd at temperatures between 650 and 850 °C. It has been reported that PdO can be stabilized in catalysts containing the rare earths La2O3, Pr2O3, and CeO2, with Pr2O3-promoted catalysts stabilizing PdO to 1,300 °C [8, 9]. Because these reactions require con
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