Surface metal sites

Key points: Very small metal particles on ceramic oxide substrates are very active catalysts for a range of reactions. Metal particles are often deposited on supports to provide a catalyst. For example, finely divided Pt/Re alloys distributed on the surface of γ-alumina particles are used to inter convert hydrocarbons, and finely divided Pt/Rh rhodium alloy particles supported on γ-alumina are used in the catalytic converters of vehicles to promote the combination of O₂ with CO and hydrocarbons to form CO₂ and reduction of nitrogen oxides to nitrogen (Box 26.1). A supported metal particle about 2.5 nm in diameter has about 40 per cent of its atoms on the surface, and the particles are protected from fusing together into bulk metal by their separation. The high proportion of exposed atoms is a great advantage for these small supported particles, particularly for metals such as platinum and the even more expensive rhodium. The metal atoms on the surface of metal clusters are capable of forming bonds such as M-CO, M-CH₂R, M-H, and M-O (Table 26.3). Often the nature of surface ligands is inferred by comparison of IR spectra with those of organometallic or inorganic complexes. Thus, both terminal and bridging CO groups can be identified on surfaces by IR spectroscopy, and the IR spectra of many hydrocarbon ligands on surfaces are similar to those of discrete organometallic complexes. The case of the N₂ ligand is an interesting contrast because coordinated N₂ was identified by IR spectroscopy on metal surfaces before dinitrogen complexes had been prepared.

The development of new techniques for studying single-crystal surfaces has greatly expanded our knowledge of the surface species that may be present in catalysis. For example, desorption of molecules from surfaces (thermally or by ion or atom impact) combined with mass spectrometric analysis of the desorbed substance provides insight into the chemical identity of surface species. Similarly, Auger and X-ray photoelectron spectroscopy (XPS, Section 8.8) provide information on the elemental composition of surfaces. Low-energy electron diffraction (LEED) provides information about the structure of single-crystal sur faces and, when adsorbate molecules are present, their arrangement on the surface. One important finding from LEED is that the adsorption of small molecules on a surface may bring about a structural modification of the surface. This surface reconstruction is often observed to reverse when desorption occurs. Scanning tunnelling microscopy (STM, Section 25.3a) provides an unrivalled method for locating adsorbates on surfaces. This striking technique provides a contour map of single-crystal surfaces at or close to atomic resolution. Although most of these modern surface techniques cannot be applied to the study of supported multiphasic catalysts, they are very helpful for revealing the range of probable surface species and circumscribing the structures that may plausibly be invoked in a mechanism of heterogeneous catalysis. The application of these techniques to heterogeneous catalysis is similar to the use of X-ray diffraction and spectroscopy for the characterization of organometallic homogeneous catalyst precursors and model compounds.

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