Strong metal-support interactions properties

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the Schwab effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-121) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

Titania-supported Metals. - After reduction at 473 K, platinum-group metals supported on Ti02 chemisorbed both hydrogen and carbon monoxide in quantities indicative of moderate-to-high dispersion, but following reduction at 773 K chemisorption was drastically lowered e.g., H/Mt <0.01 for Pt, Ir, and Rh, 0.05-0.06 for Pd and Ru, and 0.11 for Os). Agglomeration, encapsulation, and impurities were eliminated as possible causes and a strong metal-support interaction (SMSI) was proposed. Titania is not unique in its SMSI properties and 11 oxides used to support iridium were classified as follows ... 

One of the main aspeets that determine the properties of a given catalyst is the nature of the interaction between the oxide support and the dispersed active metal. The influence of this interaction on the activity, selectivity and stability of the catalyst is determined by factors such as the preparation method, the atmosphere and temperature of the calcination and reduction stages (1,2), and the specific metal-support system studied. The latter is especially important for catalysts based on transition metals supported on partially reducible oxides (3). Such catalysts displaying strong metal-support interaction (SMSI) exhibit suppression of chemisorption of H2 and CO (4-6) when reduced at temperatures up to 773 K. 

We have presented evidence that the strong metal-support interactions observed with titania and niobia are due to an oxide layer over the metal catalyst. This layer interacts chemically with the metal, as evidenced by the fact that the titania layers on Pt, Rh, and Pd do have slightly different properties. The fact that the methanation rates for a titania-covered and a niobia-covered Pt foil are identical indicates that the reason for enhanced methanation activity on the oxide-covered surface is likely due to geometric and not electronic cons id er at ions. 

Equilibrium and kinetic calculations, based on bulk properties, support the view that strong metal-support interactions in Pt/Ti02 catalysts arise from the decoration of platinum crystallites with TiOx species. 

A number of investigations have indicated that so-called "strong metal-support interactions (SMSI)" are caused by the migration of partially reduced oxide species onto the surface of titania supported metal particles (1 8). While this is an attractive theory in that it can account for the observed modifications in chemical properties of the metal particles, there is no agreed mechanism by which the postulated transport processes occur. 

This paper discusses transition metal intermetallic compounds, in the context of the reactivity and physical properties expected for materials produced via solid-solid reactions at the metal catalyst oxide-support interface. It is shown that several observable and proposed features of the so called Strong Metal-Support Interaction (SMSI) — chemisorption activity, phase segregation, and encapsulation — follow naturally from the chemistry of these materials. Both literature precedent and experimental data are presented to support the close relationship suggested above. 

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