Strong metal-support interaction ceria

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]. 

Kalakkad, Datye, and coworkers—TEM indicates strong metal-support interactions in Pt/ceria and Pt/Ce/Al catalysts. Kalakkad et al 362,369 published TEM and probe reaction studies on 0.6%Pt/CeO2 catalysts relevant to ACC catalysis. The... 

Imamura, Kaito, and coworkers—metal-support effects observed after calcination. Imamura et al 9X reported a strong metal-support interaction between Rh and Ce02, whereby high surface area ceria calcined at low temperature (550 °C) was able to transport Rh particles to the bulk, as measured by XPS. They suggested that despite the low degree of exposure of the Rh particle at the surface, the exposed Rh was highly active for the methanol decomposition reaction. 

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. 

While some stability issues have been identified on the supported Au catalysts, a major progress was made by the discovery that Au/Ce02 showed remarkable performance stability.50 Among the catalysts with several Au loadings (Fig. 6.4), it was found that 3% Au showed the most stable operation over a 3-week period of tests due to an optimal ratio of surface Au and ceria sites. Evidence of strong metal-support interaction was correlated with the enhanced reducibility of ceria in the presence of Au nanoparticles. Further research into different catalyst preparation methods for Au/Ce02 showed that Au dispersion and the WGS activity are extremely sensitive to minor variations in the preparation procedure.51... 

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

The spillover of O creates an oxygen vacancy which, as noted above, is easily accommodated in the fluorite structure. The Ce ions that are formed will interact strongly with the metal. Several studies have demonstrated strong metal-support interaction behavior in metal/reduced ceria systems. This interaction presumably aids the spillover process. 

Many authors have shown that the support could play a role, not only in changing particle size but also in modifying adsorption properties of the metals. Ceria could stabilize ionic species of platinum leading to a strong metal-support interaction. Bera et al. have compared the behavior of Pt/Ce02 and Pt/Al203 in TW catalysis." The enhanced activity observed in several reachons (CO-I-O2, CO - - NO and HC -f O2, Table 1.10) has been attributed to the formation of new sites (-0 Ce" +-0 Pt"+-0 with = 2 or 4). Ceria-supported catalysts are more active than alumina ones for all the reactions. NO as an oxidant is more sensitive in nature to support than O2. Moreover, ceria is a better promoter for oxidation of CO and propane than that of methane. Whatever the oxidant (NO or O2), methane oxidation remains difficult with a modest promotion by ceria. 

The selective hydrogenation of a,P-unsaturated aldehydes is used as a probe reaction in studying the strong metal/support interaction (SMSI). Ceria is able to form oxygen vacancies and interme-tallic compounds after reduction treatment at relatively high temperatures. 

Cheekatamarla and Lane [62, 63] studied the effect of the presence of Ni or Pd in addition to Pt in the formulation of catalysts for the ATR of synthetic diesel. For both metals, a promotional effect with respect to catalytic activity and sulfur poisoning resistance was found when either alumina or ceria was used as the support. Surface analysis of these formulations suggests that the enhanced stability is due to strong metal-metal and metal-support interactions in the catalyst. 

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