Metal-support effects

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. 

Another way of viewing these metal-support effects described above is in terms of a site I effective cage polarizability which... 

The electronic structure of a metal particle may be affected by perturbations in its environment (gases, carrier, neighboring supported material) and is larger for smaller particles. In the case of supported catalysts the latter effect is termed the support effect. Edge spectroscopy is a sensitive tool for detecting charge transfer between metal and support or adsorbates. Pure metal-support effects are difficult to observe, since these are frequently perturbed by size and electronic effects. 

Vannice MA, Garten RL (1979) Metal-support effects on the activity and selectivity of Ni catalysts in CO/H2 synthesis reactions. J Catal 56 236... 

Effects ci support Principles associated with metal-support effects and their relationships to sintoing and redispersion phenomena are addressed by Bartholomew. An important and peiht obvious concept in regard to studies of model catalysts is that investigatitni of model catalysts, because of thdr lack rtf porosity and surface roughness, provides a more definitive measure of the relative strmgths of metal-suf rt interactions than study conventional supported metals. 

Table 7 Metal-support effects in CO-H2 synthesis reactions ...

In order to minimise possibilities for spurious metal-support effects associated in the literature with use of RliCh as precursor of ceria-dispersed rhodium [5], the materials used in the present study were prepared using rhodium(in) acetylacetonate (Aldrich) as precursor. Wet impregnation at 0.5, 2 or 4 wt% onto the surfaces of the oxide powders was achieved from solution in high purity methanol or tetrahydrofuran, after which samples were dried and calcined in O2 for 2 hr at 823 K. Ceria available from Aldrich (Ce02(A), 16 m ) or Rhone-Poulenc (Ce02(r.p.),110 m g ), and AI2O3 (Conte, 200 m g ) were used as supports. 

It is clear that metal-support effects play a critical role in determining how water will affeet the performance parameters of supported cobalt catalysts when co-fed H2O is added. 

M. Baibarac, L. Mihut, G. Louam, S. Lefrant, and I. Baltog, Doping and metallic-support effect evidenced on SERS spectra of polyaniUne thin films, J. Polym. Set, Part B Polym. Phys., 38, 2599-2609 (2000). 

Metal-Support Effects and Promotion Relation to Catalyst Synthesis... 

A variety of metal-support effects can occur to alter the adsorptive and/or catalytic behavior of a metal surface, and these include 1) Incomplete reduction of the metal 2) Support-induced cluster size 3) Epitaxial growth 4) Particle morphology 5) Contamination by the support 6) Bifunctional catalysis 7) Spillover and porthole phenomena and 8) Charge transfer between a metal and a semiconductor [2]. In addition, one might cite the stabilization of extremely small (1-3 atom) metal clusters on a support [7]. 

Also, there is one additional type of metal-support effect that was originally termed SMSI (Strong Metal-Support Interactions) by the researchers at Exxon, where it was discovered [8], and its presence using a reducible oxide support was demonstrated by a marked decrease in H2 and CO chemisorption capacity, especially the former, with no increase in metal crystallite size, i.e., no decrease in dispersion [8]. This was subsequently shown to be primarily due to reduction of the support accompanied by its migration or the migration of one of its suboxides onto the metal surface, thus causing decreased chemisorption capacity due to physical blockage of... 

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