Strong metal-support interactions spectroscopy

The structure of supported rhodium catalysts has been the subject of intensive research during the last decade. Rhodium is the component of the automotive exhaust catalyst (the three-way catalyst) responsible for the reduction of NO by CO [1], In addition, it exhibits a number of fundamentally interesting phenomena, such as strong metal-support interaction after high temperature treatment in hydrogen [21, and particle disintegration under carbon monoxide [3]. In this section we illustrate how techniques such as XPS, STMS, EXAFS, TEM and infrared spectroscopy have led to a fairly detailed understanding of supported rhodium catalysts. 

Abbreviations BCC. body centered cubic DOS. density of states ESR. electron spin resonance HX.AI S, extended X-ray absorption fine structure F CC. face centered cubic (a crystal structure). FID, free induction decay FT, Fourier transform FWHM, full width at half maximum HCP, hexagonal close packed HOMO, highest occupied molecular orbital IR, Infrared or infrared spectroscopy LDOS, local density of states LUMO, lowest unoccupied molecular orbital MAS. magic angle spinning NMR. nuclear magnetic resonance PVP. poly(vinyl pyrrolidone) RF. Radiofrequency RT, room temperature SEDOR, spin echo double resonance Sf, sedor fraction SMSI, strong metal-support interaction TEM. transmission electron microscopy TOSS, total suppression of sidebands. 

It is also possible to simulate supported metal catalysts by the vapor deposition of metal on a flat surface of silica, alumina, etc. The particle size distribution can be closely controlled and the results verified by various electron spectroscopies, for example (SI). For the reverse situation of a flat metal surface decorated by oxide particles, one can simulate catalysts in the strong metal-support interaction state (32). 

Ti02 catalysts reduced at 700°K and is presently accepted as the key step in the induction of strong metal-support interaction (SMSI). The present observation appears to be the first case in which SMSI can be induced at normal (500 K) catalyst reduction temperatures. High resolution electron energy loss spectroscopy (EELS) experiments of CO on Ni/Mn0x reduced at 500°K showed dramatic reduction of the C-0 stretching frequency of adsorbed CO. These EELS results are discussed in light of... 

K. Tamura, U. Bardi, and Y. Nihei. An Investigation by Angular Resolved X-Ray Photoelectron Spectroscopy of Strong Metal-Support Interaction (SMSI) in the Pt/TiO, System. Surf. Sci. 197 L281 (1988). 

Bifunctional and Spillover Catalysis. - It has been suggested above that the active involvement of the support in a catalytic reaction could appear as a metal-support interaction, and several recent papers demonstrate in some detail how this can come about. Catalysis of the hydrogenation of CO2 by Rh on various supports shows a strong dependence of turnover number on the kind of support used (see Table 2). Dispersions, D, are similar, as are activation energies, so the considerable differences in turnover number reside principally in the pre-exponential factor. A further study of this system by i.r. spectroscopy has identified CO on the metal and the formate ion on the support (although not in the case of Si02), and the favoured mechanism for CH4 formation involves a reaction of spillover H with CO2 chemisorbed on the support by insertion into surface OH groups. 

---