Structure and morphology of vanadia-promoted Rh/SiO{sub 2}: A transmission electron microscopy study
- Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin (Germany)
- Universitaet Muenchen, Munich (Germany)
Vanadia-promoted Rh/SiO{sub 2}-catalysts have been prepared by impregnation followed by calcination at 573, 773, 973, and 1173 K. The structure and morphology of these materials in the oxidized state, and after low (523 K) and high (773 K) temperature reduction, were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), microdiffraction, X-ray diffraction (XRD), and CO chemisorption. During calcination at temperatures {ge}973 K, a RhVO{sub 4} phase is formed, which consists of well-crystallized rod-like particles after calcination at 1173 K. After reduction in H{sub 2}, the catalysts consist of highly dispersed Rh{degrees} particles with the promoter oxide V{sub 2}O{sub 3}. The well-crystallized sample (calcination at 1173 K) cannot be reduced at 523 K, but at 773 K the rhodium, originally present as RhVO{sub 4}, is quantitatively reduced to small metal particles in contact with V{sub 2}O{sub 3}. In contrast to the high dispersion derived from TEM, CO chemisorption gave unexpectedly low CO/Rh ratios, which were even zero for the catalyst calcined at 1173 K. The CO/Rh ratios decreased with increasing calcination temperature (RhVO{sub 4} formation) at constant reduction temperature and with increasing reduction temperature at a given calcination temperature. It is suggested that the surface of the highly dispersed Rh{degrees} particles is decorated and blocked by VO{sub x} species.
- OSTI ID:
- 110981
- Journal Information:
- Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 1 Vol. 149; ISSN 0021-9517; ISSN JCTLA5
- Country of Publication:
- United States
- Language:
- English
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