Zeolite-supported transition metal catalysts by design

✍ Scribed by W.M.H. Sachtler

• Publisher: Elsevier Science
• Year: 1992
• Tongue: English
• Weight: 577 KB
• Volume: 15
• Category: Article
• ISSN: 0920-5861
• DOI: 10.1016/0920-5861(92)85007-9

✦ Synopsis

Zeolite supported metal catalysts are prepared by ion exchange or incipient wetness impregnation, or by deposition of metal complexes, followed by calcination and reduction. Hydrogen reduction of encaged transition metal ions yields metal clusters or isolated atoms and protons of high Brlansted acidity. The elementary steps in the genesis of mono-and bimetallic metal clusters have been unravelled by a combination of dynamic and spectroscopic methods. Small metal clusters can be anchored to zeolite cage walls by transition metal ions or protons; however, adsorption of a CO which can displace the protons from the metal, initiates migration of the primary carbonyl clusters leading to their coalescence. At low temperature, this process is limited by the geometry of the metal core vis-a-vis the cage window. Protons of high Brsnsted acidity can reoxidize metals at elevated temperature, even selectively "leach' the less noble metal atoms out of bimetal clusters. Large metal particles, present in zeolite voids or at the external surface, can be transformed into small particles, exposing virtually all their metal atoms, by either of two novel in situ techniques which are based on oxidative dispersion with oxygen or chlorine gas.