Chloride-induced migration of supported platinum and palladium across phase boundaries

• Published in: Catalysis letters 2000-01, Vol.66 (4), p.189-195
• Main Authors: LEBEDEVA, O. E, CHIOU, W.-A, SACHTLER, W. M. H
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Ammonium chloride; Catalysis; Catalysts: preparations and properties; Chemistry; Chloride; Chloride ions; Exact sciences and technology; Ferric chloride; General and physical chemistry; High temperature; Iron chlorides; Metal particles; Migration; Organic chemistry; Palladium; Platinum; Pretreatment; Reduction; Silicon dioxide; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Transition metals; Volatile compounds; Zeolites
• ISSN: 1011-372X; 1572-879X
• DOI: 10.1023/A:1019040816115

Previous work showed that calcination in O2 of physical mixtures of Fe2O3 and supported Pt leads to a strong reduction enhancement of the Fe2O3, but that a much smaller effect was observed with supported Pd. The present results show that a strong reduction enhancement could be achieved by pretreating Pt/Al2O3 or Pd/Al2O3 with NH4Cl and then decomposing NH4Cl, before mixing the solid with Fe2O3. Such pretreatment with NH4Cl has no effect on SiO2 or zeolite‐supported metals, because only Al2O3 retains chloride ions at its surface. In the physical mixtures, chlorides migrate from Al2O3 to Fe2O3 at elevated temperature and form a volatile compound, presumably FeCl3. Layered‐bed experiments show that this FeCl3 sublimes, and that its chemical interaction with Pt or Pd on any support results in the formation of mobile Pt– or Pd–chloro complexes that reach Fe2O3 particles by surface migration. After exposure to an H2 flow, the complexes are reduced, and Pt or Pd particles are formed on the Fe2O3, enhancing its reduction by H spillover. These metal particles on the Fe2O3 have been identified by TEM and X‐ray energy dispersive spectroscopy (EDS). Abundant formation of PdFe alloys upon reduction is verified by TPR/TPD, indicating that almost all Pd has interacted with the volatile Fe chloride. In the absence of a transition metal, chloride ions retard the reduction of Fe2O3.