Generation and characterization of rhodium carbonyls formed on silica and alumina under mild conditions

• Published in: Journal of catalysis 1988-03, Vol.110 (1), p.96-102
• Main Authors: Keyes, Michael P., Watters, Kenneth L.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts: preparations and properties; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; General. Nomenclature, chemical documentation, computer chemistry; Theory of reactions, general kinetics; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/0021-9517(88)90300-4

The effect of the metal precursor on the surface structure of rhodium supported on SiO 2 and Al 2O 3 and treated under very mild conditions was examined. For both SiO 2 and Al 2O 3 supports, the mild conditions used produced samples with properties different from those that have heretofore been reported. Rh(I) on SiO 2 was obtained by sublimation of [Rh(CO) 2Cl] 2 and by mild reduction under CO of RhCl 3 SiO 2 and of ( RhNO 3) 3 SiO 2 . In each case, three v(CO) IR bands (two strong and one weak) were obtained with frequency and intensity patterns that closely matched those for molecular dirhodium complexes such as [Rh(CO) 2Cl] 2 and [Rh(CO) 2OSiMe 3] 2, indicating a dinuclear structure on the SiO 2 surface. Solvent extraction results led to the conclusion that if a dicarbonyl species was generated with ( RhNO 3) 3 SiO 2 it was in the form [Rh(CO 2O s] 2 (where O s is a surface oxide), while direct sublimation of [Rh(CO) 2Cl] 2 onto SiO 2 and reduction of RhCl 3 SiO 2 both formed physisorbed [Rh(CO) 2Cl] 2 that could be extracted readily from the surface into pentane. The [Rh(CO) 2Cl] 2 absorbed on pretreated Al 2O 3 behaved differently. Three v(CO) IR bands initially evident (two strong and one weak) displayed a pattern different from that for the parent dirhodium complex. The weak band vanished within an hour while the two strong bands remained. The remaining bands were indistinguishable from the v(CO) bands reported for monomeric surface Rh i(CO) 2 species derived from RhCl 3 Al 2O 3 and Rh(NO 3) 3 Al 2O 3 , while the short-lived species identified with the weak band is proposed to be a monocarbonyl of Rh(I). Pretreatment of the Al 2O 3 at 800 °C produced a sample with shifted v(CO) bands and color much different from that for other samples studied. The formation of the proposed monocarbonyl and the different behavior after 800 °C pretreatment are discussed in terms of the role of Cl and Lewis acid sites.