Cationic Tri-Rhenium Rafts on gamma-Alumina: Characterization by X-ray Absorption Spectroscopy

Abstract

Rhenium surface species, derived by treating [H,Re,(CO),,] adsorbed on y-A1203 in hydrogen at 400 'C, formed extremely small surface grouping of rhenium atoms having an average Re-Re coordination number of 2, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Results of X-ray photoelectron spectroscopy and threshold resonance experiments show that the rhenium was cationic, with oxidation states of about +4 to +6. Infrared spectroscopy was used to follow the decomposition of surface species derived from [H3Re3(C0)12a]d sorbed on y-A1203,t he data indicating that Re subcarbonyls such as [Re(CO)3(O-AI)2(HO-Al)] were formed as intermediates. On the basis of the Re-O and Re-Re coordination parameters determined by EXAFS spectroscopy, a structural model of cationic trirhenium rafts on the y-A1203 surface is suggested. This structure is characterized by a Re-Re distance of 2.67 A, which is greater than the distances characterizing the quadruple and triple Re bonds (2.3 A) in Re complexes but shorter than the distance in bulk metallic Re (2.74 A). The Re-Re distance suggests an intermediate bond order, roughly 1.5. The trirhenium rafts are suggested to be tethered to surface oxygens with a Re-0 distance of 2.05 A indicated by the EXAFS data. A longer Re-O distance, 2.58 A, is also indicated by the EXAFS data, and this is suggested to be indicative of interactions of the rhenium with surface hydroxyl groups. Another y-Al2O3-supported Re sample was prepared by a conventional method with aqueous NH4ReOr as the precursor, with treatment in hydrogen at 450 OC. The resulting surface species were highly nonuniform, consisting of surface rhenium oxide species and metallic rhenium particles.