Cationic trirhenium rafts on gamma-Al2O3 : characterization by X-ray absorption spectroscopy

Re surface species, derived by treating H3Re3(CO)12 adsorbed on g-Al2O3 in H at 400 Deg, formed extremely small surface grouping of Re atoms having an av. Re-Re coordination no. of 2, as detd. by EXAFS spectroscopy. Results of XPS and threshold resonance expts. show that the Re was cationic, with oxidn. states of about +4 to +6. IR spectroscopy was used to follow the decompn. of surface species derived from H3Re3(CO)12 adsorbed on g-Al2O3, the data indicating that Re subcarbonyls such as [Re(CO)3{O-Al}] were formed as intermediates. On the basis of the Re-O and Re-Re coordination parameters detd. by EXAFS spectroscopy, a structural model of cationic Re3 rafts on the g-Al2O3 surface is suggested. This structure is characterized by an Re-Re distance of 2.67 .ANG., which is greater than the distances characterizing the quadruple and triple Re bonds (2.3 .ANG.) in Re complexes but shorter than the distance in bulk metallic Re (2.74 .ANG.). The Re-Re distance suggests an intermediate bond order of .apprx.1.5. The Re3 rafts are tethered to surface O with an Re-O distance of 2.05 .ANG. indicated by the EXAFS data. A longer Re-O distance, 2.58 .ANG., is also indicated by the EXAFS data and is indicative of interactions of the Re with surface OH groups. Another g-Al2O3-supported Re sample was prepd. by a conventional method with aq. NH4ReO4 as the precursor, with treatment in H at 450 Deg. The resulting surface species were highly nonuniform, consisting of surface Re oxide species and metallic Re particles. [on SciFinder (R)