Rhenium complexes and clusters supported on γ-Al2O3: Effects of rhenium oxidation state and rhenium cluster size on catalytic activity for n-butane hydrogenolysis

Supported metals prepared from H3Re3(CO)(12) on gamma-Al2O3 were treated under conditions that led to various rhenium structures on the support and were tested as catalysts for n-butane conversion in the presence of H-2 in a flow reactor at 533 K and 1 atm. After use, two samples were characterized by X-ray absorption edge positions of approximately 5.6 eV (relative to rhenium metal), indicating that the rhenium was cationic and essentially in the same average oxidation state in each. But the Re-Re coordination numbers found by extended X-ray absorption fine structure spectroscopy (2.2 and 5.1) show that the clusters in the two samples were significantly different in average nuclearity despite their indistinguishable rhenium oxidation states. Spectra of a third sample after catalysis indicate approximately Re-3 clusters, on average, and an edge position of 4.5 eV. Thus, two samples contained clusters approximated as Re-3 (on the basis of the Re-Re coordination number), on average, with different average rhenium oxidation states. The data allow resolution of the effects of rhenium oxidation state and cluster size, both of which affect the catalytic activity; larger clusters and a greater degree of reduction lead to increased activity. (C) 2009 Elsevier Inc. All rights reserved.