JOURNAL OF CATALYSIS 176, 310–320 (1998) ARTICLE NO. CA 982053 Iridium Clusters Supported on γ -Al 2 O 3 : Structural Characterization and Catalysis of Toluene Hydrogenation O. Alexeev and B. C. Gates Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Received August 12, 1997; revised February 9, 1998; accepted February 23, 1998 Iridium clusters were prepared on γ -Al 2 O 3 that had been cal- cined at 400 ◦ C. When supported [Ir 4 (CO) 12 ], the precursor, was treated in H 2 at various temperatures, it was decarbonylated, with aggregation taking place to various degrees to give a family of sup- ported clusters and aggregates with average diameters in the range of about 9 to 33 ˚ A, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Similarly, samples were de- carbonylated in He, and the degree of aggregation of the iridium was less than in H 2 undercomparable conditions. Infrared spectra show that decarbonylation of the supported [Ir 4 (CO) 12 ] in the pres- ence of H 2 was not reversible and was accompanied by aggregation of the iridium;infrared spectra of COadsorbed on the decarbony- lated samples were characterized by a band at 2058 cm −1 , typical of terminal COadsorbed on iridium particles. The γ -Al 2 O 3 -supported iridium clusters and aggregates were used to catalyze toluene hy- drogenation at 60 ◦ C and 1 atm. Samples with nearly the same Ir–Ir first-shell coordination number(nearly the same dispersion) were characterized by essentially the same catalytic activity, independent of whether the decarbonylation was carried out in He or H 2 . The catalytic activity for the structure-insensitive toluene hydrogena- tion reaction was found to increase with increasing average cluster or aggregate diameter and to be greater for iridium on the calcined γ -Al 2 O 3 than for iridium on uncalcined γ -Al 2 O 3 . c  1998 Academic Press INTRODUCTION Supported catalysts consisting of metal clusters smaller than about 10–20 ˚ A in diameter are difficult to characterize because of the smallness and nonuniformity of the clusters, and there is little information available providing relation- ships between cluster size and catalytic activity. Clusters as small as these are important in practical catalysts, e.g., platinum clusters consisting of about 5–10 atoms each, on average, in zeolite LTL (1–5) applied for selective dehydro- cyclization of naphtha to give aromatics (6–7). Supported clustersthat appear to have relativelyhigh degreesofstruc- turaluniformityhave been made bydecarbonylation ofsup- portedmetalcarbonylclusterprecursorssuchas[Ir 4 (CO) 12 ] (8). Continuing an investigation of supported clusters made from [Ir 4 (CO) 12 ], we report the preparation of a family of γ -Al 2 O 3 -supported iridium catalysts, their characteriza- tion byextended X-rayabsorption fine-structure (EXAFS) spectroscopy, and their catalytic activities for toluene hy- drogenation.The sampleswere prepared with γ -Al 2 O 3 that had been calcined at 400 ◦ C; the results complement those reported (9) for similar samples prepared with uncalcined γ -Al 2 O 3 . The data demonstrate a dependence of catalytic activity on cluster size, even for the structure-insensitive toluene hydrogenation,and an effect ofthe calcination tem- perature of the γ -Al 2 O 3 . EXPERIMENTAL METHODS M aterials [Ir 4 (CO) 12 ] (Strem) was used as supplied. n -Pentane sol- vent (Aldrich) was refluxed under N 2 in the presence of Na/benzophenone ketyl (to remove traces of water) and deoxygenated by sparging of dry N 2 . All gases were UHP grade and additionally purified by flow through traps con- taining pellets of Al 2 O 3 -supported Cu and activated zeolite to remove traces of O 2 and H 2 O, respectively. The γ -Al 2 O 3 support was prepared by first forming a paste of porous γ - Al 2 O 3 (Degussa, Aluminum Oxide C) and deionized wa- ter, followed by overnight drying at 120 ◦ C, calcination at 400 ◦ C in flowing O 2 (Matheson) for 2 h, and evacuation (10 −3 Torr) at 400 ◦ C for 14 h. The surface area of the resul- tant material was approximately 100 m 2 /g. Preparation and Decarbonylation of γ -A l 2 O 3 -Supported [Ir 4 (CO) 12 ] The preparation procedures are as described elsewhere (10). [Ir 4 (CO) 12 ]/γ -Al 2 O 3 was decarbonylated by treat- ment in flowing H 2 as the temperature was ramped at a rate of 3 ◦ C/min from 25 ◦ C to the desired temperature and then held at that temperature for 2 h. The sample containing supported iridium clusters that had aggregated was formed by treatment of [Ir 4 (CO) 12 ]/γ -Al 2 O 3 in H 2 at 400 ◦ C for 2 h, followed by treatment with O 2 at 300 ◦ C for 1 h and then reduction in H 2 at 400 ◦ C for 2 h. 0021-9517/98 $25.00 Copyright c  1998 by Academic Press All rights of reproduction in any form reserved. 310