J. Phys. Chem. 1993,97,5375-5383
5375
Tetrairidium Clusters Supported on y-Al203: Formation from [Ir4(CO)12] and CO-Induced Morphology Changes S.Kawi, J.-R. h a g , and B. C. Gates' Center for Catalytic Science and Technology. Department of Chemical Engineering, University of Delaware, Newark, Delaware I9716 Received: December 8, 1992; In Final Form: February 23, I993
[Ir4(CO)12] was prepared on the surface of y-Al203 powder by a surface-mediated synthesis from supported [Ir(CO)z(acac)] in the presence of CO. Supported iridium clusters with nearly unique structures were prepared by decarbonylation of the iridium carbonyl clusters at 300 OC in He. The cluster formation and decarbonylation were characterized by infrared and extended X-ray absorption fine structure spectroscopies. As the sample was heated in He to about 75 OC, the infrared spectrum changed to resemble that of [HIr4(CO)11]-, which could be extracted from the surface by cation metathesis. The decarbonylation of this cluster took place without changes in the frequencies of the infrared bands in the carbonyl region, consistent with the suggestion that no other iridium carbonyls formed. The decarbonylated clusters in the presence of He had an average Ir-Ir coordination number of 2.9 with a bond distance of 2.69 A, indicating that the tetrahedral metal frame of the precursor cluster carbonyl had been largely retained after decarbonylation. Thus, the supported cluster is represented as Ir4 with a tetrahedral structure and is inferred to be one of the simplest and most uniform supported metals. When this sample was recarbonylated at 125 OC,neither [Ir4(C0)12] nor [HIr4(CO), '1was reformed. Instead, the recarbonylated clusters had an average Ir-Ir coordination number of 4.1 with an average bond distance of 2.70 A, indicating cluster agglomeration.
Introduction
Experimental Section
Microstructures dispersed on surfaces of supports (substrates) range from metals in supported catalystsl-5 to semiconductorsin potential electronic and optical materials?,' Most of these materialsare structurallynonuniformand difficult to characterize. Understanding of their chemistry and performance will be advanced by the development of methods for synthesis and characterization of discrete, structurally uniform microstructures on supports.8 Metal carbonyl clusters have been used frequently in attempts to prepare uniform metal clusters on supports, but most of the attempts have been unsuccessful because decarbonylation of the clusters typically gives fragmented and/or aggregated clusters.9 Most progress has been made by decarbonylation of relatively robust metal cluster carbonyl anions, which have been suggested to give Oslo on MgO,'O Ptl, on MgO," Ir4 on MgO,I2J3Ir6 on MgO,I33l4and Ir4in NaX ze01ite.I~Decarbonylation of neutral metal carbonyl clusters in zeolite cages has been suggested to The give Ir4 in NaY zeolite,4J6 and Ir6 in NaY decarbonylated clusters have been characterized by extended X-ray absorption fine structure (EXAFS) Spectroscopy. The cluster structures are less than fully defined, and more than one structure has typically been found; for example, Ir4on MgO has been modeled as a mixture of Ir4 tetrahedra and Ir4 rafts.13 The goal of the research summarized here was to prepare structurally uniform supported metal clusters. We have used a neutral precursor, [Ir4(CO)Iz]on 7-A1203,and characterized its decarbonylation and recarbonylation. Infrared and EXAFS spectroscopieswere used to Characterize the surface species. The results are consistent with the retention of the tetrahedral Ir4 frameupon decarbonylationof [Ir4(CO)1z];the result is evidently the most nearly uniform supported metal on an amorphous support. However, recarbonylation markedly disrupts the structure.
Materialsand Sample Preparation. Samples were prepared in a Braun MB-150M drybox purged with N2that recirculated through 02- and moisture-scavenging traps or on a Schlenk vacuum line that was purged with NZ(Matheson, 99.999%). The drybox was equipped with 02 and moisture detectors and consistently kept the concentrations of these contaminants
