Magnesia-Supported Tetrairidium Clusters Derived from [Ir4(CO)12]

Abstract

The chemistry of iridium clusters formed from [Ir4(CO)12] on basic MgO surfaces was investigated with infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The chemistry of iridium carbonyls on MgO is comparable to that of iridium carbonyls in basic solutions. The adsorption of [Ir4 (CO)12] on MgO with high, intermediate, and low surface hydroxyl group concentrations led to the formation of structures that are suggested on the basis of infrared spectra to be [Ir4(CO)llCOH{O}], [HIr4(CO)11]-, and [Ir8(C0)22l2-, respectively, where {0} represents oxygen of the MgO surface. Decarbonylation of the iridium carbonyl clusters on MgO led to the formation of iridium clusters. The reaction of [Ir4(CO)12] with highly dehydroxylated MgO followed by treatment in flowing H e at 325 OC for 2 h and flowing H2 at 300 'C for 2 h at 1 atm led to formation of clusters that are modeled on the basis of EXAFS spectra as predominantly Ir4 tetrahedra. The reaction of [Ir4(CO)12] with highly hydroxylated MgO followed by treatment in flowing H e at 325 OC for 2 h and flowing H2 at 300 OC for 2 h at 1 atm led to the formation of larger iridium clusters, with average diameters of about 12 A. Treatment of the Ir clusters on MgO in D2 saturated with D2O led to the formation of Ir particles > 100 8, in diameter. The EXAFS data also provide evidence of the structure of the metal-metal oxide interface.The results suggest how variations of metal-support interactions resulting from changes in surface O H group concentration can be used to prepare supported metal clusters of controlled nuclearity.

Near Absence of Support Effects in Toluene Hydrogenation Catalyzed by MgO-Supported Iridium Clusters

“…The observations are in qualitative agreement with numerous reports (e.g., 30,31) showing that increasing surface hydroxylation and water increase rates of aggregation or sintering of supported noble metals.…”

Supported Ir and Pt Clusters: Reactivity with Oxygen Investigated by Extended X-ray Absorption Fine Structure Spectroscopy

“…Water in the form of steam or surface hydroxyl groups has been observed to facilitate sintering of supported metals. 33,34 The EXAFS data give evidence of Pt-Pt interactions, indicating the presence of 7 Å Pt clusters on average in the sample exposed to air for 15 min. The sample had initially contained approximately 10 Å clusters on average.…”

Probing Metal Oxide Surface Reactivity with Adsorbate Organometallic Chemistry: Formation of Iridium Carbonyl Clusters on La2O3

“…La 2 O 3 -supported iridium carbonyl clusters were prepared on a Schlenk vacuum line and in a Braun glovebox with O 2 and moisture contents each <1 ppm. Samples containing 1 wt % Ir were prepared by adsorption of [Ir 4 (CO) 12 ] from a pentane slurry onto the metal oxide; details are as given elsewhere. ,

1 Infrared spectra of La 2 O 3 : (A) sample as received; (B) sample after treatment under vacuum at 400 °C for 4 h; (C) sample after treatment in flowing N 2 at 800 °C for 4 h.

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“…Extraction of Surface Organometallic Species. The surface organometallic species were extracted from La 2 O 3 by bringing the sample in contact with a solution of [bis(triphenylphosphine)nitrogen(1+) chloride], [PPN][Cl], in dry THF …”

“…Infrared Spectroscopy. Spectra were measured with a Nicolet 510M FTIR spectrometer, as reported elsewhere. − The freshly supported iridium carbonyl on La 2 O 3 was treated in an infrared cell (or in a glass reactor) in flowing CO, at progressively higher temperatures ranging from room temperature to 200 °C.…”