Iridium Clusters in NaX Zeolite Cages: Synthesis, Reactivity, and Characterization by Infrared and Extended x-ray Absorption Fine Structure Spectroscopies

Abstract

Ir(CO)~(acac)] in the pores of NaX zeolite was treated in CO and converted into indium carbonyl clusters, which are suggested on the basis of infrared spectra and comparisons with solution chemistry to be [HIr4(CO), I]-, which was subsequently converted into clusters that are suggested to be [Ir6(C0)15I2-. The former clusters in the NaX zeolite were decarbonylated by treatment in He followed by H2 at 573 K and characterized by extended X-ray absorption fine structure spectroscopy. The Ir-Ir first-shell coordination number (3.0) indicates that the clusters were predominantly Ir4 tetrahedra. The clusters postulated to be [Ir6(C0)15]2-were reversibly fragmented and reformed in the cages.

Low-cost, heated, and/or cooled flow-through cell for transmission x-ray absorption spectroscopy

“…X-ray absorption spectroscopy ͑XAS͒ is increasingly used to characterize the dispersed catalytically active species; 1 combined with transmission electron microscopy and chemisorption measurements, XAS has provided some of the most detailed characterizations of structures, such as metal clusters and particles in catalysts. [2][3][4][5] Because catalysts are reactive materials, they often must be characterized in the absence of air and other contaminants, and it is useful to characterize them after treatments in atmospheres representative of those encountered during their operation. Thus, XAS cells have been designed for flow of reactants, as well as evacuation, and for heating to high temperatures typical of catalytic processes ͑e.g., 500°C͒ and cooling to nearly liquid nitrogen temperature for optimum XAS measurements.…”

Impurity Atoms on Small Transition Metal Clusters. Insights from Density Functional Model Studies

“…In particular, average M-M distances for Ir 4 (tetrahedral) and Rh 6 (triangular prism-the most stable structure) were calculated *20 pm shorter than the corresponding experimental values, 266-272 pm for iridium [17,18,28] and 267-269 pm for rhodium [22]. Adsorption of the clusters on a zeolite support essentially did not affect the average metal-metal distances: for Ir 4 it remained unchanged and for Rh 6 it increased by 2 pm only [33,38,47].…”

“…This ''soft-landing'' approach [14,15] can be applied to open surfaces. Over the years, Gates and co-workers [16][17][18][19][20][21][22] developed an alternative, chemical procedure to prepare metal clusters both on open surfaces and inside the pores of the support, e.g. in zeolites.…”

“…We modeled in particular the influence of atomic adsorbates on the geometric and electronic structure of the clusters. A particular goal of these investigations was to clarify the difference between experimental EXAFS distances [17,18,22] and the optimized interatomic distances in bare Ir 4 and Rh 6 clusters (both in the gas phase and on the zeolite support) [33,38,46,47]. Since in all cases the values derived from EXAFS were notably longer, up to 20 pm, than the calculated ones and the computational method was confirmed to perform accurately for similar systems (see below and [33,38,47]), we considered the possible presence of impurity atoms on the experimentally prepared metal clusters.…”

Computational Modelling of Nanoporous Materials

“…Focusing on a concrete experimental problem for the structural features of Ir 4 cluster in faujasite, 95 the successive adsorption of H 2 from the gas phase on supported Ir 4 clusters was simulated. Two states of the support were considered, hydroxylated and dehydroxylated zeolite fragment.…”