Iridium Atoms Bonded to Crystalline Powder MgO: Characterization by Imaging and Spectroscopy

Abstract

Samples with low loadings of metals on well-defined supports provide some of the best opportunities to determine the metal–support structure and bonding. We illustrate methods for characterizing atomically dispersed heavy metals on metal oxide supports by aberration-corrected scanning transmission electron microscopy (STEM) complemented by fluorescence detection extended X-ray absorption fine structure and infrared spectroscopies. STEM images of Ir atoms derived from Ir­(C2H4)2(acac) (acac = acetylacetonato) on high-surface-area MgO powder were obtained with minimized electron beam damage by quickly recording images near where the focus had been established. The images show that iridium at a loading of 1.0 wt % on MgO calcined at 1073 K was atomically dispersed, populating much of the surface of the MgO particles, which had irregular shapesconsequently the Ir atoms were bonded at various sites to two or three surface O atoms. In contrast, MgO calcined at 1273 K consisted of almost perfectly cubic crystals, and Ir atoms at a loading of only 0.01 wt % on this nearly ideal support were anchored preferentially at edges and corners of the (100) faces and bonded to three surface O atoms. The latter results indicate a path forward for the determination of precise structures of atomically dispersed metals on crystalline metal oxide supports.

Isostructural Atomically Dispersed Rhodium Catalysts Supported on SAPO-37 and on HY Zeolite

“…Oven-dried sample tubes capped with a Transeal cap (Micromeritics) were tared in the glovebox and then filled with approximately 50 mg of powder. Details of the experimental methods are as reported . These data are presented for calibration purposes only; we recognize that they do not provide accurate values of internal surface areas for materials such as ours (Table S1 in the SI).…”

“…Details of the experimental methods are as reported. 37 These data are presented for calibration purposes only; we recognize…”

Supported Metal Pair-Site Catalysts

“…Thus, high- Z atoms ( Z is atomic number) can be readily observed on thin, light-element supports with relatively straightforward interpretability. Although single-atomic catalytic species have been investigated extensively with this technique, there are only a few examples pertaining to dinuclear species. ,,,, …”

Bruce Gates: A Career in Catalysis

“…Gates et al. later extended the investigation, showing that treatment of MgO at 1273 K facilitates more precise identification of surface sites because of the enhanced crystallinity of the support …”

“…Gates et al later extended the investigation, showing that treatment of MgO at 1273 K facilitates more precise identification of surface sites because of the enhanced crystallinity of the support. 73 Through collaboration with teams at Minnesota and Northwestern, the Gates group extended the work on dispersed supported metal complexes by using MOFs as supports. 74 They introduced sites with precisely defined structures by tuning MOF surface ligands and the density of MOF defects by postsynthetic treatments.…”