PNNL

Abstract

Catalysts consisting of isolated metal atoms on oxide supports have attracted wide attention because they offer unique catalytic properties, but their structures remain largely unknown because the metals are bonded at various, heterogeneous surface sites. Now, by using highly crystalline MgO as a support for metal sites made from a mononuclear organoiridium precursor and investigating the surface species with X-ray absorption spectroscopy, atomic resolution electron microscopy, and electronic structure theory, we have differentiated among the MgO surface sites for iridium bonding. The results demonstrate the contrasting structures and catalytic properties of samples, even including those incorporating iridium at loadings as low as 0.01 wt % and showing that the latter are nearly ideal in the sense of having almost all the Ir atoms at equivalent surface sites, with each Ir atom bonded to three oxygen atoms of the MgO surface. These supported molecular catalysts are modeled accurately with density functional theory. The results open the door to the precise synthesis of families of single-site catalysts. The work was supported by the U.S. Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences (BES), Grant DE-FG02-04ER15513 and the DOE BES Catalysis Center Program by a subcontract from Pacific Northwest National Laboratory (KC0301050-47319) to the University of Alabama. We thank Chevron for fellowship support of ASH and LMD. DAD thanks the Robert Ramsay Chair Fund of The University of Alabama for support. A portion of the research was performed using facilities of the Environmental Molecular Sciences Laboratory (Ringgold ID 130367), a DOE SC User Facility sponsored by the Office of Biological and Environmental Research. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by DOE SC BES under Contract No. DE-AC02- 76SF00515. We thank Chevron Energy Technology Company for performing the XRD experiments.