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Beating Heterogeneity of Single-Site Catalysts: MgO-Supported Iridium Complexes

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journal contribution
posted on 12.03.2018, 00:00 by Adam S. Hoffman, Louise M. Debefve, Shengjie Zhang, Jorge E. Perez-Aguilar, Edward T. Conley, Kimberly R. Justl, Ilke Arslan, David A. Dixon, Bruce C. Gates
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.