Homogeneity of Surface Sites in Supported Single-Site Metal Catalysts: Assessment with Band Widths of Metal Carbonyl Infrared Spectra

Determining and controlling the uniformity of isolated metal sites on surfaces of supports are central goals in investigations of single-site catalysts because well-defined species provide opportunities for fundamental understanding of the surface sites. CO is a useful probe of surface metal sites, often reacting with them to form metal carbonyls, the infrared spectra of which provide insights into the nature of the sites and the metal–support interface. Metals bonded to various support surface sites give broad bands in the spectra, and when narrow bands are observed, they indicate a high degree of uniformity of the metal sites. Much recent work on single-site catalysts has been done with supports that are inherently nonuniform, giving supported metal species that are therefore nonuniform. Herein we summarize values of ν_CO data characterizing supported iridium <i>gem</i>-dicarbonyls, showing that the most nearly uniform of them are those supported on zeolites and the least uniform are those supported on metal oxides. Guided by ν_CO data of supported iridium <i>gem</i>-dicarbonyls, we have determined new, general synthesis methods to maximize the degree of uniformity of iridium species on zeolites and on MgO. We report results for a zeolite HY-supported iridium <i>gem</i>-dicarbonyl with full width at half-maximum values of only 4.6 and 5.2 cm^–1 characterizing the symmetric and asymmetric CO stretches and implying that this is the most nearly uniform supported single-site metal catalyst

Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids

1,3-Dialkylimidazolium ionic liquid coatings act as electron donors, increasing the selectivity for partial hydrogenation of 1,3-butadiene catalyzed by iridium complexes supported on high-surface-area γ-Al₂O₃. High-energy-resolution fluorescence detection X-ray absorption near-edge structure (HERFD XANES) measurements quantify the electron donation and are correlated with the catalytic activity and selectivity. The results demonstrate broad opportunities to tune electronic environments and catalytic properties of atomically dispersed supported metal catalysts