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<sub>CO</sub> 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<sub>CO</sub> 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<sup>–1</sup> characterizing the symmetric and asymmetric CO stretches and implying
that this is the most nearly uniform supported single-site metal catalyst