Transformation of reduced graphene aerogel-supported atomically dispersed iridium into stable clusters approximated as Ir-6 during ethylene hydrogenation catalysis

Abstract

Transformation of atomically dispersed reduced graphene aerogel (rGA)-supported complexes, Ir-I(C2H4)(2)(+), with an iridium loading of 9.9 wt%, to form low-nuclearity clusters was investigated during ethylene hydrogenation catalysis. Continuous-scan X-ray absorption spectra demonstrate the formation of clusters well approximated as Ir-4 during reaction at 100 degrees C in flowing equimolar ethylene and H-2. The Ir-4 clusters transformed into clusters well approximated as Ir 6 when the feed molar ratio was switched to H-2: C2H4 = 2 and remained stable in pure H-2 at 100 degrees C. Catalyst performance data show that hydrogenation activity increased with metal nuclearity in the order of atomically dispersed iridium/rGA << Ir-4/rGA < Ir-6/ rGA. Continuous scan X-ray absorption data, complemented with aberration-corrected scanning transmission electron microscopy images, demonstrate that the supported clusters approximated as Ir-6 are stable even in H-2 at atmospheric pressure and 100 degrees C. These supported iridium clusters are among the ones having the highest metal loadings reported for a supported metal cluster catalyst.