Transformation of reduced graphene aerogel-supported atomically dispersed iridium into stable clusters approximated as Ir6 during ethylene hydrogenation catalysis

Abstract

Transformation of atomically dispersed reduced graphene aerogel (rGA)-supported complexes, IrI(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 Ir4 during reaction at 100 °C in flowing equimolar ethylene and H2. The Ir4 clusters transformed into clusters well approximated as Ir6 when the feed molar ratio was switched to H2:C2H4 = 2 and remained stable in pure H2 at 100 °C. Catalyst performance data show that hydrogenation activity increased with metal nuclearity in the order of atomically dispersed iridium/rGA≪ Ir4/rGA < Ir6/rGA. Continuous scan X-ray absorption data, complemented with aberration-corrected scanning transmission electron microscopy images, demonstrate that the supported clusters approximated as Ir6 are stable even in H2 at atmospheric pressure and 100 °C. These supported iridium clusters are among the ones having the highest metal loadings reported for a supported metal cluster catalyst.