Is it single-metal homogeneous or polymetallic heterogeneous catalysis? Further investigation of the catalytically active species in benzene hydrogenation beginning with [RhCp*Cl ₂] ₂ via operando-XAFS, kinetic and poisoning studies

Previous investigation (Hagen et al. J. Am. Chem. Soc. 2005, 127, 4423) for benzene hydrogenation starting with [RhCp*Cl ₂] ₂ at 100 °C and 50 atm initial H ₂ pressure concluded that the true catalyst was most likely Rh(0) nanoparticles on the basis of sigmoidal kinetics, Rh(0) precipitate, and positive Hg(0) poisoning studies. There was, at the time, no other plausible hypothesis for the true catalyst in light of the extant data. Further investigation via operando-XAFS provides a new hypothesis for the active catalyst: ligated Rh ₄ sub-nanometer clusters that constitute ∼98% of the total soluble Rh in solution. Kinetics and quantitative poisoning studies support this hypothesis. The results are of significant interest since determining the nature of the true catalyst continues to be a central, often vexing issue, in any and all catalytic reactions.