Electronic Structure-Reactivity Relationship on Ruthenium Step-Edge Sites from Carbonyl C Chemical Shift Analysis.

Ru nanoparticles are highly active catalysts for the Fischer-Tropsch and the Haber-Bosch processes. They show various types of surface sites upon CO adsorption according to NMR spectroscopy. Compared to terminal and bridging η adsorption modes on terraces or edges, little is known about side-on η CO species coordinated to B or B step-edges, the proposed active sites for CO and N cleavage. By using solid-state NMR and DFT calculations, we analyze C chemical shift tensors (CSTs) of carbonyl ligands on the molecular cluster model for Ru nanoparticles, Ru(η-μ-CO)(CO)(η-CMe), and show that, contrary to η carbonyls, the CST principal components parallel to the C-O bond are extremely deshielded in the η species due to the population of the C-O π* antibonding orbital, which weakens the bond prior to dissociation. The carbonyl CST is thus an indicator of the reactivity of both Ru clusters and Ru nanoparticles step-edge sites toward C-O bond cleavage.