Electronic Structure-Reactivity Relationship on Ruthenium Step-Edge Sites from Carbonyl 13C 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 ill adsorption modes on terraces or edges, little is known about side-on 172 CO species coordinated to B-5 or B-6 step-edges, the proposed active sites for CO and N-2 cleavage. By using solid-state NMR and DFT calculations, we analyze C-13 chemical shift tensors (CSTs) of carbonyl ligands on the molecular cluster model for Ru nanoparticles, Ru-6(eta(2)-mu(4)-CO)(2)(CO)(13)(eta(6)-C6Me6), and show that, contrary to eta(1) carbonyls, the CST principal components parallel to the C-O bond are extremely deshielded in the eta(2) species due to the population of the C-O pi* 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.