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 η¹ adsorption modes on terraces or edges, little is known about side-on η² CO species coordinated to B₅ or B₆ step-edges, the proposed active sites for CO and N₂ cleavage. By using solid-state NMR and DFT calculations, we analyze ¹³C chemical shift tensors (CSTs) of carbonyl ligands on the molecular cluster model for Ru nanoparticles, Ru₆(η²-μ₄-CO)₂(CO)₁₃(η⁶-C₆Me₆), and show that, contrary to η¹ carbonyls, the CST principal components parallel to the C-O bond are extremely deshielded in the η² 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.

On the origin of high-temperature phenomena in Pt/Al2O3

Treatments of Pt/γ-Al₂O₃ with H₂ under harsh conditions have long been known to strongly influence the properties of this important catalytic system, but the true causes of the high-temperature effects still remain unclear. We have performed a more detailed study of this issue, having used H₂-TPD as a sensitive probe of metal-support interactions. The experimental results are in accordance with previous studies and demonstrate strong changes in adsorption and catalytic properties of Pt/γ-Al₂O₃ after high-temperature H₂ treatments, as well as the possibility to reverse the changes, completely or in part, through O₂ and H₂O treatments. Thorough examination has shown that such behaviour is an intrinsic property of Pt/γ-Al₂O₃ and cannot be attributed to impurities or experimental artifacts. Moreover, there is no abrupt transition to a high-temperature state, but the system undergoes smooth and gradual changes upon increasing the H₂-treatment temperature (T_TR), with the changes being already apparent at a T_TR of ∼ 300 °C. The results suggest that hydrogen can generate oxygen vacancies on the surface of the support in close vicinity to the Pt particles, and the system appears under equilibrium to be kinetically driven by temperature and thermodynamically driven by the P_H2/P_H2O ratio or local concentration of surface hydroxyls near Pt particles. The generated vacancies change the properties of contacting particles, and the changes are most pronounced for sub-nanometric Pt clusters and single atoms. Implications of the phenomena for the synthesis, study, and use of Pt/γ-Al₂O₃ and its related nanosystems are discussed.