Free Energy Assessment of Water Structures and Their Dissociation on Ru(0001)

The free energy landscape of the structures and dissociation degree of the first water and heavy water adlayers on Ru(0001) surface is presented. Thermodynamically favored interconversion routes connecting different experimentally reported structures are suggested based on free energy calculations. On going from low to high water coverage, one-dimensional (1D) periodic chain-like structures with small, or zero, water dissociation degree like Chain-4a motifs are found to be very stable intermediates in the formation of dissociated and molecular ice-like bilayers, respectively. The isotopic effects on the dissociation degree of the ice-like bilayers are estimated: a preference for the half- dissociated form is found for H2O at temperatures below 275 K followed by energetic degeneration between all bilayers dissociated over that threshold. Instead, for heavy water this temperature is shifted to 225 K. Moreover, the configurational entropy due to the different arrangements of dissociated and molecular flat molecules further contributes to set the energies of all these structures within a small energy window that make experimental identification difficult

Unique Reaction Path in Heterogeneous Catalysis: The Concerted Semi-Hydrogenation of Propyne to Propene on CeO2

ABSTRACT: Despite its ubiquity in homogeneous and enzymatic catalysis, concerted mechanisms have been over- looked for heterogeneously catalyzed reactions. The elusive nature of transition states leaves Density Functional Theory, DFT, as the only robust tool for their identification and characterization. By means of this method, we show that a concerted path takes part in the recently discovered semi- hydrogenation of propyne on CeO2, for which an excellent activity and selectivity have been reported. The high surface H coverage imposed by the experimental hydrogenation con- ditions induces site isolation and drives the reaction through a six-membered ring transition state. This unprecedented pathway accounts for many of the experimental observations, such as the unique syn-stereoselectivity, the excellent alkene selectivities, or the high temperature and large H2/alkyne ratios required

Structure and Reactivity of Supported Hybrid Platinum Nanoparticles for the Flow Hydrogenation of Functionalized Nitroaromatics

This contribution targets the first comprehensive understanding of the next-generation catalyst for nitroarene hydrogenation, featuring ligand-capped 2 nm platinum nanoparticles (Pt-HHDMA, HHDMA: hexadecyl(2-hydroxyethyl)- dimethylammonium dihydrogen phosphate) deposited on carbon. Fundamental questions related to the structure, properties, and mechanistic fingerprints of the metal−organic interphase of the hybrid system were addressed through a battery of advanced characterization methods and theoretical calculations. Catalytic tests conducted in a flow reactor at variable temperature and pressure revealed the superior activity of Pt-HHDMA in comparison with the archetypal and industrially relevant Lindlar-type Pt−Pb/CaCO3, with outstanding chemoselectivity and leaching resistance. The analysis of the reaction mechanism by Density Functional Theory, which was never addressed systematically, showed that the benefits of the ligand-modified catalyst arise from the facilitated H2 activation and weak nitroarene adsorption on the HHDMA-modified surface. At the same time, the ligand isolates the platinum ensemble, reducing the possibility of unselective routes by controlling the adsorption geometry and extent of the reactant and product intermediates. These results substantially enrich the mechanistic understanding of HHDMA-modified Pt catalyst and are of fundamental relevance for future improvements of this hybrid catalyst and for extrapolating this technology to other challenging reactions

When the Solvent Locks the Cage: Theoretical Insight into the Transmetalation of MOF‐5 Lattices and Its Kinetic Limitations

Transmetalation is an innovative postsynthetic strategy for tailoring the properties of metal−organic frameworks (MOFs), allowing stable unprecedented metal coordination environments. Although the experimental synthetic protocol is well- established, the underlying mechanism for transmetalation is still unknown. In this work, we propose two different solvent-mediated reaction paths for the Ni transmetalation in Zn- MOF-5 lattices through density functional theory simulations. In both mechanisms, the bond strength between the exchanged metal and the solvent is the key descriptor that controls the degree of transmetalation. We also show that the role of the solvent in this process is twofold: it initially promotes Zn exchange, but if the metal−solvent bond is too strong, it blocks the second transmetalation cycle by restricting the lattice flexibility. The competition between these two effects leads the degree of incorporation of metal into MOF-5 to display a volcano-type dependence with respect to the metal−solvent bond strength for different transition metal ions