4 research outputs found
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 metalâorganic
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 PtâPb/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 MOFâ5 Lattices and Its Kinetic Limitations
Transmetalation is an innovative postsynthetic strategy for tailoring the properties of metalâorganic 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 metalâsolvent 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 metalâsolvent bond strength for different transition metal ions