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Density functional theory was used to examine the oxygen evolution reaction on a large number of active sites formed by doping three different surfaces of Co3O4 with various 3d transition-metal atoms. By combining the scaling and Bronsted–Evans–Polanyi (BEP) relationships that are determined for these sites, it is shown that the activity of a site is controlled by the redox potential for oxidation of the site. On the basis of this, a kinetics-based design strategy is presented for identifying the optimal active site at a given electrode potential. This design strategy is shown to be valid regardless of whether the rate-limiting water addition step occurs electrochemically or nonelectrochemically, as long as certain conditions are met. Another finding is that the BEP relations are sensitive to the structure of the active site, with sites reacting through μ3-oxo species having the most favorable relations. Finally, the kinetics-based design strategy is compared with the commonly used thermodynamics-based de...

Despite their industrial use in selective hydrogenation reactions, the surface chemistry of long-chained alkynes on transition metals is not well understood. To this end, the two C6-alkynes 1- and 3-hexyne were studied on Pt(111) using temperature-programmed desorption (TPD), electron emission spectroscopies (MIES/UPS), and infrared reflection–absorption spectroscopy (IRRAS). Besides the formation of graphitic carbon residues, both molecules mainly undergo desorption, self-hydrogenation, and dehydrocyclization to form benzene during temperature-programmed desorption, similar to the analogous alkenes. The dehydrocyclization to benzene is shown to be ubiquitous to unsaturated hydrocarbons on Pt(111) regardless of the degree of unsaturation and its position within the molecule. A reaction mechanism for dehydrocyclization is proposed based on dehydrogenation followed by ring-closure. This work extends the understanding of alkyne chemistry on Pt-based catalysts and may aid to identify additional reaction mecha...

We use the gauge-including projector augmented-waves (GIPAW) method to report, for the first time, theoretical 27Al Knight shifts in metallic systems other than metallic Al. We consider metallic Al and a set of six intermetallic compounds, for which experimental chemical shifts were recently made available in the literature. The orbital and spin components of the chemical shielding tensors are computed from the same ground-state spin-polarized electronic structure, converged under the influence of a uniform external magnetic field. A linear response formalism is used to compute the orbital part, while the spin part is approximated by the linear relationship between the external field and the Fermi contact contribution to the induced magnetic hyperfine field at the nuclear position. Core spin-polarization effects are taken into account by means of a perturbative approach. Our results show that the GIPAW approach yields reasonably acceptable chemical shifts with affordable k-meshes in the irreducible Brillo...

Self-assembled monolayers (SAMs) grown on surfaces of ferromagnetic metals have attracted increasing attention as they can act as corrosion inhibitors on easily oxidizable transition metals and are potentially relevant for application in spintronics. We have performed a model study of aromatic thiol SAMs grown on atomically flat Ni(111) by means of synchrotron-based X-ray photoelectron spectrosco- py, X-ray absorption spectroscopy, and density functional theory. Our analysis demonstrates that well-defined bonding through the sulfur headgroup of the molecules (thiolate bonding) can be established at 200 K. However, the bonding configuration is metastable: breaking of the C−S bond and subsequent chemisorption of both fragments on the Ni surface decreases the total energy. The low activation barrier for C−S dissociation hampers the formation of room-temperature-stable monolayers. In addition, we show that end groups with a strong affinity to the nickel substrate can severely modify the global pattern of interaction of the thiol molecules with the surface upon adsorption. Fil: Blobner, F.. Technische Universitat Munchen; Alemania Fil: Abufager, Paula Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina Fil: Han, R.. Technische Universitat Munchen; Alemania Fil: Bauer, J.. Technische Universitat Munchen; Alemania Fil: Duncan, D. A.. Technische Universitat Munchen; Alemania Fil: Maurer, R. J.. Technische Universitat Munchen; Alemania Fil: Reuter, K.. Technische Universitat Munchen; Alemania Fil: Feulner, P.. Technische Universitat Munchen; Alemania Fil: Allegretti, F.. Technische Universitat Munchen; Alemania

Density-functional theory based global geometry optimization is employed to systematically scrutinize the possibility of multidoping of hydrogenated Si clusters to achieve high spin states beyond the septet limit of a single-atom dopant. While our unbiased configurational search reveals that the previously suggested Si18H12 double hexagonal prism structure is generally too small to accommodate two dopants in magnetized state, the larger Si24H24 cage turns out to be suitable for such applications. For dimer dopants M2+ = Cr2+, Mn2+, and CrMn+, the structural integrity of the host cage is conserved in the ground-state structure of corresponding M2+@Si24H24 aggregates, as is the unusually high spin state of the guest dopant, which in the case of Cr2+ already exceeds the single-atom dopant septet limit by almost a factor of 2. Moreover, the possibility of further increasing the cluster spin moment by encapsulating an even larger number of dopants into a suitably sized hydrogenated Si cage is illustrated for the example of a (CrMn+)2@Si28H28 aggregate with a total number of 18 unpaired electrons. These results strongly suggest multidoping of Si clusters as a viable route to novel cluster-based materials for magneto-optic applications.

Using ab initio thermodynamics, the stability of a wide range of hydrocarbon adsorbates under various chemical vapor deposition (CVD) conditions (temperature, methane and hydrogen pressures) used in experimental graphene growth protocols at solid and liquid Cu surfaces has been explored. At the employed high growth temperatures around the melting point of Cu, we find that commonly used thermodynamic models such as the harmonic oscillator model may no longer be accurate. Instead, we account for the translational and rotational mobility of adsorbates using a recently developed hindered translator and rotator model or a two-dimensional ideal gas model. The thermodynamic considerations turn out to be crucial for explaining experimental results and allow us to improve and extend the findings of earlier theoretical studies regarding the role of hydrogen and hydrocarbon species in CVD. In particular, we find that smaller hydrocarbons will completely dehydrogenate under most CVD conditions. For larger clusters our results show that metal-terminated and hydrogen-terminated edges have very similar stabilities. While both cluster types might thus form during the experiment, we show that the low binding strength of clusters with hydrogen-terminated edges could result in instability towards desorption.