First-Principles Study of Alkoxides Adsorbed on Au(111) and Au(110) Surfaces: Assessing the Roles of Noncovalent Interactions and Molecular Structures in Catalysis

Microscopic understanding of molecular adsorption on catalytic surfaces is crucial for controlling the activity and selectivity of chemical reactions. However, for complex molecules, the adsorption process is very systemspecific and there is a clear need to elaborate systematic understanding of important factors that determine catalytic functionality. Here, we investigate the binding of eight molecules, including seven alkoxides and one carboxylate, on the Au(111) and Au(110) surfaces. Our density-functional theory calculations including long-range van der Waals interactions demonstrate the significant role of these “weak” noncovalent forces on the adsorption structures, energetics, and relative adsorbate stabilities. Interestingly, the binding energy trends are insensitive to the surface structure. Instead, the adsorption stability depends strongly on the structural and chemical characteristics of the molecules: linear vs branching configurations, number of unsaturated C−C bonds, bidentate adsorption, and the presence of electronegative atoms. Our calculations help establish the influence of individual and collective chemical factors that determine the catalytic selectivity of alkoxides

Nonadditivity of the Adsorption Energies of Linear Acenes on Au(111): Molecular Anisotropy and Many-Body Effects

Adsorption energies of chemisorbed molecules on inorganic solids usually scale linearly with molecular size and are well described by additive scaling laws. However, much less is known about scaling laws for physisorbed molecules. Our temperature-programmed desorption experiments demonstrate that the adsorption energy of acenes (benzene to pentacene) on the Au(111) surface in the limit of low coverage is highly nonadditive with respect to the molecular size. For pentacene, the deviation from an additive scaling of the adsorption energy amounts to as much as 0.7 eV. Our first-principles calculations explain the observed nonadditive behavior in terms of anisotropy of molecular polarization stemming from many-body electronic correlations. The observed nonadditivity of the adsorption energy has implications for surface-mediated intermolecular interactions and the ensuing on-surface self-assembly. Thus, future coverage-dependent studies should aim to gain insights into the impact of these complex interactions on the self-assembly of π-conjugated organic molecules on metal surfaces

Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material

Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge density wave (CDW) material 1T-TaSe$_2$. After exciting the material with a short light pulse, spatial smearing of the electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This indicates a bi-directional energy exchange between the electrons and the strongly-coupled phonons. By tuning the laser excitation fluence, we can control the magnitude of the electron temperature modulation, from ~ 200 K in the case of weak excitation, to ~ 1000 K for strong laser excitation. This is accompanied by a switching of the dominant mechanism from anharmonic phonon-phonon coupling to coherent electron-phonon coupling, as manifested by a phase change of $\pi$ in the electron temperature modulation. Our approach thus opens up possibilities for coherently manipulating the interactions and properties of quasi-2D and other quantum materials using light.Comment: 15 pages, 4 figure

Ab initio Molecular Dynamics Simulations of the Structural Response of Solids to Ultrashort Laser and XUV Pulses

Promotionsstipendium der Universität Kassel, Germany

Exploring multichannel superconductivity in ThFeAsN

We investigate theoretically the superconducting state of the undoped Fe-based superconductor ThFeAsN. Using input from ab initio calculations, we solve the Fermi-surface based, multichannel Eliashberg equations for Cooper-pair formation mediated by spin and charge fluctuations, and by the electron-phonon interaction (EPI). Our results reveal that spin fluctuations alone, when coupling only hole-like with electron-like energy bands, can account for a critical temperature Tc up to ∼7.5K with an s±-wave superconducting gap symmetry, which is a comparatively low Tc with respect to the experimental value Texpc=30K. Other combinations of interaction kernels (spin, charge, electron-phonon) lead to a suppression of Tc due to phase frustration of the superconducting gap. We qualitatively argue that the missing ingredient to explain the gap magnitude and Tc in this material might be the first-order correction to the EPI vertex. In the noninteracting state this correction adopts a form supporting the s± gap symmetry, in contrast to EPI within Migdal's approximation, i.e., EPI without vertex correction, and therefore it enhances tendencies arising from spin fluctuations

Noncovalent Bonding Controls Selectivity in Heterogeneous Catalysis: Coupling Reactions on Gold

Enhancing the selectivity of catalytic processes has potential for substantially increasing the sustainability of chemical production. Herein, we establish relationships between reaction selectivity and molecular structure for a homologous series of key intermediates for oxidative coupling of alcohols on gold using a combination of experiment and theory. We establish a scale of binding for molecules with different alkyl structures and chain lengths and thereby demonstrate the critical nature of noncovalent van der Waals interactions in determining the selectivity by modulating the stability of key reaction intermediates bound to the surface. The binding hierarchy is the same for Au(111) and Au(110), which demonstrates a relative lack of sensitivity to the surface structure. The hierarchy of binding established in this work provides guiding principles for predicting how molecular structure affects the competition for binding sites more broadly. Besides the nature of the primary surface-molecule bonding, three additional factors that affect the stabilities of the reactive intermediates are clearly established: (1) the number of C atoms in the alkyl chain, (2) the presence of C–C bond unsaturation, and (3) the degree of branching of the alkyl group of the adsorbed molecules. We suggest that this is a fundamental principle that is generally applicable to a broad range of reactions on metal catalysts