Catalysis beyond frontier molecular orbitals: Selectivity in partial hydrogenation of multi-unsaturated hydrocarbons on metal catalysts

in Science Advances (2017), 3

The mechanistic understanding and control over transformations of multi-unsaturated hydrocarbons on transition metal surfaces remains one of the major challenges of hydrogenation catalysis. To reveal the ... [more ▼]

The mechanistic understanding and control over transformations of multi-unsaturated hydrocarbons on transition metal surfaces remains one of the major challenges of hydrogenation catalysis. To reveal the microscopic origins of hydrogenation chemoselectivity, we performed a comprehensive theoretical investigation on the reactivity of two a,b-unsaturated carbonyls—isophorone and acrolein—on seven (111) metal surfaces: Pd, Pt, Rh, Ir, Cu, Ag, and Au. In doing so, we uncover a general mechanism that goes beyond the celebrated frontier molecular orbital theory, rationalizing the C--C bond activation in isophorone and acrolein as a result of significant surface-induced broadening of high-energy inner molecular orbitals. By extending our calculations to hydrogen-precovered surface and higher adsorbate surface coverage, we further confirm the validity of the “inner orbital broadening mechanism” under realistic catalytic conditions. The proposed mechanism is fully supported by our experimental reaction studies for isophorone and acrolein over Pd nanoparticles terminated with (111) facets. Although the position of the frontier molecular orbitals in these molecules, which are commonly considered to be responsible for chemical interactions, suggests preferential hydrogenation of the C--O double bond, experiments show that hydrogenation occurs at the C--C bond on Pd catalysts. The extent of broadening of inner molecular orbitals might be used as a guiding principle to predict the chemoselectivity for a wide class of catalytic reactions at metal surfaces. [less ▲]

Tuning the work function of stepped metal surfaces by adsorption of organic molecules

in Journal of Physics : Condensed Matter (2017), 29

Understanding the binding mechanisms for aromatic molecules on transition-metal surfaces, especially with defects such as vacancies, steps and kinks, is a major challenge in designing functional ... [more ▼]

Understanding the binding mechanisms for aromatic molecules on transition-metal surfaces, especially with defects such as vacancies, steps and kinks, is a major challenge in designing functional interfaces for organic devices. One important parameter in the performance of organic/inorganic devices is the barrier of charge carrier injection. In the case of a metallic electrode, tuning the electronic interface potential or the work function for electronic level alignment is crucial. Here, we use density-functional theory (DFT) calculations with van der Waals (vdW) interactions treated with both screened pairwise (vdWsurf) and many-body dispersion (MBD) methods, to systematically study the interactions of benzene with a variety of stepped surfaces. Our calculations confirm the physisorptive character of Ag(2 1 1), Ag(5 3 3), Ag(3 2 2), Ag(7 5 5) and Ag(5 4 4) surfaces upon the adsorption of benzene. The MBD effects reduce the adsorption energies by about 0.15 eV per molecule compared to the results from the DFT + vdWsurf method. In addition, we find that the higher the step density, the larger the reduction of the work function upon the adsorption of benzene. We also study the effect of vdW interactions on the electronic structure using a fully selfconsistent implementation of the vdWsurf method in the Kohn–Sham DFT framework. We find that the self-consistent vdWsurf effects increase the work function due to the lowered Fermi level and the increased vacuum level. As a result, the benzene/Ag(2 1 1) system has the lowest work function (3.67 eV) among the five adsorption systems, significantly smaller than the work function of the clean Ag(1 1 1) surface (4.74 eV). Our results provide important insights into the stability and electronic properties of molecules adsorbed on stepped metal surfaces, which could help in designing more appropriate interfaces with low work functions for electron transfer. [less ▲]

Thermal and electronic fluctuations of flexible adsorbed molecules: Azobenzene on Ag(111)

in Physical Review Letters (2016), 116

Identical Binding Energies and Work Functions for Distinct Adsorption Structures: Olympicenes on the Cu(111) Surface

in The Journal of Physical Chemistry Letters (2016), 7(6), 1022-1027

Steps or Terraces? Dynamics of Aromatic Hydrocarbons Adsorbed at Vicinal Metal Surfaces

in Physical Review Letters (2015), 115(8),

The study of how molecules adsorb, diffuse, interact, and desorb from imperfect surfaces is essential for a complete understanding of elementary surface processes under relevant pressure and temperature ... [more ▼]

The study of how molecules adsorb, diffuse, interact, and desorb from imperfect surfaces is essential for a complete understanding of elementary surface processes under relevant pressure and temperature conditions. Here we use first-principles calculations to study the adsorption of benzene and naphthalene on a vicinal Cu(443) surface with the aim to gain insight into the behavior of aromatic hydrocarbons on realistic surfaces at a finite temperature. Upon strong adsorption at step edges at a low temperature, the molecules then migrate from the step to the (111) terraces, where they can freely diffuse parallel to the step edge. This migration happens at temperatures well below the onset of desorption, suggesting a more complex dynamical picture than previously proposed from temperature-programed desorption studies. The increase of the adsorption strength observed in experiments for Cu(443) when compared to Cu(111) is explained by a stronger long-range van der Waals attraction between the hydrocarbons and the step edges of the Cu(443) surface. Our calculations highlight the need for time-resolved experimental studies to fully understand the dynamics of molecular layers on surfaces. © 2015 American Physical Society. [less ▲]

van der Waals Interactions Determine Selectivity in Catalysis by Metallic Gold

in JOURNAL OF THE AMERICAN CHEMICAL SOCIETY (2014), 136(38), 13333-13340

To achieve high selectivity for catalytic reactions between two or more reactants on a heterogeneous catalyst, the relative concentrations of the reactive intermediates on the surface must be optimized ... [more ▼]

To achieve high selectivity for catalytic reactions between two or more reactants on a heterogeneous catalyst, the relative concentrations of the reactive intermediates on the surface must be optimized. If species compete for binding sites, their concentrations depend on their relative binding strengths to the surface. In this article we describe a general framework for predicting the relative stability of organic intermediates involved in oxygen-assisted reactions on metallic gold with broad relevance to catalysis by metals. Combining theory and experiment, we establish that van der Waals interactions between the reactive intermediates and the surface, although weak, determine relative stabilities and thereby dictate the conditions for optimum selectivity. The inclusion of these interactions is essential for predicting these trends. The concepts and methods employed here have broad applicability for determining the stability of intermediates on the surfaces of catalytic metals and specifically demonstrate the critical role of weak interactions in determining reaction selectivity among reactions of complex molecules. [less ▲]

Interaction of Isophorone with Pd(111): A Combination of Infrared Reflection-Absorption Spectroscopy, Near-Edge X-ray Absorption Fine Structure, and Density Functional Theory Studies

in Journal of Physical Chemistry. C, Nanomaterials and interfaces (2014), 118(48), 27833-27842

Atomistic level understanding of interaction of alpha,beta-unsaturated carbonyls with late transition metals is a key prerequisite for rational design of new catalytic materials with the desired ... [more ▼]

Atomistic level understanding of interaction of alpha,beta-unsaturated carbonyls with late transition metals is a key prerequisite for rational design of new catalytic materials with the desired selectivity toward C-C or C-O bond hydrogenation. The interaction of this class of compounds with transition metals was investigated on alpha,beta-unsaturated ketone isophorone on Pd(111) as a prototypical system. In this study, infrared reflectionabsorption spectroscopy (IRAS), near-edge X-ray absorption fine structure (NEXAFS) experiments and density functional theory calculations including van der Waals interactions (DFT+vdW) were combined to obtain detailed information on the binding of isophorone to palladium at different coverages and on the effect of preadsorbed hydrogen on the binding and adsorption geometry. According to these experimental observations and the results of theoretical calculations, isophorone adsorbs on Pd(111) in a flat-lying geometry at low coverages. With increasing coverage, both C-C and C-O bonds of isophorone tilt with respect to the surface plane. The tilting is considerably more pronounced for the C-C bond on the pristine Pd(111) surface, indicating a prominent perturbation and structural distortion of the conjugated p system upon interaction with Pd. Preadsorbed hydrogen leads to higher tilting angles of both p bonds, which points to much weaker interaction of isophorone with hydrogen-precovered Pd and suggests the conservation of the in-plane geometry of the conjugated pi system. The results of the DFT+vdW calculations provide further insights into the perturbation of the molecular structure of isophorone on Pd(111). [less ▲]

A novel immunomodulator, FTY-720 reverses existing cardiac hypertrophy and fibrosis from pressure overload by targeting NFAT (nuclear factor of activated T-cells) signaling and periostin.

in Circulation. Heart failure (2013), 6(4), 833-44

BACKGROUND: Hypertension or aortic stenosis causes pressure overload, which evokes hypertrophic myocardial growth. Sustained cardiac hypertrophy eventually progresses to heart failure. Growing evidence ... [more ▼]

BACKGROUND: Hypertension or aortic stenosis causes pressure overload, which evokes hypertrophic myocardial growth. Sustained cardiac hypertrophy eventually progresses to heart failure. Growing evidence indicates that restraining hypertrophy could be beneficial; here, we discovered that FTY-720, an immunomodulator for treating multiple sclerosis, can reverse existing cardiac hypertrophy/fibrosis. METHODS AND RESULTS: Male C57/Bl6 mice underwent transverse aortic constriction (TAC) for 1 week followed by FTY-720 treatment for 2 weeks under continuing TAC. Compared with vehicle-treated TAC hearts, FTY-720 significantly reduced ventricular mass, ameliorated fibrosis, and improved cardiac performance. Mechanistic studies led us to discover that FTY-720 appreciably inhibited nuclear factor of activated T-cells (NFAT) activity. Moreover, we found that in primary cardiomyocytes (rat and human) pertussis toxin (Gi-coupled receptor inhibitor) substantially blocked the antihypertrophic effect of FTY-720. This observation was confirmed in a mouse model of pressure overload. Interestingly, gene array analysis of TAC hearts revealed that FTY-720 profoundly decreased gene expression of a group of matricellular proteins, of which periostin was prominent. Analysis of periostin protein expression in TAC-myocardium, as well as in rat and human cardiac fibroblasts, confirmed the array data. Moreover, we found that FTY-720 treatment or knockdown of periostin protein was able to inhibit transforming growth factor-beta responsiveness and decrease collagen expression. CONCLUSIONS: FTY-720 alleviates existing cardiac hypertrophy/fibrosis through mechanisms involving negative regulation of NFAT activity in cardiomyocytes and reduction of periostin expression allowing for a more homeostatic extracellular compartment milieu. Together, FTY-720 or its analogues could be a promising new approach for treating hypertrophic/fibrotic heart disease. [less ▲]

Density-Functional Theory with Screened van der Waals Interactions for the Modeling of Hybrid Inorganic-Organic Systems

in PHYSICAL REVIEW LETTERS (2012), 108(14),

The electronic properties and the function of hybrid inorganic-organic systems (HIOS) are intimately linked to their interface geometry. Here we show that the inclusion of the many-body collective ... [more ▼]

The electronic properties and the function of hybrid inorganic-organic systems (HIOS) are intimately linked to their interface geometry. Here we show that the inclusion of the many-body collective response of the substrate electrons inside the inorganic bulk enables us to reliably predict the HIOS geometries and energies. This is achieved by the combination of dispersion-corrected density-functional theory (the DFT+ van der Waals approach) [Phys. Rev. Lett. 102, 073005 (2009)], with the Lifshitz-Zaremba-Kohn theory for the nonlocal Coulomb screening within the bulk. Our method yields geometries in remarkable agreement (approximate to 0.1 angstrom) with normal incidence x-ray standing wave measurements for the 3, 4, 9, 10-perylene-tetracarboxylic acid dianhydride (C24O6H8, PTCDA) molecule on Cu(111), Ag(111), and Au(111) surfaces. Similarly accurate results are obtained for xenon and benzene adsorbed on metal surfaces. [less ▲]

Benzene adsorbed on metals: Concerted effect of covalency and van der Waals bonding

in Physical Review. B (2012), 86(24),

The adsorption of aromatic molecules on metal surfaces plays a key role in condensed matter physics and functional materials. Depending on the strength of the interaction between the molecule and the ... [more ▼]

The adsorption of aromatic molecules on metal surfaces plays a key role in condensed matter physics and functional materials. Depending on the strength of the interaction between the molecule and the surface, the binding is typically classified as either physisorption or chemisorption. Van der Waals (vdW) interactions contribute significantly to the binding in physisorbed systems, but the role of the vdW energy in chemisorbed systems remains unclear. Here we study the interaction of benzene with the (111) surface of transition metals, ranging from weak adsorption (Ag and Au) to strong adsorption (Pt, Pd, Ir, and Rh). When vdW interactions are accurately accounted for, the barrier to adsorption predicted by standard density-functional theory (DFT) calculations essentially vanishes, producing a metastable precursor state on Pt and Ir surfaces. Notably, vdW forces contribute more to the binding of covalently bonded benzene than they do when benzene is physisorbed. Comparison to experimental data demonstrates that some of the recently developed methods for including vdW interactions in DFT allow quantitative treatment of both weakly and strongly adsorbed aromatic molecules on metal surfaces, extending the already excellent performance found for molecules in the gas phase. [less ▲]