Mark Saeys

Highly efficient, NiAu-catalyzed hydrogenolysis of lignin into phenolic chemicals

A highly efficient, stable NiAu catalyst that exhibits unprecedented low temperature activity in lignin hydrogenolysis was for the first time developed, leading to the formation of 14 wt% aromatic monomers from organosolv lignin at 170 °C in pure water.

A Triphenylamine-Based Conjugated Polymer with Donor-π-Acceptor Architecture as Organic Sensitizer for Dye-Sensitized Solar Cells

A conjugated polymer containing an electron donating backbone (triphenylamine) and an electron accepting side chain (cyanoacetic acid) with conjugated thiophene units as the linkers has been synthesized. Dye-sensitized solar cells (DSSCs) are fabricated utilizing this material as the dye sensitizer, resulting a typical power conversion efficiency of 3.39% under AM 1.5 G illumination, which represents the highest efficiency for polymer dye-sensitized DSSCs reported so far. The results show the good promise of conjugated polymers as sensitizers for DSSC applications.

Hydrogenation kinetics of toluene on Pt/ZSM-22

Kinetic experiments on the hydrogenation of toluene were performed on 0.5 wt.% Pt/ZSM-22 at temperatures in the range 423–498 K, H2 inlet partial pressures of 100–300 kPa and toluene inlet partial pressures of 10–60 kPa. Construction of a kinetic model was based on a critical evaluation of available literature data on the hydrogenation of aromatic components together with physicochemical studies on the interaction of aromatic components and related hydrogenated products with metal surfaces as well as on quantumchemical calculations. This lead to a general kinetic model, analogous to the Horiuti Polanyi mechanism for ethylene hydrogenation, with the first four H atom addition steps not in quasi-equilibrium. Chemisorption of H2 and toluene was assumed to occur on identical sites. No dehydrogenated surface species was taken into account. The preexponential factors were calculated using transition state theory. A model with equal surface reaction rate coefficients for the H addition steps was selected as the best model. The estimated toluene and H 2 chemisorption enthalpies amounted to −70 and −42 kJ mol −1 . An activation energy in the range of 40–50 kJ mol −1 was found. Under typical reaction conditions, 60% of the surface is covered by toluene and 20% by H atoms. The remaining 20% are free. Negligible amounts of partially hydrogenated species were found to be present on the catalyst surface.

Origin of extraordinary stability of square-planar carbon atoms in surface carbides of cobalt and nickel

Surface carbides of cobalt and nickel are exceptionally stable, having stabilities competitive with those of graphitic C on these surfaces. The unusual structure of these carbides has attracted much attention: C assumes a tetracoordinate square-planar arrangement, in-plane with the metal surface, and its binding favors a spontaneous p4g clock surface reconstruction. A chemical bonding model for these systems is presented and explains the unusual structure, special stability, and the reconstruction. C promotes local two-dimensional aromaticity on the surface and the aromatic arrangement is so powerful that the required number of electrons is taken from the void M4 squares, thus leading to Peierls instability. Moreover, this model predicts a series of new transition-metal and main-group-element surface alloys: carbides, borides, and nitrides, which feature high stability, square-planar coordination, aromaticity, and a predictable degree of surface reconstruction.

Dangling-bond logic gates on a Si(100)-(2 × 1)-H surface.

Atomic-scale Boolean logic gates (LGs) with two inputs and one output (i.e. OR, NOR, AND, NAND) were designed on a Si(100)-(2 × 1)-H surface and connected to the macroscopic scale by metallic nano-pads physisorbed on the Si(100)-(2 × 1)-H surface. The logic inputs are provided by saturating and unsaturating two surface Si dangling bonds, which can, for example, be achieved by adding and extracting two hydrogen atoms per input. Quantum circuit design rules together with semi-empirical elastic-scattering quantum chemistry transport calculations were used to determine the output current intensity of the proposed switches and LGs when they are interconnected to the metallic nano-pads by surface atomic-scale wires. Our calculations demonstrate that the proposed devices can reach ON/OFF ratios of up to 2000 for a running current in the 10 µA range.

Strain-enhanced tunneling magnetoresistance in MgO magnetic tunnel junctions

While the effects of lattice mismatch-induced strain, mechanical strain, as well as the intrinsic strain of thin films are sometimes detrimental, resulting in mechanical deformation and failure, strain can also be usefully harnessed for applications such as data storage, transistors, solar cells, and strain gauges, among other things. Here, we demonstrate that quantum transport across magnetic tunnel junctions (MTJs) can be significantly affected by the introduction of controllable mechanical strain, achieving an enhancement factor of ~2 in the experimental tunneling magnetoresistance (TMR) ratio. We further correlate this strain-enhanced TMR with coherent spin tunneling through the MgO barrier. Moreover, the strain-enhanced TMR is analyzed using non-equilibrium Greens function (NEGF) quantum transport calculations. Our results help elucidate the TMR mechanism at the atomic level and can provide a new way to enhance, as well as tune, the quantum properties in nanoscale materials and devices.

Contacting a conjugated molecule with a surface dangling bond dimer on a hydrogenated Ge(001) surface allows imaging of the hidden ground electronic state.

Fabrication of single-molecule logic devices requires controlled manipulation of molecular states with atomic-scale precision. Tuning molecule-substrate coupling is achieved here by the reversible attachment of a prototypical planar conjugated organic molecule to dangling bonds on the surface of a hydrogenated semiconductor. We show that the ground electronic state resonance of a Y-shaped polyaromatic molecule physisorbed on a defect-free area of a fully hydrogenated surface cannot be observed by scanning tunneling microscopy (STM) measurements because it is decoupled from the Ge bulk states by the hydrogen-passivated surface. The state can be accessed by STM only if the molecule is contacted with the substrate by a dangling bond dimer. The reversibility of the attachment processes will be advantageous in the construction of surface atomic-scale circuits composed of single-molecule devices interconnected by the surface dangling bond wires.

Kinetic models for catalytic reactions from first principles: benzene hydrogenation

A fundamental kinetic model was constructed from first principles for the hydrogenation of benzene over a Pt(111) catalyst. Benzene adsorbs at the hollow and the bridge sites of the Pt(111) surface. Benzene at the hollow site is the reactive species, whereas benzene at the bridge site is too strongly bound. Hydrogenation follows a Horiuti–Polanyi mechanism. A reaction path analysis based on quantum chemical density functional theory calculations indicates that the fifth hydrogenation step is the rate determining step with an activation energy of 104 kJ mol−1. From the first principles reaction path analysis, a Langmuir–Hinshelwood–Hougen–Watson rate equation was constructed using first principles kinetic and thermodynamic data. Only the coverage-dependent hydrogen adsorption enthalpy was regressed to accurately (F value of 38 500) model laboratory scale data for the hydrogenation of toluene over a Pt–ZSM-22 catalyst. The optimized hydrogen adsorption enthalpy of −68.8 ± 2 kJ mol−1 is intermediate between the low and high coverage value of −94.0 and −45.0 kJ mol−1 respectively.

Flexible MgO Barrier Magnetic Tunnel Junctions.

Flexible MgO barrier magnetic tunnel junction (MTJ) devices are fabricated using a transfer printing process. The flexible MTJ devices yield significantly enhanced tunneling magnetoresistance of ≈300% and improved abruptness of switching, as residual strain in the MTJ structure is released during the transfer process. This approach could be useful for flexible electronic systems that require high-performance memory components.

Thioetherification of Chloroheteroarenes: A Binuclear Catalyst Promotes Wide Scope and High Functional-Group Tolerance

A constrained binuclear palladium catalyst system affords selective thioetherification of a wide range of functionalized arenethiols with chloroheteroaromatic partners with the highest turnover numbers (TONs) reported to date and tolerates a large variety of reactive functions. The scope of this system includes the coupling of thiophenols with six- and five-membered 2-chloroheteroarenes (i.e., functionalized pyridine, pyrazine, quinoline, pyrimidine, furane, and thiazole) and 3-bromoheteroarenes (i.e., pyridine and furane). Electron-rich congested thiophenols and fluorinated thiophenols are also suitable partners. The coupling of unprotected amino-2-chloropyridines with thiophenol and the successful employment of synthetically valuable chlorothiophenols are described with the same catalyst system. DFT studies attribute the high performance of this binuclear palladium catalyst to the decreased stability of thiolate-containing resting states. Palladium loading was as low as 0.2 mol %, which is important for industrial application and is a step forward in solving catalyst activation/deactivation problems.