Molecular-Level Understanding of CeO₂ as a Catalyst for Partial Alkyne Hydrogenation

The unique catalytic properties of ceria for the partial hydrogenation of alkynes are examined for acetylene hydrogenation. Catalytic tests over polycrystalline CeO₂ at different temperatures and H₂/C₂H₂ ratios reveal ethylene selectivities in the range of 75–85% at high degrees of acetylene conversion and hint at the crucial role of hydrogen dissociation on the overall process. Density-functional theory is applied to CeO₂(111) in order to investigate reaction intermediates and to calculate the enthalpy and energy barrier for each elementary step, taking into account different adsorption geometries and the presence of potential isomers of the intermediates. At a high hydrogen coverage, β-C₂H₂ radicals adsorbed on-top of surface oxygen atoms are the initial reactive species forming C₂H₃ species effectively barrierless. The high alkene selectivity is owed to the lower activation barrier for subsequent hydrogenation leading to gas-phase C₂H₄ compared to that for the formation of β-C₂H₄ radical species. Moreover, hydrogenation of C₂H₅ species, if formed, must overcome significantly large barriers. Oligomers are the most important byproduct of the reaction and they result from the recombination of chemisorbed C₂H_<i>x</i> species. These findings rationalize for the first time the applicability of CeO₂ as a catalyst for olefin production and potentially broaden its use for the hydrogenation of polyunsaturated and polyfunctionalized substrates containing triple bonds

Atomic-Scale Sliding Friction on Graphene in Water

The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecular dynamics (MD) and atomic force microscopy. MD simulations predict that the atomic-scale stick–slip is almost identical to that found in ultrahigh vacuum. Furthermore, they show that water plays a purely stochastic role in sliding (solid-to-solid) friction. These observations are substantiated by friction measurements on graphene grown on Cu and Ni, where, oppositely of the operation in air, lattice resolution is readily achieved. Our results promote friction force microscopy in water as a robust alternative to ultra-high-vacuum measurements

Sublattice Localized Electronic States in Atomically Resolved Graphene-Pt(111) Edge-Boundaries

Understanding the connection of graphene with metal surfaces is a necessary step for developing atomically precise graphene-based technology. Combining high-resolution STM experiments and DFT calculations, we have unambiguously unveiled the atomic structure of the boundary between a graphene zigzag edge and a Pt(111) step. The graphene edges minimize their strain by inducing a 3-fold edge-reconstruction on the metal side. We show the existence of an unoccupied electronic state that is mostly localized on the C-edge atoms of one particular graphene sublattice, which could have implications in the design of graphene based devices

Bioengineering a Single-Protein Junction

Bioelectronics moves toward designing nanoscale electronic platforms that allow <i>in vivo</i> determinations. Such devices require interfacing complex biomolecular moieties as the sensing units to an electronic platform for signal transduction. Inevitably, a systematic design goes through a bottom-up understanding of the structurally related electrical signatures of the biomolecular circuit, which will ultimately lead us to tailor its electrical properties. Toward this aim, we show here the first example of bioengineered charge transport in a single-protein electrical contact. The results reveal that a single point-site mutation at the docking hydrophobic patch of a Cu-azurin causes minor structural distortion of the protein blue Cu site and a dramatic change in the charge transport regime of the single-protein contact, which goes from the classical Cu-mediated two-step transport in this system to a direct coherent tunneling. Our extensive spectroscopic studies and molecular-dynamics simulations show that the proteins’ folding structures are preserved in the single-protein junction. The DFT-computed frontier orbital of the relevant protein segments suggests that the Cu center participation in each protein variant accounts for the different observed charge transport behavior. This work is a direct evidence of charge transport control in a protein backbone through external mutagenesis and a unique nanoscale platform to study structurally related biological electron transfer

Submolecular Imaging by Noncontact Atomic Force Microscopy with an Oxygen Atom Rigidly Connected to a Metallic Probe

In scanning probe microscopy, the imaging characteristics in the various interaction channels crucially depend on the chemical termination of the probe tip. Here we analyze the contrast signatures of an oxygen-terminated copper tip with a tetrahedral configuration of the covalently bound terminal O atom. Supported by first-principles calculations we show how this tip termination can be identified by contrast analysis in noncontact atomic force and scanning tunneling microscopy (NC-AFM, STM) on a partially oxidized Cu(110) surface. After controlled tip functionalization by soft indentations of only a few angstroms in an oxide nanodomain, we demonstrate that this tip allows imaging an organic molecule adsorbed on Cu(110) by constant-height NC-AFM in the repulsive force regime, revealing its internal bond structure. In established tip functionalization approaches where, for example, CO or Xe is deliberately picked up from a surface, these probe particles are only weakly bound to the metallic tip, leading to lateral deflections during scanning. Therefore, the contrast mechanism is subject to image distortions, artifacts, and related controversies. In contrast, our simulations for the O-terminated Cu tip show that lateral deflections of the terminating O atom are negligible. This allows a detailed discussion of the fundamental imaging mechanisms in high-resolution NC-AFM experiments. With its structural rigidity, its chemically passivated state, and a high electron density at the apex, we identify the main characteristics of the O-terminated Cu tip, making it a highly attractive complementary probe for the characterization of organic nanostructures on surfaces