For molecules to be used as components in molecular machines, methods that couple individual molecules to external energy sources and that selectively excite motion in a given direction are required. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically driven motors have not yet been built, despite several theoretical proposals for such motors. Here we report that a butyl methyl sulphide molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunnelling microscope are used to drive the directional motion of the molecule in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of both the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices.

Spillover of reactants from one active site to another is important in heterogeneous catalysis and has recently been shown to enhance hydrogen storage in a variety of materials [1][2][3][4][5][6][7] . The spillover of hydrogen is notoriously hard to detect or control 1,2,4-6 . We report herein that the hydrogen spillover pathway on a Pd/Cu alloy can be controlled by reversible adsorption of a spectator molecule. Pd atoms in the Cu surface serve as hydrogen dissociation sites from which H atoms can spillover onto surrounding Cu regions. Selective adsorption of CO at these atomic Pd sites is shown to either prevent the uptake of hydrogen on, or inhibit its desorption from, the surface. In this way, the hydrogen coverage on the whole surface can be controlled by molecular adsorption at a minority site, which we term a "molecular cork" effect. We show that the molecular cork effect is present during a surface catalyzed hydrogenation reaction and illustrate how it can be used as a method for controlling uptake and release of hydrogen in a model storage system 1,2,[4][5][6]8 .Hydrogen activation, uptake, and reaction are important phenomena in heterogeneous catalysis, fuel cells, hydrogen storage devices, materials processing and sensing [1][2][3][4][5][6][7][8][9][10][11] . Much attention has been devoted to materials that exhibit facile activation and weak binding of hydrogen, as these properties lead to the best energy landscape for storage or chemical reactivity [12][13][14] . Spillover is a common method by which a reagent can be activated at one location and then reacted at another, and it is commonly invoked to explain the synergistic relationship between metals in an alloy or metal/metal oxide mixtures 1,[3][4][5][6][7]12,15 . For example, in heterogeneous catalysis hydrogen spillover from metal particles to reducible oxide supports is implicated as an important step in a variety of reactions including hydrogenations, hydroisomerizations, and methanol synthesis 1,3,6 . Hydrogen spillover has also been shown to significantly enhance the performance of hydrogen storage materials such as metal organic frameworks, zeolites and many carbon-based nanostructures 2,4-6 . In these cases, the addition of small metal particles, typically Pt or Pd, promotes uptake by activating molecular H 2 and facilitating spillover of hydrogen atoms (H a ) onto the support. Despite these advances, the mechanism of spillover in most systems remains poorly understood, and with the exception of hydrogen bridges 16 in storage systems, methods for mediating the spillover pathway do not exist. In this paper we describe how the hydrogen spillover pathway on the Pd/Cu alloy system can be controlled via the reversible adsorption of a spectator molecule (CO) at minority Pd atom sites. The use of a model system amenable to study by scanning tunnelling microscopy (STM) was critical in order to monitor the detailed distribution of Pd atoms, H a and CO molecules, all of which are distributed heterogeneously at the atomic-scale. This information ca...

Formic acid is a potential hydrogen storage molecule which dehydrogenates to form CO2 and H2 on metal surfaces. However, it can also decompose via a competing dehydration reaction that forms CO and H2O, reducing the amount of H2 produced and poisoning the catalyst with CO. Formic acid re-formation to hydrogen is typically performed by Pt and Pd catalysts, which while highly active for dehydrogenation also catalyze dehydration. Cu is typically not utilized, as it requires prohibitively high temperatures, although Cu surfaces are very selective toward dehydrogenation. We studied the reaction of formic acid on single-atom alloys (SAAs), consisting of single Pt atoms substituted into a Cu lattice. Surface science studies allowed us to relate alloy structure to reactivity and selectivity and visualize reaction intermediates. These experiments revealed that SAAs are able to selectively dehydrogenate formic acid with a 6-fold increase in yield in comparison to Cu. This increase in conversion is due to a more facile dehydrogenation of formic acid to formate on the SAA surface (120 K vs 160 K on Cu(111)). We acquired quantitative desorption and molecular scale imaging data showing spillover of formate from Pt sites to Cu. Increasing the Pt concentration beyond the SAA regime resulted in loss of selectivity. These results prompted us to test SAA nanoparticle (NP) catalysts under realistic conditions. However, only a slight increase in conversion was observed between pure Cu and Pt-Cu SAA NPs. In our surface science studies, dehydrogenation of formate to CO2 and H2 did not occur until above 400 K on both the SAA and pure Cu surfaces, indicating that Pt sites do not catalyze this rate-limiting step. While SAAs do not offer increased reactivity for formic acid dehydrogenation, they do offer significantly lower barriers for O–H bond breaking, which holds promise for other dehydrogenation reactions.

Formic acid is an important molecule, due to its potential for hydrogen storage and the role of formate in methanol synthesis. Formic acid can decompose on metals and oxides via dehydrogenation or dehydration, although dehydrogenation is preferred for most applications. These two pathways are linked via the water−gas shift reaction (WGSR), making them hard to separate, and debate over the mechanisms still exists. Cu catalysts are known to selectively decompose formic acid via dehydrogenation to produce CO 2 and H 2 . Formic acid's interaction with Cu(110) has been extensively studied, but despite the (111) facet being predominant in many nanoparticles, Cu(111) has received little attention. Using temperature-programmed desorption/reaction (TPD/R) and scanning tunneling microscopy (STM), we have probed key steps in the decomposition of formic acid on Cu(111) at the atomic scale, observing intact adsorption and surface intermediates, as well as the surface after product desorption. Our model system allows us to investigate the reaction under conditions where WGSR is inactive. We find that Cu(111) decomposes formic acid 100% selectively through dehydrogenation. At 85 K, formic acid adsorbs molecularly on Cu(111), forming hydrogen-bonded chains in the β configuration. The acid loses a H atom by 160 K, producing the formate intermediate and surface-bound H atoms, both of which are visualized by STM. All molecules at surface step edges react to formate, but on the Cu(111) terraces desorption of formic acid competes with formate production, which limits formate production to 0.05 monolayer. H atoms formed by O−H bond cleavage recombine to form H 2 in a desorption rate limited process by 360 K. CO 2 and H 2 desorb from the surface in reaction rate limited processes at 400 and 450 K due to formate decomposition on terraces and steps, respectively.

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Methanol steam reforming is a promising reaction for on-demand hydrogen production. Copper catalysts have excellent activity and selectivity for methanol conversion to hydrogen and carbon dioxide. This product balance is dictated by the formation and weak binding of formaldehyde, the key reaction intermediate. It is widely accepted that oxygen adatoms or oxidized copper are required to activate methanol. However, we show herein by studying a well-defined metallic copper surface that water alone is capable of catalyzing the conversion of methanol to formaldehyde. Our results indicate that six or more water molecules act in concert to deprotonate methanol to methoxy. Isolated palladium atoms in the copper surface further promote this reaction. This work reveals an unexpected role of water, which is typically considered a bystander in this key chemical transformation.

In this work, our aim was to determine whether L-arginine (a known insulinotropic amino acid) can promote a shift of b-cell intermediary metabolism favoring glutathione (GSH) and glutathione disulfide (GSSG) antioxidant responses, stimulus-secretion coupling and functional integrity. Clonal BRIN-BD11 b-cells and mouse islets were cultured for 24 h at various L-arginine concentrations (0-1 . 15 mmol/l) in the absence or presence of a proinflammatory cytokine cocktail (interleukin 1b, tumour necrosis factor a and interferon g). Cells were assessed for viability, insulin secretion, GSH, GSSG, glutamate, nitric oxide (NO), superoxide, urea, lactate and for the consumption of glucose and glutamine. Protein levels of NO synthase-2, AMP-activated protein kinase (AMPK) and the heat shock protein 72 (HSP72) were also evaluated. We found that L-arginine at 1 . 15 mmol/l attenuated the loss of b-cell viability observed in the presence of proinflammatory cytokines. L-Arginine increased total cellular GSH and glutamate levels but reduced the GSSG/GSH ratio and glutamate release. The amino acid stimulated glucose consumption in the presence of cytokines while also stimulating AMPK phosphorylation and HSP72 expression. Proinflammatory cytokines reduced, by at least 50%, chronic (24 h) insulin secretion, an effect partially attenuated by L-arginine. Acute insulin secretion was robustly stimulated by L-arginine but this effect was abolished in the presence of cytokines. We conclude that L-arginine can stimulate b-cell insulin secretion, antioxidant and protective responses, enabling increased functional integrity of b-cells and islets in the presence of proinflammatory cytokines. Glucose consumption and intermediary metabolism were increased by L-arginine. These results highlight the importance of L-arginine availability for b-cells during inflammatory challenge.

Atomic and molecular self-assembly are key phenomena that underpin many important technologies. Typically, thermally enabled diffusion allows a system to sample many areas of configurational space, and ordered assemblies evolve that optimize interactions between species. Herein we describe a system in which the diffusion is quantum tunneling in nature and report the self-assembly of H atoms on a Cu(111) surface into complex arrays based on local clustering followed by larger scale islanding of these clusters. By scanning tunneling microscope tip-induced scrambling of H atom assemblies, we are able to watch the atomic scale details of H atom self-assembly in real time. The ordered arrangements we observe are complex and very different from those formed by H on other metals that occur in much simpler geometries. We contrast the diffusion and assembly of H with D, which has a much slower tunneling rate and is not able to form the large islands observed with H over equivalent time scales. Using density functional theory, we examine the interaction of H atoms on Cu(111) by calculating the differential binding energy as a function of H coverage. At the temperature of the experiments (5 K), H(D) diffusion by quantum tunneling dominates. The quantum-tunneling-enabled H and D diffusion is studied using a semiclassically corrected transition state theory coupled with density functional theory. This system constitutes the first example of quantum-tunneling-enabled self-assembly, while simultaneously demonstrating the complex ordering of H on Cu(111), a catalytically relevant surface.