Gold Supported on Graphene Oxide: An Active and Selective Catalyst for Phenylacetylene Hydrogenations at Low Temperatures

A constraint to industrial implementation of gold-catalyzed alkyne hydrogenation is that the catalytic activity was always inferior to those of other noble metals. In this work, gold was supported on graphene oxide (Au/GO) and used in a hydrogenation application. A 99% selectivity toward styrene with a 99% conversion in the hydrogenation of phenylacetylene was obtained at 60 °C, which is 100 to 200 °C lower than optimal temperatures in most previous reports on Au catalysts. A series of gold- and palladium-based reference catalysts were tested under the same conditions for phenylacetylene hydrogenation, and the performance of Au/GO was substantiated by studying the role of functionalized GO in governing the geometrical structure and thermal stability of supported Au nanoparticles under reaction conditions

Improving Formate and Methanol Fuels: Catalytic Activity of Single Pd Coated Carbon Nanotubes

The oxidations of formate and methanol on nitrogen-doped carbon nanotubes decorated with palladium nanoparticles were studied at both the single-nanotube and ensemble levels. Significant voltammetric differences were seen. Pd oxide formation as a competitive reaction with formate or methanol oxidation is significantly inhibited at high overpotentials under the high mass transport conditions associated with single-particle materials in comparison with that seen with ensembles, where slower diffusion prevails. Higher electro-oxidation efficiency for the organic fuels is achieved

New Insights into Fundamental Electron Transfer from Single Nanoparticle Voltammetry

The reductive redox behavior of oxygen in aqueous acid solution leading first to adsorbed superoxide species at single palladium coated multiwalled carbon nanotubes (of length ca. 5 μm and width 130 nm) is reported. The small dimensions of the electroactive surface create conditions of high mass-transport permitting the resolution of electrode kinetic effects. In combination with new theoretical models, it is shown that the physical location of the formed product within the double layer of the electrode profoundly influences the observed electron transfer kinetics. This <i>generically</i> important result gives new physical insights into the modeling of the many electrochemical processes involving adsorbed intermediates

Polarity-Free Epitaxial Growth of Heterostructured ZnO/ZnS Core/Shell Nanobelts

Surface-polarity-induced formation of ZnO/ZnS heterojunctions has a common characteristic that ZnS (or ZnO) is exclusively decorated on a Zn-terminated (0001) surface of ZnO (or ZnS) due to its comparatively chemically active nature to an O (or S)-terminated (000–1) surface. Here, we report a polarity-free and symmetrical growth of ZnS on both ZnO±(0001) surfaces to form a new heterostructured ZnO/ZnS core/shell nanobelt via a thermal evaporation method. Remarkably, the ZnS shell is single-crystalline and preserves the structure and orientation of the inner ZnO nanobelt with an epitaxial relationship of (0001)_ZnO//(0001)_ZnS; [2–1–10]_ZnO//[2–1–10]_ZnS. Through this case, we demonstrate that an anion-terminated polar surface could also drive the nucleation and growth of nanostructures as the cation-terminated surface by controlling the growth kinetics. Considering high-performance devices based on ZnO/ZnS heterojunctions, the current ZnO/ZnS nanobelt is advantageous for optoelectronic applications due to its single-crystalline nature and relatively more efficient charge separation along 3D heterointerfaces

Quantifying Single-Carbon Nanotube–Electrode Contact via the Nanoimpact Method

A new methodology is developed to enable the measurement of the resistance across individual carbon nanotube-electrode contacts. Carbon nanotubes (CNTs) are suspended in the solution phase and occasionally contact the electrified interface, some of which bridge a micron-sized gap between two microbands of an interdigitated gold electrode. A potential difference is applied between the contacts and the magnitude of the current increase after the arrival of the CNT gives a measure of the resistance associated with the single CNT–gold contact. These experiments reveal the presence of a high contact resistance (∼50 MΩ), which significantly dominates the charge-transfer process. Further measurements on ensembles of CNTs made using a dilute layer of CNTs affixed to the interdigitated electrode surface and measured in the absence of solvent showed responses consistent with the same high value of contact resistance

Assembly of Three-Dimensional Hetero-Epitaxial ZnO/ZnS Core/Shell Nanorod and Single Crystalline Hollow ZnS Nanotube Arrays

Hetero-epitaxial growth along three-dimensional (3D) interfaces from materials with an intrinsic large lattice mismatch is a key challenge today. In this work we report, for the first time, the controlled synthesis of vertically aligned ZnO/ZnS core/shell nanorod arrays composed of single crystalline wurtzite (WZ) ZnS conformally grown on ZnO rods along 3D interfaces through a simple two-step thermal evaporation method. Structural characterization reveals a “(01–10)_ZnO//(01–10)_ZnS and [0001]_ZnO//[0001]_ZnS” epitaxial relationship between the ZnO core and the ZnS shell. It is exciting that arrays of single crystalline hollow ZnS nanotubes are also innovatively obtained by simply etching away the inner ZnO cores. On the basis of systematic structural analysis, a rational growth mechanism for the formation of hetero-epitaxial core/shell nanorods is proposed. Optical properties are also investigated <i>via</i> cathodoluminescence and photoluminescence measurements. Remarkably, the synthesized ZnO/ZnS core/shell heterostructures exhibit a greatly reduced ultraviolet emission and dramatically enhanced green emission compared to the pure ZnO nanorods. The present single-crystalline heterostructure and hollow nanotube arrays are envisaged to be highly promising for applications in novel nanoscale optoelectronic devices, such as UV-A photodetectors, lasers, solar cells, and nanogenerators

Structure–Activity Studies on Highly Active Palladium Hydrogenation Catalysts by X‑ray Absorption Spectroscopy

Functionalized carbon nanotubes were used to produce Pd-based hydrogenation catalysts. Pd/CNT with small (1–2 nm) Pd particles showed classical catalytic behavior in propyne hydrogenation, with high propene selectivity at moderate conversion levels and propane formation near full conversion. Pd/CNT with larger (∼15 nm) nanoparticles, however, was selective (88%) toward propene even at practically full propyne conversion. An additionally prepared Pd₂Ga/CNT catalyst exhibited even higher propene selectivity at full conversion. All of these materials were studied in situ by X-ray absorption spectroscopy at the Pd K-edge. Pd₂Ga/CNT was stable under all conditions examined without variation in XANES or in the derived EXAFS parameters. Both Pd/CNT samples formed β-hydride under hydrogen, as assessed from the calculated lattice expansion and the characteristic red shift of the XANES maxima. The minor spectroscopic difference between the monometallic catalysts observed at high propyne conversion suggests the decisive role of a Pd–C (subsurface C) contribution in the structure of larger Pd particles, being absent with ultrasmall nanoparticles. In general, all factors (intermetallic phase formation, subsurface C, etc.) that reduce the surface H coverage will give rise to enhanced partial hydrogenation selectivity of palladium when secondary alkene hydrogenation at late bed segments or diffusion issues in the pores are avoided