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New nitrogen assembly carbon catalyst has potential to transform chemical manufacturing

Scientists at the U.S. Department of Energy’s Ames Laboratory have discovered a metal-free carbon-based catalyst that has the potential to be much less expensive and more efficient for many industrial concerns, including manufacturing of bio- and fossil fuels, electrocatalysis, and fuel cells.

At their most fundamental, these industry processes involve splitting strong chemical bonds, like hydrogen-hydrogen, carbon-oxygen, and carbon-hydrogen bonds. Traditionally this has been accomplished with catalysts that use transition or precious metals, many of them expensive and low in natural abundance — like platinum and palladium.

The scientists performed experiments with a type of heterogeneous catalyst, Nitrogen-Assembly Carbons (NACs), in which the design and placement of nitrogen on the carbon surface greatly influenced the catalytic activity of the material. These N atoms on carbon surfaces were previously believed to be distant from one another, as the close placement of N atoms is thermodynamically unstable. The team in Ames Lab correlated the N precursors and pyrolysis temperature for the NACs synthesis with the N distribution and discovered that meta-stable N assemblies can be made by design and deliver unexpected catalytic reactions. Such reactions include hydrogenolysis of aryl ethers, dehydrogenation of ethylbenzene and tetrahydroquinoline, and hydrogenation of common unsaturated functionalities (such as ketone, alkene, alkyne, and nitro groups). Moreover, the NACs catalysts are robust with consistent selectivity and activity for both liquid and gas phase reactions under high temperature and/or pressure.

“We discovered that how the nitrogen was distributed on the surface of these NACs really mattered, and in the process realized that this was an entirely new kind of chemical activity,” said Ames Laboratory Associate Scientist Long Qi.

“The discovery should enable scientists to design nitrogen assemblies that are able to accomplish more sophisticated and challenging chemical transformations without the need for transition metals” said Ames Laboratory scientist Wenyu Huang. “It broadly applies to many different types of chemical conversions and industries.”

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Materials provided by DOE/Ames Laboratory. Note: Content may be edited for style and length.

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‘Superdiamond’ carbon-boron cages can trap and tap into different properties

A long-sought-after class of “superdiamond” carbon-based materials with tunable mechanical and electronic properties was predicted and synthesized by Carnegie’s Li Zhu and Timothy Strobel. Their work is published by Science Advances.

Carbon is the fourth-most-abundant element in the universe and is fundamental to life as we know it. It is unrivaled in its ability to form stable structures, both alone and with other elements.

A material’s properties are determined by how its atoms are bonded and the structural arrangements that these bonds create. For carbon-based materials, the type of bonding makes the difference between the hardness of diamond, which has three-dimensional “sp3” bonds, and the softness of graphite, which has two-dimensional “sp2” bonds, for example.

Despite the enormous diversity of carbon compounds, only a handful of three-dimensionally, sp3-bonded carbon-based materials are known, including diamond. The three-dimensional bonding structure makes these materials very attractive for many practical applications due to a range of properties including strength, hardness, and thermal conductivity.

“Aside from diamond and some of its analogs that incorporate additional elements, almost no other extended sp3 carbon materials have been created, despite numerous predictions of potentially synthesizable structures with this kind of bonding,” Strobel explained. “Following a chemical principle that indicates adding boron into the structure will enhance its stability, we examined another 3D-bonded class of carbon materials called clathrates, which have a lattice structure of cages that trap other types of atoms or molecules.”

Clathrates comprised of other elements and molecules are common and have been synthesized or found in nature. However, carbon-based clathrates have not been synthesized until now, despite long-standing predictions of their existence. Researchers attempted to create them for more than 50 years.

Strobel, Zhu, and their team — Carnegie’s Gustav M. Borstad, Hanyu Liu, Piotr A. Guńka, Michael Guerette, Juli-Anna Dolyniuk, Yue Meng, and Ronald Cohen, as well as Eran Greenberg and Vitali Prakapenka from the University of Chicago and Brian L. Chaloux and Albert Epshteyn from the U.S. Naval Research Laboratory — approached the problem through a combined computational and experimental approach.

“We used advanced structure searching tools to predict the first thermodynamically stable carbon-based clathrate and then synthesized the clathrate structure, which is comprised of carbon-boron cages that trap strontium atoms, under high-pressure and high-temperature conditions,” Zhu said.

The result is a 3D, carbon-based framework with diamond-like bonding that is recoverable to ambient conditions. But unlike diamond, the strontium atoms trapped in the cages make the material metallic — meaning it conducts electricity — with potential for superconductivity at notably high temperature.

What’s more, the properties of the clathrate can change depending on the types of guest atoms within the cages.

“The trapped guest atoms interact strongly with the host cages,” Strobel remarked. “Depending on the specific guest atoms present, the clathrate can be tuned from a semiconductor to a superconductor, all while maintaining robust, diamond-like bonds. Given the large number of possible substitutions, we envision an entirely new class of carbon-based materials with highly tunable properties.”

“For anyone who is into — or whose kids are into — Pokémon, this carbon-based clathrate structure is like the Eevee of materials,” joked Zhu. “Depending which element it captures, it has different abilities.”

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Controlling the charge state of organic molecule quantum dots in a 2D nanoarray

Australian researchers have fabricated a self-assembled, carbon-based nanofilm where the charge state (ie, electronically neutral or positive) can be controlled at the level of individual molecules, on a length scale of around one nanometre.

Molecular self-assembly on a metal results in a high-density, 2D, organic quantum-dot array with electric-field-controllable charge state, with the organic molecules used as ‘nano-sized building blocks’ in fabrication of functional nanomaterials.

Achieved densities are an order of magnitude larger than conventional inorganic systems.

The atomically-thin nanofilm consists of an ordered two-dimensional (2D) array of molecules which behave as ‘zero dimensional’ entities called quantum dots (QDs).

This system has exciting implications for fields such as computer memory, light-emitting devices and quantum computing.

The School of Physics and Astronomy study shows that a single-component, self-assembled 2D array of the organic (carbon-based) molecule dicyanoanthracene can be synthesised on a metal, such that the charge state of each molecule can be controlled individually via an applied electric field.

“This discovery would enable the fabrication of 2D arrays of individually addressable (switchable) quantum dots from the bottom-up, via self-assembly, says lead author Dhaneesh Kumar.

“We would be able to achieve densities tens of times larger than state-of-the-art, top-down synthesised inorganic systems.”

QUANTUM DOTS: TINY, ‘ZERO-DIMENSIONAL’ POWERHOUSES

Quantum dots are extremely small — about one nanometre across (ie, a millionth of a millimetre).

Because their size is similar to the wavelength of electrons, their electronic properties are radically different to conventional materials.

In quantum dots, the motion of electrons is constrained by this extremely small scale, resulting in discrete electronic quantum energy levels.

Effectively, they behave as ‘zero-dimensional’ (0D) objects, where the degree of occupancy (filled or empty) of their quantised electronic states determines the charge (in this study, neutral or negative) of the quantum dot.

Ordered arrays of charge-controllable quantum dots can find application in computing memory as well as light-emitting devices (eg, low-energy TV or smartphone screens).

Arrays of quantum dots are conventionally synthesised from inorganic materials via top-down fabrication approaches. However, using such ‘top-down’ approaches, it can be challenging to achieve arrays with large densities and high homogeneity (in terms of quantum-dot size and spacing).

Because of their tunability and self-assembling capability, using organic (carbon-based) molecules as nano-sized building blocks can be particularly useful for the fabrication of functional nanomaterials, in particular well-defined scalable ensembles of quantum dots.

THE STUDY

The researchers synthesised a homogeneous, single-component, self-assembled 2D array of the organic molecule dicyanoanthracene (DCA) on a metal surface.

The study was led by Monash University’s Faculty of Science, with support by theory from the Monash Faculty of Engineering.

This atomic-scale structural and electronic properties of this nanoscale array were studied experimentally via low-temperature scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) (School of Physics and Astronomy, under Dr Agustin Schiffrin). Theoretical studies using density functional theory supported the experimental findings (Department of Material Science and Engineering, under A/Prof Nikhil Medhekar).

The researchers found that the charge of individual DCA molecules in the self-assembled 2D array can be controlled (switched from neutral to negative and vice versa) by an applied electric field. This charge state electric-field-control is enabled by an effective tunneling barrier between molecule and surface (resulting from limited metal-adsorbate interactions) and a significant DCA electron affinity.

Subtle, site-dependent variations of the molecular adsorption geometry were found to give rise to significant variations in the susceptibility for electric-field-induced charging.

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