Golden cat

A more environmentally friendly way to make ethylene (a primary feedstock for the chemical industry, which also goes by the name of ethene) would use natural gas as the raw material rather than cracking crude oil. Now, a golden opportunity in the form of a two-centred gold complex has come to light.

Ethylene is one of the chemical industry’s primary feedstock materials from which a whole range of different compounds are made, among them the coolant and antifreeze compound ethylene glycol and plastics such as polyethylene. Ethylene is currently produced from oil by steam cracking. However, direct conversion of natural gas, methane, to ethylene could also be viable on an industrial scale if a suitable catalyst and reaction setup were developed. One of the big advantages of such an approach is that methane supplies are likely to provide us with raw material long after oil has dwindled.

Scientists working with Thorsten Bernhardt at the University of Ulm, Germany and Uzi Landman at the Georgia Institute of Technology, in Atlanta, Georgia, USA, explain that the problem with the conversion of methane to ethylene is one of bond strength. The bonds between carbon and hydrogen atoms in the methane molecule, are very strong and so difficult to break. To compel the carbon in methane to form bonds with other carbon atoms requires a lot of energy. Additionally, any efforts to do so, usually produce lots of side-products containing more than the requisite two carbon atoms found in ethylene.

Activating methane to allow it to form bigger molecules, such as ethylene, is a complex process. The team explains that in order to find a suitable catalyst a clearer understanding of the process at the molecular level is needed. As such Bernhardt and colleagues investigated different catalytic metal clusters, aggregates of a few metal atoms, as model systems for the process. They found that particles containing two charged gold atoms (ions) in pairs could convert methane to ethylene at low temperatures and pressures in the gas phase without forming significant quantities of side-products.

methane to ethane using a golden cat (Credit: Bernhardt et al/Wiley-VCH)
methane to ethane using a golden cat (Credit: Bernhardt et al/Wiley-VCH)

In order to understand how this process works, the team “trapped” the reaction intermediates and used computational techniques to model the energy levels of starting materials, catalyst, intermediates, and products. They found that each gold ion in the dimer binds to a single methane molecule, this severs hydrogen from the methane and the two carbon atoms can then form a single bond to each other. This is a precursor to ethylene itself which has a strong double bond between its carbon atoms. The precursor initially remains bound to one of the gold ions, while the second gold ion, suddenly finds itself able to bond to a new methane molecule. In the final step, this second methane molecule nudges away the ethylene precursor, the carbon single bond, which can then form a carbon double bond. The newly trapped methane then pairs up with yet another methane and the cycle begins again.

“Both the activation of the carbon-hydrogen bonds of the methane and the subsequent splitting off of the ethylene molecule require cooperative action of several atoms bound to the gold dimer,” Berhnardt explains. “Our insights are not only of fundamental interest, but may also be of practical use.”

LINKS

Angew Chem Int Edn 2010, 49, 980-983
http://dx.doi.org/10.1002/anie.200905643

Bernhardt
http://www.uni-ulm.de/iok/bernhardt/

Cool for cats

Chemists in the US have demonstrated a definitive link between the size of catalyst particles on a solid surface, their electronic properties and their ability to accelerate a chemical reaction. The study could help improve the design of yet more-efficient catalysts to reduce energy requirements for countless industrial processes and cut greenhouse gas emissions.

Ideally, catalysts are substances that speed chemical reactions without themselves being consumed in the reaction. In reality, they are never 100 percent efficient, can be degraded by repeated reaction cycles, and often become poisoned by by-products. Nevertheless, they are at the heart of thousands of chemical reactions used to make everything from pharmaceuticals to plastics.

“One of the big uncertainties in catalysis is that no one really understands what size particles of the catalyst actually make a chemical reaction happen,” says Scott Anderson of the University of Utah. “If we could understand what factors control activity in catalysts, then we could make better and less expensive catalysts.”

Catalysts are commonly made from rare metals including, gold, rhodium, palladium, and platinum, and there is typically a range of catalyst particle sizes present. In almost all cases, the size of the most active particles is unknown. In gold catalysts, which have been intensively studied recently, it has been shown that the bulk of the metal in a catalyst powder exists in the form of particles that are too big to do any catalysis, and only a small fraction of the metal is active.

“If you could make a catalyst with only the right size particles, you could save 90 percent of the cost or more,” asserts Anderson. He also points out that switching to cheaper and more common metals, such as zinc, nickel, and copper, and “tuning” their properties would also let chemists reduce costs significantly. The process of tuning such base metals would involve reducing the particle size until it reaches a catalytic optimum, which is the focus of the Utah team’s work.

Previous work showed how to alter electronic and chemical properties of a catalyst in a gas, but things are different once the particles are mounted on a metal oxide surface for real-life industrial processes.

Anderson and Kaden working to accelerate chemical reactions efficiently (Credit: William Kunkel, University of Utah)
Anderson and Kaden working to accelerate chemical reactions efficiently (Credit: William Kunkel, University of Utah)

In the new study, Anderson and his students took a step toward tuning catalysts to have desired properties. In work with Bill Kaden and William Kunkel, and Tianpin Wu, the team has demonstrated, for the first time, that the size of palladium metal catalyst “nanoparticles” deposited on a titanium dioxide surface affects not only the catalyst’s level of activity in converting carbon monoxide to carbon dioxide, but also the particles’ electronic properties.

As the size of a catalyst metal particle is reduced to the nanoscale, its properties initially remain the same as bulk metal. However, when the particles are just 10 nanometres across (containing 10,000 atoms or so) the movements of electrons in the metal become confined, so boosting their energy.

When there are fewer than about 100 atoms in catalyst particles, the size variations also result in fluctuations in the electronic structure of the catalyst atoms. Those fluctuations strongly affect the particles’ ability to act as a catalyst, Anderson says.

The study not only showed how catalytic activity varies with catalyst particle size, “but we have been able to correlate that size dependence with observed electronic differences in the catalyst particles,” Kaden adds. “People had speculated this should be happening, but no one has ever seen it.”

Links:

Science, 2009, in press
Scott L. Anderson homepage

Chips are down

Graphene is a modified form of the all-carbon pencil lead material graphite and is being touted as the material of choice for a future generation of computer chips to augment, or even usurp, silicon. Now, three research teams have devised new approaches to handling graphene that could accelerate development of this material.

Carbon has several allotropes – same element, different forms. Graphite is the stuff of pencil lead and exists as layer upon layer of hexagonally patterned chicken wire type sheets with a carbon at each vertex. Diamond is the hardest known materials and exists as a robust tetrahedrally bonded network of carbon atoms. Fullerenes and nanotubes are tiny spheres, spheroids, and tubes. Amorphous carbon, which has a mixture of the trivalent and tetravalent bonded carbon atoms. Graphene is akin to single layers of graphite.

Nitin P. Padture

Nitin P. Padture

Andre Geim and colleagues at The University of Manchester and colleagues focused on experimental measurements of the intriguing electronic properties of graphene after theoreticians had predicted them. Now, research teams around the globe are further investigating this intriguing substance. However, processing graphene is not without limitations. As such, various efforts have focused on ways to simplify the handling of the material.

Rod Ruoff and his colleagues at the University of Texas at Austin have found a way to disperse chemically modified graphene in a wide variety of organic solvents. This could open the door to developing graphene in conductive films, polymer composites, ultracapacitors, batteries, paints, inks and plastic electronics, the team says.

Rod Ruoff

Rod Ruoff

By using ‘solubility parameters’ ubiquitously applied by industry to determine the solvents most likely to dissolve certain materials or to create good colloids, we have developed a set of solubility parameters for chemically modified graphenes, explains Ruoff. We believe that this approach will have exceptional utility for technology transition in use of colloidal suspensions of graphene sheets.

Tomás Palacios

Tomás Palacios

In parallel, but unconnected work, a team at Ohio State University, led by Nitin Padture are developing a technique for mass producing computer chips made from graphene. Graphene has huge potential, it’s been dubbed the new silicon, says Padture, but there hasn’t been a good process for high-throughput manufacturing it into chips.

Graphene’s chickenwire structure

Graphene’s chickenwire structure

He and his colleagues have found a way to mesh the graphene fabrication process with standard microelectronics manufacturing methods. In their first series of experiments, the team stamped high-definition features just ten graphene layers thick on to a silicon oxide substrate, making this a potential mass-production method.

A graphene frequency multiplier (Photo by Donna Coveney)

A graphene frequency multiplier (Photo by Donna Coveney)

In other work to be published in the May issue of Electron Device Letters, MIT researchers, led by Tomás Palacios, have built an experimental graphene chip known as a frequency multiplier. Frequency multipliers are widely used in telecommunications and computing applications. However, current technology suffers from noise interference that requires energy-intensive filtering. The graphene frequency multiplier system has but a single transistor and so, these researchers say, efficiently produces a very clean output that needs no filtering.

Further reading

Nano. Lett., 2009, in press
http://dx.doi.org/10.1021/nl803798y

Adv Mater, 2009, 21, 1243-1246
http://dx.doi.org/10.1002/adma.200802417

Nanoscience and Technology Lab
http://bucky-central.me.utexas.edu/

Nitin P. Padture homepage
http://www.matsceng.ohio-state.edu/faculty/padture/padturewebpage/

Tomás Palacios homepage
http://web.mit.edu/tpalacios/

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