Conversion of Carbon Dioxide into Methanol with Silanes over N-Heterocyclic Carbene Catalysts

Activate and reduce: Carbon dioxide was reduced with silane using a stable N-heterocyclic carbene organocatalyst to provide methanol under very mild conditions. Dry air can serve as the feedstock, and the organocatalyst is much more efficient than transition-metal catalysts for this reaction. This approach offers a very promising protocol for chemical CO₂ activation and fixation.

http://www3.interscience.wiley.com/journal/122295517/abstract?CRETRY=1&SRETRY=0

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Low-temperature oxidation of CO catalysed by Co₃O₄ nanorods

http://www.nature.com/nature/journal/v458/n7239/abs/nature07877.html

Low-temperature oxidation of CO, perhaps the most extensively studied reaction in the history of heterogeneous catalysis, is becoming increasingly important in the context of cleaning air and lowering automotive emissions^{1, 2}. Hopcalite catalysts (mixtures of manganese and copper oxides) were originally developed for purifying air in submarines, but they are not especially active at ambient temperatures and are also deactivated by the presence of moisture^{3, 4}. Noble metal catalysts, on the other hand, are water tolerant but usually require temperatures above 100 °C for efficient operation^{5, 6}. Gold exhibits high activity at low temperatures and superior stability under moisture, but only when deposited in nanoparticulate form on base transition-metal oxides^{7, 8, 9}. The development of active and stable catalysts without noble metals for low-temperature CO oxidation under an ambient atmosphere remains a significant challenge. Here we report that tricobalt tetraoxide nanorods not only catalyse CO oxidation at temperatures as low as –77 °C but also remain stable in a moist stream of normal feed gas. High-resolution transmission electron microscopy demonstrates that the Co₃O₄ nanorods predominantly expose their {110} planes, favouring the presence of active Co³⁺ species at the surface. Kinetic analyses reveal that the turnover frequency associated with individual Co³⁺ sites on the nanorods is similar to that of the conventional nanoparticles of this material, indicating that the significantly higher reaction rate that we have obtained with a nanorod morphology is probably due to the surface richness of active Co³⁺ sites. These results show the importance of morphology control in the preparation of base transition-metal oxides as highly efficient oxidation catalysts.

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Make methane while the sun shines

(http://www.nature.com/news/2009/090205/full/news.2009.83.html)

Nanotubes help turn carbon dioxide and water into natural gas.

Charles Choi

Researchers have used sunlight to convert carbon dioxide and water vapour into a range of fuels faster than ever before, thanks to a nanotube catalyst.

Materials scientist Craig Grimes and his colleagues at Pennsylvania State University in University Park have used hollow titania (titanium dioxide) nanotubes around 135 nanometres wide and a tenth of a millimetre long to catalyse the reaction. Scientists have used titania nanoparticles to speed up this process before, but Grimes and his colleagues were able to generate hydrocarbons around 20 times faster than that achieved in previous studies, thanks to some clever chemistry.

The researchers added a little nitrogen to their nanotubes and loaded copper and platinum nanoparticles onto the surfaces. On its own, titania works best as a catalyst for this reaction in ultraviolet light. But adding nitrogen and copper to the mix shifts the preference of the titania tubes to visible light, Grimes says. And the copper and platinum nanoparticles are thought to speed up the latter stages of the reaction.

The reaction itself also takes place inside the nanotubes, which are hollow and have a large internal surface area thanks to their thin 20-nanometer-thick walls.

Chain reaction

The researchers filled steel tubes with carbon dioxide and water vapour, covered the end of the chambers with a film of their nanotubes, and capped the containers with a quartz window to let light in. The closed chambers were then set outside on on the university campus on sunny days from July to September 2008.

When light falls on the nanotubes, they release energetic charge carriers, which split the water molecules inside them into two reactive components — hydroxide radicals and hydrogen ions. The hydrogen ions combine to form hydrogen gas. The researchers don't yet understand exactly what happens next, but they think that the carbon dioxide also splits to form oxygen and carbon monoxide, which then reacts with gaseous hydrogen to form methane and water.

The devices generated roughly 160 microlitres of the hydrocarbons per hour per gram of their titania nanotubes, a rate at least 20 times higher than in previous studies done with ultraviolet light. The findings are published online in the journal Nano Letters¹.

Physical chemist Michael Grätzel at the Federal Polytechnic School of Lausanne in Switzerland says that the results "are fundamental work that shows that nanotubes might get you a better conversion efficiency than prior approaches". He points out that the efficiency of the catalyst is still quite low, but is optimistic that further work can improve it.

Go with the flow

"This is clearly very nice work, with some excellent science," says electrochemist John Turner at the National Renewable Energy Laboratory in Golden, Colorado. But he cautions that other solutions for dealing with carbon dioxide may prove more viable.

Commercial processes are already available that use carbon dioxide to make a mixture called syngas, which can then be converted to liquid hydrocarbons in a process that can run continuously. But with the new nanotube devices, the chambers would have to be replenished with carbon dioxide and water from time to time to keep the reaction going. "Why do you want to take a nice continuous process and turn it into an intermittent one by coupling it with solar [energy]?," he asks.

But the researchers argue that their process could be made continuous if carbon dioxide and water vapour could be passed through the nanotube film and the methane fuel collected from the other side. Even with their current nanotubes, Grimes calculates that a reflector that concentrates sunlight on a square metre of the nanotube film could yield 500 litres of methane over the course of eight hours.

Grimes, however, agrees that the production rates are still quite low — "to date we are not going to save mankind", he says. But he hopes that depositing copper nanoparticles more evenly onto the surfaces of the nanotubes and making other improvements will help boost their conversion rates by a factor of several thousand. "I believe this can be commercially practical with a given concentrated carbon dioxide source such as coal plant," he says.