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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2018 год >  № 86

№ 86

 

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  • Валентин Николаевич Пармон
    – к 70-летию со дня рождения
  • 60 лет Институту катализа им. Г.К. Борескова СО РАН
  • V Международная школа-конференция по катализу
    для молодых ученых «Каталитический дизайн:
    от исследований на молекулярном уровне к практической реализации»
  • За рубежом
  • Приглашения на конференции



Валентин Николаевич Пармон
– к 70-летию со дня рождения

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60 лет Институту катализа им. Г.К. Борескова СО РАН

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V Международная школа-конференция по катализу для молодых ученых

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Catalyst frees hydrogen from seawater

New solar-powered electrolysis system avoids briny bugbears like chlorine production

Harvesting hydrogen gas from water through electrolysis could lead to a renewable source of fuel. For a small island nation like Singapore, though, fresh water is a precious resource. So electrolysis researchers there have turned their attention to the sea. They have now developed a catalyst that helps to electrolyze seawater with record-breaking efficiency, generating oxygen and hydrogen that could eventually feed fuel cells (Adv. Mater. 2018, DOI: 10.1002/adma.201707261).

The system is powered by solar electricity, producing hydrogen from sunlight with an overall efficiency of 17.9%. “As far as we know, that’s the highest efficiency for seawater,” says  Bin Liu of Nanyang Technological University, who was part of the research team.

The oceans contain a vast store of hydrogen atoms, but freeing them by electrolysis is a huge challenge. In an electrolyzer, the current used to split briny water usually turns chloride ions into unwanted chlorine gas, while other ions like calcium and magnesium form insoluble precipitates that clog vital catalysts on the electrodes. The electrolysis reactions can also cause pH changes that corrode electrodes.

A cobalt hexacyanoferrate catalyst helps to generate bubbles of oxygen gas at the anode (yellow arrow, right) of a solar-powered seawater electrolyzer. Meanwhile, a nickel molybdenum sulfide catalyst at the cathode produces hydrogen gas (arrow, left). Solar panel shown at far left.


Liu’s team had previously developed a nickel molybdenum sulfide catalyst that lowered the voltage needed to generate hydrogen gas at a cathode from seawater (Sci. Adv. 2015, DOI:10.1126/sciadv.1500259). Their new catalyst relies on earth-abundant elements to oxidize seawater at an anode, producing oxygen gas, protons, and electrons.

To make the anode, the researchers grew nanoneedles of basic cobalt carbonate on carbon fiber cloth. Then they dipped the cloth in 2-methylimidazole, which formed a thin layer of a cobalt-imidazole metal organic framework (MOF) on the outside of the needles. Adding sodium ferrocyanide transformed that layer into cobalt hexacyanoferrate, which inherited the porous nanostructure of the MOF and formed 20-nm-thick catalytic shells around the conducting nanoneedles.

With a commercial triple-junction solar cell to supply electricity, the team tested the system using local seawater, adding nothing more than a phosphate buffer to maintain a neutral pH. After 100 hours of continuous operation, the electrolyzer had made hydrogen and oxygen, but no chlorine at all. Moreover, its electrodes and catalysts were intact, and its output had fallen by just 10%. In contrast, an electrolyzer that used conventional catalysts of platinum and iridium oxide to split the local seawater lost activity much more rapidly, and also produced some chlorine.

“The fact that it is selective for oxygen evolution rather than chlorine evolution is very significant,” says  Michael E. G. Lyons, an electrocatalysis researcher at Trinity College Dublin. “It’s a very tricky thing to do.”

Peter Strasser at the Technical University of Berlin, who has worked on seawater electrolysis, points out that the system has a very low current density. To make useful amounts of hydrogen, the system would need a much higher current density, which could trigger chlorine evolution or other unwanted side reactions. “Problems arise when you go to high current densities,” he says.

Liu says that initial tests at higher current densities have not produced any chlorine. But he acknowledges that the system’s performance could be improved. Using fresh water, for example, solar-powered electrolyzers have reached solar-to-hydrogen efficiencies of more than 30% (Nat. Commun. 2016, DOI: 10.1038/ncomms13237). Liu’s team is now working with researchers at the Dalian Institute of Chemical Physics to develop their system into a prototype device for generating hydrogen.

 

Evolved enzymes serve up diverse cyclopropanes

Heme proteins make all possible isomers of prized 3-membered rings


Each engineered heme protein catalyzes formation of a different cyclopropane stereoisomer.

With some engineering in the lab, a quartet of iron-containing heme proteins from microbes can convert inert alkenes into each possible stereoisomer of cyclopropanes, which are valuable motifs in medicinally active compounds (ACS Cent. Sci. 2018,  DOI:10.1021/acscentsci.7b00548). Previous engineered proteins needed help from an artificial cofactor to complete this feat. This work suggests that heme proteins are perfectly capable of doing this chemistry on their own.

Building cyclopropanes with protein catalysts is not new, says Frances H. Arnold, the California Institute of Technology professor who led the work. However, prior heme protein catalysts made by her group and others worked best on relatively reactive alkenes. “These proteins are being commercialized, and our clients want more challenging cyclopropanations,” including transformations of unactivated alkenes, she says.

So graduate student Anders M. Knight and colleagues used directed evolution, which simulates natural selection, to find promising candidates. They optimized four heme-containing proteins from bacteria and archaea, each of which produced a different cyclopropane stereoisomer from the unactivated alkene 1-octene.

The cyclopropane-making reaction, a carbene transfer, takes place inside Escherichia coli cells and works in the presence of alcohols and other groups that might normally interfere with the reaction. Caltech has filed a provisional patent application on the technology.

“This work shows that the diversity of heme proteins in nature, coupled with the power of directed evolution, can be a route to novel stereoselective catalysts,” says  John Hartwig of the University of California, Berkeley. His team has carried out this chemistry using proteins with a nonnatural iridium cofactor. So far, the new heme proteins convert terminal alkenes only, but Hartwig thinks with more work, they could convert internal alkenes too.

Knight agrees. “These active sites are very tunable,” he says. Arnold adds, “I hope that as we do more difficult target substrates, it’ll push people in industry to give this a try.”

Chemical & Engineering News


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