Easing Atmospheric CO2 Levels Using Nanotubes and Sunlight

Feb 16, 2009 By Laura Mgrdichian feature
A digital photograph of one of the reaction chambers under natural sunlight. Photo courtesy Craig Grimes.

(PhysOrg.com) -- Researchers at The Pennsylvania State University have determined a way to use arrays of nanotubes in a solar-based process to convert carbon dioxide and water into methane and other hydrocarbon fuels. Their method may provide a new way to reduce carbon-dioxide levels in the atmosphere—rising due to our planet's heavy use of fossil fuels—as well as produce alternative fuels.

The rate of carbon dioxide (CO2) conversion using this method is 20 times higher than that of previously published research. The work is described in the January 27, 2009, online edition of Nano Letters.

"Every 12 days the world consumes about one billion barrels of oil, which represents the release of almost 1 trillion pounds of carbon dioxide into the atmosphere," said the study's lead researcher, Craig Grimes, to PhysOrg.com. "One way of dealing with this problem is by recycling the CO2 into a high-energy-content fuel, but this makes sense only if a renewable energy source, like solar energy, can be used in the process."

This type of solar-based conversion process only works if a photocatalyst—a material that reacts with light—is used to convert the CO2 into hydrocarbons. A photocatalyst that utilizes the most solar energy possible is the best option.

One popular photocatalyst candidate for the job has been titanium dioxide, also called titania, because it can powerfully react with oxygen. But so far, researchers haven't been able to make titania perform adequately despite experimenting with a variety of forms, such as nanoparticles, pellets, and multi-layer films.

Grimes and his colleagues used arrays of titania nanotubes. They created the nanotubes using a technique that incorporates nitrogen into the nanotubes' structures, which the researchers initially thought would help increase the conversion rate (this turned out to be true only in a very limited capacity).

The process also yields a high total surface area compared to other forms of the material, a property that aids in the conversion. To further boost the process, the group scattered an ultra-thin layer of platinum and/or copper "cocatalyst" nanoparticles on the surface of the array.

The nanotubes were as long as 140 micrometers (millionths of a meter) in length and a diameter of about 115 nanometers (billionths of a meter). The total size of each array sample was about 2 centimeters square and the group created several samples.

The researchers made two reaction chambers, each with a window at the top to let in sunlight. They loaded one sample into each chamber and evacuated the air out, producing a vacuum, and sealed them. Next they pumped CO2 through a tank of water and into the chambers, flushing it through via intake and outtake valves for 10 minutes.

This all took place outdoors on sunny or mostly-sunny days on the Penn State campus. The samples were left outside for 2.5 hours, up to a maxiumum of 3.5 hours, between about 12:30 and 4:00 p.m.

Analysis of the chambers' interiors showed that the predominant product of the conversion was methane, with some ethane, propane, butane, pentane, and hexane, along with other materials in very small concentrations. The conversion rates were high, although comparing the results with other published results was rather difficult, according to the group.

"Most of the previous results were achieved using nanoparticles illuminated by ultraviolet light, so we were not exactly comparing apples to apples," said Grimes. "But going by the weight of the material, we could figure out that the rate of hydrocarbon production we achieved is at least 20 times higher than those previous studies."

Grimes and his group attribute their success, in large part, to the cocatalyst particles they used. They think that a homogeneous distribution of both types could further increase the conversion rate.

More information: Nano Lett., 2009, 9 (2), pp 731-737 DOI: 10.1021/nl803258p

Copyright 2009 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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Arkaleus
4 / 5 (20) Feb 16, 2009
I think there's already an inexpensive, ready-to-market photocatalyst available for CO2 conversion. They're called plants. Try planting lots of them. Not only do you get CO2 conversion, you get oxygen, sugar, and the miraculous process we've known as AGRICULTURE for about the last 10,000 years.

The immense amount of productive capacity to make even one of these nano-can catalysts would require more energy and generate more waste than makes sense for a ecological program.

Plant trees. Plant gardens. Make the earth green. Breathe.

gmurphy
3.7 / 5 (10) Feb 16, 2009
Arkaleus, agreed but plants need water. Deserts have very little water but lots of solar power, these places could make a genuine economic contribution by producing carbon neutral fuels. Of course we would need effective gas separation technology to increase the density of CO2 from the atmosphere.
Mercury_01
2.6 / 5 (10) Feb 16, 2009
This is a grand idea, sure to work. Suppose we build these huge factories and use them to produce billions of tons of physiologically harmful carbon nanananotubes, then we put them outside in the sun and soak up all the co2 we produced in those very factories! Its brilliant I tell you, Brilliant!!!
diego
3.5 / 5 (10) Feb 16, 2009
I think that this would be a good idea but i think just using plants would be a better idea
Mercury_01
1.9 / 5 (7) Feb 16, 2009
actually, a plant can only hold the c02 for as long as its alive.
NeilFarbstein
1.7 / 5 (6) Feb 16, 2009
it's on the right track but 2.5 hours is a bit long.
NeilFarbstein
1.7 / 5 (6) Feb 16, 2009
i find 2 hours to be little too long. Itmight explode in the sunlight.
Arikin
2.6 / 5 (5) Feb 16, 2009
quote: "the predominant product of the conversion was methane, with some ethane, propane, butane, pentane, and hexane"

Aren't these products also polluting when we use them in combustion? Albeit, not as much as coal and oil...

Oh, yes plants only hold the CO2 until they die but by then we are using them to fertilize the next generation of plants... Nice cycle don't you think?
mikiwud
2 / 5 (9) Feb 17, 2009
gmurphy,
Plants can grow with less water in higher concentations of CO2.
Egnite
2.5 / 5 (2) Feb 17, 2009
So this would basically recycle the CO2 so that we can burn it and produce CO2 again. Dunno how that eases anything, the amount of CO2 in the atmosphere will stay the same. True less barrels of oil will be used but its not easing the atmospheric CO2 as stated!

Best way to ease teh CO2 would be natures way imo, plant some forrests and give them a chance to live and die and start the million yr process of turning into fossil fuels again.
Soylent
4 / 5 (4) Feb 17, 2009
Plant trees. Plant gardens. Make the earth green. Breathe.


Almost totally useless for carbon sequestration. 99.9% of that carbon is dumped right back into the atmosphere within a decade.
Sean_W
3.3 / 5 (7) Feb 17, 2009
"Plant trees. Plant gardens. Make the earth green. Breathe."

"Almost totally useless for carbon sequestration. 99.9% of that carbon is dumped right back into the atmosphere within a decade."

Make biochar from them. It releases much of the chemical energy and sequesters much of the carbon while creating a soil additive which retains water and nutrients, keeping them from running off into streams.

Not that I am against the described technology. It is worth investigating.
taisha99
3.7 / 5 (3) Feb 17, 2009
Methane is a worse greenhouse gas than CO2. I agree with the plants idea. Why, in the middle east have they used their petro dollars to build hotels, airports and towns - why haven't they turned their deserts into gardens ?

Evolution knows best ? (Unless someone can run experiments for a few hundred million years to find optimal solutions.

Have a nice day
grahf
5 / 5 (1) Feb 17, 2009
Taisha: They haven't done that because they don't have any water, and desalinization is extremely expensive. Even for those who are flush with petrodollars.
Arkaleus
1.7 / 5 (6) Feb 17, 2009
Organic solutions to organic problems. We need to stop thinking like 19th century industrialists when adapting to our changing world. We are natural, organic beings living in a natural system with cycles and systems of its own. We need to maximize the potential of integrated organic techniologies and sustain ourselves in a manner that does not destroy our habitat. Wasn't this common sense already?
Sean_W
3 / 5 (4) Feb 17, 2009
grahf: The cost of desalination is actually coming down rapidly, I have heard some estimates that it may soon compete for cost with traditional water acquisition like pumping and piping. But the larger problem with greening the deserts is that many of them are quite elevated. It is the energy needed to raise all the water needed which keep people from greening the desert. Or so I am told.
Sean_W
1 / 5 (3) Feb 17, 2009
If you don't pressurize it, methane rises. I wonder if it could be allowed to rise up an inland bound pipe, burned for energy in the desert and the vapor condensed in a cooling tower for local water use. Or would the low density of unpressurized methane gas not be economical as an energy source compared to pressurizing it for use at the lower altitudes? Maybe it is a pipe dream.
Pl0p
2 / 5 (4) Feb 18, 2009
pfs... trees: millions of years of converting the heat of the sun and CO2 into fossil fuels. We burn it up and deforest the globe: create co2 which the trees used, but we cut the trees: it can only result in overheating the planet. There are some other articles stating that that our planet will hold too many people in 2050 so planting forests is not ideal, we lack the space. If we can create a system mimicking photosynthesis and make it such way it's more efficient than trees we can solve our problems. Desalination will be based on the same principle: all problems solved ;)
lengould100
3 / 5 (4) Feb 18, 2009
This system amounts to artificial photosynthesis. Seems it should be worth carefully investigating, on the grounds that nature's photosynthesis is far too inefficient to be of much use (less that 1% conversion of solar energy into carbohydrates even when the plants are in full growth phase, most plants shut down half the year in temperate climates, plants esp high-growth-rate one are quite inefficient users of fresh water resources, etc.), meaning a field of plants neither makes very good use of the sunlight or of the available groundspace as a means to capture CO2.

Provided this method can increase areal and temporal conversion efficiency of insolation by a sufficient factor to compensate for the energy required to manufacture and oprate it, it deserves detailed evaluation.
Sean_W
1 / 5 (3) Feb 18, 2009
Deforestation? Turns out we are not so good at it.

http://www.nytime...tml?_r=1
AtlasGemini
5 / 5 (2) Feb 20, 2009
It is not about sequestration, but about not putting the CO2 in the atmosphere in the first place. If this technology could be made to work, it could reduce the need to burn fossil fuel (i.e. more methane and oil stays sequestered in the ground). One could even imagine a closed-loop whereby NatGas power plants use the methane, pipe the Co2 to solar fields using this technology which would make methane, pipe it back to the power plant, and back and forth, back and forth, ... you get the idea. In this case, the methane/CO2/water work as intermediary carriers/storage of the solar energy.
dachpyarvile
1.4 / 5 (5) Feb 21, 2009
You have got to be kidding me! Convert CO2 into CH4??? CH4 is a worse greenhouse gas than CO2! If we burn the CH4 it will return the CO2 back into the atmosphere anyway.

And, should we convert the CO2 into other hydrocarbons it will end up being burned and release the CO2 into the atmosphere all over again, anyway. What a waste of technology for supposedly saving the planet.

But, if the technology could be applied on the wide scale, it might be possible to at least decrease US dependence on foreign oil if Obama gets his way and no new drilling occurs. :)

If it is viable for use in a closed system, it might help reduce the need for people to use CH4 from other sources. Of course, one cannot get something from nothing. It might not save the planet to use a contraption based upon the technology but it might open up other avenues for the consumer.
dachpyarvile
1 / 5 (3) Feb 21, 2009
pfs... trees: millions of years of converting the heat of the sun and CO2 into fossil fuels. We burn it up and deforest the globe: create co2 which the trees used, but we cut the trees: it can only result in overheating the planet....


Nope. Most of the O2 in the atmosphere comes from the sea and from photosynthesis therein. We could deforest the entire world and still maintain at least 75% of our current atmospheric O2 levels. And, the planet would not overheat anyway. Grass is a more efficient photosynthesizer than trees. More grasses would grow in the absence of trees whose shade often interferes with the growth of grasses.
dachpyarvile
1 / 5 (3) Feb 21, 2009
In addition to this, the planet still would not overheat due to the fact that the planet did not overheat millions of years ago when the planet's CO2 levels were in the 1,000s ppm.
Damon_Hastings
3 / 5 (2) Feb 22, 2009
To the author: it would be nice to see some numbers. If you included the actual conversion rates, then we could get an idea for how practical this technology would actually be for carbon sequestration or fuel production. Too often on physorg I see an article about a fabulous new fuel production technique, only to research it offsite and discover that although it's the fastest technique in its class, it's still ten times slower than the standard technique from another class of technology. And so far as sequestration goes... how do these nanotube arrays compare against, say, a tree? Now, of course, when the tree dies it releases its stored CO2... but so will this methane if/when we burn it.

I'd be hesitant to use methane as our next-gen "fuel of choice", since it's an order of magnitude more potent as a greenhouse gas than CO2. We'd have to be very careful about leaks.
Bolandista
not rated yet Feb 23, 2009
If we are keen to remove carbon from the atmosphere and not let it return we have to find a way to make calcium carbonate(CaCO3).
dachpyarvile
1 / 5 (2) Feb 23, 2009
If we are keen to remove carbon from the atmosphere and not let it return we have to find a way to make calcium carbonate(CaCO3).


Ca(OH)2 CO2 --> CaCO3 H2O

Trouble is, we get calcium hydroxide from the following reaction:

CaO H2O --> Ca(OH)2

It gets worse. We get calcium oxide from the following reaction:

CaCO3 --> CaO CO2

One common form of the reaction is the slaking of lime, lime being calcium carbonate.

We know how to make calcium carbonate. We just need to find a better reaction/catalyst methodology that does not form a wasteful loop.
dachpyarvile
1 / 5 (2) Feb 23, 2009
What is up with the physorg board software??? It omits the plus signs! Reposting a little differently:

Ca(OH)2 plus CO2 --> CaCO3 and H2O

Trouble is, we get calcium hydroxide from the following reaction:

CaO plus H2O --> Ca(OH)2

It gets worse. We get calcium oxide from the following reaction:

CaCO3 --> CaO and CO2

One common form of the reaction is the slaking of lime, lime being calcium carbonate.

We know how to make calcium carbonate. We just need to find a better reaction/catalyst methodology that does not form a wasteful loop.
dachpyarvile
1 / 5 (2) Feb 23, 2009
Hurray for the rempant downrankers of posts that refute their nonsense... :)
HCoin
not rated yet Feb 23, 2009
This is the most exciting possible no lose energy production scenario to come down the pipe yet. Mix water and CO2, get fuel to burn and add oxygen to the air. Burn the fuel in cars, homes and businesses to replace the raw ingredients. Totally renewable, until the sun burns out. Get home from work and fill your car's tank from what's been made on the roof during the day.

More than that, use the ethanol to make solid products and so remove greenhouse gases. An answer to everyone's prayer. No cost of supply, no cost to transport the supply, neutral environmental impact, sustainable until the sun burns out. Wow.
Soylent
not rated yet Feb 24, 2009
More than that, use the ethanol to make solid products and so remove greenhouse gases.


We used to do that with oil. We made these interesting little knick-knacks, bottles and things out of plastic and when they were no longer useful we put them in a landfill.

Now we have to recycle(read waste energy making inferior products and finally burn the degraded crap for warmth).

Why? Because "plastics are like unnatural and stuff, and will last for thousands of years. You're like, hurting the Earth Mother Gaia and you're not... uhmm... you're not in tune with nature, man...".

Can we try a "war on hippies" instead of a "war on drugs" this time?
Soylent
not rated yet Feb 24, 2009
Mix water and CO2, get fuel to burn and add oxygen to the air. Burn the fuel in cars, homes and businesses to replace the raw ingredients. Totally renewable, until the sun burns out. Get home from work and fill your car's tank from what's been made on the roof during the day.


This is never going to happen.

It's unreasonable to expect more than 10% efficiency because it's costly to isolate CO2 out of the air at 380 ppm and any fairly complex molecule requires multiple steps to form; in each step you'll waste a whole bunch of energy because quanta that are above the necessary energy level cannot be used, quanta that are above can be used but all excess energy is waste.

At 10% efficiency you'll need about 25 m^2 in sunny climate for the equivalent of 1 litre of petrol per day(that's about 2400 feet^2 per gallon for you yanks).

This is not about energy. If it's ever cost effective to do this it will be used as feed stock for chemical processes.