A new artificial photosynthesis approach uses
sunlight to turn carbon dioxide into methane,
which could help make natural-gas-powered
devices carbon neutral.
Methane is the main component of natural gas.
Photosynthesis is the process through which
green plants use sunlight to make food for
themselves out of carbon dioxide and water,
releasing oxygen as a byproduct.
Artificial photosynthesis often aims to produce
hydrocarbon fuels, similar to natural gas
or gasoline, from the same starting materials.
The methane-generating method is made possible
by a new catalyst developed through a collaboration
among the University of Michigan, McGill University
and McMaster University.
A paper on the findings is published in the
journal Proceedings of the National Academy
of Sciences.
The solar-powered catalyst is made from abundant
materials and works in a configuration that
could be mass produced.
The researchers think that it could be recycling
smokestack carbon dioxide into clean-burning
fuel within 5-10 years.
Thirty percent of the energy in the U.S. comes
from natural gas.
If we can generate green methane, it’s a
big deal.
The chief advance is that the team has harnessed
relatively large electrical currents with
a device that should be possible to mass produce.
It’s also especially good at channeling
that electricity toward forming methane, with
half of the available electrons going toward
methane-producing reactions rather than toward
byproducts like hydrogen or carbon monoxide.
Previous artificial photosynthesis devices
often operate at a small fraction of the maximum
current density of a silicon device, whereas
this new approach operates at 80 or 90 percent
of the theoretical maximum using industry-ready
materials and earth abundant catalysts.
Turning carbon dioxide into methane is a very
difficult process.
The carbon must be harvested from CO2, which
requires a lot of energy because carbon dioxide
is one of the most stable molecules.
Likewise, H2O must be broken down to attach
the hydrogen to the carbon.
Each carbon needs four hydrogen atoms to become
methane, making for a complicated eight-electron
dance (each carbon-hydrogen bond has two electrons
in it, and there are four bonds).
The design of the catalyst is critical to
the success of the reaction.
The research team’s theoretical and computational
work identified the key catalyst component:
nanoparticles of copper and iron.
The copper and iron hold onto molecules by
their carbon and oxygen atoms, buying time
for hydrogen to make the leap from the water
molecule fragments onto the carbon atom.
The device is a sort of solar panel studded
with nanoparticles of copper and iron.
It can use the sun’s energy or an electrical
current to break down the carbon dioxide and
water.
The base layer is a silicon wafer, not unlike
those already in solar panels.
That wafer is topped with nanowires, each
300 nanometers tall and about 30 nanometers
wide, made of the semiconductor gallium nitride.
The arrangement creates a large surface area
over which the reactions can occur.
The nanoparticle-flecked nanowires are covered
with a thin film of water.
The device can be designed to run under solar
power alone, or the methane production can
be amped up with a supplement of electricity.
Alternatively, running on electricity, the
device could potentially operate in the dark.
In practice, the artificial photosynthesis
panel would need to be connected to a source
of concentrated carbon dioxide—for example,
carbon dioxide captured from industrial smokestacks.
The device may also be configured to produce
synthetic natural gas (syngas) or formic acid,
a common preservative in animal feed.
