Artificial Photosynthesis: A Study of Novel Copper Catalysts
Ashley Baker
Photosynthesis is among the most important chemical and biological processes on
Earth. This fundamental operation is necessary for plant and animal life, but the chemical
processes that occur within plants have yet to be completely demystified. While it is known that
photosynthesis involves the formation of carbon-carbon bonds, the specific mechanisms of
those bond formations are unknown and have never been successfully replicated in a lab.
Because it is integral to an array of scientific fields, thoroughly understanding
photosynthesis would be a gateway to environmental and technological advancement. In
addition to the broad implications of such elementary knowledge, artificial photosynthetic cells
would have specific value in closed environments, such as submarines and spacecraft. In these
situations it is impractical or impossible to carry live plants. The current systems used act like
filters that collect impurities from the air but do not actually recycle it.
Before a system can be created that recycles expended air into useful products, the
basic mechanisms of carbon-carbon bond formation must be understood and replicated
artificially. The goal of this research was to synthesize and identify compounds that can be used
to form carbon-carbon bonds. The net products of photosynthesis are oxygen and
carbohydrates, such as glucose. These products form when carbon dioxide is combined with
water and energy from the sun.
This equation is a simplified umbrella version that omits minor, less understood
reactions, including those that result in carbon-carbon bond formation. The first step in carboncarbon bond formation is the creation of a carbon dioxide radical. A “radical” molecule has an
unpaired electron. Unpaired, or radical, electrons are not stable. These unpaired electrons are
analogous to free radicals in the human body that are known to damage cells due to their high
reactivity. Because of their instability and high reactivity, two radical electrons that are near
each other will “snap” together to form a stable bond consisting of two electrons.
The red dots in the above diagram represent radical, or unpaired, electrons on two
carbon dioxide molecules. These radical electrons find each other and form the red line (a
stable bond) to make oxalate. The intent of this research was to create compounds that will first
form bonds with carbon dioxide and then release carbon dioxide radicals that can find each
other to form carbon-carbon bonds, thus mimicking photosynthesis.
The first part of this research was synthesizing a variety of potentially effective
compounds. Ligands are molecules that bind to a metal center. Previous research in our labs
has shown that the ligand di-2-pyridyl ketone is reactive in the presence of carbon dioxide when
bound to platinum. It was reasonable to experiment with copper as a substitute for platinum as a
metal center because copper is less expensive and less toxic. Di-2-pyridyl ketone (dpK) is
similar in structure to another ligand, 2,2’-dipyradylamine (dpN). For this reason, some copperdpN compounds were also synthesized for this research.
dpK
dpN
Additionally, previous research has shown that copper will form clusters with dpK.
Clusters are not the norm in chemistry, and therefore their behavior in the presence of carbon
dioxide had potential to be unusual. Clusters could capture multiple carbon dioxide molecules.
When multiple carbon dioxides are released as radicals (a highly reactive molecule with an
unpaired electron) from a cluster, they would be in close proximity, increasing the likelihood of
forming carbon-carbon bonds.
Although the chemicals of interest were already identified, the research began with a
series of eliminations. After synthesizing copper-dpK compounds using Copper (I) Chloride and
Copper (II) Chloride, it became clear that the time limitations for the research would also limit
the number of compounds that could be thoroughly investigated. Although the Copper (II)
Chloride compounds had promising preliminary test results, it was hypothesized that Copper (I)
would be more reactive in the presence of carbon dioxide. When given an electron, Copper (I)
has an odd number of electrons and is likely be highly reactive, like a radical. The Copper (I)
would be more likely than Copper (II) to give its extra electron away to carbon dioxide to make
carbon dioxide radicals.
Seven compounds were eventually identified as good candidates for further study.
These products were Copper(I)Chloride:dpK in a 1:1 ratio, Copper(I)Chloride:dpK in a 1:2 ratio,
and Copper(I)Chloride:dpN in a 1:2 ratio. Two unique products were derived from each
synthesis with dpK and three unique products were derived from the synthesis with dpN. These
seven products were chosen for their consistent reproducibility and their solubility (how easily
they dissolve into a solution).
Once the products were synthesized, the next step was to run tests to determine their
effectiveness as carbon dioxide catalysts. A catalyst is a substance that increases the rate of a
reaction without being consumed in the reaction. These compounds were to act as electrocatalysts, meaning that they should increase the rate of a reaction that occurs when electricity is
run through a solution containing them. Because radicals are not stable, they require a “push”
in order to form. In nature, this energetic “push” is provided by sunlight. In the lab, the energy
was provided by electrical energy.
The main tool used in determining a compound’s electro-catalytic effectiveness is Cyclic
Voltammetry (a CV scan). Cyclic Voltammetry measures electron movement (also known as
current) through a solution containing a compound while varying the voltage applied to an
electrode (a piece of metal in the solution). In the presence of carbon dioxide, the seven
compounds described above were expected to act as effective electro-catalysts. When in
solution and in the presence of carbon dioxide gas, it was hoped that the catalyst (one of the
copper:dpK or copper:dpN compounds) would first bind to carbon dioxide. Then, the catalyst
would accept an electron from the electrode. The catalyst would then deliver the electron to the
CO2, which would depart from the catalyst. The copper:ligand compound is then available to
accept another electron. Meanwhile, the carbon dioxide is in solution with an extra electron,
making it highly reactive and likely to form a carbon-carbon bond with another radical generated
through the same process.
A carbon dioxide catalyst will generate more current (cause more electrons to move)
when carbon dioxide is present. If carbon dioxide is not present, the electro-catalyst will accept
an electron from the electrode and have no further reaction. This would generate a small
amount of current. If carbon dioxide is present the electro-catalyst will accept an electron, pass it
to the carbon dioxide, the carbon dioxide will leave as a radical, and the catalyst is free to
accept another electron. This generates far more current by allowing more electrons to move
from the electrode onto the catalyst. In addition, the formation of radicals in solution increases
the likelihood of forming carbon-carbon bonds.
In the cyclic voltammetry experiments, several of the dpK compounds proved to be
effective electro-catalysts that were reactive in the presence of carbon dioxide. The
copper(I)chloride:dpK 1:2 was the most effective. CV scans were run with and without carbon
dioxide, and the increase in current only occurred when carbon dioxide was present. The
differences in current can be seen in the following cyclic voltammetry data. The first data set
(blue line only) shows the current generated when the compound is not in the presence of
carbon dioxide. The scan begins at 0V, runs to -2.5V, and reverses until it returns to 0V. The
increase in current that occurs around -1.3V is the movement of electrons from the electrode to
the catalyst. There is no evidence that further electron transfer occurred. The sharp peak that
begins around -2.2V is water contamination in the solution. On the reverse section of the scan,
there are several bumps around -2V, -1.5V, and -0.75V. These changes in current represent the
movement of electrons from the compound back to the electrode.
The second data set shows the differences in current that occur when carbon dioxide is
present in the system. The area under the red line, not only its height, is far greater then when
only nitrogen is present and is indicative of a large increase in current. The reverse section of
the scan shows further evidence that the 1:2 compound is a carbon dioxide catalyst. The bumps
that appear on the blue line do not appear when carbon dioxide is present. The electrons cannot
move from the copper back to the electrode in the second scan because they have left with the
carbon dioxide, as radical electrons. This is precisely the desired result because the formation of
carbon dioxide radicals leads to carbon-carbon bonds.
Further analysis of the CV scans run with the 1:2 compound became necessary because
in the presence of carbon dioxide and electrical energy, bubbles were produced in solution.
Although bubbles can be problematic in cyclic voltammetry due to their tendency to short circuit
electrodes, the gas contained in the bubbles is unknown. The equipment to analyze the gas
produced was not available, but it could potentially be oxygen or carbon monoxide and is
therefore worth further investigation.
Besides investigating the catalytic effectiveness of the compounds, the copper:ligand
products required characterization. When chemicals are combined, the resulting product is not
as simple as addition (in chemistry, one plus two often does not equal three). However, it is
important to understand the structure of a molecule including what atoms are bound together,
whether or not the molecule is a cluster, and what chemical groups have formed (alcohols,
ketones, gem-diols, etc.). Standard spectroscopic methods were used, including Infrared
Spectroscopy (IR) and Nuclear Magnetic Resonance Spectroscopy (NMR). IR is analogous to a
fingerprint of a molecule, while NMR is analogous to an MRI of a molecule.
While these spectroscopic methods are helpful in establishing identities for each
compound, the ultimate goal is to obtain x-ray crystal structures of each molecule. X-ray
crystallography produces a 3-D snapshot of a molecule. The x-ray structure not only shows
what atoms, groups, and bonds are present, but also the overall shape of the molecule,
interactions between the atoms based on bond lengths, and steric strain (suggesting how stable
the molecule likely is). Understanding these interactions can lead to tweaks in the synthesis of
the catalysts (copper:ligand compounds) that would lead to more effective molecules.
The partial crystal structure for the second copper(I):dpK 1:1 compound was obtained.
The structure contained four copper atoms, verifying that a cluster had been made. This partial
crystal structure, which was grown in the presence of carbon dioxide, showed carbon monoxide
bound in the molecule. Because there is an oxygen missing, it is reasonable to assume that
oxygen was released during this process. The ultimate goal of this research is to create a
photosynthetic mimic, meaning that this compound requires more in depth analysis. This partial
crystal structure is shown below, including the dpK ligands whose locations are speculated.
A full crystal structure was also obtained of the copper(I):dpK 1:2 secondary product.
This is the product tested in the graphs shown above that proved to be an effective carbon
dioxide catalyst. The structure shows a single copper bound to two dpK ligands. Water was also
bound on the molecule. This compound appears to have a very high affinity for water, despite
being synthesized in methanol. There is reason to believe that the molecule would bind carbon
dioxide in the same location that water is bound in the crystal structure, enabling it to act as the
carbon dioxide catalyst that it was demonstrated to be. This structure is shown below. Blue
atoms represent nitrogen, red represents oxygen, and white ellipsoid atoms are carbons. The
molecule is a chloride salt, which is why the teal Cl1 atom appears next to the structure.
It is reasonable to believe that this molecule undergoes the catalytic cycle as described
earlier in this paper. In the presence of carbon dioxide, the carbon dioxide would bind where the
water molecule was captured in the crystal structure. When electrons are added to the solution
via an electrode the electron moves to the ligand and eventually to the carbon dioxide. At that
point, it leaves as a carbon dioxide radical, which hopefully finds other radicals in solution and
forms the desired carbon-carbon bonds. The ligand is then free to accept more electrons, and
the cycle can repeat, thus generating more current as more electrons are able to be transferred.
This hypothesis is demonstrated in the following graphic, and the oxalate is reasonable
speculation.
Although there was a great deal of success in this research, there is still much to be done.
A scientific paper will be written and submitted for publication. The paper will include detailed
descriptions of each synthesis, spectroscopic data, and step-by-step experimental procedures.
The crystal structure shown above and others that are still being analyzed at the University of
Virginia will also be included in that paper. There are endless possibilities for the continuation of
this research, including experimenting with other metal centers, such as nickel. The
copper(II):dpK componds showed promise in the preliminary CV scans, and are worth further
investigation. Several of the compounds made were impure and with more time could be
isolated and tested more accurately. These are just a few examples of potential further
research.
This research was made possible by the Sweet Briar College Honors Summer Research
Program and with the help of Dr. Robert M. Granger, Dr. Michal Sabat, and the Sweet Briar
College Chemistry Department.
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