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Heterogeneous Methods for the Catalysis of CO2 Reduction

Carbon dioxide (CO2) is known to be the most abundant atmospheric greenhouse gas and continues to rise at an alarming rate. The heterogeneous catalytic conversion of CO2 to liquid fuels seems to be a feasible solution in the effort to mitigate CO2 emissions and restore carbon balance by recycling CO2 into liquid fuels. Although carbon dioxide is a challenging molecule to activate and convert to useful chemical products, CO2 can be returned to a useful state by activating and/or reducing it via heterogeneous catalytic systems. Heterogeneous methods may eventually offer a solution to growing concerns about the depletion of oil resources while also transforming overabundant atmospheric CO2 into usable products.

Electrocatalyic methods for CO2 reduction
In order to reduce CO2, an electrocatalyst attached to the surface must participate in the transfer of an electron at the working electrode and also effectively lower the activation barrier of the chemical reaction for CO2. It is ideal to use an electrocatalyst that transfers an electron as close as possible to the thermodynamic potential of a CO2 reduction reaction (see Figure 1) yielding products such as carbon monoxide, formic acid, formaldehyde, or methanol. Heterogeneous catalytic reactions often require an overpotential, which lowers the conversion efficiency from CO2 to a corresponding product. The overpotential is the difference between what the applied potential is in the electrochemical cell at a given current density versus the standard reduction potential of CO2 transformed to a new chemical product.

==Products of CO2 at various metal electrodes ==

Electrocatalytic metals for CO2 reduction can be classified according to their product selectivity. Cu electrodes are known to yield hydrocarbons and alcohols as major products. Secondly, CO is mainly evolved at Au, Ag, Zn, Pd, and Ga electrodes. Thirdly, formate is produced together with small portions of hydrogen gas at Pb, Hg, In, Sn, Cd, and Tl. Lastly, little to no product is formed from CO2 at Ni, Fe, Pt, and Ti with H2 as a side product. Remarkably, methane, ethane, ethanol, and propanol are produced from CO2 at Cu electrodes while others have reported the efficient formation of CO at metal electrodes in aqueous electrolytes. Furthermore, it is reported that CO is formed at Ni and Pt electrodes and then adsorbed on the electrode surface. The adsorbed CO poisons the electrode surface and prevents further reduction of CO2 on Ni and Pt. Hence, another simpler classification for metal electrodes is CO formation metals, which includes Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt. The formate-forming metals are Pb, Hg, In, Sn, Cd, and Tl.

The formation of a CO2 radical species is rate-determining
The reduction of CO2 on metal electrodes has been studied, and while the understanding the mechanisms for different products is still limited, the step most generally accepted is the rate-determining formation of a radical CO2 species that chemisorbs to the metal. When the bent CO2 species adsorbs to the metal electrode, the metal atoms will interact with the carbon or oxygen or both in CO2 to form a carbon-coordination, oxygen-coordination, or mixed coordination. Since the CO2 metal coordination bond is stabilized mainly by back donation from the metal into the empty CO2 π* orbital, the adsorption strength will depend upon the degree of back donation from the metal atoms of the catalyst.

Challenges associated with aqueous CO2 reduction
Investigations into the heterogeneous electrocatalytic reduction of CO2 commonly encounter issues with poor reaction kinetics, competing reactions of byproducts, surface impurities, poor solubility of CO2, and low selectivity. The greatest challenge facing the advancement of heterogeneous methods of CO2 reduction may be increasing the energy efficiency, which is due to characteristically high overpotentials for CO2 reduction. The competing reaction with the reduction of CO2 is hydrogen evolution. Hence, the ideal metal catalyst for reducing CO2 would have a high overpotential for producing hydrogen in order to greatly increase Faradaic efficiency. Researchers in the field of heterogeneous catalysis for CO2 reduction hope to find the optimal combination of reaction conditions to minimize hydrogen evolution so that catalysts give maximum faradaic efficiencies.

Environmental aspect of CO2 reduction chemistry
Given the finite amount of oil in existence, it seems reasonable we should be searching for a renewable source of carbon for use in the production of liquid hydrocarbon fuels. While the rise in atmospheric CO2 is increasing at an alarming rate, CO2 may provide the desirable abundant, renewable source of carbon for our energy-addicted society. Given in then next 50 years it's estimated the world's population will grow by 3 billion people and by 1 billion cars, studies by the Intergovernmental Panel on Climate Change (IPCC) show that to stabilize the atmospheric concentration of CO2 at 350-400 ppm and limit the global mean temperature increase to 2.0-2.4 °C, global CO2 emissions in 2050 would have to be reduced by 50-80% of the emission levels in the year 2000. The heterogeneous catalytic conversion of CO2 to liquid fuels may be a very feasible solution in the effort to create a carbon neutral cycle and ultimately, mitigate CO2 emissions and hence create a carbon neutral cycle by recycling CO2 into liquid fuels.