PHYS-P 310/510 Environmental Physics Midterm Presentation

P310/P510—Fall 2016

Carbon Dioxide Catalysis: The Carbon Cycle

Name: LeeAnn Sager Date: October 20, 2016

Why do we care about Carbon Dioxide Catalysis? We have previously learned about the negative impact of excess carbon dioxide in the atmosphere due to its transparency to solar UV-vis light and its absorbance and re-emittance of IR light radiated by the earth. [This property is a feature of a “greenhouse gas” because it leads to a net increase in the heat (and hence temperature) of the Earth similar to how the windows of a greenhouse allow for the heating of the structure.] Carbon dioxide is produced upon the burning of hydrocarbons—the major constituents of many energy sources. Using carbon fuels like gasoline and coal releases carbon dioxide into the atmosphere, which leads to an increase in temperature of the earth—an event that could be potentially catastrophic to the environment and climate of our planet. Additionally, humans are rapidly depleting our known sources of fuels, leading to a search for a new (and hopefully cleaner) energy source. Wouldn’t it be wonderful if we could use the excess carbon dioxide that we no longer want in our atmosphere for fuel? Well, we can. In fact, there are known processes for converting carbon dioxide into methane, ethylene, ethanol, n-propanol, and many other possible fuel sources.1

A Brief Aside on Electrochemistry: First, the basics. Oxidation is the loss of electrons. Reduction is the gain of electrons. A redox reaction is one in which one species loses electrons (is oxidized) and the other gains electrons (is reduced).2 In a chemical system, energy can be stored by going from a lower energy state (arrangement of atoms) to a higher energy state. This is known as an endothermic process—a process that involves the input of energy.2 One possible mechanism for this conversion is known as electrolysis. In electrolysis, a voltage difference is supplied in order to force an unfavorable (endothermic) redox reaction to occur by supplying energy.2 The minimum energy needed can be determined from the reduction half-reactions. Although an understanding of half-reactions is not needed to understand this discussion, Figure 1 below demonstrates the mathematics necessary if A is reduces and B is oxidized.

A++e-→A Eredo=x

B++e-→B Eredo=y

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A++B→A+B+ Ecello=x-y

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Ecello=Eredoreduced species-Eredooxidized species

∆Go=-nFEcello=[J]

n=#electrons transferred F=9.64853x104C/mol

Figure 1: Equation describing how to use half-reactions to determine energy stored via electrolysis (Ecello<0) or released in a galvanic cell (Ecello>0).

Carbon Dioxide Splitting: For carbon dioxide splitting, energy can be stored by going from lower energy carbon dioxide to the higher energy organic constituents of fuel. The energy is then released upon burning the fuels, which produces more carbon dioxide. In most applications of carbon dioxide-fuel conversion, this energy is supplied by photovoltaic cells. [See Ayjaz’s presentation for the pros and cons of photovoltaics.] A table of the energies required to transform CO2 to more useful forms is shown in Figure 2. A common more-activated form is CO, which is then converted into fuel using other catalytic methods3. These processes generally require less energy input. [Conversion of CO to fuels is another branch of research inside inorganic chemistry. More information on CO conversion to ethanol and propanol can be found using reference 3.]

Figure 2: Energies of conversion of CO2 to higher energy forms.4

Problems with Carbon Splitting: Additional energy beyond the theoretical minimum must be inserted into the system in order to form CO (or other higher-energy carbon forms) from CO2. This excess energy is referred to as an overpotential, and it represents the kinetic barrier for electrochemical reactions (i.e. the activation energy).2 This additional energy makes conversion from CO2 to other forms of carbon inefficient and more expensive than ideal for mass production of fuel. A possible solution to this is found via catalysis.

Catalysis: A catalyst is a substance that lowers the energy input necessary through allowing other mechanistic pathways with lower activation energies.2 There are several important classes of carbon dioxide catalysts including metal complexes with macrocyclic ligands5, metal catalysts with phosphine ligands6, metal catalysts with bipyridine ligands7, etc. Figure 3 below shows the three types of catalysts mentioned. Note that the mechanisms through which many of these catalysts form carbon monoxide are not well-defined and are current topics of active research.

Figure 3: Several classes of carbon dioxide catalysts are shown above.

However, all known catalysts have some fault that limit them from universal application and acceptance. They are either expensive, difficult to synthesize, impractical due to long-term degradation, don’t show the needed specificity towards carbon dioxide, require specific electrodes, or still have too large an overpotential to make them practical.7

REFERENCES:

1.  Jhong, H-R.; Ma, S.; Kenis, P.J., Current Opinion in Chemical Engineering, 2013, 2: 191-199.

2.  Shriver, D.; Weller, M.; Overton, T.; Rourke, J.; Armstrong, F. Chapter One: Atomic Structure. Inorganic Chemistry, 6th ed.; W.H. Freeman and Co.: New York, NY, 2014; 34-64.

3.  Stanford University. Stanford scientists discover a novel way to make ethanol without corn or other plants https://www.eurekalert.org/pub_releases/2014-04/su-ssd040514.php (accessed Oct. 2016).

4.  University of Colorado. REDOX HALF-REACTION REDUCTION POTENTIALS AND FREE ENERGIES http://ceae.colorado.edu/~silverst/cven5534/redox half reactions.pdf.

5.  E. Benson, C.P. Kubiak, A.J. Sathrum,, and J.M. Smieja. Chem. Soc. Rev., 2009, 38, 89-99

6.  J. Hawecker, J.M. Lehn, and R. Ziessel. J. Chem. Soc., Chem. Commun., 1984, 328.

7.  S. Slater and J.H. Wagenknecht. J. Am. Chem. Soc., 1984, 106, 5367.

8.  Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. Chem. Soc. Rev. 2014, 43 (2), 631–675.

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