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The role of the Cu oxidation state on CO2 reduction on copper oxide
surfacesThuy‐My Le
Department of Chemical and Biomolecular EngineeringJohns Hopkins University
Need for alternative fuels
Hubbert’s Prediction of Peak Oil
•It is predicted for oil, which is used for electricity generation and fuel, to peak in the year 2020•CO2 is a harmful product of the combustion of fossil fuels.
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Earth, M. World total CO2 emissions. 2010 [cited; Available from: http://www.climatechangeconnection.org/Emissions/WorldtotalCO2emissions.htm.
Hubbert, M.K., Energy from Fossil Fuels. Science, 1948. 108(2813): p. 589‐590
Pustovaya/iStockphoto, A. Greenhouse gas leaves message in a bottle. [cited; Available from: http://www.sciencemuseum.org.uk/antenna/wineCO2/images/CO2‐pollution.jpg.
Methanol economy
Benefits•Liquid form allows for feasible storage and transportation.• Is an intermediate to other chemicals• Can be converted into dimethyl ether and be used in place of diesel fuel.
•Emits relatively less CO2• Methanol can be mix with gasoline to make M85, which emits less CO2 than regular gasoline.
•No need for large reconstruction of infrastructure•Methanol can be produced by the reduction of CO2
Problems• CO2 reduction electrochemical synthesis is an uphill reaction that requires a stable catalyst.
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Westenhaus, B. Methanol Structure. Brian Westenhaus 2010 [cited; Available from: http://newenergyandfuel.com/http:/newenergyandfuel/com/2008/05/08/is‐methanol‐the‐up‐and‐coming‐alcohol‐for‐fuel/methanol‐structure/.
Le, M., et al. Electrochemical Reduction of CO2 to Methanol at Oxidized Copper Electrodes. 2010. University of Florida.
Current experimental research
0
1/9
1
10
100
‐2 ‐1.8 ‐1.6 ‐1.4 ‐1.2 ‐1 ‐0.8
CH3OH (µmol cm
‐2 h
‐1)
V (SCE)
Oxidized CuAnodized CuElectrodeposited Cu
CO2
e‐
CH4 H2O
O2
H+H+
e‐
•Electrodeposited Cu seems to be the best catalyst. •The amount of methanol is positively correlated with the stability of Cu(I) on surface.
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cathode anode
‐ +
Our objective
• What are the basic mechanisms of CO2reduction?
• Why does Cu(I) compared to Cu(II) provides a better catalysts?
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Materials and methods
Scientific approach: computational modeling of molecule adsorption on copper oxide surfaces
Method: Density functional theory (DFT)•No experimental input needed•Uses Schrödinger’s equation to predict outcomes
Materials: oVienna Ab initio Simulation Package (VASP)
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Possible mechanisms
CO2 (g)???CH3OHCO2(g) CO2(a) [1]
OCOH(a) OC(a) + OH(a) [2]CO2(a) CO(a) + O(a) [3]
CO2
e‐
CH4 H2O
O2
H+H+
e‐
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‐ +
cathode anode
Procedure
• Create surfaces for CuO and Cu2O by implementing low Miller indices
• Adsorb CO2 and COOH on the surfaces• Adsorb products individually to see if splitting CO2 or COOH on the surface is favorable
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Bulk structuresCu(I) vs Cu(II) with different Miller
indices
Cu2O(001) and Cu2O(111)Bulk Structure
CuO (001) and CuO (111) Bulk Structure
Cu (I)Cu (II)
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a = 4.3069 Å (4.27 Å) a = 4.7618 Å (4.6833 Å) b = 3.4795 Å (3.4208 Å)c = 5.1265 Å (5.1294 Å)β = 99.5˚ (99.567˚)
Islam, M.M., et al., Journal of Molecular Structure‐Theochem, 2009. 903(1‐3): p. 41‐48.
Brese, N.E., et al., Journal of Solid State Chemistry, 1990. 89(1): p. 184‐190.
Bare CuO(001) surfacesO‐terminatedCu‐terminated
Side View
Top View
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Bare slab Bare slab VS. Bare slab Bare slab
CO2 CO O
∆E = (ECO2/slab + Eslab) – (ECO/slab + EO/slab)
By above definition if ∆E < 0 the reactants are favored and the reaction is endothermic (obviously the inverse holds)
Adsorption energy
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OCOH(a) OC(a) + OH(a) [2]CO2(a) CO(a) + O(a) [3]
CO adsorbed on O‐terminated CuO (001)
Position 1(desorbed)
Position 2(final)
Position 3(desorbed)
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CO2 / CuO(001) Cu‐terminated
E0=‐123.54 eVE0= ‐147.12 eV
E0= ‐141.42 eV E0=‐131.43 eVΔE = (ECO2/slab + Eslab) – (ECO/slab + EO/slab)
Δ E = 2.19 eV
Slab
CO/Slab O/Slab
CO2 (a) CO (a) + O (a)
CO2/Slab
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Eads = 0.59 eV/CO2
COOH / CuO(001) Cu‐terminated
E0= ‐123.54
E0= ‐141.42 eV E0= ‐136.28 eV
E0= ‐151.78 eV
Δ E = (ECOOH/slab + Eslab) – (ECO/slab + EOH/slab)Δ E = 2.38 eVOCOH (a) OC (a) + OH (a)
SlabCOOH/Slab
CO/Slab OH/Slab
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Eads = 4.11 eV/COOH
Summary of reactions
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∆E = (ECO2/slab + Eslab) – (ECO/slab + EO/slab)
By above definition if ∆E < 0 the reactants are favored and the reaction is endothermic (obviously the inverse holds)
Though other adsorbate structures were found, the most favorable structures were used for comparisons.
∆ ECu2O(100)
Cu term
Cu2O(100)O term
Cu2O(111)
Cu term
Cu2O(111)O term
CuO(001)
Cu term
CuO(001)O term
CuO(111)
Cu term
CuO(111)O term
CO2 (g) CO2 (a) ‐0.55 1.14 2.81 0.94 0.59 ‐0.24 0.77 **
OCOH (a) CO (a) + OH (a) ‐ 0.5 ‐ 3.11 ‐‐ ‐‐ 2.38 ‐3.17 ‐‐ ‐‐
CO2 (a) CO (a) + O (a) 2.78 ‐3.51 0.26 ‐5.96 2.19 ‐2.69 1.42 ‐1.36
Conclusion
Results •Our results are coherent with experimental results. Cu (I) oxides are more favored than Cu (II) Oxides•OCOH(a) OC(a) + OH(a) is not a viable mechanism because OCOH does not adsorb to the surface.
Future Work• We can not claim that DFT results is an exact representation of CO2reduction but it does provide significant insight.
•Experimentalists can do further testing to confirm our results. •Cu2O erodes with time and is not an efficient catalyst
•Possible alternatives include Cu on ZnO•Prior DFT work can be done on Cu/ZnO surfaces to see whether the surface is worth experimental investment
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