The role of the Cu oxidation state on CO 2 reduction on copper oxide surfaces

Transcription

1 The role of the Cu oxidation state on CO 2 reduction on copper oxide surfaces Thuy My Le Department of Chemical and Biomolecular Engineering Johns Hopkins University

2 Need for alternative fuels It is predicted for oil, which is used for electricity generation and fuel, to peak in the year 2020 CO 2 is a harmful product of the combustion of fossil fuels. Hubbert s Prediction of Peak Oil Earth, M. World total CO 2 emissions [cited; Available from: missions.htm. Hubbert, M.K., Energy from Fossil Fuels. Science, (2813): p Pustovaya/iStockphoto, A. Greenhouse gas leaves message in a bottle. [cited; Available from: pollution.jpg. 2

3 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 CO 2 Methanol can be mix with gasoline to make M85, which emits less CO 2 than regular gasoline. No need for large reconstruction of infrastructure Methanol can be produced by the reduction of CO 2 Problems CO 2 reduction electrochemical synthesis is an uphill reaction that requires a stable catalyst. Westenhaus, B. Methanol Structure. Brian Westenhaus 2010 [cited; Available from: methanol the up and coming alcohol for fuel/methanolstructure/. 3

4 Current experimental research + cathode e CO 2 e H + CH 4 H2 O O 2 anode H Oxidized Cu Anodized Cu Electrodeposited Cu CH 3 OH (µmol cm 2 h 1 ) / V (SCE) Electrodeposited Cu seems to be the best catalyst. The amount of methanol is positively correlated with the stability of Cu(I) on surface. Le, M., et al. Electrochemical Reduction of CO 2 to Methanol at Oxidized Copper Electrodes University of Florida. 4

5 Our objective What are the basic mechanisms of CO 2 reduction? Why does Cu(I) compared to Cu(II) provides a better catalysts? 5

6 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) 6

7 Possible mechanisms CO 2 (g)??? CH 3 OH CO 2 (g) CO 2 (a) [1] OCOH(a) OC(a) + OH(a) [2] CO 2 (a) CO(a) + O(a) [3] + cathode e CO 2 e H + CH 4 H2 O O 2 anode H + 7

8 Procedure Create surfaces for CuO and Cu 2 O by implementing low Miller indices Adsorb CO 2 and COOH on the surfaces Adsorb products individually to see if splitting CO 2 or COOH on the surface is favorable 8

9 Bulk structures Cu(I) vs Cu(II) with different Miller Cu (II) indices Cu (I) CuO (001) and CuO (111) Bulk Structure a = Å ( Å) b = Å ( Å) c = Å ( Å) β = 99.5 ( ) Cu 2 O(001) and Cu 2 O(111) Bulk Structure a = Å (4.27 Å) Brese, N.E., et al., Journal of Solid State Chemistry, (1): p Islam, M.M., et al., Journal of Molecular Structure Theochem, (1 3): p

10 Bare CuO(001) surfaces Cu terminated O terminated Side View Top View 10

11 Adsorption energy OCOH(a) OC(a) + OH(a) [2] CO 2 (a) CO(a) + O(a) [3] CO 2 CO O Bare slab Bare slab VS. Bare slab Bare slab E = (E CO2 /slab + E slab ) (E CO/slab + E O/slab ) By above definition if E < 0 the reactants are favored and the reaction is endothermic (obviously the inverse holds) 11

12 CO adsorbed on O terminated CuO (001) Position 1 (desorbed) Position 2 (final) Position 3 (desorbed) 12

13 CO 2 / CuO(001) Cu terminated CO 2 /Slab Slab E ads = 0.59 ev/co 2 E 0 = ev E 0 = ev CO/Slab O/Slab E 0 = ev CO 2 (a) CO (a) + O (a) E 0 = ev ΔE = (E CO2 /slab + E slab ) (E CO/slab + E O/slab ) Δ E = 2.19 ev 13

14 COOH / CuO(001) Cu terminated COOH/Slab Slab E ads = 4.11 ev/cooh E 0 = ev CO/Slab E 0 = OH/Slab E 0 = ev OCOH (a) OC (a) + OH (a) E 0 = ev Δ E = (E COOH/slab + E slab ) (E CO/slab + E OH/slab ) Δ E = 2.38 ev 14

15 Summary of reactions E Cu 2 O (100) Cu term Cu 2 O (100) O term Cu 2 O (111) Cu term Cu 2 O (111) O term CuO (001) Cu term CuO (001) O term CuO (111) Cu term CuO (111) O term CO 2 (g) CO 2 (a) ** OCOH (a) CO (a) + OH (a) CO 2 (a) CO (a) + O (a) E = (E CO2 /slab + E slab ) (E CO/slab + E O/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. 15

16 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 CO 2 reduction but it does provide significant insight. Experimentalists can do further testing to confirm our results. Cu 2 O 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 16