1. DFT modeling of water oxidation: from molecular catalysts to oxide surfaces Simone Piccinin CNR-IOM, Trieste ICTP Workshop on Material Challenges in Devices for Fuel Solar Production and Employment, Trieste 19-23 May 2014

2. Figure from: Lewis and Nocera, PNAS 103, 15729 (2006) 2H 2 O → O 2 + 2H 2  G 0 = 4.92 eV, E 0 NHE = -1.23 V Ox:2H 2 O → O 2 + 4H + + 4e - E 0 NHE = 1.23 V Red:2H + + 2e - → H 2 E 0 NHE = 0.00 V Artificial photosynthesis: “solar to fuel”

3. Water oxidation catalysts Homogeneous catalystsHeterogeneous catalysts Single- and multi-center TM-based catalysts (Ru, Ir, Co, Mn) “Blue dimer” (T. Meyer 1982) Well characterized, the mechanism of reaction can be investigated with a variety of techniques Often short-lived, due to ligand oxidation Requirements Low overpotential No degradation Based on earth-abundant elements RuO 2 (110 and 0001) under UHV RuO 2, IrO 2, Co 3 O 4 The surface of the catalyst under reaction conditions is difficult to characterize Stable under suitable pH conditions

4. Outline of the talk Ru4-POM: What is the reaction mechanism? Co-Pi: What is the structure? Homogeneous catalystsHeterogeneous catalysts α-Fe 2 O 3 : What is the surface structure? Amorphous catalysts

5. Ru4-POM: What is the reaction mechanism? Co-Pi: What is the structure? α-Fe 2 O 3 : What is the surface structure? Amorphous catalysts Part 1 Homogeneous catalystsHeterogeneous catalysts

6. Co-Pi: experimental determination of the structure SEM of electrodeposited film Amorphous Electrodeposited (1.10- 1.25 V, NHE) on ITO Co(II) ions and KPi buffer at pH=7 Co:P:K ~ 2:1:1 ratio Evidence of Co(III) and Co(IV) ions in the film M.W. Kanan and D.G. Nocera, Science 321, 1072 (2008) M.W. Kanan et al., JACS 132, 13692 (2010)

7. Co-Pi: experimental determination of the structure SEM of electrodeposited film EXAFS Ext. X-ray absorption fine structure Co-Pi clusters Complete/incomplete cubanes Amorphous Electrodeposited (1.10- 1.25 V, NHE) on ITO Co(II) ions and KPi buffer at pH=7 Co:P:K ~ 2:1:1 ratio Evidence of Co(III) and Co(IV) ions in the film M.W. Kanan and D.G. Nocera, Science 321, 1072 (2008) M.W. Kanan et al., JACS 132, 13692 (2010) (c) …

8. Computational approach Metadynamics (Shell Model) DFT optimizations EXAFS simulations X. L. Hu, S. Piccinin, A. Laio and S. Fabris, ACS Nano 6, 10497-10504 (2012)

9. Co-Pi: metadynamics simulations (Shell Model) To drive the formation of Co-O and O-Co-O bonds: CV1: # Co i -O bonds CV2: # Co bridging 2 O atoms Co 40 P 20 K 20 O 120 (random positions) CV1, CV2 PO 4 3- K+K+ Co 3+

10. Co-Pi: free energy landscape

11. Co-Pi: formation of cubanes To drive the formation of Co 4 O 4 cubanes: CV3:  CN 1 (Co-Co) x … x CN 6 (Co-Co) Cubanes are terminated with phosphates

12. Co-Pi: simulated EXAFS Grain 1Grain 6

13. Co-Pi: structure Cubanes are present in Co-Pi and stable even at high T Cubanes are always connected to phosphates The cubane-rich portion of the simulated particles reproduce well the experimental EXAFS The cubane-rich portion of our amorphous nanoparticles are reliable structural models of the Co-Pi catalyst surface. X. L. Hu, S. Piccinin, A. Laio and S. Fabris, ACS Nano 6, 10497-10504 (2012)

14. Co-Pi: benchmarking Grain modelsActive site models “cubane” “ion”

15. Water oxidation mechanisms “Nucleophilic attack” Water oxidation:2H 2 O → O 2 + 4H + + 4e - Oxo-coupling: “diagonal” Oxo-coupling: “geminal”

16. Co-Pi: benchmarking Postulated mechanism: nucleophilic attack First-principles calculations: PBE, B3LYP, PBE0, PBE+U vs. CCSD(T)

17. Co-Pi: benchmarking Postulated mechanism: nucleophilic attack Method: “computational NHE” *OH 2 → *OH + e - + H + NHE:  (e - ) +  (H + ) = 1/2  (H 2 )  G 1 (V) = G(*OH) - G(*OH 2 ) -1/2  (H 2 ) – eV J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jonsson, J. Phys. Chem. B 2004, 108, 17886

18.  G/eV B3LYP PBE0 reaction step  G/eV PBE Co-Pi: benchmarking K. Kwapien, S. Piccinin and S. Fabris J. Phys. Chem. Lett. 4, 4223–4230 (2013)

19.  G/eV B3LYP PBE0 reaction step  G/eV PBE Co-Pi: benchmarking K. Kwapien, S. Piccinin and S. Fabris J. Phys. Chem. Lett. 4, 4223–4230 (2013)

20. PBE+U DFT/cubane PBE+U improves singnificantly over PBE Co-Pi: benchmarking

21. U = 0-2 eV U = 4-9 eV B3LYP Co-Pi: benchmarking PBE+U

22. Co-Pi: CPMD in solution CPMD Co-Pi cluster (44 atoms) + 169 water molecules PBE / US PP Grain models Cluster in solution

23. edge-sharingcorner-sharing Co-Pi: CPMD in solution Complete + incomplete cubanes

24. -H +, -e - Co-Pi: energetics in vacuum Method: DFT-PBE + USPP *OH 2 → *OH + e - + H + NHE:  (e - ) +  (H + ) = 1/2  (H 2 )  G 1 (V) = G(*OH) - G(*OH 2 ) -1/2  (H 2 ) – eV J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jonsson, J. Phys. Chem. B 2004, 108, 17886

25. Co-Pi: changing the oxidation state of Co O.S. G2G2 G3G3 BE(*O) Co(II)1.291.801.23 Co(III)1.391.751.47 Co(IV)1.431.591.74 G2G2 G3G3 “Overpotential”: η = 1.59-1.23 = 0.36 V (exp ~ 0.30 V)

26. Co-Pi: summary Phosphates: inhibit oxidations (e - withdrawing wrt H 2 O) Oxidation state: higher oxidation state result in lower overpotentials Coordination: low coordination Co sites lead to higher cost for PCETs

27. Ru4-POM: What is the reaction mechanism? Co-Pi: What is the structure? Homogeneous catalystsHeterogeneous catalysts α-Fe 2 O 3 : What is the surface structure? Amorphous catalysts Part 2

28. Fe 2 O 3 photocatalyst for water oxidation PROS band gap of around 1.9–2.1 eV high stability abundant non toxic CONS CBM +0.2 V NHE  external bias low electron mobility low absorption coefficient high recombination rates slow reaction kinetics (overpotential of around 0.5–0.6 V) K. Sivula, F. Le Formal, and M. Gra ̈ tzel, ChemSusChem 4, 432 (2011).

29. Fe 2 O 3 : surface structure S1S2S3S4S5S6TOP We consider 6 different terminations of the Fe 2 O 3 (0001) surface Method: DFT-PBE+U (4.2 eV) + USPP M. Nguyen, N. Seriani and R. Gebauer, J. Chem. Phys. 138, 194709 (2013) M. Nguyen, N. Seriani, S. Piccinin and R. Gebauer, J. Chem. Phys. 140, 064703 (2014)

30. S1 Fe 2 O 3 : surface structure S2 S3 S5 S6 Bias (V) GG  G (ev/Å 2 ) Varying O & H content Surface Free Energy (V) M. Nguyen, N. Seriani, S. Piccinin and R. Gebauer, J. Chem. Phys. 140, 064703 (2014)

31. Fe 2 O 3 : Bias under photoillumination 0.2 E 0 NHE (V) 2.3 E 0 (H+/H 2 ) 1.23 E 0 (H 2 O/O 2 ) 0.0 2.1 + + + + + + + Bias (V)  G (ev/Å 2 ) Surface Free Energy (V) CBM VBM - - -  O-terminated surfaces

32. Fe 2 O 3 : water oxidation mechanism H2OH2O e - + H + H2OH2O O 2 e - + H + V=0 V=1.23 V=2.3 G2G2 G3G3  G 2 = 2.05 eV,  G 3 = 1.41 eV η = 0.82 V, exp 0.6 V η = 0.77 V with 1 water layer* * P. Liao, J. A. Keith, and E. A. Carter, J. Am. Chem. Soc. 134, 13296 (2012).  G (eV) Step #

33. Fe 2 O 3 : water oxidation mechanism H2OH2O e - + H + H2OH2O O 2 e - + H + V=0 V=1.23 V=2.3 G2G2 G3G3 O.S. G2G2 G3G3 BE(*O) S12.051.413.09 O-vac1.731.792.58 Fe-vac2.180.454.33 N-dop1.861.453.10

34. Ru4-POM: What is the reaction mechanism? Co-Pi: What is the structure? Homogeneous catalystsHeterogeneous catalysts α-Fe 2 O 3 : What is the surface structure? Amorphous catalysts Part 3

35. Ru4-POM: a homogeneous catalyst Synthesized by two research groups: Sartorel et al. JACS 130, 5006 (2008) Geletii et al. Angew. Chem. 47, 3896 (2008) OX:2H 2 O → O 2 + 4H + + 4e - RED:4Ce 4+ + 4e - → 4Ce 3+ (pH 1.8) WHY Ru4-POM? Small overpotential ~ 0.35 V No deactivation over days High turnover rate (~ 450 cycles/h)

36. Toma et al., Nature Chemistry 2, 826 (2010) Ru4-POM @ CNT/graphene C. Ma, S. Piccinin and S. Fabris, PCCP 16, 5333-5341 (2014) Classical MD in explicit water Anchoring Ru4-POM to electrodes

37. Energetics of the catalytic cycle

38.  G(S0 → S4) = 3.38 << 4.92 (4.56) eV

39. Energetics of the catalytic cycle - 4 (e -, H + )  G = 3.38 eV - (e -, H + ) ???

40. Energetics of the catalytic cycle - 4 (e -, H + )  G = 3.38 eV - (e -, H + ) Higher oxidation states ?

41. Energetics of the catalytic cycle

42. Possible mechanisms of O-O bond formation Intramolecular paths (direct mechanism) oxo ligand - oxo in Ru 4 O 4 cluster (as proposed for OEC in PSII) Nucleophilic attack (acid-base mechanism) oxo ligand – oxygen in water (as proposed for the “Blue dimer”)

43. Metadynamics ~ 2ps x 8 replicas, biasing 2 CVs to drive the O-O bond formation

44. Metadynamics simulations of the O-O bond formation CV1: O-O coordination number CV2: O-H coordination number

45. Metadynamics simulations of the O-O bond formation CV1: O-O coordination number CV2: O-H coordination number PBE/B3LYP estimate: 0.79 - 0.96 eV Experimental estimate: 0.83 - 1.01 eV The intramolecular mechanism has a prohibitively high activation energy (2.2 eV)

46. Energetics of the catalytic cycle

47. 0.79

48. Energetics of the catalytic cycle

49. - In this path a single Ru atom is involved in the catalytic process

50. Energetics of the catalytic cycle - Thermodynamically the two paths are equivalent - Activation energies of the PCET steps might favor one of the two paths

51. Energetics of the catalytic cycle - Thermodynamically the two paths are equivalent - Activation energies of the PCET steps might favor one of the two paths Overpotential: 1.53-1.23 = 0.30 V Exp: = 0.35 V

52. Intermediates of the catalytic cycle Mechanism on Ru4-POM

53. Intermediates of the catalytic cycle O HH M O H O O H M M O M O O M - (H + + e - ) + H 2 O - (H + + e - ) Mechanism on metal oxides Mechanism on Ru4-POM

54. Intermediates of the catalytic cycle O HH M O H O O H M M O M O O M - (H + + e - ) + H 2 O - (H + + e - ) Mechanism on metal oxides G2G2  G 2 =  E(O) -  E(OH) +  ZPE - T  S  G 3 =  E(OOH) -  E(O) +  ZPE - T  S G3G3  G 2 +  G 3 = const (3.2 ± 0.2 eV) J. Rossmeisl, J.K. Nørskov et al. J. Electroanal. Chem. 607, 83 (2007)

55. Ru4-POM vs. RuO 2

56. Ru4-POM vs RuO 2 same key intermediates as RuO 2 same reaction mechanism same overpotential (~ 0.3 V) the core of Ru4-POM can be thought of as minimal RuO 2 unit embedded in inorganic ligands Ru4-POM vs RuO 2 same key intermediates as RuO 2 same reaction mechanism same overpotential (~ 0.3 V) the core of Ru4-POM can be thought of as minimal RuO 2 unit embedded in inorganic ligands

57. Volcano plot

58. Summary We modeled the thermodynamics of water oxidation on homogeneous and heterogeneous catalysts Careful benchmarking of theoretical approach leads to good agreement with available experimental measurements Catalytic properties can be rationalized, to a good extent, on the basis on simple descriptors

59. Summary Open issues modeling the electrified interface: effects of electric field on the structure of the electrode/electrolyte interface modeling the reactions in the liquid environment around the electrified interface thermodynamics → kinetics We modeled the thermodynamics of water oxidation on homogeneous and heterogeneous catalysts Careful benchmarking of theoretical approach leads to good agreement with available experimental measurements Catalytic properties can be rationalized, to a good extent, on the basis on simple descriptors

60. Co-Pi Karolina Kwapien, Xiaoliang Hu, Alessandro Laio, Stefano Fabris Fe 2 O 3 Manh-Thuong Nguyen, Nicola Seriani and Ralph Gebauer Ru 4 -POM Changru Ma, Stefano Fabris, Andrea Sartorel and Marcella Bonchio REFERENCES Co-Pi X. Hu, S. Piccinin, A. Laio and S. Fabris (2012), ACS Nano 6, 10497-10504 (2012) K. Kwapien, S. Piccinin and S. Fabris, J. Phys. Chem. Lett. 4, 4223-4230 (2013) Fe 2 O 3 M.-T. Nguyen, N. Seriani, S. Piccinin and R. Gebauer, J. Chem. Phys. 140, 064703 (2014) Ru 4 -POM S. Piccinin and S. Fabris, Phys. Chem. Chem. Phys. 13, 7666-7674 (2011) S. Piccinin, A. Sartorel, G. Aquilanti, A. Goldoni, M. Bonchio, S. Fabris, PNAS 110(13), 4917 (2013) C. Ma, S. Piccinin and S. Fabris, Phys. Chem. Chem. Phys. 16, 5333-5341 (2014) Acknowledgments