Ab-initio modeling of water oxidation catalysts: Co-Pi and Ru4-POM

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1 Ab-initio modeling of water oxidation catalysts: Co-Pi and Ru4-POM Simone Piccinin CNR-IOM, Trieste (Italy) CECAM Conference Energy from the Sun: Computational Chemists and Physicists Take up the Challenge Chia (CA), Sept. 2012

Artificial photosynthesis: solar to fuel 2H 2 O O 2 + 2H 2 ΔG 0 = 4.92 ev, E 0 NHE = -1.

2 Artificial photosynthesis: solar to fuel 2H 2 O O 2 + 2H 2 ΔG 0 = 4.92 ev, E 0 NHE = 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 Figure from: Lewis and Nocera, PNAS 103, (2006)

Water oxidation catalysts Heterogeneous catalysts RuO 2, IrO 2, Co 3 O 4 Homogeneous catalysts Single- and multi-center TM-based catalysts (Ru, Ir, Co, Mn) RuO 2 (110 and 0001) under UHV Blue dimer

3 Water oxidation catalysts Heterogeneous catalysts RuO 2, IrO 2, Co 3 O 4 Homogeneous catalysts Single- and multi-center TM-based catalysts (Ru, Ir, Co, Mn) RuO 2 (110 and 0001) under UHV Blue dimer (T. Meyer 1982) The surface of the catalyst under reaction conditions is difficult to characterize Stable under suitable ph conditions 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

4 Outline of the talk Heterogeneous catalysts Homogeneous catalysts Co-Pi: What is the structure? Ru4-POM: What is the reaction mechanism?

5 Co-Pi: experimental determination of the structure SEM of electrodeposited film Amorphous Electrodeposited ( 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, (2010)

6 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 ( 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, (2010) (c)

7 Computational approach Metadynamics (Shell Model) DFT optimizations EXAFS simulations

CV2: # Co bridging 2 O atoms Σ ikj CN (Co k -O i ) x CN (Co k -O j )

8 Co-Pi: metadynamics simulations (Shell Model) To drive the formation of Co-O and O-Co-O bonds: CV1: # Co i -O bonds Σ ik CN (Co k -O i ) CV2: # Co bridging 2 O atoms Σ ikj CN (Co k -O i ) x CN (Co k -O j ) PO 4 3- K + Co 40 P 20 K 20 O 120 (random positions) CV1, CV2 Co 3+

9 Co-Pi: free energy landscape

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

11 Co-Pi: formation of cubanes To drive the formation of Co 4 O 4 cubanes: CV3: ΣCN 1 (Co-Co) x x CN 6 (Co-Co) Cubane units are stable even at high temperature

12 Co-Pi: simulated EXAFS grain 5 66 grain Grain grain 1 5 grain Grain 1

13 Co-Pi: summary 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.

14 Water oxidation catalysts Heterogeneous catalysts Homogeneous catalysts Co-Pi: What is the structure? Ru4-POM: What is the reaction mechanism?

15 Ru4-POM 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 e - 4Ce 3+ (ph 1.8) WHY Ru4-POM? Small overpotential ~ 0.35 V No deactivation over days High turnover rate (~ 450 cycles/h)

16 CNT Toma et al., Nature Chemistry 2, 826 (2010)

17 Computational approach Electronic structure DFT calculations with GGA (PBE) / Hybrid (HSE06, B3LYP) functionals (CP2K) Nørskov's approach for the energetics of PCET ½ µ(h 2 ) = µ(h + ) + µ(e - ) ΔG ΔE DFT + ΔZPE DFT - TΔS H H O M G(M-OH 2 ) - (H + + e - ) V 0 M H O G(M-OH) Nørskov et al. J. Phys. Chem. B 108, (2004) ΔG(V) = G(M-OH) + µ(h + ) + µ(e - ) - G(M-OH 2 ) = G(M-OH) + 1/2 µ(h 2 ) - ev - G(M-OH 2 ) = ΔG(V=0) - ev

18 Proposed reaction cycle: 4 PCET 2H 2 O O 2 + 4H + + 4e - (?) Experimental evidence for 4 PCET 1) Raman spectra Sartorel et al. JACS 131, (2009) 2) ph variation of CV peaks Geletii et al. JACS 131, (2009) S. Piccinin and S. Fabris PCCP 13, 7666 (2011)

19 Energetics of the catalytic cycle

20 Energetics of the catalytic cycle ΔG(S0 S4) = 3.38 << 4.92 (4.56) ev

21 Energetics of the catalytic cycle - 4 (e -, H + ) ΔG = 3.38 ev??? - (e -, H + ) - (e -, H + )

22 Energetics of the catalytic cycle - 4 (e -, H + ) ΔG = 3.38 ev Higher oxidation states? - (e -, H + ) - (e -, H + )

23 Energetics of the catalytic cycle S0 -S3 exp A B C S6 S0 S1 S2 S3 S4 S5 S6 S2

24 Mechanism for O-O bond formation S6 O 2 S2 2H 2 O

25 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 )

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

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

28 Metadynamics simulations of the O-O bond formation CV1: O-O coordination number CV2: O-H coordination number PBE/B3LYP estimate: Experimental estimate: ev ev The intramolecular mechanism has a prohibitively high activation energy (2.2 ev)

29 Energetics of the catalytic cycle

30 Energetics of the catalytic cycle 0.79

31 Energetics of the catalytic cycle

32 Energetics of the catalytic cycle - In this path a single Ru atom is involved in the catalytic process

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

Energetics of the catalytic cycle Overpotential: 1.

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

35 Intermediates of the catalytic cycle Mechanism on Ru4-POM

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

(~ 0.3 V) can the core of Ru4-POM be thought of as minimal RuO 2 unit embedded in inorganic H

37 Intermediates of the catalytic cycle Mechanism on Ru4-POM Mechanism on metal oxides M H O H Ru4-POM vs RuO 2 same key intermediates as RuO 2 same reaction mechanism same overpotential (~ 0.3 V) can the core of Ru4-POM be thought of as minimal RuO 2 unit embedded in inorganic H ligands? M H O - (H + + e - ) - (H + + e-) - (H + + e - ) + H 2 O M O M O O - (H + + e - ) M O O

38 Intermediates of the catalytic cycle Mechanism on metal oxides H H O H H O - (H + + e - ) - (H + + e-) - (H + + e - ) + H 2 O O O O - (H + + e - ) O O M M M M M ΔG 2 ΔG 3 ΔG 2 = ΔE(O) - ΔE(OH) + ΔZPE - TΔS ΔG 3 = ΔE(OOH) - ΔE(O) + ΔZPE - TΔS ΔG 2 + ΔG 3 = const (3.2 ± 0.2 ev) J. Rossmeisl, J.K. Nørskov et al. J. Electroanal. Chem. 607, 83 (2007)

39 Ru4-POM vs. RuO 2 (a) G [ev] (b) RuO 2 (110) OH* H 2 O(l) O 2 (g) OOH* O* G 3 G 2 - G 2 - G 3 G [ev] E O [ev]

Ru4-POM vs. RuO 2 (a) G [ev] G [ev] (b) 6 5 4 3 2 1 0-0.5-1 -1.23-1.5-2.

40 Ru4-POM vs. RuO 2 (a) G [ev] G [ev] (b) RuO 2 (110) OH* H 2 O(l) overpotential O 2 (g) OOH* O* G 3 G 2 - G 2 - G E O [ev]

41 Ru4-POM vs. RuO 2 (a) G [ev] G [ev] (b) RuO 2 (110) OH* H 2 O(l) O 2 (g) OOH* O* G 3 G 2 - G 2 - G 3 overpotential E O [ev]

42 Ru4-POM vs. RuO 2 (a) G [ev] G [ev] (b) RuO 2 (110) OH* H 2 O(l) O 2 (g) OOH* O* G 3 G 2 - G 2 - G 3 overpotential E O [ev]

43 Ru4-POM vs. RuO 2 (a) G [ev] (b) RuO 2 (110) OH* H 2 O(l) O 2 (g) OOH* O* G 3 G 2 ΔG 3 - G 2 - G 3 G [ev] ΔG 2 RuO 2 (110) E O [ev]

44 Ru4-POM vs. RuO 2 (a) G [ev] (b) H 2 O(l) RuO 2 (110) Ru 4 -POM OH* OOH* O* G 3 G 2 O 2 (g) - G 2 - G 3 G [ev] Ru 4 -POM RuO 2 (110) E O [ev]

45 Ru4-POM: Summary Mechanism: nucleophilic attack of water molecule on an oxo intermediate Activation energies ~ 0.8 ev Paths involving a single Ru center are also possible, suggesting that the efficiency of this catalyst is not linked to cooperative effects of the four Ru centers, but to the favorable spread of the thermodynamic cost among the key intermediates The thermodynamics of the reaction cycle promoted by Ru4-POM can be understood on the basis of the energetics of metal oxides Ru4-POM is an optimal RuO 2 cluster where every single atom is a surface atom and hence an active site.

46 Summary THANKS TO: Xiaoliang Hu, Alessandro Laio, Stefano Fabris Andrea Sartorel and Marcella Bonchio EU-FP7 Marie Curie, DEISA, CINECA REFERENCES (1) S. Piccinin and S. Fabris, Phys. Chem. Chem. Phys. 13, (2011) (2) S. Piccinin, A. Sartorel, G. Aquilanti, A. Goldoni, M. Bonchio and S. Fabris (2012), submitted (3) X. Hu, S. Piccinin, A. Laio and S. Fabris (2012), submitted

47 H 2 O Ru IV OH Ru V Ru IV H 2 O Ru V OH

48 Co-Pi: fitting the empirical potential Shell Model potential V cs =1/ 2kr cs V ij = A exp( r ij / B) + z i z j / r ij k Q s + Q c Fit against DFT-PBE total energy differences for Co 3 O 4 and K 3 PO 4

Supporting Information

Supporting Information Uncovering the role of oxygen atom transfer in Ru-based catalytic water oxidation Dooshaye Moonshiram, 1,2 Yuliana Pineda-Galvan, 1 Darren Erdman, 1 Mark Palenik, 3, Ruifa Zong,