How to burn water with sunlight: insights from computational chemistry

Transcription

1 How to burn water with sunlight: insights from computational chemistry Andrzej L. Sobolewski Institute of Physics, Polish Academy of Sciences H 2 EUCMOS XXXIV, Coimbra,

2 Motivation The dream reaction: 2H 2 O + nhv vis 2H 2 + O 2 electromagnetic energy chemical energy Reaction of life

3 Photosynthesis ~500 nm The most critical issue in photocatalytic water splitting is the coupling of the one-photon light-absorption event in the chromophore to the coupled fourelectron/four-proton transfer processes which are required for the oxidation of water (2H 2 O +4hν O 2 + 4e + 4H + O 2 + 2H 2 ). Photosystem II

4 The photoelectrochemical approach to water splitting (λ 415nm) TiO 2 Pt ph=4.7 ph biased A. Fujishima, K. Honda, ature 238 (1972) 37 challenges: band gap, charge recombination losses, inefficieny of water oxidation

5 Photodissociation of H 2 O H 2 O + nhv H + OH dissociation energy of water: D 0 = cm -1 = 5.10 ev water dissociates homolytically proper photocatalyser is needed!

6 Photo-acids Chem. Phys. Lett. 321 (2000) 479 hν PyrH + H 2 O + hν Pyr + H 3 O reduction of water Photo-bases H 3 O + + {e - } aq hν (?) Prd + H 2 O + hν PrdH + OH oxidation of water PrdH + + OH - (?)

7 abs Sobolewski & Domcke, J. Phys. Chem. A, 111 (2007) n π* e - ET h + CC2 e - π* h + p x,y π h + products: PyH + H 2 oxidation of ammonia PT

8 π* Pyridine-H 2 O: reaction path for π* electron/proton transfer ADC(2) 2p x (O) PyH + +OH - n π hν conical intersections PyH +OH 2p z (O) 3.0 ev stored as chemical energy - three-state conical intersection X. Liu, A.L. Sobolewski, R. Borrelli, W. Domcke, PCCP 15 (2013) 5957 X. Liu, A.L. Sobolewski, W. Domcke, J. Phys. Chem. A, 118 (2014) 7788

9 R O [Å] Pyridine-H 2 O: barrier for water splitting Py-H 2 O =0.23eV PyH -OH R OH [Å] regeneration of photocatalyser?

10 abs Photophysics of pyridinyl radical (πσ* paradigm) π* ADC(2) IVR σ* π # 2.0 ev stored as chemical energy X. Liu et al.. PCCP 15 (2013) 5957

11 Water splitting with pyridine electron/proton transfer (2) hν hydrogen-bond breaking OH (1) hν water splitting +H 2 O (3) H-detachment H Py + H 2 O +2hν Py + OH + H X. Liu, et al., PCCP 15 (2013) 5957 J. Phys. Chem. A, 118 (2014) 7788

12 Heterocycles absorption vs. the solar spectrum Ac BA AO H 2 H 2 X. Liu et al., Chemical Physics, 464 (2016) 78

13 Chem. Mater. 2016, 28, 5191 ature Commun. 2015, 6, 8508

14 Google Scholar ature Materials 8, 76 (2009) graphitic carbon nitride (melon) g-c water + 10%triethanolamine(ED)

15 Photophysixs of heptazine-h 2 O complex LUMO π 2 * 1 p z π 1 * hν(?) π 1 * HOMO π 1 ππ * 2 1 nπ * 1 ππ * 1 HepH + + OH - π 1 * hν HepH + OH p z p z n S 0

16 abs Heptazine-H 2 O: electron-proton transfer ADC(2) ππ* nπ* ππ* CT 2.0 ev stored as chemical energy J. Ehrmaier, T..V. Karsili, A.L. Sobolewski, W. Domcke, J. Phys. Chem. A, 121 (2017) 4754

17 Heptazine-H 2 O: electron-proton transfer =0.75eV J. Ehrmaier, T..V. Karsili, A.L. Sobolewski, W. Domcke, J. Phys. Chem. A, 121 (2017) 4754

18 abs Heptazine-H 2 O: electron-proton transfer linearly interpolated reaction path ππ* nπ* ADC(2) ππ* CI S 0

19 Heptazinyl radical: H-atom photodetachment σ* hν? H-atom nπ* ππ* π 2.0 ev πσ* stored as chemical energy

20 Heptazinyl radicals: dark H 2 release MP2/cc-pVDZ H ev CO 2 catalyst(?) formic acid ev

21 Direct water splitting with carbon nitride electron/proton transfer (2) hν experimental check: OH radical detection hydrogen-bond breaking OH O 2 MnO (1) hν water splitting (3) H-detachment H Pt H 2 +H 2 O

22 Spectroscopic detection of OH radicals TA: terephtalic acid HTA: hydroxyterephtalic acid

23 Fluorescence intensity of HTA Fluorescence intensity, a.u. TiOP in water: detection of OH radicals with TA TiOP nm, HTA nm without illumination illumination 0.5 h illumination 1.5 h illumination 2.5 h illumination 3.5 h illumination 4.5 h illumination 5.5 h Ti Wavelength, nm 3.5*10-3 M aoh in water Morawski et al., PCCP 16 (2014) 15256

24 Spectroscopic detection of OH radicals non-fluorescent

25 Photochemical water splitting with g-c(h): detection of OH radicals with benzoic acid Irradiation of nm fluorescence nm O. Morawski et al., in preparation

26 Melem 3 : absorption spectrum H

27 Photoreactivity with water g-c(h) g-c(h)-pt dispersed fluorescence of melon in water nm (55 mw/cm 2) fluorescence nm dispersed fluorescence of melon in water nm (55 mw/cm 2) fluorescence nm Morawski et al., in preparation

28 Sacrified electron donors experiments 1 ππ*(ct) X 1 ππ*(le) 1 nπ*(le) O OH HO O 1 ππ*(le) O OH O OH H 2 H 3 C H 3 C X: TA BA H 2 O MeOH 4-MBA MAH

29 Summary: Existing paradigm of water splitting: ew paradigm of water splitting: (1) exciton generation (1) chromophore excitation (2) exciton dissociation (2) electron driven proton transfer (3) diffusion of charge carriers to (photo-redox reaction) solid/liquid interfaces (3) radical generation (4) reaction with water at the interfaces MnO O 2 Pt H 2 -heterocyles are promissing model systems for study fundamental mechanisms of the water splitting process at the molecular level and may provide a template for designing macromolecular systems which can photocatalyse the reaction.

30 The water splitting consortium Theory Experiment Wolfgang Domcke, TUM Olaf Morawski, PAS, Warsaw Xiaojun Liu Beijing Jiaotong University, Beijing Andreas Ehrmaier, TUM Tolga Karsili Temple University, Philadelphia Johannes Ehrmaier, TUM

31 Acknowledgements: AvH Research Award

32 Thank you for your attention! H 2 W. Domcke, J. Ehrmaier, A. L. Sobolewski Solar energy harvesting with carbon nitrides: Do we understand the mechanism? ChemPhotoChem, perspective article, submitted

33 Energy [ev] Pyridine-H 2 O: reaction path for electron/proton transfer triplet states ADC(2) singlet states 6 3 n * (a) 6 1 n * S 0 3 n 5 4 (b) S 0 PyH + + OH - 1 n * hv PyH + OH * 2 1 S S R OH [Å] - three-state conical intersection X. Liu, A.L. Sobolewski, W. Domcke, J. Phys. Chem. A, 118 (2014) 7788

34 R O [Å] Pyridine-H 2 O: barrier for water splitting triplet surface ADC(2) singlet surface Py-H 2 O (a) =0.33eV PyH-OH Py-H O 4.85 (b) R OH [Å] R OH [Å] =0.23eV PyH-OH

35 HeptazineX-H 2 O ADC(2)/cc-pVDZ@MP2 geom. X 1 ππ*(ct) X X H 2 O H 2 C 1 ππ*(le) H 2 H 2 C C C COOH C C HOOC COOH O 2 O 2 O 2 1 ππ*(le)