July 4, /07/04 Advanced Course in Photocatalytic Reaction Chemistry 1

Transcription

1 July 4, /07/04 Advanced Course in Photocatalytic Reaction Chemistry 1

2 Advanced Course in Photocatalytic Reaction Chemistry understanding chemistry by understanding photocatalysis understanding photocatalysis by understanding chemistry Division of Environmental Material Science, Graduate School of Environmental Science The first semester of Fiscal :45 10:15, Thursday at Lecture Room D103 Bunsho Ohtani, Ewa Kowalska and Mai Takase Catalysis Research Center, Hokkaido University, Sapporo , Japan (dial-in)/ (facsimile) /07/04 Advanced Course in Photocatalytic Reaction Chemistry 2

3 objectives/goal/keywords << objectives >> Understanding the mechanism of decomposition of pollutants, methods of photocatalysts preparation, design of practical photocatalytic reaction systems, and strategy for enhancement of photocatalytic activity. << goal >> To understand principle of photocatalytic reaction from the standpoint of chemistry and strategy for practical applications. To obtain scientific method for research on functional solid materials. << keywords >> Photocatalyst, Photoinduced oxidative decomposition, Superhydrophilicity, Excited electron-positive hole, Structure-activity correlation, Higher photocatalytic activity, Visible-light response 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 3

4 schedule (1) April 11 introduction of photocatalysis (2) April 18 interaction between substances and light (3) April 25 electronic structure and photoabsorption (4) May 2 thermodynamics: electron and positive hole (5) May 9 adsorption (6) May 16 (Professor Ewa Kowalska) (7) May 23 kinetic analysis of photocatalysis (8) May 30 steady-state approximation (9) June 6 kinetics and photocatalytic activity (10) June 13 kinetic analysis (11) June 20 action spectrum analysis (1) (12) June 27 (Professor Mai Takase) (13) July 4 design and development of photocatalysts (1) (14) July 11 design and development of photocatalysts (2) (15) July 18 design and development of photocatalysts (3) July 25 deadline for submission of special report 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 4

5 special report special report for extra score (20 point) report on critical review on "photocatalysis" in Wikipedia, pointing out errors, misunderstanding and speculations 光触媒 Japanese or English A4 size 2 pages submission by attachment a PDF file is more preferable than a Word file title: pc xxxxxxxx file name: pc xxxxxxxx.pdf (or.docx or.dox) deadline of submission: July 25, : /07/04 Advanced Course in Photocatalytic Reaction Chemistry 5

6 comments on this lecture Please send in Japanese or English within 48 hours to: subject: pc2013mmdd-xxxxxxxx body: (full name) (nickname) (comments and/or questions on today's lecture) 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 6

7 action spectrum analysis action spectrum analysis statistical analysis 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 7

8 super hydrophilicity & oxidative decomposition Photoirradiation onto titania surface induces super hydrophilicity to make watercontact angle to be almost zero. titania-coated : ordinary glass Photoirradiation onto titania film decomposes adsorbed methylene blue. 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 8

9 photocatalytic reaction Photocatalytic reaction is a kind of photoreaction and therefore cannot be a series reaction: a parallel reaction initiated by photoabsorption with short-live species, e.g., photoexcited electrons and positive holes photoabsorption electron-hole pair 2 redox (chemical) reaction 1 3 recombination 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 9

10 necessary conditions for photocatalytic reactions reaction initiated by photoabsorption of photocatalyst (generally accepted) blank test: Copresence of 3 requisites, photoirradiation, photocatalyst (solid material) and reaction substrate(s) is indispensable. Photoreaction initiated by photoabsorption of a compound adsorbed by a solid surface and subsequent electron injection also requires 3 requisites. action spectrum analyses: possible sole technique to prove what absorbs light to initiate the photoreaction checking product(s): adsorption can decrease the amount of substrate(s); stoichiometry photoabsorber (= photocatalyst) remaining unchanged checking turnover frequency: molar ratio of product(s) to photocatalyst to be more than unity 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 10

11 visible-light responsive photocatalyst white titania (TiO 2 ) absorbing only ultraviolet light giving color by SOME treatment(s) activity under visible-light irradiation? titania titania with color =/ visible light responsive titania 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 11

12 measurement of action spectrum plots of apparent quantum efficiency (response normalized by number of incident photons) versus wavelength response (product, current...) apparent quantum efficiency photoreaction/ photocatalytic reaction wavelength wavelength 1 wavelength 2 wavelength 3 wavelength /07/04 Advanced Course in Photocatalytic Reaction Chemistry 12

13 action spectrum photoabsorption efficiency quantum efficiency wavelength/nm wavelength/nm example: discrimination of active crystalline phase in anatase-rutile mixtures [T. Torimoto, et al., Phys. Chem. Chem. Phys., 4, (2002)]. apparent quantum efficiency wavelength/nm action spectrum = apparent quantum efficiency 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 13

14 action spectrum analysis J. Chem. Soc., FaradayTrans.1, 81, 2467 (1985). apparent 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 14

15 action spectrum measurement (1) light source / monochromator / reaction cell reaction cell ca. 0.1 mw cm -2 FWHM: ca. 20 nm cell holder monochromator xenon arc 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 15

16 action spectrum measurement (2) simultaneous irradiation mw cm -2 FWHM ca. 20 nm thermopile xenon arc power meter cell holders wavelength adjustment irradiation port 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 16

17 non and S-doped titania for MB decomposition Yan, X.; Ohno, T.; Nishijima, K.; Abe, R.; Ohtani, B., Chem. Phys. Lett., 429, (2006). S-doped TiO 2 MB P-25 S-doped TiO /07/04 Advanced Course in Photocatalytic Reaction Chemistry 17

18 photochemical MB decomposition MB in suspension S-doped TiO 2 P /07/04 Advanced Course in Photocatalytic Reaction Chemistry 18

19 titania photocatalysts for AcOH decomposition S-doped titania clearly showed the activity under visible-light irradiation. S-doped S-doped TiO 2 P-25 P /07/04 Advanced Course in Photocatalytic Reaction Chemistry 19

20 action spectrum measurement (3) liquid-phase photocatalytic reaction "sharp-cut" optical filter lens optical filter reaction cell (example) L-42 stirrer actual limiting wavelength center of 5 and 72% trans-mission water bath skip wavelength 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 20

21 pseudo action spectrum action spectrum measured by "sharp-cut filters": pseudo action spectrum corresponding to integral of "true" action spectrum plateau part of "pseudo action spectrum" suggests no photoreaction occurring at that wavelength region action spectrum apparent quantum yield pseudo action spectrum wavelength 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 21

22 P-25/MB photocatalytic reaction pseudo action spectrum corresponds to integral of "true" action spectrum action spectrum app integral of action spectrum pseudo action spectrum r /µmol min /nm 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 22

23 collection of papers of less expected citations Proving that MB is inappropriate as a test compound for the reaction under visiblelight irradiation by action spectrum analyses. cited 60 times by June /07/04 Advanced Course in Photocatalytic Reaction Chemistry 23

24 anatase-rutile mixture diffuse reflectance spectra in the unit of absorption normalized at 350 nm 1/2 : wavelength giving half value to that at 350 nm absorption (normalized) Merck P-25 CR-EL(1473 K) TIO Wavelength / nm 1/2 / nm Merck+CR-EL 360 Wako(A) CR-EL Wako(A) +CR-EL P-25(1473K) TIO-5 Aldrich(A<R) CR-EL(1473K) CR-EL Wako(R) 0.5 Wako(A) +CR-EL f anatase Hombikat TIO-2 Merck P-25 f anatase anatase content estimated from XRD patterns skip 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 24

25 test photocatalytic reactions for 35 titanias (a) oxygen evolution along with silver metal deposition 4Ag + + 2H 2 O = 4Ag + O 2 + 4H + (b) methanol dehydrogenation CH 3 OH = HCHO + H 2 (c) oxidative decomposition of acetic acid in water CH 3 COOH + 2O 2 = 2CO 2 + 2H 2 O (d) oxidative decomposition of acetaldehyde in air CH 3 CHO + 5/2O 2 = 2CO 2 + 2H 2 O (e) synthesis of pipecolinic acid from L-lysine L-lysine = PCA + NH /07/04 Advanced Course in Photocatalytic Reaction Chemistry 25

26 action spectra of photocatalytic reaction app (normalized) 1 (a) (b) (c) Wavelength / nm CH 3 OH HCHO + H 2 4Ag + + 2H 2 O 4Ag + O 2 + 4H + CH 3 COOH + 2O 2 2CO 2 + 2H 2 O 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 26

27 1/2 versus f anatase 1/2 / nm absorption edge wavelength anatase: ca. 370 nm rutile: ca. 410 nm oxygen evolution & silver metal deposition P25 (1473 K) CR-EL Wako(A)+CR-EL TIO-5 Aldrich(A<R) Wako(R) CR-EL(1473 K) Merck+CR-EL R >> A f anatase P25 Wako(A) Hombikat Merck 1/2 / nm inner-filter effect by rutile TIO-2 1/2 / nm CR-EL Aldrich(A<R) TIO-5 Wako(R) Wako(A)+CR-EL CR-EL(1473 K) P25 (1473 K) Merck+CR-EL TIO dehyderogenation of methanol f anatase CR-EL(1473 K) P25 (1473 K) Wako(A) Merck 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 27 P Wako(R) Hombikat CR-EL A >> R TIO Merck Merck+CR-EL P25 (1:1) 370 Aldrich(A<R) TIO-5 Wako(A) f anatase R A decomposition of acetic acid

28 action spectra of anatase/rutile/p25 oxidative decomposition of acetic acid in an aqueous solution: carbon dioxide liberation apparent quantum efficiency (%) P25 rutile anatase wavelength/nm P25 amorphous pure anatase pure rutile reconstructed mixture CH 3 COOH (CO 2 ) 91 CH 3 CHO (CO 2 ) 100 CH 3 OH (H 2 ) <Pt> Ag + (Ag/O 2 ) Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. J. Photochem. Photobiol. A Chem. 216 (2010) skip 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 28

29 hydrogen evolution from methanol isolated anatase: A isolated rutile: R platinization: photodeposition (0.2 or 2wt% loading) negligible activity of all bare samples 0.2wt%-Pt loaded P25 ~ A + Pt/R (85:15) 2wt%-Pt loaded P25 ~ Pt/A + Pt/R (85:15) comparable activity of R with A when platinized photodeposition occurs preferentially on rutile particles apparent quantum efficiency wt%Pt/R 0.2wt%Pt/P25 A + Pt/R(85:15) 2wt%Pt/P25 A/Pt + Pt/R (85:15) 2wt%Pt/A wavelength/nm 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 29

30 action spectrum: case 2 Change of (intrinsic) quantum efficiency, i.e., efficiency of electron-hole utilization depending on the irradiation wavelength may induce wavelength/nm photoabsorption efficiency quantum efficiency wavelength/nm shift of action spectrum apparent quantum efficiency wavelength/nm action spectrum skip 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 30

31 wavelength dependence 100% anatase titania powders dehydrogenation of methanol <platinum-loaded/under argon> mineralization of acetic acid <under air> CH 3 OH HCHO + H 2 CH3COOH + 2O2 2CO2 + 2H2O 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 31

32 light intensity dependence 1 CH 3 OH HCHO + H nm Merck (anatase) titania apparent quantum efficiency ( app ) nm 380 nm 395 nm CH3COOH + 2O2 2CO2 + 2H2O dehydrogenation of methanol <platinum-loaded/under argon> 0th = linear mineralization of acetic acid <under air> -0.5th = square root at higher intensity -1st = constant rate, due to diffusion (of O 2?) limited process light intensity/mw cm /07/04 Advanced Course in Photocatalytic Reaction Chemistry 32

33 square-root law of light intensity previous reports: due to enhanced recombination at higher rate of formation of photoexcited electrons and positive holes (photocatalyst) e - + h + I e - + h + (heat) k r [e][h] = k r [e] 2 e - + (substrate) (product) k e [e][s] (h - + (substrate) (product) k h [h][s] ) d[e]/dt = 0 = I - k r [e] 2 - k e [e][s] [e] = { -k e [S] + ( k e2 [S] 2 + 4k r I ) 1/2 }/2k r if 4k r I >> k e2 [S] 2, k e [S] [e] = k r -1/2 1/2 I 1/2 rate 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 33

34 roles of oxygen in photocatalysis reaction with (capture of) photoexcited electron reacts with photoexcited electron to yield superoxide anion radical (O 2 + e - = O 2 - ) unknown fate of superoxide anion radical accelerates the reaction with holes (oxidation) participation in radical chain mechanism reacts with radical species, which is liberated by positive hole reaction, to give peroxy species radical chain reaction to give higher efficiency = one photon produces several products RH + h + R + H + (H 2 O) RH + HO R + H 2 O R + O 2 RO 2 RO 2 + RH RO 2 H + R RO 2 H (decomposition) (RH + R R + RH) 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 34

35 possible radical-chain mechanism peroxy radical as a chain carrier I : incident photon flux; : photoabsorption efficiency; : intrinsic quantum efficiency RH R R + O 2 RO 2 RO 2 + RH RO 2 H + R 2RO 2 (deactivation) I k 1 k 2 k 3 stationary (steady) state approximation for RO 2 and R d[r ]/dt = 0 = I -k 1 [R ][O 2 ]+k 2 [RO 2 ][RH] d[ro 2 ]/dt = 0 = k 1 [R ][O 2 ]-k 2 [RO 2 ][RH] - k 3 [RO 2 ] 2 [RO 2 ] 2 = I k 3 -d[rh]/dt = I k 2 [RH][RO 2 ] app = + k 2 [RH]( / k 3 ) 0.5 I /07/04 Advanced Course in Photocatalytic Reaction Chemistry 35

36 plots of app against square root of light intensity Linear relations are obtained. Y-intersect corresponding to was negligible, i.e., very low intrinsic quantum efficiency. Appreciable photocatalytic activity might be due to long chain length. app at 350 nm app = + k 2 [RH]( / k 3 ) 0.5 I TIO Merck I -0.5 / (mw cm -2 ) /07/04 Advanced Course in Photocatalytic Reaction Chemistry 36

37 necessary conditions for photocatalytic reactions reaction initiated by photoabsorption of photocatalyst (generally accepted) blank test: Copresence of 3 requisites, photoirradiation, photocatalyst (solid material) and reaction substrate(s) is indispensable. Photoreaction initiated by photoabsorption of a compound adsorbed by a solid surface and subsequent electron injection also requires 3 requisites. action spectrum analyses: possible sole technique to prove what absorbs light to initiate the photoreaction checking product(s): adsorption can decrease the amount of substrate(s); stoichiometry photoabsorber (= photocatalyst) remaining unchanged checking turnover frequency: molar ratio of product(s) to photocatalyst to be more than unity skip 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 37

38 platinum-loaded tungsten(iv) oxide photocatalyst 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 38

39 visible-light active bismuth tungstate (Bi 2 WO 6 ) CH 3 CHO + 5/2 O 2 light irradiation (>400 nm) CH 3 CHO 2CO 2 + 2H 2 O light irradiation (>400 nm) CO K 433 K 463 K 493 K 493 K 463 K 433 K 403 K photocatalyst loading: 50 mg AcH in feed: 2000 ppm (ca. 30 µmol) light source: 300-W xenon lamp with a cutoff filter (>400 nm) N-TiO 2 : nitrogen-doped titania (87 m 2 g -1 ) prepared by nitrization by ammonia of Ishihara ST-01 at 723 K for 3 h. 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 39

40 diffuse reflectance and action spectra K calcined at 403 K N-TiO 2 TiO 2 (P25) flake-ball bismuth tungstate photocata 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 40

41 a gold-loaded titania photocatalyst Action spectrum analysis revealed that photoabsorp-tion of gold initiates the reaction Reaction rate was increased with the increase in gold particle size. 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 41

42 super hydrophilicity & oxidative decomposition Photoirradiation onto titania surface induces super hydrophilicity to make water-contact angle to be almost zero. titania-coated : ordinary glass Photoirradiation onto titania film decomposes adsorbed methylene blue. 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 42

43 proposed mechanism of superhydrophilicity Increase in the number of surface hydroxyls by the reaction of positive holes at the surface [Professor Hashimoto's group (Tokyo University)] dark 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 43

44 thin film of TiO 2 coated on glass plates (without preheating) small CA before irradiation order of the onset wavelength = that of absorption M-6 6 nm PC nm NTB nm NTB nm Thin Solid Films, 516 (2008) /07/04 Advanced Course in Photocatalytic Reaction Chemistry 44

45 change of CA on irradiated S-TiO 2 films kept at 393 K overnight in an oven before irradiation change of rate of CA decrease at around 10 degrees 50 water contact angle/degree nm 365 nm 380 nm nm nm monochromatic irradiation/min Stage 1 Stage /07/04 Advanced Course in Photocatalytic Reaction Chemistry 45

46 reciprocal of contact angle Sakai et al., reported linear relations between; 1/CA vs. time They proposed that; 1/CA number of surface hydroxyls J. Phys. Chem. B 2003, 107, UV range: nm 2013/07/04 Advanced Course in Photocatalytic Reaction Chemistry 46

47 wavelength dependence of superhydrophilicity different behavior depending on the water contact angle: Taki M6 (anatase) > 10 degree ordinary oxidation of surface contaminants <= 10 degree different mechanism, presumably including surface bound hydrogen peroxide or peroxy species sulfur-doped titania (anatase) Thin Solid Films, 516 (2008) /07/04 Advanced Course in Photocatalytic Reaction Chemistry 47

48 comments on this lecture Please send in Japanese or English within 48 hours to: subject: pc xxxxxxxx body: full name nickname comments on this lecture question(s) JPY1,200 (77%) JPY3,500 (79%) pc /07/04 Advanced Course in Photocatalytic Reaction Chemistry 48