May 7, /05/07 Advanced Course in Environmental Catalytic Chemistry I 1

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1 May 7, /05/07 Advanced Course in Environmental Catalytic Chemistry I 1

2 Advanced Course in Environmental Catalytic Chemistry I 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 and Ewa Kowalska Catalysis Research Center, Hokkaido University, Sapporo , Japan (dial-in)/ (facsimile) ohtani@cat.hokudai.ac.jp /05/07 Advanced Course in Environmental Catalytic Chemistry I 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 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 3

4 schedule (1) Apr 9 introduction of photocatalysis (2) Apr 16 interaction between substances and light (3) Apr 23 electronic structure and photoabsorption (4) Apr 30 thermodynamics: electron and positive hole (5) May 7 adsorption (6) May 14 kinetic analysis of photocatalysis (7) May 21 steady-state approximation (8) May 28 (Environmental application of photocatalysis) Kowalska (9) Jun 4 kinetics and photocatalytic activity (10) Jun 11 action spectrum analysis (1) (11) Jun 18 action spectrum analysis (2) (12) Jun 25 crystal structure (1) (13) Jul 2 crystal structure (2) (14) Jul 9 design and development of photocatalysts (1) (15) Jul 16 design and development of photocatalysts (2) 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 4

5 comments on this lecture Please send in Japanese or English within 48 hours to: subject: pc2015mmdd-xxxxxxxx pc2015mmdd-xxxxxxxx (full name) (nickname) (comments and/or questions on today's lecture) 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 5

6 principle of photocatalytic reaction electronic structure of semiconductors and insulators conduction & valence bands separated by bandgap photoexcitation beyond the bandgap 1. photoexcitation 2. electron and hole 3. relaxation 4. reduction & oxidation 5. recombination conduction band e - relaxation e - ONLY thermodynamic aspects NOT excitation kinetic aspects h + valence band h + reduction recombination photoabsorption relaxation oxidation 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 6

7 Honda-Fujishima effect: splitting of water Photoirradiation of a titania electrode short-circuited with a platinum counter electrode DOES NOT induce splitting of water into hydrogen and oxygen 1) Application of bias potential which does not induce electrolysis of water in the dark 2) Use of higher and lower ph electrolytes for titania and platinum electrodes, respectively, i.e., chemical bias e - e - O 2 H 2 O 2 H 2 UV UV TiO 2 Pt TiO 2 Pt high ph low ph 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 7

8 absorption edge wavelength absorption edge: corresponding to band gap apparently the edge is not SHARP due to distribution of "density of states" DOS in the bands are not homogeneously distributed negligible DOS at the edges = less photoabsorption DOS Density Of States 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 8

9 localized/delocalized Band model tells photoexcitation of electron in VB to CB. "delocalized" electrons and positive holes How electrons and positive holes migrate in a photocatalyst? cf. organic molecules LUMO absorption HOMO 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 9

10 hypothesis for excitation to give electron-hole pairs Electron traps, i.e., vacant electronic level just below the CB bottom Excitation of electron in VB to CB occurs nonlocalized fashion. Non-localized electron in CB is localized at an electron trap without migration, i.e., just after excitation. Assuming higher density of electron traps on particle SURFACES, e Electrostatic interaction of electron and hole Traps for positive hole may be present. What is the SURFACE traps? Possible structure of anatase titania may be an adsorbed O /05/07 Advanced Course in Environmental Catalytic Chemistry I 10

11 relaxation and recombination femtosecond pump-probe photoabsorption spectroscopy relaxation < 1 ps -> pump-probe 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 11

12 electron-hole and relaxation photoexcitation: excited electron (e - ) in a conduction band (CB) and positive hole (h + ) in a valence band (VB) relaxation: e - at the bottom of CB and h + at the top of VB levels of energy of relaxed e - and h + not depending on the irradiation wavelength 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 12

13 photocatalytic reaction known in old times photoadsorption/desorption of oxygen chalking (not "choking") titania particles binder (organic compounds) photocatalytic reaction 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 13

14 photocatalytic air purifier unit active air purification: decontamination and deodorization of circulating air air conditioner Hitachi Home and Life Solution air purifier Daikin Ltd /05/07 Advanced Course in Environmental Catalytic Chemistry I 14

15 pavements containing titania photocatalyst "Photoroad" Nitrogen oxides (NO x ) are oxidized by photocatalyst Resulting nitrate (NO 3- ) ions are captured by calcium ions Calcium nitrate (Ca(NO 3 ) 2 ) is washed out by rain water Fujita /05/07 Advanced Course in Environmental Catalytic Chemistry I 15

16 photocatalytic reaction photoabsorption to yield photoexcited electron (e - ) and positive hole (h + ) reaction of SURFACE-ADSORBED compounds with e - and h +. In other words, only adsorbed compounds can be reacted. 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 16

17 adsorption isotherm interaction of compounds with surfaces van der Waals force: physical adsorption chemical bond: chemisorption concentration (pressure) dependence of adsorbed amount: isotherm adsorbate adsorbent physical adsorption chemisorption 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 17

18 adsorption isotherms named after proposers Q Answer the names of these isotherms. A Henry Langmuir B amount of adsorption equilibrium pressure/concentration Brunauer Freundlich Emmett Teller C D equilibrium pressure/concentration 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 18

19 three assumptions for derivation of Langmuir eq. des. des. O ads. V O O ads. V V 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 19

20 three assumptions for derivation of Langmuir eq. All the adsorption sites are same in quality, i.e., strength of capturing an adsorbate is constant Only one adsorbate is adsorbed by one site. There is no interaction between sites, i.e., adsorption not influenced by adsorption of neighboring sites des. des. O ads. V O O ads. V V 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 20

21 adsorption equilibrium adsorption equilibrium: same rates of adsorption and desorption Adsorption rate depends on (1) concentration in bulk and (2) number (density) of vacant sites (V) Desorption rate depends on number (density) of occupied sites (O). des. des. O ads. V O O ads. V V 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 21

22 kinetics rate (r) of reaction for A B is expressed using k (rate constant) and concentrations of A and B ([A] and [B], respectively, as r f = k f [A] r b = k b [B] E a < E' a For equilibrium, r f = r b, then K = k f /k b = [B]/[A] energy E a source G <0 product E' a 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 22

23 Q: Derive the Langmuir isotherm. Same rate of adsorption and desorption at equilibrium rate of adsorption: r a = k a [v]c rate of desorption: r d = k d [o] k a [v]c = k d [o] k a C [v]/([v]+[o]) = k d [o]/([v]+[o]) k a C (1 - ) = k d KC (1 - ) = =KC/(1 + KC ) vacant site: [v], occupied site: [o], coverage = [o]/([v] + [o]), concentration at equilibrium: C, rate constants: k a and k d, adsorption equilibrium constant: K = k a /k d 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 23

24 adsorption and photocatalytic activity the larger the adsorbed substrate(s), the higher the activity the larger the surface area, the larger the adsorbed amount example linear relation between the rate and adsorbed silver ion (J. Phys. Chem., 87 (1997) /05/07 Advanced Course in Environmental Catalytic Chemistry I 24

25 two limits for Langmuir isotherm relation between actual adsorption amount and : saturation amount s [S] ads = s = skc 1+ KC two limits When C is so small that KC can be neglected: [S] ads When C is large enough to neglect "1": [S] ads = 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 25

26 two limits for Langmuir isotherm relation between actual adsorption amount and : saturation amount s [S] ads = s = skc 1+ KC two limits When C is so small that KC can be neglected: [S] ads C When C is large enough to neglect "1": [S] ads = s 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 26

27 Langmuir isotherm There are two limits: linear part and saturated part. amount of adsorption [S] ads approaching to the limit, s = s = skc 1+ KC proportional to concentration equilibrium concentration 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 27

28 adsorption isotherms named after proposers Q Answer the names of these isotherms. Henry Langmuir amount of adsorption equilibrium pressure/concentration BET Freundlich equilibrium pressure/concentration 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 28

29 Q: Who are BET? v m : saturation amount of first layer of adsorption c: a constant x: relative pressure (0-1) 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 29

30 Brunauer Emmett Teller isotherm vacuum pressure gauge gas v m : saturation amount of first layer of adsorption c: a constant x: relative pressure (0-1) joint cell thermometer a Dewar flask with liquid nitrogen a kind of "agglomeration" 得られた 1 層めの飽和吸着量に窒素の吸着断面積 (0.162 nm 2 ) をかけて表面積 (m 2 ) をもとめ, さらにつかった固体量でわって比表面積 (m 2 g -1 ) とする. 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 30

31 Q: How is relative pressure, x, measured? vacuum pressure gauge gas v m : saturation amount of first layer of adsorption c: a constant x: relative pressure (0-1) joint cell thermometer x : pressure of nitrogen measured under the same CONDITIONS = saturated vapor pressure of nitrogen Dewar bottle with liquid nitrogen The condition: boiling point of nitrogen at the temperature of liquid nitrogen in a Dewar bottle, and the vapor pressure is the same at atmospheric pressure around an instrument. Therefore, atmospheric pressure has to be measured. 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 31

32 kinetics Q If a compound adsorbed on a photocatalyst surface in Langmuirian fashion is oxidized (or reduced) by the positive hole (or photoexcited electron) in the photoirradiated photocatalyst, what is a rate expression for the reaction? 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 32

33 comments on this lecture Please send in Japanese or English within 48 hours to: subject: pc xxxxxxxx pc xxxxxxxx <full name> <nickname> <comments on this lecture> <question(s) if any> 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 33

34 sample mail 2015/05/07 Advanced Course in Environmental Catalytic Chemistry I 34