20150716 BO Sapporo - 北海道大学触媒科学研究所 ...pcat.cat.hokudai.ac.jp/class/pc2015/20150716_BO_Sapporo.pdf · special report special report for extra ... activation

2015/07/16─Advanced Course in Environmental Catalytic Chemistry 1

July 16, 2015


Advanced Course in Environmental Catalytic Chemistry I

understanding chemistry by understanding photocatalysisunderstanding photocatalysis by understanding chemistry

Division of Environmental Material Science, Graduate School of Environmental ScienceThe first semester of Fiscal 201508:45─10:15, Thursday at Lecture Room D103

Bunsho Ohtani and Ewa Kowalska

Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan011-706-9132 (dial-in)/011-706-9133 (facsimile)

[email protected]://www.hucc.hokudai.ac.jp/~k15391/


objectives/goal/keywords

<< objectives >>Understanding the mechanism of decomposition of pollutants, methodsof photocatalysts preparation, design of practical photocatalytic reactionsystems, and strategy for enhancement of photocatalytic activity.

<< goal >>To understand principle of photocatalytic reaction from the standpointof chemistry and strategy for practical applications. To obtain scientificmethod for research on functional solid materials.

<< keywords >>Photocatalyst, Photoinduced oxidative decomposition, Superhydro-philicity, Excited electron-positive hole, Structure-activity correlation,Higher photocatalytic activity, Visible-light response


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(15) Jul 16 summary: photocatalysis A--Z


comments on this lecture

Please send email in Japanese or English within 48 hours

to: [email protected]: pc2015MMDD-XXXXXXXX

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special report

special report for extra (bonus) score (20 point)report on critical review on "photocatalysis" in Wikipedia, pointing out errors, misunderstanding and speculationshttp://en.wikipedia.org/wiki/Photocatalysishttp://ja.wikipedia.org/wiki/光触媒

• Japanese or English• A4 size 2 pages• submission by email attachment• a PDF file is more preferable than a Word file• email title: pc20150723-XXXXXXXX• file name: pc20150723-XXXXXXXX.pdf (or .docx or .dox)• deadline of submission: July 23, 2015 23:59

photocatalysis A to Z 7J. Photochem. Photobiol. C: Photochem. Rev., 11 (2010) 157-178.

Photocatalysis A to Z [JPPC, 11 (2010) 157-178]


A: activation energy for photocatalysis

activation energy for ordinary chemical reactions: a few hundred kJ mol-1

reduced by CATALYSTSmeasured activation energy for photocatalytic reactions: a few ten kJ mol-1

(a) Vorontsov, A. V.; Stoyanova, I. V.; Kozlov, D. V.; Simagina, V. I.; Savinov, E. N. J. Catal. 2000, 189, 360. [10.9 (acetone)] (b) Xu, Y. M. Chem. J. Chin. Univ. Chin. 2000, 21, 1539. [4.2-4.6 (acetophenone)] (c) Su, W. Y.; Fu, X. Z.; Wei, K. M. Chem. J. Chin. Univ. Chin. 2001, 22, 272. [13.7 (bromomethane)] (d) Lea, J.; Adesina, A. A. J. Chem. Tech. Biotech. 2001, 76, 803. [7.83 (nitrophenol)] (e) Kartal, O. E.; Erol, M.; Oguz, H. Chem. Eng. Tech. 2001, 24, 645. [16.2 (phenol)] (f) Okte, A. N.; Resat, M. S.; Inel, Y. J. Catal. 2001, 198, 172. [17.1 (1,3-dihydroxy-5-methoxybenzene)] (g) Tada, H.; Suzuki, F.; Yoneda, S.; Ito, S.; Kobayashi, H. Phys. Chem. Chem. Phys. 2001, 3, 1376. [19.7/29.4 (bis(2-dipyridil)disulfide)] (h) Lee, N. C.; Choi, W. Y. J. Phys. Chem. B 2002, 106, 11818. [18.7 (soot)] (i) Cui, W. Q.; Feng, L. R.; Xu, C. H.; Lu, S. J.; Qiu, F. Chin. J. Catal. 2003, 24, 937. [8.46 (methanol)] (j) Kozlov, D. V.; Panchenko, A. A.; Bavykin, D. V.; Savinov, E. N.; Smirniotis, P. G. Russ. Chem. Bull. 2003, 52, 1100. [6.3 + 0.4 (benzene)] (k) Mills, A.; Hill, G.; Bhopal, S.; Parkin, I. P.; O'Neill, S. A. J. Photochem. Photobiol. A Chem. 2003, 160, 185. [19(stearic acid)] (l) Al-Rasheed, R.; Cardin, D. J. Chemosphere 2003, 51, 925. [17 + 0.6 (fumic acid)] (m) Garcia, J. C.; Takashima, K. J. Photochem. Photobiol. A Chem. 2003, 155, 215. [24.8 (imazaquin)] (n) Machado, A. E. H.; de Miranda, J. A.; de Freitas, R. F.; Duarte, E. T. F. M.; Ferreira, L. F.; Albuquerque, Y. D. T.; Ruggiero, R.; Sattler, C.; de Oliveira, L. J. Photochem. Photobiol. A Chem. 2003, 155, 231. [7.9–10.5 (organic matter)] (o) Parra, S.; Stanca, S. E.; Guasaquillo, I.; Thampi, K. R. Appl. Catal. B: Environ. 2004, 51, 107. [10.9 (atrazine)]


A: activity

• Known: Rate of photocatalytic reactions under given conditions, i.e., relative photocatalytic activity and general empirical trends.

• Unknown: Intrinsic photocatalytic activity, overall kinetic equation, and true correlation between physical or structural properties and photocatalytic reaction rate.


A: anatase and rutile

commercial titania Crystalline fraction of anatase or rutile : Rietveld analysisIntensity ratios: most intense anatase and rutile peaks

1.36

Xa

Ohtani, B.; Sano, M.; Takase, M., Anal. Chem. to be published.


A: amorphous• amorphous-anatase mixture• f(amorphous) determined by DSC

crystallization of amorphous to anatase

f(ana

tase

) fro

m X

RD

f(ana

tase

)from

DS

C

smaller XRD peakintensity at < 30 nm

J. Phys. Chem., 101, 3746 (1997)


B: band structure

• Known: Positions of conduction-band bottom and valence-band top of semiconducting materials and thereby band gaps.

• Unknown: Photoexcited states of particulate photocatalysts, actual form of photoexcited electron and positive hole in the initial stage of photocatalysis, and mechanism of their migration.


gν

ν EhCe

Jh

B: band position

Scaife's plot

D. E. Scaife, Solar Energy, 25, 41-54 (1980).

flat-band potential = conduction band bottom

-1

band gap = distance between CB and VB

VB

CB


C: crystallinity

• Known: Relative extent of crystal growth.• Unknown: Methods for determination of precise composition of crystals and

actual structure of an amorphous phase.


D: doping

• Known: Something included in samples.

• Unknown: Methods for determination of the precise structure, density and spatial distribution of doped materials and of the effect of doping.


E: energy conversion

• Known: Method for calculating energy conversion efficiency for a simple non-biased photocatalytic or photoelectrochemical system.

• Unknown: An appropriate method for calculation of energy conversion efficiency for electrochemically or chemically biased systems.


F: first-order kinetics (1)

• Known: Method for kinetic analysis for the first-order rate law.

• Unknown: Meaning of a linear relation in a plot of logarithmic concentration of a substrate or a product against the concentration of substrate in the bulk.



• two possible cases:(1) adsorption equilibrium during the

reaction in Henry fashion (or low-concentration part of Langmuirianfashion) for the equation

• r = I keh[S]/ kr = aI kehC/ kr

(2) non-equilibrium due to faster consumption of substrate on the surface= diffusion-limited process: The reaction rate is determined by the rate of diffusion with a constant a.

• [S] ~ 0• r = aC

What the observed rate constant kmeans?



• two possible cases:(1) adsorption equilibrium during the

reaction in Henry fashion (or low-concentration part of Langmuirian fashion) for the equation

• r = I keh[S]/ kr = (aI keh/kr)C

(2) non-equilibrium due to faster consumption of substrate on the surface= diffusion-limited process: The reaction rate is determined by the rate of diffusion with a constant a.

• [S] ~ 0• r = aC = bSC

• S: specific surface area

light-intensity dependence

first order

vs.

at higher intensity region

zeroth order


G: Gibbs energy

• Known: A necessary condition of band structure of a photocatalyst for photocatalytic reactions.

• Unknown: (none)


H: hydroxyl radical

• Known: Positive holes are produced in photoirradiated photocatalysts, and they may give hydroxyl radicals.

• Unknown: Chemical form of positive hole at the interface.


H: Honda-Fujishima effect (1)

Fujishima, A.; Honda, K., Nature 238, 37 (1972).• order of authors' names: not

"Honda and Fujishima"• not an origin of "photocatalysis by

titania" in a strict bibliographic sense, but actually promoted studies on photocatalysis, i.e., showed the possibility of water splitting by solar light

• suggestion of water splitting by photoirradiated semiconducting materials


TiO2 Pt

↑O2 ↑Ｈ2

e-

UV

TiO2

↑O2

e-

↑Ｈ2

UV

Pt

high pH low pH

H: Honda-Fujishima effect (2)• 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

hidden messagerequirement of BIAS potential, i.e., electric FIELD to separate photoexcited electron and positive hole (= charge separation) to drive the reactions of positive Gibbs energy change


I: identification

• Known: Methods for product identification in an organic chemical sense.

• Unknown: Sufficient conditions for product identification in a strict scientific sense.


J: junction

• Known: Electronic structure of semiconductor–metal and solution interface in electrode systems.

• Unknown: Actual contact of particle–solution interface and electron (positive hole) transfer through this junction.


K: Kubelka–Munk function• Known: Method for conversion of data of reflectance to Kubelka–Munk function.• Unknown: Meaning of K–M function as concentration of a target compound

mixed with a matrix.


L: laboratory


L: Langmuir–Hinshelwood mechanism

• Known: Method for double-reciprocal plotting of reaction rate and substrate concentration.

• Unknown: Possibility of reaction mechanism not related to Langmuir-type isotherm of adsorption.

kSCkKSr1111

kKSC

kSrC 11


L: light intensity dependence (1)

Merck (anatase) titania

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 O2?) limited process

light intensity/mW cm-2

appa

rent

qua

ntum

effi

cien

cy (

app)

1

0.1

10

1

0.1

10

0.1 101

CH3COOH + 2O2

2CO2 + 2H2O

CH3OH HCHO + H2

290 nm

350 nm

380 nm

395 nm



• peroxy radical as a chain carrier• I : incident photon flux; : photoabsorption efficiency; : intrinsic quantum efficiency

• stationary (steady) state approximation for RO2· and R·

RH R·R· + O2 RO2·RO2· + RH RO2H + R·2RO2· (deactivation)

I k1

k2

k3

d[R·]/dt = 0 = I - k1[R·][O2] + k2[RO2·][RH]d[RO2·]/dt = 0 = k1[R·][O2] - k2[RO2·][RH] - k3[RO2·]2

[RO2·]2 = I k3

-d[RH]/dt = I k2[RH][RO2·]

app = + k2[RH]( / k3)0.5I-0.5



• 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.

ap

p at

350

nm

I-0.5 / (mW cm-2)-0.5

TIO-2

Merck

0

0.2

0.4

0.6

0.8

0 0.5 1 1.5 2 2.5 3 3.5

app = + k2[RH]( / k3)0.5I-0.5


M: mutual recombination

• Known: Behavior of recombination of e– and h+ depending on their density.• Unknown: Absolute rate constant of mutual recombination occurring during

photocatalytic reaction under ordinary photoirradiation systems.


N: nanostructure

• Known: Wide variety of photocatalysts with characteristic morphology can be obtained by changing reaction conditions, especially under hydrothermal conditions.

• Unknown: How the nano-or other size-range structures themselves affect photocatalytic activities.


N: necessary conditions for photocatalysisElectrode potential of substrate to be reduced must be more anodic compared with the bottom of conduction band of photocatalysts.Electrode potential of substrate to be oxidized must be more cathodic compared with the top of valence band of photocatalysts

Reactions by photoexcited electrons and positive holes occur separately,i.e., charge separation, especially for the reactions of positive Gibbs energy change.

photocatalyst

CB

VB

∆G < 0 absorption

e–

h+

∆Ge < 0

∆Gh < 0

∆G > 0 absorption

CB

VB

h+

e– ∆Ge < 0

∆Gh < 0

ener

gy


O: optical band gap

• Known: A method for band-gap energy estimation by plotting data using the Kubelka–Munk function of a photocatalyst.

• Unknown: A method for judgment of the mode of transition and for exclusion of the influence of absorption by impurities or surface electronic states.


O: original papers on photocatalysis (before 1980)

• Ohtani, B. Electrochemistry 2014, 82, 414-425.


P: particle size

• Known: Several methods to determine particle size.• Unknown: Mode of average size, number, surface-area or volume (or others)-

based one.


P: particle size and specific surface area

(particle size/nm) • (specific surface area/m2 g-1) = ca. 1500

y = 1500x-1

0

10

20

30

40

50

60

70

80

90

0 100 200 300

crys

tallit

e si

ze/ n

m

specific surface area/ m2 g-1

anatase

mixture(a)

mixture(r)


Q: quantum efficiency (1)• Known: Practical method for determination of

apparent quantum efficiency.• Unknown: Intrinsic quantum efficiency, i.e., the

fraction of electron-positive hole pairs that escape from their mutual recombination.


Q: quantum efficiency (2)

the first principle of photochemistry: only molecules absorbing a photon can reactnumber ratio of reacted molecules to absorbed photons, assuming single photon process:

n(reacted molecules)/n(absorbed photons)Processes of heterogeneous photocatalysis may contain reactions with multiple electrons or holes, e.g., water photolysis to give oxygen.quantum efficiency for heterogeneous photocatalysis:

n(electrons or holes used in reaction)/n(adsorbed photons)

r(electrons or holes used in reaction)/r(adsorbed photons)apparent quantum efficiency

r(electrons or holes used in reaction)/r(incident photons)

where r(incident photons) is a light flux (I).

I

I


R: rate-determining step

• Known: Inappropriateness of discussion on rates of reduction by photoexcited electrons and oxidation by positive holes.

• Unknown: Intrinsic rate of recombination governing the overall reaction rate.


S: steady-state approximation for photocatalysis

• the simplest mechanism1) photoabsorption to yield photogenerated electron-positive hole pair (e-h):

II: photon flux in mol s-1 and : photoabsorption efficiency 2) reaction of e-h with a substrate to give product(s): keh[e-h][S]3) recombination of e-h: kr[e-h]

• approximationlife time of an intermediate, e-h, is small and its concentration is constant

during the reaction

d[e-h]/dt = 0 =[e-h] = r =

I - keh[e-h][S] - kr[e-h]I / (keh[S] + kr)

keh[e-h][S] = I keh[S]/ (keh[S] + kr)


S: synergetic effect

• Known: How to confirm a synergetic effect when two components are obtained in pure forms.

• Unknown: How to confirm a synergetic effect when two components are originally in a mixture and each pure component cannot be obtained.


T: thermodynamics

• Known: Thermodynamic explanation using a band model of semiconducting materials.• Unknown: General description of thermodynamics of photocatalysis defined for materials

including molecular or highly dispersed compounds.


U: ultraviolet in solar radiation

• Known: Content of ultraviolet light in solar radiation based on energy.• Unknown: Content of ultraviolet light in solar radiation based on photon

number.


V: visible light-induced photocatalysis

• Known: Strategy to prepare visible light-sensitive materials.

• Unknown (partly): How to prove a given reaction to be visible light-photocatalytic one.


W: wavelength dependence

• Known: Importance of discussion with wavelength dependence, i.e., an action spectrum.

• Unknown: (partly) Difference in an action spectrum and a pseudo action spectrum


X: X-ray photoelectron spectroscopy

• Known: Peak shift (chemical shift) in an XP spectrum shows valency of a given atom, but not a bond between atoms.

• Unknown: Method for showing the existence of specific bonds (presumably by X-ray absorption spectroscopy).


Y: yield

• Known: Importance of determination of absolute yield.• Unknown: (none)


Z: z-scheme photocatalysis

• Known: Thermodynamic requirements for photocatalysts and a redox mediator.

• Unknown: Strategy for inhibition of undesired possible e– and h+ transfer processes and of spontaneous backward reaction of redox products.


comments on this lecture

Please send email in Japanese or English within 48 hoursto: [email protected]: pc20150716-XXXXXXXX

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subject: pc20150716-12345678

pc20150716-12345678

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大谷文章

某教授

光触媒についてだけではなく，物理化学をはじめとする化学の広い分野のさまざまなことについて復習になり，また多くのことを学びました．

絶版になっている「光触媒標準研究法」はどこかで入手可能ですか．

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20150716 BO Sapporo - 北海道大学触媒科学研究所 ...pcat.cat.hokudai.ac.jp/class/pc2015/20150716_BO_Sapporo.pdf · special report special report for extra ... activation