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2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 1

June 12, 2014


Advanced Course in Photocatalytic Reaction Chemistry

understanding chemistry by understanding photocatalysisunderstanding photocatalysis by understanding chemistry

Division of Environmental Material Science, Graduate School of Environmental ScienceThe first semester of Fiscal 201408:45─10:15, Thursday at Lecture Room D103

Bunsho Ohtani, Ewa Kowalska and Mai Takase

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) April 10 introduction of photocatalysis(2) April 17 interaction between substances and light(3) April 24 electronic structure and photoabsorption(4) May 1 thermodynamics: electron and positive hole(5) May 8 adsorption(6) May 15 (Professor Ewa Kowalska)(7) May 22 kinetic analysis of photocatalysis (8) May 29 steady-state approximation(9) June 5 kinetics and photocatalytic activity(10) June 12 kinetic analysis(11) June 19 (Professor Mai Takase)(12) June 26 action spectrum analysis (1)(13) July 3 action spectrum analysis (2)(14) July 10 crystal structure of titania(15) July 17 design and development of photocatalysts

July 24 deadline for submission of special report


comments on this lecture

Please send email in Japanese or English within 48 hours

to: [email protected]: pc2014MMDD-XXXXXXXXbody:

(full name)(nickname)(comments and/or questions on today's lecture)


electron-holepair

recombi-nation

photo-absorption

redox(chemical)reaction

photocatalytic reaction

photocatalytic reaction: a kind of photoreactionnot a series reaction, but a parallel reactioninitiated by photoabsorption with short-live speciese.g., photoexcited electrons and positive holes

11

22

33


steady-state approximation for photocatalysis

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

I I: 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)Derive an equation showing the rate r,

applying steady-state approximation to electron-positive hole pairs.


steady-state approximation for photocatalysis

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

I I: 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)


kinetics of photoinduced reaction

There are two limits: linear part and saturated part.

concentration of subsrate(s)

rate

of r

eact

ion

proportional to concentration

approaching to the limit, I

keh[S] + krr =

I keh[S]

I


concentration of substrate

• overall rate of photocatalytic reaction based on steady-state approximation for electron-hole pairs

r = I keh[S] / (keh[S] + kr) or

r = I keh[S] / kr (when keh[S] << kr)

• meaning of keh[S]: rate of SURFACE REACTION with electron-hole pairs with surface-adsorbed substrate

• two possible cases:(1) adsorption equilibrium during the reaction(2) non-equilibrium due to faster consumption of substrate on the surface

= diffusion-limited process


adsorption and photocatalytic activity

• the larger the adsorbed substrate(s), the higher the activity

• the larger the surface area, the larger the adsorbed amount

an examplelinear relation between the rate and adsorbed silver ion (J. Phys. Chem., 87 (1997) 3550.

Sr

eh kkIr S

r

eh kkIr


first-order kinetics in photocatalysis

Q What kind of reaction kinetics can interpret the experimental results, if a first-order kinetics, like plots below, is observed?


first-order kinetics

• 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 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] ~ 0r = aC How are these discriminated?How are these discriminated?





r = I keh[S]/ kr = aI kehC/ kr


[S] ~ 0r = aC

What the observed rate constant kmeans?

What the observed rate constant kmeans?





r = I keh[S]/ kr = (aI keh/kr)C


[S] ~ 0r = aC = bSC

S: specific surface area

light-intensity dependence

first order

vs.

at higher intensity region

zeroth order

light-intensity dependence

first order

vs.

at higher intensity region

zeroth order


Fick's law of diffusion

• rate (flux; J) of diffusion

• diffusion constant D include area of "hypothetical wall".

• J = DC if surface concentration is zero.

• for particles,

hypothetical wall = thin diffusion layer surrounding the surface

hypotheticalwall

x axis

xCDJ

xCDJ

lowconcentration

side

hypothetical wall high concentration

side


kinetic analysis

(1) first-order kinetics for a substrate: a linear relation between logarithm of product yield (substrate consumption) and time– adsorption equilibrium during the reaction in Henry fashion (or low-

concentration part of Langmuirian fashion) for the equation– non-equilibrium due to faster consumption of substrate on the surface

= diffusion-limited processrate constant: Ikeh/kr and a (diffusion constant)Checking light intensity dependence, these may be discriminated: first order =

Henry-type adsorption and zeroth order at the higher intensity = diffusion-limited process

(2) reciprocal relation with concentration of substrate: a linear relation between rate and concentration of a substrate– adsorption equilibrium constant K can be estimated and compared with that

obtained in the dark adsorption equilibrium measurement– kS (= Ikeh/kr) can be estimated.


meaning of constants

0 2 4 6 8 100

5x106

1x107

1.5x107

2x107

1/r

1/C

bC

ar

11

0 0.2 0.4 0.6 0.8 10

2x106

4x106

6x106

8x106

C/r

C

'' bCarC

Q What do two parameters, a (a') and b (b'), obtained from the slope and intercept of a linear plot, mean?


Langmuir adsorption equilibrium

• overall rate of photocatalytic reaction based on steady-state approximation for electron-hole pairs

r = I keh[S]/ (keh[S] + kr) or

r = I keh[S]/ kr (when keh[S] << kr, i.e., LOW quantum efficiency)

• meaning of keh[S]: rate of SURFACE REACTION with electron-hole pairs with surface-adsorbed substrate

• possible adsorption equilibrium during the reaction:• assuming Langmuir adsorption isotherm (S: saturation adsorption, C: equilibrium

concentration, K: adsorption equilibrium constant)

[S] = SKC/(1 + KC)r = I kehSKC/ kr(1 + KC) = I (keh/kr)SKC/ (1 + KC)


data analysis for photocatalysis

0 2 4 6 8 100

5x106

1x107

1.5x107

2x107

1/r

1/C

kSCkKSr1111

0 0.2 0.4 0.6 0.8 10

2x106

4x106

6x106

8x106

C/r

C

kKSC

kSrC 11

r = I kehSKC/ kr(1 + KC) = I (keh/kr)SKC/ (1 + KC)1/r = (1/kKS)(1/C) + 1/kS, where k = I (keh/kr)

• Plots (left and right) may give K and kS, but not k or kr, ke-h.


an example

1 µm


photocatalytic activity

• Assuming the definition of "photocatalytic activity" to be INTRINSIC ability of a photocatalyst to drive photocatalytic reaction, what is(are) the term(s) showing photocatalytic activity?

• C, I: reaction condition adjusted freely• S, K, : properties of solid (extrinsic ability)• keh, kr (or their ratio, keh/kr): intrinsic ability

Can we measure keh and kr from experimental results?

KC

SKCkkI

r r

1

eh

KC

SKCkkI

r r

1

eh


Langmuirian-adsorption mechanism in catalyses

0 2 4 6 8 100

5x106

1x107

1.5x107

2x107

1/r

1/C

kSCkKSr1111

0 0.2 0.4 0.6 0.8 10

2x106

4x106

6x106

8x106

C/r

C

kKSC

kSrC 11

r = kSKC/ (1 + KC)1/r = (1/kKS)(1/C) + 1/kS

• Plots (left and right) may give K and kS, but not k or S.


Langmuir-Hinshelwood mechanism

• bimolecular reaction: reaction of two substrates, A and B adsorbed on surface with a reaction rate constant k.

• Common surface cites adsorb substrates A and B with equilibrium constants, KA and KB, respectively.

• Both A and B are adsorbed on the surface in Langmuirian fashion, with a total (saturated) concentration of the surface sites, S.

• Assuming the bulk concentration of A and B, CA and CB, respectively, rate r is proportional to surface concentrations of A and B, and then:

2BBAA

BBAA2

1 CKCKCKCKkSr

2BBAA

BBAA2

1 CKCKCKCKkSr


Eley-Rideal mechanism

• bimolecular reaction: reaction of two substrates, A and B, adsorbed on surface and coming from the bulk, respectively, with a reaction rate constant k.

• Surface cites adsorb substrates A with equilibrium constants, KA.• A is adsorbed on the surface in Langmuirian fashion, with a total

(saturated) concentration of the surface sites, S.• Assuming the bulk concentration of A and B, CA and CB, respectively,

rate r is proportional to surface concentration of A and B in the bulk, and then:

AA

BAA

1 CKCCkSKr

AA

BAA

1 CKCCkSKr


comments on this lecture

Please send email in Japanese or English within 48 hours

to: [email protected]: pc20140612-XXXXXXXXbody:

full namenicknamecomments on this lecturequestion(s) JPY1,200 (77%) JPY3,500 (79%)

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