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2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 2
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/
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 3
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 4
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 5
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)
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 6
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 7
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.
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 8
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)
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 9
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 10
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 11
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 12
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?
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 13
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?
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 14
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
What the observed rate constant kmeans?
What the observed rate constant kmeans?
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 15
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 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] ~ 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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 16
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 17
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.
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 18
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?
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 19
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)
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 20
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.
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 22
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 23
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.
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 24
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 25
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
2014/06/12─Advanced Course in Photocatalytic Reaction Chemistry 26
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%)
pc20140612-12345678