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2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 1
June 20, 2013
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 2
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 kinetics and action spectrum analysis (1)(11) June 20 action spectrum analysis (2) and crystal
structure of photocatalysts(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 25August 1
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 3
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 201308:45─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/
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 4
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 5
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 kinetics and action spectrum analysis (1)(11) June 20 action spectrum analysis (2) and crystal
structure of photocatalysts(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 25August 1
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 6
comments on this lecture
Please send email in Japanese or English within 48 hours
to: [email protected]: pc2013MMDD-XXXXXXXXbody:(full name)(nickname)(comments and/or questions on today's lecture)
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 7
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 8
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 9
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 10
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 11
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 12
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 13
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.
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 14
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 15
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 16
action spectrum analysis
action spectrum analysisstatistical analysis
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 17
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
electron-holepair
recombi-nation
photo-absorption
redox(chemical)reaction
11
22
33
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 18
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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 19
visible-light responsive photocatalyst
white titania (TiO2) absorbing only ultraviolet light giving color by SOME treatment(s)activity under visible-light irradiation?
titaniawithcolor
titaniawithcolor
titaniatitaniavisible lightresponsive
titania
visible lightresponsive
titania=/
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 20
photoreaction/photocatalytic
reaction
photoreaction/photocatalytic
reaction
wavelength 1
wavelength 2
wavelength 3
wavelength 4
response (product, current...)
・・・
measurement of action spectrum
• plots of apparent quantum efficiency (response normalized by number of incident photons) versus wavelength
wavelength
appa
rent
qua
ntum
effic
ienc
y
1 23
4
5
6
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 21
action spectrum
wavelength/nm wavelength/nm
wavelength/nm
photoabsorptionefficiency
appa
rent
qua
ntum
effic
ienc
y
quantumefficiency
example: discrimination of active crystalline phase in anatase-rutile mixtures[T. Torimoto, et al., Phys. Chem. Chem. Phys., 4, 5910-5914 (2002)].
example: discrimination of active crystalline phase in anatase-rutile mixtures[T. Torimoto, et al., Phys. Chem. Chem. Phys., 4, 5910-5914 (2002)].
action spectrum= apparent quantum
efficiency
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 22quantum efficiency 22
quantum efficiency (yield)
• the first principle of photochemistry: only molecules absorbing a photon can react• number 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
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 23quantum efficiency 23
number of electrons or holes for reaction
example 1: acetic acid decomposition
CH3COOH + 2O2 2CO2 + 2H2OAssuming the reduction of 1 mol of oxygen (O2) into 2 mol of water requires 4 positive holes, 8 mol of electron-hole pairs are used in this stoichiometry. Therefore, 1 mol of carbon dioxide production corresponds to 4 mol of photons, at minimum.
example 2: acetaldehyde decomposition
CH3CHO + 5/2 O2 2CO2 + 2H2OAssuming the reduction of 1 mol of oxygen (O2) into 2 mol of water requires 4 positive holes, 10 mol of electron-hole pairs are used in this stoichiometry. Therefore, 1 mol of carbon dioxide production corresponds to 5 mol of photons, at minimum.
example 3: water splitting
2H2O O2 + 2H2Assuming the production of 1 mol of oxygen (O2) from water requires 4 positive holes, 4 mol of electron-hole pairs are used in this stoichiometry. Therefore, 1 mol or 2 mol of oxygen or hydrogen production corresponds to 4 mol of photons, at minimum.
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 24
action spectrum analysis
J. Chem. Soc., FaradayTrans.1, 81, 2467 (1985).
appa
rent
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 25
action spectrum measurement (1)
light source / monochromator / reaction cell
reaction cell
monochromator
xenon arc
cell holder
ca. 0.1 mW cm-2
FWHM: ca. 20 nm
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 26
simultaneous irradiation
action spectrum measurement (2)
power meter
irradiation port
thermopile
wavelength adjustment
xenon arc
cell holders
0.1-18 mW cm-2
FWHM ca. 20 nm0.1-18 mW cm-2
FWHM ca. 20 nm
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 27
non and S-doped titania for MB decomposition
S-doped TiO2S-doped TiO2
P-25P-25
S-doped TiO2S-doped TiO2
MBMB
Yan, X.; Ohno, T.; Nishijima, K.; Abe, R.; Ohtani, B., Chem. Phys. Lett., 429, 606-610 (2006).
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 28
photochemical MB decomposition
S-doped TiO2S-doped TiO2
P-25P-25
MB insuspension
MB insuspension
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 29
titania photocatalysts for AcOH decomposition
S-dopedS-doped
P-25P-25
S-doped titania clearly showed the activity under visible-light irradiation.
P-25P-25
S-doped TiO2S-doped TiO2
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 30
action spectrum measurement (3)
lens optical filter
reaction cell
stirrer
water bath
liquid-phase photocatalytic reaction
"sharp-cut"optical filter
(example)L-42
actual limiting wavelength
wavelength
center of 5 and 72% trans-mission
skipskip
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 31
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
pseudoaction spectrum
wavelength
appa
rent
qua
ntum
yie
ld
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 32
P-25/MB photocatalytic reactionpseudo action spectrum corresponds to integral of "true" action spectrum
pseudo action spectrum
300 400 500 600 7000.000
0.001
0.002
0.003
0.004
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
r /µm
ol m
in-1
app
/nm
action spectrum
integral of action spectrum
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 33
collection of papers of less expected citations
Proving that MB is inappropriate as a test compound for the reaction under visible-light irradiation by action spectrum analyses.
cited 60 times by June 2011
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 34
diffuse reflectance spectra in the unit of absorption normalized at 350 nm
1/2: wavelength giving half value to that at 350 nm
anatase-rutile mixture
fanataseanatase content
estimated from XRD patterns
0
0.2
0.4
0.6
0.8
1
350 360 370 380 390 400 410 420
abso
rptio
n (n
orm
aliz
ed)
Wavelength / nm
Merck P-25
Wako(A)+CR-EL
CR-ELTIO-5
CR-EL(1473 K)360
370
380
390
400
0 0.2 0.4 0.6 0.8 1
1/
2/ n
m
fanatase
Merck
HombikatTIO-2
P-25
Wako(A)Merck+CR-EL
Wako(A)+CR-EL
TIO-5Aldrich(A<R)
Wako(R)CR-EL
CR-EL(1473K)
P-25(1473K)
0.5
skipskip
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 35
test photocatalytic reactions for 35 titanias
(a) oxygen evolution along with silver metal deposition
4Ag+ + 2H2O = 4Ag + O2 + 4H+
(b) methanol dehydrogenation
CH3OH = HCHO + H2
(c) oxidative decomposition of acetic acid in water
CH3COOH + 2O2 = 2CO2 + 2H2O
(d) oxidative decomposition of acetaldehyde in air
CH3CHO + 5/2O2 = 2CO2 + 2H2O
(e) synthesis of pipecolinic acid from L-lysine
L-lysine = PCA + NH3
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 36
CH3OH HCHO + H2
4Ag+ + 2H2O 4Ag + O2 + 4H+
CH3COOH + 2O2 2CO2 + 2H2O
action spectra of photocatalytic reaction
app
(nor
mal
ized
)0.5
00.2
0.40.60.8
1 (a)
00.2
0.40.60.8
1 (b)
0
0.2
0.4
0.6
0.8
1
350 360 370 380 390 400 410
Wavelength / nm
(c)
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 37
360
370
380
390
400
410
0 0.2 0.4 0.6 0.8 1
1/
2 / n
m
fanatase
Merck
CR-EL
Merck+CR-ELP25TIO-5
Aldrich(A<R)
Wako(A)+CR-EL
Wako(R)
Hombikat
CR-EL(1473 K)
P25 (1473 K)
TIO-2Wako(A)
370
380
390
400
410
0 0.2 0.4 0.6 0.8 1
1/
2/ n
m
fanatase
MerckP25
Wako(A)
Merck+CR-EL
Aldrich(A<R)TIO-5
CR-EL
Wako(R)Wako(A)+CR-EL
CR-EL(1473 K)P25 (1473 K)
TIO-2
360
370
380
390
400
410
0 0.2 0.4 0.6 0.8 1
1/
2 / n
m
fanatase
Wako(R)CR-EL
Hombikat
Merck
Wako(A)
Merck+CR-EL(1:1)Aldrich(A<R)
TIO-5
TIO-2
CR-EL(1473 K)P25 (1473 K)
P25
dehyderogenation of methanol1/2 versus fanatase
absorption edge wavelengthanatase: ca. 370 nmrutile: ca. 410 nm
R >> AR >> A
R AR A
A >> RA >> R
oxygen evolution & silver metal deposition
decomposition of acetic acid
inner-filter effectby rutile
inner-filter effectby rutile
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 38
action spectra of anatase/rutile/P25
oxidative decomposition of acetic acid in an aqueous solution: carbon dioxide liberation
0
0.005
0.01
0.015
0.02
0.025
360 390 420 450
appa
rent
qua
ntum
effi
cien
cy (%
)
wavelength/nm
0
50
100
150
P25pure anatase pure rutile
reconstructed mixture
CH3COOH(CO2)
CH3COOH(CO2)
CH3CHO(CO2)
CH3CHO(CO2)
CH3OH(H2) <Pt>
CH3OH(H2) <Pt>
Ag+
(Ag/O2)Ag+
(Ag/O2)
amorphous
91
100
120 56
anatase
rutile
P25
skipskip
Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. J. Photochem. Photobiol. A Chem.
216 (2010) 179-182.
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 39
hydrogen evolution from methanol
isolated anatase: Aisolated rutile: Rplatinization:
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 Rwith A when platinized
photodeposition occurs preferentially on rutile particles
0
0.2
0.4
0.6
0.8
1
360 390 420
appa
rent
qua
ntum
effi
cien
cy
wavelength/nm
2wt%Pt/R
0.2wt%Pt/P25
2wt%Pt/P25
A + Pt/R(85:15)
A/Pt + Pt/R(85:15)
2wt%Pt/A
0
0.2
0.4
0.6
0.8
1
360 390 420
appa
rent
qua
ntum
effi
cien
cy
wavelength/nm
2wt%Pt/R
0.2wt%Pt/P25
2wt%Pt/P25
A + Pt/R(85:15)
A/Pt + Pt/R(85:15)
2wt%Pt/A
0
0.2
0.4
0.6
0.8
1
360 390 420
2wt%Pt/R
0.2wt%Pt/P25
2wt%Pt/P25
A + Pt/R(85:15)
A/Pt + Pt/R(85:15)
2wt%Pt/A
0
0.2
0.4
0.6
0.8
1
360 390 420
2wt%Pt/R
0.2wt%Pt/P25
2wt%Pt/P25
A + Pt/R(85:15)
A/Pt + Pt/R(85:15)
2wt%Pt/A
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 40
action spectrum: case 2
wavelength/nm wavelength/nm
wavelength/nm
photoabsorptionefficiency
action spectrum
appa
rent
qua
ntum
effic
ienc
y
quantumefficiency
Change of (intrinsic) quantum efficiency, i.e., efficiency of electron-hole utilization depending on the irradiation wavelength
may induce
shift of action spectrum
skipskip
2013/06/20─Advanced Course in Photocatalytic Reaction Chemistry 41
comments on this lecture
Please send email in Japanese or English within 48 hours
to: [email protected]: pc20130620-XXXXXXXXbody:
full namenicknamecomments on this lecturequestion(s) JPY1,200 (77%) JPY3,500 (79%)
pc20130620-12345678