Kuwauchi, Y., Yoshida, H., Akita, T., Haruta, M. and
Takeda, S. Angew. Chem. Int. Ed., 2012, 51: 7729–7733
Metallic Supported Nanoparticles as Catalysts
Jan. 26th, 2016
Malek Ibrahim
Group meeting
1
Outline
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Catalytic properties of metallic SNP
Factors affecting the catalytic properties of SNP
Support effects on NP reactivity
SNP synthesis
SNP characterization
• Design of SNP catalyst for ethanol Guerbet reaction
2
SNP as heterogeneous catalysts
Widely used in industry
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hydrogenation
dehydrogenation
polymerization
ammonia synthesis
desulfurization
cyclization
steam reforming
http://inano.au.dk/research/research-areas/nano-materials-materials-science/nanocatalysis/
Other applications include automotive catalytic
converters, industrial emission treaters, fuel cells…
Pd/C, Pt/SiO2, Au/TiO2, NiMo/Al2O3, Ni/LaFeO3
Advantages
http://vts.uni-ulm.de/docs/2012/7900/vts_7900_11445.pdf
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Higher activity than the bulk metal analog
High metal dispersion (surface area per unit volume)
High thermal stability
No need for catalyst separation and recovery (up to 30% production cost)
Less corrosion and hazardous waste problems
3
Surface reaction mechanisms
Adsorption of at least one of the reactants is necessary for the reaction to proceed
Langmuir-Hinshelwood
Eley-Rideal
http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Complex_Reactions/Catalysis
Adsorption energy of different surface species (reactants, intermediates, products,
inhibitors..) affects reaction rate
Adsorption configuration affects reaction rate and selectivity
4
Catalytic properties of SNP
A small number of atoms arranged in a specific order
Higher surface roughness leads to exposure of
surface under-coordinated atoms
Electronic and catalytic properties of these specific
atoms are different from those exposed by bulk metal
surfaces
M. Haruta, Faraday Discuss., 2011, 152, 11
Example 1: O2 dissociation on AuNP but not Au bulk
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under coordinated Au atoms have localized HOMO with charge
density “sticking out” in the vacuum
facile charge transfer to the π* orbital of O2
G. Mills, M.S. Gordon, H. Metiu, J. Chem. Phys. 2003 118, 4198.
Kuwauchi, Y., Yoshida, H., Akita, T., Haruta, M. and
Takeda, S. Angew. Chem. Int. Ed., 2012, 51: 7729–7733
5
Catalytic properties of SNP
Example 2: Au/TiO2 WGS reactivity-size relationship
Ribeiro et al. J. Am. Chem. Soc., 2010, 132 (40), pp 14018–14020
Corner atoms are the active sites for WGS
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Catalytic properties of SNP
“Bare” supports don’t show any
activity; metal catalyzed reactions
but changing support type affects
the NP activity of the same size
Ribeiro et al. J. Am. Chem. Soc., 2012, 134 (10), pp 4700–4708
Deposition of TiO2 on Au/Al2O3
significantly enhanced catalyst
Synergetic effect between AuNP and
TiO2
Junling Lu et al. J.Phys.Chem.C 2016, 120, 478−486
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Outline
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP
Support effects on NP activity
SNP synthesis
SNP characterization
• Supported CuNP as catalyst for ethanol Guerbet reaction
8
Effect of reaction conditions on NP size
Catalyst deactivation is observed from cycle to cycle
The increase in NP size leads to a decrease in the number of active sites
9
Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
Effect of NP spacing on size
Deactivation can be mitigated by increasing the NP spacing (lower metal loading)
10
Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
Effect of NP shape on activity
Cuboctahedron
Reaction rate can be facet
dependent
11
Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
Effect of reaction condition on NP shape
Reaction conditions can alter the NP shape
12
Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
Outline
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP…environment, shape, loading
Support effects on NP activity
SNP synthesis
SNP characterization
• Supported CuNP as catalyst for ethanol Guerbet reaction
13
Support effect on NP activity
Support can be metal oxides, carbide, activated carbon, or polymer
Support affects NP activity by:
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Setting NP shape (surface wetting)
http://www.chem.qmul.ac.uk/surfaces/scc/scat1_7.htm
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Support effect on NP activity
Support can be metal oxides, carbide, activated carbon, or polymer
Support affects NP activity by:
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Setting NP shape (surface wetting)
Stabilizing against coarsening
Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
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Support effect on NP activity
Support can be metal oxides, carbide, activated carbon, or polymer
Support affects NP activity by:
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Setting NP shape (surface wetting)
Stabilizing against coarsening
Stabilizing certain metal oxidation states
Pt-O-Zr
PtCe
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Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
Support effect on NP activity
Support can be metal oxides, carbide, activated carbon, or polymer
Support affects NP activity by:
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Setting NP shape (surface wetting)
Stabilizing against coarsening
Stabilizing certain metal oxidation states
Encapsulating the metal NP
Cabaellero et al., Chem. Commun., 2010,46, 1097-1099
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Beatriz Cuenya, Thin Solid Films, 518 (2010) 3127–3150
Inter-connected factors affecting SNP reactivity
Inter-particles distance
Shape
Coordination
number
Size
Environment
Metal loading
Support
Oxidation
state
Unknown!
Catalytic properties
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Outline
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP…environment, shape, loading
Support effects on NP activity
SNP synthesis
SNP characterization
• Supported CuNP as catalyst for ethanol Guerbet reaction
19
Additional Support effects: acid-base properties
Metal oxides exhibit acid-base properties
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Surface metal cation acts as Lewis acid
Surface oxygen anion acts as Lewis base (or Brønsted acid)
The relative acid-base strength depends on the cation size and metal-oxygen bond
strength
Acid/base strength is measured by desorption temperature of pre adsorbed “probe”
molecules (ammonia and carbon dioxide typically)
Surface relative acid-base strength favors certain adsorption configurations
Selectivity
90%
Basic support
Tanabe et al. Applied Catalysis A: General 181 (1999) 399-434
17%
Acidic support
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Additional Support effects: crystal phase
A metal oxide can exist in more than one crystallographic structure, ex. TiO2
surface view
unit cell
anatase
rutile
A. Bouzoubaa et al. / Surface Science 583 (2005) 107–117
The change in ions coordination and bond length leads to a change in surface catalytic
properties
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Outline
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP…environment, shape, loading
Support effects on NP activity…acid-base properties
SNP synthesis
SNP characterization
• Supported CuNP as catalyst for ethanol Guerbet reaction
22
SNP synthesis: metal loading methods
Incipient Wetness (IW): filling the support pores with metal solution followed by drying
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Accurate metal loading
Poor size control
White et al., Chem. Soc. Rev., 2009,38, 481-494
Electrostatic adsorption (EA): Adjust pH of solution to electrically charge the
support surface followed by electrostatic adsorption of the metal containing anion/cation
• Good size controlled
• Limited metal loading capacity
Regalbuto et al., J. Catal., 2008, 260, 329-341
Oxide
PZC
MgO
12.4
TiO2 (rutile)
5.7
TiO2 (anatase)
6.2
WO3
0.4
ZrO2
7.6
SiO2
2
ZnO
9.2
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SNP synthesis: metal loading methods cont.,
Ion Exchange (IE): Exchange of the metal containing ion with surface protons
• Small NP size
• Limited metal loading capacity
Deposition precipitation (DP): Increase the pH of the solution to form insoluble
metal salt that precipitates on the support surface
Solution gelation (solgel): simultaneous formation of the support metal oxide and
the supported metal from their complexes/salts
Other methods
Reverse Micelle
Dendrimer
Microwave assisted reduction
Au/TiO2
Method
IW
EA
IE
DP
Metal loading Avg. d
(wt%)
(nm)
0.9
4.9
1.1
2.1
1
5.8
1.8
1.8
Zanella et al. J. Phys. Chem. B, Vol. 106, No. 31, 2002
Trace impurities such as carbon or halides from synthesis can affect catalytic properties of NP
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SNP synthesis: heat treatment
Drying: removal of solvent
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medium T <120 oC, low pressure
low drying rate yields smaller NP
Heat treatment: decomposition of the surface metal precursor and agglomeration of
metal atoms to form NP
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High T >250 oC, reductive or inert environment
Low ramp rate yields smaller NP
Optimize conditions to avoid catalyst deactivation
Cabaellero et al., Chem. Commun., 2010,46, 1097-1099
Ma et al. Heterogeneous Gold Catalysts and Catalysis, 2014, pp. 1-26
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Outline
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•
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP…environment, shape, loading
Support effects on NP activity…acid-base properties
SNP synthesis…metal loading methods, heat treatment
SNP characterization
• Supported CuNP as catalyst for ethanol Guerbet reaction
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SNP Characterization
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Size: TEM
www.warwick.ac.uk
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Metal content: ICP-OES/MS
Oxidation state: XPS, XANES
Crystallographic structure: XRD
Surface properties: TPD-MS/IR/TGA…
Morphology: AFM
and many other techniques!
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SNP Characterization: group activity
The maximum amount of hydrogen adsorbed on 0.5 g Ni/Al2O3 catalyst
containing 1wt% Ni was found to be 15 micromoles. Assuming no hydrogen
adsorption occurs on Al2O3 and that the adsorption stoichiometry is 1
hydrogen molecule to every surface Ni atom, what is the NiNP average
diameter in nm?
Givens:
Ni density 7.81 g/cm3
occupied surface area by Ni atom 6.7X10-16 cm2
Hint: assume NP geometry as a perfect hemisphere
Area= π*d2
Volume=π*d3/12
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Outline
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•
•
•
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP…environment, shape, loading
Support effects on NP activity…acid-base properties
SNP synthesis…metal loading methods, heat treatment
SNP characterization…TEM, AFM, XPS, XRD, ICP-OES
• Supported CuNP as catalyst for ethanol Guerbet reaction
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Bioethanol: going beyond the gasoline blend limit
Straight chain alkanes
Biomass
Guerbet…HDO
Fermentation
Ethanol
Branched alkanes
10-15% max.
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Guerbet reaction on metal oxide catalysts
III
I
II
Pd/MgxAlyOz1
K-Cu/CexMgyOz2
Srx(PO4)y(OH)23
1-Toste et al. Nat. Protoc. (2015) 10, 528
2-Iglesia et al. J. Catal. (1998) 176, 155
3-Onda et al. Appl. Catal. A (2011) 402, 188
Problems:
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low selectivity
poor stability
Continuous, steady state,
vapor phase reaction
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Ethanol possible reactions on metal oxide catalysts
Catalyst
MgO
TiO2
Al2O3
Sel.% @ 10%
conv.
80
15
5
Strong basic sites are required for
dehydrogenation while strong acid sites
catalyze dehydration
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Catalyst
MgO
TiO2
Al2O3
Sel.% @ 10%
conv.
80
15
5
Butanol formation rate (mole)(min)-1(m)-2
Ethanol possible reactions on metal oxide catalysts
7x10-8
7x10-8
6x10-8
6x10-8
5x10-8
5x10-8
4x10-8
4x10-8
3x10-8
3x10-8
2x10-8
2x10-8
0
5
10
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TOS (h)
Butanol formation rate on MgO versus time on stream (TOS, h) at 3 kPa
EtOH, 60 kPa H2, bal. He, and 633 K reaction temperature
Strong basic sites are required for
dehydrogenation while strong acid sites
catalyze dehydration
Strong basic sites get deactivated quickly
Mg(OH)2
Can strong basic sites be replaced?
inactive
MgO
360 0C
CxHy/MgO
inactive
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How does Guerbet reaction proceed on mildly basic catalyst, TiO2?
Aldol rate (mole) (min)-1 (g. cat.)-1
3.5x10-5
r =𝑲𝒂𝒑𝒑 (𝑷𝒂𝒄𝒆𝒕𝒂𝒍𝒅𝒆𝒉𝒚𝒅𝒆 )𝟐
3.0x10-5
III
2.5x10-5
I
II
2.0x10-5
1.5x10-5
1.0x10-5
503 K
523 K
5.0x10-6
0.0
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Acetaldehyde PP (kPa)
2nd order on acetaldehyde pressure @ constant ethanol pressure
4x10-5
3x10-5
-1
Aldol rate (mole) (min) (g. cat.)
-1
r ≈𝑲𝒂𝒑𝒑 (𝑷𝒆𝒕𝒉𝒂𝒏𝒐𝒍 )−𝟐
2x10-5
523 K
1x10-5
503 K
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Maximize the ratio of adsorbed aldehyde to alkoxide to
maximize selectivity
Ethanol PP (kPa)
-2nd order on ethanol pressure @ constant acetaldehyde pressure
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Multifunctional catalyst design
Addition of a metallic function to facilitate dehydrogenation
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103 USD/Kg
Which metal works best?
0.006
0.013
0.5
25
38.2
36.8
CH3
3 kPa EtOH, 15 kPa H2, bal. He, and 503 K reaction temperature
C
4.5
Decarbonylation
Dehydration
Etherification
Esterification
Ketonization
Aldolization
Side products Selectivity (%)
4.0
3.5
3.0
H
O
CH3
H
C O
2.5
2.0
1.5
η1 mode
1.0
η2 mode
0.5
0.0
8
10
12
14
16
18
20
Ethanol Conversion (%)
22
24
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Minimize Cu catalyzed esterification
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X
Inhibits aldol condensation
Low energy content
No further chain growth
Hydro-deox. to small molecules
Metal loading method optimization
C-C formation rate (mole)(min)-1(m)-2
Ethyl ester:
2x10-7
1x10-7
1x10-7
1x10-7
1x10-7
1x10-7
9x10-8
8x10-8
0
1
2
3
4
5
6
Ethyl ester PP (kPa)
3 kPa EtOH, 0.4 kPa Acet.,15 kPa H2, bal. He, and 503 K reaction temperature
IW
EA
3 kPa EtOH, 15 kPa H2, bal. He, and 503 K reaction temperature
20 nm
50 nm
Catalyst made by EA yields smaller, more selective
catalyst to the desired products
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Minimize Cu catalyzed esterification
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X
C-C formation rate (mole)(min)-1(m)-2
Ethyl ester:
2x10-7
Inhibits aldol condensation
Low energy content
No further chain growth
Hydro-deox. to small molecules
1x10-7
1x10-7
1x10-7
1x10-7
1x10-7
9x10-8
8x10-8
0
1
2
3
4
5
6
Ethyl ester PP (kPa)
3 kPa EtOH, 0.4 kPa Acet.,15 kPa H2, bal. He, and 503 K reaction temperature
Bimetallic SNP different products selectivity at 15% ethanol conversion at 230 0C
Acetaldehyde
Methane
Ethane+
Diethyl ether
Ethyl ester
Ketones
C-C products
ethylene
Cu3Ag1
92.01
0.30
0.25
2.28
0.58
1.26
3.32
Cu3Pd1
94.73
4.32
0.00
0.22
0.30
0.20
0.23
Cu10Au1
100.00
0.00
0.00
0.00
0.00
0.00
0.00
Cu
97.09
0.00
0.00
0.13
1.94
0.75
0.09
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Minimize Cu catalyzed esterification
CuNP different products selectivity at 15% ethanol conversion at 230 0C
Acetaldehyde
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Methane
Ethane+
Diethyl
ethylene
ether
Ethyl ester
Ketones
C-C
products
Cu/Al2O3
84.58
0.00
0.53
8.99
2.45
1.97
1.47
Cu/MgO
94.09
0.24
0.00
0.00
0.28
0.65
4.74
Cu/C
90.18
0.21
0.00
0.48
0.40
0.70
8.03
Cu powder
98.36
0.08
0.00
0.10
0.33
1.13
0.00
Cu/SiO2
96.62
0.00
0.16
0.15
1.59
1.34
0.14
Cu/TiO2
93.21
0.00
0.11
0.21
0.24
0.38
5.86
K-Cu/Al2O3
93.64
0.27
0.00
1.52
1.18
1.29
2.10
K-Cu/SiO2
98.35
0.10
0.00
0.00
0.72
0.08
0.75
The higher the support acidity, the lower the acetaldehyde selectivity
Doping supports with alkali metal (K) attenuates surface acidity and reduces
esterification selectivity
Unsupported Cu is highly selective to acetaldehyde but deactivates quickly
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Synergy effect of Cu supported on TiO2
Metal catalyzed
C2H5OHg + *A*B ⇌ C2H5O*A + H*B
C2H5O*A + *B ⇌ CH3CHO*A + H*B
CH3CHO*A ⇌ CH3CHOg + *A
2H*B ⇌ H2 + 2*B
CH3CHO*A + *B ⇌ -CH2CHO*A + H*B
CH3
6. CH3CHO*A + CH2CHO*A ⇌ *A OCHCH2CHO*A
1.
2.
3.
4.
5.
CH3
Catalyst system
C-C formation activation barrier
kJ.mole-1
Cu/SiO2 + TiO2
106.4
TiO2
100.9
Cu/TiO2
68.8
CH3
7. *AOCHCH2CHO*A + H*B ⇌ HOCHCH2CHO*A + *A*B
CH3
8. HOCHCH2CHO*A + H*B ⇌ CH3CH=CHCHO*A + H2O + *B
9. CH3CH=CHCHO*A ⇌ CH3CH=CHCHOg + *A
10.CH3CH=CHCHO*A + 2H*B ⇌ CH3CH2CH2CHO*A + 2*B
11.CH3CH2CH2CHO*A ⇌ CH3CH2CH2CHOg + *A
12.CH3CH2CH2CHO*A + 2H*B ⇌ CH3CH2CH2CH2OHg + 2*B
Metal
Weaker adsorption
R.D.S
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Group activity
Crotonaldehyde hydrogenation to butanol can proceed through either the less
thermodynamically favored butenol as an intermediate or through butyraldehyde.
On which catalyst out of these three would you expect butenol route to be more favorable
in absence of metal function, MgO, TiO2, Al2O3?
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Conclusion
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Catalytic properties of metallic SNP…size and support
Factors affecting catalytic properties of NP…environment, shape, loading
Support effects on NP activity…acid-base properties
SNP synthesis…metal loading methods, heat treatment
SNP characterization…TEM, AFM, XPS, XRD, ICP-OES
• Supported CuNP as catalyst for ethanol Guerbet reaction
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Mildly basic sites are more stable catalysts
Addition of metal function facilitates ethanol dehydrogenation
Acetaldehyde undergoes 2nd order aldol condensation to form C-C bonds
Cu is the best performing metal but catalyzes esterification
Optimization of catalyst synthesis and addition of Au and K enhances selectivity
Addition of Cu to the TiO2 surface facilitates product desorption which is the R.D.S
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Q&A