electrocatalysis: catalysis of redoxreactions
TRANSCRIPT
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Electrocatalysis:catalysis of redox reactions
Marc Koper
Leiden University (NL)
Summerschool Norderney21-26 July 2013
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Redox reactions
e-
Ox
Red
Ox + e- � Red
electrode electrolyte
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Redox reactions of water
E (vs.RHE)
currentdensity
0 1.23
2 H+ + 2 e-→ H2
H2 → 2 H+ + 2 e-diffusion-limitedcurrent
2 H2O → O2 + 4 H+ + 4 e-
O2 + 4 H+ + 4 e- → 2 H2O
diffusion-limitedcurrent
PtNilaccase
RuO2
PSIIplatinumhydrogenase
overpotential
M.T.M.Koper, H.A.Heering, in “Fuel Cell Science”, Eds. A.Wieckowski, J.K.Nørskov, Wiley (2010), p.71-110
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Catalysis of multi-step reactions
Practically every (interesting) chemical reaction happens in a series of steps;
catalysis is about optimizing that sequence
1 e- / 1 step / 0 intermediate
2 e- / 2 steps / 1 intermediate
>2 e- / >2 steps / >1 intermediate
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Single electron transfer
• Marcus Theory
• Activation energy determined by solvent reorganization energy λ(a difficult quantity to calculate accurately)
• Marcus Theory does not account for bond breaking, proton transfer, or catalysis.
C.Hartnig, M.T.M.Koper, J.Am.Chem.Soc. 125 (2003) 9840
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Two electron transfer
2 H+ + 2 e- � H2
H+ + e- � Hads (Volmer)
Hads + H+ + e-� H2 (Heyrovsky)
H+ + e- Hads H2freeenergy
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Thermodynamics
2 H+ + 2 e- � H2 E0 = 0 V
H+ + e- � Hads E10 = - ∆Gads(H)/e0
Hads + H+ + e-� H2 E2
0 = ∆Gads(H)/e0
Thermodynamic restriction: (E10 + E2
0)/2 = E0
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Thermodynamic volcano plot
R.Parsons,Trans.Faraday Soc. (1958); H.Gerischer (1958)J.K.Nørskov et al., J.Electrochem.Soc. (2004)
M.T.M.Koper, H.A.Heering, In Fuel Science Science
M.T.M.Koper, E.Bouwman, Angew.Chem.Int.Ed. (2010)
zero thermodynamic overpotential
0
0
∆Gads(H)
Potential of thermodynamicallyleast favorable reaction
H+ + e- ���� HadsH+ + Hads + e- ���� H2
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Experimental volcano for H2 evolution
J.Greeley, J.K.Nørskov, L.A.Kibler, A.M.El-Aziz, D.M.Kolb, ChemPhysChem 7 (2006) 1032
H binding energy= ca. 220 kJ/mol
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More than 2 electron transfers
O2 + 4 H+ + 4 e- � 2 H2O E0 = 1.23 V
O2 + H+ + e- � OOHads E1
0
OOHads� Oads + OHads ∆G
Oads + H+ + e- � OHads E2
0
OHads + H+ + e- � H2O E3
0
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The optimal catalyst
ΔG(OHads) = CO = 1.23 eV
ΔG(Oads) = 2 × CO = 2.46 eV
ΔG(OOHads) = 3 × CO = 3.69 eV
ΔG(O2) = 4 × CO = 4.92 eV
Independent of the mechanism
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However: scaling relationships
F.Abild-Petersen, J.Greeley, F.Studt, P.G.Moses, J.Rossmeisl, T.Munter, T.Bligaard, J.K. Nørskov,
Phys.Rev.Lett. 99 (2007) 016105
For (111) metal surfaces
F.Calle-Vallejo, J.I.Martinez, J.M.Garcia-Lastra, J.Rossmeisl, M.T.M.Koper, Phys.Rev.Lett. 108 (2012) 116103
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The optimal scaling relations
ΔG(OHads) (≈ 0.50×ΔG(Oads) + 0.05 eV)
= 0.5×ΔG(Oads) + KOHΔG(OOHads) (≈ 0.53×ΔG(Oads) + 3.18 eV)
= 0.5×ΔG(Oads) + KOOH
KOH = 0 eV
KOOH = 2.46 eV
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The optimal volcano
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0-0.5
0.0
0.5
1.0
1.5
2.0
2.5
O2+ H
++ e
- = OOH
adsoptimal
E0 / V
∆G(Oads) / eV
1.23
optimalOH
ads+ H
++ e
- = H
2O optimal catalyst
sub-optimal catalyst
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Does optimal scaling exist?
Metals:
ΔG(OHads) ≈ 0.50×ΔG(Oads) + 0.05 eV
ΔG(OOHads) ≈ 0.53×ΔG(Oads) + 3.18 eV
Oxides:
ΔG(OHads) ≈ 0.61×ΔG(Oads) - 0.90 eV
ΔG(OOHads) ≈ 0.64×ΔG(Oads) + 2.03 eV
KOOH – KOH = 3.13 eV, 2.93 eV; Optimal = 2.46 eV
M.T.M.Koper, J.Electroanal.Chem. 660 (2011) 254
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“Fundamental” overpotential?
ηT (ORR,OER) =KOOH – KOH – 2.46 eV
2 e
~ 3.15 eV
= ~ 0.35 V
One does not even need to know the catalyst-oxygen interaction…
∆G[HO2(aq)] - ∆G[OH (aq)] = 3.4 eV
I.Man, J.Rossmeisl et al., ChemCatChem 3 (2011) 1159M.T.M.Koper, J.Electroanal.Chem. 660 (2011) 254; Chem.Sci. 4 (2013) 2710
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Proton-coupled electron transfer
2 H+ + 2 e- � H2
O2 + 4 H+ + 4 e- � 2 H2O
CO2 + 6 H+ + 6 e- � CH3OH + H2O
• Are proton and electron transfer always coupled?
• How does (de-)coupled proton-electron transfer manifest?
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Proton-coupled electron transfer
S. Hammes-Schiffer, A.A.Stuchebrukhov, Chem.Rev.110 (2010) 6939
solvent coordinatecoupled to ET
solvent coordinatecoupled to PT
AH+
A
AH
A-
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Hamiltonian of PCET
j.Grimminger, S.Bartenschlager, W.Schmickler, Chem.Phys.Lett. 416 (2005) 316 M.T.M.Koper, Phys.Chem.Chem.Phys. 15 (2013) 1399
( )
2
22
12
*
2122,11,
*
222
),(
nnqqqqqq
ccVccVnnccVccVnnqqH
Appeepeppee
ptptpp
k
kAkAkkk
k
kAApe
βλλλλλ
εεεε
+−−−++
+++++++= ++++∑∑
Electron affinity + solvation energy
-
Proton affinity = pKa(AH+)
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Proton-coupled electron transfer
A + 2H+ + 2e- A- + 2H+ + e-ETET
CPET PTPT
A2- + 2H+
AH+ + H+ + 2e- AH + H+ + e-
PT
PT
AH22+ + 2e-
CPETCPET
CPET
AH- + H+
PT PT
AH2+ + e- AH2
ET ET
ET ET
M.T.M.Koper, Phys.Chem.Chem.Phys. 15 (2013) 1399M.T.M.Koper, Chem.Sci. 4 (2013) 2710
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When PT and ET decouple
• PT and ET are concerted if off-diagonal states are energetically unfavorable. Reaction rate is independent of pH on the RHE potential scale.
• For a reduction reaction, ET happens first if the intermediate has a high electron affinity.
• For an oxidation reaction, PT happens first if the intermediate has a low proton affinity.
• If PT and ET decouple, the reaction rate becomes pH dependent on the RHE potential scale. Optimal activity is at pH=pKa
M.T.M.Koper, Phys.Chem.Chem.Phys. 15 (2013) 1399M.T.M.Koper, Chem.Sci. 4 (2013) 2710
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Formic acid oxidation on Pt
J.Joo, T.Uchida, A.Cuesta, M.T.M.Koper, M.Osawa, J.Am.Chem.Soc. 135 (2013) 9991
Formic acid oxidation prefers intermediate pH
pKa
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Oxidation of poly-ols
Hβ
R-OHα
� Hβ
R-O- + Hα
+
“sugar alcohols”, poly-ols mono-ols
Y.Kwon, S.C.S.Lai, P.Rodriguez, M.T.M.Koper, J.Am.Chem.Soc. 133 (2011) 6914
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Hammond relationship
Y.Kwon, S.C.S.Lai, P.Rodriguez, M.T.M.Koper, J.Am.Chem.Soc. 133 (2011) 6914
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More detailed mechanism
Overall: R-CHHβ-OHα → R-CH=O + Hα+ + Hβ
+ + 2 e-
1. First proton: R-CHHβ-OHα + OH- → R-CHHβ-O
- + H2O
“base catalyzed”
2. OH adsorption on gold:
OH- + * ⇌ OHδ-ads + (1-δ) e
-
COads favors OHads formation
3. Second proton:
R-CHHβ-O- + OHads →
R-CH=O + HαOHβ + e-
gold catalyzed
P.Rodriguez, Y.Kwon, M.T.M.Koper, Nature Chem. 4 (2012) 177
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CPET in oxygen reduction on Pt
-0.6 -0.3 0.0 0.3 0.6 0.9
-6
-4
-2
0
0.0 0.2 0.4 0.6 0.8 1.0
-6
-4
-2
0
E / V vs SHE
a
b
j / m
A c
m-2
E / V vs RHE
0.1 M HClO4
35 mM HClO4
7 mM HClO4
20 mM NaOH
0.1 M NaOH
The ORR rate on Pt is independent of pH on the RHE scale.
Concerted proton-electron transfer.
M.F.Li, L.W.Liao, D.F.Yuan, D.Mei, Y.-X.Chen, Electrochim.Acta (2013), in press
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Decoupled PCET in ORR on Au
0.0 0.3 0.6 0.9 1.2
-1.2
-0.9
-0.6
-0.3
0.0
1 mM HClO4+NaClO4
10 mM HClO4+NaClO4
100 mM HClO4
0.5 mM NaOH +NaClO4
2 mM NaOH +NaClO4
10 mM NaOH +NaClO4
100 mM NaOH +NaClO4
b
i / uA
E / V vs RHE
-0.8 -0.4 0.0 0.4 0.8 1.2
-1.2
-0.9
-0.6
-0.3
0.0
a
E / V vs SHE
The ORR rate on Au is dependent of pH on the RHE scale.
Decoupled proton-electron transfer.
Q.J.Chen, Y.L.Zheng, L.W.Liao, J.Kang, Y.-X.Chen, Scientia Sinica Chimica 41 (2011) 1777
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Mechanism of ORR
O2
H2O2
O2(ads)
O2H(ads)OOHads
OHads + OH 2 OH
H2O
Oads + OH
ET
ET
ET ET
PT
PT
PT
CPET
CPET CPET
CPET
CPET
CPET
DIS
Oads + OHads
CPET
DIS
Pt
Au, Hg, Ag, C, [Au(100)]
M.T.M.Koper, Chem.Sci. 4 (2013) 2710
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Pt
Rh
HNO3
NO3-
pH dependence of nitrate reduction
J.Yang, P.Pascual, M.Duca, M.T.M.Koper, submitted
pKa[HNO3] = -1.3
Two reactive forms of nitrate:
HNO3 is the only reactive species on Pt.
Both HNO3 and NO3- can
be reduced on the more reactive Rh.
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Electrocatalytic CO2 reduction
CO2
CO
HCOOH
C2O42-
2e-
2e-
2e-
CH4, C2H4, CxHy
Cu
highoverpotential
aldehyde
Calvin cycle
alcohol
difficult
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CO2 and CO reduction on copper electrodes produces H2, HCOOH,CH4 and C2H4.
Y.Hori, Mod.Asp.Electrochem (2008) K.J.P.Schouten, Y.Kwon, C.J.M.van der Ham, Z.Qin, M.T.M.Koper, Chem.Sci. (2011)
CO and CO2 reduction on copper
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K.J.P.Schouten, Z.Qin, E..Perez Gallent, M.T.M.Koper, J.Am.Chem.Soc. 134 (2012) 9864
CO + 6 H+ + 6 e- → CH4 + H2O 2 CO + 8 H+ + 8 e- → C2H4 + 2 H2O
CO reduction on Cu(111) and Cu(100)
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(100) terraces - not (100) steps
K.J.P.Schouten, E.Perez Gallent, M.T.M.Koper, ACS Catal. 3 (2013) 1292
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A consistent mechanism
K.J.P.Schouten, Y.Kwon, C.J.M.van der Ham, Z.Qin, M.T.M.Koper, Chem.Sci. 2 (2011) 1902F.Calle Vallejo, M.T.M.Koper, Angew.Chem.Int.Ed 52 (2013) 7282
CO2 COads
HCOOH
2H+,2e-
2H+,2e-
HCOads HCads CH4
HCHO CH2OHads CH3OH
OC
O-
C
H+,e-
e-=CHO
HCO
CH2
O CH
C2H4
OHC = CH
reductive CO couplingMcMurry couplingepoxide reduction
?
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A. Paets van Troostwijk, J.R. Deiman, Obs. Phys. 35 (1789) 369.
1789: birth of water splitting
“Sur une manière de décomposer l'Eau en Air inflammable et en Air vital”
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O.Diaz-Morales, F.Calle-Vallejo, C.de Munck, M.T.M.Koper, Chem.Sci. 4 (2013) 2334
Oxygen evolution on gold: SERS
oxide formation
oxygen evolution
O-O stretch?
H216O
H218O
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Oxygen evolution from a Au electrode with an oxide layer formed in 18O-water: 36O2 is evolved first.
Suggests an oxide decomposition/dispropor-tionation mechanism
O.Diaz-Morales, F.Calle-Vallejo, C.de Munck, M.T.M.Koper, Chem.Sci. 4 (2013) 2334
O2 evolution on gold: online MS
Teyler’s Museum, Haarlem (NL)
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Conclusions
• Transfer 2 electrons at a time
• If you insist on transferring more than 2 electrons with 1 catalyst, be prepared to deal with scaling relationships…
• Unfavorable scaling between OOH and OH leads to irreversible kinetics of the oxygen electrode
• Proton-decoupled electron transfer leads to strong pH dependence of catalysis
• Each PCET reaction has an optimal pH, and an optimal catalyst at the optimal pH
• Many important electrocatalytic reactions are pH dependent
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Acknowledgements
• Paramaconi Rodriguez, Youngkook Kwon, Klaas Jan Schouten, Zhisheng Qin, Elena Perez Gallent, Stanley Lai, Federico Calle-Vallejo, Oscar Diaz-Morales, Jian Yang
• Jiyong Joo, Masatoshi Osawa (Sapporo)• Netherlands Organization for
Scientific Research (NWO)• National Research School Catalysis (NRSC)• BioSolarCells consortium (NL)• European Commission (FP7 ELCAT Network)• Japan Society for the Promotion of Science (JSPS)