1 2007 ACS NORM Kinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT Kenichi Shimizu;...

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2007 ACS NORM2007 ACS NORM

Kinetic Study of Formic Acid Kinetic Study of Formic Acid Oxidation using PtRu-CNT Oxidation using PtRu-CNT

and PtBi-CNTand PtBi-CNTKenichi Shimizu; I. Frank Cheng; Clive Yen; Byounghoon Kenichi Shimizu; I. Frank Cheng; Clive Yen; Byounghoon

Yoon; Chien M. WaiYoon; Chien M. Wai

Dept. Chemistry University of Idaho, Moscow, ID Dept. Chemistry University of Idaho, Moscow, ID [email protected]@vandals.uidaho.edu

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Direct Formic Acid Fuel CellDirect Formic Acid Fuel Cell Largely available in nature Largely available in nature

(Renewable).(Renewable). Higher fuel concentration than Higher fuel concentration than

DMFC.DMFC. Up to ~20 M HCOOH vs. up to ~2 M Up to ~20 M HCOOH vs. up to ~2 M

CHCH33OHOH Less fuel crossover than Less fuel crossover than

methanol.methanol. Higher theoretical cell potential.Higher theoretical cell potential.

1.45 V for DFAFC, 1.2 V for DMFC1.45 V for DFAFC, 1.2 V for DMFC

Kang, S.; et al.; J. Phys. Chem. B, 2006, 110, 7270.Rice, C.; et al.; J. Power Sources, 2003, 115, 229.

http://www.tekion.com/business/index.htm

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Formic acid oxidation as a part of Formic acid oxidation as a part of methanol oxidation processmethanol oxidation process

Cao, D.; et al. J. Phys. Chem. 2005, 109, 11622.

44

HCOOH CO2 + 2H+ + 2e- E0 = -0.25 VNHE

COads + H2O

COads + OHads CO2 +H+ +e-

H2O OHabs+ H+ + e-

Rxn 1

Rxn 2

Formic acid oxidationFormic acid oxidation

-0.05

0

0.05

0.1

0.15

0.2

0.25

-100 400 900E /(mV vs. Ag/AgCl)

A/m

gP

t

55

Binary catalysts and their effectsBinary catalysts and their effects

Bi-functional effect (Active secondary Bi-functional effect (Active secondary catalyst).catalyst).Pt, PtPd, PtRuPt, PtPd, PtRu

Third body effect (Inert secondary Third body effect (Inert secondary catalyst).catalyst).PtBi, PtPb, PtAuPtBi, PtPb, PtAu

Catalyst support is either Pt itself or carbon black.Inert to oxidation of small organic solvent.

Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614Conway, B.E.; et al. Zeitschrift für physikalische Chemie Neue Folge 1978, 112, 195-214.

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Bi-functional EffectBi-functional Effect

Active secondary metal catalyst such as Ru.Active secondary metal catalyst such as Ru.Dissociative absorption of CO onto Pt.Dissociative absorption of CO onto Pt.

1.1. Pt-CHPt-CH33OHOHadsads Pt-CO Pt-COadsads + 4H + 4H++ + 4e + 4e--

Or (Pt + HCOOH Or (Pt + HCOOH Pt-CO Pt-COadsads + H + H22O)O)

Absorption of OH onto Ru through dissociation of HAbsorption of OH onto Ru through dissociation of H22O.O.2.2. HH22O + Ru O + Ru Ru-OH Ru-OHadsads + H + H++ +e +e--

2.2. Pt-COPt-COadsads + Ru-OH + Ru-OHadsads Pt + Ru + CO Pt + Ru + CO22 +H +H++ + e + e--

Secondary catalyst must have lower dissociation Secondary catalyst must have lower dissociation potential than Pt. potential than Pt. (Ru has 0.2 -0.3 V lower reaction potential (Ru has 0.2 -0.3 V lower reaction potential than Pt)than Pt)

Rate determining step is 3.Rate determining step is 3.

Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614.Christensen, P.A. et al. J. Electroanal. Chem. 1993, 362, 207-218.

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Third Body EffectThird Body Effect Dissociative absorption on Pt takes more than one Dissociative absorption on Pt takes more than one

active siteactive site Pt + CHPt + CH33OH OH Pt-CH Pt-CH22OH + H +eOH + H +e--

Pt-CHPt-CH22OH OH Pt Pt22-CHOH + H-CHOH + H++ + e + e--

PtPt22-CHOH -CHOH Pt Pt33-COH + H-COH + H++ + e + e--

PtPt33-COH -COH Pt-CO + 2Pt +H Pt-CO + 2Pt +H++ + e + e--

Catalytically inert catalyst, such as Catalytically inert catalyst, such as Bi, sterically hinders absorption of Bi, sterically hinders absorption of poisonous carbon species.poisonous carbon species.

Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614

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How can overall reaction be improved.How can overall reaction be improved.

Chen, X.-Y., et al., J. Angew. Chem., Int. Ed. 2006, 45, 981.

Research questionResearch question

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PtRu and PtBi CNT PtRu and PtBi CNT

Atomic ratio of Pt:Ru is 1:1.4.Atomic ratio of Pt:Bi is 1:1.6.

Pt42Ru58CNT Pt38Bi62CNT

1010

Catalytic effect of PtRuCNTCatalytic effect of PtRuCNT

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-100 100 300 500 700 900

E /(mV vs. Ag/AgCl)

A/m

g P

t (

PtR

uC

NT

)

-0.05

0

0.05

0.1

0.15

0.2

0.25

A/m

g P

t (P

tCN

T)

PtRu CNTPt CNT

1 M H1 M H22SOSO44

0.1 M HCOOH0.1 M HCOOH Forward peaks are not Forward peaks are not

resolved as well as resolved as well as using PtCNT.using PtCNT.

Peak current was Peak current was enhanced with PtRu enhanced with PtRu CNT. CNT.

1111

-0.05

0

0.05

0.1

0.15

0.2

0.25

-100 400 900E /(mV vs. Ag/AgCl)

A/m

gP

t

(1)(1) (2)(2) (3)(3)PtCNTPtCNT 665 mV665 mV 396396 457457

PtCB* PtCB* 696696 424424 331331

PtRuCNTPtRuCNT 642642 N/AN/A 395395

PtRuCB*PtRuCB* 654654 N/AN/A 424424

CNT supported catalysts CNT supported catalysts resulted in slightly lower resulted in slightly lower oxidation potential.oxidation potential.

Peak 2 and 3 might be due Peak 2 and 3 might be due to the same reaction.to the same reaction.

(1)

(2)

(3)

Summary of Peak PotentialsSummary of Peak Potentials

*Carbon black supported Pt and PtRu from ETEK

Pt CNT

1212

-0.05

0

0.05

0.1

0.15

0.2

0.25

-100 400 900E /(mV vs. Ag/AgCl)

A/m

gP

t

(1)(1) (2)(2) (3)(3) (1)/(3)(1)/(3)

PtCNTPtCNT 0.150.15 0.0850.085 0.280.28 0.550.55

PtCB*PtCB* 0.220.22 0.0590.059 0.670.67 0.330.33

PtRuCNTPtRuCNT 0.520.52 N/AN/A 0.400.40 1.31.3

PtRuCB*PtRuCB* 0.180.18 N/AN/A 0.180.18 1.01.0

Higher reverse peak than Higher reverse peak than forward peak may indicate forward peak may indicate sluggish kinetic activity for Pt sluggish kinetic activity for Pt CNT and PtCB.CNT and PtCB.

(1)

(2)

(3)

Summary of Peak CurrentsSummary of Peak Currents

A/mg Pt

Pt CNT


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Influence of TemperatureInfluence of Temperature

0

1

2

3

-100 400 900

PtRu CB 30%

0

0.6

1.2

-100 400 900

Pt CNT

0

0.5

1

1.5

2

-100 400 900

E/(mV vs. Ag/AgCl)

mA

PtRu CNT

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Activation Energy for intermediate Activation Energy for intermediate oxidationoxidation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-100 400 900E /(mV vs. Ag/AgCl)

mA

Pt CB

-10

-9

-8

-7

-6

0.0028 0.00371/T (1/K)

ln (

ip)

PtRu CB

PtCB

PtRuCNT

PtCNT

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Summary of Activation EnergySummary of Activation Energy

EEacac (kJ/mole) (kJ/mole) CI (90%)CI (90%)

PtCB**PtCB** 20.420.4 N/AN/A

Pt***Pt*** 20.920.9 1.61.6

PtCB*PtCB* 21.121.1 3.23.2

PtCNTPtCNT 17.517.5 3.63.6

PtRuCB*PtRuCB* 21.021.0 5.75.7

PtRuCNTPtRuCNT 22.222.2 3.93.9

**Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.***Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

90% confidence interval


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EEacac

(kJ/mole)(kJ/mole)CI (90%)CI (90%)


Pt***Pt*** 20.920.9 1.61.6

PtCB*PtCB* 21.121.1 3.23.2

PtCNTPtCNT 17.517.5 3.63.6



Results are agreeable Results are agreeable to the others.to the others.

Mean activation Mean activation energy was the energy was the smallest with Pt CNT.smallest with Pt CNT.


90% confidence interval


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Summary of PtRuCNTSummary of PtRuCNT

It is effective towards formic acid It is effective towards formic acid oxidation. (Highest peak current with oxidation. (Highest peak current with PtRuCNT).PtRuCNT).

There is no significant difference in There is no significant difference in activation energies.activation energies.

PtRuCNT has higher turn over rate.PtRuCNT has higher turn over rate.

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Catalytic Effect of PtBiCNTCatalytic Effect of PtBiCNT

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

-100 100 300 500 700 900

E /(mV vs. Ag/AgCl)

A/m

g P

t (P

tBiC

NT

)

-0.05

0

0.05

0.1

0.15

0.2

0.25

A/m

g P

t (P

tCN

T)

PtBi CNTPt CNT 1 M H1 M H22SOSO44

0.1 M HCOOH0.1 M HCOOH Position of the forward Position of the forward

peak of PtBiCNT was peak of PtBiCNT was almost same as the almost same as the backward peak.backward peak.

PtBiCNT had lower PtBiCNT had lower current output than current output than PtCNT. PtCNT.

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Summary of Peak PotentialsSummary of Peak Potentials

Peak (1) was not Peak (1) was not observed for PtBi observed for PtBi catalyst; no formation catalyst; no formation of Pt-CO.of Pt-CO.

(1)(1) (2)(2) (3)(3)PtCNTPtCNT 665mV665mV 396396 457457

PtCB*PtCB* 696696 424424 331331

PtRuCNTPtRuCNT 642642 N/AN/A 395395

PtRuCB*PtRuCB* 654654 N/AN/A 424424

PtBiCNTPtBiCNT N/AN/A 467467 443443

-0.05

0

0.05

0.1

0.15

0.2

0.25

-100 400 900E /(mV vs. Ag/AgCl)

A/m

gP

t

(2)

(1)

(3)Pt CNT


2020

Summary of Peak CurrentsSummary of Peak Currents

Low catalytic activity of PtBi Low catalytic activity of PtBi may be attributed to the may be attributed to the larger particle size.larger particle size.

Better Efficiency than PtCNT Better Efficiency than PtCNT and PtCB catalysts.and PtCB catalysts.

(1)(1) (2)(2) (3)(3) (2)/(3)(2)/(3)

PtCNTPtCNT 0.150.15 0.0850.085 0.280.28 0.300.30

PtCB*PtCB* 0.220.22 0.0590.059 0.670.67 0.090.09

PtRuCNTPtRuCNT 0.520.52 N/AN/A 0.400.40 N/AN/A

PtRuCB*PtRuCB* 0.180.18 N/AN/A 0.180.18 N/AN/A

PtBiCNTPtBiCNT N/AN/A 0.0430.043 0.0310.031 1.391.39

-0.05

0

0.05

0.1

0.15

0.2

0.25

-100 400 900E /(mV vs. Ag/AgCl)

A/m

gP

t

(2)

(1)

(3)Pt CNT


2121

Influence of TemperatureInfluence of Temperature

-12

-11

-10

-9

-8

-7

0.0027 0.0037

1/T (1/K)

ln (

ip)

PtCBPtBiCNT

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-100 100 300 500 700 900

E /(mV vs. Ag/AgCl)

mA

PtBi CNT

Peak current leveled off Peak current leveled off above 23 above 23 °C.°C.

*Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.**Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

2222


Bi secondary Bi secondary catalyst is not catalyst is not taking part of taking part of electro-oxidation electro-oxidation process itself.process itself.

Restriction of Pt Restriction of Pt reaction site by Bi reaction site by Bi resulted in high resulted in high activation energy.activation energy.

EEacac

(kJ/mole)(kJ/mole)CI (90%)CI (90%)


Pt***Pt*** 20.920.9 1.61.6

PtCB*PtCB* 21.121.1 3.23.2

PtCNTPtCNT 17.517.5 3.63.6



PtBiCNTPtBiCNT 45.545.5 4.74.7

PtBi(III) ***PtBi(III) *** 42.342.3 7.27.2



2323

Summary of PtBiCNTSummary of PtBiCNT

Only peak associated with reaction 1 was Only peak associated with reaction 1 was observed.observed.

Low catalytic activity (low peak current). Low catalytic activity (low peak current). Activation energy was significantly larger Activation energy was significantly larger

than Pt and PtRu electrocatalysts.than Pt and PtRu electrocatalysts.

Bi suppresses reaction 2.Bi suppresses reaction 2.

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Tafel analysisTafel analysis

-8

-7

-6

-5

-4

-3

-0.1 0.4 0.9

E /(mV vs. Ag/AgCl)

Lo

g l

i l

PtRuCNTPtCNT

Slope (mV/dec)Slope (mV/dec)

PtCB*PtCB* 150150

PtCNTPtCNT 271 (140)271 (140)

PtCB*PtCB* 198198

PtRuCNTPtRuCNT 6161

PtRuCB*PtRuCB* 8888

PtBiCNTPtBiCNT 331331

Higher tafel slope indicated that Higher tafel slope indicated that the first electron transfer likely be the first electron transfer likely be the rate determining step.the rate determining step.

*Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.Maciá M.D.; et al.; J. Electroanal. Chem. 2003, 554-555, 25.


2525

ConclusionConclusion PtRu was possible to enhance formic acid oxidation through PtRu was possible to enhance formic acid oxidation through

reaction 2.reaction 2. Binary catalyst with bi-functional effect, i.e. PtRuCNT, did not Binary catalyst with bi-functional effect, i.e. PtRuCNT, did not

affect on the activation energy of formic acid oxidation.affect on the activation energy of formic acid oxidation.

With PtBiCNT, major reaction was reaction 1, which had the With PtBiCNT, major reaction was reaction 1, which had the lower oxidation potential than reaction 2. lower oxidation potential than reaction 2.

PtBiCNT catalyst did not improve the catalytic activity towards PtBiCNT catalyst did not improve the catalytic activity towards formic acid oxidation. formic acid oxidation.

PtBiCNT caused large increase in activation energy indicating PtBiCNT caused large increase in activation energy indicating effective suppression of reaction 2.effective suppression of reaction 2.

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Improvement of formic acid oxidation can Improvement of formic acid oxidation can be achieved by using catalyst with third be achieved by using catalyst with third body effect. body effect.

We need to prepare more active PtBi We need to prepare more active PtBi CNT.CNT.

ConclusionConclusion

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AcknowledgementAcknowledgement

Dr. I. Frank ChengDr. I. Frank Cheng Chris RoskeChris Roske Dr. Chen M. WaiDr. Chen M. Wai Dr. Byunghoon YoonDr. Byunghoon Yoon Dr. Clive H. YenDr. Clive H. Yen Dept of Chemistry at Dept of Chemistry at

the University of Idahothe University of Idaho

Financial supportFinancial support Electric Power Research Electric Power Research

Institute (EPRI) Institute (EPRI) Innovative Small Grants Innovative Small Grants ProgramProgram

Dr. and Mrs. Renfrew Dr. and Mrs. Renfrew Summer ScholarshipSummer Scholarship

1 2007 ACS NORM Kinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT Kenichi Shimizu;...

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