the effect of carbon structure and properties on the ...€¦ · • porosity - microporosity...

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The effect of carbon structure and properties on the preparation of carbon

nanofiber supported Pt- catalysts

Ingvar KvandeTrondheim, 09.06.09

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Outline

• Introduction• Conventional carbons and common viewpoints with regards to catalyst

preparation• Carbon nanofibers (CNFs)• Preparation of Pt supported CNF supported catalysts

– Carbon nanofiber (CNFs) synthesis– Suitability of different preparation methods

• Ion-Exchange, Deposition-Precipitation, Impregnation, Colloids– Governing CNF properties for metal oxide colloid deposition– Preparation of Pd supported CNF supported catalysts – supporting data– The adsorption site – Stability issues

• Results from catalytic test: H2-oxidation with and without CO• Summary

Overall scope of joint project:Develop electrocatalysts for use in high temperature polymer and alkaline

fuel-cellsOverall scope of this work:

Improved understanding of the properties of CNFs and CNF supported Pt catalysts with regards to their performance for fuel-cells systems.

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Introduction• Conventional carbon support in fuel cells

– Carbon black (Vulcan XC-72R)• consist generally of shells made up of small quasi-

graphitic crystallites• significant microporosity. Ash content

• Common problems/Driving force for new materials– Loss of Pt surface area

• Dissolution and sintering of Pt• Carbon corrosion

– More stable and active catalysts needed• Slow cathode reactions in DMFCs and PEMFCs• Poisoning by CO and methanolic residues in anode

reaction– Large scale implementation of fuel cells dependent on

improved utilization of Pt

• Promising support alternatives– CNFs

• Graphite layers stacked in an ordered manner– Platelet

• Perpendicularly with respect to the fiber axis

– Fishbone• Angle with fiber axis between 0 and

90°– Carbon nanotubes (CNTs)

• Graphite layers rolled into a cylinder

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Physical properties• Surface area – Pt dispersion found to increase with increasing surface

area• Porosity - Microporosity responsible for high surface area act as a

physical barrier to deposited metal particles thus increasing dispersion• Increasing metal loading – decreasing dispersion

– Part of the microporosity not available to the metal precursor– And/or deposition will be forced into larger pores

Conventional carbons and common viewpoints with regards to catalyst preparation

F. Rodriguez-Reinoso, Carbon, 36 (1998) 159.

S.Galvagno, C.Milone, A. Donato, G. Neri, R. Pitropaolo. Heterogeneous catalysis and Fine Chemicals III, ed. M. Guisnet et al, Elsevier, Amsterdam, (1993), p 163

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F. Rodriguez-Reinoso, C. Salinas-Martinez de Lecea, A. Sepulveda-Escribano and J.D. Lopez-Gonzalez, Catal. Today, 7 (1990) 287.



Fe/C catalysts with different metal precursor(iron nitrate, iron pentacarbonyl)Iron loading a function of metal precursor

Access of the metal solution related to thechemical character of the surface

A large surface area and right pore size distribution necessary but notsufficient to explain the properties of carbon-supported catalysts

Surface chemistry

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Surface ChemistryOxygen surface groups have beenfound to both enhance and suppressthe deposition of metal precursors.

C. Prado-Burguete, A. Linares-Solano, F. Rodríguez-Reinoso and C.S.-M. de Lecea, J. Catal., 115 (1989) 98


Supports with similar porosity and surface

area but different amount of surface oxygenMore oxygen

Increasing metal dispersion

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Surface Chemistry– Amphoteric nature

Maximum catalyst dispersionfavoured when the entirecarbon surface is chemicallyaccessible

Electrostatic attraction

Stability?• Some oxygen groups may not be chemically stable

under reduction conditions


9

Carbon surface chemistry – non-oxygen and basic sites?


10

Carbon nanofibers (CNFs)• High conductivity• Well defined structure and

high purity– sulphur free

• High external surface area– As high as 250 m2/g

• Predominantly mesoporous– No diffusional limitations

• Edges available for interaction

– Different interaction on structures with different stacking angles?

– Electronic or geometric interaction?

• Bessel and co-workers (J. Phys. Chem. B 105 (2001) 1115

– Similar activity in methanol oxidation for 5wt% Pt supported on platelet and “ribbon”CNFs compared to 25wt% Pt supported on Vulcan XC-72R

• Peculiar geometric interaction?• More efficient removal of adsorbed

species?• Higher conductivity?

• Other studies on CNF/CNT supported catalysts

– Higher electro active surface area?– Significant metal-support electronic

interaction?

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CNF-synthesisTable 1. Supports, synthesis conditions and post treatment conditions.

Structure Catalyst Reactants Feed ratio Reactor, scale Temperature

[°C]

Used in

paper

Platelet Fe3O4 CO/H2 4:1 Quartz, small 600 IV,VI

Platelet Fe3O4 CO/H2 4:1 Quartz, large 600 IV, V

Fishbone Ni/Al2O3 CH4/H2 4:1 Quartz, small 600 II

Fishbone Ni/Al2O3 CH4/H2 4:1 Ceramic, large 600 III, IV,VI

Fishbone Fe/SiO2 CO/H2/N2 80:27:160 Quartz, small 550 IV, VI

Fishbone Ni/SiO2 CO/H2/N2 80:27:160 Quartz, small 550 IV, V, VI

P1P2

FB Ni-CH4

FB Fe-COFB Ni-CO

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SupportsCatalyst preparation, Pt/CNF

Platelet 1 as-grown Platelet 2

Fishbone Fe-COFishbone Ni-CO

Fishbone Ni-CH4MWNTs

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Catalyst preparation, Pt/CNF, methodsI. Kvande, S.T. Briskeby, M. Tsypkin, M. Ronning, S. Sunde, R. Tunold and D. Chen, Top. Catal., 45 (2007) 81.

The homogeneous deposition-precipitation method (HDP) – aqueous solvent

50 ml of deionised water and 0.250 g of CNF was mixed in a bottle. The solution was acidified and heated to 90 °C in an oil bath under nitrogen protection. At 90 °C, Pt(NH3)4(OH)2 and urea was added under vigorous stirring. The reaction was stopped when the pH reached a constant level (pH ca 6.5) after 18 h. The catalyst was studied by TEM after reduction at 250 °C for 1 h.

The modified polyol method – organic solvent

Chloroplatinic acid was dissolved in ethylene glycol (EG). The mixture was stirred at room temperature for 30 min. The solution was then stirred at 140–180 °C for 3 h before being cooled down to 80°C. The carbon support was then dispersed in EG by sonication before being added to the Pt-containing solution. This solution was stirred for 18 h at 80 °C. It was then cooled down to room temperature before being filtered and washed with copious amounts of distilled water. The product was washed until no detection of chlorine-ions. The catalyst was dried in a vacuum oven at 80 °C for 6 h.

Incipient wetness impregnation – aqueous solvent

Ion-exchange – aqueous solvent

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Catalyst preparation, Pt/CNF, methods

– Deposition-precipitation and ion exchangeare dependent on oxygen surface groups for immobilization of the precursor

• limited to low Pt loadings (< 3wt% Pt)– Incipient wetness impregnation can reach

higher loading, but particle growth is pronounced already at 10 wt% Pt

M.L. Toebes, M.K. van der Lee, L.M. Tang, M.H. Huis in't Veld, J.H. Bitter, A.J. van Dillen and K.P. de Jong, J. Phys. Chem. B, 108 (2004) 11611.

I. Kvande, S.T. Briskeby, M. Tsypkin, M. Ronning, S. Sunde, R. Tunold and D. Chen, Top. Catal., 45 (2007) 81.

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Catalyst preparation, Pt/CNF, methods

– Deposition-precipitation and ion exchangeare dependent on oxygen surface groups for immobilization of the precursor

• limited to low Pt loadings (< 3wt% Pt)– Incipient wetness impregnation can reach

higher loading, but particle growth is pronounced already at 10 wt% Pt

– Modified polyol method• Quick nucleation and complete separation of

the nucleation and growth steps together with the controlled sedimentation step give effective control of particle size and distribution

• Organic solution• 24 wt% Pt readily achieved

I. Kvande, S.T. Briskeby, M. Tsypkin, M. Ronning, S. Sunde, R. Tunold and D. Chen, Top. Catal., 45 (2007) 81.

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Metal oxide colloid method– Metal oxide colloids prepared by base hydrolysis (Li2CO3) of water-soluble metal

salts. H2PtCl6 [PtOx] [OH]-

Hydrolysis and condensation or cocondensation to form colloidal monometal oxidesColloid stabilized by negatively charged [OH]—groups

– Advantages:• Water as solvent• No additional stabiliser• Do not need costly reducing agents• Large number of accessible mixed metal systems available (Pt ,Ir, Os, Ru, Sn)

M.T. Reetz, M.G Koch, J. Am. Chem. Soc, 1999, 121, 7933M.T. Reetz, M. Lopez: U.S. Patent 7,244,688 (2003)

Catalyst preparation, Pt/CNF, MO-colloid

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Metal oxide colloid method, SupportsCatalyst preparation, Pt/CNF, MO-colloid

Platelet 1 as-grown Platelet 2

Fishbone Fe-COFishbone Ni-CO

Fishbone Ni-CH4MWNTs

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Metal oxide colloid method, SupportsCatalyst preparation, Pt/CNF, MO-colloid

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CNF + PEI

Li2CO3 + H2PtCl6 (20wt%)60°C, pH 9-10

12h

Dropwise addition45 min

70°C

In situ Immobilization

CNF

12h60°C, pH 9-10

70°C

Filtration,Drying

Filtration,Drying

8h (2h)Two-step deposition


20

0

0,1

0,2

0,3

0,4

200 250 300 350 400 450

Wavelength (nm)

Abs

orba

nce

UV-vis

The peak of Pt4+ doesnot disappearcompletely

Experiment endedwhen no change inpeak with time


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Metal oxide colloid method, Catalysts

Pt-loading, temperature at the highest rate of oxidation for both support and catalyst and their difference, Tmax.

Samle Conc. [mM]

Time [h]

Pt loading [Wt%]

T max catalyst

[°C]

ΔTmax [°C]

MO/P1 ag 8.2 8 11.3 494 136

MO/P1 ag 2 2.5 2 6.5 520 110

MO/P1 ox 2.5 2 5.4 514 97

MO/P2 8.2 8 17.1 484 101

MO/FB Fe-CO 8.2 8 3.1 555 52

MO/FB Ni-CO 8.2 8 8.9 - -

MO/FB Ni-CH4 ag 2.5 2 3.0 - -

MO/FB Ni-CH4 ox 2.5 2 1.4 545 65

MO/MWNT 8.2 8 8.1 428 194

MO/XC72 8.2 8 19.6 400 260


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Metal oxide colloid method, Supports, Zeta-potentialCatalyst preparation, Pt/CNF, MO-colloid

-30

-20

-10

0

10

20

30

0 2 4 6 8

pH

Zeta

-pot

entia

l [m

V]

-50-40-30-20-10

01020304050

0 2 4 6 8 10 12

pH

Zeta

-pot

entia

l [m

V]

s

Oxidized supports negatively charged for all pHFishbone Fe-CO also negatively charged for all pHAs-grown and purified supports are positively charged at low pH (<5)

Platelet 1 as-grown

Platelet 2

Platelet 1 oxidized

Fishbone Ni-CO

Fishbone Fe-CO

Fishbone Ni-CH4 as-grown

Fishbone Ni-CH4 oxidized

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282284286288290292

XC72

MWNT

FB Ni-CH4

FB Ni-CO

FB Fe-CO

P2

P1ag

Binding energy [eV]

Inte

nsity

(a.u

.)

530531532533534535536537

Binding energy [eV]In

tens

ity n

orm

aliz

ed b

y C

1s a

rea

(a.u

.)Metal oxide colloid method, Supports, XPSCatalyst preparation, Pt/CNF, MO-colloid

Clear differences in the high binding energy side for the supports- Oxygen type and content

Fishbone Fe-CO

Vulcan XC-72

P1agP2

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Metal oxide colloid method, Supports


282284286288290292294

Binding energy [eV]

Inte

nsity

(a.u

.)

530532534536538

Binding energy [eV]

Inte

nsity

(a.u

.)

a) b)

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Metal oxide colloid method, Governing CNF properties

Defect or edge sites identified as necessary for Pt particle deposition

•More abundant in platelet CNFs

For CNF supports a negative effect of oxygen groupsA lowered strength of A-sites in the presence of oxygen reported in literature

•Higher abundance of small carbon crystallites and thereby strong A sites in Vulcan XC-72R

0

5

10

15

20

0,33 0,34 0,35 0,36 0,37

d002

Pt L

oadi

ng [W

t%]

b)

0

5

10

15

20

0,8 0,9 1 1,1 1,2 1,3

C1s FWHM

Pt lo

adin

g [W

t%]

0

5

10

15

20

15 20 25 30 35sp3/amorphous carbon and/or edge structures

[%]

Pt l

oadi

ng [W

t%]

0

5

10

15

20

0 0,05 0,1 0,15

O1s/C1s

Pt L

oadi

ng [W

t%]

e) f)

0

5

10

15

20

0 2 4 6 8 10

Lc

Pt lo

adin

g [W

t%]

0

5

10

15

20

0 50 100 150 200 250 300

Surface area [m2/g]

Pt l

oadi

ng [W

t%]

a)

c) d)

Catalyst symbols: Platelet CNFs Fishbone CNFs MWNTs Vulcan XC-72R


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Preparation of CNF supported Pd catalysts – supporting data

Pd particles supported on bothcarbon nanotubes and fishboneand platelet CNFs

Purification:

1. 6M NaOH 3x3h 2. 4M HCl

Control of surface chemistry

1. No treatment2. Oxidation in stagnant air at 500°C

for 2h3. Oxidation in concentrated HNO3

for 12h4. Annealing in Ar at 800°C for 1h

Characterization

• Zeta-potential measurements• TPD-MS

Platelet Fishbone

TubeJ. Zhu, T. Zhao, I. Kvande, D. Chen, X. Zhou and W. Yuan, Chinese Journal of Catalysis, 29 (2008) 1145

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Pd particles supported on bothcarbon nanotubes and fishboneand platelet CNFs

Purification:

1. 6M NaOH 3x3h 2. 4M HCl

Control of surface chemistry

1. No treatment2. Oxidation in stagnant air at 500°C

for 2h3. Oxidation in concentrated HNO3

for 12h4. Annealing in Ar at 800°C for 1h

Characterization

• Zeta-potential measurements• TPD-MS

J. Zhu, T. Zhao, I. Kvande, D. Chen, X. Zhou and W. Yuan, Chinese Journal of Catalysis, 29 (2008) 1145

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Catalyst preparation by the polyol method. Depositionaccomplished with pH-adjustment to pH = 3

For the synthesis of spherical Pd colloid/CNF, 15 ml of 1mol/L NaOH/ethylene glycol (EG) was placed in athree-necked flask and heated in air at 80 °C for 1 h. Meanwhile,0.048 g of Na2PdCl4 and 0.240 g of polyvinylpyrrolidone(PVP) were separately dissolved in 5 ml of EG at roomtemperature, which were simultaneously injected into thethree-neck flask at a rate of 10 ml/h. The reaction mixture washeated at 90 °C in air for 6 h. Then, 0.33 g CNF suspended in 5ml EG was added into the mixture and the pH was adjusted to3.0 by 0.5 mol/L HCl. After stirring for 12 h, the product wascentrifuged and washed with acetone once and then withethanol thrice to remove EG and excess PVP.

The electrostatic attraction caused by thepotential difference between the CNFsurfaces and the palladium colloids is themost important parameter for controlling thePd loading and the particle size


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Surface Chemistry – Non-oxygenanchoring sitesFragments containing >C=C< bonds

• Definition of sites

A1-site : hexagon in a graphene layerA2-site : plane steps and edge sitesA3-site : micropore sites

• Abundancy conventional carbons: – A1, A2 >> A3

• Abundancy CNFs: – No A3 sites

• Site strength : decreasing linearly withincreasing amount of oxygen P.A. Simonov, S.Y. Troitskii, V.A. Likholobov, Kinet. Catal. 41 (2000) 255

P.A. Simonov, V.A. Likholobov, Catal. Electrocal. Nanopart. Surf. (2003) 409

Carbon surface chemistry – non-oxygen sites

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Basic surface sites – not clearly identified but highly important respect to catalyst preparation!

• pyron-type of structure. In this case two non-neighbouring oxygen atoms, preferentially at different rings of the graphene layer, stabilise the positive charge by resonance

• oxygen free sites rich in π-electrons

“At ambient conditions a significant fraction of the oxygen-free edge sites are neither H-terminated nor unadulterated ó free radicals, as universally assumed”

Carbyne or carbene type sites

Carbon surface chemistry – non-oxygen sites

L.R. Radovic and B. Bockrath, J. Am. Chem. Soc., 127 (2005) 5917.

C.A.Leon y Leon, J.M.Solar, V.Calemma and L.R.Radovic, Carbon, 30(5), (1992), 797.

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Stability issues• Acidic surface groups like carboxylic acid groups typically used for anchoring metal precursors are

unstable and decompose at relatively low temperatures (< 350°C).

• An enhanced stability of Pd-particles with an increasing amount of edge sites was indicated through asuppression of leaching during the Heck reaction


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Results from catalytic test: H2-oxidation with and without CO

Fixed bed catalyst setup•Catalyst bed, 100mg total

• Pt/CNF catalyst (0.08 wt% Pt) diluted with SiC

•Reduction•5% H2 at 175°C for 30 min

•Reaction conditions•Gas composition

•H2 and O22 fed in stoichiometric amounts (8 ml/min and 4 ml/min)•The effect of CO was studied feeding H2 with 1 mole% CO•Diluted with He (800 ml/min) in order to control the level of reaction

•T-scanning procedure•25-200°C, 1°C/min ramp rate

•The O2-concentration from a GC coupled to the reaction setup was used to calculate the conversion.

H2/CO

He

H2

MFC

Air

MicroGC4 TCD

CondenserMFC

MFC

MFC

PM1

1

2

3

4

PC

Vent

Reactor

Furnace

PM2

TC

H2/CO

HeHe

H2 H2

MFC

AirAir

MicroGC4 TCD

CondenserMFC

MFC

MFC

PM1

1

2

3

4

PC

Vent

Reactor

Furnace

PM2

TC

Experimental setup

I. Kvande, D. Chen, T.-J. Zhao, I. Skoe, J. Walmsley and M. Rønning, Top. Catal., 52 (2009) 664.

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Catalysts

CNFs State/treatment Carbon source

Growth Catalyst

CatalystResiude[wt%]

Catalyst name

Preparation Method

Loading, TPO

[Wt%]

Particle size

[nm]

Platelet as-grown CO/H2 Fe3O4 0.9 MO-Pag Metal-Oxide Colloid 3.5 1.7

(2.1)*

Platelet oxidized* 1h HNO3

CO/H2 Fe3O4 0.4 MO-Pox Metal-Oxide Colloid 3.0 1.9

(2.2)*

Fishbone 4 M NaOH overnight CO/H2

20 wt% Fe/SiO2

1.1 MO-FB1 Metal-Oxide Colloid 2.2 1.8

Fishbone as-grown CH4/H2 77.5wt% Ni/Al2O3

4.9 MO-FB2ag Metal-Oxide Colloid 3.0 1.8

Fishbone HNO3/H2SO4* 1:3 30 min CH4/H2

77.5wt% Ni/Al2O3

1.8 MO-FB2ox Metal-Oxide Colloid 1.4 2.7


77.5wt% Ni/Al2O3

1.8 DP-FB2ox Deposition-Precipitation 2.3 2.0


77.5wt% Ni/Al2O3

1.8 IW-FB2ox Impregnation 4.1 3.2d

* Particle size, TEM, after H2-ox

The effect of CNF structure, oxygen groups and catalyst preparation method on catalytic activity were investigated

Results from catalytic test: H2-oxidation with and without CO I. Kvande, D. Chen, T.-J. Zhao, I. Skoe, J. Walmsley and M. Rønning, Top. Catal., 52

(2009) 664.

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Results from catalytic test: H2-oxidation with and without CO Results without CO

-11.50

-11.00

-10.50

-10.00

-9.50

-9.00

-8.50

-8.00

0.002 0.003 0.003 0.003 0.003

I/T [K-1]

ln r0

([mol

/g P

t s])

-12.00

-11.50

-11.00

-10.50

-10.00

-9.50

-9.00

-8.50

-8.00

0.0024 0.0029

1/T [K-1]

ln r0

([mol

/g P

t s])

Arrhenius plot (ln ro as a function of 1/T) for MO-Pag (■), MO-Pox ( ), MO-FB1 ( ), MO-FB2ag (●), MO-FB2ox (○), DP-FB2ox ( ) and IW-FB2ox (x).

Activation energy, Ea, and turnover frequency (TOFTOS: 50 min.-1h 30 min. Sample name Ea [kJ/mol]a TOF [s-1]b

MO-Pag 19 16.8

MO-Pox 26 3.4

MO-FB1 29 3.8

MO-FB2ag 32 3.4

MO-FB2ox 30 2.6

DP-FB2ox 35 2.4

IW-FB2ox 32 3.9 a Activation energy. 1st order dependence with respect to H2

b Turnover frequency at 70°C based on particle size from TEM-resu

By treating the reactor as a isothermal plug flow reactor, aa first order dependence with respect to H2 fitted the data


35

Results from catalytic test: H2-oxidation with and without CO Results with CO

0.0

0.2

0.4

0.6

0.8

1.0

50 75 100 125 150 175

Temperature [C]

Con

vers

ion,

x

MO-P

MO-FB1

MO-FB2ox

IW-FB2ox

DP-FB2ox

Conversion as a function of temperature for hydrogen oxidation with 1mol% CO


36

XPS study of Pt/CNFs in the presence and absence of CO

The XPS measurements were carried out at the SGM1-Scienta beamline at the synchrotron radiation source ASTRID at Institute for Storage Ring Facilities (ISA), University of Aarhus, Denmark.

C1s shoulder:CO more weakly bonded to Pt supported on purified than on oxidized CNFs

Photon energy: 340 eV

C1s-spectra

Peak 1Peak 2

282283284285286287288289

Pt/Pp annealed 130°C

Pt/Pp 100L CO

Pt/Pp

Binding energy [eV]

Inte

nsity

(a.u

)

37

XPS study of Pt/CNFs in the presence and absence of CO

The XPS measurements were carried out at the SGM1-Scienta beamline at the synchrotron radiation source ASTRID at Institute for Storage Ring Facilities (ISA), University of Aarhus, Denmark.

C1s shoulder:CO more weakly bonded to Pt supported on purified than on oxidized CNFs

Photon energy: 340 eV

C1s-spectra

282283284285286287288289

Pt/Pox annealed 130°C

Pt/Pox 100L CO

Pt/Pox

Binding energy [eV]

Inte

nsity

(a.u

.)

Peak 1Peak 2

38

SummaryPreparation of fishbone CNF supported Pt catalysts

– Aqueous methods require surface oxygen surface groups for deposition• The relatively low surface area of CNFs restrict the amount of surface

groups and thereby the loading when applying Ion-exchange, deposition-precipitation and impregnation

– Modified polyol method : An organic solvent and the controlled deposition of the colloids improves Pt loading

Governing parameters in catalyst preparation by the metal oxide colloid method– Defect and edge sites were identified as immobilization sites

• Non-oxygen type sites – carbene?– There is a negative effect of surface oxygen on the Pt loading

• Not as predominant for Vulcan XC-72

The deposition site is different for CNFs/CNTs and XC-72?!

39

Summary

Results from catalytic test: H2-oxidation with and without CO• Pt supported on as-grown platelet CNFs significantly more active than the other

catalysts both in the absence and presence of CO– CO more weakly bonded to Pt supported on purified than on oxidized CNFs

the effect of carbon structure and properties on the ...€¦ · • porosity - microporosity...

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