CARBON NANOFIBRE GROWTH FROM LOW TEMPERATURE METHANE DECOMPOSITION

OVER SKELETAL TRANSITION METAL CATALYSTSJames Highfield1, Yook Si Loo1, Ziyi Zhong1, Ruijiang Li1 & Benjamin Grushko2

1 Applied Catalysis Technology, Institute of Chemical & Engineering Sciences, 1 Pesek Road, Jurong Island, SINGAPORE S627833. 2 Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 GERMANY

CH4 C + 2 H2 H298 K = + 74.5 kJ mol-1

A. Direct eco-friendly route to “CO-free” H2 and speciality carbons.

B. Single-metal & multinary (alloy ?) skeletal catalysts from quasicrystals (QC)

Route: Al65-75(TM/Cu)35-25 arc melt/anneal XRD ideally single-phase QC

P

N2

T

Dual-set pointTemperature

Controller

Dual Lock Gas valve

To vacuum

Gas cylinder with pressure gauge

T

Heating Block

Gas Diffuser and Filter

Top lid

Pre-calibrated volume (100 ml) for calculation of gas consumption

Purge line

Gas cylinder with pressure gauge

P

N2O

/He

5M NaOH Chemical Liquid Feed pump

Knock-out pot

Drain line

P

< 5 barg

[Selective leach of Al 5 M NaOH under N2]

In-situ washed, dried, “passivated” catalyst

custom leaching rig

characterization

TEMXRD

XRFBET

catalytic testing [TG-FTIR/MS]

TEM micrograph of fresh skeletal Co (ex Al13Co4) Typical TG curve for CH4 decompositon (skeletal Co)

1. abrupt onset of weight gain (blue curve) above 350 C;

2. rapid establishment of fixed rate (10% per h @ 400 C)

250 C

300 C

350 C

400 C

TEM micrographs of carbon nanofibres on skeletal cobalt deposited at 400 C (up to 50 wt. % as carbon)

TG%

0.0

1.0

2.0

3.0

Time/h 1.0 2.0 3.0 4.0

on carburization Co “needles” broken into fine “teardrops”

metal dusting corrosion?

more proof of irreversible change

new activity below 300 C !

0.00160 0.00165 0.00170 0.00175 0.00180 0.00185 0.00190 0.00195-4

-2

0

2 Ln rate Linear Fit of Data1_E

ln r

ate

1/T K

Methane Coking kinetics: 250-330 C Co new2: 11/04/05

Eapp

= 129 +/- 6 kJ.mol-1330 C

300 C

280 C

250 C

Oven Temp.

(C)

Rate of wt. gain (%) per hour in CH4/H2 flow‡

Ni ex Al2Ni

GF

Ni9Cu

ex

Al22Ni9Cu

Ni/SA(65% Ni)

Aldrich

Fe21Cu5

ex

Al74Fe21Cu5

Feex

Al5Fe2

Co20Cu14

ex

Al67Co20Cu14

Coex

Al13Co4

Ru22Cu7

ex

Al71Ru22Cu7

Ruex

Al76Ru24

250 0.01 0.01 0.03 0.08 -- 0.003 0.05 0.005 0.001

280 0.28

300 0.06 0.017 0.080 0.35 0.004 0.02 0.82

330 1.4 2.50

350 0.15 0.030 0.065 zero 0.35 0.07 0.02 1.7 3.30 0.02

360 2.6 -

380 5.3 -

400 8.80 2.20 0.001 1.5 4.4 0.57 8.1 10.6 11.25 0.80 0.07

TG analysis of CH4 decomposition: in-situ pre-reduced samples & controls

‡ CH4 + 2 % H2 [12 ml/min; 1:1 N2]

Red: start at 400 C, then T [new low-T activity]

Blue: as for Red, then switch to CH4/N2 at 250 C & T [dramatic inhibition by H2!]

Green: mean of increasing rate (Fe-containing samples) [long induction phase?]

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.00.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

dTG

(m

g/m

in)

H2 v

ol.

%

Time (h)

450 C hold

300 C

StoichiometryH2 : C = 2.15 : 1

1.6 ml min-1 H2 71 mol. min-1

0.40 mg min-1 C or 33 g at. min-1

Eapp = 129 +/- 6 kJ mol-1

Surface area = 30–160 m2 g-1

Al = 5-10 wt.%Na < 1 wt.%

Amorphous (except Ni)

Proof of unimolecular decompn: CH4 C + 2 H2

Rate of C deposition vs. H2 level [T = 450 C; cat. Fe19Ni9; CH4: 80 ml min-1]

Summary

1. Skeletal metals made from quasicrystalline precursors are “triggered” into CH4 conversion at T > 350 C, yielding nanofibrous carbons & H2 in the ratio C:H2 = 1:2;

2. Pre-carburization leads to irreversible metal decrepitation, akin to “metal dusting corrosion”, creating particles in the range 20-50 nm well suited for filament growth;

3. 1st-row TMs Co, Ni, Fe, & their combinations most active, while Cu moderates activity;

4. Despite remarkable low-T activity, CH4 conversion is still quite low (< 2% at 400 C);

5. Process operation would need high recycle ratios and rapid (in-situ?) removal of product H2, a powerful inhibitor.